WO2024246115A1 - Powder-conveying system for conveying raw-material powder to an installation for manufacturing a three-dimensional workpiece - Google Patents

Abstract

The invention relates to a powder-conveying system for conveying raw-material powder to an installation for manufacturing a three-dimensional workpiece by irradiating layers of the raw-material powder with electromagnetic radiation or particle radiation. The powder-conveying system comprises: a conveyor line which is designed to convey a gas flow, at least along sections, and a powder flow driven by the gas flow, at least along sections; and a conveyor device which is designed to convey the gas flow through the conveyor line. The powder-conveying system also comprises: a first tank, which is connected to the conveyor line, for supplying the installation with powder for an additive manufacturing process; and at least one overflow container, which is connected to the conveyor line, for receiving excess powder accumulating during the additive manufacturing process. The powder-conveying system also comprises: a buffer container which is connected to the conveyor line for supplying a sieve device with powder to be sieved; and the sieve device for sieving the powder to be sieved and for dispensing sieved powder. The powder-conveying system comprises an interface for an external tank, which interface is connected to the conveyor line, for introducing fresh or impure powder into the powder-conveying system. The powder-conveying system also comprises a controller for controlling the powder-conveying system, so that the system carries out at least one of the following conveying processes: a. conveying the sieved powder into the first tank; b. conveying powder from the at least one overflow container into the buffer container; and c. conveying powder from the external tank into the buffer container.

Description

Powder conveying system for conveying raw material powder to a system for producing a three-dimensional workpiece

The invention relates to a powder conveying system for conveying raw material powder to a system for producing a three-dimensional workpiece, in particular for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation (for example a system for selective laser melting or sintering).

In additive (or generative) processes for producing three-dimensional workpieces and in particular in generative layer construction processes, it is known to apply an initially shapeless or shape-neutral molding compound of a raw material (for example a raw material powder) layer by layer to a carrier and to solidify it by site-specific irradiation (for example by melting or sintering) in order to ultimately obtain a workpiece of the desired shape. The irradiation can be carried out using electromagnetic radiation, for example in the form of laser radiation, or using particle radiation, for example in the form of electron radiation. In an initial state, the molding compound can initially be present as granules, as powder or as liquid molding compound and can be solidified selectively or, in other words, site-specifically as a result of the irradiation. In particular, the molding compound can be a bulk material such as raw material powder. The molding compound can comprise, for example, ceramic, metal or plastic materials and also material mixtures thereof. A variant of generative layer construction processes concerns the so-called laser beam melting in the powder bed (also: selective laser melting), in which metallic and/or ceramic raw material powder materials in particular are solidified into three-dimensional workpieces by irradiation with a laser beam.

In order to produce individual workpiece layers in the context of selective laser melting, it is also known to apply raw material powder material in the form of a raw material powder layer to a carrier and to irradiate it selectively and in accordance with the geometry of the workpiece layer currently being produced. The laser radiation penetrates the raw material powder material and solidifies it, for example as a result of heating, which causes melting or sintering. Once a workpiece layer has solidified, a new layer of unprocessed raw material powder material is applied to the workpiece layer already produced. Known coating arrangements or powder application devices can be used for this purpose. The now topmost and still unprocessed raw material powder layer is then irradiated again. The workpiece is then built up layer by layer, with each layer defining a cross-sectional area and/or a contour of the workpiece. In this context, it is also known to use CAD or comparable workpiece data in order to produce the workpieces essentially automatically.

In order to be able to carry out the three-dimensional manufacturing process in a process chamber of the plant, it is necessary that a coater (i.e. a powder application device) of the plant is provided with fresh, uncontaminated raw material powder. Furthermore, it is known that raw material powder that was already in the process chamber is collected in one or more overflow tanks, then sieved and finally made available to the process once again as sieved raw material powder.

In known systems, the powder is transported manually at least in sections and/or the handling of the powder requires that an operator of a powder conveying system has to intervene manually in the conveying process. However, this entails health risks, e.g. through inhalation of the powder, and increases the risk of the powder becoming contaminated, e.g. through contact of the powder with ambient air. It is therefore desirable to provide a powder conveying system which enables powder to be transported as automatically as possible, in particular between several source or destination tanks and/or which is low-maintenance and/or requires as little intervention by a user as possible and/or which enables powder to be transported in which the powder does not come into contact with the ambient atmosphere or comes into contact with it as little as possible.

The object of the invention is therefore to provide an improved powder conveying system which solves at least one of the problems described above or a problem related thereto.

This object is achieved by a powder conveying system having the features of the independent patent claim. Further embodiments are specified in the subclaims.

The invention therefore relates, according to a first aspect, to a powder conveying system for conveying raw material powder to a system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. The powder conveying system comprises a conveying line which is designed to convey a gas flow at least in sections and a powder flow driven by the gas flow at least in sections, and a conveying device which is designed to convey the gas flow through the conveying line. The powder conveying system further comprises a first tank connected to the conveying line for supplying the system with powder for an additive manufacturing process and at least one overflow container connected to the conveying line for receiving excess powder from the additive manufacturing process. The powder conveying system further comprises a buffer container connected to the conveying line for supplying a sieving device with powder to be sieved, and it comprises the sieving device for sieving the powder to be sieved and for dispensing sieved powder. The powder conveying system comprises an interface connected to the conveying line for an external tank for introducing fresh or contaminated powder into the powder conveying system. The powder conveying system comprises a control device for controlling the powder conveying system so that it carries out at least one of the following conveying processes: a powder conveying of the sieved powder into the first tank; b powder conveying from the at least one overflow container into the buffer container; and c powder conveying from the external tank into the buffer container.

The powder conveying system can be set up to convey the powder pneumatically, at least in sections or completely. One or more conveying sections can therefore also be designed, for example, mechanically (e.g. in the form of a screw conveyor) or as a gravity conveyor. The system can in particular be a system for selective laser melting or sintering, which has, for example, one or more of the features described above. Alternatively, the system can be a system for selective electron beam melting.

The conveyor line can, for example, comprise one or more pipes and/or one or more hoses and/or one or more connectors. The conveyor line can be powder-tight and in particular gas- and powder-tight, so that no gas or powder can enter or leave the conveyor line (from the side) - except through the openings in the conveyor line.

The fact that the conveyor line is designed in sections to convey a powder flow driven by the gas flow can mean that only a section of an entire conveyor circuit formed by the conveyor line is designed to convey a gas-powder mixture. The remaining part of the conveying circuit essentially only conveys gas (conveying gas), for example air, protective gas or an air-protective gas mixture.

The control device can, for example, comprise a computer. The control device can comprise a processor and a memory, wherein a program is stored in the memory which, when executed, causes the processor to carry out a method in accordance with the details described below. In particular, it should be noted that the method steps described below can all be carried out by the control unit. Thus, for example, if it is described below that the conveyor device is stopped, this can mean that the control device is set up to control the conveyor device so that it stops. As a further example, it can be described that a measured value from a sensor is taken into account. This can mean that the control unit is set up to receive the value from the respective sensor and, if necessary, to react depending on the control value.

The terms "tank", "storage" and "container" are used synonymously herein. In particular, the terms "overflow container" and "overflow tank" have the same meaning herein. All of the above-mentioned terms ("tank", "storage" and "container") each refer to a container which is designed to hold a predetermined maximum amount of powder, whereby the use of the respective term does not make any (limiting) statement as to how large this maximum amount of powder in the respective container is. However, reference is made to the different containers using specific terms, for example "first tank", "buffer container", "main storage" and "external tank". However, the individual terms are not to be understood as limiting, but merely serve to differentiate the different containers from one another. For example, the "buffer container" could also be referred to as a "second tank". There are therefore no structural differences intended, for example, between a "tank" and a "container". In particular, a corresponding tank can also be formed by a section of the conveyor line or a section of a cyclone if this section is designed to receive a predetermined amount of powder.

For each of the conveying processes a, b and c, associated sections of the conveying line can be provided. In other words, each conveying process a, b and c can be assigned an associated section of the conveying line through which Conveying gas flows during the respective conveying process. In connection with the conveying processes a, b and c, one can therefore also speak of corresponding conveying circuits a, b and c. The conveying circuits a, b and c can share sections of the conveying line. In other words, there can be at least one section of the conveying line that is used by at least two of the conveying processes a, b and c.

The following therefore applies: Each of the conveying processes a, b or c can be assigned an associated conveying circuit a, b or c. Each conveying circuit corresponds to a closed section of the conveying line. At least two of the conveying circuits can share one or more sections of the conveying line, i.e. can use this section(s) together.

Given the above understanding, the terms support group and support process are used synonymously in some places in this disclosure. When a specific support process is mentioned, it is clear that a corresponding support group is implicitly present and vice versa.

The conveying according to the conveying processes a, b and c can be initiated or enabled by opening and/or closing corresponding valves. For this purpose, corresponding valves can be provided at suitable points in the conveying line. If, for example, conveying gas is to be conveyed through a certain conveying circuit, valves located in the conveying line along this conveying circuit can be opened. At the same time, valves that separate the certain conveying circuit from other conveying circuits can be closed so that no conveying gas flows through one or more conveying circuits through which no powder is to be conveyed. Additionally or alternatively, sections of the conveying processes a, b and c or one or more of the conveying processes a, b and c can be designed entirely as a non-pneumatic conveying process. In this case, conveying by means of a conveyor screw can be started, for example, by starting a motor of the conveyor screw and ended by stopping the motor. Conveying by means of gravity conveying can be started, for example, by opening a flap or a valve and ended by closing the flap or the valve.

The first tank can be arranged above a process chamber of the system. The first tank can be arranged inside a housing of the system or outside of it. In addition to the first tank, a further container (herein also The intermediate tank can be arranged above the process chamber, whereby powder can be supplied to the intermediate tank from the first tank. In particular, the first tank can be located above the intermediate tank. The first tank can be located outside a housing of the system and the intermediate tank can be located inside the housing.

Either (a) all of the conveying processes a-c may contain pneumatic conveying processes, wherein a conveyed powder is conveyed by the gas flow, or (b) at least one of the conveying processes a-c, in particular the conveying process b, may not contain a pneumatic conveying process.

In case (b), the conveying process which does not include a pneumatic conveying process may comprise conveying by a screw conveyor and/or conveying by gravity conveying.

Thus, there is at least one embodiment in which all of the conveying processes a-c are designed as pneumatic conveying lines, which are set up to convey the gas flow and, at least in sections, a powder flow driven by the gas flow. However, powder can be fed (in particular a metered feed) into the respective conveying process a-c by means of a conveyor screw. Furthermore, one of the conveying processes a-c, for example process b, can be carried out completely non-pneumatically, for example by means of a correspondingly provided conveyor screw and/or via gravity conveying.

The powder conveying system may further comprise a main reservoir connected to the conveying line for receiving the sieved powder. The conveying process a may convey the sieved powder from the main reservoir into the first tank.

If there is no main storage facility, the conveying process a can, for example, remove the sieved powder directly from the sieve and convey it to the first tank, in particular by means of pneumatic conveying. To remove the powder from the sieve, the sieve can have a corresponding container for the sieved powder.

The following statements apply in particular to a powder conveying system in which all of the conveying processes are designed as pneumatic conveying processes. However, if technically reasonable, the following statements can also be applied to a powder conveying system in which at least one of the Conveying processes ac are designed as non-pneumatic conveying (e.g. as screw conveying).

The powder conveying system can enable closed powder conveying. In particular, the powder conveying system can enable powder conveying in which conveyed powder does not leave the powder conveying system over several consecutive additive manufacturing processes of the system. The term "closed powder conveying" can thus be understood and replaced by the description of powder conveying in which conveyed powder does not leave the powder conveying system over several consecutive additive manufacturing processes of the system. In this way, operator interventions can be minimized.

The powder conveying system can be designed to maintain an inert gas atmosphere within the conveying line, particularly during one of the conveying processes a to c. The inert gas can be nitrogen or argon. The conveying line can thus be inert gas-tight and one or more inert gas sources can be provided for flooding the conveying line with inert gas.

The powder conveying system can further comprise a pressure equalization vessel coupled to the conveying line. The pressure equalization vessel can reduce an overpressure within the conveying line.

The pressure equalization tank can be coupled to the conveying line downstream of the conveying device and upstream of the connections of the first tank, the at least one overflow tank, the buffer tank, the main storage tank and the external tank.

The powder conveying system can further comprise at least one metering device, in particular comprising a conveyor screw, for the metered feeding of the powder to be conveyed into the conveying line from the at least one overflow tank, from the main storage and/or from the external tank.

As an alternative to a screw conveyor, the dosing device can, for example, comprise a dosing screw, a vibrating conveyor or a tube chain conveyor.

The powder conveying system may further comprise at least one separating device, in particular comprising a cyclone, for separating the conveyed powder from the conveyor line and for feeding the powder into the first tank and/or the buffer container.

As an alternative to a cyclone, the separation device may comprise, for example, a centrifugal separator, a filter separator or an electrostatic precipitator.

At least one of the following tanks may be coupled to a pressure equalization line: the first tank, the at least one overflow tank, the buffer tank, the main storage tank and the external tank.

The processes presented below, in particular for controlling the powder conveying system according to the first aspect, can also be used independently of the system of the first aspect. In particular, one or more of the following aspects can be used in a powder conveying system which enables powder to be conveyed from a source tank to a target tank by means of pneumatic conveying.

The control device may be configured to perform a flow check, comprising checking a flow through the conveyor line.

The powder conveying system may further comprise a speed sensor for measuring a speed of the gas flow in the conveying line. The flow check comprises opening valves of the powder conveying system so that a flow is possible through at least one conveying circuit associated with one of the conveying processes a, b and c. The flow check further comprises setting a conveying speed of the gas flow to a predetermined range of values or value and determining whether a speed value of the gas flow measured by the speed sensor is greater than a predetermined limit value.

Adjusting the conveyor speed may include determining whether a speed value measured by the speed sensor is within the predetermined value range, if not, determining whether the speed value is below the predetermined value range, if the speed value is below the predetermined value range, increasing a power of the conveyor device, and if the speed value is not below the predetermined value range, decreasing the power of the conveyor device. The powder conveying system may further comprise a pressure sensor for measuring a pressure of the gas stream and a filter for filtering remaining powder particles from the gas stream. The flow check may comprise: determining a total pressure loss based on sensor data from the pressure sensor if it is determined that the total pressure loss exceeds a predetermined limit, performing a filter cleaning of the filter and determining a total pressure loss again, and if it is determined that the total pressure loss does not exceed the predetermined limit, conveying powder in at least one of the conveying processes a, b and c.

The flow check performed by the controller may further include, if it is determined that the total pressure loss exceeds the predetermined limit, issuing an error before performing the filter cleaning.

The powder conveying system may comprise at least one oxygen sensor for measuring an oxygen content of the gas stream. The controller may be configured to perform an oxygen content check, and the oxygen content check may comprise measuring an oxygen content using the at least one oxygen sensor.

The powder conveying system may comprise a first oxygen sensor for measuring the oxygen content of the gas stream and a second oxygen sensor, redundant to the first oxygen sensor, for measuring the oxygen content of the gas stream.

The control device can be designed to carry out a pipeline inerting of the conveying line if the measurement of the oxygen content of the gas stream shows that the oxygen content is above a predetermined limit value.

Alternatively or additionally, the material can also be flooded with protective gas.

The powder conveying system may further comprise a vent valve for venting gas from the conveying line to an environment or an external volume. Pipeline inerting may comprise opening the vent valve. A filter may be provided behind the drain valve (ie downstream of the drain valve).

The pipeline inerting may further comprise: evacuating at least a portion of the conveying line, leak testing of the conveying line, normalizing the conveying line, and checking an oxygen content in the conveying line.

Evacuating may include checking whether a measured pressure within the conveying line is below a predetermined evacuation pressure.

The powder conveying system may further comprise a speed sensor for measuring a speed of the gas flow in the conveying line. Evacuating may comprise: determining whether a speed value measured by the speed sensor is within a predetermined range of values, if this is not the case, determining whether the speed value is below the predetermined range of values, if the speed value is below the predetermined range of values, increasing a power of the conveying device, and if the speed value is not below the predetermined range of values, decreasing the power of the conveying device.

The leak test may include: closing the drain valve, stopping the conveyor, determining if a system pressure rise per time exceeds a predefined limit, if the system pressure rise per time exceeds the predefined limit, issuing an error message, and if the system pressure rise per time does not exceed the predefined limit, proceeding to normalize the conveyor line.

The powder conveying system may include a first pressure sensor on an inlet side of the conveyor and a second pressure sensor on an outlet side of the conveyor. Determining the system pressure increase may include considering a sum of the readings from the first and second pressure sensors.

Normalizing the production line may include: flooding the production line with inert gas and measuring an oxygen content in the production line.

The powder conveying system may further comprise restarting the pipeline inerting if the measured oxygen content in the conveying line exceeds a predetermined limit. Optionally, before restarting the Pipeline inertization can be flooded with inert gas. If the predetermined limit is still exceeded, the pipeline inertization is restarted. If the limit is no longer exceeded, the conveying process can be started, for example, or the pipeline inertization can be omitted.

Checking the oxygen content in the production line may include restarting the pipeline inerting if the measured oxygen content exceeds a predetermined limit a predetermined number of times.

The control device can be configured to carry out a conveying operation according to at least one of the conveying processes a, b and c, wherein the conveying operation comprises opening at least one valve which is arranged in a conveying circuit associated with the respective conveying process.

The control device can be configured to carry out a conveying control during conveying, which comprises: monitoring an oxygen content in the conveying line, monitoring a pressure in the conveying line, monitoring a conveying speed of the gas flow, monitoring at least one measured characteristic of the powder conveying system, and checking a termination condition for terminating the conveying.

Monitoring the oxygen content in the production line may include determining whether a measured oxygen content exceeds a predetermined limit, and if the measured oxygen content exceeds the predetermined limit, opening a valve to supply inert gas to the production line.

Monitoring the pressure in the production line may include determining whether a measured pressure in the production line is within a predetermined range, if the measured pressure in the production line is less than the predetermined range, opening a valve to supply inert gas, and if the measured pressure in the production line is higher than the predetermined range, opening a vent valve to vent gas from the production line.

Monitoring the conveying speed of the gas flow may include: determining a gas density of the gas flow based on at least one measured characteristic of the gas flow, determining a powder mass flow conveyed through the conveying line based on a flow rate fed to a metering device of a control value applied to the source tank of the conveyance, determining a target velocity of the gas flow based on the gas density and based on the bulk material mass flow, and controlling the conveying device to convey the gas flow at the determined target velocity.

Monitoring at least one measured characteristic of the powder conveying system can include: controlling a dosing device of a source tank of the conveyance with a predetermined control value, determining whether the at least one measured characteristic of the powder conveying system is above a predetermined maximum value for the respective characteristic, and if the at least one measured characteristic is above the predetermined maximum value, reducing the control value of the dosing device by a predetermined value so that the dosing device delivers a smaller dose of powder per unit of time into the gas stream.

The control device can further be configured to, if the at least one measured parameter is below the predetermined maximum value, increase the control value of the dosing device by a predetermined value so that the dosing device releases a higher dose of powder per unit of time into the gas stream.

The at least one parameter may include at least one of the following parameters: conveying speed, pump outlet pressure, pump power and dose of powder per time.

Checking a termination condition for ending the conveying may include ending the conveying by stopping the conveying device if at least one of the following events is detected: a fill level of a source tank from which powder is taken for conveying falls below a predetermined limit, a fill level of a target tank into which the powder is conveyed exceeds a predetermined limit, a predetermined maximum conveying time is exceeded and a total pressure loss of a conveying gas exceeds a predetermined limit.

The control device can be configured to carry out a pipe cleaning and a filter cleaning after completion of a conveying process according to conveying process a, b and/or c, wherein the pipe cleaning comprises a gas stream flowing through the conveying line, and wherein the filter cleaning comprises blowing off a filter provided in the conveying line with compressed air. The powder conveying system can further comprise at least one dew point sensor for measuring a relative humidity within the conveying line and/or within a tank of the powder conveying system. The control unit can be configured to initiate automatic powder drying when the measured value of the relative humidity exceeds a predetermined limit value.

The powder drying can be carried out using a drying unit of the powder conveying system. For this purpose, the drying unit can comprise a heating unit and/or a moisture absorption means. Furthermore, a vacuum generation unit can be used as a drying unit. The powder drying can be stopped when the dew point sensor measures a predetermined residual moisture, i.e. when a measured moisture value falls below a predetermined threshold value.

A dew point sensor may be provided on at least one of the following components of the powder conveying system: main tank, external tank, and conveyor.

At least one oxygen sensor may be provided for measuring an oxygen concentration at at least one of the following components of the powder conveying system: first tank, overflow tank, buffer tank, main storage, external tank and screening device. The control device may be configured to initiate flooding of the conveying line with inert gas if it is determined that at least one of the provided oxygen sensors measures an oxygen concentration above a predetermined limit value.

At least one pressure sensor for measuring a pressure can be provided on at least one of the following components of the powder conveying system: first tank, overflow tank, buffer container, main storage, external tank and sieving device. The control device can be set up to carry out at least one of the following steps if it is determined that at least one of the pressure sensors provided measures a pressure increase per predetermined unit of time above a predetermined limit value: issuing a warning, flooding the conveying line or an affected section of the powder conveying system with inert gas, measuring an oxygen content in the affected section of the powder conveying system, and carrying out a filter cleaning. At least one temperature sensor for measuring a temperature can be provided on at least one motor of the powder conveying system, in particular a motor of a conveyor screw and/or the conveying device. The control device can be set up to initiate a shutdown of the conveying device if it is determined that at least one of the temperature sensors provided measures a temperature value above a predetermined limit value.

The control device can be configured to initiate a shutdown of the conveying device if at least one of the following events is detected: a torque of a motor of a dosing device exceeds a predetermined limit value, a valve of the powder conveying system is in an actual position that does not correspond to its target position, and an error in a sensor is detected.

A fill level sensor for measuring a fill level of the at least one overflow tank can be provided on the at least one overflow tank, in particular in the form of one or more load cells, wherein a fill level sensor for measuring a fill level of the intermediate tank is provided on an intermediate tank which is arranged above the process chamber and which is designed to supply the process chamber with powder and which is designed to be supplied with powder from the first tank, in particular in the form of one or more load cells. The control device can be designed to continue an additive manufacturing process of the system in the event of a failure of the powder conveyance by the conveying device until at least one of the following events occurs: the fill level sensor of the overflow tank detects that the fill level of the overflow tank exceeds a predetermined limit value, and the fill level sensor of the intermediate tank detects that the fill level of the intermediate tank falls below a predetermined limit value.

The control device can be configured to carry out a powder conveyance of a predetermined powder quantity according to one of the conveying processes a, b or c and then to stop the conveyance.

The control device can be set up to determine a priority value for each of the conveying processes after each conveying process according to one of the conveying processes a, b and c, based on predetermined fill level limits of the source tanks and the target tanks. The control device can be set up to to then carry out funding in accordance with the funding process with the highest priority value.

The system described here for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation can, for example, comprise a carrier device for applying the powder in several layers so that a powder bed is formed. Furthermore, one or more powder application devices can be provided for applying the powder and, if necessary, for applying powder of different materials. A separate powder application device can be provided for each material. The carrier device can be moved vertically downwards by means of a lifting device so that the topmost powder layer always remains at the same height in relation to a construction chamber of the system. Furthermore, the system can comprise one or more irradiation units. The irradiation units each comprise a beam source (in particular a laser beam source) and an optics with one or more optical components for shaping and deflecting the beam (e.g. beam expander, focusing unit, scanner device, F-theta lens). Alternatively, the beam source can be located outside the respective irradiation unit, with the beam being fed to the irradiation unit via an optical conductor (e.g. fiber optic).

The invention is explained below with reference to the attached figures. They show:

Figure 1: a schematic overview of a powder conveying system for conveying raw material powder to a system for producing a three-dimensional workpiece;

Figure 2: a flow chart of a higher-level function of the powder conveying system;

Figure 3: a flow test of the conveyor circuit a or a conveying in the conveyor circuit a, with active elements of the powder conveying system according to Fig. 1 highlighted in bold;

Figure 4: a flow test of the conveyor circuit b or a conveying in the conveyor circuit b, with active elements of the powder conveying system according to Fig. 1 highlighted in bold; Figure 5: a flow test of the conveyor circuit c or a conveying in the conveyor circuit c, with active elements of the powder conveying system according to Fig. 1 highlighted in bold;

Figure 5a: a schematic overview of an alternative embodiment of a powder conveying system for conveying raw material powder to a system for producing a three-dimensional workpiece;

Figure 6: a flow chart of a leak test;

Figure 7: a pipe evacuation, with active elements of the powder conveying system according to Fig. 1 highlighted in bold;

Figure 8: a flow chart of an oxygen content check;

Figure 9: a flow chart of an adjustment of a conveying gas velocity;

Figure 10: a flow chart of a limit control

Figure 11: a plant for producing a three-dimensional workpiece by irradiating layers of a raw material powder with electromagnetic radiation or particle radiation, which is equipped with a bulk material conveying system;

Figure 12: a schematic representation of a dosing device in the form of a conveyor screw, wherein parameters of the conveyor screw are given for calculating a mass flow conveyed by the conveyor screw; and

Figure 13: a flow chart of a limit control process;

Figure 14: a plant for producing three-dimensional workpieces by exposing raw material powder layers to electromagnetic radiation or particle radiation, which is equipped with a powder preparation system comprising a sieving device; Figure 15: a detailed view of the sieving device used in the powder processing system according to Figure 14;

Figure 16: a loose powder bed on a level;

Figure 17: the screening device according to Figure 15 with a continuous drive of the screen with a low (second) drive power;

Figure 18: the screening device according to Figure 15 with a continuous drive of the screen with a high (first) drive power;

Figure 19: the screening device according to Figure 15 in operation, wherein the screening device is periodically driven alternately with the first drive power and the second drive power;

Figure 20: the development of the drive power (top) and the dosing mass flow (bottom) as a function of time when the dosing mass flow is controlled as a function of the drive power;

Figure 21: the development of a step response of a sum of a sieved powder mass flow and an oversize mass flow to a dosing mass flow in the event of a defect in a sieve fabric; and

Figure 22: an alternative embodiment of a screening device comprising a housing and a screen mounted so as to be tiltable relative to the housing, the angle of inclination of which can be adjusted relative to the horizontal plane.

Fig. 1 shows a schematic representation of a powder conveying system which is used for conveying raw material powder in a system 1100 for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation.

In the system shown in Fig. 1, several conveying processes can be carried out within the system, ie conveying processes from different source tanks to different destination tanks. The exact structure and functioning of the powder conveying system is described below. Not shown in Fig. 1 (but part of the system) is a control device for the powder conveying system. This control device controls the individual components of the system in Fig. 1, in particular the valves, dosing devices and the conveying device described below. The control device also receives and processes data from the sensors described below.

The system 1100 corresponds, for example, to a generally known system for additive manufacturing using selective laser melting or selective laser sintering. In Fig. 1, only the process chamber 1011 of the system 1100 is shown. Raw material powder is fed by means of a loader 2104 from an intermediate tank 2102 (also: hopper) of the system 1100 to the process chamber 1011 (more precisely, a powder application device or a coater in the process chamber 1011).

Excess powder from the additive manufacturing process can be collected in a front overflow tank 2 or a rear overflow tank 3. This is in particular powder that is not needed when coating a new powder layer and is thus pushed by a powder application device into one of the overflow tanks 2 or 3. An inlet valve 19 or 17 is located at an inlet of the respective powder tanks 2 or 3.

The overflow tanks 2 and 3 each comprise an outlet valve 34 or 25. Furthermore, a powder sensor 35 or 26 is provided in the area of the respective outlet of the overflow tanks 2 and 3. This monitors the inlet of an associated dosing device in the form of a dosing screw (or conveyor screw) 36 or 27. A powder sensor 37 or 28 is also provided at the outlet of the respective dosing screw 36 or 27.

The overflow tanks 2 and 3 each comprise an upper powder sensor 31 or 22 and a lower powder sensor 32 or 23 for monitoring the fill level of the respective overflow tank 2 or 3. In addition, weighing cells 33 or 24 are provided on the respective overflow tanks 2 or 3. To carry out pressure equalization, a valve 30 or 21 is located on a respective upper side of the overflow tanks 2 or 3. The respective overflow tank 2 or 3 is connected to the pressure sensor 30 or 21.

3 is connected to a delivery line. For this purpose, there is another valve 38 or 29 at the respective access to the delivery line. The process chamber is connected via corresponding valves 20 or 18 for pressure equalization to a line which connects the valve 30 or 21 with the valve 38 or 29.

The powder is conveyed from the overflow tanks 2 and 3 into a buffer tank 4 via the conveying line. A cyclone 42 is located above the buffer tank 4 to separate the powder.

After the powder has been separated in the cyclone 42, the conveying gas flows through the conveying line into a filter 41, where remaining powder particles are filtered out. To clean the filter, a compressed gas supply 39 is in contact with the filter 41, which can be switched on by the control device. There is also a valve 40 downstream of the filter 41. The conveying circuit leads back to the conveying device 79, which is provided in the form of a pump 79. The path of the conveying gas, starting from the valve 40, leads in this order via a pump protection filter 74, a dew point sensor 75, an oxygen sensor 76, a redundant oxygen sensor 77 and a pressure sensor 78, more precisely a pressure sensor 78 on the pump suction side.

Starting from the pump 79, the conveying gas flows further via a pressure sensor 80 on the pump pressure side (not shown separately on the pump pressure side in the schematic representation of Fig. 1). A pressure compensation tank 84 is provided downstream of the pressure sensor 80. This is equipped with an inert gas supply 82 and a safety valve 83. The pressure compensation tank 84 serves to prevent excessive overpressure on the pump pressure side, i.e. downstream of the pump 79.

Between pressure sensor 80 and pressure compensation tank 84, an exhaust valve 81 is provided, via which gas can be released from the conveying line. Further downstream of pressure compensation tank 84 there is a speed and temperature sensor 86. After venting access via valves 88 and 87, there is a possibility of directing the conveying gas to convey powder from a main reservoir 6 by opening a valve 72 and/or to convey powder from an external tank 8 and/or from one or both of the overflow tanks 2 and 3 by opening a valve 73.

When valve 72 is opened, the conveying gas flows to a cyclone 9 after powder from the main reservoir 6 has been introduced into the gas stream via valve 60. which is located above a first tank 1. Powder can be separated there for conveying into the first tank 1. The conveying gas flows via the cyclone 42 and through filter 41 back to the pump 79, on the previously described path (past several sensors, among other things).

When valve 73 is opened, powder from an external tank 8 can be added to the flow of conveying gas by a metering device 70 (conveyor screw 70). Furthermore, powder from the overflow tank 3 and/or powder from the overflow tank 2 can be added via the conveyor screw 27 and/or via the conveyor screw 36. The conveying gas-powder mixture then flows - as previously described - to the cyclone 42, where it can be conveyed into the buffer container 4.

Powder leaving the conveyor screw 58 of the main storage 6 can be fed via a valve 60 either to the conveying gas of the conveying line which is associated with the valve 72, or alternatively or additionally via a valve 61 to the conveying gas of the conveying line which is associated with the valve 73.

Powder that is separated by the cyclone 9 enters the first tank 1 via an inlet valve 12. From there it is conveyed via an outlet valve 16 into an intermediate tank 2102 (also: hopper), for example by means of gravity conveying. From the intermediate tank 2102 it can be fed via the loader of the process chamber 1011 of the system 1100 and used in the additive manufacturing process.

Powder which is separated by the cyclone 42 passes through an inlet valve 44 into a buffer container 4. From the buffer container 4, the powder can be fed through an outlet valve 48 to a sieving device 5 (by means of gravity conveyance). The sieving device 5 serves to filter out larger particles and impurities from the powder, in particular if the powder which is fed to the sieving device 5 has already been used in an additive manufacturing process and, for example, comes from one or both of the overflow tanks 2 or 3.

The screening device 5 comprises a dosing device 49 in the form of a conveyor screw 49 in order to feed powder to be screened in a metered manner to an ultrasonic sieve 50 of the screening device 5. Sieved powder or sieved particles pass through a line into an oversize container 7.

The sieved powder enters the main storage 6 via an inlet valve 52. The powder can be discharged from the main storage 6 via an outlet valve 56 and via the Conveyor screw 58 of the conveyor line for pneumatic conveying into the first tank 1, as described above.

Furthermore, an external tank 8 can be connected to the conveyor line, which can be a movable tank, for example, which can be connected to the conveyor line by means of a suitable interface. Alternatively, a movable tank can be connected to the external tank 8 via an inlet valve 63 of the external tank 8, so that, for example, fresh powder can be fed to the system.

Powder from the external tank 8 is fed to the conveyor screw 70 via an outlet valve 67 of the external tank 8 and via an inlet valve 68 of a conveyor screw 70. From there, the powder enters the conveyor line and is conveyed by the gas flow when valve 73 is open.

Each of the powder containers 1, 2, 3, 4, 6 and 8 has an inlet valve 12, 19, 17, 44, 52 and 63 and an outlet valve 14, 34, 25, 48, 56 and 67. For pressure equalization, each of the powder containers 1, 2, 3, 4, 6 and 8 also has a valve 10 or 11, 30, 21, 43, 51 and 62. Via these pressure equalization valves, the respective powder container is connected via a respective pressure equalization line to the conveying line in which the gas flow of the conveying gas flows, except in the case of the first tank 1, whose pressure equalization line is connected to the intermediate tank.

The pressure equalization lines of the powder containers 2, 3, 4, 6 and 8 are connected to the conveyor line via an associated valve 38, 29, 8 (the same for containers 4 and 6) and 87.

For measuring a fill level of the powder containers 1, 2, 3, 4, 6 and 8, each of these powder containers 1, 2, 3, 4, 6 and 8 comprises an upper powder sensor 13, 31, 22, 45, 53 and 64 and a lower powder sensor 14, 32, 23, 46, 54 and 65.

Furthermore, each of the powder containers 1, 2, 3, 4, 6 and 8 is provided with load cells 15, 33, 24, 47, 55 and 66. These are used to weigh the respective container and thus also serve to determine the fill level of the respective container.

In particular to prevent and/or detect blockages, a powder sensor 35, 26, 57 and 69 is provided at the inlet of the conveyor screws 36, 27, 58 and 70 and a powder sensor 37, 28, 59 and 71 is provided at the outlet of the conveyor screws 36, 27, 58 and 70. The individual elements of the powder conveying system shown in Fig. 1 are listed again below using their reference symbols, although this list does not claim to be complete.

1 - First Tank

2 - front overflow tank

3 - rear overflow tank

4 - Lock

5 - Sieve

6 - Main memory

7 - Oversize barrel (oversize container)

8 - external tank module (external tank)

9 - Cyclone

10 - Valve pressure equalization first tank

11 - Valve pressure equalization first tank machine side

12 - Valve inlet first tank

13 - upper powder sensor first tank

14 - lower powder sensor first tank

15 - Weighing cells First tank

16 - Valve Outlet First Tank

17 - Rear overflow tank inlet valve

18 - Valve pressure equalization rear overflow tank machine side

19 - Front overflow tank inlet valve

20 - Valve pressure equalization front overflow tank machine side

21 - Rear overflow tank pressure equalization valve

22 - upper powder sensor rear overflow tank

23 - lower powder sensor rear overflow tank

24 - Weighing cells rear overflow tank

25 - Rear overflow tank outlet valve

26 - Powder sensor inlet dosing screw rear overflow tank

27 - Dosing screw rear overflow tank

28 - Powder sensor outlet dosing screw rear overflow tank

29 - Valve pressure equalization delivery line rear overflow tank

30 - Front overflow tank pressure equalization valve

31 - upper powder sensor front overflow tank

32 - lower powder sensor front overflow tank

33 - Front overflow tank load cells 34 - Front overflow tank outlet valve

35 - Powder sensor inlet dosing screw front overflow tank

36 - Dosing screw front overflow tank

37 - Powder sensor outlet dosing screw front overflow tank

38 - Valve pressure equalization delivery line front overflow tank

39 - Pressure gas supply filter cleaning

40 - Valve suction side filter

41 - Filter

42 - Zyklon

43 - Valve pressure equalization lock

44 - Valve inlet lock

45 - upper powder sensor lock

46 - lower powder sensor lock

47 - Load cells lock

48 - Valve outlet lock

49 - Dosing screw sieve

50 - Ultrasonic sieve

51 - Main accumulator pressure equalization valve

52 - Valve inlet main reservoir

53 - upper powder sensor main storage

54 - lower powder sensor main storage

55 - Load cells main memory

56 - Valve outlet main reservoir

57 - Powder sensor inlet dosing screw main reservoir

58 - Dosing screw main storage

59 - Powder sensor outlet dosing screw main reservoir

60 - Valve powder outlet “a”

61 - Valve powder outlet “b”

62 - Valve pressure equalization external tank module

63 - Valve inlet external tank module

64 - upper powder sensor external tank module

65 - lower powder sensor external tank module

66 - Load cells external tank module

67 - Valve outlet external tank module

68 - Valve inlet dosing screw external tank module

69 - Powder sensor inlet dosing screw external tank module

70 - Dosing screw external tank module

71 - Powder sensor outlet dosing screw external tank module 72 - Gas supply valve "a"

73 - Gas supply valve "b" & "c"

74 - Pump protection filter

75 - Dew point sensor

76 - Oxygen sensor

77 - redundant oxygen sensor

78 - Pressure sensor pump suction side

80 - Pressure sensor pump pressure side

81 - Exhaust valve

82 - Inert gas supply pressure compensation tank

83 - Safety valve pressure equalization tank

84 - Pressure equalization tank

85 - Inert gas supply delivery line

86 - Speed & temperature sensor

87 - Valve pressure equalization delivery line external tank module

88 - Valve pressure equalization delivery line main storage

In connection with the arrangement of a powder conveying system described above and shown in Fig. 1, at least three conveying processes a to c can be implemented. In other words, the bulk material conveying system comprises at least three conveying circuits a to c. The conveying circuits share the conveying line in which the conveying gas circulates, at least in sections. Furthermore, two of the conveying processes can run in parallel, in particular the conveying processes b and c described below. This leads to a mixing (blending) of powder from the overflow tanks 2 and 3 and the external tank 8, with the powder mixing in the buffer container 4.

The individual conveying processes or conveying circuits are explained below. They are shown in Figures 3 to 5. Fig. 3 shows conveying process a, Fig. 4 shows conveying process b and Fig. 5 shows conveying process c. Active elements are shown in bold so that the powder or gas flow can be followed.

Conveying process a (Fig. 3):

In the conveying process a, powder is conveyed from the main storage 6 (source tank) into the first tank 1 (target tank). The conveying in the conveying process a includes the prior opening of the valves 56 and 60. The powder in the main storage 6 is It is powder that was previously sieved by the sieving device 5. The powder can then be fed from the first tank 1 to the additive manufacturing process in the process chamber 1011. The powder is fed into the conveying line via the conveyor screw 58 and is separated in the cyclone 9. The conveying gas flows via the filter 41 back to the pump 79.

Conveying process b (Fig. 4):

In the conveying process b, powder is conveyed from the overflow tank 2 (original tank) and/or the overflow tank 3 (original tank) into the buffer tank 4 (target tank). The conveying in the conveying process b includes the prior opening of the valves 25 and 34. The powder in the overflow tanks 2 and 3 is contaminated powder that was not solidified in the additive manufacturing process. The contaminated powder is fed from the buffer tank 4 to the sieve device 5. The powder is brought into the conveying line via the conveyor screws 36 and 27 and is separated in the cyclone 42. The conveying gas flows back to the pump 79 via the filter 41.

Conveying process c (Fig. 5):

In the conveying process c, powder is conveyed from the external tank 8 (source tank) into the buffer tank 4 (target tank). The conveying in the conveying process c includes the prior opening of the valves 67 and 68. The powder in the external tank 8 is possibly contaminated powder that is fed into the process from outside. However, it can also be uncontaminated, fresh powder. The powder conveyed from the buffer tank 4 into the buffer tank 4 is fed to the sieve device 5. The powder is brought into the conveying line via the conveyor screw 69 and is separated in the cyclone 42. The conveying gas flows back to the pump 79 via the filter 41.

Fig. 5a shows a schematic representation of an alternative powder conveying system which is used for conveying raw material powder in a system 1100 for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. Since the powder conveying system of Fig. 5a is constructed similarly to that of Fig. 1, differing aspects of the two embodiments are described in particular below. Aspects of the powder conveying system of Fig. 5a which are not explained can correspond to those of the system of Fig. 1, unless otherwise stated. In particular, the same or comparable elements of the powder conveying system of Fig. 5 are identified with the same reference numerals as in Fig. 1.

In the system shown in Fig. 5a, several conveying processes can be carried out within the system, i.e. conveying processes from different source tanks to different target tanks. The exact structure and functioning of the powder conveying system is described below.

Not shown in Fig. 5a (but part of the system) is a control device for the powder conveying system. This control device controls the individual components of the system in Fig. 5a, in particular the valves, metering devices, conveyor screws and the conveying device described below. The control device also receives and processes data from the sensors of the system in Fig. 5a, which correspond to those described above in connection with Fig. 1. To avoid repetition, the sensors are therefore not described again.

The system 1100 corresponds, for example, to a generally known system for additive manufacturing by means of selective laser melting or selective laser sintering. In Fig. 5a, only the process chamber 1011 of the system 1100 is shown. Raw material powder is fed by means of a loader from a first tank 1 of the system 1100 to the process chamber 1011 (more precisely, a powder application device or a coater in the process chamber 1011). Alternatively, according to the above description of Fig. 1, an additional intermediate tank can be provided between the first tank 1 and the process chamber 1011.

Excess powder from the additive manufacturing process can be collected in a front overflow tank 2 or a rear overflow tank 3, with the overflow tanks 2, 3 being shown together in Fig. 5a. This is in particular powder that is not needed when coating a new powder layer and is thus pushed into one of the overflow tanks 2 or 3 by a powder application device.

The powder is conveyed from the overflow tanks 2 and 3 into a buffer tank 4 via a conveyor screw 991, which is part of the conveyor line. In particular, two conveyor screws 991 can be provided - one for each of the overflow tanks 2, 3. A valve 992 is provided at the inlet of the buffer tank 4. In the In the area of the valve 992, the powder can fall into the buffer container 4 by gravity.

The powder contained in the buffer container 4 can be fed to the screening device 5 by means of a conveyor screw 49. At the entrance to the screening device 5 there is a valve 993 which must be open for the purpose of feeding powder into the screening device. Starting from one end of the conveyor screw 49, the powder reaches the screening device 5 by means of gravity conveyance.

The screened out oversize grain is conveyed by pneumatic conveying via a cyclone 994 into the oversize grain container 7. A valve 995 is located at the inlet of the oversize grain container. Screened powder collects on the underside of the screening device in a volume provided for this purpose and is transported away from there by pneumatic conveying and separated in the cyclone 9 and fed to the first tank 1.

The gas flow of the pneumatic conveying circuit is driven by a conveying device 79, which is provided in the form of a pump 79. Downstream of the pump 79, an exhaust valve 81 is provided, via which gas can be discharged from the conveying line.

When valve 996 is opened, powder from an external tank 8 can be added to the flow of conveying gas by a metering device 70 (conveyor screw 70). The conveying gas-powder mixture then flows to the cyclone 997, where it can be conveyed into the buffer container 4. New powder (i.e. powder newly fed into the system) can thus first be sieved by the sieving device 5 before it is processed in the process chamber 1011.

Powder separated by the cyclone 9 enters the first tank 1 via an inlet valve 12. From there, it can be fed via the loader of the process chamber 1011 of the system 1100 and used in the additive manufacturing process.

Powder which is separated by the cyclone 997 passes through an inlet valve 998 into a buffer container 4. From the buffer container 4, the powder can be fed to the screening device 5 through the conveyor screw 49. Furthermore, it is possible to empty the powder conveying system, in particular via the powder screw 49 and the Zyklop 994, by appropriately controlling the respective valves.

A valve 9911 is provided at the inlet of cyclone 9. A valve 9910 is provided at the inlet of cyclone 997. A valve 999 is provided at the inlet of cyclone 994. By opening these valves 999, 9910 and 9911, separation can be switched on via the respective cyclone. Thus, a corresponding conveying process can be switched on and off via these valves 999, 9910 and 9911 in particular.

In connection with the arrangement of a powder conveying system described above and shown in Fig. 5a, at least three conveying processes a to c can be implemented. In other words, the bulk material conveying system comprises at least three conveying circuits a to c. The conveying circuits share the conveying line in which the conveying gas circulates, at least in sections. Furthermore, two of the conveying processes can run in parallel, in particular the conveying processes b and c described below. This leads to a mixing (blending) of powder from the overflow tanks 2 and 3 and the external tank 8, whereby the powder mixes in the buffer container 4. In the example shown in Fig. 5a, the conveying process b is implemented as non-pneumatic conveying.

The individual funding processes and funding groups are explained below.

funding process a:

In the conveying process a, sieved powder is conveyed from a volume on the underside of the sieving device 5 (original tank) into the first tank 1 (target tank). The conveying in the conveying process a includes at least the prior opening of the valve 9911. The sieved powder is powder that was previously sieved by the sieving device 5. The powder can then be fed from the first tank 1 to the additive manufacturing process in the process chamber 1011. The conveying gas flows back to the pump 79 via the valve 9911.

funding process b:

In the conveying process b, powder is conveyed from the overflow tank 2 (source tank) and/or the overflow tank 3 (source tank) into the buffer tank 4 (target tank). For this purpose, the conveyor screw 991 is switched on and the conveying is not pneumatic, but mechanical via the conveyor screw 991. At one end of the conveyor screw 991, the powder falls by gravity via the open valve 992 into the buffer container 4. The conveying in the conveying process b can include the prior opening of the valve 992. The powder in the overflow tanks 2 and 3 is contaminated powder that was not solidified in the additive manufacturing process. The contaminated powder is fed from the buffer container 4 to the sieve device 5 by means of the conveyor screw 49.

funding process c:

In the conveying process c, powder is conveyed from the external tank 8 (source tank) into the buffer tank 4 (target tank). The conveying in the conveying process c includes the prior opening of the valves 996 and 9910. The powder in the external tank 8 is possibly contaminated powder that is fed into the process from outside. However, it can also be uncontaminated, fresh powder. The powder conveyed from the buffer tank 4 into the buffer tank 4 is fed to the sieve device 5. The powder is brought into the conveying line via the conveyor screw 70 and is separated in the cyclone 997. The conveying gas flows back to the pump 79 via the valve 9910.

Further details of the powder conveying system of Fig. 1 are explained below and, in particular, processes or methods are explained which can be carried out in connection with the powder conveying system. The methods described are carried out by a control unit (not shown) of the powder conveying system. For this purpose, the control device comprises a processor and a memory on which corresponding commands for carrying out the individual methods are stored. The control device also comprises a human-machine interface and one or more interfaces for receiving sensor data. Finally, the control device comprises one or more interfaces for (electrically) controlling individual components of the powder conveying system (such as the valves, the dosing devices, etc.).

Although the following explanations are directed in particular to the powder conveying system of Fig. 1, they can also be applied to the powder conveying system of Fig. 5a. Even if not explicitly shown in Fig. 5a, the containers shown therein and described above also include corresponding sensors and/or valves for carrying out the processes described below. Embodiments of the technology presented here can in particular enable inert metal powder conveying, which, by entering specific powder parameters (grain size, bulk density, etc.), universally approaches the optimal conveying point (i.e. conveying gas velocity) for the common powder types and reacts to changing system influences (clogging of filters, leakage, 02 rise, etc.).

The powder conveying system has an HMI (Human Machine Interface) and a PLC (Programmable Logic Controller) which can store recipes for different types of powder, especially metal powder. These contain material-specific characteristics such as:

solid density

bulk density

Grain size distribution & mean grain size

Using these material properties, a conveyor control system can move to the optimal conveyor point. This is defined by the saltation speed + safety distance. See "Adjustment of conveyor gas speed" in Fig. 9.

Apart from the adjustment of the conveying gas velocity according to Fig. 9, several processes are presented here and in particular below, each of which can run in isolation from the other processes and offer independent advantages. In particular, the adjustment of the conveying gas velocity is only optional and both the arrangement of several conveying circuits according to Fig. 1 and the provision of different control processes offer independent advantages.

Furthermore, it is pointed out at this point that the individual control processes presented below (for example the process of adjusting the conveying gas velocity) can also be used in a pneumatic conveying system independently of the specific arrangement according to Fig. 1.

The system of Fig. 1 has three conveying processes: a - conveying powder from the main tank 6 to the first tank 1 b - conveying powder from the front overflow tank 2 and the rear overflow tank 3 to lock 4 (buffer tank 4) c - conveying powder from the external tank 8 to lock 4 The conveying processes b & c can also run in parallel, so-called "blending" (mixing of powder from tanks 2 or 3 and 8).

Each container (i.e. in particular containers 1, 2, 3, 4, 6 and 8) has pressure equalization lines to the conveyor system to ensure optimal powder discharge.

A higher-level function of the powder conveying system of Fig. 1 is shown in Fig. 2.

Before each conveying cycle ("conveying" in Fig. 2), an attempt is made to build up a gas flow in the respective target pipe system and the total pressure loss is determined. If this exceeds a defined limit, the automatic filter cleaning is triggered and another attempt is made to build up a gas flow. If this does not succeed again within the defined operating conditions, an error is issued which indicates a clogged system.

If the system differential pressure is below the defined limit, the oxygen content of the gas flow is determined ("Initial O2 test" in Fig. 2). If this is below a defined limit, metal powder conveying can begin ("Conveying" in Fig. 2). If the oxygen value is above a defined limit, initial pipeline inerting is triggered.

During the initial pipeline inerting, the pipeline circuit is interrupted at valves 72 & 73 and the drain valve 81 is opened. The pipelines are evacuated either separately ("a" & "b") or together. If the pressure at sensor 78 falls below a defined limit value, the pipeline system is tested for leaks (see Fig. 6).

During the leak test (Fig. 6), the pressure sensors 78 & 80 are evaluated. The system pressure increase per time must not exceed a defined limit. If the limit is undershot, normalization can continue. If the limit is exceeded, an error message is displayed on the HMI.

During normalization, the drain valve 81 is closed, the shut-off valves 72 & 73 are opened and the inert gas supply 82 is opened. The pressure in the piping system is increased with inert gas up to a defined total system pressure and then the valve 82 is closed. During the process, the oxygen content of the pipe system is monitored 76 & 77. If this repeatedly exceeds the defined limit, the initial pipe inerting is repeated. After a definable number of repetitions, the pipe inerting can be considered to have failed. An error message is displayed on the HMI.

If the oxygen content is below the defined limit value, the metal powder conveying process ("conveying" in Fig. 2) can continue.

In the metal powder conveying cycle, the corresponding valves of the required conveying circuit are opened. These include the powder outlet valves of the source tank, as well as the inlet valves of the target tank and the internal machine lock 4. The pressure equalization lines of the source tanks to the conveying line are also opened. The motors of the respective dosing elements are activated and their speed is changed via the conveyor control system.

The conveyor control is structured in several stages and runs in a loop until defined termination criteria are reached.

The first step is the oxygen control. If the measured oxygen content on sensors 76 & 77 exceeds a defined limit, the valve 85 is opened and inert gas is supplied. If the oxygen limit is not reached, the valve 85 is closed. However, if a defined time value is exceeded by then, the valve 85 remains open and the next step of pressure control is carried out.

During pressure control, sensors 78 & 80 are used to check whether the total system pressure is within the defined range. If this is not reached, valve 85 is opened and inert gas is supplied. If this is exceeded, valve 81 is opened and conveying gas is released. If the defined range for the total system pressure is reached, the calculation of the target speed for the gas flow continues (see also Fig. 9).

The calculation of the target speed for the gas flow is structured in 3 stages (see Fig. 9). The first partial calculation determines the density of the conveying gas ("Calculation of gas density"). This depends on the mixing ratio of inert gas, residual air and moisture content, as well as their standard parameters (e.g. density). The Mixing ratio is determined via the oxygen sensors 76 & 77 and the dew point sensor 75. Using the mixing ratio of the gas pressure at sensor 80 and the gas temperature at sensor 86, the gas density at the pump outlet can be determined. For powder feed points further along the pipeline section, additional pressure sensors should preferably be attached to the feed points; alternatively, determined correction factors can be used. The second partial calculation determines the metal powder mass flow ("mass flow calculation") using the motor speed of the respective dosing device, the metal powder parameters from the recipe, the geometric properties of the dosing device and a determined efficiency. The third partial calculation ("saltation speed calculation") brings together all the results and determines the target speed of the gas flow for the respective prevailing metal powder mass flow with the help of other geometric parameters such as the pipeline cross-section and metal powder parameters such as average grain size.

The target speed is controlled by a PID pump control and the speed sensor 86.

The last part of the conveying control is the limit control (see Fig. 10) under the aspect of machine protection, which results in an adjustment of the metal powder mass flow. The limit values for the following are checked: maximum conveying speed (wear protection of the piping system); maximum pump outlet pressure (pump protection, component-specific); maximum pump output (service life); and maximum dosing output (service life).

If all limit values are exceeded, the motor speed of the respective dosing device (i.e. the respective screw conveyor) is increased by a defined value. If at least one limit value is exceeded, the motor speed of the respective dosing device is reduced by a defined value. Once the limit value control is complete, the termination conditions for the metal powder conveyor are checked. If these are not met, the conveyor control loop starts again from the beginning.

A delivery cycle is terminated when: the source tank is empty, or the target tank is full, or a defined maximum delivery time has been exceeded, or the total pressure loss exceeds a defined limit.

The fact that the source tank is empty can mean that a level sensor in the source tank of the respective conveying process reports a level that is below a predetermined limit. The fact that the target tank is full can mean that a level sensor in the target tank of the respective conveying process reports a level that is above a predetermined limit.

When a conveying cycle is terminated, the respective pipe system is cleaned by a gas flow so that no blockages due to falling powder can occur in vertical sections when the gas flow is terminated (see "Pipe cleaning" in Fig. 2). Cleaning of the pipe system is completed when the load cells of the target tank detect a defined limit value for the change in weight over time.

Once the pipe cleaning cycle is finished, the automatic filter cleaning (39, 40) is finally triggered (see "Filter cleaning" in Fig. 2).

The system has a pressure compensation tank 84, which is designed to reduce the overpressure at the pump outlet. When using conventional vacuum pumps, the maximum overpressure is specified by the manufacturer and must not be exceeded to protect the component. This is ensured by the limit value control (see above). A pressure compensation tank is used to increase the delivery capacity.

Embodiments of the powder conveying system described herein may be able to adapt to changing system conditions (e.g. filter wear), changing materials (e.g. material characteristics) and/or changing environmental conditions without requiring empirically determined, different conveying parameter sets. This also means maximum performance for the respective machine, material and environmental conditions.

In addition or alternatively to the processes described above, in particular the higher-level function shown in Fig. 2 and its subroutines, the powder conveying system of the present disclosure can have the following advantageous features or carry out the following advantageous processes. In the following, aspects of automatic condition monitoring are described which can be used in particular in connection with the powder conveying system of Fig. 1.

A dew point sensor is provided on at least one of the following components of the powder conveying system: main tank 6, external tank 8, and conveying device 79. The dew point sensors are used to measure humidity during vacuum conveying and within the tanks 6 and 8 via an exhaust unit. If a certain limit value (according to one embodiment > 5%) is exceeded, automatic powder drying is initiated.

An oxygen sensor for measuring an oxygen concentration is provided on at least one of the following components of the powder conveying system: first tank 1, overflow tank 2 or 3, buffer tank 4, main storage 6, external tank 8 and sieve device 5. The oxygen sensors are used to monitor the oxygen concentration during vacuum conveying and within all tanks & sieves. If the corresponding limit values are exceeded, automatic inerting is started. The measurement during conveying takes place continuously, the measurement in the tanks & sieves discontinuously at set intervals.

A pressure sensor for measuring pressure is provided on at least one of the following components of the powder conveying system: first tank 1, overflow tank 2 or 3, buffer tank 4, main storage 6, external tank 8 and sieve device 5. The pressure sensors in the tanks and sieve are used for leakage control. If a limit value for pressure loss/increase per time is measured, a warning is issued and the inerting/O2 measurement of the corresponding part of the system is carried out to compensate for any missing inerting. Furthermore, the pressure sensors in the conveyor can be used for filter monitoring. If the differential pressure rises above a defined limit value, automatic filter cleaning is triggered. If the increased differential pressure is not remedied by filter cleaning, the piping system may be blocked and a warning is issued.

The motors of the dosing elements (conveyor screws) and the vacuum pump (conveying device) are temperature-monitored (using appropriate temperature sensors). If a temperature limit is exceeded, the system is switched off to protect the components. There is also a temperature sensor in the conveyor line. The motors of the dosing devices are torque-monitored. If a defined torque limit is exceeded, the device is switched off and a warning is issued.

All pipeline valves have position feedback. If a discrepancy occurs between the target position and the actual position, the machine goes into a safe state and a warning is issued for the corresponding component.

All sensors are checked for plausibility/are monitored. If an error occurs, the machine goes into a safe state and a warning is issued for the corresponding component.

In the following, aspects of a remaining runtime-optimized prioritization are described, which can be used in particular in connection with the powder conveying system of Fig. 1.

At least one or more of the containers and in particular all containers (1, 2102, 2, 3, 4, 6, 7, 8) are equipped with weighing cells which ensure continuous level measurement of the respective container.

If there is a disruption in closed loop operation, the tanks (2102, 2, 3) form the safety reserve for the additive manufacturing process. The intermediate tank 2102 (hopper) stores the powder for the additive manufacturing process. If this is empty, the process stops. The overflow tanks (2, 3) collect the excess process powder. The empty volume forms the reserve here. If at least one overflow tank is full, the process stops.

The aim is to optimise the remaining running time in relation to the hopper filling level and overflow reserve. It should be noted that the amount emptied per time from the hopper is variable and is related to the exposure time, layer thickness and overdosing factor. The same applies to the filling quantity per time of the overflows. Accordingly, the weight changes over time, measured on the load cells, are dynamic inputs for the algorithm.

There are 3 different conveying circuits or conveying processes: a. Conveying from tank 6 through lock 1 into the hopper 2102 b. Conveying overflows 2 or 3 through lock 4 onto the screen 50 in tank 6 c. Conveying from external tank 8 through lock 4 to screen 50 in tank 6

A conveying cycle conveys a predetermined amount of powder (e.g. 25 l) into lock 1 or lock 4 and is terminated.

After each funding cycle, the algorithm calculates the priorities of funding circuits a, b and c. The funding process with the highest priority is executed.

There are fixed min and max limits for each tank. Tank 6 forms the node between all cycles; according to one embodiment, it may not be filled to the maximum by "c", as otherwise the volume is lost to discharge the overflows "b". According to one embodiment, the delivery circuit "b" always has priority over the delivery circuit "c".

The powder conveying system of the present disclosure (in particular the powder conveying system of Fig. 1) can also be a special case of a bulk material conveying system according to the following description. Individual aspects of the powder conveying system described herein can be combined and/or supplemented in any way with the aspects of the bulk material conveying system described below. In particular, in connection with the powder conveying system of the present disclosure, an adjustment of the conveying speed can be carried out according to the following description.

However, the aspects of a bulk material conveying system described below can also be advantageous in themselves and represent one or more inventions. Thus, the bulk material conveying system described below can also be used independently (i.e. independently of the powder conveying system described above).

The following disclosure relates to a bulk material conveying system and a method for conveying bulk material. The following disclosure relates in particular to the conveying of raw material powder in a system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation (for example a system for selective laser melting).

In additive (or generative) processes for producing three-dimensional workpieces and in particular in generative layer construction processes, it is known that to apply initially shapeless or shape-neutral molding compound of a raw material (for example a raw material powder) layer by layer to a carrier and to solidify it by site-specific irradiation (e.g. by melting or sintering) in order to ultimately obtain a workpiece of the desired shape. The irradiation can be carried out by means of electromagnetic radiation, for example in the form of laser radiation, or by means of particle radiation, for example in the form of electron radiation. In an initial state, the molding compound can initially be present as granules, as powder or as liquid molding compound and can be solidified selectively or, in other words, site-specifically as a result of the irradiation. In particular, the molding compound can be a bulk material such as raw material powder. The molding compound can, for example, comprise ceramic, metal or plastic materials and also material mixtures thereof. A variant of generative layer construction processes concerns the so-called laser beam melting in the powder bed (also: selective laser melting), in which metallic and/or ceramic raw material powder materials in particular are solidified into three-dimensional workpieces by irradiation with a laser beam.

In order to produce individual workpiece layers in the context of selective laser melting, it is also known to apply raw material powder material in the form of a raw material powder layer to a carrier and to irradiate it selectively and in accordance with the geometry of the workpiece layer currently being produced. The laser radiation penetrates the raw material powder material and solidifies it, for example as a result of heating, which causes melting or sintering. Once a workpiece layer has solidified, a new layer of unprocessed raw material powder material is applied to the workpiece layer already produced. Known coating arrangements or powder application devices can be used for this purpose. The now uppermost and still unprocessed raw material powder layer is then irradiated again. Consequently, the workpiece is built up successively layer by layer, with each layer defining a cross-sectional area and/or a contour of the workpiece. In this context, it is also known to use CAD or comparable workpiece data in order to produce the workpieces essentially automatically.

The present disclosure is generally directed to a bulk material conveying system for conveying bulk material. In particular, the bulk material can be raw material powder for use in one of the systems described above for producing a three-dimensional workpiece. In particular for the provision of raw material powder in an additive manufacturing process of the type described above, but also for other applications, it may be necessary to convey bulk material from a source tank to a target tank.

For this purpose, a so-called pneumatic conveying system is typically used. In this case, a gas flow of conveying gas (for example air or a protective gas such as argon or an air-protective gas mixture) is generated through a conveying line by means of a conveying device (e.g. a pump or a blower), whereby the conveying line can in particular form a closed circuit so that the gas flow is conveyed in a circle (so-called conveying circuit). For example, a predetermined dose of bulk material per unit time is fed into the gas flow from a source tank by means of a dosing device. It is also possible for bulk material from several source tanks to be fed into the gas flow at the same time. From the point where the bulk material is fed in, a gas-bulk material mixture is conveyed through the conveying line to a point in the conveying circuit where a separating device (for example a cyclone) is located. With the help of the separating device, the bulk material is separated from the gas-bulk material mixture as completely as possible and fed to a target tank. For example, a cyclone can be located above the target tank, whereby the separated bulk material falls into the target tank due to gravity (so-called gravity conveying).

One problem with pneumatic bulk material conveying is that the operating parameters of the system and/or the properties of the bulk material (for example, the material of a bulk material powder used and its associated conveying properties) can change during operation or between individual conveying processes. In this case, certain operating parameters of the bulk material conveying system must be adjusted, in particular the speed of the gas flow, which is determined by the conveying device. Another operating parameter of the bulk material conveying system that may need to be adjusted is the dose of bulk material per unit time that is conveyed into the gas flow.

In particular, it is a problem to determine a suitable velocity of a gas flow for pneumatic bulk material conveying under changing conditions (e.g. operating parameters of the system and/or nature of the bulk material). The object of the present disclosure is therefore to provide a bulk material conveying system and a corresponding method for conveying bulk material, which solve at least one of the problems described above or a problem related thereto. In particular, a reliable and simple determination of a target value of the gas velocity is desirable, which can react to a change in one or more operating parameters.

This object is achieved by a bulk material conveying system and a method for conveying bulk material with the features of the independent aspects below. Further embodiments are given in the sub-aspects.

According to a first aspect, the disclosure therefore relates to a bulk material conveying system, in particular for conveying raw material powder in a system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. The bulk material conveying system comprises a conveying line which is designed to convey a gas flow and, at least in sections, a bulk material flow driven by the gas flow, and a dosing device which is designed to supply the gas flow with a predetermined dose of bulk material per unit of time. The dose is determined by a control value applied to the dosing device. The bulk material conveying system further comprises a conveying device which is designed to convey the gas flow through the conveying line and at least one measuring device for measuring at least one characteristic of the gas flow. Furthermore, the bulk material conveying system comprises a control device which is configured to determine a gas density of the gas flow based on the measured at least one characteristic variable of the gas flow, determine a bulk material mass flow of the bulk material flow based on the control value applied to the metering device, determine a target speed of the gas flow based on the gas density and based on the bulk material mass flow, and control the conveying device to convey the gas flow at the determined target speed.

The bulk material conveying system can be set up to convey the bulk material pneumatically. The system can in particular be a system for selective laser melting or sintering, which has, for example, one or more of the features described above. Alternatively, the system can be a system for selective electron beam melting. The conveyor line can, for example, comprise one or more pipes and/or one or more hoses and/or one or more connectors. The conveyor line can be powder-tight and in particular gas- and powder-tight, so that no gas or powder can enter or leave the conveyor line (from the side) - except through the openings in the conveyor line.

The fact that the conveying line is designed in sections to convey a bulk material flow driven by the gas flow can mean that only one section of an entire conveying circuit formed by the conveying line is designed to convey a gas-bulk material mixture. The remaining part of the conveying circuit essentially only conveys gas (conveying gas), for example air, protective gas or an air-protective gas mixture.

The control value can be a voltage, a current, a pulse or another suitable signal (in particular an electrical signal) which can be varied in order to vary the dose delivered by the dosing device. The conveying device can be a pump or a blower. Furthermore, one or more filter devices can be provided in the conveying circuit which is formed by the conveying line, in particular for filtering bulk material or bulk material residues remaining in the gas flow.

The control device can comprise a computer, for example. The control device can comprise a processor and a memory, wherein a program is stored on the memory which, when executed, causes the processor to carry out the method according to the second aspect.

The term bulk material mass flow is used here to indicate a physically quantifiable mass flow of the bulk material being conveyed (in kg/s). In contrast, the term bulk material flow describes (only) the presence of bulk material being conveyed through the conveying line.

The steps of determining can each comprise one or more calculations. Measured variables, but also stored variables (for example standard values) can be included in the respective calculation. The conveying device can be controlled in such a way that it reduces or increases the speed of the conveyed gas flow. In particular, for example, increasing the voltage applied to the conveying device can lead to the speed of the gas flow being increased. Since the gas flow conveys the bulk material, the speed of the gas flow also affects the speed of the bulk material being conveyed.

The gas density can be determined based on at least one of the following parameters of the gas flow: oxygen content, pressure, temperature, dew point and humidity.

For each of the parameters mentioned, a corresponding sensor can be provided in the gas stream, which is set up to measure the respective parameter.

wobei pmess ein gemessener Druck des Gasstroms ist, Tmess eine gemessene Temperatur des Gasstroms ist, p_n ein vorbestimmter Normaldruck ist, T_n eine vorbestimmte Normaltemperatur ist und sich pGemisch aus der folgenden Formel ergibt:

wobei pLufteine bekannte Dichte von Luft ist, welche Bestandteil des Gasstroms ist, Pschutzgas eine bekannte Dichte eines Schutzgases ist, welches Bestandteil des Gasstroms ist, Sauerstoffgehaltiuft ein bekannter Sauerstoffgehalt der Luft ist und Sauerstoffgehaltmess ein gemessener Sauerstoffgehalt des Gasstroms ist. The gas density p can be determined using the following formula:

where pmess is a measured pressure of the gas stream, Tmess is a measured temperature of the gas stream, p _n is a predetermined normal pressure, T _n is a predetermined normal temperature and pmixture is given by the following formula:

where pair is a known density of air which is part of the gas stream, Pprotective gas is a known density of a protective gas which is part of the gas stream, oxygen contentair is a known oxygen content of the air and oxygen contentmeasured is a measured oxygen content of the gas stream.

Pmess and Tmess can each be measured by suitable sensors arranged in the gas flow. At least one or both of the sensors required for this can be arranged downstream of the conveying device, in particular between the conveying device and a source tank from which the bulk material is conveyed. In particular, at least one or both of the sensors can be arranged immediately downstream of the conveying device. The normal pressure and the normal temperature can be predetermined standard values, for example the normal pressure can be 1013.25 mbar and for example the normal temperature can be 293.15 K. However, normal pressure and/or normal temperature can be also measured values of the ambient pressure or the ambient temperature.

The protective gas can be argon, for example, where the density Pprotective gas corresponds to a known density of the gas argon. The density of air, the density of the protective gas used and the oxygen content of the air can be looked up in the relevant tables in the specialist literature. The oxygen content of the air can also be a measured oxygen content of the ambient air.

Oxygen content measurement can be carried out by a suitable sensor arranged in the gas flow. The sensor required for this can be arranged upstream of the conveying device, between a target tank into which the bulk material is conveyed, and the conveying device. In particular, the sensor can be arranged immediately upstream of the conveying device.

The control value applied to the dosing device can be a motor speed of a motor of the dosing device, in particular a motor for driving a conveyor screw of the dosing device.

However, the control value can also be a current or a voltage that is applied to the motor. Generally speaking, the control value can represent any (e.g. electrical) signal that is suitable for changing the dose (more precisely: dose per time) output by the dosing device and in particular for setting it to a predetermined value. The dosing device can, as described above, comprise a conveyor screw that is designed to convey bulk material from the original tank. Furthermore, the dosing device can comprise at least one of the following elements: a rotary valve, a dosing slide and a valve, in particular a valve with a variable opening diameter.

Determining the target velocity v _S0 n of the gas flow may include calculating a saltation velocity based on the determined gas density and the determined bulk material mass flow. Determining the target velocity v _S0 n may be done using the following formula: target ^saltation "F ^safety where Vsaitation is the calculated saltation velocity of the conveyed bulk material and vsafety is a predetermined safety velocity.

The saltation speed can be a speed below which particles of the bulk material being conveyed (for example powder) start to fall down and accumulate on a bottom of the conveying line. To ensure that this does not happen, a predetermined safety speed can be added to the calculated saltation speed. In a way, this forms a safety margin from the saltation speed, which ensures that no deposition of bulk material takes place in the conveying line. As an alternative to adding the safety speed, the saltation speed can be multiplied by a safety factor.

wobei M_s der bestimmte Schüttgutmassestrom in kg/s ist, g eine vorbestimmte Fallbeschleunigung in m/s² ist, D ein Durchmesser der Förderleitung in m ist, p die bestimmte Gasdichte des Gasstroms in kg/m³ ist und a und b jeweils von einem Partikeldurchmesser d des Schüttguts abhängige Parameter sind. Calculating the saltation rate can be done using the following formula:

where M _s is the determined bulk material mass flow in kg/s, g is a predetermined acceleration due to gravity in m/s ² , D is a diameter of the conveying line in m, p is the determined gas density of the gas flow in kg/m ³ and a and b are parameters each dependent on a particle diameter d of the bulk material.

The predetermined acceleration due to gravity can correspond to the acceleration due to gravity on the earth's surface of 9.81 m/s ^2. The parameters a and b can each be a predetermined and/or pre-stored constant for a respective bulk material. Furthermore, the parameter a and/or the parameter b can be determined depending on the particle diameter d of the bulk material used. In other words, a formula for calculating the parameter a and/or a formula for calculating the parameter b can be dependent on the particle diameter d. The particle diameter d can be taken from a specification of the bulk material used. The particle diameter d can be stored, for example, in a memory of the control device. In particular, a table can be stored in a memory of the control device in which the respective values for the particle diameter d of the respective powder are stored for different powder materials. In this way, when calculating the target speed can be used to refer to a respective particle diameter d.

The control of the conveying device for conveying the gas flow can be carried out at the determined target speed using a speed sensor for measuring a speed of the gas flow and a control loop, in particular comprising a PID controller.

A PID controller is a proportional-integral-derivative controller, whereby PID control is well known in control engineering for setting and maintaining a predetermined value (in this case the gas velocity) constant.

The control device can further be configured to determine whether at least one measured characteristic of the bulk material conveying system is above a predetermined maximum value for the respective characteristic, and if the at least one measured characteristic is above the predetermined maximum value, reducing the control value of the dosing device by a predetermined value.

Several parameters can be determined (in particular measured) and if at least one of the determined parameters is above a maximum value predetermined for this parameter, the control value is reduced by the predetermined value.

The bulk material conveying system can comprise one or more sensors for measuring the respective parameter. Determining whether the at least one measured parameter is above the predetermined maximum value and the corresponding adjustment of the control value can be carried out after the steps of determining a gas density, determining a bulk material mass flow, determining a target speed and controlling the conveying device. These steps can be carried out again after adjusting the control value with the new control value. Thus, the determination (and if necessary adjustment) of the target speed and - if necessary - the adjustment of the control value can be carried out alternately. Reducing the control value means that the dosing device outputs a smaller dose of bulk material per unit time. The control device can further be configured to increase the control value of the dosing device by a predetermined value if the at least one measured parameter is below the predetermined maximum value.

Several parameters can be determined (in particular measured) and if all of the determined parameters are below a maximum value predetermined for the respective parameter, the control value is increased by the predetermined value. Thus, the control value can only be increased if all of the determined parameters do not exceed their associated maximum value. Increasing the control value means that the dosing device dispenses a higher dose of bulk material per unit of time.

The at least one parameter may include at least one of the following parameters: conveying speed, pump outlet pressure, pump power and dose of bulk material per unit time.

The respective parameter can be measured, for example, using a sensor provided for this purpose or determined in another way. For example, the parameter can be calculated based on at least one input value (e.g. an applied voltage) which is applied to an element of the powder conveying system (e.g. the conveying device).

The control device can be configured to terminate a conveying process by stopping the conveying device if at least one of the following events is detected: a source tank from which bulk material is removed by means of the dosing device is empty, a target tank into which the bulk material is conveyed is full, a predetermined maximum conveying time is exceeded and a total pressure loss of the conveying gas exceeds a predetermined limit value.

The respective event can be detected, for example, using a corresponding sensor. In detail, the events can be detected as follows. A source tank from which bulk material is taken using the dosing device is empty, this can be determined using a corresponding level sensor (e.g. a capacitive sensor, a radar sensor, an ultrasonic sensor or an optical sensor). A target tank into which the bulk material is conveyed is full, this can be determined using a corresponding level sensor (e.g. a capacitive sensor, a radar sensor, an ultrasonic sensor or an optical sensor). A predetermined maximum conveying time has been exceeded, this can be determined using a corresponding timer. which is started when the production is started. A total pressure loss of the conveying gas exceeds a predetermined limit can be determined via one or more pressure sensors in the conveying line.

The bulk material conveying system may further comprise a pressure equalization tank coupled to the conveying line downstream of the conveying device and upstream of the dosing device.

The pressure compensation vessel can be designed to reduce an overpressure at an outlet of the conveying device (e.g. a pump). Thus, the overpressure vessel can be provided at the outlet of the conveying device.

The bulk material conveying system can comprise at least a first conveying circuit for conveying raw material powder from a main storage facility into a first tank of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. A lower outlet of the first tank can be coupled to an upper inlet of an intermediate tank of the system, wherein a manufacturing process in a process chamber of the system is fed with powder from the intermediate tank. An upper inlet of the main storage facility can be coupled to an outlet of a sieve for sieving the raw material powder.

The main storage can, for example, be a storage or tank permanently installed in the bulk material conveying system. The main storage can be located at a lower height than the first tank. The first conveying circuit can be activated by the control device by opening at least one valve so that the conveying device conveys gas through this first conveying circuit. The bulk material conveying system can be designed in such a way that powder can be fed to the sieve from a buffer container which is located above the sieve. Powder can, for example, be conveyed into the buffer container by means of a second conveying circuit from an overflow tank of the system and/or from an external tank.

The terms "tank", "storage" and "container" are used synonymously herein. However, the different containers are referred to using specific terms, such as "tank", "buffer container", "main storage" and "external tank". However, the individual terms are not to be understood as restrictive, but merely serve to differentiate the different containers from one another. For example, the "buffer tank" could also be called a "second tank".

The bulk material conveying system can comprise at least a second conveying circuit for conveying raw material powder from an overflow tank into a buffer tank. The overflow tank can be designed to receive excess powder from the process chamber of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. A lower outlet of the buffer tank can be coupled to an inlet of a sieve for sieving the raw material powder.

The second conveying circuit can be activated by the control device by opening at least one valve so that the conveying device conveys gas through this second conveying circuit. The overflow tank can be arranged, for example, in a lateral area next to a construction cylinder of the system, with the overflow tank being open at the top so that the excess powder can be pushed into the overflow tank, for example by means of a powder application device.

The bulk material conveying system may comprise at least a third conveying circuit for conveying raw material powder from an external tank into a buffer container. The external tank may be detachably coupled or coupled to the bulk material conveying system. A lower outlet of the buffer container may be coupled to an inlet of a sieve for sieving the raw material powder.

The third conveying circuit can be activated by the control device by opening at least one valve so that the conveying device conveys gas through this third conveying circuit. The third conveying circuit can be operated simultaneously with the second conveying circuit so that powder from the external tank and powder from the overflow tank are mixed together in an adjustable mixing ratio. The mixing ratio can be set, for example, by the respective dosing devices of the respective containers (external tank, overflow tank).

The dosing device can be located at the outlet of each source tank and a cyclone can be located at the inlet of each target tank for separating the raw material powder from the gas stream and for feeding the raw material powder into the target tank. The dosing device allows the bulk material flow to be introduced into the gas flow and the cyclone allows the bulk material to be removed from the gas flow again.

According to a second aspect, the disclosure relates to a method for conveying bulk material, in particular for conveying raw material powder in a system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. The method comprises conveying a gas flow and a bulk material flow driven by the gas flow through a conveying line and supplying a predetermined dose of bulk material per unit of time through a dosing device. The dose can be determined by a control value applied to the dosing device. The method further comprises conveying the gas flow through the conveying line using a conveying device and measuring at least one characteristic of the gas flow using a measuring device. The method further comprises determining a gas density of the gas flow based on the measured at least one characteristic of the gas flow and determining a bulk material mass flow of the bulk material flow based on the control value applied to the dosing device. The method further comprises determining a desired velocity of the gas flow based on the gas density and based on the bulk material mass flow and controlling the conveying device to convey the gas flow at the determined desired velocity.

All of the above-described details and features of the first aspect (bulk material conveying system) can apply in connection with the method for conveying bulk material according to the second aspect (individually or in any combination with one another). The bulk material conveying system according to the first aspect can be designed to carry out the method for conveying bulk material according to the second aspect.

The gas density can be determined based on at least one of the following parameters of the gas flow: oxygen content, pressure, temperature, dew point and humidity.

wobei pmess ein gemessener Druck des Gasstroms ist, Tmess eine gemessene Temperatur des Gasstroms ist, p_n ein vorbestimmter Normaldruck ist, T_n eine vorbestimmte Normaltemperatur ist und sich pGemisch aus der folgenden Formel ergibt:

wobei pLufteine bekannte Dichte von Luft ist, welche Bestandteil des Gasstroms ist, Pschutzgas eine bekannte Dichte eines Schutzgases ist, welches Bestandteil des Gasstroms ist, Sauerstoffgehaltiuft ein bekannter Sauerstoffgehalt der Luft ist und Sauerstoffgehaltmess ein gemessener Sauerstoffgehalt des Gasstroms ist. The gas density p can be determined using the following formula:

where pmess is a measured pressure of the gas stream, Tmess is a measured temperature of the gas stream, p _n is a predetermined normal pressure, T _n is a predetermined normal temperature and pmixture is given by the following formula:

where pair is a known density of air which is part of the gas stream, Pprotective gas is a known density of a protective gas which is part of the gas stream, oxygen contentair is a known oxygen content of the air and oxygen contentmeasured is a measured oxygen content of the gas stream.

The control value applied to the dosing device can be a motor speed of a motor of the dosing device, in particular a motor for driving a conveyor screw of the dosing device.

Determining the desired velocity v _S0 n of the gas flow may comprise calculating a saltation velocity based on the determined gas density and the determined bulk material mass flow, and determining the desired velocity v _S0 n may be performed using the following formula: desired ^saltation "F ^safety where Vsaitation is the calculated saltation velocity of the bulk material being conveyed and vsafety is a predetermined safety velocity.

wobei M_s der bestimmte Schüttgutmassestrom in kg/s ist, g eine vorbestimmte Fallbeschleunigung in m/s² ist, D ein Durchmesser der Förderleitung in m ist, p die bestimmte Gasdichte des Gasstroms in kg/m³ ist und a und b jeweils von einem Partikeldurchmesser d des Schüttguts abhängige Parameter sind. Calculating the saltation rate can be done using the following formula:

where M _s is the determined bulk material mass flow in kg/s, g is a predetermined gravitational acceleration in m/s ² , D is a diameter of the conveyor line in m, p is the is a certain gas density of the gas stream in kg/m ³ and a and b are parameters depending on a particle diameter d of the bulk material.

Controlling the conveying device for conveying the gas flow at the determined target speed can be carried out using a speed sensor for measuring a speed of the gas flow and a control loop, in particular comprising a PID controller.

The method may further comprise: determining whether at least one measured characteristic of the bulk material conveying system is above a predetermined maximum value for the respective characteristic, and if the at least one measured characteristic is above the predetermined maximum value, reducing the control value of the dosing device by a predetermined value.

The method may further comprise, if the at least one measured parameter is below the predetermined maximum value, increasing the control value of the dosing device by a predetermined value.

The at least one parameter may include at least one of the following parameters: conveying speed, pump outlet pressure, pump power and dose of bulk material per unit time.

The method may further comprise terminating a conveying operation by stopping the conveying device if at least one of the following events is detected: a source tank from which bulk material is removed by means of the dosing device is empty, a target tank into which the bulk material is conveyed is full, a predetermined maximum conveying time is exceeded and a total pressure loss of the conveying gas exceeds a predetermined limit value.

The method may comprise conveying raw material powder through at least a first conveying circuit from a main storage facility into a first tank of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. A lower outlet of the first tank may be coupled to an upper inlet of an intermediate tank of the system, wherein a production process in a process chamber of the system is fed with powder from the intermediate tank. An upper inlet of the main storage facility may be coupled to an outlet of a sieve for sieving the raw material powder. The method may comprise conveying raw material powder through at least one second conveying circuit from an overflow tank into a buffer tank. The overflow tank may be configured to receive excess powder from the process chamber of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. A lower outlet of the buffer tank may be coupled to an inlet of a sieve for sieving the raw material powder.

The method may include conveying raw material powder through at least a third conveying circuit from an external tank into a buffer container. The external tank may be detachably coupled or coupled to the bulk material conveying system. A lower outlet of the buffer container may be coupled to an inlet of a sieve for sieving the raw material powder.

The dosing device can be located at the outlet of each source tank. A cyclone can be located at the inlet of each target tank to separate the raw material powder from the gas stream and to feed the raw material powder into the target tank.

The system described here for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation can, for example, comprise a carrier device for applying the powder in several layers so that a powder bed is formed. Furthermore, one or more powder application devices can be provided for applying the powder and, if necessary, for applying powder of different materials. A separate powder application device can be provided for each material. The carrier device can be moved vertically downwards by means of a lifting device so that the topmost powder layer always remains at the same height in relation to a construction chamber of the system. Furthermore, the system can comprise one or more irradiation units. The irradiation units each comprise a beam source (in particular a laser beam source) and an optics with one or more optical components for shaping and deflecting the beam (e.g. beam expander, focusing unit, scanner device, F-theta lens). Alternatively, the beam source can be located outside the respective irradiation unit, with the beam being fed to the irradiation unit via an optical conductor (e.g. fiber optic). Fig. 11 shows a system 1100 for producing a three-dimensional workpiece by irradiating layers of a raw material powder with electromagnetic radiation or particle radiation. In other words, the system 1100 is a system for producing a three-dimensional workpiece by means of an additive manufacturing process in which a raw material powder is used, for example selective laser melting or selective laser sintering.

Although the bulk material conveying system of the present disclosure is described below in connection with an above-mentioned system, the bulk material conveying system or the method for conveying bulk material is not limited to use in connection with an additive manufacturing system. The advantages resulting from the bulk material conveying system or the associated method presented here are generally applicable to situations in which bulk material is conveyed by means of pneumatic conveying.

The system 1100 comprises a carrier 1002 and a powder application device 1003 for applying a raw material powder 1004 to the carrier 1002. The carrier 1002 and the powder application device 1003 are located within a process chamber 1011, which can be sealed from the ambient atmosphere, i.e., from the environment of the process chamber 1011. The system 1100 further comprises an irradiation device 1005 for selectively irradiating electromagnetic radiation or particle radiation onto the raw material powder 1004, which has been applied to the carrier 1002.

The process chamber 1011, i.e. the powder application device 1003, is supplied with raw material powder 1004 by means of a bulk material conveying system 1001, which is described in detail below. In the present case of powder conveying, the bulk material conveying system 1001 is a powder conveying system. The bulk material conveying system 1001 comprises a powder storage 1006 in which the raw material powder 1004, which is supplied to the process chamber 1011, is stored. The powder storage 1006 is connected to a conveying line 1007 via a dosing device 1008. A gas flow 1009 is conveyed through the conveying line 1007 by means of a conveying device 1019 along a direction which is indicated by an arrow in Fig. 11. In the exemplary powder conveying system 1001 shown in Fig. 11, the conveying device 1009 is designed in the form of a vacuum pump. The dosing device 1008 is designed to dispense a desired dose of raw material powder 1004 into the gas stream 1009 flowing through the conveyor line 1007. In particular, the dosing device 1008 comprises a first powder valve 1021, which is provided with a continuously variable flow cross-section, so that the amount of powder 1004 that is introduced into the gas stream 1009 per unit of time via a dosing opening 1023 of the dosing device 1008 can be continuously varied. In other words, a control value can be applied to the dosing device 1008 by a control device 1040 described further below, which determines the dose dispensed per time. In the case of the valve 1021 with a variable cross-section described above, the control value determines the cross-section and thus the dose. Alternatively, the valve 1021 can be replaced by a conveyor screw with an associated motor for driving the conveyor screw. In this case, the control value determines the speed of the motor and thus the dose delivered. By applying a predetermined control value, a desired dose of powder delivered per unit of time can be determined.

The raw material powder-gas mixture flowing through the conveying line 1007 downstream of the dosing device 1008 is conveyed to a cyclone 1010 which serves as a separating device. The cyclone 1010 comprises an inlet 1020 which enables a tangential inflow of the raw material powder-gas mixture into a conical separation chamber 1022. Powder particles 1004 which fall out of the rotating flow of the raw material powder-gas mixture which forms within the conical separation chamber 1022 are discharged from the cyclone 1010 by means of a powder outlet 1024 which is located in a lower region of the cyclone 1010. These powder particles 1004 are fed to the process chamber 1011, i.e. the powder application device 1003, by means of a connecting line 1026 which connects the powder outlet 1024 of the cyclone 1010 with a powder inlet 1028 of the process chamber 1011.

A second powder valve 1030 is provided in the connecting line 1026. Just like the first powder valve 1021 of the dosing device 1008, the second powder valve 1030 is also equipped with a continuously variable flow cross-section, so that the amount of powder 1004 that is fed to the process chamber 1011 from the powder outlet 1024 of the cyclone 1010 can be continuously varied. The second powder valve 1030 can also (like the first powder valve) be replaced by a conveyor screw with an associated motor. In the case shown in Fig. 11, powder is thus conveyed from an original tank formed by the powder reservoir 1006 into a target tank, wherein the target tank can be viewed, for example, as the volume of the connecting line 1026 below the powder valve 1030. In the schematic representation of Fig. 11, this volume represents a powder container for the powder application device 1003, from which the powder application device 1003 is supplied with powder for the application of individual layers. Alternatively, this powder reservoir (and thus the target tank) can be integrated into the powder application device 1003.

The gas which was separated from the powder particles 1004 in the cyclone 1010 is fed back into the conveying line 1007 via a gas outlet 1032 of the cyclone 1010. The gas outlet 1032 is arranged in an upper part of the cyclone 1010.

Since the gas stream 1009 leaving the gas outlet 1032 of the cyclone 1010 may contain remaining raw material powder particles 1004, a filter unit 1014 is arranged in the conveyor line 1007 downstream of the cyclone 1010. The filter unit 1014 comprises a replaceable filter 1013 which is adapted to filter out remaining raw material powder particles 1004 which are in the gas stream 1009 leaving the gas outlet 1032.

The bulk material conveying system 1001 further comprises a measuring device 1050 for measuring a characteristic of the gas flow which is conveyed in the conveying line 1007. In the illustrated embodiment of Fig. 11, the measuring device 1050 is arranged immediately downstream of the conveying device 1019 and the measuring device 1050 is a pressure sensor for measuring a pressure (gas pressure) within the conveying line 1007. In addition to the measuring device 1050, further measuring devices 1015, 1016, 1017 and 1018 are provided along the conveying line 1007. The measuring device 1018 is a temperature sensor for measuring a temperature of the gas flow. The measuring device 1015 is an oxygen sensor for measuring an oxygen content of the gas flow. The measuring devices 1016 and 1017 can each be further sensors for measuring one of the aforementioned properties (pressure, temperature, oxygen content). Furthermore, the measuring devices 1016 and 1017 can each be, independently of one another, a sensor for measuring a flow rate, a presence of bulk material (e.g. capacitive sensor or ultrasonic sensor), a dew point, etc. Apart from the measuring devices 1050 and 1015 to 1018, fewer or several sensors can be provided. Furthermore, the measuring devices can be provided at another location in the conveyor line.

The pressure sensor 1050, the temperature sensor 1018 and the oxygen sensor 1015 are particularly relevant for the functioning of the control device 1040 described here (see below).

Finally, the bulk material conveying system 1001 comprises a control device 1040.

The control device 1040 controls the operation of the elements of the bulk material conveying system, in particular all elements that can be controlled. For example, the control device 1040 controls the conveying device 1019 and can control it in particular in such a way that a predetermined flow rate of the conveyed gas is set, for example by applying a voltage value set by the control device 1040. Furthermore, the control device 1040 is able to increase or decrease the flow rate by a predetermined value. The control device 1040 also controls the dosing device 1008. In particular, the control device 1040 applies a predetermined control value to the dosing device 1008, which results in the dosing device releasing a predetermined dose of raw material powder per unit of time into the gas flow. Furthermore, the control device 1040 is able to increase or decrease a currently conveyed dose by a predetermined value. The control device 1040 also receives the measurement data from all measuring devices. The control device 1040 also controls the valve 1030.

The control device 1040 can also be configured to control the entire operation of the system 1010, i.e. the exposure by means of the irradiation device 1005, the powder application by means of the powder application device 1003, lowering of the carrier 1002, etc.

To determine a suitable setpoint of the gas velocity in the delivery line 1007 and to set a velocity corresponding to the setpoint, the controller 1040 performs the following steps:

A. determining a gas density of the gas stream based on the measured at least one characteristic of the gas stream;

B. determining a bulk material mass flow rate of the bulk material stream based on the control value applied to the dosing device; C. Determining a target gas flow velocity based on the gas density and based on the bulk material mass flow; and

D. Controlling the conveying device to convey the gas flow at the determined target speed.

The individual steps:

Step A:

The control device 1040 calculates a gas density of the gas flow based on a measured characteristic of the gas flow. There are several ways in which a gas density of the gas flow can be determined (for example calculated or estimated) using one or more measured characteristics. One possibility is shown below as an example. The control device 1040 uses a measured pressure (measured by the pressure sensor 1050), a measured temperature (measured by the temperature sensor 1018) and a measured oxygen content (measured by the oxygen sensor 1015).

wobei pmess der vom Drucksensor 1050 gemessene Druck des Gasstroms ist, Tmess die vom Temperatursensor 1018 gemessene Temperatur des Gasstroms ist, p_n ein vorbestimmter Normaldruck ist (im Beispiel ein Normaldruck von p_n = 1013,25 mbar), und T_n eine vorbestimmte Normaltemperatur ist (im Beispiel eine Normaltemperatur von 293,15 K). Die Werte für Normaldruck und Normaltemperatur können auch durch gemessene Werte der Umgebung (umgebende Atmosphäre) des Schüttgutfördersystems ersetzt werden. pGemisch ergibt sich aus der folgenden Formel:

wobei pLufteine bekannte Dichte von Luft ist, welche Bestandteil des Gasstroms ist, Pschutzgas eine bekannte Dichte eines Schutzgases ist, welches Bestandteil des Gasstroms ist, Sauerstoffgehaltiuft ein bekannter Sauerstoffgehalt der Luft ist und Sauerstoffgehaltmess der vom Sauerstoffsensor 1015 gemessene Sauerstoffgehalt des Gasstroms ist. Als Dichte der Luft kann beispielsweise pLutt = 1,225 kg/m³ eingesetzt werden und für den Fall, dass als Schutzgas Argon verwendet wird, kann als Dichte des Schutzgases pschutzgas = 1,784 kg/m³ eingesetzt werden. Als Sauerstoffgehalt der Luft kann Sauerstoffgehaltiuft = 20,94 % eingesetzt werden. Alternativ kann der Sauerstoffgehalt der Umgebungsatmosphäre gemessen und verwendet werden. The gas density p is determined using the following formula:

where pmess is the pressure of the gas flow measured by the pressure sensor 1050, Tmess is the temperature of the gas flow measured by the temperature sensor 1018, p _n is a predetermined normal pressure (in the example a normal pressure of p _n = 1013.25 mbar), and T _n is a predetermined normal temperature (in the example a normal temperature of 293.15 K). The values for normal pressure and normal temperature can also be replaced by measured values of the environment (surrounding atmosphere) of the bulk material conveying system. pmixture is obtained from the following formula:

where pair is a known density of air which is part of the gas stream, Pprotective gas is a known density of a protective gas which is part of the gas stream, oxygen content air is a known oxygen content of the air and oxygen content measurement is the oxygen content of the gas flow. For example, pLutt = 1.225 kg/m ³ can be used as the density of the air and in the case that argon is used as the shielding gas, the density of the shielding gas can be pschutzgas = 1.784 kg/m ^3. The oxygen content of the air can be oxygen contentiair = 20.94%. Alternatively, the oxygen content of the ambient atmosphere can be measured and used.

Step B:

The control device 1040 determines a bulk material mass flow of the bulk material flow based on the control value applied to the dosing device 1008. In the case of the described conveying of powder, the bulk material mass flow is a powder mass flow. The mass flow is specified in kg/s, for example.

The determination of the bulk material mass flow depends on the dosing device 1009 used and in particular on its geometry. In the case of a dosing device 1009 that has already been calibrated, an associated volume flow of the bulk material being conveyed can be calculated or read from a table, for example, depending on an applied control value (e.g. rotation speed of a conveyor screw). In particular, there can be a linear dependency between the control value and the volume flow, whereby a factor that characterizes the linear dependency can have been determined beforehand by means of calibration. From the determined volume flow (m ³ /s), the desired mass flow in kg/s can be calculated using a known density (kg/m ³ ) of the bulk material being conveyed. A calibration table or a way of calculating the volume flow based on a given control value can be found, for example, in a manual or a specification of the dosing device 1009.

If no corresponding calibration data is available, the dosing device 1009 can be calibrated to determine a dependency between control value and volume flow.

Furthermore, the bulk material mass flow can also be calculated. A corresponding calculation is shown below using the example of a screw conveyor.

The bulk material mass flow is calculated using the following formula:

Here, M _s is the bulk material mass flow, D _z is an outer diameter of the screw conveyor, Gw is a height of the screw wall, H _w ,i is a first screw spacing, e is a thickness of the screw wall, n _{m is} a rotation speed of the screw conveyor, i is a transmission ratio and p _s is a bulk density.

Details on the parameters given above in the context of a screw conveyor can also be found in Fig. 12, which shows a screw conveyor and associated parameters.

Step C:

The controller 1040 determines a desired velocity of the gas flow based on the gas density determined in step A and based on the bulk material mass flow determined in step B.

Determining the target velocity v _S0 n of the gas flow may include calculating a saltation velocity based on the determined gas density and the determined bulk material mass flow. Determining the target velocity v _S0 n is carried out according to one embodiment using the following formula: target ^saltation " ^safety where Vsaitation is the calculated saltation velocity of the conveyed bulk material and vsafety is a predetermined safety velocity.

The saltation speed is a speed below which particles of the bulk material being conveyed (e.g. powder) begin to fall down and accumulate on a bottom of the conveying line 1007. To ensure that this does not happen, a predetermined safety speed is added to the calculated saltation speed. In a way, this forms a safety margin from the saltation speed, which ensures that no deposition of bulk material takes place in the conveying line 1007. As an alternative to adding the safety speed, a predetermined safety factor can be multiplied by the saltation speed (e.g. 1.1 or 1.2). The saltation rate is calculated by the controller 1040 using the following formula:

Here, M _s is the specific bulk material mass flow in kg/s, g is a predetermined acceleration due to gravity in m/s ² , D is a diameter of the conveyor line in m, p is the specific gas density of the gas flow in kg/m ³ and a and b are parameters that are each dependent on a particle diameter d of the bulk material. The parameters a and b can, for example, each be stored as constants for the bulk material used in a memory of the control device. Furthermore, at least one of the two parameters a and b can be calculated, whereby the respective calculation is dependent on the particle diameter d of the bulk material used.

The particle diameter d is taken from a specification of the bulk material used, for example a data sheet of the powder used. The particle diameter d can also be stored, for example, in a memory of the control device 1040. In particular, a table can be stored in the memory of the control device 1040 in which the respective values for the particle diameter d of the respective powder are stored for different powder materials. In this way, a respective particle diameter d can be used when calculating the target speed by entering or selecting the powder used via a user interface of the control device 1040.

Step D:

The control device 1040 controls the conveying device 1019 to convey the gas flow at the target speed determined in step C. In this case, for example, a signal with a predetermined voltage value or a predetermined frequency is applied to the conveying device 1019, it being known that the voltage value or the frequency leads to the desired conveying speed of the conveying device 1019.

In the event that the conveyor 1019 is not calibrated accordingly, a control loop can be used which provides a measured gas velocity value a speed sensor is taken into account. For example, sensor 1016 or 1017 can be a corresponding speed sensor. For example, a PID controller can be used to regulate the desired value of the target speed.

Using the technology described above, an operating point for bulk material conveyance (i.e. a conveying speed of the conveying gas and thus a conveying speed of the conveyed bulk material) can be set in a quick, simple and uncomplicated manner. On the one hand, this involves using data that is stored for different types of powder in a memory of the control device 1040 (for example, the bulk density and the particle diameter of the powder). On the other hand, it is possible to react to changing operating parameters, in particular to changes in measured values, for example the oxygen content, the pressure and/or the temperature within the conveying line 1007. It is also possible to react to a change in the conveyed mass flow of the bulk material by adjusting the speed of the gas flow.

According to one embodiment, the above steps A to D, which are carried out by the control device 1040, can be followed by a so-called limit value control. However, this is optional and only the determination of the target speed can be carried out according to steps A to D.

Details of the limit control are shown in Fig. 13. The limit control in Fig. 13 corresponds to the limit control described above in connection with Fig.

10 for the powder conveying system of Fig. 1. In this regard, the control device 1040 is set up to determine whether at least one measured parameter of the bulk material conveying system 1001 is above a predetermined maximum value for the respective parameter. If the at least one measured parameter is above the predetermined maximum value, the control device 1040 reduces the control value of the dosing device 1008 by a predetermined value so that it releases a smaller dose of bulk material per unit of time into the gas stream.

An example of limit control is shown in Fig. 13 using specific parameters. Fig. 13 shows a flow chart of a process performed by the controller 1040 after determining the target speed and controlling the conveyor. The process starts with step 1202, in which it is checked whether a measured conveying speed is below a predetermined maximum value. To measure the conveying speed, a speed sensor in the gas stream or in the gas-powder mixture stream is used, for example the sensor 1016 or 1017 in Fig. 11.

If yes, the process proceeds to step 1204 where it is queried whether a measured pump outlet pressure is below a predetermined maximum value. To measure the pump outlet pressure, a pressure sensor in the gas stream downstream of the pump 1019 is used, for example the sensor 1050 in Fig. 11.

If "yes", the process proceeds to step 1206, where it is queried whether a pump power consumed by the pump 1019 is below a predetermined maximum value.

In the case of "yes", the process proceeds to step 1208, in which it is queried whether a dose of bulk material dispensed by the dosing device 1008 per time is within a predetermined maximum range.

If the answer is "yes", the process is complete. You can then ask whether the conveying should be stopped. If this is not the case, steps A to D discussed above are carried out again and the conveying speed is adjusted if necessary.

If it is determined in step 1208 that the dose of bulk material dispensed per time is not within the predetermined maximum range ("no" after step 1208), then in step 1210 it is checked whether the dose is below the predetermined maximum range. If this is the case ("yes"), then in step 1212 the dose is increased by a predetermined value. If this is not the case ("no" after step 1210), then in step 1214 the dose is reduced by a predetermined value.

Following step 1214, step 1216 checks whether the dose is less than a predetermined minimum. If this is not the case ("no"), the process is terminated. However, if this is the case ("yes" after step 1216), filter cleaning is carried out in step 1218. The process also proceeds to the reduction of the dose of the dosing device 1008 according to step 1214 if the result in at least one of the queries 1202, 1204 and 1206 is "no" and thus a corresponding maximum value has been reached or exceeded.

After the process of Fig. 13, it is checked whether the conveying process is terminated and thus whether in particular the conveying device 1019 is stopped.

This is the case when at least one of the following events is detected: a source tank 1006 from which bulk material is removed by means of the dosing device 1008 is empty, a target tank into which the bulk material is conveyed (or a corresponding cyclone 1010) is full, a predetermined maximum conveying time is exceeded and a total pressure loss of the conveying gas exceeds a predetermined limit value.

If the conveying process is not terminated, the method performed by the control device 1040 returns to step A (see above).

Fig. 1 shows a schematic representation of a bulk material conveying system which is used for conveying raw material powder in a system 1100 for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation.

The bulk material conveying system of Fig. 1 represents a more detailed representation of a bulk material conveying system compared to Fig. 11, wherein certain components are shown and provided with reference numerals that may also be present in the system of Fig. 11, but are not explicitly described.

Furthermore, however, the system of Fig. 1 also represents a more complex embodiment of a bulk material conveying system of the type described herein, in particular since several conveying processes can be carried out within the system, i.e. conveying processes from different source tanks to different target tanks. The exact structure and functioning of the bulk material conveying system has already been described above in connection with Fig. 1 (as a powder conveying system).

In particular, the method comprising steps A to D for adjusting a conveyor speed can also be carried out in connection with the system of Fig. 1. The same applies to the limit value control of Fig. 13. For this purpose For this purpose, the bulk material conveying system of Fig. 1 also includes a control device (not shown) for controlling the individual components of the system of Fig. 1.

The system 1100 corresponds, for example, to the system 1100 of Fig. 11 and - generally speaking - for example to a generally known system for additive manufacturing by means of selective laser melting or selective laser sintering.

During operation (i.e. conveying operation) of each of the conveying processes a to c described in connection with Fig. 1, a method can be carried out which determines the gas velocity (target velocity) and which adjusts the pump 79 so that it conveys the conveying gas at the determined target velocity. For this purpose, the steps A to D explained above can be carried out by a control device of the bulk material conveying system.

In one embodiment, for example, sensor data from the pressure sensor 80, the temperature sensor 86 and the oxygen sensor 76 are used to determine the gas density of the gas flow. To determine the bulk material mass flow, the geometry of the respective conveyor screw, which is located at the outlet of the respective source tank, is taken into account. The speed of the gas conveyed by the pump 79 is regulated using the speed sensor 86, for example by means of a PID control.

In addition, a limit control according to Fig. 13 can be implemented in connection with each of the conveying processes a to c (see the description above).

The closed powder circuits according to the conveying processes a to c can have the advantage, among other things, that a system operator has to intervene in the process manually as little as possible and comes into contact with the powder as little as possible, which, for example, poses health risks and can pose a risk of explosion or ignition for certain types of powder. The automatic adjustment of the conveying speed can have the advantage that an optimal operating point (i.e. an optimal conveying speed) can be set quickly and easily for different types of powder and/or different process conditions.

Aspects which can be used alone and/or in combination with the powder conveying system of Fig. 1 are as follows: 1. Bulk material conveying system, in particular for conveying raw material powder in a system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, wherein the bulk material conveying system comprises: a conveying line which is designed to convey a gas flow and, at least in sections, a bulk material flow driven by the gas flow; a dosing device which is designed to supply the gas flow with a predetermined dose of bulk material per unit of time, wherein the dose is determined by a control value applied to the dosing device; a conveying device which is designed to convey the gas flow through the conveying line; at least one measuring device for measuring at least one characteristic of the gas flow; and a control device which is designed to:

Determining a gas density of the gas flow based on the measured at least one characteristic of the gas flow;

Determining a bulk material mass flow of the bulk material stream based on the control value applied to the dosing device;

Determining a target velocity of the gas flow based on the gas density and based on the bulk material mass flow; and

Controlling the conveying device to convey the gas flow at the specified target speed.

2. Bulk material conveying system according to aspect 1, wherein the gas density is determined based on at least one of the following parameters of the gas flow: oxygen content, pressure, temperature, dew point and humidity.

wobei pmess ein gemessener Druck des Gasstroms ist, Tmess eine gemessene Temperatur des Gasstroms ist, p_n ein vorbestimmter Normaldruck ist, T_n eine vorbestimmte Normaltemperatur ist und sich pGemisch aus der folgenden Formel ergibt:

wobei pLufteine bekannte Dichte von Luft ist, welche Bestandteil des Gasstroms ist, Pschutzgas eine bekannte Dichte eines Schutzgases ist, welches Bestandteil des Gasstroms ist, Sauerstoffgehaltiuft ein bekannter Sauerstoffgehalt der Luft ist und Sauerstoffgehaltmess ein gemessener Sauerstoffgehalt des Gasstroms ist. 3. Bulk material conveying system according to aspect 2, wherein the gas density p is determined using the following formula:

where pmess is a measured pressure of the gas stream, Tmess is a measured temperature of the gas stream, p _n is a predetermined normal pressure, T _n is a predetermined normal temperature and pmixture is given by the following formula:

where pair is a known density of air which is part of the gas stream, Pprotective gas is a known density of a protective gas which is part of the gas stream, oxygen contentair is a known oxygen content of the air and oxygen contentmeasured is a measured oxygen content of the gas stream.

4. Bulk material conveying system according to one of aspects 1 to 3, wherein the control value applied to the dosing device is a motor speed of a motor of the dosing device, in particular a motor for driving a conveyor screw of the dosing device.

5. Bulk material conveying system according to one of aspects 1 to 4, wherein determining the target velocity v _S0 n of the gas flow comprises calculating a saltation velocity based on the determined gas density and the determined bulk material mass flow, and wherein determining the target velocity v _S0 n is carried out using the following formula: target ^saltation "F ^safety where Vsaitation is the calculated saltation velocity of the bulk material being conveyed and vsafety is a predetermined safety velocity.

wobei M_s der bestimmte Schüttgutmassestrom in kg/s ist, g eine vorbestimmte Fallbeschleunigung in m/s² ist, D ein Durchmesser der Förderleitung in m ist, p die bestimmte Gasdichte des Gasstroms in kg/m³ ist und a und b jeweils von einem Partikeldurchmesser d des Schüttguts abhängige Parameter sind. 6. Bulk material conveying system according to aspect 5, wherein the saltation rate is calculated using the following formula:

where M _s is the determined bulk material mass flow in kg/s, g is a predetermined acceleration due to gravity in m/s ² , D is a diameter of the conveying line in m, p is the determined gas density of the gas flow in kg/m ³ and a and b are parameters each dependent on a particle diameter d of the bulk material.

7. Bulk material conveying system according to one of aspects 1 to 6, wherein the control of the conveying device for conveying the gas flow with the determined Target speed is carried out using a speed sensor for measuring a speed of the gas flow and a control loop, in particular comprising a PID controller.

8. Bulk material conveying system according to one of aspects 1 to 7, wherein the control device is further configured to:

Determining whether at least one measured characteristic of the bulk material conveying system is above a predetermined maximum value for the respective characteristic; and if the at least one measured characteristic is above the predetermined maximum value, reducing the control value of the dosing device by a predetermined value.

9. Bulk material conveying system according to aspect 8, wherein the control device is further configured to: if the at least one measured parameter is below the predetermined maximum value, increase the control value of the dosing device by a predetermined value.

10. Bulk material conveying system according to claim 8 or 9, wherein the at least one parameter comprises at least one of the following parameters:

Conveying speed, pump outlet pressure, pump power and dose of bulk material per time.

11. Bulk material conveying system according to one of aspects 1 to 10, wherein the control device is arranged to:

Terminating a conveying process by stopping the conveying device when at least one of the following events is detected: a source tank from which bulk material is removed by means of the dosing device is empty, a target tank into which the bulk material is conveyed is full, a predetermined maximum conveying time is exceeded and a total pressure loss of the conveying gas exceeds a predetermined limit.

12. Bulk material conveying system according to one of aspects 1 to 11, further comprising a pressure equalization tank coupled to the conveying line downstream of the conveying device and upstream of the dosing device. 13. Bulk material conveying system according to one of aspects 1 to 12, comprising at least a first conveying circuit for conveying raw material powder from a main storage into a first tank of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, wherein a lower outlet of the first tank is coupled to an upper inlet of an intermediate tank of the system, wherein a manufacturing process in a process chamber of the system is fed with powder from the intermediate tank, and wherein an upper inlet of the main storage is coupled to an outlet of a sieve for sieving the raw material powder.

14. Bulk material conveying system according to one of aspects 1 to 13, comprising at least one second conveying circuit for conveying raw material powder from an overflow tank into a buffer container, wherein the overflow tank is configured to receive excess powder from the process chamber of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, and wherein a lower outlet of the buffer container is coupled to an inlet of a sieve for sieving the raw material powder.

15. Bulk material conveying system according to one of aspects 1 to 14, comprising at least a third conveying circuit for conveying raw material powder from an external tank into a buffer container, wherein the external tank is detachably coupleable or coupled to the bulk material conveying system, and wherein a lower outlet of the buffer container is coupled to an inlet of a sieve for sieving the raw material powder.

16. Bulk material conveying system according to one of aspects 1 to 15, wherein the dosing device is located at the outlet of a respective source tank and wherein a cyclone for separating the raw material powder from the gas stream and for feeding the raw material powder into the target tank is located at the inlet of a respective target tank.

17. Method for conveying bulk material, in particular for conveying raw material powder in a plant for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, the method comprising:

Conveying a gas stream and a bulk material stream driven by the gas stream through a conveying line;

Supplying a predetermined dose of bulk material per unit time through a dosing device, the dose being determined by a control value applied to the dosing device;

conveying the gas flow through the conveying line by means of a conveying device;

Measuring at least one characteristic of the gas flow using a measuring device;

Determining a gas density of the gas flow based on the measured at least one characteristic of the gas flow;

Determining a bulk material mass flow of the bulk material stream based on the control value applied to the dosing device;

Determining a target velocity of the gas flow based on the gas density and based on the bulk material mass flow; and

Controlling the conveying device to convey the gas flow at the specified target speed.

18. The method according to aspect 17, wherein the gas density is determined based on at least one of the following characteristics of the gas flow: oxygen content, pressure, temperature, dew point and humidity.

wobei pmess ein gemessener Druck des Gasstroms ist, Tmess eine gemessene Temperatur des Gasstroms ist, p_n ein vorbestimmter Normaldruck ist, T_n eine vorbestimmte Normaltemperatur ist und sich pGemisch aus der folgenden Formel ergibt:

wobei ßLufteine bekannte Dichte von Luft ist, welche Bestandteil des Gasstroms ist, Pschutzgas eine bekannte Dichte eines Schutzgases ist, welches Bestandteil des Gasstroms ist, Sauerstoffgehaltiuft ein bekannter Sauerstoffgehalt der Luft ist und Sauerstoffgehaltmess ein gemessener Sauerstoffgehalt des Gasstroms ist. 19. The method of aspect 18, wherein the gas density p is determined using the following formula:

where pmess is a measured pressure of the gas stream, Tmess is a measured temperature of the gas stream, p _n is a predetermined normal pressure, T _n is a predetermined normal temperature and pmixture is given by the following formula:

where ßair is a known density of air which is part of the gas stream, Pprotective gas is a known density of a protective gas which is part of the gas stream, oxygen contentiair is a known oxygen content of the air and oxygen contentmeasurement is a measured oxygen content of the gas stream.

20. Method according to one of aspects 17 to 19, wherein the control value applied to the dosing device is a motor speed of a motor of the dosing device, in particular a motor for driving a conveyor screw of the dosing device.

21. The method of any of aspects 17 to 20, wherein determining the target velocity Vsoii of the gas flow comprises calculating a saltation velocity based on the determined gas density and the determined bulk material mass flow, and wherein determining the target velocity v _S0 n is performed using the following formula: soll ^saltation "F ^Sicherheit where Vsaitation is the calculated saltation velocity of the conveyed bulk material and vsicherheit is a predetermined safety velocity.

wobei M_s der bestimmte Schüttgutmassestrom in kg/s ist, g eine vorbestimmte Fallbeschleunigung in m/s² ist, D ein Durchmesser der Förderleitung in m ist, p die bestimmte Gasdichte des Gasstroms in kg/m³ ist und a und b jeweils von einem Partikeldurchmesser d des Schüttguts abhängige Parameter sind. 22. The method of aspect 21, wherein calculating the saltation rate is performed using the following formula:

where M _s is the determined bulk material mass flow in kg/s, g is a predetermined acceleration due to gravity in m/s ² , D is a diameter of the conveying line in m, p is the determined gas density of the gas flow in kg/m ³ and a and b are parameters each dependent on a particle diameter d of the bulk material.

23. Method according to one of aspects 17 to 22, wherein the control of the conveying device for conveying the gas flow at the determined target speed is carried out using a speed sensor for measuring a speed of the gas flow and a control loop, in particular comprising a PID controller. 24. Method according to one of aspects 17 to 23, further comprising:

Determining whether at least one measured characteristic of the bulk material conveying system is above a predetermined maximum value for the respective characteristic; and if the at least one measured characteristic is above the predetermined maximum value, reducing the control value of the dosing device by a predetermined value.

25. Method according to aspect 24, further comprising: if the at least one measured parameter is below the predetermined maximum value, increasing the control value of the dosing device by a predetermined value.

26. Method according to aspect 24 or 25, wherein the at least one parameter comprises at least one of the following parameters:

Conveying speed, pump outlet pressure, pump power and dose of bulk material per time.

27. Method according to one of aspects 17 to 26, further comprising:

Terminating a conveying process by stopping the conveying device when at least one of the following events is detected: a source tank from which bulk material is removed by means of the dosing device is empty, a target tank into which the bulk material is conveyed is full, a predetermined maximum conveying time is exceeded and a total pressure loss of the conveying gas exceeds a predetermined limit.

28. A method according to any one of aspects 17 to 27, comprising:

Conveying raw material powder through at least a first conveyor circuit from a main storage tank into a first tank of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, wherein a lower outlet of the first tank is coupled to an upper inlet of an intermediate tank of the system, wherein a production process in a process chamber of the system is fed with powder from the intermediate tank, and wherein an upper inlet of the main storage tank is coupled to an outlet of a sieve for sieving the raw material powder. 29. A method according to any one of aspects 17 to 28, comprising:

Conveying raw material powder through at least one second conveying circuit from an overflow tank into a buffer container, wherein the overflow tank is configured to receive excess powder from the process chamber of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, and wherein a lower outlet of the buffer container is coupled to an inlet of a sieve for sieving the raw material powder.

30. A method according to any one of aspects 17 to 29, comprising:

Conveying raw material powder through at least a third conveying circuit from an external tank into a buffer container, wherein the external tank is detachably coupleable or coupled to the bulk material conveying system, and wherein a lower outlet of the buffer container is coupled to an inlet of a sieve for sieving the raw material powder.

31. Method according to one of aspects 17 to 30, wherein the dosing device is located at the outlet of a respective source tank and wherein a cyclone for separating the raw material powder from the gas stream and for feeding the raw material powder into the target tank is located at the inlet of a respective target tank.

The screening device of the present disclosure (in particular, the screening device 5 of FIG. 1) may further be configured according to the screening device of the following disclosure. A method performed by the screening device 5 of FIG. 1 may correspond to one of the methods of the following disclosure.

Individual aspects of the screening device and/or the method described below can be applied to the screening device 5 of the above description.

However, the aspects of a screening device or an associated method described below can also be advantageous in themselves and represent one or more inventions. Thus, the screening device and/or the method described below can also be used independently (ie independently of the powder conveying system described above). The following disclosure relates to a method for controlling the operation of a screening device and a screening device which is suitable, for example, for use in a powder conveying system of a system for producing three-dimensional workpieces using a generative layer construction process. Furthermore, the following disclosure relates to a system equipped with such a screening device for producing three-dimensional workpieces using a generative layer construction process.

In generative layer construction processes for producing three-dimensional workpieces, in particular so-called powder bed fusion, a raw material powder or granulate is applied layer by layer to a carrier and, depending on the desired geometry of the workpiece to be created, is exposed to electromagnetic radiation, for example laser radiation or particle radiation, in a location-selective manner. The radiation penetrating the powder layer causes heating and consequently a fusion or sintering of the raw material powder particles. Subsequently, further layers of raw material powder are successively applied to the already radiation-treated and solidified layer on the carrier until the workpiece has the desired shape and size. The raw material powder can comprise ceramic, metal or plastic materials, but also material mixtures thereof. Generative layer construction processes and in particular powder bed fusion processes can be used, for example, to produce prototypes, tools, spare parts or medical prostheses, such as dental or orthopedic prostheses, as well as to repair components using CAD data.

A system known for example from EP 2 335 848 B1 for producing three-dimensional workpieces by selectively irradiating a raw material powder comprises a process chamber sealed from the ambient atmosphere and a carrier arranged in the process chamber for receiving the raw material powder to be irradiated. The system also comprises an irradiation device equipped with a radiation source, in particular a laser source, and an optical unit. The optical unit serves to selectively guide an irradiation beam generated by the radiation source over the raw material powder layers applied to the carrier depending on the geometry of the workpiece to be produced. When building a three-dimensional workpiece by selectively irradiating the powder layers applied to the carrier, the radiation energy introduced into the raw material powder causes the powder particles to melt and/or sinter. During the additive manufacturing of three-dimensional components in a powder bed (particularly through selective laser melting and/or selective laser sintering), excess powder is generated when the individual powder layers are applied, which can be collected in one or more collecting containers. Furthermore, non-excess, solidified powder cannot be obtained when the workpieces are unpacked. The excess powder can be reused in an additive manufacturing process after appropriate reprocessing. For example, the reprocessed powder can be reused as raw material powder for a selective laser melting process or laser sintering process or mixed with the fresh raw material powder used in this process.

However, particulate impurities as well as glued or sintered powder agglomerates in the excess powder could lead to contamination of the powder bed and thus to reduced quality of the workpiece if the excess powder is reused in an additive manufacturing process.

An essential step in the recycling of the excess powder is therefore sieving, which removes unwanted particulate contaminants whose particle size is larger than the particle size of the powder from the excess powder. In addition, the excess powder can be subjected to further powder recycling steps, e.g. drying, cleaning, separation of components, etc., which can be carried out before and/or after sieving the powder.

A device for sieving powder, which is suitable for use in a system for producing a three-dimensional workpiece by means of selective electron beam melting, selective laser melting, laser cladding, laser metal deposition or selective laser sintering, is described in DE 20 2021 102 494. This device comprises a base unit with a vibration generator and a control unit for controlling the vibration generator. The base unit has an interface for connecting a replacement module, which comprises a sieve and a housing for holding the sieve. The replacement module can be connected to the base unit via the interface.

The following disclosure is directed to the object of providing a method for controlling the operation of a screening device and a screening device which enable efficient screening of powder and are therefore particularly well suited for use in a powder conveying system of a plant for producing three-dimensional Workpieces using a generative layer construction process. Furthermore, the following disclosure is directed to the task of specifying a system for producing three-dimensional workpieces using a generative layer construction process, which enables efficient production of high-quality workpieces.

This object is achieved by a method for controlling the operation of a screening device having the features of the following aspect 1, a screening device having the features of aspect 13 and a system for producing three-dimensional workpieces using a generative layer construction process having the features of aspect 21.

In a method for controlling the operation of a sieving device, in a step (i), powder to be sieved is fed to a sieve through a powder inlet. The sieve can comprise a sieve frame onto which a sieve fabric defining a sieve surface is stretched. The sieve frame can be connected to a sieve container or placed on the sieve container. Sieved powder can then be collected in the sieve container after passing through the sieve fabric. The sieve container can have a cross-section that tapers downwards, so that sieved powder received in the sieve container can be discharged from the sieve container by gravity via a sieved powder outlet arranged in the region of a lower section of the sieve container. To discharge oversize grain that is too coarse to pass through the sieve fabric in the direction of the sieve container, the sieve device can have an oversize grain outlet. The powder inlet and the oversize grain outlet are preferably arranged in the region of opposite side edges of the sieve surface.

The screening device is preferably equipped with a drive device for driving the screen. The drive device preferably acts on the screen frame, so that when the screening device is in operation, the screen frame and thus the screen fabric are set into vibration by the drive device. The drive device can be a mechanical drive device that vibrates the screen. Preferably, however, the drive device is designed in the form of an ultrasonic drive device and is designed to subject the screen to ultrasonic vibrations. The use of an ultrasonic drive device instead of a mechanical drive device enables an increase in the screening powder throughput, i.e. a higher screening performance, and thus an increase in the efficiency of the screening device. In particular, the Sieve powder throughput can be increased by a factor of approximately 4 by equipping the sieving device with an ultrasonic drive device compared to a mechanically driven sieving device.

In an ultrasonic drive device, the frequency primarily determines the drive power. Although the amplitude also influences the power consumption of the drive motor, the frequency is the most important parameter for the drive power, as it influences the vibrations of the sieve mesh and thus the separation effect of the sieve. A higher frequency normally leads to an improved separation effect and thus to a higher sieve powder throughput, i.e. a higher sieving performance.

When a powder is loosely poured onto a plane, an angle of repose forms. The angle of repose is the angle between the horizontal plane and the maximum inclination that the powder can assume without further influences, i.e. the angle at which the powder begins to flow when it is poured onto an inclined surface. The angle of repose depends on various factors, such as the size and shape of the powder particles. The angle of repose is also influenced by process parameters, such as the relative humidity. A powder poured onto a powder to be sieved using the sieving device can, for example, have an angle of repose between approx. 20° and approx. 40°, preferably between approx. 25° and approx. 35°, and particularly preferably approx. 30°.

The wall friction angle, on the other hand, is an angle between a horizontal plane and the inclination at which the powder sticks to the wall and does not flow further. The wall friction angle depends on the type and nature of the wall as well as on the surface quality of the powder particles. A powder bed of a powder to be sieved using the sieving device can, for example, have a wall friction angle of between approx. 10° and approx. 30°, preferably between approx. 15° and approx. 25° and particularly preferably of approx. 20°.

When powder is fed through the powder inlet of the sieve device, a cone of repose forms on the sieve, i.e. on the sieve fabric, the shape and angle of repose of which is influenced by the vibrations acting on the cone of repose and consequently by the drive power of the drive device driving the sieve. In particular, the angle of repose is reduced as the drive power of the drive device increases. At the same time, the vibrations triggered by the drive device reduce the wall friction, which prevents the Powder pouring over the sieve surface is made easier. If the sieve is driven with a low drive power of the drive device, the pouring cone that forms on the sieve below the powder inlet resembles a pouring cone that forms on a stationary plane, the base area of which only takes up a small section of the sieve surface of the sieve. In such an operating state of the sieve device, the sieve surface utilization is low, since only a small section of the sieve surface adjacent to the powder inlet is actually charged with powder.

If, however, the sieve is driven with a high drive power, the powder spreads out over the sieve surface, i.e. the angle of repose of the cone of repose, which the powder fed through the powder inlet onto the sieve forms on the sieve surface, decreases with increasing drive power of the drive device driving the sieve, as does the wall friction angle. This increases the base area of the cone of repose, so that the sieve surface utilization increases and sections of the sieve surface further away from the powder inlet and closer to the oversize grain outlet are also exposed to powder. However, if the base area of the cone of repose becomes too large, and in particular so large that the powder flows over the entire sieve surface and thus the entire sieve surface is exposed to powder, there is a risk that powder will flow unscreened into the oversize grain outlet. As a result, powder that is actually fine-grained enough to pass through the sieve mesh is lost unused and the sieve efficiency decreases.

In the method for controlling the operation of a screening device, the screen is therefore driven in a step (ii) for a first time interval with a first drive power which is dimensioned such that the powder to be screened flows over an entire screening surface of the screen and/or into an oversize grain outlet when the screen is continuously driven with the first drive power. During the first time interval, the screening device is therefore driven with such a high drive power that, if it were continuously maintained, on the one hand would ensure maximum screen surface utilization, but on the other hand would at least be associated with a high risk of powder being lost unscreened through the oversize grain outlet.

After the first time interval has elapsed, the sieve is therefore driven in a step (Hi) for a second time interval with a second drive power which is lower than the first drive power. After the second time interval has elapsed, steps (ii) and (iii) are repeated, ie the sieve is periodically alternated with the first higher drive power and the second lower drive power.

The first drive power can take on different values for different powder types and different process parameters, such as temperature, particle size distribution of the powder, moisture content of the powder, etc. The first drive power can therefore either be determined empirically before a sieving process or taken from a previously created drive power value table for different powder types and process parameters. Alternatively, the first drive power can also be selected based on empirical values.

During the first time interval, you can benefit from the high screen surface utilization resulting from the high drive power and consequently from a high screening performance. In addition, during the first time interval, oversize grain is specifically conveyed towards the oversize grain outlet. In particular, due to the "spreading" of the cone of material over the entire screen surface during the periodically recurring first time interval, oversize grain that collects inside the cone of material during the second time interval and "builds up" in columns can also be discharged from the screen surface via the oversize grain outlet. This can counteract the formation of so-called stuck grains by powder particles that are pressed into the screen fabric by back pressure and the vibration of the screen and clog the screen fabric. The reduction in drive power during the second time interval, on the other hand, ensures that the loss of unscreened powder through the oversize grain outlet is minimized.

The method described here for controlling the operation of a sieving device therefore enables very efficient sieving of powder and is therefore particularly suitable for use in a powder conveying system of a system for producing three-dimensional workpieces using a generative layer construction process, in which large quantities of comparatively expensive (metal) powder may also have to be sieved. In addition, the sieve mesh of the sieve is exposed to a smaller powder mass overall due to the periodically changing drive power and is therefore less stressed. This can result in less wear and tear and consequently a longer service life of the sieve mesh.

Preferably, the second drive power is dimensioned such that the powder to be sieved when the sieve is continuously driven with the second drive power in the area of the powder inlet on the sieve surface of the sieve, a cone of material is formed which essentially corresponds to a cone of material forming on a stationary plane. The cone of material which forms on the sieve surface when the sieve is driven with the second drive power has in particular an angle of material which is a maximum of 30%, preferably a maximum of 20% and particularly preferably a maximum of 10% greater than the angle of material of a cone of material forming on a stationary plane. The base area of the cone of material which forms on the sieve surface when the sieve is continuously driven with the second drive power takes up only a small section of the sieve surface of the sieve arranged in the area of the powder inlet. During the second time interval, the screening device can therefore be driven with such a low drive power that, if it were maintained continuously, on the one hand would ensure that no or almost no powder flows unsifted into the oversize grain outlet, but on the other hand would result in a low screening throughput and consequently a low screening performance due to the low utilization of the screening area. Thus, the second drive power can be lower than the first drive power, but greater than zero.

However, in one or more embodiments, the second drive power can also be zero. In this case, the sieve is not driven in the second time interval.

The second drive power can take on different values for different types of powder and different process parameters, such as temperature, particle size distribution of the powder, moisture content of the powder, etc. The second drive power can therefore either be determined empirically before a sieving process or taken from a previously created drive power value table for different types of powder and process parameters. Alternatively, the second drive power can also be selected based on empirical values.

In principle, the sieve surface of the sieve can be aligned coplanarly to a horizontal plane. Alternatively, however, it is also conceivable that the sieve surface of the sieve is inclined relative to a horizontal plane. An angle of repose of the cone of repose, which forms on the sieve surface of the sieve when the sieve is driven with the second drive power in the area of the powder inlet, is preferably adapted to an orientation of the sieve surface. This means that when the sieve surface of the sieve is aligned coplanarly to a horizontal plane, the powder fed to the sieve through the powder inlet preferably forms a symmetrical cone of repose with a constant angle of repose along a circumference of the cone of repose. If the If, on the other hand, the sieve surface of the sieve is inclined relative to a horizontal plane, the powder fed through the powder inlet onto the sieve preferably forms a cone of repose whose angle of repose varies along its circumference depending on the direction of inclination and the angle of inclination of the sieve surface.

Preferably, the sieve surface of the sieve is inclined relative to a horizontal plane in such a way that the flow of the powder fed through the powder inlet in the direction of the oversize grain outlet is supported by gravity. In other words, the sieve surface of the sieve is preferably designed to slope downwards from an area below the powder inlet in the direction of the oversize grain outlet. This not only promotes the spread of the powder over the sieve surface, but also the removal of oversize grain from the sieve surface into the oversize grain outlet. Accordingly, the angle of repose of the repose cone, which forms on the sieve surface when the sieve is driven with the second drive power, is preferably smaller in a peripheral section of the repose cone facing the oversize grain outlet than in a peripheral section of the repose cone facing away from the oversize grain outlet.

However, an angle of inclination of the sieve surface of the sieve relative to the horizontal plane is preferably smaller than an angle of repose of a repose cone that the powder to be sieved forms on a horizontal plane. This ensures that at least when the sieve is driven with the second drive power in the area of the powder inlet, a stable repose cone is still formed and the powder does not flow uncontrollably over the sieve surface. For example, the angle of inclination of the sieve surface of the sieve relative to the horizontal plane can be approximately 10° to approximately 25°, preferably approximately 15° to approximately 20° and particularly preferably approximately 17°.

Similar to the first drive power, the first time interval can also take on different values for different types of powder and different process parameters, such as temperature, particle size distribution of the powder, moisture content of the powder, etc. The first time interval is therefore preferably a value determined empirically for the powder to be sieved before a sieving process. However, the first time interval can also be taken from a previously created table of values that contains different values of the first time interval for different types of powder and process parameters, or can be selected based on empirical values. When determining the first time interval, the first time interval is preferably ended at the latest when powder to be screened flows into the oversize grain outlet. The flow of powder to be screened into the oversize grain outlet can be detected by means of an oversize grain sensor. The oversize grain sensor is preferably arranged in the area of the oversize grain outlet.

Preferably, the first time interval is dimensioned such that a sieve surface utilization of approximately 70% to approximately 90%, preferably approximately 75% to approximately 85% and particularly preferably approximately 80% of the total sieve surface of the sieve is not exceeded by the end of the first time interval. In other words, the first time interval preferably provides a "time safety reserve" so that the powder does not spread over the entire sieve surface during the first time interval. This particularly reliably prevents unscreened powder from being lost through the oversize grain outlet.

The second time interval can also take on different values for different types of powder and different process parameters, such as temperature, particle size distribution of the powder, moisture content of the powder, etc. The second time interval is therefore preferably also a value determined empirically for the powder to be sieved before a sieving process. The second time interval can also be taken from a previously created table of values that contains different values of the second time interval for different types of powder and process parameters, or can be selected based on empirical values.

When determining the second time interval, the second time interval is preferably ended at the latest when powder to be sieved forms a cone of material with a defined size in the area of the powder inlet on the sieve surface of the sieve. On the one hand, this prevents the cone of material from becoming too large and clogging the powder inlet. On the other hand, the efficiency of the sieving process is increased by the transition to the first time interval, during which the sieve is driven with increased drive power and the sieve throughput is consequently increased. The formation of a cone of material with a defined size can be detected, for example, by means of a dosing sensor. The dosing sensor is preferably arranged in the area of the powder inlet.

The following explains the control of the powder feed through the powder inlet of the sieve, which can also be used independently of the drive control described above. In an independently claimable method for controlling the operation of a screening device, powder to be screened is fed at least temporarily continuously through the powder inlet onto the screen with a metering mass flow rhdos = 0.5 * (rh(amin) + rh(a _m ax)). The metering mass flow can be set, for example, by appropriately controlling a metering device assigned to the powder inlet, which can comprise a metering screw, for example, and/or by controlling a valve assigned to the powder inlet. The parameters a _m ax and rh(a _m ax) define a screen area utilization and a screen throughput when driving the screen with a first drive power, which is dimensioned such that the powder to be screened flows over an entire screen area of the screen and/or into an oversize grain outlet when the screen is continuously driven with the first drive power. The parameters a _m in and rh(a _m in), on the other hand, define a sieve area utilization and a sieve throughput when the sieve is driven with a second drive power that is lower than the first drive power. The continuous dosing mass flow can therefore be determined by forming an average of the sieve throughput when the sieve is driven with the first drive power and the sieve throughput when the sieve is driven with the second drive power.

The sieve throughput when driving the sieve with a second drive power rh(amin) is preferably determined by increasing the metering mass flow of the powder to be sieved through the powder inlet when driving the sieve with the second drive power until the powder to be sieved has formed a cone of material with a defined size on the sieve surface of the sieve in the area of the powder inlet. The formation of the cone of material with the defined size can be detected, for example, by means of the metering sensor arranged in the area of the powder inlet. In other words, when determining the sieve throughput when driving the sieve with a second drive power rh(a _m in), the metering mass flow of the powder to be sieved through the powder inlet is preferably increased until the metering sensor triggers.

The sieve throughput when driving the sieve with the first drive power rh(amax) is preferably determined by dividing the dosing mass flow of the powder to be sieved by the powder inlet is increased until the powder to be screened has formed a cone of material with a defined size in the area of the powder inlet on the screen surface of the screen and powder to be screened flows into the oversize grain outlet. The formation of the cone of material with the defined size can, for example, be detected by means of the dosing sensor arranged in the area of the powder inlet. The flow of powder to be screened into the oversize grain outlet can, on the other hand, be detected by means of the oversize grain sensor arranged in the area of the oversize grain outlet. In other words, when determining the screen throughput when driving the screen with the first drive power rh(a _m ax), the dosing mass flow of the powder to be screened through the powder inlet is preferably increased until the dosing sensor and the oversize grain sensor trigger.

The value of rh(a _m ax) thus obtained is preferably multiplied by a safety factor to prevent accidental overdosing of powder. The safety factor may be, for example, 0.8, 0.7, 0.6 or 0.5.

During the sieving process, the sieve fabric can become clogged, for example, by particles that are formed by pressing powder particles into the sieve fabric, or by cold welding. This reduces the sieve throughput. To take this phenomenon into account, the dosing mass flow rhdos can be reduced if the powder to be sieved forms a cone of material with a defined size on the sieve surface of the sieve in the area of the powder inlet. The formation of the cone of material with the defined size can, for example, be detected by means of the dosing sensor arranged in the area of the powder inlet. In other words, if the dosing sensor is triggered during an ongoing sieving process, this can be interpreted as a sign of a blockage in the sieve fabric and a resulting reduced sieve throughput, and the dosing mass flow can therefore be reduced.

Additionally or alternatively, a sieve cleaning can be initiated if the powder to be sieved forms a cone of material with a defined size in the area of the powder inlet on the sieve surface of the sieve and/or the dosing mass flow rhdos falls below a limit value. In other words, if a cone of material with a defined size still forms in the area of the powder inlet on the sieve surface of the sieve even with a low dosing mass flow that falls below the limit value, this can be interpreted as an indication that the sieve fabric is so clogged that sieve cleaning is required. Additionally or alternatively, sieve cleaning can also be initiated after the end of each sieving process. A sieving process can be terminated, for example, when a powder feed container for powder to be sieved, which can be connected to the powder inlet of the sieving device, is empty.

In addition or as an alternative to the above options, a sieve cleaning can also be triggered manually, i.e. initiated by a user input. In addition or as an alternative to this, a sieve cleaning can be initiated in a time-controlled manner, for example whenever a predetermined time (i.e., absolute time) or a predetermined operating time of the sieve device (i.e., a time during which the sieve device was in operation) has passed since a last sieve cleaning.

When initiating a sieve cleaning, the powder supply through the powder inlet is preferably stopped. Furthermore, any powder still present in the sieve can be sieved before the sieve cleaning starts. After the sieve cleaning starts, the sieve can be driven with maximum drive power. Additionally or alternatively, after the sieve cleaning starts, a vibrator can be activated that drives the sieve independently of the drive device of the sieve device. An angle of attack of the vibrator on the sieve, a drive amplitude of the vibrator and/or a drive frequency of the vibrator can preferably be variably adjusted.

Alternatively or in addition to the continuous powder dosing described above, the powder to be sieved can be fed discontinuously through the powder inlet onto the sieve, at least at times. With discontinuous powder feeding, powder to be sieved can first be fed through the powder inlet onto the sieve when the sieve is not driven, until the powder to be sieved has formed a cone of material with a defined size on the sieve surface in the area of the powder inlet. The formation of the cone of material with a defined size can in turn be detected using the dosing sensor provided in the area of the powder inlet. After the powder feed has ended, the sieve can be driven and the powder fed onto the sieve surface can be sieved.

The sieving process can be terminated when mfed — movergrain + msieved, where _mfed is the mass of the powder fed, movergrain is the mass of the powder in the oversize grain outlet and m _sieved is the mass of the sieved powder. The mass of the powder fed m _fed can be determined, for example, by means of a first measuring device which is provided in a powder feed container which can be connected to the powder inlet of the sieving device. The mass of the powder moberkorn which flowed into the oversize grain container via the oversize grain outlet can be determined by means of a second measuring device which is provided in an oversize grain container which can be connected to the oversize grain outlet of the sieving device. The mass of the sieved powder m _sieved can finally be determined by means of a third measuring device which is provided in a sieved powder container which can be connected to the sieved powder outlet of the sieving device.

In the method for controlling the operation of a screening device, the powder supply can be either exclusively continuous or exclusively discontinuous. However, it is also conceivable that the powder supply is sometimes continuous and sometimes discontinuous.

Alternatively or in addition to the continuous or discontinuous powder metering described above, the powder to be screened can be fed at least temporarily through the powder inlet onto the screen with a metering mass flow that is determined as a function of the drive power used to drive the screen. Preferably, a first metering mass flow with which the powder to be screened is fed through the powder inlet onto the screen during the first time interval is greater than a second metering mass flow with which the powder to be screened is fed through the powder inlet onto the screen during the second time interval. In other words, if the screen is driven with a higher drive power during the first time interval, more powder is fed through the powder inlet onto the screen than during the second time interval, during which the screen is driven with a lower drive power. This can prevent a backlog of powder and/or a state in which a larger amount of powder lies on the screen fabric and reduces the screen performance. This enables an increase in the powder throughput.

The first and/or the second dosing mass flow can be adjusted, as can the length of the first and the second time interval and the first and the second drive power, depending on the properties of the powder to be screened. The first and the second dosing mass flow can each have a positive value > 0. However, it is also conceivable that the second dosing mass flow has a value = 0, i.e. that during the second time interval during which the Sieve is driven with the lower second drive power, no powder is fed through the powder inlet onto the sieve.

The oversize grain rate is a parameter that indicates the ratio between the mass of oversize grain flowing into the oversize grain outlet in a defined time unit and the mass of the total powder processed in the screening device in the defined time unit. In particular, the oversize grain rate Qoversize grain can be determined according to

Calculate Qoversize ⁼ 1 ^— [rhsieved/ (rhsieved + rhoversize)]* 100%, where rhsieved is the mass flow of the sieved powder that flows into the sieved powder container that can be connected to the sieved powder outlet of the sieving device during the sieving process and rhoversize is the mass flow of the oversize that flows into the oversize container that can be connected to the oversize outlet of the sieving device during the sieving process. The parameters rhsieved and rhoversize can be monitored continuously during the sieving process, for example by means of the second measuring device provided in the oversize container and the third measuring device provided in the sieved powder container. Accordingly, the oversize rate Qoversize can also be determined continuously.

Preferably, in the method for controlling the operation of a screening device, a warning is issued if the oversize grain rate exceeds a limit value. If the screening device is used to prepare raw material powder that is intended for processing in a system for producing three-dimensional workpieces using a generative layering process, an oversize grain rate that is too high can be an indicator of unfavorable process parameters of the system. For example, an oversize grain rate that is too high can indicate that large welding spatters are generated when the powder is irradiated, which remain in the powder bed and can therefore impair the quality of the workpiece to be produced. Monitoring the oversize grain rate can therefore be used to monitor the process parameters in the system for producing three-dimensional workpieces using a generative layering process.

In a preferred embodiment of the method for controlling the operation of a screening device, the screening device is sealed from the ambient atmosphere and is flooded with protective gas during operation. This prevents undesired oxidation of the powder to be screened by means of the screening device. Preferably, additional protective gas can be fed into the screening device if an inert gas pressure in the screening device falls below a limit value. This allows leaks in the screening device to be detected and compensated.

Furthermore, during the sieving process, a step response of a sum of a sieved powder mass flow and an oversize mass flow to a dosing mass flow can be monitored. The "step response" of the sum of the sieved powder mass flow and the oversize mass flow to the metering mass flow is understood to be a time difference between a point in time at which a defined metering mass flow has been supplied to the sieving device and a point in time at which a corresponding sieved powder mass flow has passed through the sieve fabric. The step response is therefore a time parameter that indicates the duration of the sieving process for a specific powder mass flow. The metering mass flow can be recorded, for example, using the first measuring device that is provided in the powder feed container that can be connected to the powder inlet of the sieving device. The sum of the sieved powder mass flow and the oversize mass flow can be recorded, for example, using the second and third measuring devices that are provided in the oversize container that can be connected to the oversize outlet of the sieving device and in the sieved powder container that can be connected to the sieved powder outlet of the sieving device. The dosing mass flow and the sum of the sieved powder mass flow and the oversize mass flow can be continuously recorded. Accordingly, the step response of the sum of the sieved powder mass flow and the oversize mass flow to the dosing mass flow can also be continuously monitored.

If the sieve fabric becomes clogged during the sieving process, the step response of the sum of the sieved powder mass flow and the oversize mass flow to the dosing mass flow increases. In contrast, a shortening of the step response of the sum of the sieved powder mass flow and the oversize mass flow to the dosing mass flow and in particular a shortening below a certain limit value represents an indicator of a defect, for example a tear in the sieve fabric. Therefore, a warning is preferably issued if the step response of the sum of the sieved powder mass flow and the oversize mass flow to the dosing mass flow falls below a first limit value. In addition or alternatively, sieve cleaning can be initiated if the step response of the sum of the sieved powder mass flow and the oversize mass flow to the dosing mass flow falls below a second limit value. This enables additional, redundant monitoring of the sieve throughput in addition to the monitoring of the formation of a cone of material exceeding a certain size in the area of the powder inlet described above and, if necessary, the initiation of sieve cleaning as a result.

The method for controlling the operation of a screening device may further comprise changing an angle of inclination of the screening surface of the screen relative to a horizontal plane.

This aspect, as well as all aspects described below in connection with a change in the angle of inclination, can be applied on the one hand to one of the methods described above and/or on the other hand independently thereof to a method for controlling the operation of a screening device.

Thus, a method for controlling the operation of a sieving device, in particular for sieving powder of an additive manufacturing device (for example a device for producing a three-dimensional workpiece by means of selective electron beam melting, selective laser melting, laser cladding, laser metal deposition or selective laser sintering), may comprise the steps of: feeding powder to be sieved onto a sieve through a powder inlet; driving the sieve; and changing an inclination angle of the sieve surface of the sieve relative to a horizontal plane.

Changing the angle of inclination can, for example, comprise an initial change and thus adjustment of the angle of inclination, which is carried out in particular before the step of feeding in powder to be sieved. Additionally or alternatively, the angle of inclination can also be changed during ongoing operation of the sieving device, for example at the beginning of the first and/or the second time interval.

The sieve can be arranged inside a housing that is particularly gas-tight and the sieve can be rotated relative to the housing, thereby changing the angle of inclination. The gas-tight housing can be sealed gas-tight using a flap. The sieve can be inserted into the housing through the flap.

A first inclination angle can be set during the first time interval and a second inclination angle may be set during the second time interval. Either (a) the first inclination angle is less than the second inclination angle or (b) the first inclination angle is greater than the second inclination angle.

The inclination angle can be changed during the first and/or the second time interval.

The method can further comprise detecting a spreading speed of the powder to be screened on the screen and/or a position of a powder front of the powder to be screened on the screen. Changing the angle of inclination of the screen surface of the screen relative to the horizontal plane can take place depending on the detected spreading speed and/or depending on the detected position.

The detection can be carried out, for example, using a sensor, wherein the sensor can in particular comprise a camera, an inductive sensor and/or a light barrier. The sensor can be attached to the housing, in particular to an upper wall of the housing. The detection can also be carried out using the control unit. The spreading speed of the powder to be sieved can be a spreading speed of the powder front.

The angle of inclination can be changed in such a way that the angle of inclination is reduced when the detected spreading speed of the powder to be screened and/or the detected position of the powder front exceeds a predetermined threshold value. The angle of inclination can also be changed in such a way that the angle of inclination is increased when the detected spreading speed of the powder to be screened and/or the detected position of the powder front falls below a predetermined threshold value. The angle of inclination can also be adjusted permanently, for example within the framework of a closed control loop in which a constant spreading speed of the powder to be screened is set by adjusting the angle of inclination.

A sieving device comprises a powder inlet and a drive device which is configured to drive the sieve. Furthermore, the sieving device comprises a control unit which is configured to control the powder inlet and the drive device such that in a step (i) powder to be sieved is fed through the powder inlet onto the sieve and in a step (ii) the sieve is opened for a first time interval with a first drive power, wherein the first drive power is dimensioned such that the powder to be sieved flows over an entire sieve surface of the sieve and/or into an oversize grain outlet when the sieve is continuously driven with the first drive power. Furthermore, the control unit is configured to control the powder inlet and the drive device such that in a step (iii) after the expiration of the first time interval, the sieve is driven for a second time interval with a second drive power which is less than the first drive power and in a step (iv) after the expiration of the second time interval, steps (ii) and (iii) are repeated.

The second drive power is preferably dimensioned such that the powder to be sieved, when the sieve is continuously driven with the second drive power, forms a cone of repose on the sieve surface of the sieve in the region of the powder inlet, which essentially corresponds to a cone of repose forming on a stationary plane, wherein an angle of repose of the cone of repose is adapted in particular to an orientation of the sieve surface.

Additionally or alternatively, the sieve surface of the sieve may be inclined relative to a horizontal plane such that the flow of the powder fed through the powder inlet towards the oversize grain outlet is assisted by gravity, wherein an angle of inclination of the sieve surface of the sieve relative to the horizontal plane is preferably smaller than an angle of repose of a cone of repose formed by the powder to be sieved on a horizontal plane.

The first time interval can be a value empirically determined for the powder to be sieved. In addition or alternatively, the control unit can be configured to end the first time interval when determining the first time interval at the latest when powder to be sieved flows into the oversize grain outlet. The sieving device can comprise an oversize grain sensor provided in the region of the oversize grain outlet for monitoring the flow of powder to be sieved into the oversize grain outlet. The control unit can be configured to dimension the first time interval such that a sieve area utilization of approximately 70% to approximately 90%, preferably approximately 75% to approximately 85% and particularly preferably approximately 80% of the total sieve area of the sieve is not exceeded by the end of the first time interval.

The second time interval can be a value determined empirically for the powder to be sieved. In addition or alternatively, the control unit can be configured to start the second time interval at the latest to be terminated when the powder to be sieved forms a cone of material with a defined size in the area of the powder inlet on the sieve surface of the sieve. The sieving device can comprise a dosing sensor provided in the area of the powder inlet for detecting the formation of a cone of material with a defined size.

In the following, an embodiment of a sieving device is explained which is equipped with a control unit for controlling the powder supply through the powder inlet of the sieve and can also be used independently of the sieving device described above with a control unit carrying out a drive control.

An independently loadable sieving device comprises a powder inlet and a control unit which is configured to control the powder inlet such that the powder to be sieved is fed at least temporarily continuously through the powder inlet to the sieve with a metering mass flow rhdos = 0.5 * (rh(amin) + rh(a _m ax)). To adjust the metering mass flow, the control unit can, for example, be configured to control a metering device assigned to the powder inlet, which can, for example, comprise a metering screw, and/or a valve assigned to the powder inlet accordingly. The parameters a _m ax and rh(a _m ax) define a sieve surface utilization and a sieve throughput when driving the sieve with a first drive power which is dimensioned such that the powder to be sieved flows over an entire sieve surface of the sieve and/or into an oversize grain outlet when the sieve is continuously driven with the first drive power. The parameters a _m in and rh(a _m in), on the other hand, define a screen area utilization and a screen throughput when driving the screen with a second drive power that is lower than the first drive power.

The control unit can further be configured to determine rh(a _m in) by increasing the metering mass flow of the powder to be sieved through the powder inlet when driving the sieve with the second drive power until the powder to be sieved has formed a cone of material with a defined size in the region of the powder inlet on the sieve surface of the sieve.

Alternatively or additionally, the control unit may be configured to rh(a _m ax) determine by increasing the metering mass flow of the powder to be screened through the powder inlet when driving the sieve with the first drive power until the powder to be screened has formed a cone of material with a defined size in the region of the powder inlet on the sieve surface of the sieve and powder to be screened flows into the oversize grain outlet, wherein the value of rh(a _m ax) thus obtained is preferably multiplied by a safety factor of 0.8, 0.7, 0.6 or 0.5. The control unit can further be configured to reduce the metering mass flow rhdos when the powder to be screened forms a cone of material with a defined size in the region of the powder inlet on the sieve surface of the sieve. Furthermore, the control unit can be configured to initiate a sieve cleaning when the powder to be sieved forms a cone of material with a defined size in the area of the powder inlet on the sieve surface of the sieve and/or the dosing mass flow rhdos falls below a limit value.

The control unit can be configured, when a sieve cleaning is initiated, to control the powder inlet such that the powder supply through the powder inlet is stopped, and/or to control the drive device such that any powder still present in the sieve is sieved before the sieve cleaning starts, and/or to control the drive device such that after the sieve cleaning starts, the sieve is driven with a maximum drive power, and/or to activate a vibrator after the sieve cleaning starts, which is configured to drive the sieve independently of the drive device of the sieve device. Preferably, an angle of attack of the vibrator on the sieve, a drive amplitude of the vibrator and/or a drive frequency of the vibrator can be variably adjusted.

The control unit can also be configured to control the powder inlet such that the powder to be sieved is fed at least temporarily discontinuously through the powder inlet onto the sieve. In particular, the control unit can be configured to control the drive device and the powder inlet such that initially, when the sieve is not driven, powder to be sieved is fed through the powder inlet onto the sieve until the powder to be sieved has formed a cone of material with a defined size in the region of the powder inlet on the sieve surface of the sieve, and after the powder feed has ended, the sieve is driven and the powder fed onto the sieve surface of the sieve is sieved. Furthermore, the control unit can be configured to control the drive device and the powder inlet such that the sieving process is ended when fflfed - movergrain + fflsieved, where mfed is the mass of the powder fed in, movergrain is the mass of the oversize grain flowing into the oversize grain outlet and m _sieved is the mass of the sieved powder.

The sieving device can comprise a first measuring device for determining m _ZU guided, which is provided in a powder feed container that can be connected to the powder inlet of the sieving device, a second measuring device for determining moberkorn, which is provided in an oversize container that can be connected to the oversize outlet of the sieving device, and/or a third measuring device for determining m _ge sieved, which is provided in a sieved powder container that can be connected to a sieved powder outlet of the sieving device.

The control unit can further be configured to control the powder inlet such that the powder to be sieved is fed at least temporarily through the powder inlet onto the sieve with a metering mass flow that is determined as a function of the drive power used to drive the sieve, wherein in particular a first metering mass flow with which the powder to be sieved is fed through the powder inlet onto the sieve during the first time interval is greater than a second metering mass flow with which the powder to be sieved is fed through the powder inlet onto the sieve during the second time interval.

The control unit can be configured to issue a warning when an oversize grain rate, in particular a continuously measured one, exceeds a limit value.

Preferably, the screening device is sealed from the ambient atmosphere and flooded with a protective gas during operation. Additionally or alternatively, the control unit can be configured to supply additional protective gas to the screening device when an inert gas pressure in the screening device falls below a limit value. For this purpose, the control unit can, for example, control a valve that controls the supply of inert gas to the screening device.

The control unit is preferably further configured to monitor a step response of a sum of a sieved powder mass flow and an oversize mass flow to a dosing mass flow during the sieving process. The control unit can also be configured to output a warning if the step response of the sum of the sieved powder mass flow and the oversize mass flow to the dosing mass flow falls below a first limit value. Finally, the control unit can be configured to initiate a sieve cleaning when the step response of the sum of the sieved powder mass flow and the oversize mass flow to the dosing mass flow exceeds a second limit value.

The sieve device can further comprise a lid that can be detached from a sieve container. A seal can be arranged in the lid. For example, the seal can be arranged in the region of a side of the lid facing the sieve container and can serve to seal the sieve container from the ambient atmosphere when the lid is closed. The sieve device can further comprise a clamping device that is configured to exert a clamping force on the seal that holds the seal in its position in the lid. The clamping device can, for example, comprise a clamping piece that can be pressed against the seal using an adjusting screw. The clamping device advantageously prevents the seal from falling out of the lid when the lid is removed from the sieve container.

The screening device may comprise an inclination device for changing an inclination angle of the screening surface of the screen relative to a horizontal plane.

This aspect, as well as all aspects described below in connection with a change in the angle of inclination, can be applied on the one hand to one of the screening devices described above and/or on the other hand independently thereof to a screening device.

Thus, a sieving device, in particular for sieving powder of an additive manufacturing device (for example a device for producing a three-dimensional workpiece by means of selective electron beam melting, selective laser melting, laser cladding, laser metal deposition or selective laser sintering), may comprise: a powder inlet; a drive device configured to drive the sieve; and an inclination device for changing an inclination angle of the sieve surface of the sieve relative to a horizontal plane.

Changing the angle of inclination can, for example, comprise an initial change and thus adjustment of the angle of inclination, which is carried out in particular before the step of feeding powder to be sieved. Additionally or alternatively, the angle of inclination can also be changed during ongoing operation of the sieving device, for example at the beginning of the first and/or the second time interval.

The screening device can comprise a housing that can be closed in a gas-tight manner, wherein the screening device is arranged within the housing and wherein the inclination device is designed to rotate the screening device relative to the housing and thereby change the angle of inclination.

The powder inlet and the oversize grain outlet may be fixedly attached to the housing. The powder inlet may be attached to a top of the housing and the oversize grain outlet may be attached to a bottom of the housing. Furthermore, a sieve container may be fixedly attached to the bottom of the housing.

The sieve device can comprise a sieve holder for receiving, in particular for inserting, the sieve. The tilting device can be attached to the sieve holder and be configured to rotate the sieve holder.

The controller may be configured to set a first tilt angle during the first time interval and set a second tilt angle during the second time interval. Either (a) the first tilt angle is less than the second tilt angle or (b) the first tilt angle is greater than the second tilt angle. The first tilt angle and the second tilt angle may be kept constant during the first and second time intervals, respectively.

The control unit may be configured to change the tilt angle during the first and/or during the second time interval.

The sieving device can further comprise at least one sensor for detecting a spreading speed of the powder to be sieved on the sieve and/or a position of a powder front of the powder to be sieved on the sieve. The control unit can be configured to change the angle of inclination of the sieve surface of the sieve relative to the horizontal plane depending on the detected spreading speed and/or depending on the detected position.

The sensor can in particular comprise a camera, an inductive sensor and/or a light barrier. The sensor can be attached to the housing, in particular to an upper wall of the housing. The detection can also be carried out using the control unit. At the propagation speed of the sieving powder can be a propagation velocity of the powder front.

The control unit can be configured to change the angle of inclination in such a way that the angle of inclination is reduced when the detected spreading speed of the powder to be screened and/or the detected position of the powder front exceeds a predetermined threshold value. Furthermore, the control unit can be configured to change the angle of inclination in such a way that the angle of inclination is increased when the detected spreading speed of the powder to be screened and/or the detected position of the powder front falls below a predetermined threshold value. The control unit can also adjust the angle of inclination permanently, for example, e.g. within the framework of a closed control loop in which a constant spreading speed of the powder to be screened is set by adjusting the angle of inclination.

A powder processing system comprises a sieving device as described above. The powder processing system can be designed, for example, in the form of a closed system sealed from the ambient atmosphere. Furthermore, the powder processing system can be flooded with a protective gas in parts or entirely during operation. The powder processing system can comprise a powder feed container that can be connected to the powder inlet of the sieving device, a sieved powder container that can be connected to the sieved powder outlet of the sieving device, and an oversize container that can be connected to the oversize grain outlet of the sieving device. The powder processing system is intended in particular for use in a system for producing three-dimensional workpieces by exposing raw material powder layers to electromagnetic radiation or particle radiation.

A plant for producing three-dimensional workpieces by exposing raw material powder layers to electromagnetic radiation or particle radiation comprises a screening device and/or a powder preparation system as described above.

Furthermore, the system can comprise a process chamber that is sealed against the ambient atmosphere and a carrier for holding the raw material powder to be irradiated. Any excess powder that accrues when the individual powder layers are applied to the carrier can be collected in one or more collecting containers. The process chamber can comprise a gas inlet for supplying a gas, in particular an inert gas, into the process chamber and a gas outlet for discharging gas, possibly laden with particulate contaminants, from the process chamber. The carrier can be arranged in the process chamber. However, it is also conceivable for the process chamber to be movable via the carrier. The carrier can be a rigidly fixed carrier. Preferably, however, the carrier is displaceable in the vertical direction, so that the carrier can be moved step by step downwards in the vertical direction as the height of a workpiece built on the carrier increases.

The raw material powder applied to the carrier, for example by means of a powder application device that can be moved over the carrier, is preferably a metal powder, in particular a metal alloy powder. However, the raw material powder can also be a ceramic powder or a powder containing various materials. The powder can have any suitable particle size or particle size distribution. However, it is preferred to process powder with a particle size of less than 100 pm.

The system preferably also comprises an irradiation device which serves to selectively direct electromagnetic radiation or particle radiation onto the powder bed applied to the carrier. The system can also comprise an unpacking station into which a workpiece accommodated in a construction chamber can be transferred after it has been completed. In the unpacking station, the workpiece can be removed from the construction chamber and the non-solidified powder surrounding it in the construction chamber, if necessary after a cooling time.

A system 100s shown in Figure 14 for producing three-dimensional workpieces by exposing raw material powder layers to electromagnetic radiation or particle radiation comprises a process chamber 102s which is sealed from the ambient atmosphere. A powder application device 104s arranged in the process chamber 102s serves to apply raw material powder layers to a carrier 106s. Excess powder which accrues when the individual powder layers are applied to the carrier 106s is collected in a collecting container 107s. The carrier 106s can be displaced in the vertical direction so that the carrier 106s can be moved step by step in the vertical direction downwards into a construction chamber 109s as the height of a workpiece 108s built on the carrier 106s increases. The process chamber 102s is provided with a gas inlet 110s for supplying an inert gas (e.g. argon) into the process chamber 102s. A gas outlet 112s is also provided so that a continuous gas flow can be generated through the process chamber 102s. The gas flow can serve to remove melt splashes and/or other undesirable dirt particles, such as welding fumes, from the process chamber 102s.

The system 100s further comprises an irradiation device 112s, which serves to selectively direct electromagnetic radiation or particle radiation onto the powder bed applied to the carrier 106s. The exemplary system 100s shown in Figure 14 comprises only one irradiation device 112s. The system 100s can, however, also have a plurality of irradiation devices 112s.

The irradiation device 112s comprises a radiation source 114s, which here is designed in particular in the form of a laser source. The radiation source 114s, which can comprise, for example, a diode-pumped ytterbium fiber laser which emits laser light with a wavelength of approximately 1070 to 1080 nm, can be integrated into the irradiation device 112s. In the system 100s shown in Figure 14, however, the radiation source 114s is arranged outside the irradiation device 112s, wherein a laser beam 116s emitted by the radiation source 114s is guided into the irradiation device 112s via an optical fiber 118s.

The irradiation unit 112s further comprises two lenses 120s and 122s. In the embodiment of an irradiation unit 112s shown in Figure 14, both lenses 120s and 122s have a positive refractive power. The lens 120s collimates the laser light emitted by the optical fiber 118s so that a collimated or substantially collimated laser beam 116s is generated. The lens 122s, on the other hand, is configured to focus the collimated (or substantially collimated) laser beam 116s to a desired z-position along a z-axis.

Finally, the irradiation unit 112s comprises a scanner system with a scanner mirror 124s that can be pivoted about a pivot axis S. During operation of the system 100s, the scanner system and in particular the scanner mirror 124s serve to deflect the laser beam 116s emitted by the radiation source 114s in such a way that the beam 116s strikes the raw material powder layer applied to the carrier 106s at a desired position. The system 100s further comprises an unpacking station 126s. A construction chamber 109s with a workpiece 108s arranged therein is transferred to the unpacking station 126s when the construction of the workpiece 108s is completed. The irradiation device 112s and the process chamber 102s can then be used to build a new workpiece without any further delay. In the unpacking station 126s, the workpiece 108s can be cooled if necessary and is then unpacked, ie removed from the construction chamber 109s. This may also produce a large amount of non-solidified powder in which the workpiece 108s is embedded before unpacking.

Both the powder collected in the collecting container 107s and the powder recovered in the unpacking station 126s can contain particulate contaminants as well as glued or sintered powder agglomerates. If the powder is reused in an additive manufacturing process in the system 100s, these contaminants could lead to contamination of the powder bed and consequently to a reduced quality of the workpiece 108s. The system 100s therefore comprises a powder preparation system 128s, which is connected to the collecting container 107s and the unpacking station 126s via a powder line 130s. For example, a blower, a conveyor belt or another suitable conveyor device (not shown in Figure 14) can be used to convey powder from the collecting container 107s and the unpacking station 126s into the powder preparation system 128s. The powder preparation system is designed as a closed system, sealed against the ambient atmosphere and is completely flooded with a protective gas, such as argon, during operation.

The powder preparation system 128s comprises a powder feed container 132s connected to the powder line 130s. The powder feed container 132s thus serves to receive the powder to be prepared from the collecting container 107s and the unpacking station 126s. The powder preparation system 128s also comprises a sieve device 10s with a sieve that is formed by a sieve frame 14s and a sieve fabric 16s stretched over the sieve frame. Sieved powder is collected in a sieve container 18s after passing through the sieve fabric 16s.

A cover 20s is placed on the sieve frame 14s so that the sieve container 18s, as well as the other components of the powder preparation system 128s, is sealed against the ambient atmosphere and can be flooded with an inert gas during operation of the sieve device 10s. Argon, for example, can be used as an inert protective gas, which prevents undesirable oxidation of the powder in the Sieving device 10s prevents powder 56s to be sieved. The inert gas pressure in the sieving device 10s is continuously monitored by means of a pressure sensor not shown in the figures. If the inert gas pressure in the sieving device 10s falls below a limit value, additional protective gas is fed into the sieving device 10s under the control of the control unit 40s.

A detailed view of the screening device 10s is shown in Figure 15. The screening device 10s of Figure 15 is used in the system shown in Figure 14. Alternatively, the screening device 10s can be used as the screening device 5 in the system of Figure 1.

A powder inlet 22s of the sieve device 10s comprises a metering device 24s and a valve 26s, so that a controlled supply of powder from the powder supply container 132s to the sieve is possible via the powder inlet 22s. A metering sensor 23s is provided in the area of the powder inlet 22s, the function of which is explained in more detail below. The sieve container 18s has a cross-section that tapers downwards. Sieved powder received in the sieve container 18s can therefore be discharged from the sieve container by gravity via a sieved powder outlet 28s arranged in the area of a lower section of the sieve container 18s. The sieved powder outlet 28s is connected to a sieved powder container 134s of the powder preparation system 128s and comprises a valve 30s, so that a controlled discharge of sieved powder from the sieve container 18s into a sieved powder container 134s of the powder preparation system 128s is possible via the sieved powder outlet 28s.

The screening device 10s further comprises an oversize grain outlet 32s, to which a valve 34s is assigned. In the area of the oversize grain outlet 32s, an oversize grain sensor 33s is provided, which can detect powder particles flowing into the oversize grain outlet 32s. Oversize grain that is too coarse to pass through the screen fabric 16s in the direction of the screen container 18s can be removed from the screening device 10s in a controlled manner via the oversize grain outlet 24s and fed to an oversize grain container 136s of the powder preparation system 128s. The powder inlet 22s and the oversize grain outlet 32s are arranged in the area of opposite side edges of the screen surface defined by the screen fabric 16s. Furthermore, the sieve fabric 16s and thus the sieve surface of the sieve defined by the sieve fabric 16s is inclined relative to a horizontal plane E in such a way that the flow of the powder fed through the powder inlet in the direction of the oversize grain outlet 32s is supported by gravity. In other words, the sieve surface of the sieve is surrounded by a The area located between the powder inlet 22s and the oversize grain outlet 32s is designed to slope downwards, which promotes the spreading of powder fed in via the powder inlet 22s over the sieve surface and the removal of oversize grain into the oversize grain outlet 32s. Just like the removal of sieved powder, the removal of oversize grain from the sieving device 10s is also gravity-driven.

The screening device 10s is also equipped with a drive device 36s for driving the screen. The drive device 36s engages the screen frame and causes the screen frame 14s and thus the screen fabric 16s to vibrate when the screening device 10s is in operation. The preferred embodiment of a screening device 10s shown here is equipped with a drive device 36s in the form of an ultrasonic drive device, which is designed to subject the screen to ultrasonic vibrations. Furthermore, a vibrator 38s is also provided, which also engages the screen frame 14s and serves to cause the screen frame 14s and thus the screen fabric 16s to vibrate for the purpose of cleaning the screen.

The operation of the screening device 10s is controlled by means of a control unit 40s. The control unit 40s can be a control unit that is exclusively assigned to the screening device 10s. Alternatively, however, it is also conceivable that the control unit 40s is integrated into a higher-level control unit, for example a control unit for controlling the powder preparation system 128s and/or a control unit for controlling the system 100s for producing a three-dimensional workpiece.

Finally, the screening device 10s comprises a first measuring device 42s, a second measuring device 40s and a third measuring device 46s. The first measuring device 42s, which is designed here in the form of one or more weighing measuring cells, is arranged in the powder feed container 132s of the powder preparation system 28s and serves to record the mass of the powder fed to the powder inlet 22s of the screening device 10s from the powder feed container 32s. The second measuring device 44s, which is also designed here in the form of one or more weighing measuring cells, is arranged in the oversize container 136s of the powder preparation system 28s and serves to record the mass of the oversize that has flowed into the oversize container 136s via the oversize outlet 32s of the screening device 10s. The third measuring device 46s, which is again designed in the form of one or more weighing cells, is finally arranged in the sieved powder container 134s of the powder preparation system 28s and serves to measure the mass of the sieved powder flowing from the sieved powder outlet 28s of the sieving device 10s into the sieved powder container 134s.

As can be seen in Figure 16, when a powder 56s is loosely packed on a plane, an angle of repose a is formed which is influenced by various factors, such as the shape, density, size distribution and surface properties of the powder particles as well as by process parameters such as the relative humidity and temperature as well as by vibrations and movements acting on the cone of repose.

When powder 56s to be screened is fed through the powder inlet 22s onto the screen of the screening device 10s, a cone of repose forms on the screen, i.e. on the screen fabric 16s, the shape and angle of repose of which is influenced by the vibrations acting on the cone of repose and consequently by the drive power of the drive device 36s driving the screen. In addition, the shape and angle of repose of the cone of repose are influenced by the angle of inclination of the screen surface.

If the sieve, as shown in Figure 17, is driven with a low drive power of the drive device 36s, the cone of material forming on the sieve below the powder inlet 22s resembles a cone of material forming on a stationary plane, the base area of which only occupies a small section a of the sieve surface of the sieve. In such an operating state of the sieve device 10s, the sieve surface utilization is correspondingly low, since only a small section a of the sieve surface adjacent to the powder inlet 22s is actually charged with powder 56s. In contrast, a section b of the sieve surface that is not charged with powder and consequently defines a "safety distance" between the section a of the sieve surface charged with powder and the oversize grain outlet 32s is comparatively large.

The inclination of the sieve surface in the direction of the oversize grain outlet 32s means that the repose cone is no longer symmetrically shaped, as in Figure 16, but is adapted to the orientation of the sieve surface and has a variable angle of repose, which is smaller in a peripheral section of the repose cone facing the oversize grain outlet 32s than in a peripheral section of the repose cone facing away from the oversize grain outlet 32s. An angle of inclination aa-w of the sieve surface of the sieve relative to the horizontal plane E (see Figure 15) is, however, smaller than the angle of repose aa of the repose cone that the powder 56s to be sieved forms on a horizontal plane, so that it is ensured that when the sieve is driven with a low drive power in the area of the powder inlet 22s still forms a stable cone of material and the powder 56s does not flow uncontrollably over the sieve surface.

If, however, the sieve is driven with a high drive power, as shown in Figure 18, the powder 56s spreads over the sieve surface, i.e. an angle of repose aar of the cone of repose that the powder 56s fed to the sieve through the powder inlet forms on the sieve surface decreases with increasing drive power of the drive device 36s driving the sieve. At the same time, the base area of the cone of repose increases, so that the section a of the sieve surface that is loaded with powder 56s and consequently the sieve surface utilization increases until the powder finally flows over the entire sieve surface and thus the entire sieve surface is loaded with powder. A section b of the sieve surface that is not loaded with powder is then no longer present, so that the section a of the sieve surface that is loaded with powder no longer has a "safety distance" from the oversize grain outlet 32s. This means that when the sieve is continuously operated with a high drive power, powder 56s, as shown in Figure 18, flows unscreened into the oversize grain outlet 32s. As a result, powder 56s, which is actually fine-grained enough to pass through the sieve mesh 16s, is lost unused.

In a method for controlling the operation of the sieving device 10s, which is illustrated in more detail in Figure 19, the powder 56s to be sieved is therefore first fed to the sieve through the powder inlet 22s in a step (i). The sieve is then driven in a step (ii) for a first time interval with a first drive power (see Figure 19 top and middle). The first drive power is dimensioned such that the powder 56s to be sieved would flow over the entire sieve surface of the sieve and/or into the oversize grain outlet if the sieve were continuously driven with the first drive power, as shown in Figure 18. The first drive power is therefore so high that if it were continuously maintained, maximum sieve surface utilization would be guaranteed, but there would at least be a high risk that powder 56s would be lost unsieved through the oversize grain outlet 32s. The driving of the sieve with the first drive power is therefore limited in time.

Therefore, after the expiry of the first time interval, the sieve is driven in a step (iii) for a second time interval with a second drive power that is lower than the first drive power (see Figure 19 below). The second drive power is in particular dimensioned such that the powder to be sieved 56s at a Continuous driving of the sieve with the second drive power, as shown in Figure 17, forms a cone of material in the area of the powder inlet 22s on the sieve surface of the sieve, which essentially corresponds to a cone of material forming on a stationary plane and whose base area only occupies a small section a of the sieve surface of the sieve arranged in the area of the powder inlet 22s. If the sieve were driven continuously with the second drive power, it would be ensured that no or almost no powder 56s flows unsifted into the oversize grain outlet. However, due to the low utilization of the sieve surface, the sieve throughput and consequently the sieve performance would be low. Therefore, the driving of the sieve with the second drive power is also limited in time.

In the case described above, the second drive power is greater than zero and less than the first drive power. In a special case, the second drive power can also be zero, so that no active sieving takes place during the second time interval. However, passive sieving can also take place in the non-driven state, with powder trickling through the sieve due to gravity.

After the second time interval has elapsed, steps (ii) and (iii) are repeated, i.e. the sieve is driven periodically alternately with the first higher drive power and the second lower drive power. In the first time interval in which the sieve is driven with the first (high) drive power, the powder particles of the powder 56 to be sieved, including the oversize grain 50s contained in the powder 56s, are distributed over the sieve surface (see Figure 19, top), so that the sieving process can then take place with a high sieve surface utilization, with the oversize grain 50s being transported by gravity in the direction of the oversize grain outlet 32s (see Figure 19, middle). In contrast, during the second time interval, the sieve surface utilization is low. In addition, the oversize grain 50s "accumulates" in the form of a column inside the cone. This means that powder particles that are pressed into the 16s sieve mesh by the dynamic pressure and the vibration of the sieve can form stuck grains and clog the sieve mesh.

Since the first and second time intervals alternate periodically and the sieve is driven periodically alternately with the first higher drive power and the second lower drive power, the advantages of both drive powers can be combined. By limiting the time of the first time interval, a good distribution of the powder particles over the sieve surface and an efficient removal of the oversize grain 50s in the direction of the oversize grain outlet 32s can be ensured. However, there always remains a space not loaded with powder. Section b of the sieve surface and consequently a "safety distance" between the section A of the sieve surface loaded with powder and the oversize grain outlet 32s. Furthermore, the sieve fabric 16s is loaded with a smaller powder mass overall and is therefore less stressed.

The first and second drive power can take on different values for different types of powder and different process parameters, such as temperature, particle size distribution of the powder, moisture content of the powder, etc. The first and second drive power are therefore either determined empirically before a sieving process, taken from a drive power value table for different types of powder and process parameters, or selected based on empirical values. The first and second time intervals can also take on different values for different types of powder and different process parameters, such as temperature, particle size distribution of the powder, moisture content of the powder, etc. The first and second time intervals are therefore empirically determined values for the powder to be sieved before a sieving process, taken from a value table, or selected based on empirical values.

When determining the first time interval, the first time interval ends at the latest when powder to be screened 56s flows into the oversize grain outlet 32s, i.e. the first time interval is selected such that there is no loss of powder to be screened during the driving of the sieve with the first drive power within the first time interval. The flow of powder to be screened into the oversize grain outlet 32s is detected by means of the oversize grain sensor 33s. In particular, the first time interval is dimensioned such that a sieve surface utilization of approx. 70% to approx. 90%, preferably approx. 75% to approx. 85% and particularly preferably approx. 80% of the total sieve surface of the sieve is not exceeded by the end of the first time interval. The first time interval therefore provides a "time safety reserve" so that the powder does not spread over the entire sieve surface during the first time interval.

When determining the second time interval, the second time interval is terminated at the latest when powder 56s to be sieved forms a cone of material with a defined size in the area of the powder inlet 22s on the sieve surface of the sieve. The formation of a cone of material with a defined size is detected by means of the dosing sensor 23s, which is triggered when a tip of the cone of material protrudes into a detection area of the dosing sensor 23s. By limiting the second time interval prevents the material cone from becoming too large during the second time interval and clogging the powder inlet 22s.

During operation of the sieving device 10s, the powder 56s to be sieved is fed to the sieve at least temporarily continuously through the powder inlet 22s. A metering mass flow is set by a corresponding control of the metering device 24s and/or a corresponding control of the valve 26s by the control unit 40s. In particular, a continuous metering mass flow rhdos is set which follows the equation rhdos = 0.5 * (rh(amin) + rh(a _m ax)), wherein the parameters a _m ax and rh(a _m ax) define a sieve area utilization and a sieve throughput when the sieve is driven with the first drive power and the parameters a _m in and rh(a _m in) define a sieve area utilization and a sieve throughput when the sieve is driven with the second drive power.

The sieve throughput when driving the sieve with a second drive power rh(amin) is determined by increasing the metering mass flow of the powder 56s to be sieved through the powder inlet 22s when driving the sieve with the second drive power until the powder 56s to be sieved has formed a cone of material with a defined size in the area of the powder inlet 22s on the sieve surface of the sieve and consequently the metering sensor 23s is triggered. The sieve throughput when driving the sieve with the first drive power rh(a _max ), on the other hand, is determined by increasing the metering mass flow of the powder 56s to be sieved through the powder inlet 22s when driving the sieve with the first drive power until the powder 56s to be sieved has formed a cone of material with a defined size on the sieve surface of the sieve in the area of the powder inlet 22s and powder 56s to be sieved flows into the oversized grain outlet. When determining the sieve throughput when driving the sieve with the first drive power rh(amax), the metering mass flow of the powder 56s to be sieved through the powder inlet 22s is consequently increased until the metering sensor 23s and the oversized grain sensor 33s are triggered. The value of rh(a _m ax) obtained in this way is multiplied by a safety factor of, for example, 0.8, 0.7, 0.6 or 0.5 to counteract an unintentional overdose of powder.

If the sieve mesh 16s becomes clogged during the sieving process, for example due to stuck grains or cold welding, the sieve throughput decreases. the continuous dosing mass flow rhdos is reduced if, during the sieving process, the powder 56s to be sieved forms a cone of material with a defined size in the area of the powder inlet 22s on the sieve surface of the sieve and the dosing sensor 23s is triggered accordingly. However, if the continuous dosing mass flow rhdos has already been reduced to such an extent that it falls below a limit value and the powder 56s to be sieved nevertheless forms a cone of material with a defined size in the area of the powder inlet 22s on the sieve surface of the sieve and the dosing sensor 23s is triggered accordingly, this is interpreted by the control unit 40s as an indication that the sieve fabric 16s is clogged and initiates sieve cleaning.

If the control unit 40s has detected that a sieve cleaning is to be initiated, the powder supply through the powder inlet 22s is first stopped. Powder 56s still present in the sieve is sieved before the sieve cleaning starts. After the sieve cleaning starts, the sieve is driven with maximum drive power. In addition or alternatively, the vibrator 38s can be activated after the sieve cleaning starts, whereby an angle of attack of the vibrator 38s on the sieve, a drive amplitude of the vibrator 38s and a drive frequency of the vibrator 38s can be variably adjusted. In addition or alternatively to a sieve cleaning initiated as a result of a blockage of the sieve fabric 16s, a sieve cleaning can also be initiated after the end of each sieving process when the powder feed container 132s is empty.

Alternatively or in addition to the continuous powder dosing described above, the powder 56s to be sieved can also be fed discontinuously through the powder inlet 22s onto the sieve, at least at times. With discontinuous powder feeding, powder 56s to be sieved is first fed through the powder inlet 22s onto the sieve in a non-driven state of the sieve until the powder 56s to be sieved has formed a cone of material with a defined size on the sieve surface in the area of the powder inlet 22s and the dosing sensor 23s is triggered. The powder feed is then stopped and the sieve is driven so that the powder 56s fed onto the sieve surface is sieved.

Alternatively or in addition to the continuous or discontinuous powder metering described above, the powder 56s to be sieved can also be fed to the sieve at least temporarily with a metering mass flow through the powder inlet 22s, which is determined depending on the drive power used to drive the sieve. In particular, a first metering mass flow, with with which the powder to be sieved is fed to the sieve through the powder inlet 22s during the first time interval, be greater than a second dosing mass flow with which the powder to be sieved is fed to the sieve through the powder inlet 22s during the second time interval.

In the diagrams of Figure 20, the drive power L is plotted against time t at the top and the metering mass flow rhdos is plotted against time t at the bottom. During the periodically recurring first time intervals tl, the sieve is driven with the higher first drive power LI, whereas during the periodically recurring second time intervals t2, the sieve is driven with the lower second drive power L2. Accordingly, powder 56s is fed through the powder inlet 22s to the sieve during the first time intervals tl with a higher first metering mass flow rhdosi and during the second time intervals t2 with a lower second metering mass flow rhdos2. In particular, the second metering mass flow rhdos2 here has the value 0, i.e. during the second time intervals t2, no powder is fed through the powder inlet to the sieve.

The "sinusoidal" course of the dosing mass flow shown in the lower diagram of Figure 20 results from the response behavior of the dosing device when starting or stopping the movement of the dosing device, such as a dosing screw. A "rectangular" course of the dosing mass flow is also conceivable. If the first and second time intervals, as shown in Figure 20, are each of the same length, an average dosing capacity of rhdosi/2 is achieved.

The sieving process is terminated when msupplied ⁼ movergrain + msieved, where msupplied is the mass of the powder supplied, movergrain is the mass of the overgrain that flowed into the overgrain outlet 32s and m _sieved is the mass of the sieved powder. The mass of the powder supplied _mSUPPLIED is determined by means of a first measuring device 42s, which records the powder outflow from the powder feed container 132s. The mass of the powder movergrain that flowed into the overgrain container 136s via the overgrain outlet is determined by means of the second measuring device 44s, which records the inflow of overgrain into the overgrain container 38s. The mass of the sieved powder msieved is finally determined by means of the third measuring device 46s, which records the inflow of sieved powder into the sieved powder container 134s. The oversize grain rate is a parameter that indicates the ratio between the mass of the oversize grain flowing into the oversize grain outlet 32s in a defined time unit and the mass of the powder processed in the sieving device 10s in the defined time unit. In particular, the oversize grain rate Qoversize grain can be determined according to

Calculate Qoversize ⁼ 1 ^— [rhsieved/earlysieved + rhoversize]* 100%, where rhsieved is the mass flow of the sieved powder that flows during the sieving process via the sieved powder outlet 28s of the sieving device 10s into the sieved powder container 134s and rhoversize is the mass flow of the oversize that flows during the sieving process via the oversize outlet 32s of the sieving device 10s into the oversize container 136s.

An oversize grain rate that is too high can be an indicator of unfavorable process parameters of the system 100s when using the screening device 10s described here to prepare raw material powder that is intended for processing in the system 100s for producing three-dimensional workpieces using a generative layer construction process. For example, an oversize grain rate that is too high can indicate that when the powder is irradiated using the irradiation device 112s, large welding spatters are created that remain in the powder bed and can thus impair the quality of the workpiece 108s to be produced. The parameters rhsieved and rhoversize are therefore continuously monitored during the screening process using the second and third measuring devices 44s, 46s. The control unit 40s continuously determines the oversize grain rate Qoversize from these values. Furthermore, a warning is issued under the control of the control unit 40s if the oversize grain rate exceeds a limit value so that the process parameters of the system 100s can be checked if necessary.

Furthermore, during the sieving process, a step response of a sum of a sieved powder mass flow rh sieved and an oversize mass flow rh oversize to a metering mass flow rh dos is monitored, whereby the metering mass flow rh dos is continuously recorded by the first measuring device 42s, the oversize mass flow rh oversize is continuously recorded by the second measuring device 44s and the sieved powder mass flow rh sieved is continuously recorded by the third measuring device 46s. The diagrams shown in Figure 21 show the development of the sum of the sieved powder mass flow rh sieved and the The oversize mass flow rhüberkom as a function of time t is shown by the dotted curves, while the development of the dosing mass flow rhdos as a function of time t is shown by the dashed curves.

The "step response" of the sum of the sieved powder mass flow rhgesiebt and the oversize mass flow rhüberkom to the dosing mass flow rhdos is the time difference Δt between a point in time tl at which a defined dosing mass flow rhdos has been fed to the sieving device for 10s and a point in time t2 at which a corresponding sum of the sieved powder mass flow rhgesiebt and the oversize mass flow rhüberkom has passed the sieve fabric for 16s. The step response is therefore a time parameter that indicates the duration of the sieving process for a specific powder mass flow.

If the step response shortens from the value Atl illustrated in the upper diagram in Figure 21 to the value At2 illustrated in the lower diagram in Figure 21, i.e. the curves in the diagram move closer together, this can be interpreted as an indicator of a defect, for example a tear in the sieve fabric 16s, if the step response falls below a first limit value. Therefore, a warning is issued under the control unit 40s if the step response of the sum of the sieved powder mass flow rhsieved and the oversize mass flow rhovercome to the dosing mass flow rhdos falls below the first limit value.

If, on the other hand, the step response becomes longer, this indicates that the sieving process is being extended and can be interpreted as an indicator that the sieve fabric has become clogged. Therefore, a sieve cleaning is initiated under the control of the control unit 40s when the step response of the sum of the sieved powder mass flow rhsieved and the oversize mass flow rhovercome to the dosing mass flow rhdos exceeds the second limit value.

Figure 22 shows a schematic side view of a screening device 10s, which can be considered as an alternative embodiment or as a further development of the screening device 10s of Figure 14. The screening device 10s can be used in conjunction with all of the above-described embodiments of a screening device 10s, a powder preparation system 128s and/or a system 100s for producing three-dimensional workpieces. The elements and/or functions of the screening device 10s not described below correspond to those of the screening device 10s described above, in particular the screening device 10s of - Ill -

Figure 15. Thus, some of the elements of the screening device 10s of Figure 22 are not shown or are only indicated schematically, since they correspond to the elements of the screening device 10s of Figure 15 already explained in detail above.

In the upper section (a) of Figure 22, the screening device 10s is shown in a state in which the screen (consisting of screen frame 14s and screen mesh 16s) is not yet installed in a housing 60s of the screening device 10s. The lower section (b) of Figure 22 shows the screening device in the installed state of the screen 14s, 16s.

The sieving device 10s of Figure 22 comprises a housing 60s which - similar to the lid 20s - is suitable for hermetically sealing the sieving device 10s. A sieving process can thus be carried out in a closed inert gas atmosphere. A powder inlet 22s is located in an upper side of the housing 60s. An oversize grain outlet 32s and a merely indicated sieving container 18s are provided on an underside of the housing. The above-mentioned elements 22s, 32s and 18s are attached to the housing 60s and are thus fastened independently of a change in the angle of inclination aa-w (see below).

The sieve, consisting of sieve frame 14s and sieve mesh 16s, can be inserted into the housing from the side via a flap 62s. The flap 62s can be closed and, when closed, seals the housing 60s gas-tight. A sieve holder 66s is also provided, into which the sieve 14s, 16s can be inserted and, if necessary, fastened. The sieve 14s, 16s can thus be easily removed and reinserted or, if necessary, replaced.

An inclination angle aa-w of the sieve 14s, 16s (more precisely, of the sieve fabric 16s) relative to the horizontal plane E is adjustable. For this purpose, an inclination device 64s is provided which is designed to change the inclination angle aa-w. The inclination device can comprise a motor, in particular a servo motor. In the example shown in Figure 22, the inclination device 64s is attached to the sieve holder 66s and designed to incline it relative to the housing 60s. More precisely, the inclination device according to the example shown is arranged in the middle of the sieve 14s, 16s and designed to rotate the sieve 14s, 16s along a horizontal axis of rotation.

In particular, the inclination device 64s can be controlled by the control unit 40s so that any inclination angle aa-w within a predetermined angle range (e.g. 0° to 45°). According to some embodiments, a change in the angle of inclination aa-w can take place so quickly that a first angle of inclination is set in the first time interval and a second angle of inclination is set in the second time interval. In other words, the change in the angle can take place in a change interval which is shorter than the shorter of the first and second time intervals, in particular a maximum of half as long, a maximum of 1/4 as long, a maximum of 1/8 as long, a maximum of 1/10 as long, a maximum of 1/50 as long or a maximum of 1/100 as long.

However, it is also possible that a change in the inclination angle aa-w occurs continuously during the first time interval and/or during the second time interval.

Thus, the control of the screening performance (i.e. different screening performance in the first and second time intervals) can be supported by different inclination angles aa-w in the respective time intervals. In particular, a larger inclination angle can be set for the first time interval than for the second time interval. However, conversely, a smaller inclination angle can also be set for the first time interval than for the second time interval. Both options can be advantageous - depending on the situation and goal. A smaller inclination angle aa-w leads to less oversize being removed and the powder possibly accumulating on the screen mesh 16s. A higher inclination angle aa-w leads to better removal of the oversize, but also to any "good" powder that could pass through the screen mesh 16s reaching the oversize outlet 32s.

According to some embodiments, the angle of inclination can be set depending on the powder used. This can, for example, be done initially before the start of a sieving process so that the angle of inclination remains constant during the sieving process. For example, a higher angle of inclination can be set for heavier materials than for lighter materials. A higher angle of inclination can also be set for powder material with non-round and/or spiky powder particles, which therefore has lower flowability, than for powder material which has round powder particles and therefore higher flowability. The angle of inclination can thus be optimized with regard to the flow properties of the material used.

Furthermore, a sensor (not shown) can be provided which is designed to measure a spreading speed of the powder to be sieved on the sieve and/or to detect a position of a powder front of the powder to be sieved on the sieve. For this purpose, the sensor can comprise, for example, a camera, an inductive sensor and/or a light barrier. The sensor can, for example, be attached to an inner side of the housing 60s, in particular to an upper wall of the housing 60s.

The control unit can be configured such that the inclination angle aa-w of the sieve surface of the sieve relative to the horizontal plane is changed depending on the detected spreading speed and/or depending on the detected position. In particular, the inclination angle aa-w can be changed such that the inclination angle aa-w is reduced when the detected spreading speed of the powder to be sieved and/or the detected position of the powder front exceeds a predetermined threshold value. Similarly, the inclination angle aa-w can be changed such that the inclination angle aa-w is increased when the detected spreading speed of the powder to be sieved and/or the detected position of the powder front falls below a predetermined threshold value.

This means that the angle of inclination can be controlled automatically and time-consuming tests to determine an optimal and powder-dependent angle of inclination can be avoided.

Aspects which can be used alone and/or in combination with the powder conveying system described above (see, for example, Fig. 1) are as follows:

1. A method for controlling the operation of a screening device (10s), comprising the steps of:

(i) feeding powder to be sieved (56s) onto a sieve through a powder inlet (22s);

(ii) driving the sieve for a first time interval with a first drive power, wherein the first drive power is dimensioned such that the powder to be sieved (56s) flows over an entire sieve surface of the sieve and/or into an oversize grain outlet (32s) when the sieve is continuously driven with the first drive power;

(iii) after expiration of the first time interval, driving the sieve for a second time interval with a second drive power which is less than the first drive power; (iv) after expiry of the second time interval, repeating steps (ii) and (iii).

2. Method for controlling a sieving device (10s) according to aspect 1, wherein the second drive power is dimensioned such that the powder to be sieved (56s) forms a repose cone on the sieve surface of the sieve when the sieve is continuously driven with the second drive power in the region of the powder inlet (22s), which substantially corresponds to a repose cone forming on a stationary plane, wherein an angle of repose (aa) of the repose cone is adapted in particular to an orientation of the sieve surface.

3. Method for controlling the operation of a screening device (10s) according to aspect 1 or 2, wherein the screening surface of the screen is inclined relative to a horizontal plane (E) such that the flow of the powder fed through the powder inlet (22s) in the direction of the oversize grain outlet (32s) is assisted by gravity, wherein an angle of inclination (aa-w) of the screening surface of the screen relative to the horizontal plane (E) is preferably smaller than an angle of repose (aa) of a repose cone formed by the powder (56s) to be screened on a horizontal plane (E).

4. A method for controlling the operation of a screening device (lOss) according to any one of aspects 1 to 3, wherein:

- the first time interval is an empirically determined value for the powder to be sieved (56); and/or

- when determining the first time interval, the first time interval ends at the latest when powder to be screened (56s) flows into the oversize grain outlet (32s), wherein the flow of powder to be screened (56s) into the oversize grain outlet (32s) is detected in particular by means of an oversize grain sensor (33s) provided in the region of the oversize grain outlet; and/or

- the first time interval is dimensioned such that by the end of the first time interval a screen area utilization of approx. 70% to approx. 90%, preferably approx.

75% to about 85% and particularly preferably about 80% of the total sieve area of the sieve is not exceeded; and/or

- the second time interval is an empirically determined value for the powder to be sieved (56s); and/or

- when determining the second time interval, the second time interval is terminated at the latest when the powder to be sieved (56s) forms a cone of material with a defined size in the area of the powder inlet (22s) on the sieve surface of the sieve wherein the formation of a pouring cone with a defined size is detected in particular by means of a dosing sensor (23s) provided in the region of the powder inlet (22s).

5. Method for controlling the operation of a screening device (10s) according to one of aspects 1 to 4, wherein the powder to be screened (56s) is fed at least temporarily continuously with a metering mass flow rhdos = 0.5 * (rh(amin) + rh(a _m ax)) through the powder inlet (22s) onto the screen, wherein a _m in is a screen area utilization and rh(amin) is a screen throughput when driving the screen with the second drive power, and wherein a _m ax is a screen area utilization and rh(amax) is a screen throughput when driving the screen with the first drive power.

6. A method for controlling the operation of a screening device (10s) according to aspect 5, wherein:

- rh(amine) is determined by increasing the metering mass flow of the powder to be screened (56s) through the powder inlet (22s) when the sieve is driven with the second drive power until the powder to be screened (56s) has formed a cone of material with a defined size in the region of the powder inlet (22s) on the sieve surface of the sieve; and/or

- rh(amax) is determined by increasing the metering mass flow of the powder to be screened (56s) through the powder inlet (22s) when driving the sieve with the first drive power until the powder to be screened (56s) has formed a cone of material with a defined size on the sieve surface of the sieve in the region of the powder inlet (22s) and powder to be screened (56s) flows into the oversize grain outlet (32s), wherein the value of rh(a _m ax) thus obtained is preferably multiplied by a safety factor of 0.8, 0.7, 0.6 or 0.5.

7. A method for controlling the operation of a screening device (10s) according to aspect 5 or 6, wherein:

- the dosing mass flow (rhdos) is reduced when the powder to be screened (56s) forms a cone of material on the screen surface in the area of the powder inlet (22s) with a defined size; and/or

- a sieve cleaning is initiated when the powder to be sieved (56s) forms a cone of material with a defined size on the sieve surface of the sieve in the area of the powder inlet (22s) and/or the dosing mass flow rhdos falls below a limit value.

8. A method for controlling the operation of a screening device (10s) according to aspect 7, wherein upon initiation of a screening cleaning

- the powder supply through the powder inlet (22s) is stopped; and/or

- any powder still present in the sieve is sieved before starting the sieve cleaning; and/or

- after the start of the sieve cleaning, the sieve is driven with a maximum drive power; and/or

- after the start of the sieve cleaning, a vibrator (38s) is activated which drives the sieve independently of a drive device (36s) of the sieve device (10s), wherein preferably an angle of attack of the vibrator (38s) on the sieve, a drive amplitude of the vibrator (38s) and/or a drive frequency of the vibrator (38s) are variably adjustable.

9. Method for controlling the operation of a screening device (10s) according to one of aspects 1 to 8, wherein the powder (56ss) to be screened is fed at least temporarily discontinuously through the powder inlet (22s) onto the screen, wherein in particular

- initially, when the sieve is not driven, powder (56s) to be sieved is fed onto the sieve through the powder inlet (22s) until the powder (56s) to be sieved has formed a cone of material with a defined size on the sieve surface of the sieve in the region of the powder inlet (22s);

- after the powder feed has been completed, the sieve is driven and the powder fed onto the sieve surface is sieved; and

- the sieving process is terminated when msupplied ⁼ movergrain + msieved, where msupplied is the mass of the powder supplied, _movergrain is the mass of the powder flowing into the overgrain outlet (32s) and m sieved is the mass of the sieved powder, and where m _Supplied is determined in particular by means of a first measuring device (42s) which is arranged in a connection with the powder inlet (22s) of the sieving device (10s) connectable powder feed container (132s), oversize is determined in particular by means of a second measuring device (44s) which is provided in an oversize container (136s) which can be connected to the oversize outlet (32s) of the sieving device (10s), and/or _oversieved is determined in particular by means of a third measuring device (46s) which is provided in a sieved powder container (134s) which can be connected to a sieved powder outlet (28s) of the sieving device (10s).

10. Method for controlling the operation of a sieving device (10s) according to one of aspects 1 to 9, wherein the powder (56s) to be sieved is fed to the sieve at least temporarily with a metering mass flow through the powder inlet (22s) which is determined as a function of the drive power used to drive the sieve, wherein in particular a first metering mass flow with which the powder (56s) to be sieved is fed to the sieve through the powder inlet (22s) during the first time interval is greater than a second metering mass flow with which the powder (56) to be sieved is fed to the sieve through the powder inlet (22s) during the second time interval.

11. Method for controlling the operation of a screening device (10s) according to one of aspects 1 to 10, wherein a warning is issued when an oversize grain rate exceeds a limit value, wherein the oversize grain rate is measured in particular continuously.

12. A method for controlling the operation of a screening device (10s) according to any one of aspects 1 to 11, wherein:

- the screening device (10s) is sealed against the ambient atmosphere and is flooded with a protective gas during operation; and/or

- additional protective gas is supplied to the screening device (10s) when an inert gas pressure in the screening device (10s) falls below a limit value.

13. A method for controlling the operation of a screening device (10s) according to any one of aspects 1 to 12, wherein:

- during the sieving process, a step response of a sum of a sieved powder mass flow (rhsieved) and an oversize mass flow (rhoversize) to a dosing mass flow (rhdos) is monitored; and/or

- a warning is issued if the step response of the sum of the sieved powder mass flow (rhsieved) and the oversize mass flow (rhoversize) to the dosing mass flow (rhdos) falls below a first limit value; and/or

- a sieve cleaning is initiated when the step response of the sum of the sieved powder mass flow (rhsieved) and the oversize mass flow (rhoversize) to the dosing mass flow (rhdos) exceeds a second limit value.

14. A method for controlling the operation of a screening device (10s) according to any one of aspects 1 to 13, further comprising:

- Changing an inclination angle (aa-w) of the sieve surface of the sieve relative to a horizontal plane (E).

15. Method for controlling the operation of a screening device (10s) according to aspect 14, wherein the screen is arranged within a housing (60s) which is in particular sealed in a gas-tight manner and wherein the screen is rotated relative to the housing (60s) and the angle of inclination (aa-w) is thereby changed.

16. A method for controlling the operation of a screening device (10s) according to aspect 14 or 15, wherein a first inclination angle (aa-w) is set during the first time interval and a second inclination angle (aa-w) is set during the second time interval and wherein either (a) the first inclination angle is less than the second inclination angle or (b) the first inclination angle is greater than the second inclination angle.

17. A method for controlling the operation of a screening device (10s) according to aspect 14 or 15, wherein the angle of inclination is changed during the first and/or during the second time interval.

18. A method for controlling the operation of a screening device (10s) according to any one of aspects 14 to 17, further comprising:

- detecting a spreading speed of the powder to be sieved (56s) on the sieve and/or a position of a powder front of the powder to be sieved (56s) on the sieve, wherein the changing of the angle of inclination (aa-w) of the sieve surface of the sieve relative to the horizontal plane (E) takes place depending on the detected spreading speed and/or depending on the detected position.

19. A method for controlling the operation of a screening device (10s) according to aspect 18, wherein the changing of the inclination angle (aa-w) is carried out such that the Inclination angle (aa-w) is reduced when the detected propagation speed of the powder to be sieved (56s) and/or the detected position of the powder front exceeds a predetermined threshold value.

20. Screening device (10s) comprising:

- a powder inlet (22s);

- a drive device (36s) configured to drive the sieve; and

- a control unit (40s) configured to control the powder inlet (22s) and the drive device (36s) such that:

(i) powder (56s) to be sieved is fed onto the sieve through the powder inlet (22s);

(ii) the sieve is driven for a first time interval with a first drive power, wherein the first drive power is dimensioned such that the powder to be sieved (56s) flows over an entire sieve surface of the sieve and/or into an oversize grain outlet (32s) when the sieve is continuously driven with the first drive power;

(iii) after expiry of the first time interval, the sieve is driven for a second time interval with a second drive power which is less than the first drive power;

(iv) after expiry of the second time interval, steps (ii) and (iii) are repeated.

21. A screening device (10s) according to aspect 20, wherein:

- the second drive power is dimensioned such that the powder to be sieved (56s) forms a cone of material on the sieve surface of the sieve when the sieve is continuously driven with the second drive power in the region of the powder inlet (22s), which essentially corresponds to a cone of material forming on a stationary plane, wherein an angle of material (aa) of the cone of material is adapted in particular to an orientation of the sieve surface; and/or

- the sieve surface of the sieve is inclined relative to a horizontal plane (E) such that the flow of the powder fed through the powder inlet (22s) in the direction of the oversize grain outlet (32s) is assisted by gravity, wherein an angle of inclination (aa-w) of the sieve surface of the sieve relative to the horizontal plane (E) is preferably smaller than an angle of repose (aa) of a repose cone formed by the powder (56s) to be sieved on a horizontal plane (E); and/or

- the first time interval is an empirically determined value for the powder to be sieved (56s); and/or - the control unit (40s) is configured, when determining the first time interval, to end the first time interval at the latest when powder (56s) to be screened flows into the oversize grain outlet (32s), wherein the screening device (10s) in particular comprises an oversize grain sensor provided in the region of the oversize grain outlet for monitoring the flow of powder (56s) to be screened into the oversize grain outlet (32s); and/or

- the control unit (40s) is configured to dimension the first time interval such that a sieve surface utilization of approximately 70% to approximately 90%, preferably approximately 75% to approximately 85% and particularly preferably approximately 80% of the total sieve surface of the sieve is not exceeded by the end of the first time interval; and/or

- the second time interval is an empirically determined value for the powder to be sieved (56s); and/or

- the control unit (40s) is configured, when determining the second time interval, to end the second time interval at the latest when the powder (56s) to be sieved forms a pouring cone with a defined size in the region of the powder inlet (22s) on the sieve surface of the sieve, wherein the sieving device (10s) in particular comprises a dosing sensor (23s) provided in the region of the powder inlet (22s) for detecting the formation of a pouring cone with a defined size.

22. Sieving device (10s) according to aspect 20 or 21, wherein the control unit (40s) is configured to control the powder inlet (22s) such that the powder to be sieved (56s) is fed to the sieve through the powder inlet (22s) at least temporarily continuously with a metering mass flow rhdos = 0.5 * (rh(amin) + rh(a _m ax)), wherein a _m in is a sieve area utilization and rh(amin) is a sieve throughput when driving the sieve with the second drive power, and wherein a _m ax is a sieve area utilization and rh(amax) is a sieve throughput when driving the sieve with the first drive power, wherein the control unit (40s) is configured in particular:

- to determine rh(amine) by increasing the metering mass flow of the powder to be screened (56s) through the powder inlet (22s) when driving the sieve with the second drive power until the powder to be screened (56s) has formed a cone of material with a defined size in the region of the powder inlet (22s) on the sieve surface of the sieve; and/or

- rh(amax) is to be determined by driving the sieve with the first drive power, the metering mass flow of the powder to be sieved (56s) through the powder inlet (22s) is increased until the powder to be screened (56s) has formed a cone of material with a defined size in the area of the powder inlet (22s) on the sieve surface of the sieve and powder to be screened (56s) flows into the oversize grain outlet (32s), the value of rh(a _m ax) thus obtained is preferably multiplied by a safety factor of 0.8, 0.7, 0.6 or 0.5; and/or

- to reduce the dosing mass flow rhdos if the powder to be screened (56s) forms a cone of material with a defined size in the area of the powder inlet (22s) on the screen surface of the screen; and/or

- to initiate a sieve cleaning when the powder to be sieved (56s) forms a cone of material with a defined size on the sieve surface of the sieve in the area of the powder inlet (22s) and/or the dosing mass flow rhdos falls below a limit value.

23. Screening device (10s) according to aspect 22, wherein the control unit (40s) is configured, upon initiation of a screen cleaning

- to control the powder inlet (22s) so that the powder supply through the powder inlet (22s) is stopped; and/or

- to control the drive device (36s) so that any powder still present in the sieve is sieved before the start of the sieve cleaning; and/or

- to control the drive device (36s) so that after the start of the sieve cleaning the sieve is driven with a maximum drive power; and/or

- after the start of the sieve cleaning, to activate a vibrator (38s) which is configured to drive the sieve independently of the drive device (38s) of the sieve device (10s), wherein preferably an angle of attack of the vibrator (38s) on the sieve, a drive amplitude of the vibrator (38s) and/or a drive frequency of the vibrator (38s) is variably adjustable.

24. Sieving device (10s) according to one of aspects 20 to 23, wherein the control unit (40s) is configured to control the powder inlet (22s) such that the powder to be sieved (56s) is fed at least temporarily discontinuously through the powder inlet (22s) onto the sieve, wherein the control unit (40s) is in particular configured to control the drive device (36s) and the powder inlet (22s) such that:

- initially, when the sieve is not driven, powder (56s) to be sieved is fed through the powder inlet (22s) onto the sieve until the powder (56s) to be sieved has formed a cone of material with a defined size on the sieve surface of the sieve in the region of the powder inlet (22s);

- after completion of the powder supply, the sieve is driven and the powder on the sieve surface the powder fed into the sieve is sieved; and

- the sieving process is terminated when ni fed - ni oversize + Hl sieved, where m fed is the mass of the powder fed, mo oversize is the mass of the powder flowing into the oversize outlet (32s) and m _sieved is the mass of the sieved powder, and wherein the sieving device (10s) in particular comprises a first measuring device (42s) for determining m _fed , which is provided in a powder feed container (132s) connectable to the powder inlet (22s) of the sieving device (10s), in particular a second measuring device (44s) for determining mo oversize, which is provided in an oversize container (136s) connectable to the oversize outlet (32s) of the sieving device (10s), and/or and/or in particular a third measuring device (46s) for determining m _sieved , which is provided in a sieved powder outlet (28s) of the sieving device (10s) connectable sieved powder container (134s) is provided.

25. Sieving device (10s) according to one of aspects 20 to 24, wherein the control unit (40s) is configured to control the powder inlet (22s) such that the powder to be sieved (56s) is fed to the sieve at least temporarily with a metering mass flow through the powder inlet (22s) which is determined as a function of the drive power used to drive the sieve, wherein in particular a first metering mass flow with which the powder to be sieved (56s) is fed to the sieve through the powder inlet (22s) during the first time interval is greater than a second metering mass flow with which the powder to be sieved (56s) is fed to the sieve through the powder inlet (22s) during the second time interval.

26. Screening device (10s) according to one of aspects 20 to 25, wherein the control unit (40s) is configured to issue a warning when an oversize grain rate, in particular continuously measured, exceeds a limit value.

27. Screening device (10s) according to one of aspects 20 to 26, wherein:

- the screening device (40s) is sealed against the ambient atmosphere and is flooded with a protective gas during operation; and/or

- the control unit (40s) is configured to supply additional shielding gas into the screening device (10s) when an inert gas pressure in the screening device (10s) falls below a limit value.

28. Screening device (10s) according to one of aspects 20 to 27, wherein the control unit (40s) is configured:

- to monitor a step response of a sum of a sieved powder mass flow (rhsieved) and an oversize mass flow (rhoversize) to a dosing mass flow (rhdos) during the sieving process; and/or

- a warning will be issued if the step response of the sum of the sieved powder mass flow (rhsieved) and the oversize mass flow (rhoversize) to the dosing mass flow (rhdos) falls below a first limit value; and/or

- to initiate a sieve cleaning if the step response of the sum of the sieved powder mass flow (rhsieved) and the oversize mass flow (rhoversize) to the dosing mass flow (rhdos) exceeds a second limit value.

29. Screening device (10s) according to any one of aspects 20 to 28, comprising:

- a lid (20s) detachable from a sieve container (18s);

- a seal (48s) arranged in the cover (20s); and

- a clamping device (50s) configured to exert a clamping force on the seal (20s) that holds the seal (48s) in position in the lid (20s).

30. Screening device (10s) according to any one of aspects 20 to 29, comprising:

- an inclination device (64s) for changing an inclination angle (aa-w) of the sieve surface of the sieve relative to a horizontal plane (E).

31. A screening device (10s) according to aspect 30, comprising:

- a housing (60s) which can be closed in a gas-tight manner, in particular, wherein the sieve is arranged within the housing (60s) and wherein the inclination device (64s) is designed to rotate the sieve relative to the housing (60s) and thereby change the angle of inclination (aa-w).

32. A screening device according to aspect 30 or 31, wherein the powder inlet (22s) and the oversize grain outlet (32s) are fixedly attached to the housing (60s).

33. A screening device according to any one of aspects 30 to 32, comprising:

- a sieve holder (66s) for receiving, in particular for inserting, the Sieve, wherein the inclination device (64s) is attached to the sieve holder (66s) and is adapted to rotate the sieve holder (66s).

34. A screening device (10s) according to any one of aspects 30 to 33, wherein the control unit (40s) is configured to set a first inclination angle (aa-w) during the first time interval and to set a second inclination angle (aa-w) during the second time interval, and wherein either (a) the first inclination angle is less than the second inclination angle or (b) the first inclination angle is greater than the second inclination angle.

35. Screening device (10s) according to one of aspects 30 to 33, wherein the control unit (40s) is configured to change the inclination angle during the first and/or during the second time interval.

36. Screening device (10s) according to one of aspects 30 to 35, further comprising:

- at least one sensor for detecting a spreading speed of the powder to be screened (56s) on the screen and/or a position of a powder front of the powder to be screened (56s) on the screen, wherein the control unit (40s) is configured to change the angle of inclination (aa- w) of the screen surface of the screen relative to the horizontal plane (E) depending on the detected spreading speed and/or depending on the detected position.

37. Sieving device (10s) according to aspect 36, wherein the control unit (40s) is configured to change the angle of inclination (aa-w) such that the angle of inclination (aa-w) is reduced when the detected propagation speed of the powder to be sieved (56s) and/or the detected position of the powder front exceeds a predetermined threshold value.

38. Powder processing system (128s) comprising a screening device (10s) according to any one of aspects 30 to 37.

39. System (100s) for producing three-dimensional workpieces by exposing raw material powder layers to electromagnetic radiation or particle radiation, which comprises a screening device (10s) according to one of aspects 30 to 37 and/or a powder preparation system (128s) according to aspect 38.

Claims

patent claims 1. Powder conveying system for conveying raw material powder to a system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, the powder conveying system comprising: a conveying line which is designed to convey a gas flow at least in sections and a powder flow driven by the gas flow at least in sections; a conveying device which is designed to convey the gas flow through the conveying line; a first tank connected to the conveying line for supplying the system with powder for an additive manufacturing process; at least one overflow container connected to the conveying line for receiving excess powder from the additive manufacturing process; a buffer container connected to the conveying line for supplying a sieving device with powder to be sieved; the sieving device for sieving the powder to be sieved and for dispensing sieved powder; an interface connected to the conveying line for an external tank for introducing fresh or contaminated powder into the powder conveying system; and a control device for controlling the powder conveying system so that it carries out at least one of the following conveying processes: a powder conveying of the sieved powder into the first tank; b powder conveying from the at least one overflow container into the buffer container; and c powder conveying from the external tank into the buffer container. 2. Powder conveying system according to claim 1, wherein either (a) all of the conveying processes a-c contain pneumatic conveying processes, wherein a conveyed powder is conveyed by the gas flow, or (b) at least one of the conveying processes a-c, in particular the conveying process b, does not contain a pneumatic conveying process. 3. Powder conveying system according to claim 2, wherein in case (b) the conveying process which does not include a pneumatic conveying process comprises conveying by a conveyor screw and/or conveying by means of gravity conveying. 4. A powder conveying system according to any one of claims 1 to 3, further comprising a main reservoir connected to the conveying line for receiving the sieved powder, wherein the conveying process a conveys the sieved powder from the main reservoir into the first tank. 5. Powder conveying system according to one of claims 1 to 4, wherein the powder conveying system enables a closed powder conveying, in particular a powder conveying in which conveyed powder does not leave the powder conveying system over several consecutive additive manufacturing processes of the system. 6. Powder conveying system according to one of claims 1 to 5, wherein the powder conveying system is designed to maintain an inert gas atmosphere within the conveying line, in particular during one of the conveying processes a to c. 7. Powder conveying system according to one of claims 1 to 6 in combination with claim 4, further comprising a pressure equalization container coupled to the conveying line, which is coupled to the conveying line in particular downstream of the conveying device and upstream of the connections of the first tank, the at least one overflow container, the buffer container, the main storage and the external tank. 8. Powder conveying system according to one of claims 1 to 7 in combination with claim 4, further comprising at least one metering device, in particular comprising a conveyor screw, for the metered feeding of the powder to be conveyed into the conveyor line from the at least one overflow container, from the main storage and/or from the external tank. 9. Powder conveying system according to one of claims 1 to 8, further comprising at least one separation device, in particular comprising a cyclone, for separating the conveyed powder from the conveying line and for feeding the powder into the first tank and/or into the buffer container. 10. Powder conveying system according to one of claims 1 to 9 in combination with claim 4, wherein at least one of the following containers is coupled to a pressure equalization line: the first tank, the at least one overflow container, the buffer container, the main storage and the external tank. 11. Powder conveying system according to one of claims 1 to 10, wherein the control device is arranged to carry out a flow check, comprising checking a flow through the conveying line. 12. The powder conveying system of claim 11, further comprising a velocity meter for measuring a velocity of the gas flow in the conveying line, wherein the flow check comprises: Opening valves of the powder conveying system so that flow is possible through at least one conveying circuit belonging to one of the conveying processes a, b and c; Setting a conveying speed of the gas flow to a predetermined range or value; and Determining whether a gas flow velocity value measured by the velocity sensor is greater than a predetermined limit. 13. Powder conveying system according to claim 12, wherein adjusting the conveying speed comprises: Determining whether a speed value measured by the speed sensor is within the predetermined value range; if this is not the case, determining whether the speed value is below the predetermined value range; if the speed value is below the predetermined value range, increasing an output of the conveyor device; and if the speed value is not below the predetermined value range, decreasing the output of the conveyor device. 14. A powder conveying system according to any one of claims 11 to 13, further comprising: a pressure sensor for measuring a pressure of the gas stream; and a filter for filtering remaining powder particles from the gas stream, wherein the flow check comprises: Determining a total pressure loss based on sensor data from the pressure sensor; if it is determined that the total pressure loss exceeds a predetermined limit value, carrying out a filter cleaning of the filter and determining a total pressure loss again; and if it is determined that the total pressure loss does not exceed the predetermined limit value, conveying powder in at least one of the conveying processes a, b and c. 15. The powder conveying system of claim 14, wherein the flow check performed by the controller further comprises: if it is determined that the total pressure loss exceeds the predetermined limit, prior to performing the filter cleaning, outputting an error. 16. Powder conveying system according to one of claims 1 to 15, comprising at least one oxygen sensor for measuring an oxygen content of the gas stream, wherein the control device is configured to carry out an oxygen content check, and wherein the oxygen content check comprises measuring an oxygen content using the at least one oxygen sensor. 17. Powder conveying system according to claim 16, wherein the control device is arranged to carry out a pipeline inerting of the conveying line if the measurement of the oxygen content of the gas stream shows that the oxygen content is above a predetermined limit value. 18. The powder conveying system of claim 17, further comprising a vent valve for venting gas from the conveying line to an environment or an external volume, wherein the pipeline inerting comprises opening the vent valve. 19. A powder conveying system according to claim 18, wherein the pipeline inerting further comprises: Evacuating at least a section of the conveyor line; Leakage test of the conveyor line; Normalizing the conveyor line; and Checking the oxygen content in the conveyor line. 20. The powder conveying system of claim 19, wherein evacuating includes checking whether a measured pressure within the conveying line is below a predetermined evacuation pressure. 21. A powder conveying system according to claim 20, further comprising a velocity meter for measuring a velocity of the gas flow in the conveying line, wherein the evacuating comprises: Determining whether a speed value measured by the speed sensor is within a predetermined range of values; if this is not the case, determining whether the speed value is below the predetermined range of values; if the speed value is below the predetermined range of values, increasing an output of the conveyor device; and if the speed value is not below the predetermined range of values, decreasing the output of the conveyor device. 22. Powder conveying system according to one of claims 19 to 21, wherein the leakage test comprises: closing the drain valve; Stopping the conveyor; Determine whether a system pressure rise per time exceeds a predefined limit; if the system pressure rise per time exceeds the predefined limit, issue an error message; and if the system pressure rise per time does not exceed the predefined limit, proceed to normalize the delivery line. 23. The powder conveying system of claim 22, comprising a first pressure sensor on an inlet side of the conveyor and a second pressure sensor on an outlet side of the conveyor, wherein determining the system pressure increase comprises considering a sum of the readings of the first and second pressure sensors. 24. A powder conveying system according to any one of claims 19 to 23, wherein normalizing the conveying line comprises: Flooding the conveyor line with inert gas; and Measuring the oxygen content in the production line. 25. The powder conveying system of claim 24, further comprising: if the measured oxygen content in the conveying line exceeds a predetermined limit, restarting the pipeline inerting. 26. Powder conveying system according to one of claims 19 to 25, wherein checking the oxygen content in the conveying line comprises: If the measured oxygen content exceeds a predetermined limit a predetermined number of times, restart the pipeline inerting process. 27. Powder conveying system according to one of claims 1 to 26, wherein the control unit is configured to carry out conveying according to at least one of the conveying processes a, b and c, wherein the conveying comprises opening at least one valve which is arranged in a conveying circuit associated with the respective conveying process. 28. Powder conveying system according to claim 27, wherein the control unit is arranged to carry out a conveying control during conveying, which comprises: Monitoring the oxygen content in the production line; Monitoring a pressure in the delivery line; Monitoring a conveying speed of the gas flow; Monitoring at least one measured parameter of the powder conveying system; and Checking a termination condition to end the funding. 29. A powder conveying system according to claim 28, wherein monitoring the oxygen content in the conveying line comprises: Determining whether a measured oxygen content exceeds a predetermined limit; and if the measured oxygen content exceeds the predetermined limit, opening a valve to supply inert gas to the production line. 30. A powder conveying system according to claim 28 or 29, wherein monitoring the pressure in the conveying line comprises: Determining whether a measured pressure in the conveying line is within a predetermined range; if the measured pressure in the conveying line is less than the predetermined range, opening a valve to supply inert gas; and if the measured pressure in the conveying line is higher than the predetermined range, opening a vent valve to vent gas from the conveying line. 31. A powder conveying system according to any one of claims 28 to 30, wherein monitoring the conveying speed of the gas flow comprises: Determining a gas density of the gas flow based on at least one measured characteristic of the gas flow; Determining a powder mass flow conveyed through the conveying line based on a control value applied to a dosing device of an originating tank of the conveyance; Determining a target velocity of the gas flow based on the gas density and based on the bulk material mass flow; and Controlling the conveying device to convey the gas flow at the specified target speed 32. Powder conveying system according to one of claims 28 to 31, wherein monitoring at least one measured characteristic of the powder conveying system comprises: Controlling a dosing device of a source tank of the production with a predetermined control value; Determining whether the at least one measured characteristic of the powder conveying system is above a predetermined maximum value for the respective characteristic; and if the at least one measured characteristic is above the predetermined maximum value, reducing the control value of the dosing device by a predetermined value so that the dosing device delivers a smaller dose of powder per unit of time into the gas stream. 33. Powder conveying system according to claim 32, wherein the control device is further configured to: if the at least one measured parameter is below the predetermined maximum value, increase the control value of the dosing device by a predetermined value so that the dosing device delivers a higher dose of powder per unit of time into the gas stream. 34. Powder conveying system according to claim 32 or 33, wherein the at least one characteristic comprises at least one of the following characteristics: Conveying speed, pump outlet pressure, pump power and dose of powder per time. 35. Powder conveying system according to one of claims 28 to 34, wherein checking a termination condition for terminating the conveying comprises: Terminating the conveying by stopping the conveying device when at least one of the following events is detected: a fill level of a source tank from which powder is taken for conveying falls below a predetermined limit, a fill level of a target tank into which the powder is conveyed exceeds a predetermined limit, a predetermined maximum conveying time is exceeded and a total pressure loss of a conveying gas exceeds a predetermined limit. 36. Powder conveying system according to one of claims 1 to 35, wherein the control device is designed to carry out a pipe cleaning and a filter cleaning after completion of a conveying according to conveying process a, b and/or c, wherein the pipe cleaning comprises a gas stream flowing through the conveying line, and wherein the filter cleaning comprises blowing off a filter provided in the conveying line with compressed air. 37. Powder conveying system according to one of claims 1 to 36, further comprising at least one dew point sensor for measuring a relative humidity within the conveying line and/or within a tank of the powder conveying system, wherein the control unit is configured to initiate automatic powder drying when the measured value of the relative humidity exceeds a predetermined limit value. 38. A powder conveying system according to claim 37 in combination with claim 4, wherein a dew point sensor is provided on at least one of the following components of the powder conveying system: main reservoir, external tank, and conveyor. 39. Powder conveying system according to one of claims 1 to 38 in combination with claim 4, wherein at least one oxygen sensor is provided for measuring an oxygen concentration on at least one of the following components of the powder conveying system: first tank, overflow container, buffer container, main storage, external tank and sieving device, wherein the control device is adapted to initiate flooding of the conveying line with inert gas when it is determined that at least one of the oxygen sensors provided measures an oxygen concentration above a predetermined limit value. 40. Powder conveying system according to one of claims 1 to 39 in combination with claim 4, wherein at least one pressure sensor is provided for measuring a pressure on at least one of the following components of the powder conveying system: first tank, overflow container, buffer container, main storage, external tank and sieving device, wherein the control device is set up to carry out at least one of the following steps if it is determined that at least one of the pressure sensors provided measures a pressure increase per predetermined unit of time above a predetermined limit value: issuing a warning; Flooding the conveyor line or an affected section of the powder conveyor system with inert gas; Measuring an oxygen content in the affected section of the powder conveying system; and Performing a filter cleaning. 41. Powder conveying system according to one of claims 1 to 40, wherein at least one temperature sensor is provided for measuring a temperature on at least one motor of the powder conveying system, in particular a motor of a conveyor screw and/or the conveying device, wherein the control device is designed to initiate a shutdown of the conveying device if it is determined that at least one of the temperature sensors provided measures a temperature value above a predetermined limit value. 42. Powder conveying system according to one of claims 1 to 41, wherein the control device is designed to initiate a shutdown of the conveying device if at least one of the following events is detected: a torque of a motor of a metering device exceeds a predetermined limit value; a valve of the powder conveying system is in an actual position that does not correspond to its target position; an error in a sensor is detected. 43. Powder conveying system according to one of claims 1 to 42, wherein a filling level sensor for measuring a filling level of the at least one overflow container is provided on the at least one overflow container, in particular in the form of one or more load cells, wherein a fill level sensor for measuring a fill level of the intermediate tank is provided on an intermediate tank which is arranged above the process chamber and which is configured to supply the process chamber with powder and which is configured to be supplied with powder from the first tank, in particular in the form of one or more load cells, wherein the control device is configured to continue an additive manufacturing process of the system in the event of a failure of the powder conveyance by the conveying device until at least one of the following events occurs: the fill level sensor of the overflow container detects that the fill level of the overflow container exceeds a predetermined limit value; and the fill level sensor of the intermediate tank detects that the fill level of the intermediate tank falls below a predetermined limit value. 44. Powder conveying system according to one of claims 1 to 43, wherein the control device is arranged to carry out a powder conveyance of a predetermined powder quantity according to one of the conveying processes a, b or c and then to stop the conveyance. 45. Powder conveying system according to claim 44, wherein the control device is configured to determine a priority value for each of the conveying processes after each conveying operation according to one of the conveying processes a, b and c, based on predetermined fill level limits of the source tanks and the target tanks and wherein the control device is configured to subsequently carry out conveying according to the conveying process with the highest priority value.