DE29920207U1 - Device for regulating the temperature of a plastic mass in an extruder - Google Patents

Description

» -W VV WW

The present invention relates to a device for controlling the temperature of a plastic mass in an extruder.

When manufacturing plastic profiles and pipes, a plastic material is plasticized in an extruder and fed through one or more extruder screws to an extrusion nozzle, which gives the profile the desired cross-sectional shape, after which the profile is ejected. The extruder screw is arranged in an extruder cylinder, which is an essential component of such an extruder. The temperature of the plastic material inside the extruder cylinder is critical for the extrusion process. On the one hand, a certain minimum temperature must be present for a proper extrusion process, but on the other hand, the plastic material must not be heated too high to avoid burning. If the plastic material, which is generally PVC, is heated too much, hydrochloric acid is released, which destroys the extruder cylinder and the screw. During the extrusion process, maintaining a constant temperature is extremely important for uniform extrusion quality.

Furthermore, when starting up the extruder, it is necessary to reach a temperature inside the extruder cylinder as quickly as possible that lies within the permissible range between the minimum temperature and the maximum temperature.

In known methods for controlling the internal temperature, a temperature sensor is provided which measures the internal temperature as deep as possible inside the extruder cylinder in the area of the opening for the extruder screw. The amount of heat required to achieve the desired internal temperature is supplied via heating devices which are attached to the outside of the extruder cylinder. This enables reasonably precise and reliable control of the internal temperature in the stationary state. If the internal temperature falls below the setpoint, the heating devices are switched to full power until the setpoint is reached. If the setpoint is exceeded, the heating devices are switched off, with cooling being used if necessary to prevent the plastic mass from overheating. However, this known solution has a number of disadvantages. Even with carefully coordinated control of the internal temperature, the internal temperature fluctuates around the setpoint, so that part of the heat supplied must be periodically dissipated by cooling on the outside of the cylinder in order to prevent the material from overheating. This means a waste of energy.

However, the start-up process of the extruder is particularly critical in known devices. For reasons of productivity, it is necessary to bring the extruder to the target temperature as quickly as possible when starting up. This means that the heating devices must have a corresponding output in order to supply the required amount of heat in the shortest possible time. This causes a relatively steep increase in the internal temperature of the extruder cylinder, which is actually desirable. Due to the relatively large amount of heat that

-&Igr;

·

ν, the system, the internal temperature continues to rise even after the heating device has been switched off, so that there is a risk of the plastic mass overheating. In addition, the maximum temperature permitted by the material on the outer surface of the extruder cylinder can be exceeded in such a case, resulting in damage to the extruder cylinder. It has also been found that if massive heat is introduced into the extruder cylinder, irregularities in the heat distribution occur, which lead to different deformation and, in particular, to different expansion of the extruder cylinder in the axial direction. This means that the axis of the extruder cylinder bends, which in turn can damage the extruder screws arranged therein.

The object of the present invention is to avoid these disadvantages and to provide a device for controlling the temperature which, on the one hand, enables precise maintenance of the desired target temperature inside the extruder cylinder and, on the other hand, ensures rapid achievement of this target temperature during the start-up process.

According to the invention, a temperature measurement is also carried out on the outside of the cylinder and this outside temperature is taken into account as a correction value when regulating the inside temperature. It has been found that an improved regulation of the inside temperature is possible if the respective outside temperature is taken into account. This is taken into account in such a way that, all other things being equal, a lower heating output is applied when the outside temperature is significantly higher than the inside temperature and, conversely, a higher heating output is applied when the outside temperature is low than would be the case if only the inside temperature is taken into account. If, for example, the inside temperature is a certain amount below the setpoint, a conventional controller switches to full heating output. In the method according to which the device according to the invention works, this is also basically the case, but the outside temperature is also taken into account. If this is already significantly higher than the inside temperature, the heating output is throttled by a predetermined amount or, if necessary, set to zero entirely. The control is carried out in such a way that the target temperature is reached as quickly as possible, but if possible, no overshoot occurs. The corresponding control parameters can be optimized best by means of adaptive control, which runs according to a self-learning control algorithm.

Additional protection against undesirable material stresses can be achieved by having the controller limit the outside temperature to a predetermined maximum value.

The most reliable way to measure the internal temperature is by using a sensor inserted into the extruder barrel from above. This solution is also practical because the sensor is easily accessible for maintenance purposes. However, the external temperature should be measured in the lower area, as this is critical for the stress on the extruder barrel.

·

·

A particularly simple control system is obtained if the difference between the measured value of the indoor temperature and the measured value of the outdoor temperature is taken into account as a correction element when controlling the indoor temperature.

In a particularly advantageous design variant, the power of the heating devices in the lower half of the extruder cylinder is greater than in the upper half of the extruder cylinder. Surprisingly, it has been found that a uniform heating power distribution in the circumferential direction does not lead to a uniform temperature distribution in the extruder cylinder. Therefore, the thermally induced expansions of the extruder cylinder in the axial direction are different, so that the extruder axis bends, which is extremely undesirable and can lead to the destruction of the extruder screws. It has been found that it is advantageous to apply a larger amount of heat per unit area in the lower area of the extruder cylinder than in the upper area. In this way, a uniform temperature distribution can be achieved, which prevents unacceptable thermal deformation of the extruder cylinder.

An improvement in the control dynamics can be achieved by providing a cooling device for cooling the extruder cylinder, which is preferably designed as a fan that is directed at the extruder cylinder. It is particularly advantageous if the heating device has several heating bands, each of which is arranged in a cooling body and which extend in the circumferential direction on the outside of the extruder cylinder, and if the additional sensor is arranged on a cooling body. This enables the additional sensor to be attached easily on the one hand and enables precise measurement on the other, since it has been found that the outside temperature measured in this way corresponds very well to the temperature peaks that actually occur.

Furthermore, the invention relates to an extruder for producing plastic profiles with an extruder cylinder in which at least one screw is provided for plasticizing and conveying plasticized plastic material, which has a device for controlling the temperature as described above.

In the following, the present invention is explained in more detail with reference to the embodiments shown in the figures.

Show it:

Fig. 1 shows schematically a section through an extruder according to the invention,

Fig. 2 and 3 Diagrams showing the temperature profile in the wall of the extruder barrel,

Fig. 4 a detail of the heating device and

Fig. 5 is a block diagram of the temperature control.

The extruder of Fig. 1 has an extruder cylinder 1 in which a recess 2 is arranged, which is intended to accommodate two extruder screws (not shown). The extruder cylinder 1 is surrounded by a heating device 3, which consists of an upper half 3a and a lower half 3b. The heating device 3 has in the area of the

lower half 3b a power that is about 10% higher than in the upper half 3a.

A first temperature sensor 4 measures the temperature inside the extruder cylinder 1 as close as possible to the recess 2. The temperature sensor 4 is inserted vertically from above. Opposite the first temperature sensor 4, a further temperature sensor 5 is arranged on the underside of the extruder cylinder 1. The temperature sensor 5 is preferably arranged on a heat sink of a heating band.

For uniform cooling, a split fan is used, with a separate air duct in an outer jacket chamber 6. The cooling air flows through an upper opening 7 along the arrows 8 around the cylinder 1 and outwards at the bottom through further openings 9. This design achieves a particularly symmetrical cooling effect because the cooling air flow is split and guided evenly on both outer sides.

The structure of the heating device 3 is shown in detail in Fig. 4. Cooling rings 10 with a U-shaped cross section are applied in the circumferential direction to the cylinder 1, which is only indicated. Heating bands 11 are arranged between the protruding legs 10a. The temperature sensor 5 is attached to the leg 10b of a cooling ring 10 that rests against the cylinder 1.

In conventional control devices, the internal temperature Ti is controlled by a PID controller to a specific setpoint that is optimal for the extrusion process. The PID controller is usually implemented in software in a corresponding control circuit. In the present invention, the same PID control algorithm is used in principle, but instead of the measured internal temperature Ti, a fictitious value Ti* is used as the control variable, which is calculated, for example, according to the following formula (1):

Ti* = Ti+k(AT-A) with AT = Ta-Ti (1)

The factors k and A are changed from predetermined initial values until an optimal control behavior is achieved. This can be optimized particularly preferably by a self-learning control algorithm. As can be seen from the formula, the temperature difference Δ�T between the outside temperature Ta and the inside temperature Ti is included in the correction term. This temperature difference is corrected with a factor A that corresponds to a normal temperature difference between the outside temperature and the inside temperature in the equilibrium state. This ensures that the fictitious temperature Ti* in the equilibrium state is equal to the actual inside temperature Ti. The factor A will generally be positive because part of the applied heating power is radiated outwards. In certain cases in which a relatively large amount of heat is generated inside the extruder barrel due to the mechanical friction of the extruded material and the extruder screws, the factor A can also assume a negative value. Depending on the extruder, the profile produced, the material and the operating conditions, the value of A can fluctuate between -20° and 80°.

The proportionality coefficient k controls the extent of the correction and is optimized within an allowable range of 0.1 to 0.8.

Fig. 2 and 3 show typical temperature profiles across the wall thickness of the extruder cylinder 1. The radial dimension r is plotted on the vertical axis, with I designating the inner surface and A the outer surface of the extruder cylinder 1. The temperature is plotted on the horizontal axis. As already mentioned above, the internal temperature Ti is not measured directly on the inner surface, but at a distance from it. However, the error caused by this distance is generally negligible. Fig. 2 shows a typical unsteady process during heating. The internal temperature Ti is still relatively low, while the external temperature Ta is relatively high due to the heating power applied from the outside. In contrast, Fig. 3 shows a state that occurs after massive use of cooling. The external temperature Ta is significantly lower than the internal temperature Ti.

In Fig. 5, a control device is shown schematically in a block diagram. In a function block B, the above formula (1) is evaluated and T* is calculated. In an actual PID controller, T* is compared with T _S0LL and a controlled variable is calculated. In a subsequent element MAX, the controlled variable is corrected in order to prevent a maximum permissible outside temperature from being exceeded. The corrected controlled variable P is a measure of the heating energy to be supplied or the cooling required in the negative case.

The present invention makes it possible to control the internal temperature in an extruder cylinder more precisely to a certain setpoint and at the same time to achieve faster heating during start-up operation.

ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ ϕ •ϕ ■ φ ψ φ
ϕ ϕ ϕ ϕ ϕ
&phgr; *
&phgr;
&phgr; •ϕ
ΦΦ· φ I
ΦΦ I ϕ ϕ ϕ ΦΦΦ· •Φ; ··

Monthly ··

Claims (7)

1. Device for controlling the temperature of a plastic mass in an extruder with a heating device to supply heat to the outside of the extruder cylinder ( 1 ) and with a first sensor ( 4 ) for measuring the temperature inside the extruder cylinder and with a controller to keep the internal temperature Ti at a predetermined setpoint, characterized in that a further sensor ( 5 ) for measuring the external temperature Ta is provided on the outside of the extruder cylinder ( 1 ). 2. Device according to claim 1, characterized in that the first sensor ( 4 ) is provided in the upper region of the extruder cylinder ( 1 ) and that the further sensor ( 5 ) is provided in the lower region of the extruder cylinder ( 1 ). 3. Device according to one of claims 1 or 2, characterized in that the heating device has two sections, one of which is arranged on the underside of the extruder cylinder ( 1 ) and the other is arranged on the upper side of the extruder cylinder ( 1 ), and that the heating power of the section arranged on the lower side is greater than that of the section arranged on the upper side of the extruder cylinder ( 1 ). 4. Device according to one of claims 1 to 3, characterized in that a cooling device for cooling the extruder cylinder ( 1 ) is also provided, which is preferably designed as a fan directed towards the extruder cylinder ( 1 ). 5. Device according to one of claims 1 to 4, characterized in that the controller is designed to limit the outside temperature Ta to a predetermined maximum value. 6. Device according to one of claims 1 to 4, characterized in that the heating device has a plurality of heating bands, each of which is arranged in a cooling body and which extend in the circumferential direction on the outside of the extruder cylinder ( 1 ), and that the further sensor ( 5 ) is arranged on a cooling body. 7. Extruder for producing plastic profiles with an extruder cylinder ( 1 ) in which at least one screw is provided for plasticizing and conveying plastic material, characterized in that the extruder has a device for regulating the temperature according to one of the preceding claims.