WO2020200674A1 - Method and device for producing plastic particles using a laser beam - Google Patents

Abstract

The invention relates to a method and a device for producing plastic particles (P), wherein a strand (S) of molten plastic or a filament consisting of plastic is cut to form plastic particles (P) using at least one laser beam (L).

Description

METHOD AND DEVICE FOR MANUFACTURING

PLASTIC PARTICLES USED BY LASER BEAM

DESCRIPTION

The invention relates to a method and a device for producing plastic particles.

The production of plastic powder is relevant for different fields of application, especially for additive manufacturing, in which different

Manufacturing processes require powders that consist of thermoplastics and, for example, have particle diameters smaller than 200 μm.

The mean particle diameter currently most frequently used is in the range from 40 μm to 80 μm. In principle, smaller particles can also be used.

Various processes are known for producing these powders, for example the bead polymerization described in WO99 / 58317A1, which is described in the

Seed suspension polymerization described in DE 698 28 426 T2, the emulsified grinding described in DE 101 22 492 A1, various

Precipitation processes, such as in DE 29 06 647 B1, grinding processes, melt spray processes or spray drying. The latter three processes are known, for example, from EP 2 115 043 B1. Furthermore, the processes of underwater pelletizing and strand pelletizing are for example

WO 2017 / 112723A1, known.

US Pat. No. 9,205,590 B2 describes a process in which molten plastic is discharged from a nozzle, in principle similar to the meltblown process (see also Pinchuk, L. S .; Goldade, V. A .; Makarevich, A. V .;

Kestelman, VN: Melt Blowing - Equipment, Technology, and Polymer Fibrous Materials, Springer, 2002). The resulting melt strand is accelerated and thus accelerated by an air stream flowing out in the discharge direction next to the nozzle also rejuvenated and additionally stimulated to vibrations, which lead to a detachment of melt strands or drops from the melt strand. As in "A novel Extrusion Process for the Production of Polymer Micropellets", Osswald et al., Polym. Closely. If DOI 10.1002 / pen is executed, this technology can partially lead to a successful pulverization and partially the detachment of drops fails, so that the drops are still connected by threads. Targeted manipulation of the drop size is also not known.

The invention is based on the object of specifying an improved method and an improved device for producing plastic particles.

The object is achieved according to the invention by a method having the features of claim 1 and by a device having the features of

Claim 11.

Advantageous embodiments of the invention are the subject of the subclaims.

In a method according to the invention for producing plastic particles, a strand of melted plastic or a filament made of plastic is cut by means of at least one laser beam to form plastic particles.

In one embodiment, the strand is conveyed continuously or discontinuously from a melt nozzle, exits essentially vertically downwards and is then cut by means of the at least one laser beam.

In a further embodiment, the filament is conveyed continuously or discontinuously and cut at a free end, in particular a freely hanging end, by means of the at least one laser beam. In a further embodiment, the filament is conveyed discontinuously and tensioned at a free end and cut by means of at least two laser beams. In one embodiment, a fluid stream emerges from a fluid nozzle arranged around the melt nozzle in such a way that the strand is accelerated and tapered. In the simplest case, the fluid can be air, but it can also comprise other gases, for example nitrogen or other inert gases, in order to avoid oxidation or possible thermo-oxidative degradation of the melt. Fluids with a higher viscosity than air are helpful in obtaining flows with less tendency to turbulence. The job of the fluid flow is to accelerate the strand so that it tapers. In this way, the diameter of the strand can be achieved which is considerably smaller than the diameter of the melt nozzle.

In one embodiment, the fluid flow is tempered in such a way that the strand does not or only slightly cools or that the strand is heated, for example by prior heating with a heating register or the like. The fluid flow can

for example have temperatures in the range of +/- 50 ° C around the temperature of the melt.

In one embodiment, the strand is guided in a tube below the fluid nozzle. In this way, a fluid flow that is as uniform as possible is supported with as constant a speed as possible in the discharge direction of the melt and with as little turbulence as possible. A tapering of the tube can be used to further accelerate the fluid flow and thus the

To aid stretching of the strand.

In one embodiment, energy is introduced into the strand by means of the laser beam in such a way that the plastic at the relevant point on the strand suddenly evaporates or decomposes in gaseous form. In this way, thread formation can be avoided. A parameter configuration can be selected in such a way that sufficient energy is introduced in order to achieve a clean, that is, thread-free, detachment of drops, in particular without any loss of material through evaporation. With the temperature control of the strand and the fluid flow and the choice of intensity, distribution and duration of the laser beam, the The roundness of the drops can be influenced. As long as the drop is liquid, the surface tension will strive for an even rounding of the drop.

In one embodiment, a CO2 laser is used to generate the laser beam. The wavelength used, for example 10.6 pm, is usually sufficiently well absorbed by plastics.

In one embodiment, the plastic is or is colored in such a way that its absorption spectrum is adapted to the wavelength of the laser used to generate the laser beam. The coloring of the plastics can be used to adapt the absorption spectrum of the plastics to the wavelength of the laser beam. For example, plastics colored black absorb wavelengths in the range of most common laser sources, for example by adding carbon black. Fiber lasers (typical wavelength in the range of 1064 nm) or other laser sources with shorter wavelengths than those of the C0 ₂ laser can thus also be used. This is beneficial to the smallest possible spot sizes

(Focus diameter) of the laser beam to keep the zone of energy input small. The smaller these zones, the more favorable the separation of a drop or strand section.

In one embodiment, the laser beam is pulsed.

In one embodiment, the laser beam is operated as a laser scanner so that the energy input can be controlled by guiding the laser beam or the pulsed laser radiation. For example, it is then also possible with a spot diameter that is smaller than the strand diameter, one

To achieve energy input over the entire width of the strand. Is the

If the scanning speed is sufficiently high and the strand discharge speed is correspondingly slow, a quasi-simultaneous detachment of several strand sections can also be achieved by means of scanning. In one embodiment, the tube has at least one laterally arranged inlet that is transparent to the laser beam and through which the laser beam is guided.

A device according to the invention for producing plastic particles comprises a melt nozzle, from which a strand of melted plastic can be carried, and at least one laser, by means of which at least one laser beam can be guided onto the strand in order to cut the strand to form plastic particles.

In the present application, cutting the strand can be understood to mean, on the one hand, that the energy input from the laser merely promotes the breakdown of the strand into droplets, and on the other hand that the energy input by the laser is so high that conventional laser cutting is present the energy input partially evaporates the plastic, so that a section is separated. In addition, mixed forms of these two options should be included under the term of cutting the strand.

In one embodiment, the melt nozzle is arranged such that the strand emerges essentially vertically downwards.

In one embodiment, a fluid nozzle is provided around the melt nozzle for ejecting a fluid stream in order to accelerate and taper the strand.

In one embodiment, a heating device for controlling the temperature of the

Fluid flow provided.

In one embodiment, a tube, in which the strand can be guided, is provided below the outlet of the fluid flow. In one embodiment, the tube has at least one laterally arranged inlet that is transparent to the laser beam and through which the laser beam can be guided.

The principle of the invention is based on the particularly continuous strand-shaped ejection of a plastic melt from a nozzle, whereupon this strand is locally heated in a pulsed manner with the aid of laser radiation in such a way that it breaks up into several parts, for example drops. These drops fall

then downwards and cool down while falling, so that the plastic is now available as a powder, also known as microgranulate.

By means of the method according to the invention, the detachment of drops from the melt strand can be influenced in a more targeted manner and vibration excitation can be dispensed with.

Embodiments of the invention are described below with reference to

Drawings explained in more detail.

Show in it:

Figure 1 is a schematic view of an apparatus for producing plastic particles, and

Figure 2 is a schematic view of a further embodiment of a

Device for the production of plastic particles.

Corresponding parts are provided with the same reference symbols in all figures.

FIG. 1 shows a schematic view of a device 1 for producing plastic particles P. The device 1 comprises a melt nozzle 2, from which a strand S of melted plastic is discharged. Melting the plastic and pumping the melt can be carried out, for example, by a single-screw or multi-screw extruder. Surrounding the melt nozzle 2 is an annular gap-shaped fluid nozzle 3 for ejecting a fluid flow F. In the simplest case, the fluid can be air, but it can also include other gases, for example nitrogen or other inert gases, in order to avoid oxidation or possible thermo-oxidative degradation of the melt . Fluids with a higher viscosity than air are helpful in obtaining flows with less tendency to turbulence. The task of the fluid flow F is to accelerate the string S so that it tapers. In this way, the diameter of the strand S can be achieved which are considerably smaller than the diameter of the melt nozzle 2. For this, and for the following step, it is useful that the strand S does not cool down or that the strand S is even heated. The fluid flow F can therefore be temperature-controllable, for example by prior heating with a heating register or the like, and

Have temperatures in the range of, for example, +/- 50 ° C around the temperature of the melt. In contrast to US Pat. No. 9,205,590 B2, the fluid flow F should therefore not strongly excite the strand S and make it vibrate, so that the

Strand S disintegrates. In order to support a fluid flow F which is as uniform as possible, with a speed that is as constant as possible in the discharge direction of the melt and as little turbulence as possible, the area below the discharge of the strand S from the melt nozzle 2 and below the outlet of the

Fluid flow F be sheathed with a tube (not shown). The tapered strand S is subsequently locally excited in a pulse-like manner with one or more laser beams L. This results in an energy input into the strand S, which the

Temperature in a small section of the strand S greatly increased. Depending on

Intensity of the laser radiation, the temperature of the strand S increases, which in turn results in a decrease in viscosity and melt strength, so that at the point of energy input in the strand S, a

Drop T is favored. The higher the local energy input, the more the droplet detachment is favored. The energy input can be so strong that the plastic suddenly evaporates or decomposes in gaseous form at this point. On in this way thread formation can be avoided. It can be a

Parameter configuration can be selected such that sufficient energy is introduced to achieve a clean, that is, thread-free detachment of droplets T. The roundness of the droplets T can also be influenced by controlling the temperature of the strand S and of the fluid flow F and by choosing the intensity, distribution and duration of the laser beam L. As long as the drop T is liquid, the surface tension will strive for an even rounding of the drop T. The flight phase of the droplets T after the cut can thus be carried out as in a downer reactor (Sachs et al .: Characterization of a downer reactor for particle rounding, doi: 10.1016 / j.powtec.2017.01.006; Sachs et al .: Rounding of Irregular Polymer Parti cles in a Downer Reactor, doi: 10.1016 / j.proeng.2015.01.119) can be used to influence the particle shape. The cooled drop T then forms a particle P.

However, the method does not necessarily require the use of the fluid flow F.

The laser beam L can be guided to the strand S from different sides and / or angles.

By using several laser sources, beam splitters or diaphragms, several cuts can also be carried out by laser beams L simultaneously.

The laser beam L should be sufficiently absorbed by the processed plastic. For example, a CO2 laser can be used whose wavelength used, for example 10.6 pm, is usually sufficiently well absorbed by plastics. The coloring of the plastics can serve to improve the

The absorption spectrum of the plastic to match the wavelength of the laser beam L. For example, plastics colored black absorb wavelengths in the range of most common laser sources, for example by adding carbon black.

About the parameters diameter of the melt nozzle 2, melt temperature, temperature of the fluid flow F and speed of the fluid flow F is the achieved diameter of the strand S adjustable. The time interval between

Laser pulses determine the length of the strand sections produced, which contract into drops T. The method therefore makes it possible to influence the powder grain shape and size distribution more directly than in US Pat. No. 9,205,590 B2.

FIG. 2 shows a schematic view of a further embodiment of an apparatus 1 for producing plastic particles P.

A circular melt nozzle 2 is used for the plastic melt, for example with a diameter D3 = 200 μm. The fluid nozzle 3 in the form of an annular gap from which the fluid flow F emerges can, for example, have a diameter D1 -D2 of 1.5 mm.

A tube 4, for example with an inside diameter Dl = 3 mm, which continues the fluid flow F, develops seamlessly from this annular gap. Below the outlet of the melt nozzle 2, for example approx. 20 cm below, there is an inlet 5 in the tube 4 which is transparent to the laser beam L. For example, a pulsable CO ₂ laser with a wavelength of 10.6 pm is used. The tube 4 ends below the inlet 5, for example 2 cm below.

Subsequently, a suction with a lower fluid velocity and a larger pipe cross-section, for example at least a factor of ten larger than the pipe 4, can be provided so that cool ambient air, for example at a temperature of about 20 ° C, can be sucked in and the detached droplets T solidify can and can be fed to a collecting container. The discharge direction is vertical, so that gravity supports the material flow.

For example, a polypropylene homopolymer with a melt flow

Rate (MFR) of 50 g / min (measured at 230 ° C, 2.16 kg) was used. A single-screw extruder, for example, is used to melt and convey the plastic. The temperature of the melt is, for example, 240 ° C., as is the temperature of the air blown out in the fluid nozzle 3. At a Plastic mass throughput of, for example, 0.05 kg / h, a fluid volume flow of, for example, 8.6 1 / min is used. The laser beam L is, for example, pulsed at a frequency of 20 kHz, for example with a pulse duration of 5 ps and a power of 1100 mW. The strand S is thus stretched to such an extent that a particle size of, for example, approx. 50 μm is reached when the particles P have cooled down.

The focus diameter of the laser beam L on the strand S is, for example, 100 gm. The intensity distribution of the laser beam in the focus can approach a Gaussian distribution.

In a further embodiment, instead of the air-assisted take-off, provision can be made for the strand S to be deposited on a roll and taken off with the aid of an adjustable roll speed. Extreme stretching can thus be achieved. The strand S cools down on the roll to form a thread (monofilament). However, it is not wound up on the roll, as is known from the manufacture of staple fibers (CH000000273678A), but continues to a section where it hangs freely while it is continuously conveyed. On this one

The filament is then used in the same way as in the

Melt strand cut with the help of laser pulses.

In principle, the cutting can also be decoupled from the filament production and can also be carried out discontinuously.

In principle, the following discontinuous design can also be used for a particularly reproducible cut: The filament is unwound from a roll, the free end being tensioned, for example between two clamping jaws or with a gripper. Unwinding is stopped so that the filament is taut. This filament is cut at several points by laser pulses arriving at the same time, so that numerous sections are created. There must be at least two laser pulses. However, there can also be any number of laser pulses. After the cutting process, a does not remain cut rest on the grippers / fixtures. With the method modified in this way, the filament can be controlled more easily than a strand of melt guided with an air stream.

REFERENCE LIST

1 device

2 melt nozzle

3 fluid nozzle

4 pipe

5 inlet

Dl diameter

D2 diameter

D3 diameter

D4 diameter

F fluid flow

L laser beam

P particles, plastic particles

S strand

T drop

Claims

PATENT CLAIMS 1. A method for the production of plastic particles (P), wherein a Strand (S) of melted plastic or a filament made of plastic to form plastic particles (P) is cut by means of at least one laser beam (L). 2. The method of claim 1, wherein the strand (S) consists of a Melt nozzle (2) is conveyed continuously or discontinuously and exits essentially vertically downwards and is then cut by means of the at least one laser beam (L) or the filament is conveyed continuously or discontinuously and in a section in which it hangs freely by means of the at least one Laser beam (L) is cut or the filament discontinuously conveyed and clamped at one free end and cut using at least two laser beams (L). 3. The method according to claim 2, wherein a fluid stream (F) emerges from a fluid nozzle (3) arranged around the melt nozzle (2) in such a way that the strand (S) is accelerated and tapered. 4. The method according to claim 3, wherein the fluid flow (F) is tempered so that the strand (S) does not or only slightly cools or that the strand (S) is heated. 5. The method according to any one of claims 3 or 4, wherein the strand (S) is guided below the fluid nozzle (3) in a tube (4). 6. The method according to any one of the preceding claims, wherein by means of the laser beam (L) an energy input into the strand (S) takes place such that the plastic at the relevant point on the strand (S) evaporates suddenly or decomposes in gaseous form. 7. The method according to any one of the preceding claims, wherein a CO2 laser is used to generate the laser beam. 8. The method according to any one of the preceding claims, wherein the Plastic is or is colored so that its absorption spectrum is matched to the wavelength of a laser used to generate the laser beam. 9. The method according to any one of the preceding claims, wherein the Laser beam (L) is pulsed. 10. The method according to any one of claims 5 to 9, wherein the tube (4) at least one laterally arranged for the laser beam (L) has transparent inlet (5) through which the laser beam (L) is guided. 11. Device (1) for producing plastic particles (P), comprising a melt nozzle (2) from which a strand (S) is melted Plastic can be discharged, and at least one laser by means of which at least one laser beam (L) can be guided onto the strand (S) in order to cut the strand (S) to form plastic particles (P). 12. The device (1) according to claim 11, wherein the melt nozzle (2) so is arranged that the strand (S) emerges substantially vertically downwards. 13. The device (1) according to claim 11 or 12, wherein the surrounding area Melt nozzle (2) a fluid nozzle (3) is provided for the ejection of a fluid stream (F) in order to accelerate and taper the strand (S). 14. Device (1) according to claim 13, wherein a heating device is provided for controlling the temperature of the fluid flow (F). 15. Device (1) according to one of claims 13 or 14, a tube (4) in which the strand (S) can be guided is provided below the outlet of the fluid flow (F). 16. The device (1) according to claim 15, wherein the tube (4) has at least one laterally arranged, for the laser beam (L) transparent inlet (5) through which the laser beam (L) can be guided.