DE102019109005A1 - Method and device for the production of plastic particles - Google Patents

Abstract

The invention relates to a method and a device for producing 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).

Description

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

The production of plastic powders is relevant for different fields of application, in particular for additive manufacturing, in which different manufacturing processes require powders that are made of thermoplastics and have, for example, 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 those in FIG WO99 / 58317A1 Bead polymerization described in the DE 698 28 426 T2 seed suspension described in the DE 101 22 492 A1 emulsified milling described, various precipitation processes, such as in the DE 29 06 647 B1 , Grinding process, melt spray process or spray drying. The latter three methods are known from, for example EP 2 115 043 B1 . Furthermore, the methods of underwater pelletizing and strand pelletizing are, for example WO 2017 / 112723A1 , known.

In the US 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, LS; Goldade, VA; Makarevich, AV; Kestelman, VN: Melt Blowing - Equipment, Technology, and Polymer Fibrous Materials, Springer, 2002). The resulting melt strand is accelerated by an air stream flowing out in the discharge direction next to the nozzle and thus also tapered and also excited to vibrate, 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. Sci, DOI 10.1002 / pen, 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 with the features of claim 1 and by a device with 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 to support the 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. The roundness of the droplets can also be influenced by controlling the temperature of the strand and the fluid flow and by selecting the intensity, distribution and duration of the laser beam. As long as the drop is liquid, the surface tension will strive for an even rounding of the drop.

In one embodiment, a CO ₂ laser is used to generate the laser beam. The wavelength used, for example 10.6 µm, 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. This means that fiber lasers (typical wavelength in the range of 1064 nm) or other laser sources with shorter wavelengths than those of the CO ₂ laser can also be used. This is advantageous in order to use the smallest possible spot sizes (focus diameter) of the laser beam, which keep the energy input zone 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, with a spot diameter that is smaller than the strand diameter, it is then also possible to achieve an energy input over the entire width of the strand. 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 the production of plastic particles comprises a melt nozzle from which a strand of melted plastic can be discharged, 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 is provided for controlling the temperature of the fluid flow.

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 then fall down and cool down as they fall, 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 excitation of vibrations can be dispensed with.

Embodiments of the invention are explained in more detail below with reference to drawings.

• 1 eine schematische Ansicht einer Vorrichtung zur Herstellung von Kunststoffpartikeln, und
• 2 eine schematische Ansicht einer weiteren Ausführungsform einer Vorrichtung zur Herstellung von Kunststoffpartikeln.

Show in it:

• 1 a schematic view of an apparatus for producing plastic particles, and
• 2 a schematic view of a further embodiment of an apparatus for producing plastic particles.

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

1 shows a schematic view of a device 1 for the production of plastic particles P .

The device 1 includes a melt nozzle 2 , of which one strand S. melted plastic is discharged. Melting the plastic and pumping the melt can be carried out, for example, by a single or multi-screw extruder. Surrounding the melt nozzle 2 there is an annular gap-shaped fluid nozzle 3 for ejecting a fluid stream F. . 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 F. is the strand S. to accelerate, so that it tapers. Thus, the diameter of the strand S. can be achieved that are significantly smaller than the diameter of the melt nozzle 2 . For this, and for the following step, it is useful to have the strand S. not cold or that the strand S. is even heated. The fluid flow F. can therefore be temperature-controlled, 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 9,205,590 B2 should the fluid flow F. the strand S. so do not strongly stimulate and vibrate, so that the strand S. disintegrates. To support a fluid flow that is as uniform as possible F. with the most constant speed possible in the discharge direction of the melt and the lowest possible turbulence, the area below the discharge of the strand S. from the melt nozzle 2 and below the exit of the fluid stream F. be sheathed with a tube (not shown). The tapered strand S. is further used with one or more laser beams L. impulse-like locally excited. This results in an energy input into the string S. showing the temperature in a small section of the strand S. greatly increased. Depending on the intensity of the laser radiation, the temperature of the strand increases S. instead, which in turn leads to a reduction in viscosity and melt strength, so that at the point of energy input in the strand S. the detachment of 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. In this way, thread formation can be avoided. A parameter configuration can be selected in such a way that sufficient energy is introduced to ensure a clean, that is, thread-free, detachment of drops T to achieve. With the temperature control of the strand S. as well as the fluid flow F. and the choice of intensity, distribution and duration of the laser beam L. can also change the roundness of the drops T to be influenced. As long as the drop T is liquid, the surface tension is a uniform rounding of the drop T strive for. The flight phase of the drops T after the cut, 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 Particles 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 attached to the strand from different sides and / or angles S. be guided.

By using several laser sources, beam splitters or diaphragms, several cuts can be made simultaneously with laser beams L. be performed.

The laser beam L. should be sufficiently absorbed by the processed plastic. For example, a CO ₂ laser can be used whose wavelength used, for example 10.6 μm, is generally sufficiently well absorbed by plastics. The coloring of the plastics can serve to change the absorption spectrum of the plastics to the wavelength of the laser beam L. adapt. For example, plastics colored black absorb wavelengths in the range of most common laser sources, for example by adding carbon black.

Via the parameters diameter of the melt nozzle 2 , Melt temperature, temperature of the fluid flow F. and velocity of fluid flow F. is the achieved diameter of the strand S. adjustable. The time interval between the laser pulses determines the length of the produced Sections of strands that become drops T contract. The process thus makes it possible to influence the powder grain shape and size distribution more directly than in the US 9,205,590 B2 .

2 shows a schematic view of a further embodiment of a device 1 for the production of plastic particles P .

A circular melt nozzle is used 2 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, for example, a diameter D1-D2 of 1.5 mm.

A pipe develops seamlessly from this annular gap 4th , for example with an inner diameter D1 = 3 mm, that the fluid flow F. continues. Below the outlet of the melt nozzle 2 , for example approx. 20 cm below, there is an inlet 5 in the pipe 4th that for the laser beam L. is transparent. For example, a pulsable CO ₂ laser with a wavelength of 10.6 µm is used. The pipe 4th ends below the inlet 5 , for example 2 cm below. Subsequently, suction with a lower fluid velocity and a larger pipe cross-section, for example at least a factor of ten larger than the pipe 4th , be provided so that cool ambient air, for example at a temperature of about 20 ° C, can be sucked in, and thus the detached drops T can solidify 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) is 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., the temperature in the fluid nozzle 3 blown air as well. With a plastic mass throughput of, for example, 0.05 kg / h, a fluid volume flow of, for example, 8.6 l / min is used. The laser beam L. is pulsed, for example, with a frequency of 20 kHz, for example with a pulse duration of 5 μs and a power of 1100 mW. This becomes the strand S. stretched so much that a particle size of, for example, approx. 50 µm is achieved if the particles P are cold.

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

In a further embodiment, instead of the air-assisted vent, the strand can be provided S. put on a roll and pulled off with the help of 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 onto the roll, as is known from the manufacture of staple fibers ( CH000000273678A ), but continues to a section where it hangs freely while being continuously conveyed. On this section of the line, the filament is then cut with the help of laser pulses, analogous to the process for the melt strand.

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 simultaneously arriving laser pulses, 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, an uncut residue remains 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.

List of reference symbols

1

contraption

2

Melt nozzle

3

Fluid nozzle

4th

pipe

5

inlet

D1

diameter

D2

diameter

D3

diameter

D4

diameter

F.

Fluid flow

L.

laser beam

P

Particles, plastic particles

S.

strand

T

drops

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• WO 9958317 A1 [0003]
• DE 69828426 T2 [0003]
• DE 10122492 A1 [0003]
• DE 2906647 B1 [0003]
• EP 2115043 B1 [0003]
• WO 2017/112723 A1 [0003]
• US 9205590 B2 [0004, 0033, 0038]
• CH 000000273678 A [0044]

Claims (16)

Process for the production of plastic particles (P), wherein a strand (S) of melted plastic or a filament made of plastic is cut by means of at least one laser beam (L) to form plastic particles (P). Procedure according to Claim 1 , the strand (S) being conveyed continuously or discontinuously from a melt nozzle (2) and exiting essentially vertically downwards and then being cut by means of the at least one laser beam (L) or the filament being conveyed continuously or discontinuously and in one section , in which it hangs freely, is cut by means of the at least one laser beam (L) or the filament is conveyed discontinuously and tensioned at a free end and cut by means of at least two laser beams (L). Procedure 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. Procedure 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. Method according to one of the Claims 3 or 4th , the strand (S) being guided in a tube (4) below the fluid nozzle (3). Method according to one of the preceding claims, wherein the laser beam (L) is used to introduce energy into the strand (S) in such a way that the plastic suddenly evaporates or gaseously decomposes at the relevant point on the strand (S). Method according to one of the preceding claims, wherein a CO ₂ laser is used to generate the laser beam. Method according to one of the preceding claims, wherein the plastic is or is colored in such a way that its absorption spectrum is adapted to the wavelength of a laser used to generate the laser beam. Method according to one of the preceding claims, wherein the laser beam (L) is pulsed. Method according to one of the Claims 5 to 9 wherein the tube (4) has at least one laterally arranged inlet (5) which is transparent to the laser beam (L) and through which the laser beam (L) is guided. Device (1) for producing plastic particles (P), comprising a melt nozzle (2) from which a strand (S) of 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) is to cut the strand (S) to form plastic particles (P). Device (1) according to Claim 11 , wherein the melt nozzle (2) is arranged so that the strand (S) emerges essentially vertically downwards. Device (1) according to Claim 11 or 12 , a fluid nozzle (3) for ejecting a fluid stream (F) being provided around the melt nozzle (2) in order to accelerate and taper the strand (S). Device (1) according to Claim 13 , wherein a heating device is provided for controlling the temperature of the fluid flow (F). Device (1) according to one of the Claims 13 or 14th , a tube (4) in which the strand (S) can be guided is provided below the outlet of the fluid flow (F). Device (1) according to Claim 15 wherein the tube (4) has at least one laterally arranged inlet (5) transparent to the laser beam (L) through which the laser beam (L) can be guided.

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