CZ20013851A3 - Process for producing model bodies, material used in this process and production process of such material - Google Patents

Abstract

Model bodies of any form or shape can be produced from special plastic powders with the aid of selective sintering using infrared lasers. Also described are plastic powders containing an infrared absorber and suitable for laser-assisted model production.

Description

Technical field

The present invention relates to a process for the manufacture of model bodies by which plastics in the form of selected plastic powders can be formed into any three-dimensional structure, the method comprising selective sintering by laser light in the infrared region. The present invention also relates to a special plastic powder which contains an infrared absorbing agent and which is particularly suitable for sintering by infrared laser light.

In particular, the present invention relates to a method for producing three-dimensional plastic models according to stored geometric data by means of a computer-controlled device that is operable to be directly capable of being infrared laser beams. to produce prototypes and models (hereafter referred to as rapid prototyping equipment).

BACKGROUND OF THE INVENTION

The term "rapid prototyping" includes computer-controlled, additive, automated modeling techniques that are currently known. The term "laser sintering" refers to a method of rapid prototyping in which a loose pile of certain powdered materials can be heated and sintered at precisely defined locations on different planes by laser beams, which are preferably computer controlled.

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The use of plastic powders for CO ₂ laser sintering is known and has been described in: A. Gebhardt, Rapid Prototyping, Carl Hanser Verlag, Munich, Vienna 1996, pp. 115-116. This publication describes a method for manufacturing model bodies in which any three-dimensional structure can be formed by selective sintering of plastics using light emanating from a CO ₂ laser.

One disadvantage of the hitherto known processes is the limited accuracy of the molded articles obtained by these processes. Due to this lack of precision, molded articles that are currently obtained in this way must in many cases undergo costly manual reprocessing. This low degree of accuracy is due in part to the use of a CO ₂ laser whose wavelength is 10.6 micrometers and which can be very in-focus. Focused lasers, such as the Nd-YAG laser, whose wavelength is 1064 nanometers, have not been used for laser sintering since conventional plastics do not absorb radiation at this wavelength.

SUMMARY OF THE INVENTION

The present invention relates to a method for producing three-dimensional plastic models according to stored geometrical data by means of laser beams regulated according to these data and having a wavelength in the range of 500 nanometers to 1500 nanometers, preferably in the range of 800 nanometers to 1200 nanometers, the laser beams are directed according to the given geometric data to defined three-dimensional zones of a loose bed of fine powder consisting of plastic particles, thereby causing welding or welding of 000 000 · · · · · · ··· Sintering of this material, characterized by the fact that said plastic powder is composed of particles whose mean the size ranges from 2 microns to 200 microns, and contains an infrared absorbing agent.

In a particularly preferred embodiment of the present invention, the plastic powder is substantially spherical.

The present invention also relates to plastic powders for use as starting materials for laser modeling, wherein said powder is comprised of particles whose mean size (i.e., weighted average diameter) is in the range of 2 microns to 200 microns and contains an agent. for infrared absorption.

Various types of laser can be used in the method of the present invention. Particularly suitable for use herein are solid lasers and lasers using semiconductor diodes. Examples of solid lasers include Nd-YAG lasers with a wavelength of 1064 nanometers and Nd-YLF lasers with a wavelength of 1053 nanometers. Suitable diode lasers are those emitting radiation having a wavelength in the range of 823 nanometers to 985 nanometers.

During radiation, the energy that is radiated to the surface of said loosened powder pile is preferably in the range of 0.01 millioule / mm ² to 100 millioules / mm ² , more preferably in the range of 1 millioule / mm ² to 50 millioules / mm ² .

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According to the application, the effective diameter of the laser beam is preferably in the range of 0.001 millimeter to

0.05 millimeter, more preferably in the range of 0.01 millimeter to

0.05 millimeter.

Preferably, pulsed lasers are used, and it has been found that lasers with a high pulse frequency ranging from 1 kilohertz to 100 kilohertz have been found to be particularly suitable.

The preferred process of the present invention can be described as follows:

the laser beam impinges on the uppermost layer of the loose bed of a material suitable for use in the present invention and welds or sinters the material to a precisely defined layer thickness. Said layer thickness is in the range of 0.005 millimeter to 1 millimeter, preferably in the range of 0.01 millimeter to 0.5 millimeter.

The first layer of the desired component is formed as described above. The working volume is then reduced by an amount which is less than the thickness of said sintered layer. The working volume is filled with additional polymeric material up to the original bed height. The second layer of said component is sintered by repeated laser irradiation and bonded to the preceding layer. Additional layers to be formed to complete the manufacture of the component are made by repeating the above process.

· 9 * * »» »* * * * * *

The speed of the laser beam used is in the range from 1 millimeter / second to 1000 millimeter / second, preferably from 10 millimeter / second to 100 millimeter / second.

Plastic powders suitable for use in the present invention may belong to different classes of polymers. Examples are: polyolefins such as polyethylene and polypropylene, polyamides such as polyamide-β and polyamide-6,6, polyesters such as polyethylene terephthalate, polycarbonates, weldable polyurethanes, polystyrene, styrene acrylonitrile copolymers and polyacrylates. Particularly suitable for the production of model bodies without pores are plastic powders based on partially crystalline polymers.

The size of the particles forming said powder is particularly important for the process of the invention. Generally, the average particle diameter is in the range of from 2 microns to 200 microns, preferably from 5 microns to 100 microns, most preferably in the range of 5 microns to 50 microns. The mean particle diameter (particle size) herein refers to the weighted average particle diameter.

In order to produce particles the size of which is so important to the process of the present invention, it is possible to comminute plastics, which are usually in the form of coarse granules. However, in certain circumstances sharp or angular plastic particles are obtained in this way. Such particles, whose surface is irregular or uneven, sometimes exhibit poor flowability, which has a disadvantageous effect on their processing in laser sintering devices. For this reason, agents for increasing ♦ and β must usually be added to the plastics

In order to improve the flowability of such comminuted plastics, and to ensure that they can be used in automated equipment.

Particularly suitably used in the process of the present invention are polymers having a substantially spherical shape (also referred to hereinafter as bead polymers).

Bead polymers have also been shown to be suitable for the laser sintering process of the present invention. In addition, these polymers exhibit a flowability that is much better than the flowability of other comminuted particles, and therefore it is not even necessary to add flowability enhancers to improve their flowability.

Another important advantage of the bead polymers is that they do not leave problematic residues behind when they are burned, for example when they form the core of a hollow ceramic molded article. In the case of comminuted plastic particles to which flowability enhancers are added, it has been observed that such particles cannot be incinerated without leaving residues.

This phenomenon is particularly important when plastic models, which are primarily produced by laser sintering, are further processed in successive steps to produce a precise molded article. Therefore, for example, a pattern made according to the present invention is first coated with a wax, which coating is performed to further improve the surface of the pattern, then immersed in a ceramic material suspension and the resulting pattern that is coated with a ceramic

The material is annealed in the furnace. Materiál materiál tt tt tt tt tt tt tt tt materiál tt materiál materiál Said model should burn completely during firing to form a exposed hollow shaped ceramic article. Since commonly used ground plastics do not decompose completely by incineration due to their flowability enhancers, metal patterns often cast in the ceramic form exhibit surface inaccuracies.

Another advantage of using bead polymers is the surface accuracy and surface roughness of the models produced by the method of the present invention. Due to their spherical shape and their good flowability, the models produced using the preferred bead polymers are smoother and therefore more accurate.

The bead polymers of the present invention preferably include homopolymers or copolymers of monoethylenically unsaturated compounds (monomers). For the purposes of the present invention, copolymers are polymers which are synthesized from two or more different monomers. Examples of suitable monomers include styrene, α-methylstyrene, chlorostyrene, acrylic esters such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyl acrylate, esters of methacrylic acid such as methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, isopropyl methacrylate, isopropyl methacrylate, , hexyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, dodecyl methacrylate and stearyl methacrylate, as well as acrylonitrile, methacrylonitrile, methacrylamide and vinyl acetate.

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The molecular weight of the bead polymers of the present invention has been shown to be an important consideration for the suitability of a given bead polymer for use in the method of the present invention. Specifically, the molecular weight (weight average molecular weight, _Mw) should preferably be in the range from 10,000 daltons to 500,000 daltons, preferably from 20,000 daltons to 250,000 daltons. Thus, during the manufacture of the bead polymers, molecular weight control agents can be used to obtain the desired molecular weight polymer. A group of molecular weight control agents that are particularly suitable for use in the present invention include sulfur compounds such as n-butyl mercaptans, dodecyl mercaptan, ethyl thioglycolate and diisopropylxanthogene disulfide. Furthermore, the agents described in the German patent DE 3 010 373, such as the enol ether of formula (I), are also very suitable for controlling the molecular weight:

Particularly suitable bead polymers can be prepared by known methods. Thus, it is possible to obtain bead polymers having particle sizes ranging from about 10 microns to 200 microns by slurry or bead polymerization. By "slurry polymerization" is meant a method wherein a monomer or monomer-containing mixture that contains an initiator soluble in said monomer (s) is dispersed in the form of droplets, optionally mixed with small, solid particles, in a substantially immiscible phase which contains a dispersing agent and subsequently cures by raising the temperature and stirring the mixture. A more detailed description of suspension polymerization can be found, for example, in Ullmanns Encyclopedia of

Industrial Chemistry, Volume A21, 5th edition, VCH,

Weinheim 1992, pp. 363-373.

Bead polymers having particle sizes ranging from 2 microns to 10 microns can be made by a process known as dispersion polymerization. In dispersion polymerization, a solvent is used in which the monomers used are soluble but in which the polymer formed is not soluble. Dispersion polymerization typically produces bead polymers with a narrow particle size distribution.

Essentially all compounds that absorb light with a wavelength in the range of 500 nanometers to 1500 nanometers, preferably in the range of 800 nanometers to 1200 nanometers, are suitable as infrared absorption agents. Both infrared pigments and infrared dyes can be used independently.

Carbon black, in particular synthetically produced carbon black, is preferably used as the infrared pigment of the present invention.

The carbon blacks that are preferably used have a specific surface area in the range of 10 m ² / gram to 500 m ² / gram as measured by the BET method. Suitable types of carbon black include channel carbon black, retort black, and flame black carbon black.

In addition, pigments based on mixed metal oxides of the rutile or spinel type are suitable according to the invention.

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Examples of suitable metal oxide pigments are the commercially available HEUCODUR®-Brown 859 a

HEUCODUR®-Black 953

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Infrared dyes (i.e. infrared absorbing dyes, also referred to as IRDs) are known in the art. Various types of infrared dyes are suitable for use in the present invention, such as, for example, an indaniline dyes, oxonol dyes, porphine derivatives, anthraquinone dyes, mesostyryl dyes, pyrrile compounds and squarylium derivatives.

The infrared dyes of DE-OS 4,331,622 are also particularly suitable for use in the present invention because they absorb visible radiation only within a narrow wavelength range and thus allow 3D models to be produced by the method of the present invention which are not colored or which are only very brightly colored.

An example is the infrared dye of formula (II):

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The amount of infrared absorbing agent according to the present invention is in the range of

0.01 weight percent to 10 weight percent, preferably in the range of 0.05 weight percent to 5 weight percent, based on the weight of the plastic powder.

The manufacture of plastic powders containing an infrared absorption agent can be carried out in various ways. Thus, it is possible to mix the plastic material with an infrared absorption agent in the melt by means of an extruder and comminute the resulting extrudate into particles of the desired size in a mill. It is also possible to add an infrared absorption agent to the compositions during manufacture of the plastic material, so that said infrared absorption agent is contained in the resulting plastic material. In the manufacture of bead polymers by suspension polymerization, the infrared absorption agent can be added to the monomers.

It has been found that plastic particles can be easily doped with infrared dyes. In this process, the plastic particles are dispersed in a liquid phase, preferably water, in which the plastic material does not dissolve, and it is also possible to use a dipping agent or surfactant. Examples of suitable surfactants for this application include sodium salts of alkyl sulfonates, sodium isooctyl esters of sulfosuccinic acid and ethoxylated nonylphenol. An infrared dye solution is added to the dispersion thus obtained, preferably a water-miscible solvent such as ethyl acetate, toluene, butanone, chloroform, dichloroethane or methyl isobutyl ether is added to said dispersion. During this stage «» • · f φ φ

9 ··

The solvent, which contains the infrared dye, swells said plastic particles by the action of this solvent. Subsequently, water or water can be removed from the mixture by filtration or decantation, and the solvent can be removed by evaporation, for example under reduced pressure, wherein said infrared dye remains in said plastic particles.

The plastic powders of the present invention containing an infrared absorption agent are particularly suitable for laser sintering using infrared lasers, especially Nd-YAG lasers, and can be used to obtain models or components that are characterized by particularly good detail accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically shows a device for rapid prototype production.

DETAILED DESCRIPTION OF THE INVENTION

The following examples illustrate the manufacture of finely divided plastic materials and attempts to sinter these materials using an infrared laser. These examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

• ·

Production of a Bead Polymer according to the Invention That Contained a Carbon Black

Example 1

a) Dispersion of carbon black grams of carbon black (Printex G, supplied by Degussa, BET surface area of 30 m ² / gram), 24 grams of polymethyl methacrylate and 216 grams of methyl methacrylate were milled in a ball mill for 2 hours to obtain a homogeneous dispersion.

b) Bead polymer

A mixture of 200 grams of the dispersion from a) and 2.0 grams of 2,2'-azobis (isobutyronitrile) was vigorously stirred and the resulting mixture was transferred to a stirred reactor pre-charged with 1.0 liter of 1 wt.% Aqueous alkaline copolymer solution. %, wherein the copolymer was 50 weight percent methacrylic acid and 50 weight percent methyl methacrylate, and the pH of the solution was adjusted to 8 with sodium hydroxide solution. The stirrer speed was set to 700 rpm and the mixture was maintained at 60 ° C for 3 hours. C, then at 78 ° C for 10 hours, at 85 ° C for 2 hours, and then allowed to cool to room temperature. The resulting bead polymer was isolated by decantation, washed repeatedly with water and dried under vacuum at 50 ° C. 168 grams of an intense black bead polymer were obtained with a coloring having an average particle size of 18 microns and a weight average molecular weight M _{w of} 230,000.

Example 2

Production of a bead polymer according to the present invention containing carbon black

a) Dispersion of carbon black grams of carbon black (Printex G, supplied by Degussa), 28.8 grams of polymethyl methacrylate, 216 grams of methyl methacrylate and 43.2 grams of n-butyl methacrylate were milled in a ball mill for 2 hours to obtain a homogeneous dispersion.

b) Bead polymer

A mixture of 250 grams of the dispersion from a) and 2.5 grams of 2,2'-azobis (isobutyronitrile) was vigorously stirred and the resulting mixture was transferred to a stirred reactor pre-charged with 1.25 liter of 1 wt.% Aqueous alkaline copolymer solution. %, wherein the copolymer was 50 weight percent methacrylic acid and 50 weight percent methyl methacrylate, and the pH of the solution was adjusted to 8 with sodium hydroxide solution. The stirrer speed was set at 600 rpm and the mixture was maintained at 78 ° C for 10 hours. C, then at 85 ° C for 2 hours and then allowed to cool to room temperature over 2 hours. The resulting bead polymer was isolated by decantation, repeatedly

999

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199 9 99 washed with water and dried under vacuum at 50 ° C. It was obtained

205 grams of an intense black color bead polymer having an average particle size of 25 microns and a weight average molecular weight M _{w of} 220,000.

Example 3

Production of a bead polymer according to the present invention which contains an infrared dye

(a) Production of bead polymer

2340 grams of methanol, 180 grams of polyvinylpyrrolidone, 210 grams of methyl methacrylate and 90 grams of ethyl methacrylate were mixed in a 4-liter reactor equipped with a grid stirrer to form a homogeneous solution. This solution was heated to 55 ° C under nitrogen at 100 rpm for one hour. A solution of 6 grams of 2,2'-azobis (isobutyronitrile) in 165 grams of methanol was then added to the reactor. The polymerization mixture was stirred for an additional 20 hours at 55 ° C at 100 rpm. The resulting polymer dispersion was then allowed to cool to room temperature and the resulting bead polymer was isolated by settling. 193 grams of a bead polymer having a weight average molecular weight M _{w of} 75,000, an average particle size of 12 microns and a ratio of 0 (90) / 0 (10) of 1.18 were obtained. The ratio of 90% (0 (90)) to 10% (0 (10)) of the volume distribution of the particles, namely the value of the ratio 0 (90) / 0 (10), was a good measure of the width of the particle size distribution. A 0 (90) / 0 (10) ratio of less than 2.0 was indicative of a narrow particle size distribution.

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b) Doping the bead polymer with an infrared dye

100 grams of the bead polymer of step a) was dispersed in a solution of 900 grams of water and 200 milligrams of sodium isooctyl sulfosuccinate. To this solution was added dropwise, with stirring, a solution containing 284 mg of infrared dye of formula (II) and 100 g of ethyl acetate. The resulting mixture was stirred at room temperature for 4 hours and then sonicated for 10 minutes. The bead polymer was filtered off from the mixture, which was repeatedly washed with water and dried under vacuum at 50 ° C to constant weight to remove all ethyl acetate.

Example 4

Production of a bead polymer according to the present invention which contains an infrared dye

(a) Production of bead polymer

In a 4 L reactor equipped with a grid stirrer, 2500 grams of methanol, 64 grams of polyvinylpyrrolidone, 240 grams of styrene and 60 grams of ethyl methacrylate were mixed to form a homogeneous solution. This solution was heated under nitrogen at 70 rpm at 100 rpm for one hour. A solution of 3.75 grams of 2,2'-azobis (isobutyronitrile) in 75 grams of styrene was then added to the reactor. The polymerization mixture was stirred for an additional 15 hours at 70 ° C at 100 rpm. The resulting polymer dispersion was subsequently

The mixture was allowed to cool to room temperature and the resulting bead polymer was isolated by settling. Yielding 247 g of the pearl polymer whose weight average molecular weight _Mw was 60,000, the average particle size of microns and the ratio value 0 (90) / 0 (10 ) was 1.6.

b) Doping the bead polymer with the infrared dye grams of the bead polymer of step a) was dispersed in a solution of 90 grams of water and 20 milligrams of sodium isooctyl sulfosuccinate. To this solution was added dropwise with stirring

28.4 mg of infrared dye of formula (II) and 10 grams of ethyl acetate. The resulting mixture was stirred at room temperature for 4 hours. The bead polymer was filtered from the mixture, which was repeatedly washed with water and dried under vacuum at 50 ° C to constant weight to remove all ethyl acetate.

Example 5

The sintering tests performed with the bead polymers of Examples 1 to 4 grams of the given bead polymer of Examples 1 to 4 were introduced into the support vessel of the experimental apparatus shown in Figure 1.

The laser beam Nd-YAG _1 (cross-section 5 mm ² , pulse frequency 10 hertz) was directed freely to the surface with a deflection mirror 2 at a speed of 10 millimeters / second 0 ♦ * ♦ «* · • 0 0 0 · · · ·

0000 9 4 • 0

loose bed 2 ^of polymer bead, wherein said laser beam were scanned area of 20 x 20 mm. The radiated energy corresponded to 40 millijoules / mm ² . In this way, solid molded articles were obtained from bead polymers 1b), 2b), 3b) and 4b). These test specimens were mechanically broken in liquid nitrogen and the surfaces of individual fractures were examined by a scanning electron microscope. In the case of the bead polymer of Example 3a) (which served as a comparative example without the use of an infrared absorption agent), it was found that the product structure was not altered in the manner described above and still had the form of a powder.

The results are summarized in the following table:

Example Creation molded product Surface Evaluation structure quarry (lb) according to this invention Yes smooth smooth with pores 2b) according to this invention Yes smooth smooth with pores 3b) according to this invention Yes smooth, shiny smooth without pores 4b) according to this invention Yes smooth, shiny smooth without pores 3a) comparative No - -

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Claims (12)

PATENTOVÉ Claims Process for producing three-dimensional plastic models according to stored geometry data by laser beams controlled therefrom and having a wavelength in the range of 500 nanometers to 1500 nanometers, preferably in the range of 800 nanometers to 1200 nanometers, said laser beams corresponding to said laser beams the geometric data is directed to defined three-dimensional zones of a loose bed of fine powder consisting of plastic particles, thereby welding or caking the material, characterized in that said plastic powder is comprised of particles having a mean size ranging from 2 microns to 200 microns, and contains an infrared absorbing agent. The method of claim 1, wherein said plastic powder is a bead polymer. Method according to claim 1 or 2, characterized in that the laser used is a solid laser, in particular a 1064 nanometer Nd-YAG laser or a 1053 nanometer Nd-YLF laser, or a semiconductor diode laser, in particular such a laser. a semiconductor diode laser that emits radiation with a wavelength of 823 nanometers; or 985 nanometers. Production method according to any one of claims 1 to 3, characterized in that the energy density on the surface of said free-flowing powder bed during irradiation. 4 · 4444 44 achieves a value in the range of 0.01 milliliter / mm ² to 100 millioules / mm ² , preferably values in the range of 1 milijoule / mm ² to 50 milijoules / mm ² . Production method according to any one of claims 1 to 4, characterized in that the effective diameter of said laser beam is in the range of 0.001 millimeter to 0.05 millimeter, preferably in the range of 0.01 millimeter to 0.05 millimeter. Method according to any one of claims 1 to 4, characterized in that it uses a pulsed laser whose pulse frequency ranges from 1 kilohertz to 100 kilohertz. Plastic powders for use as a starting material in the manufacture of laser models, characterized in that the average diameter of the particles constituting said powders is in the range of 2 microns to 200 microns, further comprising an infrared absorption agent. Plastic powders according to claim 7, characterized in that they comprise bead polymers. Plastic powders according to claim 7 or 8, characterized in that they contain, as infrared absorbing agents, light absorbing compounds having a wavelength in the range from 500 nanometers to 1500 nanometers, preferably in the range from 800 nanometers to 1200 nanometers, in particular infrared pigments. and / or infrared dyes. 4 * 9 · 9 · 9 * 9 · 9 * 9 * · Plastic powders according to claim 9, characterized in that they contain carbon black as the infrared pigment or as a dye selected from the group consisting of the indaniline dyes, oxonol dyes, porphine derivatives, anthraquinone dyes, mesostyryl dyes, pyrrile compounds and squarylium derivatives. Plastic powders according to any one of claims 7 to 10, characterized in that they contain infrared absorption agents in an amount of from 0.01 to 10% by weight, preferably of from 0.05 to 5% by weight, based on the weight of the plastic powder. 12. A process for producing a plastic powder containing an infrared dye, wherein the aqueous dispersion of the plastic powder is contacted with a solution of the infrared dye in a water-miscible solvent, and subsequently water and said solvent are removed from the mixture.