EP1085972A1

EP1085972A1 - Method and material for producing model elements

Info

Publication number: EP1085972A1
Application number: EP99950332A
Authority: EP
Inventors: Wolfgang Podszun; David Bryan Harrison; Gabriele Alscher
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1998-05-11
Filing date: 1999-05-06
Publication date: 2001-03-28
Also published as: DE19820725A1; BR9910364A; JP2002514527A; AU4258799A; WO1999058317A1; CA2331528A1; TW453947B

Abstract

The invention relates to a method for producing model elements, according to which any desired three-dimensional structure can be obtained using plastic materials, especially selected bead polymers, by means of selective sintering with laser light. The invention also relates to a special material which is especially well suited for laser sintering. A bead polymer consisting of a homo- or copolymer of monoethylenically unsaturated compounds with an average particle diameter of 2 to 200 °m is used as a plastic powder.

Description

Process and material for the production of model bodies

The invention relates to a process for the production of model bodies, in which, using plastics, in the form of selected bead polymers, any three-dimensional structure can be built up using selective sintering using laser light. The invention also relates to a special material that is particularly suitable for laser sintering.

The invention particularly relates to a method for producing three-dimensional

Plastic models according to stored, geometric data with the help of a computer-aided laser-based system for the direct production of prototypes and models (rapid prototyping system).

Rapid prototyping is the term used to summarize the computer-controlled, additive, automatic model building processes known today. Laser sintering is a rapid prototyping process in which fillings made of certain powdery materials are heated and fused or sintered at certain points in the room under the influence of laser beams, preferably controlled by a program.

Such a method is described for example in the patent DE 19 701 078 Cl. Low-melting metals are sintered in a process for the production of three-dimensional tools for the shaping of thermoplastic plastic using a rapid prototyping system. Low-melting metals and / or metal alloys with a melting point below 200 ° C. are used in the form of metal powder or metal foils free from plastic binders or metallic binders. The energy of the laser radiation used is set up in accordance with the melting point of the metals and / or metal alloys used. With this procedure none Models are created from plastics. It is also not possible to obtain metal models from high-melting metals.

The use of plastic powders for laser sintering is also known (A. Gebhardt: "Rapid Prototyping", Carl Hanser Verlag, Munich, Vienna, 1996, p. 115-

116). The process serves both for the production of plastic models and for the production of positive preforms for ceramic casting molds.

A disadvantage of the known plastic powders is their poor flowability, which can only be partially reduced by the use of flow aids.

Because of the poor free-flowing properties, transportation in the laser sintering system is difficult.

The following additional problems arise in the manufacture of positive preforms for ceramics. The sintering of ground polymer, e.g. of

Polystyrene for the preform is feasible, but the surface quality of the preform is not fully satisfactory. The polymer preform is then surrounded with ceramic material that is fired at high temperatures for solidification. The polymer material is volatilized during this process. Complete volatilization is desired. Most polymer powders, however, cannot be removed without residue when burning due to the use of flow aids.

The object of the invention was to find a material suitable for the sintering process by means of lasers which has a smooth to fine-grained surface after

Sintering forms and which can be almost completely incinerated, if necessary, at conventional burner temperatures for firing ceramic, in particular at a temperature of more than 1100 ° C.

The task is solved by using special plastic powders, in

Form of polymer beads from homo- or copolymer of monoethylenic unsaturated compounds, preferably copolymers with methyl methacrylate or styrene units, with an average particle diameter of 2 to 200 μm, as sintered material during sintering by means of lasers.

The invention relates to a method for producing three-dimensional

Models made of plastic in accordance with stored, geometric data with the aid of laser beams of a wavelength of 200 to 20,000 nm, preferably 500 to 10,000 nm, controlled according to this data, laser beams being directed to certain spatial zones of a bed of a fine-grained plastic powder in accordance with the geometric data, and the material is fused or sintered, characterized in that a bead polymer made from homo- or copolymer of monoethylenically unsaturated compounds, preferably a copolymer made from methyl methacrylate or styrene, with an average particle diameter of 2 to 200 μm is used as the plastic powder.

When determining the particle diameter (particle size), the weight average is given here.

Bead polymers with an average particle diameter of 5 to 100 μm are particularly suitable for the process according to the invention.

The bead polymers to be used according to the invention have much more favorable flow properties than ground other plastics and therefore do not require any flow aids to improve their flow properties.

Another important advantage of the bead polymer is that it does not leave any troublesome residues when it is incinerated, for example as the core of a hollow ceramic mold. In the case of ground plastic particles mixed with free-flowing auxiliaries, it was observed that they do not incinerate without residues. This is of particular importance if the plastic models, which are primarily created using laser sintering, are further processed in subsequent processes for investment casting. For this purpose, for example after brushing the model produced with the method according to the invention with wax to further improve the model surface, the model is immersed in a slurried ceramic mass and the model coated with ceramic material is fired in the furnace. The model should burn completely when fired and leave the free hollow shape made of ceramic. Since conventional ground plastics do not burn completely due to the flow aid, the metallic models subsequently cast in the ceramic mold often have surface inaccuracies.

Another advantage of the use of bead polymers results in the surface accuracy and surface roughness of the models produced by the process according to the invention. Due to their round shape and their good flow properties, the models made with the preferred bead polymers are smoother and therefore more precise.

The bead polymers for the purposes of the present invention are polymer particles which are largely spherical. Different processes for the production of spherical particles are known, for example polymerization processes such as suspension or. Bead polymerization, dispersion polymerization, seed feed polymerization, further atomization techniques and precipitation processes. So bead polymers with a particle size of about 10 to 200 microns can be obtained by suspension polymerization or bead polymerization. The term suspension polymerization is understood to mean a process in which a monomer or a monomer-containing mixture which contains an initiator soluble in the monomer (s) in a phase which is essentially immiscible with the monomer (s) and which contains a dispersant, in the form of droplets, optionally in a mixture with small, solid particles, and is cured by increasing the temperature with stirring. Further details of the suspension polymerization are published, for example, in the publication "Polymer Processes" CE Schildknecht, published in 1956 by Interscience Publishers, Inc. New York, described in the chapter "Polymerization in Suspension" on pages 69 to 109.

Bead polymers with a particle size of 2 to 10 μm can be produced by the so-called dispersion polymerization. A suitable method is described, for example, in EP-A-610 522. Dispersion polymerization uses a solvent in which the monomers used are soluble but the polymer formed is insoluble. Dispersion polymerization generally provides bead polymers with a narrow particle size distribution.

The bead polymers to be used according to the invention preferably consist of homopolymers or copolymers of monoethylenically unsaturated compounds (monomers). For the purposes of the invention, copolymers are understood to be polymers which are composed of two or more different monomers. Suitable monomers are e.g. Styrene, alpha-methylstyrene, chlorostyrene,

Acrylic acid esters, such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyl acrylate, methacrylic acid esters, such as methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, iso-butyl methacrylate, hexyl methacrylate, 2-ethyl decyl methyl acrylate, methacrylate methacrylate, Methacrylonitrile, methacrylamide and vinyl acetate.

Homo- and copolymers of methacrylic acid esters and / or acrylic acid esters are particularly preferred. Polymethyl methacrylate and copolymers with a proportion of more than 60% by weight of methyl methacrylate units are particularly preferred. Well-suited copolymers are, for example, those which are 60 to 98

% By weight of methyl methacrylate units and 2 to 40% by weight of units of acrylic acid esters and / or methacrylic acid esters having 4 to 18 carbon atoms in the alcohol part, in particular copolymers of methyl methacrylate with one or more monomers from the group: n-butyl acrylate, iso- Butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyl acrylate, n-butyl methacrylate, iso-butyl methacrylate acrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, dodecyl methacrylate, stearyl methacrylate.

It has been shown that the molecular weight of the bead polymers can be important for the suitability for the process according to the invention. The molecular weight (weight average, Mw) should in particular be from 10,000 to 1,000,000, preferably from 10,000 to 500,000, particularly preferably from 20,000 to 250,000 g / mol. To set the desired molecular weight, molecular weight regulators can be used in the preparation of the bead polymers. Suitable molecular weight regulators are especially sulfur compounds, e.g. n-butyl mercaptan, dodecyl mercaptan, ethyl thioglycolate and diisopropyl xanthogen disulfide. The sulfur-free regulators mentioned in DE 3 010 373 are also very suitable for adjusting the molecular weight, for example the enol ethers of the formula I.

Suitable laser types are all that bring the bead polymer to sintering, fusing or crosslinking, in particular CO2 laser (10 μm) ND-YAG laser (1,060 nm) He-Ne laser (633 nm) or dye laser (350-1,000 nm). A CO ₂ laser is used before.

The energy density in the bed is preferably from 0.1 to 10 J / mm ³ during irradiation.

The effective diameter of the laser beam is preferably from 0.01 to 0.5 nm, preferably 0.1 to 0.5 nm, depending on the application. Pulsed lasers are preferably used, a high pulse frequency, in particular from 1 to 100 kHz, having proven particularly suitable.

The preferred procedure can be described as follows: The laser beam strikes the uppermost layer of the bed of the material to be used according to the invention and sinters the material in a certain layer thickness. This layer thickness can be from 0.01 mm to 1 mm, preferably from 0.05 to 0.5 mm. In this way, the first layer of the desired component is created. The working space is then lowered by an amount which is less than the thickness of the sintered layer. The work space is filled up to the original height with additional polymer material. By re-irradiation with the laser, the second layer of the component is sintered and connected to the previous layer. By repeating the process, the additional layers are created until the component is finished.

The exposure speed when scanning the laser is preferably 1 to 1,000 mm / s. Typically a speed of about 100 mm / s is used.

The invention also relates to the models obtainable by the process according to the invention.

Another object of the invention is the use of the models which are produced by the method according to the invention for the production of preforms, in particular of ceramic, for the investment casting of metals.

The invention is explained in more detail below by way of example with reference to FIG. 1, without the invention being restricted thereby in detail.

Figure 1 shows the simplified schematic representation of a rapid prototyping

Investment. Examples

Examples 1 to 6 below show the production of suitable fine-particle plastic material for laser sintering.

The rapid prototyping system used to manufacture the models has the following basic structure (FIG. 1).

The beam from an IR laser 1 is directed via a deflecting mirror 2 according to the specification of a scanner unit (not shown) onto the surface of the bed 4 of a bead polymer, which is held in a round shape 5 with a movable lower punch 6.

Layers 3a, 3b of sintered plastic material are formed by the exposure. After each exposure and generation of a layer (e.g. 3b), the

Stamp 6 lowered by a layer thickness and the bed 4 supplemented with new plastic material, which is exposed in the next step, whereby the next layer 3 a of the model body is generated.

example 1

Production of a bead polymer with a particle size of 5.3 μm by dispersion polymerization

In a reaction reactor with reflux condenser, stirrer and thermometer

560 g of polyvinylpyrrolidone, 80 g of methyltricaprylammonium chloride, 6.4 g of azodiisobutyronitrile and 0.64 g of dodecyl mercaptan dissolved in a solvent mixture of 14 l of methanol and 2 l of ethanol. 950 g of methyl methacrylate and 50 g of n-butyl acrylate were added to the solution. The resulting mixture was refluxed with stirring for 5 hours and then cooled to 25 ° C. The educated

Bead polymer was isolated by centrifugation, washed with methanol and dried at 50 ° C. 780 g of a bead polymer with an average particle size of 5.3 μm were obtained. The average molecular weight M _w was 110,000 g / mol.

Example 2

Production of a bead polymer with a particle size of 45 μm by bead polymerization.

450 g methyl methacrylate, 50 g ethyl hexyl acrylate, 5 g dibenzoyl peroxide and 1 g

Enol ethers according to formula I were mixed to form a homogeneous solution. The solution was transferred to a stirred reactor which had previously been treated with 1.5 liters of a 1% strength by weight aqueous alkaline solution of a copolymer of 50% by weight methacrylic acid and 50% by weight adjusted to pH 8 with sodium hydroxide solution .-% methyl methacrylate had been filled. The stirring speed was set at 420 revolutions per minute. The temperature was held at 78 ° C for 8 hours and then at 85 ° C for 1 hour. The mixture was then cooled to room temperature, the bead polymer obtained was isolated by decanting, washed several times with water and dried at 60.degree. 465 g of a bead polymer with an average particle size of 45 μm were obtained. The middle one

Molecular weight M _w was 125,000 g / mol.

Example 3

Production of a bead polymer with a particle size of 50 μm by bead polymerization.

600 g of methyl methacrylate, 9 g of dibenzoyl peroxide (75% in water) and 12 g of enol ether according to formula I were mixed to form a homogeneous solution. The solution was transferred to a stirred reactor which previously consisted of a solution

2.0 liters of water and 8 g of polyvinyl pyrrolidone K90 had been filled. The The speed of the blade stirrer was set at 600 revolutions per minute. The temperature was held at 55 ° C for one hour, then at 75 ° C for 12 hours and then at 90 ° C for 4 hours. The mixture was then cooled to room temperature, the bead polymer obtained was isolated by filtration through a 32 μm filter cloth, washed several times with water and dried at 35 ° C. under 20 mbar vacuum. After sieving through a 125 μm sieve, a bead polymer with a Staudinger index, [η], (Ubbelohde capillary viscometer in chloroform at 25 ° C.) of 0.50 dl / g was obtained, which has a molecular weight M _w of 110,000 g / Mol corresponds.

Example 4

Production of a bead polymer with a particle size of 40 μm by bead polymerization.

510 g methyl methacrylate, 90 g n-butyl methacrylate, 9 g dibenzoyl peroxide (75% in

Water) and 12 g of enol ether according to formula I were mixed to a homogeneous solution. The solution was transferred to a stirred reactor which had previously been filled with a solution consisting of 2.0 liters of water and 8 g of polyvinylpyrrolidone K90. The speed of the blade stirrer was set to 600 revolutions per minute. The temperature was held at 55 ° C for one hour, then at 75 ° C for 12 hours and then at 90 ° C for 4 hours. The mixture was then cooled to room temperature, the bead polymer obtained was isolated by filtration through a 32 μm filter cloth, washed several times with water and dried at 35 ° C. under 20 mbar vacuum. After sieving through a 125 μm sieve, a bead polymer with a Staudinger index, [η], (Ubbelohde capillary viscometer in

Chloroform at 25 ° C) of 0.48 dl / g, which corresponds to a molecular weight M _w of 100,000 g / mol. Example 5

600 g of methyl methacrylate, 6 g of dibenzoyl peroxide (75% in water) and 3 g of enol ether according to formula I were mixed to form a homogeneous solution. The solution was transferred to a stirred reactor which had previously been filled with a solution consisting of 2.0 liters of water and 20 g of K90 polyvinylpyrrolidone. The speed of the blade stirrer was set to 600 revolutions per minute.

The temperature was held at 55 ° C for one hour, then at 75 ° C for 12 hours and then at 90 ° C for 4 hours. The mixture was then cooled to room temperature, the bead polymer obtained was isolated by filtration through a 32 μm filter cloth, washed several times with water and dried at 35 ° C. under 20 mbar vacuum. After sieving through a 125 μm sieve, a bead polymer with a Staudinger index, [η], (Ubbelohde capillary viscometer in chloroform at 25 ° C.) of 1.01 dl / g was obtained, which had a molecular weight M _w of 250,000 g / Mol corresponds.

Example 6

510 g of methyl methacrylate, 90 g of n-butyl methacrylate, 6 g of dibenzoyl peroxide (75% in water) and 3 g of enol ether according to formula I were mixed to form a homogeneous solution. The solution was transferred to a stirred reactor which had previously been filled with a solution consisting of 2.0 liters of water and 20 g of K90 polyvinylpyrrolidone. The speed of the blade stirrer was set to 600 revolutions per minute. The temperature was held at 55 ° C for one hour, then at 75 ° C for 12 hours and then at 90 ° C for 4 hours. The mixture was then cooled to room temperature, the bead polymer obtained by filtering through a 32 μm Filter cloth isolated, washed several times with water and dried at 35 ° C under 20 mbar vacuum. After sieving through a 125 μm sieve, a bead polymer with a Staudinger index, [η], (Ubbelohde capillary viscometer in chloroform at 25 ° C.) of 0.96 dl / g was obtained, which has a molecular weight M _w of 230,000 g / Mol corresponds.

Example 7

The flowability of some polymers was quantified by means of determination of the angle of repose and determination of the flow angle. The angle of repose is here

Understand the angle between the flank of a pouring cone of the powdery polymer and the vertical on the base of the pouring cone. The cone is created by discharging the bulk material from a funnel with a narrow outlet onto a flat surface.

The flow angle is the angle that an inclined surface makes in relation to the horizontal, and at which bulk material spread over the surface begins to flow under the influence of gravity.

As can be seen from Table 1, it can be seen that a bead polymer according to Example 4

Has advantages over amorphous ground materials such as polycarbonate and polystyrene.

The addition of pouring aids, as is the case with the ground materials, can be dispensed with due to the round shape. Angle of repose determinations confirm the good processability in the sintering plants. It has been shown that, because of their shape, bead polymers are more flowable than ground polymer granules with added flow aids. Table 1 Free-flowing determination of sintered materials

*) Polycarbonate with an average particle size of 40 μm **) Polystyrene with an average particle size of 40 μm

The smaller the flow angle and angle of repose, the more flowable the material. The flowability increases with increasing diameter of the pouring cone.

Example 8

The ashing of some polymers was also examined (for results see Table 2).

The ashing of the materials has shown that no residues remain when the bead polymer from Example 4 is used. As could be shown for the ground polymer materials with additives, the plastics with free-flowing aids do not ashes completely free of residues. Table 2 Ashing of model building materials

Example 9

Production of model bodies. To produce the model bodies with the dimensions of 50x10x4mm ³ , the plastic powder is sintered in a model system. For this purpose, the plastic powder is exposed in layers at a speed of 1 mm s under an infrared laser with a wavelength of 10,000 nm (CO 2 laser) at a maximum temperature of 500 ° C. The plastic powders tested from Examples 7 and 8 were sintered.

Example 10

The quality of the model bodies from Example 9 was also checked.

The fundamentally suitable materials polycarbonate and polystyrene also deliver satisfactory results (see Table 3). However, there is a particular advantage for the surface accuracy and roughness when using bead polymers. Due to the round shape, the surfaces of the models become smoother, as the following results show using a perthemeter. This not only has the advantage that more precise plastic models (preform) can be built, but also that the investment casting models cast on the basis of the preform become more precise.

Table 3 Surface roughness of sintered samples

Claims

claims

1. Process for the production of three-dimensional models from plastic in accordance with stored, geometric data with the aid of laser beams of a wavelength from 200 to controlled according to this data

10,000 nm, wherein one or more laser beams are directed according to the geometric data to certain spatial zones of a bed of a fine-grained plastic powder and the material is fused or sintered, characterized in that as a plastic powder, a bead polymer made of homo- or copolymer of monoethylenically unsaturated compounds with a average particle diameter of 2 to 200 microns is used.

2. The method according to claim 1, characterized in that as the plastic material bead polymer with an average particle diameter of 5 to

100 μm is used.

3. The method according to claim 2, characterized in that homopolymer or copolymer of monoethylenically unsaturated compounds, in particular styrene, alpha-methylstyrene, chlorostyrene, acrylic acid esters, such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyl acrylate, methacrylic acid acrylate, as the pearl polymer , such as methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, iso-butyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, dodecyl methacrylate, stearyl methacrylate, furthermore acrylonitrile, methacrylonitrile, methacrylamide and vinyl acetate are used.

4. The method according to claim 3, characterized in that the bead polymer is a homo- or copolymer of methacrylic acid esters and / or acrylic acid esters, preferably polymethyl methacrylate or copolymers with a proportion of more than 60 wt .-% methyl methacrylate units.

. A method according to claim 3, characterized in that the bead polymer is a copolymer containing 60 to 98 wt .-% methyl methacrylate units and 2 to 40 wt .-% units of acrylic acid esters and / or methacrylic acid esters with 4 to 18 carbon atoms in the alcohol part has, in particular copolymers of methyl methacrylate with one or more monomers from the group: n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyl acrylate, n-butyl methacrylate, isobutyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, decyl - acrylate, stearyl methacrylate.

6. The method according to any one of claims 2 to 5, characterized in that the molecular weight (weight average, Mw) of the bead polymer from 10,000 to 1,000,000, preferably from 10,000 to 500,000, particularly preferably from 20,000 to 250,000 g / Mole.

7. Three-dimensional models obtainable by the method according to the invention according to one of claims 1 to 6.

8. Use of the models according to claim 7 for the production of preforms, in particular of ceramic, for the investment casting of metals.

9. Use of bead polymers with an average particle diameter of 2 to 200 microns, preferably from 5 to 100 microns, as a material for laser sintering.

10. Use according to claim 9, characterized in that homopolymer or copolymer of monoethylenically unsaturated compounds, in particular styrene, alpha-methylstyrene, chlorostyrene, acrylic acid esters, such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, as bead polymer.

Dodecyl acrylate, methacrylic acid esters, such as methyl methacrylate, ethyl meth acrylate, isopropyl methacrylate, n-butyl methacrylate, iso-butyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, dodecyl methacrylate, stearyl methacrylate, furthermore acrylonitrile, methacrylonitrile, methacrylamide and vinyl acetate is used.

11. Use according to claim 10, characterized in that the bead polymer is a homo- or copolymer of methacrylic acid esters and / or acrylic acid esters, preferably polymethyl methacrylate or copolymers with a proportion of more than 60% by weight of methyl methacrylate units.

12. Use according to claim 10, characterized in that the bead polymer is a copolymer containing 60 to 98 wt .-% methyl methacrylate units and 2 to 40 wt .-% units of acrylic acid esters and / or methacrylic acid esters with 4 to 18 carbon atoms in the alcohol part has, in particular copolymers of methyl methacrylate with one or more monomers from the group: n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyl acrylate, n-butyl methacrylate, isobutyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, decyl methyl acrylate, decyl methacrylate, decyl methacrylate, .

13. Use according to one of claims 9 to 12, characterized in that the molecular weight (weight average, Mw) of the bead polymer from 10,000 to 1,000,000, preferably from 10,000 to 500,000, particularly preferably from 20,000 to 250,000 g / Mole.