METHOD FOR SELECTIVE LASER SINTERING, USING THERMOPLASTIC POLYMER POWDERS

Abstract

The present invention relates to a process for producing a three-dimensional component by means of selective laser sintering (SLS), wherein a processing temperature T.sub.x is established in a build chamber, and a powder layer consisting of a thermoplastic polymer powder is provided in the build chamber. The thermoplastic polymer powder comprises a blend of a semicrystalline polymer, an amorphous polymer and a polymeric compatibilizer. The polymer powder is then melted in a spatially resolved manner by means of a directed beam of electromagnetic radiation, wherein binding of the regions of the melted and resolidified polymer layer by layer in multiple steps affords a three-dimensional component. In the process of the invention, the temperature in the build chamber during the performance of the individual steps varies by not more than +/−10% from the processing temperature T.sub.x set. In addition, the processing temperature T.sub.x differs by not more than +/−20 K from the processing temperature T.sub.x(A) of a polymer powder comprising the corresponding semicrystalline polymer as the sole polymeric component.

Claims

1. A process for producing a three-dimensional component by means of selective laser sintering, comprising the steps of: x) setting a processing temperature T.sub.x in a build chamber and providing a powder layer consisting of a thermoplastic polymer powder P in the build chamber, where the thermoplastic polymer powder P comprises: (A) 10% to 89.9% by weight, based on the overall polymer powder P, of at least one semicrystalline polymer A; (B) 10% to 89.9% by weight, based on the overall polymer powder P, of at least one amorphous polymer B; (C) 0.1% to 20% by weight, based on the overall polymer powder P, of at least one compatibilizer C; (D) optionally 0% to 5% by weight, based on the overall polymer powder P, of at least one additive and/or auxiliary; where the sum total of the percentages by weight of components A, B, C and optionally D together is 100% by weight; and where the semicrystalline polymer A, the amorphous polymer B and the compatibilizer C are in the form of a polymer blend; xi) spatially resolved melting by means of a directed beam of electromagnetic radiation, followed by solidification of the thermoplastic polymer powder P in a defined region; where steps x) and xi) are performed repeatedly, such that binding of the regions of the melted and resolidified polymer forms a three-dimensional component layer by layer; (i) where the temperature in the build chamber during the performance of the individual steps x) and xi) of the process varies by not more than +/−10% from the processing temperature T.sub.x set; (ii) and where the processing temperature T.sub.x for the polymer powder P differs by not more than +/−20 K from the processing temperature T.sub.x(A) of a polymer powder comprising the corresponding semicrystalline polymer A as the sole polymeric component.

2. The process for producing a three-dimensional component according to claim 1, characterized in that the powder layer has a thickness in the range from 10 to 400 μm.

3. The process for producing a three-dimensional component according to claim 1 or 2, characterized in that the polymer powder P has a median particle diameter D50 in the range from 5 to 200 μm.

4. The process for producing a three-dimensional component according to any of claims 1 to 3, characterized in that the processing temperature T.sub.x is in the range from 80 to 250° C.

5. The process for producing a three-dimensional component according to any of claims 1 to 4, characterized in that volume shrinkage in the course of production of the three-dimensional component is reduced by at least 10% by means of selective laser sintering using the polymer powder P compared to the volume shrinkage when using a polymer powder comprising the corresponding semicrystalline polymer A as the sole polymeric component.

6. The process for producing a three-dimensional component according to any of claims 1 to 5, characterized in that warpage in the course of production of the three-dimensional component is reduced by at least 10% by means of selective laser sintering using the polymer powder P compared to the warpage when using a polymer powder comprising the corresponding semicrystalline polymer A as the sole polymeric component.

7. The process for producing a three-dimensional component according to any of claims 1 to 6, characterized in that the processing window of the polymer powder P in the process of selective laser sintering is from 10 to 80 K.

8. The process for producing a three-dimensional component according to any of claims 1 to 7, characterized in that the porosity of the three-dimensional component produced from the polymer powder P is at least 10% lower than the porosity of a component produced from the corresponding amorphous polymer B as the sole polymeric component.

9. The process for producing a three-dimensional component according to any of claims 1 to 8, characterized in that the semicrystalline polymer A is at least one polymer selected from polyamides, polyoxymethylene, polyether ketones, polylactides, semicrystalline polystyrene, polyethylene terephthalate, polybutylene terephthalate and semicrystalline polyolefins.

10. The process for producing a three-dimensional component according to any of claims 1 to 9, characterized in that the amorphous polymer B is at least one polymer selected from the group consisting of styrene-acrylonitrile copolymers, acrylonitrile-butadiene-styrene copolymers, acrylate-styrene-acrylonitrile copolymers, methyl methacrylate-acrylonitrile-butadiene-styrene copolymers, methyl methacrylate-butadiene-styrene copolymers, α(alpha)-methylstyrene-acrylonitrile copolymers, styrene-methyl methacrylate copolymers, amorphous polystyrene and impact-modified polystyrene.

11. The process for producing a three-dimensional component according to any of claims 1 to 10, characterized in that the compatibilizer C is at least one copolymer selected from the group consisting of styrene-maleic anhydride copolymers, styrene-acrylonitrile-maleic anhydride terpolymers, styrene-N-phenylmaleimide-maleic anhydride terpolymers, methyl methacrylate-maleic anhydride copolymers, styrene-butadiene block copolymers, styrene-polyolefin copolymers, styrene-butadiene-polyolefin copolymers, acrylonitrile-styrene-polyolefin copolymers and acrylonitrile-styrene-butadiene-polyolefin copolymers.

12. The process for producing a three-dimensional component according to any of claims 1 to 11, characterized in that the amorphous polymer B is at least one styrene polymer or styrene copolymer having a melt volume flow rate, measured to ISO 1133, in the range from 2 to 60 cm.sup.3/10 min.

13. The process for producing a three-dimensional component according to any of claims 1 to 12, characterized in that the polymer powder P comprises: (A) 20% to 79.9% by weight, based on the overall polymer powder P, of at least one polymer selected from polyamides, polyoxymethylene, polyether ketones, polylactides, semicrystalline polystyrene, polyethylene terephthalate, polybutylene terephthalate and semicrystalline polyolefins as semicrystalline polymer A; (B) 20% to 79.9% by weight, based on the overall polymer powder P, of at least one polymer selected from the group consisting of styrene-acrylonitrile copolymers (SAN), acrylonitrile-butadiene-styrene copolymers (ABS), acrylate-styrene-acrylonitrile copolymers (ASA), methyl methacrylate-acrylonitrile-butadiene-styrene copolymers (MABS), methyl methacrylate-butadiene-styrene copolymers (MBS), α(alpha)-methylstyrene-acrylonitrile copolymers (AMSAN), styrene-methyl methacrylate copolymers (SMMA), amorphous polystyrene (PS) and impact-modified polystyrene (HIPS) as amorphous polymer B; (C) 0.1% to 20% by weight, based on the overall polymer powder P, of a copolymer selected from the group consisting of styrene-acrylonitrile-maleic anhydride terpolymers, styrene-N-phenylmaleimide-maleic anhydride terpolymers, methyl methacrylate-maleic anhydride copolymers, styrene-butadiene block copolymers, styrene-polyolefin copolymers, acrylonitrile-styrene-polyolefin copolymers and acrylonitrile-styrene-butadiene-polyolefin copolymers as compatibilizer C; (D1) optionally 0% to 3% by weight of at least one silicon dioxide nanoparticle powder or silicone additive as free-flow aid, and (D2) optionally 0% to 3% by weight, based on the overall polymer powder P, of at least one further additive and/or auxiliary as further component D.

14. A thermoplastic polymer powder P comprising: (A) 10% to 89.9% by weight, based on the overall polymer powder P, of at least one semicrystalline polymer A; (B) 10% to 89.9% by weight, based on the overall polymer powder P, of at least one amorphous polymer B; (C) 0.1% to 20% by weight, based on the overall polymer powder P, of at least one compatibilizer C; (D) optionally 0% to 5% by weight, based on the overall polymer powder P, of at least one additive and/or auxiliary; where the sum total of the percentages by weight of components A, B, C and optionally D together is 100% by weight; where the semicrystalline polymer A, the amorphous polymer B and the compatibilizer C are in the form of a polymer blend; where the thermoplastic polymer powder P has a median particle diameter D50 in the range from 5 to 200 μm; wherein the processing window of the thermoplastic polymer powder P in the process of selective laser sintering is in the range from 80 to 250° C., and wherein the processing window of the thermoplastic polymer powder P differs by not more than +/−20 K from the processing window of a polymer powder comprising the corresponding semicrystalline polymer A as the sole polymeric component.

15. The use of the thermoplastic polymer powder P according to claim 14 for production of a three-dimensional component by means of selective laser sintering or related methods of additive manufacture.

Description

ELUCIDATION OF THE DRAWINGS

[0167] FIG. 1 shows an illustrative representation of the particle size distribution of a powder of the invention. What is shown is the particle size distribution density q.sub.3(x) or cumulative particle size distribution Q.sub.3(x) as a function of particle size x in μm (micrometers). The D.sub.10 (x.sub.10,3), D.sub.50 (x.sub.50,3) and D.sub.90 (x.sub.90,3) values are stated.

[0168] FIG. 2 shows an illustrative representation of diffuse reflection R (in %) as a function of wavenumber k (in cm.sup.−1) for some of the powders of the invention.

[0169] FIG. 3 shows the DSC curves of powder P1 of the invention with a first heating operation (1. AH), cooling (K) and second heating operation (2. AH). What is plotted is the amount of heat supplied or removed (in mW/mg of sample) as a function of temperature T (in ° C.). The results for the cooling (K) are: peak (crystallization) 118.3° C.; onset 110.5° C.; end 122.9° C.; area −52.91 J/g; glass transition T.sub.g 67.5° C. The results for the first heating operation (1. AH) are: peak (melting) 165.7° C.; onset 151.7° C.; end 171.1° C.; area 47.75 J/g; glass transition T.sub.g 100.6° C. The results for the second heating operation (2. AH) are: peak (melting) 162.1° C.; onset 153.6° C.; end 168.4° C.; area 50.63 J/g; glass transition T.sub.g 101.5° C.

[0170] FIG. 4 shows the DSC curves of powder P4 of the invention with a first heating operation (1. AH), cooling (K) and second heating operation (2. AH). What is plotted is the amount of heat supplied or removed (in mW/mg of sample) as a function of temperature T (in ° C.). The results for the cooling K are: peak (crystallization) 118.2° C.; onset 113.3° C.; end 123.0° C. The results for the first heating operation (1. AH) are: peak (melting) 165.5° C.; onset 151.6° C.; end 171.3° C.

[0171] The results for the second heating operation (2. AH) are: peak (melting) 162.2° C.; onset 155.5° C.; end 168.5° C.

[0172] FIG. 5 shows transmission electron micrographs of specimens that have been produced from the blends of the invention: 5a) illustrative polymer blend (example P1) with good compatibilization, 5b) comparative image of an uncompatibilized polymer blend (comparative example V1).

[0173] The invention is elucidated further by the examples and claims that follow.

EXAMPLES

1.1 Components Used

[0174] The following semicrystalline polyolefins A1 and A2 were used as component A: [0175] A1 isotactic PP (100-HR25, INEOS Olefins & Polymers) [0176] A2 LD-PE (18R430, INEOS Olefins & Polymers)

[0177] Component B1 used was a highly impact-resistant acrylonitrile-butadiene-styrene (ABS) polymer of the Terluran® type (INEOS Styrolution, Frankfurt) having a melt volume flow rate (MVR 220° C./load 10 kg, ISO 1133) of about 6 cm.sup.3/10 min.

[0178] Component B2 used was an impact-resistant amorphous polystyrene (HIPS) (INEOS Styrolution, Frankfurt) having a melt volume flow rate (melt volume rate 200° C./load 5 kg, ISO 1133) of about 4 cm.sup.3/10 min.

[0179] The following compatibilizers are used as component C: [0180] C1 star-shaped styrene-butadiene block copolymer, Styrolux® type (INEOS Styrolution), butadiene content 25% by weight, melt volume flow rate (MVR) to ISO 1133 of 11 cm.sup.3/10 min; [0181] C2 linear styrene-butadiene block copolymer, Styroflex® type (INEOS Styrolution) of the S-(B/S)-S structure, butadiene content 35% by weight, melt volume flow rate (MVR) to ISO 1133 of 13 cm.sup.3/10 min; [0182] C3 styrene-ethylene-propylene block copolymer (Septon 2104, Kuraray Europe); [0183] C4 styrene-ethylene-butylene block copolymer (G 1650 E, Kraton Polymers); [0184] C5 styrene-ethylene-propylene block copolymer (G 1701 E, Kraton Polymers); [0185] C6 polyethylene-acrylonitrile-styrene copolymer (Modiper AS100, NOF Corp.); [0186] C7 polypropylene-acrylonitrile-styrene copolymer (Modiper A3400, NOF Corp.)

[0187] An antistat was used as component D.

[0188] The polymer mixtures (polymer blends) P1 to P23 and V1 to V8 were produced as described under 1.2. The illustrative polymer blends are summarized in table 1 below. Compositions V1 to V8 are comparative experiments (without addition of the compatibilizer C).

TABLE-US-00001 TABLE 1 Compositions of the polymer blends (all values in % by weight based on the overall polymer blend) Ex. A1 A2 B1 B2 C1 C2 C3 C4 C5 C6 C7 D P1 46.0 46.0 8.0 P2 46.0 46.0 8.0 P3 23.0 69.0 8.0 P4 69.0 23.0 8.0 P5 45.5 45.5 8.0 1.0 P6 45.5 45.5 8.0 1.0 P7 46.0 46.0 8.0 P8 49.5 49.5 1.0 P9 48.5 48.5 3.0 P10 47.5 47.5 5.0 P11 46.0 46.0 8.0 P12 49.5 49.5 1.0 P13 48.5 48.5 3.0 P14 47.5 47.5 5.0 P15 49.5 49.5 1.0 P16 48.5 48.5 3.0 P17 47.5 47.5 5.0 P18 49.5 49.5 1.0 P19 48.5 48.5 3.0 P20 47.5 47.5 5.0 P21 46.0 46.0 8.0 P22 46.0 46.0 4.0 4.0 P23 46.0 46.0 4.0 4.0 V1 50.0 50.0 V2 50.0 50.0 V3 49.5 49.5 1.0 V4 49.5 49.5 1.0 V5 100 V6 100 V7 100 V8 100

1.2 Production of the Polymer Blends

[0189] All materials were predried at 80° C. for 14 hours. The semicrystalline polyolefin A, the amorphous polymer B, the compatibilizer C and any component D were compounded in a corotating twin-screw extruder of the Process 11 brand, manufacturer: Thermo Scientific, at a melt temperature of 220° C. to 240° C. The screw diameter of the twin-screw extruder was 11 mm; the screw speed was 220 rpm. Subsequently, the material was extruded through an extrusion die having a diameter of 2.2 mm into a water bath and pelletized. The throughput was between 1.5 and 2.3 kg/h.

1.3 Characterization of the Blends

[0190] The polymer blends were characterized using tensile specimens of the 1A type to ISO 527 that were produced by means of injection molding.

[0191] The notched impact resistance a.sub.k of the polymer blends was determined to ISO 179 1eA. Tensile tests were conducted to ISO 527. The results of the tests are listed in table 2.

[0192] The mechanical properties thus determined on the injection-molded tensile specimens are considered to be an indication of the quality of the polymer blends. Transmission electron micrographs were taken as a further indication of good compatibilization of the blends. FIG. 5a shows an illustrative image of polymer blend P1 of the invention, and FIG. 5b, by way of comparison, an image of the uncompatibilized polymer blend V1.

TABLE-US-00002 TABLE 2 Characterization of the polymer blends Notched impact Modulus of Tensile Elongation resistance elasticity strength σ.sub.M at break Sample a.sub.k [kJ/m.sup.2] E.sub.t [MPa] [MPa] ε.sub.B [%] P1 4.1 1200 25.7 52.9 P2 1.9 420 10.8 8.1 P3 5.6 1490 25.9 23.8 P4 5.8 1460 28.8 101.9 P5 14.4 241 12.6 23.9 P6 2.7 438 9.9 15.5 P7 2.1 1470 25.7 9.1 P8 1.9 1460 24.5 2.3 P9 1.9 1460 24.6 2.3 P10 2.0 1450 24.8 2.4 P11 5.0 962 18.1 3.2 P12 2.9 1500 25.8 4.3 P13 2.1 1530 25.6 3.9 P14 2.2 1530 25.9 3.8 P15 1.3 1500 24.9 2.1 P16 1.3 1460 25.3 3.5 P17 1.4 1440 23.2 1.9 P18 2.3 1500 24.8 3.9 P19 2.1 1440 24.3 4.0 P20 2.3 1410 23.0 3.6 P21 4.2 1410 27.2 10.6 P22 4.2 1400 27.3 14.6 P23 7.8 1220 25.1 75.5 V1 1.9 1440 24.4 3.1 V2 2.3 1490 24.2 4.8 V3 3.1 665 12.5 4.8 V4 2.6 506 7.7 7.6

1.4 Production of the Polymer Powders P

[0193] The polymer blends (pellets produced according to 1.2) were micronized in two stages. First of all, the pellets that had been precooled with liquid nitrogen were comminuted in a high-speed rotor mill (Pulverisette 14, manufacturer: Fritsch).

[0194] Thereafter, the powders thus obtained were ground to ultrafine powders in a stirred ball mill (PE5, manufacturer: Netzsch) with ZrO.sub.2 grinding balls in ethanol.

1.5 Characterization of the Polymer Powders P

1.5.1 Particle Size Distribution

[0195] Particle size distribution was measured by means of laser diffractometry in a Mastersizer 2000 (manufacturer: Malvern Instruments). The measurement for sample P1 is shown by way of example in FIG. 1. All the polymer powders produced had a median particle diameter D50 in the range from 25 to 55 μm.

1.5.2 Optical Properties

[0196] An important optical property of the polymer powders is their ability to absorb the energy introduced by the laser.

[0197] The absorption of the powders was analyzed by means of diffuse reflection infrared Fourier transformation spectroscopy (DRIFTS). The wavenumber of the laser used in the SLS process was 943 cm.sup.−1, and so the absorption of the polymer in this range was of particular relevance. For the analysis, an FTIR spectrometer (Nicolet 6700, manufacturer: Thermo Scientific) with DRIFTS accessory from PIKE technologies was used. FIG. 2 shows illustrative measurements that show a low reflection and hence high absorption of the powders of the invention at 943 cm.sup.−1.

1.5.3 Thermal Properties

[0198] The estimation of the processing temperature and the determination of the processing window in the SLS process are typically effected on the basis of DSC measurements in accordance with DIN EN ISO 11357. For this purpose, a Q 2000 DSC instrument (manufacturer: TA Instruments) was used. The measurements were conducted with a heating and cooling rate of 10 K/min under a nitrogen atmosphere. The sample mass was about 5 mg.

[0199] FIG. 3 and FIG. 4 show, by way of example, the DSC curves of the polymer powders P1 and P4. It becomes clear that both polymer powders have an advantageously large processing window (difference between crystallization temperature and melting temperature) of about 44 K (kelvin).

2.1 Performance of the Laser Sintering Experiments

[0200] The polymer powders P1, P3, P4, P5, P12, V5-V8 produced according to 1.4 were used to conduct various selective laser sintering methods in order to test the suitability of the powders for selective laser sintering. Additionally tested as comparison V9 was a commercial PA12 powder for the SLS method (PA 2200, manufacturer: EOS GmbH). The results are compiled in table 3.

[0201] The experiments were conducted on a Formiga P110 (manufacturer: EOS) and on a DTM 2000 sintering station (manufacturer: 3D Systems). All tests were conducted under a nitrogen atmosphere.

[0202] The laser power was varied between 4 and 25 W. The scan speed, i.e. the speed with which the laser beam was moved over the powder bed, was varied between 1.0 and 3.4 m/s. The hatch distance (also called trace width) is defined as the distance between the intensity maxima of two laser lines running alongside one another, and was varied between 0.08 and 0.25 mm. The energy density per unit area was 0.01 to 0.085 J/mm.sup.2. The energy density per unit area is typically calculated from the laser power divided by the scan speed and the hatch distance.

[0203] The powder beds with the compositions P1 to P23 described in table 1 had a smooth surface and clear lines around the exposed polymer particles.

[0204] For assessment of the powders and for discovery of the optimal settings for laser power, scan speed and hatch distance, individual layers having an edge length of 40×40×0.1 mm were first produced. The use of individual layers as specimens generally allows the influence of the material application on the resultant melt depths to be balanced out, and the beam-material interaction to be analyzed directly. The small amount of sample of a few grams required additionally usually enables efficient analysis of the samples.

[0205] Once the optimal settings for laser power, scan speed and hatch distance had been found, tensile specimens of the 1A type to ISO 527 were produced.

2.2 Assessment of the Tensile Specimens Obtained by SLS

[0206] Notched impact resistance a.sub.k was determined to ISO 179 1eA on the test specimens obtained according to 2.1. Tensile tests were conducted to ISO 527. The values measured with regard to breaking strength and modulus of elasticity were compared with injection-molded test specimens made from the same polymer blends. The following classification was used for the mechanical properties:

TABLE-US-00003 + good 30% below injection molding ◯ moderate 50% below injection molding − poor 80% below injection molding

[0207] The assessment of some selected samples and comparative samples is detailed in table 3.

[0208] The surface quality of the test specimens was determined visually and with a microscope (Profilm3D Optical Profiler, manufacturer: Filmetrics, with 5× objective). The following classification was used here:

TABLE-US-00004 ++ very good smooth, small visual difference from injection- molded parts + good slightly corrugated, some visual difference from in- jection-molded parts ◯ moderate rough, high visual difference from injection-molded parts − poor very rough, very high visual difference from injec- tion-molded parts

[0209] The processing window indicates the difference between crystallization temperature and melting temperature, and was determined by means of differential scanning calorimetry (DSC).

[0210] Volume shrinkage, defined as the decrease in volume of a component in the course of cooling from the processing temperature T.sub.x (process temperature) to room temperature (especially 20° C.), was determined by measuring the geometric change in length in x, y and z direction and multiplying these three values.

[0211] Warpage, defined as the change in shape of a component in the course of cooling from the processing temperature T.sub.x (process temperature) to room temperature (especially 20° C.), was determined by measuring the geometric variance of a component edge from a straight line. This is dependent on the component geometry, for example on the length of the component edge, and so the assessment was made relative to a tensile specimen according to ISO 179 1eA. The comparative sample (reference) used was sample V9.

TABLE-US-00005 TABLE 3 Assessment of SLS tensile specimens Processing Volume Warpage Surface window shrinkage (relative to V9) Mechanical Ex. quality [° C.] C. [%] [% vs. V9] properties P1 ++ 118-162 44 1.0 10% lower + P3 ++ 118-162 44 0.8 15% lower + P4 ++ 118-162 44 1.2  5% lower + P5 ++  90-105 15 1.0 10% lower + P12 ++ 117-161 44 1.0 10% lower + V5 ∘ 116-163 47 1.5   5% higher ∘ V6 ∘  92-106 14 1.5   5% higher ∘ V7 − no 0.6 20% lower − crystallization V8 − no 0.6 20% lower − crystallization V9 + 147-184 37 1.5 Reference + (V9: PA12 powder, PA 2200, manufacturer: EOS GmbH)