Process for shaping a polymeric object

Abstract

The present invention relates to a process for moulding a polymeric object, the process comprising the steps of: (a) providing a substantially flat horizontally positioned layer of a material, the layer being a film or sheet; (b) providing a layer of a thermoplastic powder onto the layer provided under (a); (c) subjecting a pre-defined part of the thermoplastic powder to irradiation to heat the pre-defined part of the thermoplastic powder to a temperature at which the thermoplastic fuses or sinters onto at least a part of the flat layer of material; (d) terminating the exposure to irradiation; (e) providing a further layer of the thermoplastic powder onto the layer provided under (b); (f) subjecting a pre-defined part of the thermoplastic powder of the layer provided under (e) to irradiation to heat the pre-defined part of the thermoplastic powder to a temperature at which the thermoplastic fuses or sinters onto at least a part of the flat layer of the material fused under (c); (g) terminating the exposure to irradiation; (h) repeating steps (e)-(g) for as many cycles as required to complete the shape of the desired polymeric object; (i) removing the fraction of thermoplastic powder that has not been fused or sintered; and (j) optionally removing the substantially flat layer as provided under (a) from the polymeric object; wherein the surface area of the substantially flat layer of material is at least 200% of the surface area of that layer produced under steps (c)-(h) having the largest surface area. Such process allows for the production of small series or uniquely designed objects at low energy consumption wherein the objects exhibit high dimensional stability.

Claims

1. A process for shaping a polymeric object, the process comprising the steps of: (a) providing a substantially flat horizontally positioned layer of a material, the layer being a film or sheet; (b) providing a layer of a thermoplastic powder onto the substantially flat horizontally positioned layer provided under (a); (c) exposing a pre-defined part of the thermoplastic powder to irradiation to heat the pre-defined part of the thermoplastic powder to a temperature at which the thermoplastic fuses or sinters onto at least a part of the substantially flat horizontally positioned layer of material; (d) terminating the exposure to irradiation; (e) providing a further layer of the thermoplastic powder onto the layer provided in the previous step of providing a layer of the thermoplastic powder; (f) exposing a pre-defined part of the thermoplastic powder of the layer provided under (e) to irradiation to heat the pre-defined part of the thermoplastic powder to a temperature at which the thermoplastic fuses or sinters onto at least a part of the flat layer of the material fused under the previous step of fusing or sintering; (g) terminating the exposure to irradiation; (h) repeating steps (e)-(g) for as many cycles as required to complete the shape of the desired polymeric object; (i) removing a fraction of the thermoplastic powder that has not been fused or sintered; and (j) optionally removing the substantially flat horizontally positioned layer as provided under (a) from the polymeric object; wherein the surface area of the substantially flat horizontally positioned layer of material is larger than the surface area of the layer produced under steps (c)-(h) having the largest surface area, wherein the process involves repeating steps (a)-(h) after step (h) for one or more further cycles, wherein the substantially flat horizontally positioned layer(s) of a material provided in the subsequent cycle(s) (a)-(h) is(are) positioned on top of the fused thermoplastic obtained from the preceding step (h).

2. The process according to claim 1, wherein the substantially flat horizontally positioned layer of material is produced by providing a layer of the thermoplastic powder onto a surface; subjecting a pre-defined part of the thermoplastic powder to irradiation to heat the pre-defined part of the thermoplastic powder to a temperature at which the thermoplastic fuses or sinters; and terminating the exposure to irradiation, to form a film or sheet.

3. The process according to claim 1, wherein the substantially flat horizontally positioned layer is produced from the same material as the thermoplastic powder.

4. The process according to claim 1, wherein the thermoplastic powder that is provided under steps (b) and (e) is heated to a temperature of at most 50 C. below the peak melting temperature as determined via differential scanning calorimetry (DSC), first heating run, according to ISO 11357-3 (2011), prior to subjecting to the irradiation.

5. The process according to claim 1, wherein the thermoplastic powder is produced from a composition comprising one or more thermoplastic material(s) selected from semi-crystalline thermoplastics.

6. The process according to claim 5, wherein the composition comprises 80.0 wt % of the one or more thermoplastic material, with regard to the total weight of the composition.

7. The process according to claim 5, wherein the composition further comprises 0.01 and 5.0 wt % of one or more flow agent selected from silica, alumina, phosphate, borate, titania, talc, mica, kaolin, attapulgite, calcium silicate, and magnesium silicate, with regard to the total weight of the composition.

8. The process according to claim 1, wherein the process is performed using a moulding device comprising a horizontal flat surface surrounded by a wall allowing for the powder that is provided under steps (b) and (e) to be contained by the volume V defined by the horizontal flat surface and the wall, wherein the horizontal flat surface can be lowered stepwise to extend the height of the wall thereby increasing the volume V.

9. The process according to claim 8, wherein the horizontal flat surface is positioned prior to step (b) such that the height of the wall surrounding the surface is 25-250 m.

10. The process according to claim 8, wherein the horizontal flat surface can be lowered in steps of 25-250 m.

11. The process according to claim 8, wherein the horizontal flat surface is lowered following each of steps (d) and (g).

12. The process according to claim 1, wherein the irradiation is provided by a localised energy source.

13. The process according to claim 1, wherein: the thermoplastic powder has a mean particle volume size of 10 and 300 m as determined in accordance with ISO 9276-2 (2014); and/or the thermoplastic powder has a D.sub.10 of 5 and 50 m, a D.sub.50 of 50 and 150 m, and a D.sub.90 of 160 and 300 m, as determined in accordance with ISO 9276-2 (2014).

14. The process according to claim 12 wherein the localized energy source is a laser energy source or a moving infrared lamp.

15. The process according to claim 5 wherein the composition comprises poly(ethylene terephthalate), poly(ethylene naphthalate), poly(butylene terephthalate), polyamide-11, polylaurolactam, or polyacetal.

Description

(1) The invention will now be illustrated by the following non-limiting examples.

(2) Shaping experiments were performed using a polyethylene terephthalate homopolymer (PET) having an intrinsic viscosity of 1.12 dl/g. The PET had a particle size distribution defined by having a D.sub.10 of 39 m, a D.sub.50 of 94 m, a D.sub.90 of 188 m, and a mean particle volume size of 107 m, determined in accordance with ISO 9276-2 (2014). The PET has a peak melt temperature, first heating run T.sub.p,m as determined in accordance with ISO 11357-3 (2011) of 251 C.

(3) The above materials were subjected to a selective laser sintering process using a Mini-SLS machine comprising a CO.sub.2 laser source. During each laser sintering process, 4 square plates of 30302 mm were produced. 0.05 wt % Aerosil 200 flow promoter was added to the powder. The materials were pre-dried prior to processing via SLS. The SLS process was conducted in an atmosphere having an oxygen content of 1.0 wt %. The SLS process conditions are presented in table 1.

(4) TABLE-US-00001 TABLE 1 Standard high temperature SLS process conditions for PET powder, used in Comparative Example 1. All four plates printed satisfactorily with all four powers, and only a single scan was needed. Operational Setting Build bed temperature 228 C. Piston temperature 185 C. Cylinder temperature 145 C. Feed temperature 160 C. Laser source CO.sub.2 Laser power, plates 1 12 W Laser power, plate 2 18 W Laser power, plate 3 24 W Laser power, plate 4 30 W Laser scan speed 5 m/s Laser hatch distance 150 m Number of scans 1 Layer thickness 100 m Cooling time (min) 60 Article (square plate) 30 mm 30 mm 2 mm dimensions, L W T In Table 1, the following definitions pertain: Build bed temperature: the temperature of the powder at the surface of the build area. Piston temperature: the temperature of the platform (up/down) of the building area Cylinder temperature: the temperature of the surrounding cylinder of the building area Feed temperature: the temperature of the powder at the surface of the feeding area (powder reservoir) from which the roller feeds the build area.

(5) Experiments were performed with PET powder, lowering in Table 2 only the build bed temperatures. As the build bed temperature is lowered below 200 C., the printing can only be done by pre-locating an in situ printed anchor film. Note that the piston temperature also decreases when the build area temperature lower than 185 C. The piston temperature becomes the same as the build area temperature when it is lower than 185 C.

(6) In the inventive experiments, a first powder layer covering the piston was subjected to laser sintering in its entirety, thereby providing a base or anchoring layer of 1212 cm in the form of a film having a thickness of 100 m, onto which further shaping of the objects was performed by the SLS process, the shaping to be done in the form of four squares of 33 cm with thickness of 2 mm each, positioned centrally onto the base layer, with a 2 cm gap between the four squares, so that 4 cm of the base layer extended beyond the printed samples. Note that the base layer was printed at the same lowered temperature as used for the article building. However, it is also possible to pre-print the base layer at the standard temperature (that is, high temperature, 228 C. for PET, see Table 1), and then continue with subsequent printing of the article at the low temperature. For the comparative examples, no base layer was provided, and the sample squares were directly printed onto the powder in the build area.

(7) Table 2 shows the results of lowering the build-bed temperature for PET powder, with and without, base layer film. Comparative Example 2 shows when the build bed temperature was lowered to 200 C. from the standard setting of 228 C. (Comparative Example 1), it was not possible to print plates 1-4 even with a double scan, due to curl.

(8) TABLE-US-00002 TABLE 2 The PET plates were scanned with different powers and number of scans. When a single number is given for the laser power, it means a single scan. A double scan is indicated by two numbers. For example, 40 + 45 means a first scan with 40 W and a repeat scan over the selected area with 45 W. All other parameters were kept as in Table 1. When print completion was not possible, it was due to curl. Building area Laser power (watts) temperature Base film Print completion Example ( C.) printing Plate 1 Plate 2 Plate 3 Plate 4 with good quality Comp. 200 0 (no film) 25 + 25 30 + 30 35 + 35 40 + 40 Not possible for ex. #2 plates 1-4 # 3 200 16.5 25 30 35 40 Possible for plates 3 and 4 #4 200 16.5 25 + 25 30 + 30 35 + 35 40 + 40 Possible for plates 3 and 4 #5 190 16.5 25 + 40 30 + 40 35 + 40 40 + 40 Possible for plates 3 and 4 #6 180 25 45 + 45 30 + 45 35 + 45 40 + 45 Possible for plates 1 and 4 #7 170 25 45 + 45 30 + 45 35 + 45 40 + 45 Possible for plates 1 and 4 #8 160 25 45 + 45 30 + 45 35 + 45 40 + 45 Print completed but entire part curved due to fast cooling. #9 150 25 45 + 45 30 + 45 35 + 45 40 + 45 Print completed but entire part curved due to fast cooling.

(9) Table 2, Example 3 shows the effect of the invention, whereby a build-bed temperature of 200 C. was used as in Comparative Example 2, but a base film was printed first at 200 C., and then the plates were printed over it. Now with single scans of 35 or 40 W, it was possible to print good plates. In Example 4, a build-bed temperature of 200 C. was used, but a base film was printed first, and then the plates were printed over it with double scans; with scans of 35 W+35 W and 40 W+40 W, it was possible to print good squares.

(10) In Table 2, Example 5, the bed temperature was lowered to 190 C., a base film was printed at 190 C., and with double scans of 35 W+40 W and 40 W+40 W, it was possible to complete the printing with good parts.

(11) In Table 2, Example 6, the bed temperature was lowered further to 180 C., a base film was printed at 180 C. according to the inventive step, and with double scans of 45 W+45 W or 45 W+40 W, it was possible to complete the printing with good parts.

(12) In Table 2, Example 7, the bed temperature was lowered further to 170 C., a base film was printed at 170 C. according to the inventive step, and with double scans of 45 W+45 W or 45 W+40 W, it was possible to complete the printing with good parts.

(13) Table 2, Examples 8 and 9, the bed temperature was lowered further to 160 C. and 150 C. respectively, a base film was printed according to the inventive step at the same temperatures, and with double scans of 45 W+45 W and 45 W+40 W, it was possible to complete the printing of the square plates. However, the entire parts curved due to rapid cooling. It may be possible with some optimisation to even work with such low temperatures.

(14) Thus, the examples 3-7 in Table 2 for PET show that the build bed print temperatures can be reduced very substantially. Prolonged temperatures of the powder in the build bed can cause changes in the polymer. In the case of PET, temperatures above 180 C. cause molecular weight increase due to solid state polycondensation; the higher the temperature, the faster the rise in molecular weight. A double scan doubles the build time, however reaction rates approximately halve for every 10 C. decrease, thus by decreasing the build temperature from 228 C. to 200 C., the molecular weight build up is substantially reduced. Example 3 in Table 2 shows for PET at 200 C., a single scan is sufficient to build the part.

(15) Upon completion of the printing process, the base layers of the samples in experiments 3-9 were easily peeled off, without damaging the plates.

(16) The invention is further illustrated with polyamide 12 (PA 12), a well established material for SLS and HSS. PA 12 suffers even faster rise in molecular weight in heat exposed powder, thus limiting its re-use. Lower build temperature for printing would thus be very beneficial for extending the use of the unsintered but heat-exposed powder. First Table 3 shows the standard or optimal process for SLS with PA 12. Curl is not an issue. Table 4 shows the results for lowering the build bed temperature for PA 12, with and without the base film.

(17) TABLE-US-00003 TABLE 3 Comparative Example 10 (C), standard SLS processing conditions for PA 12 powder (without anchor film). Operational Setting Build bed temperature 170 C. Piston temperature 135 C. Cylinder temperature 135 C. Feed temperature 120 C. Laser source CO.sub.2 Laser power 20-22 W Laser scan speed 5 m/s Laser hatch distance 150 m Number of scans 1 Layer thickness 100 m Cooling time (min) Same as printing time

(18) TABLE-US-00004 TABLE 4 results of lowering build bed temperature for PA 12 powder, with and without base layer film. Only a single scan was needed for PA 12 even when the build bed temperature was lowered below the standard one (170 C.). Building area Laser power (watts) temperature Base film Print completion Example ( C.) printing* Plate 1 Plate 2 Plate 3 Plate 4 with good quality Comp. 160 0 (no film) 17 20 23 26 Not possible for ex. #11 plates 1-4 #12 160 15 17 20 23 26 Possible for plates 1-4 #13 155 15 17 20 23 26 Possible for plates 2-4 #14 150 15 17 20 23 26 Possible for plates 2-4 #15 145 15 17 20 23 26 Possible for plates 3 and 4 #16 140 17 17 20 23 26 Possible for plates 3 and 4 #8 130 17 20 23 26 29 Not possible for plates 1-4

(19) Polyamide 12 is a very good material for SLS, but it shows a marked rise in molecular weight; for example the molecular weight approximately doubles within 1-2 h at the optimal or standard processing conditions, where the bed is at 170 C. Comparative Example 10 (C) in Table 3 shows the standard processing conditions for polyamide 12, with the build bed temperature at 170 C. where the print can be completed and the part dimensions are good. Comparative Example 11 (C) in Table 4 shows the effect of lowering the build bed temperature from 170 C. to 160 C. Due to curl, it was not possible to complete the printing of the square plates at any power setting.

(20) Example 12 shows the effect of printing an in situ film layer first as taught in the invention, and then printing the plates over it, with the build temperature at 160 C. Unlike Comparative Example 11 (C), the curl was not a problem and the square plate could be printed satisfactorily with all four powers.

(21) Example 13 shows the effect of printing an in situ film layer first as taught in the invention, and then printing the plates over it, with the build temperature lowered to 155 C. Still, the curl was not a problem and the square plate could be printed satisfactorily for plates 2-4.

(22) Example 14 shows the effect of printing an in situ film layer first as taught in the invention, and then printing the plates over it, with the build temperature lowered to 150 C. Still, the curl was not a problem and the square plate could be printed satisfactorily for plates 2-4.

(23) Example 15 shows the effect of printing an in situ film layer first as taught in the invention, and then printing the plates over it, with the build temperature lowered to 145 C. Still, the curl was not a problem and the square plate could be printed satisfactorily for power settings 3 and 4.

(24) Example 16 shows the effect of printing an in situ film layer first as taught in the invention, and then printing the plates over it, with the build temperature lowered to 140 C. Still, the curl was not a problem for power settings 3 and 4.

(25) For polyamide 12, not only is there energy saving in having a lower build-bed temperature, there is much scope to reduce the molecular weight rise in the unsintered but heat-exposed powder left after a building episode.

(26) In general, as the build bed temperature is lowered, and the difference between it and Tm increases, higher laser powers are needed. However, as the difference between the lowered bed temperature and the Tm was relatively smaller for PA 12 than for PET, the PA 12 part could be printed with a single scan, providing the base film was pre-printed before the part building.

(27) Accordingly, it can be understood from the above that producing a shaped object according to the method of the invention, results in objects having desirable shape stability properties while printing can be performed at reduced powder bed temperatures, thereby reducing energy consumption as well as preventing the unfused powder to be subjected to high heat. This increases the refresh rate for the powder, being the ability to re-use the unfused powder in a subsequent SLS cycle, and thereby increasing the material efficiency of the process.