METHOD FOR PRODUCING AN OBJECT BY MELTING A POLYMER POWDER IN A POWDER SINTERING DEVICE

Abstract

A method for producing an object by melting a polymer powder in a powder sintering device. For example, a method for producing an object by melting a polymer powder in a powder sintering device under a laser beam, implementing a rheological analysis of the polymers, in order to determine the conditions for producing the object by melting polymer powders.

Claims

1. A process for manufacturing an object by melting a polymer powder in a laser-beam powder sintering device comprising the following steps: manufacturing the object in a laser-beam powder sintering device at the initial temperature of the powder bed, said initial temperature of the powder bed being selected in the following manner: establishing the viscosity behavior of the polymer powder as a function of the time and of a descending temperature gradient starting from a supercooling temperature, corresponding to the temperature of the powder bed, that the layer melted at T>Tm by the laser rapidly returns to via cooling, the viscosity measurements being carried out in a plate-plate rheometer device at a stress frequency of less than 5 rad/s. selecting the initial temperature corresponding to the temperature of the powder bed allowing a viscosity of the supercooled polymer of between 800 and 20 000 Pa.Math.s during a time range of more than 5 minutes.

2. The process as claimed in claim 1, wherein the polymer is semicrystalline.

3. The process as claimed in claim 1, wherein the polymer is amorphous.

4. The process as claimed in claim 1, wherein the polymer is a polyaryletherketone.

5. The process as claimed in claim 1, wherein the polymer is a polyamide.

6. A composition comprising a semicrystalline polymer having a viscosity of between 800 and 20 000 Pa.Math.s, the viscosity measurements being carried out in a plate-plate rheometer device at a stress frequency of less than 5 rad/s, said composition being supercooled to a temperature T below the melting temperature after exposure to a temperature T max above its melting point and that can be used in a device for manufacturing objects by melting polymer powders.

7. The composition of claim 6, wherein the composition is a polyamide.

8. The composition of claim 6, wherein the composition is a filled polyamide.

9. The composition of claim 6, wherein the composition is a polyaryletherketone.

10. The composition of claim 6, wherein the composition is a filled polyaryletherketone.

Description

DETAILED DESCRIPTION

[0019] The process of the invention applies to any thermoplastic polymer powder intended for the laser sintering process.

[0020] Such powders are usually characterized by: [0021] a Dv50 of around 50 microns, or more specifically of between 40 and 60 microns.

[0022] The Dv50 referred to here is the median diameter by volume, which corresponds to the value of the particle size which divides the population of particles examined exactly into two. The Dv50 is measured according to the standard ISO 9276—parts 1 to 6. In the present description, a Malvern particle sizer, Mastersizer 2000, is used and the measurement is carried out by the liquid route by laser diffraction on the powder. [0023] sufficient flowability to enable machine layering, which translates into a certain sphericity of the powder particles.

[0024] The process that is the subject of the invention relates to any type of thermoplastic polymer, whether it is amorphous or semicrystalline, and preferably semicrystalline. It may be a question of polyolefins (PE, PP), polyvinyls (PVC, PVDC), styrenes (PS), polyacrylonitrile butadiene styrene (ABS), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polycarbonate (PC), ethylene-vinyl acetate copolymer (EVA, EVM), ethylene-vinyl alcohol copolymer (EVOH), aliphatic or aromatic polyamides or copolyamides (PA 6-6, PA 6, PA 11, PA 12, PA 4-6, PA 6-10, PA 6-12), polyether block amides (PEBA), polyamide-imide (PAI), poly(meth)acrylics (PMMA), polymethyl methacrylate butadiene styrene (MBS), polyacrylonitrile (PAN), polyaryletherketones (PEK, PEEK, PEKK, PEKEKK, etc.), polyimides (PI), fluoropolymers (PTFE, PVDF), aromatic polyethers (PPO), polylactic acid (PLA), silicones, thermoplastic elastomers taken alone or as mixtures. Other polymers or copolymers, whether they are statistical, gradient or block polymers or copolymers, cannot be excluded.

[0025] The polymers could contain the following additives: stabilizer (heat or UV stabilizer), conductive fillers (in particular carbon fibers, carbon nanotubes, carbon black), reinforcing fillers (carbon fibers, glass fibers, beads or any other inorganic filler), milled fibers having a length of less than 300 μm and preferably of less than 100 μm.

[0026] Preferably, they are polyamides or polyaryletherketones, more particularly PA 11, PA 12, PEK, PEEK, PEKK.

[0027] The process for manufacturing an object by fusion-coalescence of polymer powders, and in particular laser sintering, requires particular viscosity conditions of the sintered layer.

[0028] Specifically, for the manufacture of an object with maximum performance, in particular for the mechanical properties, the viscosity should be neither too low, nor too high. In addition, this optimal viscosity should last for the time required for a good coalescence of the powders.

[0029] In this laser sintering process, the powder before laser sintering is at the temperature of the powder bed (denoted by Tbed). Next, the laser supplies the energy required for melting the powder, which therefore rapidly rises to a temperature above its Tm (in the case of semicrystalline polymers, temperature denoted by Tmax). Then, the sintered powder rapidly returns to Tbed and then undergoes a slow cooling (cooling of between 0.1° C./min and 1° C./min). In what follows it has been chosen to set the cooling rate at 0.5° C./min. In summary, the heat cycle experienced by the powder bed may therefore be summarized as a function of the time by the diagram visible in FIG. 1.

[0030] During this heat cycle, the phases of coalescence of the powder and of the layers with one another take place during cooling phases after Tmax (firstly the very short phase from Tmax to Tbed then a longer phase during the slow cooling starting from Tbed).

[0031] At the end of numerous tests under real conditions in a laser sintering device, and by reproducing these conditions in a plate-plate rheometer, the applicant observed that this viscosity should be between 800 and 20 000 Pa.Math.s, and preferably between 1000 and 10 000 Pa.Math.s. Throughout the time necessary for the correct coalescence of the powders, this viscosity should remain between 800 and 20 000 Pa.Math.s, and preferably between 1000 and 10 000 Pa.Math.s. This time referred to as “open time” should be more than 5 minutes, preferably more than 10 minutes.

[0032] The invention also relates to compositions comprising a semicrystalline polymer having a viscosity of between 800 and 20 000 Pa.Math.s, and preferably of between 1000 and 10 000 Pa.Math.s over a time of more than 5 minutes and preferably of more than 10 min, the viscosity measurements being carried out in a plate-plate rheometer device at a stress frequency of less than 5 rad/s, said composition being supercooled to a temperature T below the melting temperature after exposure to a temperature Tmax above its melting point and that can be used in a device for manufacturing objects by melting polymer powders and in particular the laser sintering of polymers.

[0033] The compositions that are the subject of the invention relate to any type of thermoplastic polymer, whether it is amorphous or semicrystalline. It may be a question of polyolefins (PE, PP), polyvinyls (PVC, PVDC), styrenes (PS), polyacrylonitrile butadiene styrene (ABS), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polycarbonate (PC), ethylene-vinyl acetate copolymer (EVA, EVM), ethylene-vinyl alcohol copolymer (EVOH), aliphatic or aromatic polyamides or copolyamides (PA 6-6, PA 6, PA 11, PA 12, PA 4-6, PA 6-10, PA 6-12), polyether block amides (PEBA), polyamide-imide (PAI), poly(meth)acrylics (polymethyl methacrylate butadiene styrene polyacrylonitrile (PAN), polyaryletherketones (PEK, PEEK, PEKK, PEKEKK, etc.), polyimides (PI), fluoropolymers (PTFE, PVDF), aromatic polyethers (PPO), polylactic acid (PLA), silicones, thermoplastic elastomers taken alone or as mixtures. Other polymers or copolymers, whether they are statistical, gradient or block polymers or copolymers, cannot be excluded.

[0034] The polymers could contain the following additives: stabilizer (heat or UV stabilizer), conductive fillers (in particular carbon fibers, carbon nanotubes, carbon black), reinforcing fillers (carbon fibers, glass fibers, beads or any other inorganic filler), milled fibers having a length of less than 300 μm and preferably of less than 100 μm.

[0035] The preferred compositions are polyamides or polyaryletherketones, more particularly PA 11, PA 12, PEK, PEEK, or else PEKK.

[0036] The rheometers that can be used are rheometers of plate-plate or cone-plate type, that is to say rheometers for measuring high viscosities (typically of entangled thermoplastic polymers) at low frequencies, typically of less than 5 rad/s.

Example 1

[0037] The curves in FIG. 2 provide the heat cycles experienced by the PEEK and PA 12 powders during the cooling cycle, therefore during the cycle that has to enable the coalescence of the powders and of the layers with one another, and also the viscosity of the two polymers.

Case of PEEK HP3 (PEEK Available from the Company EOS)

[0038] The cycle is the following: Tmax=400° C. then Tbed of 362° C. as applied in a sintering machine.

Case of PA 12 PA 2200 (PA 12 Available from the Company EOS).

[0039] The cycle is the following: Tmax=230° C. then Tbed of 174° C. as applied in a sintering machine.

[0040] The measurements are carried out in an ARES rheometer (from Rheometric Scientific), at a stress frequency of 1 rad/s between parallel plates (diameter 25 mm) and with a cooling rate of 0.5° C./min.

[0041] The viscosity curve shows that under the experimental conditions (Tbed) for sintering these 2 materials, these 2 materials have a viscosity of the same order of magnitude and less than 10 000 Pa.Math.s for 10 minutes. This viscosity range and the open time (at least 10 minutes here) enables the coalescence of the powder and of the layers with one another. For such bed temperatures, the mechanical properties of the parts are optimal.

Example 2

[0042] The curves in FIG. 3 provide the heat cycles at three different bed temperatures experienced by the PEKK powder during the cooling cycle, therefore during the cycles that have to enable the coalescence of the powders and of the layers with one another, and also the respective viscosities of the polymers.

[0043] A PEKK powder, Kepstan® 6003 from the company Arkema, the particle size of which has a dv50 of 50 μm±5 μm (denoted by Kepstan 6003 PL) is subjected to a rheological test according to 3 temperature cycles (3 different bed temperatures).

[0044] Cycle 1: Tmax=340° C. then Tbed of 285° C. (hollow square for the temperature, solid square for the viscosity).

[0045] Cycle 2: Tmax=340° C. then Tbed of 265° C. (hollow circle for the temperature, solid circle for the viscosity).

[0046] Cycle 3: Tmax=340° C. then Tbed of 245° C. (hollow triangle for the temperature, solid triangle for the viscosity).

[0047] The measurements are carried out in an ARES rheometer, at a stress frequency of 1 rad/s between parallel plates (diameter 25 mm) and with a cooling rate of 0.5° C./min.

[0048] It is observed that the viscosity and the open time are greatly influenced by the temperature of the powder bed. For this grade, the optimal bed temperature for ensuring the coalescence of the powder and of the layers with one another is between 265° C. and 285° C. (between cycle 1 and cycle 2) in order to have a viscosity of less than 10 000 Pa.Math.s for an open time of more than 10 minutes.

[0049] A machine test at a bed temperature of 240° C. showed that the successive layers had not coalesced due to their excessively high viscosities.

[0050] On the other hand, a machine test at a bed temperature of 300° C. showed that the melted layer collapsed through the powder bed.

Example 3

[0051] The curves in FIG. 4 provide the heat cycle experienced by two PEKK powders of different molecular weights during the cooling cycle, therefore during the cycle that has to enable the coalescence of the powder and of the layers with one another, and also the viscosities of the two polymers.

[0052] Kepstan® 6003 and Kepstan® 6002 powders from the company Arkema, the particle size of which has a dv50 of 50 μm±5 μm (denoted by 6003 PL and 6002 PL) are subjected to a rheological test according to the following cycle.

[0053] Cycle: Tmax=340° C. then Tbed of 265° C. (viscosity: solid triangle for 6003 PL and solid square for 6002 PL).

[0054] Kepstan® 6003 PL and 6002 PL are commercial products from the company Arkema. They correspond to PEKKs having the same chemical structure but with two different molecular weights (6003 PL has a lower molecular weight than 6002 PL). Kepstan® 6003 PL: MVI (under 1 kg at 380° C.) between 8 and 16 cm.sup.3/10 min.

[0055] Kepstan® 6002 PL: MVI (under 5 kg at 380° C.) between 24 and 50 cm.sup.3/10 min.

[0056] The measurements are carried out in an ARES rheometer, at a stress frequency of 1 rad/s between parallel plates (diameter 25 mm) and with a cooling rate of 0.5° C./min.

[0057] This curve shows that the molecular weight has a significant influence on the viscosity of the material at the temperature of the bed and also on the open time.

[0058] This methodology therefore makes it possible to demonstrate that the ideal temperature of the powder bed will not depend solely on the chemical structure of the polymer but also on its molecular weight.