WO2024126965A1 - Method for manufacturing sintered objects with improved roughness - Google Patents

Abstract

The invention mainly relates to a polyamide powder composition used in powder-bed additive manufacturing, the composition consisting of: (a) 10 to 90% by weight of a polyamide powder A having an inherent viscosity calculated according to formula (1) on the basis of the flow velocity of a solution of the polymer and the solvent measured in an Ubbelohde viscometer according to ISO 307:2019, except when m-cresol is used as the solvent and a temperature of 20°C is set, of 0.8 to 2.8, and having a span of between 0.4 and 1.40; (b) 90 to 10% by weight of a polyamide powder B that is distinct from the polyamide powder A, which powder B has an inherent viscosity, calculated as indicated above, of 0.8 to 2.8, and a span of between 0.4 and 1.40; (c) 0 to 40% by weight of additives chosen from among flow agents, stabilising agents, optical brighteners and energy-absorbing additives and/or fillers, wherein the polyamide powder B is derived from the polyamide powder A, it being understood that the difference between the respective inherent viscosities of the powders A and B does not exceed 0.6 in terms of absolute value.

Description

Description

Title: PROCESS FOR MANUFACTURING SINTERED OBJ ETS WITH IMPROVED ROUGHNESS

[Technical area]

The present patent application relates to a process for manufacturing sintered objects with improved roughness by 3D printing, such sintered objects as well as a composition useful for their manufacture.

[Prior art]

Additive manufacturing on a thermoplastic polymer powder bed (SLS, MJF, HSS, etc.) makes it possible to build parts with complex geometry, including in series. It allows the production of a large number of parts simultaneously, with excellent resolution and very good mechanical properties, which gives them an advantage over other additive manufacturing processes such as fused wire deposition.

However, these processes are very demanding in terms of the properties of the polymer powders. For example, the powders must be able to form a powder bed with the properties required in the additive manufacturing process. Indeed, the powders must both be able to be spread in a thin layer of uniform thickness and form a cohesive powder bed capable of supporting the weight of the parts being constructed.

The properties of polymer powders are also important in order to obtain objects without defects, particularly with low roughness. Indeed, printed parts often present a surface roughness which is not suitable for certain applications, such as applications requiring parts with precise dimensions such as bearing cages, seals, turbines, pistons, rings, compressor valves or engine parts. This problem arises in particular in the context of the recycling of the residual powder recovered at the end of each print which is necessary for economic reasons. In particular, we note that the recycling of such powders does not ensure the manufacture of quality parts and in particular constant roughness.

It is known to improve the surface condition of parts obtained by additive manufacturing through physical and/or chemical treatment. However, these treatments substantially increase production time and therefore its cost.

Application US 2011/0237731 discloses a powder for 3D printing by laser sintering comprising 10 to 30% by weight of a polyamide 12 powder that is not very scalable in terms of viscosity under the effect of heat mixed with a more scalable polyamide 12 powder. However, some of these mixtures have components with very different properties, which can cause inhomogeneities in the manufactured article and affect its mechanical properties.

Patent application US 2018/094103 proposes polyamide powders for 3D printing by sintering with a viscosity in solution according to ISO 307 of 1.55 to 1.75, this viscosity varying from 10 to 40% after 24 hours under nitrogen at a temperature 10°C below its melting point. 3D printing with these powders does not always make it possible to obtain parts with satisfactory roughness for applications requiring parts with precise dimensions such as bearing cages, seals, turbines, pistons, rings, compressor valves or parts of engine.

[Summary of the invention]

The invention therefore aims to propose a composition of polyamide powders for 3D printing by laser sintering, in particular partially recycled, which allows the manufacture of parts with good surface quality, in particular low roughness.

Indeed, the present invention is based on the observation that a mixture of polyamide powders specifically selected and having in particular a difference in inherent viscosity of less than 0.6 allows the manufacture of articles having good surface quality, and in particular a low roughness.

Also, according to a first aspect, the invention relates to a composition of polyamide powders comprising:

(a) 10 to 90% by weight of a polyamide A powder having an inherent viscosity, calculated according to formula (1) from the flow velocity of a solution of the polymer and the solvent, measured in a viscometer Ubbelohde type according to ISO 307:2019, except for using m-cresol as a solvent and a temperature of 20°C, from 0.8 to 2.8, and having a span of between 0.4 and 1.40 ;

(b) 90 to 10% by weight of a polyamide B powder, distinct from the polyamide A powder, having an inherent viscosity, calculated as indicated above, of 0.8 to 2.8, and having a span between 0.4 and 1.40;

(c) 0 to 40% by weight of additives and/or fillers, it being understood that the difference between the respective inherent viscosities of powder A and B does not exceed 0.6 in absolute value.

According to one embodiment, the polyamide A powder and the polyamide B powder are chosen respectively from PA 6, PA 66, PA613, PA1010, PA 1012, PA 11, PA 12, PA 11/10T and their copolymers.

According to one embodiment, the polyamide B powder comes from the polyamide A powder.

According to one embodiment, the polyamide A and/or B powder has an inherent viscosity of between 0.90 and 1.50.

According to one embodiment, the difference between the inherent viscosity of the polyamide A powder and the polyamide B powder does not exceed in absolute value 0.5, preferably 0.4 and very particularly 0.3.

According to one embodiment, the composition of polyamide powders comprises 20 to 80%, preferably 30 to 70% and very particularly 40 to 60% by weight of polyamide A powder.

According to one embodiment, the composition of polyamide powders comprises 20 to 80%, preferably 30 to 70% and very particularly 40 to 60% by weight of polyamide B powder.

According to a second aspect, the invention relates to a process for manufacturing said powder composition, by mixing:

(i) a polyamide A powder having an inherent viscosity, calculated according to formula (1) from the flow speed of a solution of the polymer and the solvent, measured in an Ubbelohde type viscometer according to standard ISO 307 :2019, except for using m-cresol as a solvent and a temperature of 20°C, from 0.8 to 2.8, and having a span of between 0.4 and 1.40, said powder A having never been used;

(ii) a polyamide B powder both have an inherent viscosity, calculated as indicated above, of 0.8 to 2.8, and having a span of between 0.4 and 1.40, said powder B having been used in 3D printing by laser sintering beforehand; as well as

(iii) possible additives and/or fillers; it being understood that the difference between their respective inherent viscosities does not exceed 0.6 in absolute value. According to yet another aspect, the invention relates to a method of manufacturing a sintered object by 3D printing by Laser sintering, comprising the steps consisting of:

(i) Preparation of a composition of polyamide powders as defined above;

(ii) 3D printing by laser sintering using said batch of polyamide powder so as to obtain a sintered object.

According to one embodiment, the polyamide A powder and the polyamide B powder are chosen respectively from PA 6, PA 66, PA613, PA1010, PA 1012, PA 11, PA 12, PA 11/10T and their copolymers.

According to one embodiment, the polyamide B powder comes from the polyamide A powder.

According to one embodiment, the difference between the inherent viscosity of the polyamide A powder and the polyamide B powder does not exceed in absolute value 0.5, preferably 0.4 and very particularly 0.3.

According to one embodiment, the polyamide A and/or B powder has an inherent viscosity of between 0.9 and 1.5.

According to a final aspect finally, the invention aims at a sintered object comprising particles of polyamide powder composition as defined above, assembled by partial fusion, characterized in that it has a surface roughness, as measured in the conditions explained in the examples, characterized by an Rz < 50 pm and a Ra < 10 pm.

[Description of embodiments]

Definition of terms

The nomenclature used to designate polyamides follows the ISO 1874-1 standard. In particular, in PA X notation, X represents the number of carbon atoms of the polyamide units resulting from the condensation of an amino acid or lactam. In the PA XY notation designating a polyamide resulting from the condensation of a diamine with a dicarboxylic acid or an acid derivative having di-functionality, X represents the number of carbon atoms of the diamine and Y represents the number of carbon atoms of the dicarboxylic acid or acid derivative. The notation PA X/Y refers to copolyamides in which X and Y are two distinct monomers.

The term “powder bed additive manufacturing” is understood to designate processes in which a layer of polymer powder, the powder bed, is irradiated by electromagnetic radiation (for example laser beam, infrared radiation, UV radiation), so to selectively melt powder particles impacted by radiation. The molten particles coalesce and solidify to lead to the formation of a solid mass. This process can produce articles by repeated irradiation of a succession of freshly applied powder layers.

The term "volume average diameter" or "Dv" is also understood to mean the volume average diameter of a powdery material, as measured by Laser diffraction according to the ISO 13320: 2009 standard, for example on a Malvern type diffractometer. Insitec®. Likewise, “Dv10” and “Dv90” are respectively the corresponding diameters so that the cumulative function of the diameters of the particles, weighted by the volume, is equal to 10%, and respectively, to 90%. More specifically, Dv50 designates the median diameter in volume. The rules for representing the results of a particle size distribution are given by the ISO 9276 standard - parts 1 to 6.

dans Laquelle : Dv1O désigne Le diamètre en-dessous duquel se trouvent 10% en volume des particules de La poudre de polymère ; The term “span” is understood to designate a ratio describing the width of the particle size distribution of a powder, of the following formula: [Math 1]

in which : Dv1O designates the diameter below which there are 10% by volume of the particles of the polymer powder;

Dv50 designates the diameter below which 50% by volume of the particles of the polymer powder are located (by definition Dv50 is also the median diameter by volume), and

Dv90 designates the diameter below which 90% by volume of the particles of the polymer powder are located, these diameters being measured as indicated above.

The term “inherent viscosity” means the viscosity as calculated from the flow speed of a solution of the polymer and the solvent, measured in an Ubbelohde type viscometer according to ISO 307:2019, except for use as a solvent m-cresol and a temperature of 20°C. The inherent viscosity is calculated according to formula (1) below: [Math 2]

The inherent viscosity is equal to the natural logarithm of the ratio between the flow time of the polymer solution t divided by that of the solvent t ₀ , all divided by the concentration c of polymer dissolved in the solvent, as a mass percentage. The dimension of the inherent viscosity is the inverse of the concentration, and without unit when the concentration is given as here in mass percentage.

A. Composition of polyamide powders

The composition of polyamide powders useful for additive manufacturing on a powder bed according to the invention comprises:

(a) 10 to 90% by weight of a polyamide A powder having an inherent viscosity, calculated according to formula (1) from the flow velocity of a solution of the polymer and the solvent, measured in a viscometer Ubbelohde type according to standard ISO 307:2019, except when using m-cresol as a solvent and a temperature of 20°C, from 0.8 to 2.8 and having a span of between 0.4 and 1.40;

(b) 90 to 10% by weight of a polyamide B powder, distinct from the polyamide A powder, having an inherent viscosity, calculated as indicated above, of 0.8 to 2.8 and having a span included between 0.4 and 1.40; (c) 0 to 40% by weight of additives and/or fillers, it being understood that the difference between the respective inherent viscosities of powder A and B does not exceed 0.6 in absolute value.

Polyamide A powder and Polyamide B powder are distinct. These may be powders of the same polyamide or of a separate polyamide. Preferably, these are powders of the same polyamide. Polyamide powders differ in at least one of their properties. These properties may in particular be their appearance, for example their color, the inherent viscosity, the particle size, the crystallinity or their thermal properties.

In particular, one of the polyamide powders in the composition can be obtained from the other polyamide powder in the composition, in particular by recycling.

Polyamide powders A and B have an inherent viscosity such that their difference remains moderate, so as to avoid inhomogeneities in the sintered parts. Thus, according to the invention, these polyamide powders have a deviation in the inherent viscosity which in absolute value does not exceed 0.6, in particular 0.5, more preferably 0.4 and very particularly 0.3. Preferably, the inherent viscosity difference in absolute value is between 0 and 0.6, and in particular 0.05 to 0.5, and very particularly 0.1 to 0.3.

Furthermore, according to the invention, these polyamide powders have an inherent viscosity of at least 0.8, preferably at least 1 and more preferably at least 1.2. According to the invention, these polyamide powders have an inherent viscosity of at most 2.8, preferably at most 2.5.

These viscosity ranges are particularly advantageous and make it possible to obtain a good compromise to have both good coalescence properties during sintering (sufficiently low viscosity) and good mechanical properties of the sintered object (sufficiently high viscosity).

The viscosity of a polyamide powder depends, in addition to the nature of the polyamide, in particular on its molecular weight. Thus, a polyamide of high molecular weight will have a higher viscosity than the same polyamide of lower molecular weight.

Most commercially available polyamides come in different grades, with different viscosities. It is therefore easy to choose from the ranges of polyamides offered those which present a difference in viscosity as specified.

Furthermore, when powders A and B are powders of the same polyamide but distinct in that one has been recycled while the other has not or less summer, these powders generally also have a difference in viscosity. This difference in viscosity can vary in particular depending on the formulation of the powder and depending on the temperature and duration of use of the recycled powder, and is easily measurable.

According to the invention, the composition of polyamide powders also has a specific particle size, characterized in particular by its span.

Indeed, the particle size distribution of the polyamide powder can have a significant impact on performance in additive manufacturing by sintering, and in particular on roughness.

Thus, according to the invention, the polyamide powders have a span, as defined above, of between 0.4 and 1.40, and in particular between 0.6 and 1.10, in particular between 0.8 and 1.0.

According to one embodiment, the volume diameter Dv10 of the polyamide powders in the composition is preferably greater than 15 μm. According to certain embodiments, the polyamide powder has a volume diameter Dv10 of between 15 and 50 pm, or between 25 and 45 pm or between 30 and 40 pm.

According to one embodiment, the volume diameter Dv50 of the powder is preferably between 30 and 60 pm. According to certain embodiments, the polyamide powder has a volume diameter Dv50 of between 35 and 55 pm, or between 40 and 50 pm.

According to one embodiment, the polyamide powder has a volume diameter Dv90 of less than 120 pm, in particular less than 100 pm, and very particularly less than 90 pm. According to certain embodiments, the polyamide powder has a volume diameter Dv90 of between 40 and 120 pm, or between 45 and 100 pm, or between 50 and 80 pm.

Finally, the polyamide powder of the invention advantageously has a volume average diameter Dv of less than 55 pm. According to certain embodiments, the polyamide powder has a volume average diameter Dv of between 30 and 55 pm, in particular between 35 and 50 pm and very particularly between 40 and 45 pm.

The polyamide may in particular be chosen from aliphatic polyamides, semi-crystalline polyamides and their copolymers and mixtures.

Mention may in particular be made of aliphatic polyamides such as PA 6, PA 66, PA613, PA1010, PA 1012, PA 11, PA 12 and their copolymers, and semi-aromatic polyamides such as PA 11/10T for example. Advantageously, the composition comprises 20 to 80%, preferably 25 to 70% by weight and very particularly 30 to 60% by weight of polyamide A powder. According to certain embodiments, the polyamide powder composition comprises 10 to 20% , or 20 to 30%, or 30 to 40%, or 40 to 50%, or 50 to 60%, or 60 to 70%, or 70 to 80%, or 80 to 90% by weight of polyamide A powder per relative to the total weight of the polyamide powder composition.

Advantageously, the composition comprises 20 to 80%, preferably 25 to 70% by weight and very particularly 30 to 60% by weight of polyamide B powder. According to certain embodiments, the polyamide powder composition comprises 10 to 20% , or 20 to 30%, or 30 to 40%, or 40 to 50%, or 50 to 60%, or 60 to 70%, or 70 to 80%, or 80 to 90% by weight of polyamide B powder per relative to the total weight of the polyamide powder composition.

Preferably, the polyamide powder composition does not contain any other polyamide.

The rest of the composition may consist of other compounds, in particular fillers and/or additives. The composition of polyamide powders may in particular comprise, in addition to polyamide powders A and B, additionally 0 to 40% by weight of one or more usual additives and fillers.

The additives generally represent less than 5% by weight relative to the total weight of the composition. Preferably, the additives represent less than 1% by weight of the total powder weight. Among the additives, we can cite flow agents, stabilizing agents (light, in particular UV, and heat), optical brighteners, dyes, pigments, energy absorber additives (including UV absorbers) . Advantageously, the composition is free of pigments or dyes.

Among the flow agents, mention may be made, for example, of a hydrophilic or hydrophobic silica. Advantageously, the flow agent represents 0.01 to 0.4% by weight relative to the total weight of composition. In other embodiments, the powder composition does not include a flow agent. The composition of polyamide powders may also include one or more fillers. The fillers generally represent less than 40% by weight, in particular less than 30% by weight, and preferably less than 25% by weight relative to the total weight of the final powder composition. Among the fillers, let us cite reinforcing fillers, in particular mineral fillers such as carbon black, talc, nanotubes, carbon or not, and fibers, in particular glass or carbon fibers, crushed or not, or even glass in another form, for example in the form of flakes or balls, hollow or not.

B. 3D printing process by laser sintering

The process which is the subject of the invention may in particular be a selective laser sintering process (SLS, Selective Laser Sintering, in English), a sintering process of the MJF (Multi Jet Fusion) type or a sintering process of the HSS type ( High Speed Sintering).

The SLS process is widely known. In this context, reference may in particular be made to documents US 6,136,948 and WO 96/06881.

In this type of process, a thin layer of powder is deposited on a horizontal plate held in an enclosure heated to a temperature called construction temperature. Most often, heating to construction temperature is carried out by means of IR radiation lamps, for example halogen lamps, which generally have a maximum emission at a wavelength between 750 nm and 1250 nm. The construction temperature designates the temperature to which the powder bed, of a constituent layer of a three-dimensional article under construction, is heated during the layer-by-layer sintering process of the powder. Electromagnetic radiation, for example in the form of a laser, subsequently provides the energy necessary to sinter the powder particles at different points of the powder layer according to a geometry corresponding to an object, for example using a computer having in memory the shape of an object and restoring the latter in the form of slices. Then, the horizontal plate is lowered by a height corresponding to the thickness of a layer of powder, and a new layer of powder is spread, heated and then sintered in the same way. The procedure is repeated until the object has been made.

The layer of powder deposited on a horizontal plate can have, before sintering, for example a thickness of 20 to 200 μm, and preferably of 50 to 150 μm. After sintering, the thickness of the layer of agglomerated material is a little lower, and can for example have a thickness of 10 to 150 μm, and preferably of 30 to 120 μm.

For the MJF and HSS process, the entire layer of the build material is exposed to radiation, but only a portion coated with a fusing agent is melted to become one layer of a 3D part. Fusion agent is a compound capable of absorbing radiation and converting it into energy thermal, for example black ink. It is selectively applied in the selected region of the building material. The melting agent is able to penetrate the layer of the building material and transmits the absorbed energy to the neighboring building material, thereby causing it to melt or be sintered. By melting, bonding and subsequent hardening of each layer of the building material, the object is formed.

In the particular case of MJF, a detailing agent is also added to the edges of the area to be melted to allow the parts to have better definition.

Advantageously, the use of the polyamide powder composition described below in these processes does not require any particular modification. On the other hand, it makes it possible to obtain parts with a good surface appearance, in particular lower roughness and better definition.

Advantageously, the process makes it possible to use the composition of polyamide powders in several successive constructions. In this case, it can be reused alone or mixed with other powders, recycled or not.

C. Process for manufacturing polyamide powder

The polyamide powder contained in the composition can in particular be obtained by grinding polyamide in the form of extruded granules or scales, according to conventional techniques.

Grinding can be carried out on equipment known for this purpose, for example by means of a mill with counter-rotating pins (pin mill), a hammer mill (hammer mill) or in a whirling mill (whirl mill).

When the powder comprises, in addition to the polyamide powders, certain additives and/or reinforcing fillers, these additives and/or fillers can be incorporated by mixing in the molten state, for example by extrusion (compounding) and granulation followed by grinding of the granules . Alternatively, it is also possible to add other polymers and/or certain additives and/or certain reinforcing fillers by dry blending. Preferably, the flow agent is added by dry mixing.

According to one embodiment, the process for manufacturing the polyamide powder comprises the steps of:

(i) prepolymerization of the polyamide monomer(s) and subsequent granulation; (ii) grinding into a powder;

(iii) possible subsequent sieving of the prepolymer powder obtained;

(iv) Treatment of the prepolymer powder by bringing the prepolymer powder in the solid state into contact with water or water vapor at a temperature close to its crystallization temperature Te for a sufficient time to increasing its melting temperature and/or its melting enthalpy, separation of water or water vapor from the prepolymer powder and drying; this step allows you to add the powder to the desired level of phosphoric acid

(v) subjecting the resulting treated prepolymer powder to solid phase polycondensation to obtain a polymer powder.

The process for treating prepolymer powder is described in particular in patent application EP 1413595 A1.

Alternatively, the polyamide powder can also be manufactured by other processes known in the art, for example by precipitating anionic polymerization as described for example in EP 1 814 931 B1 or FR 06.56024 B1.

The inherent viscosity of the polyamide powder thus obtained will depend in particular on the parameters of the polycondensation, and in the case of the process described, in particular on the solid phase polycondensation.

Furthermore, it will be observed that the inherent viscosity of a polyamide powder can increase during the aging of the powder, that is to say in particular with the time of exposure to heat. The speed with which the inherent viscosity of a polyamide powder increases then depends in particular on the temperature and also on the possible presence of antioxidants or catalysts. Catalysts may in particular be phosphorus acids, in particular hypophosphorous acid, phosphorous acid as well as phosphoric acid.

As mentioned above, it is particularly preferred to combine in the composition a polyamide powder with a recycled polyamide powder, in particular having been used in 3D printing by laser sintering and having therefore been exposed for a substantial time to a temperature close to the melting temperature. Particularly advantageous is a composition comprising a polyamide powder with a recycled powder of the same polyamide.

Such a composition of polyamide powders can in particular be manufactured by combining a virgin polyamide powder having the required properties and a recycled polyamide powder having the required properties, the inherent viscosity respective being adjusted so that the difference in viscosity between the powders does not exceed a certain value.

The additives and/or reinforcing fillers can be added to the prepolymer, by melt mixing (compounding) or dry mixing, between steps (i) and (ii) of the process. Alternatively, they can be added to the composition later, in particular by dry mixing.

When the span of a powder is too low, it can be increased by adding particles with a finer or larger particle size.

When, on the other hand, the span of a powder is too high, it can be reduced by removing the finest or largest particles, for example by sieving or refining.

D. Part capable of being manufactured

The use of a composition according to the invention thus makes it possible to manufacture three-dimensional parts of good quality, particularly of surface quality. In particular, these articles may have low roughness.

The composition of polyamide powders allows the additive manufacturing by sintering of parts which have properties, particularly surface properties, and in particular in terms of roughness, at least similar to the parts obtained if not superior compared to conventional polyamide powders.

According to the present application, the roughness of the sintered object is evaluated by means of a roughness meter, in particular a non-contact roughness meter such as for example an AltiSurf® 500 surface condition characterization station from AltiMet, using a sensor Alti Probe Optic optic with a point taking rate of 1000 Hz. The roughness thus measured is expressed by the usual parameters Ra which designates the arithmetic average roughness of the profile and Rz which represents the maximum roughness of the profile.

Thus, the sintered object may comprise particles of polyamide powder composition as described above, assembled by partial fusion, characterized in that it has a surface roughness, as measured under conditions explained in the examples , characterized by an Rz < 50 pm and a Ra < 10 pm.

The invention will be explained in more detail in the examples which follow.

Unless otherwise indicated, the percentages mentioned are percentages by weight relative to the weight of the final composition. Examples

A. Particle size

The powders were characterized in terms of particle size using an Insitec type laser diffractometer from Malvern with RT Sizer type software, according to the ISO 13320: 2009 standard.

Light from a laser is shined on particles that are moving in the air. Particles scatter and re-emit light, with smaller particles scattering light more than larger ones. The diffracted (scattered) light is received by a series of photodetectors placed at different angles. This will constitute the diffraction image of the sample. This image is used to measure particle size via the theory of light scattering called Mie theory.

The measurement is carried out on 30 g of powder. We note on the measured particle size distribution the parameters Dv10, Dv50, Dv90 and the span.

B. Viscosity measurement

The inherent viscosity of the powders was determined from the flow times of a solution and the solvent, measured in a viscometer with micro-Ubbelohde type 538-23 I IC tubes according to ISO 307:2019, unless used as solvent m-cresol and a temperature of 20°C.

Polyamide 11 A: Rilsan® Invent Natural marketed by the company Arkema, additive with 6000 ppm of H ₃ PO ₄ , inherent viscosity 1.2;

Polyamide 11 B: Polyamide 11 A having been subjected to aging in a vacuum oven at a temperature of 180°C for 7 hours, inherent viscosity 1.7;

Polyamide 11 C: Rilsan® Invent Natural marketed by the company Arkema, additive with 10,000 ppm of H ₃ PO ₄ , inherent viscosity 1.2

Polyamide 11 D: Polyamide 11 C having been subjected to aging in a vacuum study at a temperature of 180°C for 7 hours, inherent viscosity 1.3;

Polyamide 11 E: Rilsan® Invent Natural marketed by the company Arkema, additive with 600 ppm of H ₃ PO ₄ , inherent viscosity 1.2

Polyamide 11 F: Polyamide 11 E having been subjected to aging in a vacuum study at a temperature of 180°C for 7 hours, inherent viscosity 2.5; Polyamide 12 G: Orgasol® Invent Smooth marketed by the company Arkema, inherent viscosity 1.3;

Polyamide 12 H: Polyamide 12 G having been subjected to aging in a vacuum study at a temperature of 170°C for 7 hours; inherent viscosity 1.3;

Polyamide 11 I: Polyamide 11 F having been subjected to aging in a vacuum study at a temperature of 180°C for 7 hours, inherent viscosity 3.0.

[Table 1]

Example 1

50% by weight of polyamide 11 A powder and 50% by weight of polyamide 11 B powder were mixed dry in a Henschel mixer at a rotation speed of 900 rpm for 100 seconds. The properties of the respective powders are shown in Table 1 above.

The resulting powder composition was characterized with respect to particle size and inherent viscosity as indicated above.

Dv10 = 7 p.m.

Dv50 = 45 m

Dv90 = 79 p.m.

Span = 1.33

- IV = 1.45 The composition of polyamide powders obtained was used to manufacture by 3D printing by laser sintering a 1A XY specimen (1A specimen according to ISO 527-2, called “XY” because printed in the plane of the printer, it is - that is to say horizontally) on a P100 machine (marketed by the company EOS) by adjusting the thickness of the powder layer to 100 pm. The print settings used are as follows:

Laser power: 24W

Laser speed: 3000mm/s

Distance between two laser passes: 0.25mm

The roughness of the specimen was determined using a non-contact roughness meter (AltiSurf® 500 surface condition characterization station from AltiMet), using an Alti Probe Optic optical sensor with a point taking rate of 1000 Hz. Roughness is expressed by the usual parameters Ra and Rz. The roughness of the upper face of the horizontally sintered specimen is measured. The results are summarized in Table 2 below. It can be seen that this powder makes it possible to obtain sintered parts with low roughness.

Example 2

50% by weight of polyamide 11 C powder and 50% by weight of polyamide 11 D powder were mixed dry in a Henschel mixer at a rotation speed of 900 rpm for 100 seconds. The properties of the respective powders are shown in Table 1 above.

The resulting powder composition was characterized with respect to particle size and inherent viscosity as indicated above.

Dv10 = 7 p.m.

Dv50 = 45 pm

Dv90 = 79 p.m.

Span = 1.33

- IV = 1.25

The composition of polyamide powders obtained was used to manufacture by 3D printing by laser sintering, a 1A XY test piece as indicated in Example 1.

The roughness of the specimen was determined as indicated in Example 1. The results are summarized in Table 2 below. This powder therefore makes it possible to obtain sintered parts with very low roughness. Example 3

50% by weight of polyamide 12 G powder and 50% by weight of polyamide 12 H powder were mixed dry in a Henschel mixer at a rotation speed of 900 rpm for 100 seconds. The properties of the respective powders are shown in Table 1 above.

The resulting powder composition was characterized with respect to particle size and inherent viscosity as indicated above.

Dv10 = 33 p.m.

Dv50 = 40 m

Dv90 = 50 pm

Span = 0.43

- IV = 1.3

The composition of polyamide powders obtained was used to manufacture by 3D printing by laser sintering, a 1A XY test piece as indicated in Example 1.

The roughness of the specimen was determined as indicated in Example 1. The results are summarized in Table 2 below. This powder therefore makes it possible to obtain sintered parts with very low roughness.

Example 4 (comparative)

50% by weight of polyamide 12 F powder and 50% by weight of polyamide 12 I powder were mixed dry in a Henschel mixer at a rotation speed of 900 rpm for 100 seconds. The properties of the respective powders are shown in Table 1 above.

The resulting powder composition was characterized with respect to particle size and inherent viscosity as indicated above.

Dv10 = 7 p.m.

Dv50 = 45 pm

Dv90 = 79 p.m.

Span = 1.33

- IV = 2.75

The composition of polyamide powders obtained was used to manufacture by 3D printing by laser sintering, a 1A XY test piece as indicated in Example 1. The roughness of the specimen was determined as indicated in Example 1. The results are summarized in Table 2 below. It is observed that this powder results in obtaining sintered parts having high roughness.

Example 5

70% by weight of polyamide 11 C powder and 30% by weight of polyamide 11 D powder were mixed dry in a Henschel mixer at a rotation speed of 900 rpm for 100 seconds. The properties of the respective powders are shown in Table 1 above.

The resulting powder composition was characterized with respect to particle size and inherent viscosity as indicated above.

Dv10 = 7 p.m.

Dv50 = 45 m

Dv90 = 79 p.m.

Span = 1.33

- IV = 1.3

The composition of polyamide powders obtained was used to manufacture by 3D printing by laser sintering, a 1A XY test piece as indicated in Example 1.

The roughness of the specimen was determined as indicated in Example 1. The results are summarized in Table 2 below. This powder therefore makes it possible to obtain sintered parts with very low roughness.

Example 6

70% by weight of polyamide 12 G powder and 30% by weight of polyamide 12 H powder were mixed dry in a Henschel mixer at a rotation speed of 900 rpm for 100 seconds. The properties of the respective powders are shown in Table 1 above.

The resulting powder composition was characterized with respect to particle size and inherent viscosity as indicated above.

Dv10 = 33 p.m.

Dv50 = 40 pm

Dv90 = 50 pm

Span = 0.43

- IV = 1.3 The composition of polyamide powders obtained was used to manufacture by 3D printing by laser sintering, a 1A XY test piece as indicated in Example 1.

The roughness of the specimen was determined as indicated in Example 1. The results are summarized in Table 2 below. This powder therefore makes it possible to obtain sintered parts with very low roughness.

Example 7 (comparative)

50% by weight of polyamide 11 E powder and 50% by weight of polyamide 11 F powder were mixed dry in a Henschel mixer at a rotation speed of 9000 rpm for 100 seconds. The properties of the respective powders are shown in Table 1 above.

The resulting powder composition was characterized with respect to particle size and inherent viscosity as indicated above.

Dv10 = 7 p.m.

Dv50 = 45 m

Dv90 = 79 p.m.

Span = 1.33

- IV = 1.85

The composition of polyamide powders obtained was used to manufacture by 3D printing by laser sintering, a 1A XY test piece as indicated in Example 1.

The roughness of the specimen was determined as indicated in Example 1. The results are summarized in Table 2 below. We observe that this powder makes it possible to obtain sintered parts with high roughness.

[Table 2]

The surface properties of the sintered parts are considered acceptable When The roughness of a 1A XY specimen is characterized by an Rz less than 50 pm and a Ra less than 10 pm. Conversely, they are considered bad when Rz is greater than 50 pm and Ra is greater than 10 pm.

The results in Table 2 above highlight the interest of a composition of polyamide powders having an inherent viscosity and a specific span and whose variation in inherent viscosity is also limited. Indeed, the sintered objects obtained from these powders have good surface properties, and in particular low roughness.

[List of cited documents]

[US 2011/0237731]

[US 2018/094103]

Claims

REVEN DICATIONS 1. Composition of polyamide powders consisting of: (a) 10 to 90% by weight of a polyamide A powder having an inherent viscosity, calculated according to formula (1): [Math 3]

from the flow speed of a solution of the polymer and the solvent, measured in an Ubbelohde type viscometer according to standard ISO 307:2019, unless m-cresol is used as solvent and a temperature of 20°C, from 0.8 to 2.8, and having a span of between 0.4 and 1.40; (b) 90 to 10% by weight of a polyamide B powder, distinct from the polyamide A powder, having an inherent viscosity, calculated as indicated above, of 0.8 to 2.8, and having a span between 0.4 and 1.40; (c) 0 to 40% by weight of additives chosen from flow agents, stabilizing agents, optical brighteners and energy absorbing additives and/or fillers, in which the polyamide B powder comes from the polyamide powder A, it being understood that the difference between the respective inherent viscosities of powder A and B does not exceed 0.6 in absolute value. 2. Composition of polyamide powders according to claim 1, in which the polyamide A powder and the polyamide B powder are chosen respectively from PA 6, PA 66, PA613, PA1010, PA 1012, PA 11, PA 12, PA 11/10T and their copolymers. 3. Composition of polyamide powders according to one of claims 1 to 2, in which the polyamide powder A and/or B has a diameter Dv50 of between 30 and 60 pm. 4. Composition of polyamide powders according to one of claims 1 to 3, in which the polyamide A and/or B powder has an inherent viscosity of between 0.90 and 1.50. 5. Composition of polyamide powders according to one of claims 1 to 4, in which the difference between the inherent viscosity of the polyamide A powder and the polyamide B powder does not exceed in absolute value 0.5, of preferably 0.4 and especially 0.3. 6. Composition of polyamide powders according to one of claims 1 to 5, comprising 20 to 80%, preferably 30 to 70% and very particularly 40 to 60% by weight of polyamide A powder. 7. Composition of polyamide powders according to one of claims 1 to 6, comprising 20 to 80%, preferably 30 to 70% and very particularly 40 to 60% by weight of polyamide B powder. 8. Process for manufacturing the polyamide powder composition according to one of claims 1 to 7, by mixing: (i) a polyamide A powder having an inherent viscosity, calculated according to formula (1): [Math 4]

from the flow speed of a solution of the polymer and the solvent, measured in an Ubbelohde type viscometer according to standard ISO 307:2019, unless m-cresol is used as solvent and a temperature of 20°C, from 0.8 to 2.8, and having a span of between 0.4 and 1.40, said powder A having never been used; (ii) a polyamide B powder both have an inherent viscosity, calculated as indicated above, of 0.8 to 2.8, and having a span of between 0.4 and 1.40, said powder B having been used in 3D printing by laser sintering beforehand; as well as (iii) possible additives and/or fillers; in which the polyamide B powder comes from the polyamide A powder, it being understood that the difference between their respective inherent viscosities does not exceed 0.6 in absolute value. 9. Process for manufacturing a sintered object by 3D printing using Laser sintering, comprising the steps consisting of: (i) Preparation of a composition of polyamide powders as defined in one of claims 1 to 7; And (ii) 3D printing by laser sintering using said batch of polyamide powder so as to obtain a sintered object. 10. Manufacturing process according to claim 9, in which the polyamide A powder and the polyamide B powder are chosen respectively from PA 6, PA 66, PA613, PA1010, PA 1012, PA 11, PA 12, PA 11/ 10T and their copolymers. 11. Manufacturing process according to claim 9 or 10, in which the polyamide B powder comes from the polyamide A powder. 12. Manufacturing method according to one of claims 9 to 11, in which the difference between the inherent viscosity of the polyamide A powder and the polyamide B powder does not exceed in absolute value 0.5, preferably 0.4 and especially 0.3. 13. Manufacturing process according to one of claims 9 to 12, in which the polyamide A and/or B powder has an inherent viscosity of between 0.9 and 1.5. 14. Sintered object comprising particles of polyamide powder composition as defined in Claims 1 to 7, assembled by partial fusion, characterized in that it has a surface roughness, as measured by means of a roughness meter without contact (AltiSurf® 500 surface condition characterization station from ALtiMet), using an Alti Probe Optic optical sensor with a point taking rate of 1000 Hz, characterized by an Rz < 50 pm and a Ra < 10 pm.