Additive manufacturing process, polymer powder composition containing detection additives, and articles obtained thereby

Abstract

The present invention relates to a method for producing a three-dimensional object, comprising locally increasing the temperature of a powder using electromagnetic radiation in a heating chamber and locally melting / coalescing to form a layer of a predetermined thickness after cooling, the powder comprising, relative to the total weight of the composition, 60% to 99% by weight of polyamide; 1% to 40% by weight of an additive for optical and / or magnetic detection selected from the group consisting of pigments with a spinel structure containing transition metal cations, transition metal oxides, transition metal sulfides; 0% to 5% by weight of a flow agent, the powder having a particle size distribution D of 35 μm to 55 μm. 50 ; and particle size distribution D above 15 μm 10 ; and particle size distribution D below 100 μm 90 has.

Description

[Technical field]

[0001] The present invention relates to the manufacture of parts made of polymeric material, a process for agglomerating, layer by layer, in particular by melting or sintering, a polymer powder containing an optical and / or magnetic detection additive. The invention also relates to such a polymer powder fed to this process and consumed during the process. The invention is finally aimed at an object obtained by the process having particularly advantageous properties in the field of safety of the food production chain. [Background technology]

[0002] Among the wide variety of additive manufacturing techniques for manufacturing parts made of polymeric materials currently available, the present application falls within the framework of techniques that involve the agglomeration of powders layer by layer with the aim of obtaining a three-dimensional object. Therefore, in the context of this document, these methods will be designated exclusively by the term "additive manufacturing methods" or "3D printing". Objects obtained by such 3D printing methods are called "3D objects".

[0003] In this context, the agglomeration of powders by fusion, coalescence and / or "sintering" is caused by radiation that melts the agglomerating materials. For example, selective laser sintering (SLS) consists of locally densifying a powdered material by melting it under the action of a laser. Other sources of electromagnetic radiation that allow the melting of the powder can also be used, for example infrared, visible or UV radiation. Other notable methods of additive manufacturing by fusion of powder beds include laser sintering, multijet fusion, infrared sintering and rapid sintering.

[0004] The use of 3D objects made of thermoplastics is advantageous in industrial production chains, especially since such objects can be produced in small batches for specific applications or exhibit specific structural features. However, 3D objects made of thermoplastics are difficult to detect by foreign object detection measures that are usually implemented as part of online quality control, especially in the food industry. Therefore, if a 3D thermoplastic object breaks, its fragments may get into the product and pose a food safety risk.

[0005] The additive manufacturing processes known from the prior art and the powders they use are not capable of producing 3D objects that have both sufficient mechanical properties for industrial use and a strong detectability by foreign object detection means, e.g. magnetic detection or detection of abnormal colors. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention aims to remedy all or part of these drawbacks.

[0007] To this end, according to a first aspect, the invention relates to a method for the manufacture of a three-dimensional object, comprising a local increase in the temperature of a polyamide-based powder by electromagnetic radiation in a heated enclosure, causing the local fusion of a layer of a given thickness so as to form, after cooling, a solid layer of polyamide, said process being characterized in that said powder is present in an amount, relative to the total weight of the composition, of: 60% to 99% by weight of polyamide; 1% to 40% by weight of an optical and / or magnetic detection additive selected from the group consisting of pigments comprising a spinel structure containing cations of transition metals, sulfides of transition metals; 0% to 5% by weight, preferably 0.1% to 4.5% by weight, of a flow agent; The present invention is characterized in that it comprises The powder is Particle size distribution D within the range of 35μm to 55μm 50 ; Particle size distribution D over 15μm 10 , and Particle size distribution below 100μm D 90 Shows.

[0008] In an embodiment, the powder comprises: Particle size distribution D within the range of 35μm to 55μm 50 , Particle size distribution D within the range of 15μm to 25μm 10 , and Particle size distribution D within the range of 80μm to 100μm 90 has.

[0009] In one embodiment, the particle size distribution D 10 is greater than 10 μm, preferably greater than 15 μm, preferably greater than 17 μm, preferably greater than 20 μm.

[0010] In one embodiment, the particle size distribution D 50 is less than 110 μm, preferably less than 100 μm, preferably less than 95 μm, preferably less than 93 μm, preferably less than 90 μm. 50 is less than 80 μm.

[0011] In an embodiment, between 30% and 70% by mass of the powder is fresh polyamide powder, and between 30% and 70% by mass of the powder is polyamide powder recovered in the heated enclosure at the end of a previous manufacturing process, and the fresh polyamide powder has an inherent viscosity number of between 0.9 dL / g and 1.4 dL / g measured according to ISO 307:2019 at 25°C.

[0012] In one embodiment, the electromagnetic radiation causing local melting of the layer is 25 mJ / mm 2 It is laser radiation having an energy density of at least 1000 keV. The method used in this case is preferably called selective laser sintering, more commonly SLS (short for "selective laser sintering").

[0013] In a second aspect, the invention relates to a powder composition for additive manufacturing processes, comprising, based on the total weight of the composition: 60% to 99% by weight of polyamide; 1% to 40% by weight of a magnetic detection additive, preferably an optical detection additive and / or a magnetic detection additive selected from the group consisting of pigments comprising a spinel structure containing transition metal cations, transition metal oxides, transition metal sulfides; - 0% to 5% by weight, preferably 0.1% to 4.5% by weight, of a flow agent; The powder is Particle size distribution D within the range of 35μm to 55μm 50 ; Particle size distribution D over 15μm 10 , and Particle size distribution below 100μm D 90 Shows.

[0014] In one embodiment, the powder composition of the present invention is obtained by dry blending a natural polyamide powder with a polyamide powder containing a detection additive.

[0015] In one embodiment, the powder composition of the present invention comprises: 0.05% to 5% by weight of an optical detection additive selected from pigments containing a spinel structure containing a cation of a transition metal, and 1% to 35% by weight of a magnetic detection additive selected from transition metal oxides Includes.

[0016] Alternatively, the optical or magnetic detection additive is selected from sulfides of transition metals.

[0017] In one embodiment, the powder composition that is the subject of the present invention has an inherent viscosity number of 0.9 dL / g to 1.4 dL / g, measured according to ISO 307:2019.

[0018] In one embodiment, the powder composition that is the subject of the present invention has a value ΔT=(T m -T c ) onset has.

[0019] In certain embodiments, the powder compositions of the present subject matter include an optical detection additive, said optical detection additive including cobalt blue.

[0020] According to a third aspect, the invention relates to a three-dimensional object obtained by additive manufacturing from the composition that is the subject of the invention.

[0021] In one embodiment, the three-dimensional object is generally colored blue by an optical detection additive, preferably allowing optical detection in the wavelength range comprised between 0.5 μm and 12 μm.

[0022] In some embodiments, the three-dimensional object has a modulus of elasticity of 1700 MPa or greater, a tensile strength of 30 MPa or greater, and an elongation at break of 20% or greater in a first direction and 35% or greater in a second direction perpendicular to the first direction.

[0023] Other advantages, objects and particular features of the present invention will become apparent from the following non-limiting description of at least one particular embodiment of an additive manufacturing process, a powder composition for said process and a three-dimensional object obtained by said process, all of which are the subject of the present invention, with reference to the accompanying drawings. [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 represents the particle size distribution density curves as a function of particle size for two powder compositions according to the invention and for a natural polyamide 11 powder. [Diagram 2] FIG. 2 represents the cumulative distribution curves as a function of circularity for two powder compositions according to the invention and for a natural polyamide 11 powder. [Diagram 3]FIG. 3 shows a scanning electron micrograph of a powder composition of the present invention. [Figure 4] FIG. 4 represents a photograph obtained by X-ray tomography of a cross section of a 3D object obtained from an additive manufacturing process using the powder composition shown in FIG. [Diagram 5] FIG. 5 shows a schematic cross-section of a 3D object obtained by sintering the powder composition shown in FIG. [Figure 6] FIG. 6 shows a photograph of the powder composition for additive manufacturing taken with a scanning electron microscope. [Figure 7] FIG. 7 shows a photograph obtained by X-ray tomography of a cross section of a 3D object obtained from a sintering process using the powder composition shown in FIG. [Figure 8] FIG. 8 is a schematic cross-sectional view of a 3D model obtained by sintering the powder composition shown in FIG. [Figure 9] FIG. 9 shows a differential scanning calorimetry (DSC) of a particular powder composition according to the present invention. [Figure 10] FIG. 10 represents a graph of the force in MPa as a function of the elongation in the xy direction, expressed in %, obtained from an elongation test on a 3D object obtained by sintering the composition of Powder A, in a particular embodiment of the invention. [Figure 11] FIG. 11 is a graph of the force in MPa as a function of the elongation in xz direction, expressed in %, obtained at the end of an elongation test on a 3D object obtained by sintering powder composition A, in a particular embodiment of the invention. [Figure 12] FIG. 12 is a graph showing the volume particle size distribution as a function of particle size for powder compositions of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] This description is non-limiting, and each feature of one embodiment can be advantageously combined with other features of other embodiments.

[0026] The inherent viscosity number values ​​of the polyamides disclosed herein are measured at 25°C according to ISO 307:2019.

[0027] The powder composition for the additive manufacturing process by sintering according to the invention comprises, relative to the total weight of the composition: 60% to 99% by weight of polyamide; 1% to 40% by weight of a magnetic detection additive, which may be an optical detection additive and / or a magnetic detection additive, preferably selected from the group consisting of pigments comprising a spinel structure containing transition metal cations, transition metal oxides, transition metal sulfides; 0% to 5% by weight, preferably 0.1% to 4.5% by weight, of a flow agent; Contains The powder is Particle size distribution D within the range of 35μm to 55μm 50 ; Particle size distribution D over 15μm 10 , and Particle size distribution below 100μm D 90 Shows.

[0028] The powder contains primarily polyamide and is therefore called "polyamide-based."

[0029] The characteristics and components of a powder composition for additive manufacturing by sintering (hereinafter referred to as "sintered powder") will be described in detail.

[0030] The particles of the sintered powder preferably have a spherical shape.

[0031] According to the present invention, the detection additive can be selected to allow for magnetic or optical detection, or two additives can be used, a first additive that allows for magnetic detection and a second additive that allows for optical detection, or an additive that allows for both optical and magnetic detection.

[0032] Selection of polyamide or polyamide blend The polyamide can be selected from any available polyamide, or mixture of polyamides, that makes it possible to obtain the particle size characteristics of the composition of the present invention.

[0033] Preferably, the polyamide is chosen from polyamides comprising one of the following monomers: PA6, PA10, PA11, PA12 and mixtures thereof.

[0034] In particular, PA11 can be used due to its advantageous properties and bio-based origin. A "bio-based" product is one that is made wholly or partly from materials of biological origin.

[0035] Characteristics of sintered powder and its use temperature (T 2 ) Preferably, the sintered powder has a service temperature range of 160° C. to 210° C. The service temperature window is determined by the initial temperature (T ei.m or T m,onset ) and the final temperature (T ef,c or T c,onset ) The difference between these two temperatures is called ΔT, and ΔT=T ei.m -T ef.c or ΔT = (T m -T c ) onset It is expressed as:

[0036] Melting peak T m,onset The initial temperature estimated from the crystallization peak T c,onset The final temperature estimated from the above can be better understood by referring to the paper "Polymers Applicable for Laser Sintering (LS)" by Schmid M. & Wegener K, published in 2016, Additive Manufacturing: Procedia Engineering, 149, 457-464, especially the relevant part in FIG. 9.

[0037] Preferably, ΔT=(Tm -T c ) onset is included in the range of 30°C to 50°C. This ΔT is advantageous because it allows the use temperature T2 to be defined. More preferably, ΔT=(T m -T c ) onset is included in the range of 30℃ to 35℃.

[0038] If ΔT is less than 30° C., the polymer may be overly sensitive to changes in state when energy is added.

[0039] If ΔT exceeds 50° C., a stable use temperature T2 cannot be defined, which may result in total agglomeration of the powder bed and recovery problems.

[0040] Similarly, this range ΔT = (T m -T c ) onset The energy supply must be adjusted depending on the operating temperature T2 selected in. Excessive energy input can have detrimental consequences, such as deformation of the 3D printed object.

[0041] Detection Additives According to an essential feature of the present invention, the powder composition comprises a detection additive, which is advantageously a water-insoluble and non-toxic inorganic compound, preferably an inorganic compound of spinel type. The powder composition of the present invention comprises 1% by weight to 40% by weight of the detection additive, relative to the total weight of the composition.

[0042] The detection additive may be an optical detection additive. More specifically, the powder composition of the invention may comprise 0.05% to 5% by weight, for example 0.05% to 0.5% by weight, of an optical detection additive, relative to the total weight of the composition. The latter is advantageously selected from pigments comprising a spinel structure containing cations of transition metals. This type of pigment has the advantage of being non-toxic. In particular, the transition metal cations remain trapped within the spinel structure and cannot be solubilized under normal contact conditions with food and drink, nor in the case of accidental ingestion passing through the intestinal tract. Spinels provide excellent thermal stability under the laser beam implemented in the SLS laser sintering process technology. The use of these pigments is therefore particularly preferred for powder compositions intended for this application.

[0043] According to one embodiment, the pigment is a blue pigment, preferably cobalt aluminate available under the trade name PB28 (CAS number: 1345-16-0). Preferably, the optical detection additive used allows optical detection, optionally by infrared light, for example, the optical detection additive used allows optical detection in the wavelength range from 0.5 μm to 12 μm.

[0044] According to another embodiment, the pigment comprises an olivine or rutile structure.

[0045] It should be noted that since the optical detection additive is substantially uniformly present in the powder composition, parts additively manufactured from this powder will be colored throughout.

[0046] The detection additive may be a magnetic detection additive. More specifically, the powder composition of the present invention may contain 1 wt % to 40 wt % of the magnetic detection additive, based on the total weight of the composition.

[0047] The magnetically detectable additive is preferably selected from oxides containing transition metals. For example, the magnetically detectable additive is an iron oxide, such as natural or synthetic magnetite (Fe3O4). This spinel oxide is insoluble in water and is non-toxic. Furthermore, it does not tend to form metal salts that may be released from parts obtained by additive manufacturing from this powder. Natural magnetite is preferred over synthetic magnetite.

[0048] It is also possible to use non-spinel transition metal oxides or transition metal sulfides as either the optical or magnetic detection additives. Obviously, the magnetic detection additive must be selected to exhibit specific magnetic properties that are easily detectable.

[0049] In a preferred embodiment, the powder composition of the present invention comprises both 0.05% to 5% by weight of an optical detection additive selected from pigments and 1% to 40% by weight of a magnetic detection additive selected from transition metal oxides.

[0050] Flow agent selection The composition according to the invention further comprises a sufficient amount of flow agent so that the composition remains free flowing and fluid and forms a uniform, homogenous and flat layer during the layer building process in the powder bed (PBF - Powder Bed Fusion), also called Layer-by-Layer Sintering (SLS, LS) of polymers.

[0051] The composition according to the invention contains from 0 to 5% by weight of flow agent relative to the total weight of the composition, preferably from 0.1 to 4.5% by weight.

[0052] The flow agents are selected from those commonly used in the field of sintering of polymer powders, such as silica, precipitated silica, silica fume, hydrated silica, silica glass, fumed silica, vitreous phosphates, vitreous oxides.

[0053] Preferably, the flow agent has a small contact surface.

[0054] Preparation of the Powder Composition According to one embodiment, the powder composition according to the invention is obtained according to a manufacturing method comprising a first step of mixing a so-called "natural" polyamide powder with a flow agent and at least one of the following steps: mixing the previously obtained composition with a polyamide powder composition containing an optical detection additive; Mixing the previously obtained composition with a composition containing a magnetically detectable additive.

[0055] In some embodiments, these last two mixing steps with the composition containing the detection additive are performed sequentially; note that the order can be reversed.

[0056] The natural polyamide powder is a powder composition comprising 95% to 100% by weight of polyamide, preferably at least 99% by weight of polyamide.

[0057] The polyamide powder composition containing the optical detection additive can be obtained by conversion of a homogeneous liquid or solid mass containing the polyamide and the above-mentioned optical additive into a powder or by solid state polycondensation, drying followed by selective comminution.

[0058] The composition containing the magnetically detectable additive may be the magnetically detectable additive in pure form (i.e., containing at least 95% of the magnetically detectable additive) or may be a composition containing a polyamide homogenized by dry mixing with the magnetically detectable additive.

[0059] The above-mentioned mixing step can be carried out by dry mixing (known under the English term "dry blend") or by a compounding process (known under the English term "master batch"). Compounding requires the subsequent steps of selective grinding of the resulting mass and adjustment of the viscosity by solid-state polycondensation and drying; for this reason, dry mixtures are preferred.

[0060] Dispersion of flow agents requires the application of significant mixing energy to obtain good homogenization, which can damage the detection additive. Therefore, it is preferred to select a dry premix of flow agent and natural polyamide powder during the first mixing step, prior to at least one mixing step with a composition containing a detection additive, which is of lower strength than the first mixing step.

[0061] In one embodiment, the mixing step is performed by cryogenic grinding, a method well known to those skilled in the art and will not be described in detail herein.

[0062] In these other embodiments, the method of obtaining a uniform and dispersed powder dry mix of all components is to determine the initial distribution and the final target distribution, i.e. Particle size distribution D within the range of 35μm to 55μm 50 , Particle size distribution D within the range of 15μm to 25μm 10 , and Particle size distribution D within the range of 80μm to 100μm 90 is adapted according to

[0063] According to one embodiment, the final target particle size distribution of the powder is as follows: Particle size distribution D within the range of 35μm to 55μm 50 , Particle size distribution D over 20μm 10 , and Particle size distribution D below 80μm 90

[0064] Particle size distribution of the powder composition Particle size distribution D of the powder composition 10 is greater than 10 μm, preferably greater than 15 μm, preferably greater than 17 μm, preferably greater than 20 μm.

[0065] Such a particle size distribution D of the powder composition 10is advantageous in avoiding the presence of large amounts of fine particles or dust that may volatilize into the air and pose health risks if inhaled and accumulated, or cause irritation to the eyes and skin upon contact with these fine dust particles.

[0066] Particle size distribution D of the powder composition 50 is included in the range of 35 μm to 55 μm. Preferably, the particle size distribution D 50 is comprised between 38 μm and 45 μm, very preferably it is comprised between 38 μm and 40 μm.

[0067] Applicant has determined during testing that these particle size distribution ranges D 50 It was noticed that this allows to obtain the best performance in terms of final resolution, geometric definition of the resulting part, together with better coverage and good flowability of the powder at temperature for the PBF powder bed process using layers of 80 μm to 120 μm.

[0068] Particle size distribution D of the powder composition 90 is less than 110 μm, preferably less than 100 μm, preferably less than 95 μm, preferably less than 93 μm, preferably less than 90 μm. 50 is less than 80 μm.

[0069] Such a particle size distribution D 90 is advantageous for using the powder in additive manufacturing processes with layer thicknesses between 80 μm and 160 μm, for example with layer thicknesses of 100 μm. 90 The value is chosen to be smaller than the layer size expected in the additive manufacturing process.

[0070] According to an embodiment, the powder composition comprises: Particle size distribution D within the range of 35μm to 55μm 50 , Particle size distribution D within the range of 15μm to 25μm 10 , and Particle size distribution D within the range of 80μm to 100μm 90 has.

[0071] For additive manufacturing by sintering, it is advantageous to have a dense distribution and uniform morphology, but if the particle size of the powder composition is too homogeneous, the geometric arrangement will make the powder more cohesive, resulting in the phenomenon of "caking" (i.e., powder clumping). Therefore, the applicant considers that such a particle size distribution D 10 , D 50 , and D. 90 It was noted that these particle size distributions D 10 , D 50 and D. 90 is advantageous since it limits the phenomenon of powder agglomeration and facilitates powder removal from the parts obtained by additive manufacturing by sintering.

[0072] In addition, the above-mentioned particle size distribution value D of the powder composition 10 , D 50 and D. 90 is determined by still image analysis methods according to the ISO 13322-1:2014 standard.

[0073] FIG. 1 shows the distribution density curves as a function of particle size (expressed in micrometers (abbreviated μm)) for the three powders. Curve 105 shows the particle size distribution of a so-called natural PA11 powder, i.e. a powder containing at least 99% PA11. Curve 110 shows the particle size distribution of a powder composition A according to the invention, in which the polyamide in the powder composition is PA11 and which comprises an optical detection additive. Curve 115 shows the particle size distribution of a powder composition B according to the invention, where the polyamide in the powder composition is polyamide 11 and the powder composition contains both an optical detection additive and a magnetic detection additive.

[0074] It should be emphasized that the optical detection additive and / or the magnetic detection additive are selected to achieve a particle size distribution of the powder composition as detailed above. As can be seen from Figure 1, the particle size distribution of the powder composition according to the invention including the detection additive remains close to that of the natural PA11 powder, with a density peak of about 50 μm.

[0075] Advantageously, the powder composition that is the subject of the present invention comprises 90%, and more preferably 99%, of particles having a size between 10 μm and 120 μm, preferably between 20 μm and 90 μm, and more preferably between 20 μm and 80 μm.

[0076] FIG. 12 is a graph showing the particle size distribution as a function of particle size for powder composition A, in which the polyamide is PA11 and the powder composition A includes an optical detection additive. The data shown in FIG. 12 was obtained from particle size measurements performed using a Malvern Panalytical Mastersizer 3000® particle size analyzer. The graph shows histogram bars 140 showing the percentage of particles in the powder composition associated with each size expressed in μm. Curve 150 represents the cumulative percentage of particles having a size below a threshold value expressed in μm. As can be seen from the graph in FIG. 12, powder composition A includes 90% of particles with a size between 10 μm and 120 μm.

[0077] Powder Composition Shape Factor The shape factor is a dimensionless quantity used in image analysis and microscopy as a numerical description of a particle's shape, regardless of the particle's size.

[0078] Circularity index f circ is the formation factor calculated as follows:

[0079]

number

[0080] P is the perimeter and A is the surface area of ​​the image of the powder particle.

[0081] Thus, the circularity of a parallelepiped mica is close to 0, while the circularity index of a sphere is 1.

[0082] The rules and nomenclature for the description and quantitative expression of particle shape and morphology specified in ISO 9276-6:2008 are explained below.

[0083] Preferably, the cumulative distribution f of the powder composition according to the present invention 10 is less than or equal to 0.15. Highly preferably, the cumulative distribution f 10 is less than or equal to 0.10. In other words, only 10% of the powder particles have a circularity index less than or equal to 0.15, preferably less than or equal to 0.10. In other words, 90% of the particles have a circularity index greater than 0.1, preferably greater than 0.15.

[0084] Cumulative distribution of powder composition f 50 is 0.6 or less. Preferably, the cumulative distribution f 50 is less than or equal to 0.55. In other words, only 50% of the powder particles have a circularity index less than or equal to 0.6, preferably less than or equal to 0.55. In other words, 50% of the powder particles have a circularity index greater than 0.55, preferably greater than 0.6.

[0085] Cumulative distribution of powder composition f 90 is 0.8 or less. Preferably, the cumulative distribution f 90 is less than or equal to 0.75. In other words, 90% of the powder particles have a circularity index less than or equal to 0.8, preferably less than or equal to 0.75. In other words, 10% of the powder particles have a circularity index greater than 0.75, preferably greater than 0.8.

[0086] Figure 2 plots the cumulative distribution (expressed as a percentage) on the vertical axis as a function of circularity (a unitless index) on the horizontal axis. The following can be seen from Figure 2: Curve 205 shows the cumulative distribution of a so-called natural PA11 powder, i.e. a powder containing at least 99% by weight of PA11. Curve 210 shows the cumulative distribution of a powder composition according to the present invention, in which the polyamide in the powder composition is PA11 and the powder composition includes an optical detection additive. Curve 215 shows the cumulative distribution of a powder composition according to the present invention, in which the polyamide in the powder composition is polyamide 11, and in which the powder composition includes both an optical sensing additive and a magnetic sensing additive.

[0087] The optical and / or magnetic detection additives are preferably selected with a view to obtaining the cumulative distribution of the powder composition as detailed above. From Figure 2 it can be seen that the cumulative distribution of the powder composition according to the invention containing the detection additive remains close to the particle size distribution of the distribution of the natural PA11 powder.

[0088] The morphology of the particles is important for the flowability of the mixture and for the densification of the powder bed during successive coatings, but also for the residual porosity in the final part obtained. A good sphericity of the powder, in combination with a very strict distribution, i.e. a cumulative distribution of the type described above, makes it possible to obtain a natural densification of the powder bed by compression and geometric arrangement of the layer. This layer is then exposed to the laser energy for fusion and coalescence, which promotes the densification of parts with low residual porosity. Conversely, a more widely dispersed and very inhomogeneous powder tends to be more chaotically organized, since some of the larger particles may not melt, which may result in a lower densification of the powder bed.

[0089] To illustrate this point, Figures 3 and 6 show two views of two powders taken under a scanning electron microscope at the same magnification: Figure 3 shows a powder with a sphericity comparable to that of the powder composition that is the subject of the present invention, while the powder in Figure 6 is shown for comparison.

[0090] These powders have a capacity of 34 mJ / mm 2(550 and 850) are subjected to an SLS laser sintering process. X-ray tomography of a cross section of the 3D object obtained after the sintering process is shown in Figures 4 and 7. Schematic diagrams of powders 30 and 60 shown in Figures 3 and 6 and cross sections of 3D objects obtained by sintering these powders are shown in Figures 5 and 8.

[0091] The powder 30 shown in FIG. 3 is a powder with good circularity uniformity, with a particle circularity of 0.4-0.8, with an average equal to 0.65. The powder 30 is placed on a pre-solidified layer 505 and is subjected to a 34 mJ / mm 2 which is obtained by tomography of the 3D object, as shown in section 510 of FIG. 5, and which allows to obtain a low and distributed residual porosity, as shown in section 410 of FIG. 4. The porous areas 420, shown in black in FIG. 4, are shown in the form of white cavities in FIG. 5. As shown in FIGS. 7 and 8, these porous areas are fewer in number and better distributed than those observed in the cross-sections of 3D objects obtained by sintering from powders with a lower uniformity of grain circularity.

[0092] 6 is less homogeneous than powder 30, with a particle circularity between 0.1 and 0.8, averaging equal to 0.55. Powder 60 is placed on a pre-consolidated layer 805 and is heated to 34 mJ / mm 2 5, resulting in a 3D object with low homogeneity and high residual porosity, as shown in section 710 of FIG. 7 obtained by tomography and shown as a schematic cross-sectional view 810 of FIG. 8.

[0093] 3D object manufacturing process It is recalled that the present patent application is within the framework of techniques using powder beds with layer-by-layer aggregation to obtain three-dimensional objects. In the present context, these methods are referred to exclusively by the terms "additive manufacturing" or "3D printing". Objects obtained by such 3D printing methods will be called "3D objects".

[0094] More specifically, the present invention relates to a process for additive manufacturing by powder bed fusion (PBF) layer by layer from polyamide powder in a heated enclosure. These methods include laser sintering (LS), selective laser sintering (SLS), multi-jet fusion (MJF), infrared sintering (IRS), and high speed sintering (HSS), among others.

[0095] Whatever additive manufacturing method is chosen, the process according to the invention aims to produce a 3D polyamide object obtained from a polyamide powder composition and containing a detection additive.

[0096] The process according to the invention is carried out in a closed vessel preheated to a set temperature T1. The atmosphere in the vessel is rich in nitrogen (or under vacuum) and low in oxygen to limit oxidation of the polymer powder, which gradually elongates the polymers that make up the polymer powder particles and is the main ageing mechanism for said powder. This elongation of the polymers tends to increase the inherent viscosity of the polymer. Limiting the temperature oxidation of the powder facilitates recycling of unused powder, which contributes greatly to the economy of the process according to the invention. Preferably, the oxygen level is less than 5% by volume, preferably less than 2% by volume, more preferably less than 1% by volume.

[0097] The holding temperature T1 is the crystallization temperature T c It is advantageous to set the preheating temperature T1 at about 20 to 30° C. According to an advantageous embodiment, for powders based on polyamide PA11 and / or PA12, the preheating temperature T1 is advantageously between about 140° C. and about 160° C., preferably between about 142° C. and about 158° C. According to a particular embodiment, the heating temperature is equal to the holding temperature.

[0098] More generally, the holding temperature T1 is preferably 150°C to 185°C.

[0099] The process that is the subject of this invention involves depositing a uniform layer of a bed of polyamide powder in a preheated enclosure.

[0100] Immediately after the deposition of each layer, the surface of the powder bed is examined, usually by infrared, to determine the T m Approximately 8% to 14% lower than the melting point T m Heating to this temperature T2 allows the polyamide powder to be kept quite close to its melting point without reaching it. This temperature is also called the operating temperature of the PBF system.

[0101] According to an advantageous embodiment, for powders based on polyamide PA11 and / or PA6, the temperature T2 is between about 183°C and about 204°C.

[0102] More typically, the temperature T2 is between 168°C and 206°C.

[0103] To obtain dense parts, the melting of the powder is necessary. In order to avoid uncontrolled flow of the liquid polymer, this melting must be temporary, rapid, localized and controlled, and for this reason must be short, i.e. the melting temperature T of the polymer must not be exceeded immediately after the local melting. m The temperature T below which a polymer can recrystallize from the melt R The cooling must be performed to the temperature T R can be around temperature T2, T R is included between T1 and T2.

[0104] To obtain said localized and controlled fusion of selected portions of the powder bed, electromagnetic radiation is applied to a target area of ​​the polyamide powder, locally increasing the temperature and making it possible to agglomerate the polyamide particles in the target area. Depending on the method selected, the electromagnetic radiation is for example visible, infrared or near infrared laser radiation. The local temperature T of the melting zone is L is the polyamide T m Approximately 8% to 14% higher than the melting point T m In this way, a temporary liquid phase is formed, but T LIf it is too high, the viscosity of the molten polymer becomes too low, which may cause sagging.

[0105] As an example, the temperatures T1 and T2 carried out during the sintering process according to the invention are summarized in Table 1 below, and the melting points T m and the crystallization temperature T c Compare with.

[0106] [Table 1]

[0107] The samples are as follows: Powder A is a powder composition according to the present invention, wherein the polyamide in the powder composition is PA11 and the powder composition comprises an optical detection additive. Powder B is a powder composition according to the present invention, wherein the polyamide in the powder composition is polyamide 11, and the powder composition comprises both an optical detection additive and a magnetic detection additive.

[0108] To determine any interval around the melting or crystallization temperature, use the initial temperature (T m,onset ) and the final temperature (T c,onset ) is preferably utilized, and two methods for determining these reference values ​​can be implemented without departing from the invention.

[0109] To illustrate this point, Figure 9 shows a DSC (differential scanning calorimetry) of a powder composition according to the invention based on PA11, which shows an initial temperature rise curve 910 and a cooling curve 920. The melting and crystallization temperatures are indicated by the corresponding peaks (T c and T m The initial temperature (T m,onset ) and the extrapolated final temperature for the crystallization peak (T c,onset) are shown in this graph.

[0110] Once all of the target areas of the powder bed layer have been scanned by the electromagnetic radiation source, a new powder bed is deposited and leveled on top of the previous powder bed. It must be remembered that the powder is self-supporting, i.e. it builds on the powder previously deposited during the process. Thus, each time a new powder bed is deposited, solidification of a part of the new powder bed begins. Each solidified part of the powder bed corresponds to a layer or slice of the 3D object obtained at the end of the process.

[0111] The thickness of each layer is typically about 50 μm to about 150 μm, preferably about 70 μm to about 120 μm, and more preferably about 80 μm to about 110 μm. After deposition of each layer, it is heated to a temperature T2 as described above.

[0112] According to one embodiment of the method, the sintering that is the subject of the invention is carried out by SLS, and in order to achieve an operating temperature T2 between 180° C. and 199° C., for example equal to 188° C., the electromagnetic radiation causing the local fusion of the layers must be 25 mJ / mm 2 Laser radiation with an energy density of more than 25 mJ / mm 2 These energy densities make it possible to avoid delamination, i.e. separation between two successive layers of solidified polyamide.

[0113] The energy density is calculated using a simplified version of Morgan's rule, which is given as:

[0114]

number

[0115] P is the power of the laser, expressed in watts. S is the scan spacing (hatch deviation) and is expressed in millimeters (mm). v is the laser speed, expressed in mm / sec. r is the laser radius, expressed in mm.

[0116] By way of example, the operating conditions of the sintering process according to the invention using different SLS systems are summarized below in Table 2. These operating conditions are carried out for a sintered powder composition containing PA11, at an operating temperature T2 approximately equal to 188° C. and with a fixed layer thickness of 100 μm.

[0117] [Table 2]

[0118] The 3D object resulting from the sintering process is still covered with non-agglomerated powder, which is removed by mechanical and / or chemical means well known to those skilled in the art (air or water jets, brushing, sanding, solvent phase treatment, ultrasonic bath, treatment with HF solution, etc.) and not described in detail here.

[0119] Reuse of the polyamide powder composition which is the subject of the present invention During the process as described above, part of the powder composition for additive manufacturing processes using a PBF (Powder Bed Fusion) powder bed, introduced into the heated chamber, does not solidify, and advantageously this powder is collected and sieved with the aim of reusing it by mixing it with a composition of fresh polyamide powder, i.e. powder that has not yet been used in the sintering process.

[0120] Preferably, the powder composition according to the invention comprises a mass fraction of fresh polyamide powder composition between 20% and 70% and a mass fraction of polyamide powder recovered after the previous production between 80% and 30%. More preferably, the deposited powder bed comprises a mass fraction of fresh polyamide powder composition between 25% and 55% and a mass fraction of polyamide powder recovered at the end of the previous production between 75% and 45%.

[0121] Adding fresh powder to used powder adds undamaged (non-thermo-oxidized) polyamide particles that have not already been damaged or deformed by the sintering process prior to inducing thermal oxidation, thus lowering the inherent viscosity of the mixture with each spreading cycle, thereby maintaining the inherent viscosity in the desired range.

[0122] Preferably, the fresh polyamide powder has an inherent viscosity number of 0.9 dl / g to 1.4 dl / g, measured in accordance with ISO 307:2019. An inherent viscosity number of less than 1.4 dl / g, preferably less than 1.2 dl / g, makes it possible to keep the inherent viscosity of the polyamide powder composition sufficiently low, even if this composition is obtained by mixing fresh powder with recycled powder used in a PBF powder bed process.

[0123] The method for measuring the inherent viscosity number of plastics and polyamides complies with ISO 307:2019 and is based on the determination of the viscosity number of dilute solutions of polyamides in certain solvents specified in the aforementioned standard.

[0124] This viscosity is responsible for the rheology of the melting and / or coalescence phenomena; the deposited particles must melt and coalesce to form a dense, non-porous mass without uncontrolled creep. The internal viscosity affects the mechanical properties of the part, its appearance, and the surface finish of the finished product.

[0125] For optimal use of the powder composition, it is recommended not to exceed a certain number of recycles for the same powder, i.e. not to recycle a mixture of powders that have been subjected to multiple thermal cycles in the PBF powder bed process. Collection of the recycled powder and its sieving should be carried out before mixing with fresh polyamide powder in order to remove agglomerates of powder particles.

[0126] The number of possible cycles depends on the degree of oxidation of the recycled powder, since inherent viscosity is known to increase with the degree of oxidation. The inventors note that on average the same powder can be reused for 8-10 recycling cycles, but this depends primarily on the duration of exposure of the powder to high temperatures throughout the thermal cycles (pre-heating, manufacturing at constant temperature, and cooling) it undergoes throughout the manufacturing process in the PBF or during cooling to temperatures below 60°C, and the oxygen levels within the enclosure.

[0127] Recycling is facilitated by the fact that fresh powders have the abovementioned inherent viscosity. In fact, to produce high-quality parts by the method according to the invention, it is possible to use powders whose inherent viscosity number lies slightly outside this zone between 0.9 dl / g and 1.4 dl / g, so that fresh powders can be recycled in the PBF process under attractive economic conditions and according to the abovementioned technical conditions (mixed with fresh powder in a proportion of 30% to 60%), preferably observing for fresh powder a continuous regeneration at 50% and a correct sieving of the powders already used.

[0128] As an example, a composition according to the invention is Powder A (already mentioned above). Fresh Powder A has an inherent viscosity equal to 1.3 dl / g. After carrying out the sintering process, a new powder composition according to the present invention is formed by mixing half fresh powder and half recycled powder. After two cycles, the inherent viscosity number of the powder composition is around 1.7 dl / g. After 3-6 cycles, the powder composition has an inherent viscosity of about 2.05 deciliters / gram.

[0129] [Table 3] Instead of the number of cycles, one can consider the heating time, i.e. the time the powder composition is exposed to heating in a heated enclosure. This approach may be more accurate since the manufacturing cycles may be longer or shorter. The table above also indicates the temperature-time value used to arrive at the inherent viscosity number. This heating time is equal to 0 for fresh powders, greater than 20 hours for powders that have undergone 1-2 cycles, and greater than 50 hours for powders that have undergone 3-6 recycling cycles.

[0130] Magnetic detection of the resulting 3D object The presence of optical or magnetic detection additives in the powder composition of the present invention allows detection of 3D objects obtained by sintering of this powder.

[0131] In the case of magnetic detection, the 3D object obtained from the powder containing the magnetic detection additive is detected, for example, by electromagnetic induction or according to any other method of detecting magnetic objects, which are well known to those skilled in the art and will not be described here.

[0132] Optical detection of the resulting 3D object The 3D objects obtained by additive manufacturing of powder compositions are colored throughout the mass, that is, the material that constitutes the 3D object is colored, and the object does not only exhibit coloration on its outer surface.

[0133] This property causes all sides of the fragments of a broken 3D object to display a color that corresponds to the optical detection additive used, so that the fragments of the object, which are colored chunks, can be detected by optical detection methods when the object breaks.

[0134] Preferably, the 3D object is entirely blue. Blue is a rare color in food, so that it stands out more than other colors when it is in food. In particular, infrared detection is possible with irradiation in the wavelength range of 0.5 μm to 12 μm. Preferably, the optical detection additive allows optical detection in the wavelength range comprised between 0.5 μm and 12 μm. These optical detection methods, even those applied to pieces of plastic material, are well known to those skilled in the art and will not be described in detail here.

[0135] Mechanical properties of 3D objects obtained by sintering according to the invention Preferably, the 3D object obtained by sintering according to the invention has a minimum tensile strength of at least 40 MPa (megapascals), preferably at least 44 MPa.

[0136] When the 3D object is obtained by sintering a powder containing a magnetically detectable additive, the 4D object preferably has a minimum tensile strength of at least 30 MPa, very preferably at least 35 MPa.

[0137] Preferably, the 3D object obtained by sintering according to the invention has a minimum elastic modulus of greater than or equal to 1600 MPa, preferably greater than or equal to 1750 MPa.

[0138] In particular, standardized specimens of 3D objects obtained from the sintering process according to the invention were tested for tensile strength and modulus expressed in megapascals (MPa) and for elongation at break expressed as a percentage. The 3D objects tested are obtained from a sintered powder composition according to the invention, comprising an optical detection additive and in which the polyamide is PA11. The test method carried out complies with the ISO 527-1:2019 standard for the measurement of tensile properties.

[0139] [Table 4]

[0140] Preferably, the 3D object obtained by the method according to the invention has an elongation at break of 20% or more in a first orientation and 35% or more in a second orientation perpendicular to the first orientation.

[0141] For tests performed according to ISO 527-1:2019, the results of which are shown in Table 4 above, these elongations at break were measured to be 25% and 40%.

[0142] Figures 10 and 11 show graphs corresponding to the test results shown above for elongation at break: Figure 10 shows the results of tensile elongation tests in the xy orientation, and Figure 11 shows the results of tensile elongation tests in the xz orientation.

Claims

1. 1. A process for manufacturing a three-dimensional object, comprising causing a local increase in the temperature of a polyamide-based powder by electromagnetic radiation in a heated enclosure, causing local fusion of a layer of a predetermined thickness so as to form a solid layer of polyamide after cooling, said process comprising, with respect to the total weight of the composition, from 60% to 99% by weight of polyamide; - 1% to 40% by weight of an optical and / or magnetic detection additive selected from the group consisting of pigments comprising a spinel structure containing transition metal cations, transition metal oxides, transition metal sulfides; - 0% to 5% by weight, preferably 0.1% to 4.5% by weight, of a flow agent; The present invention is characterized in that it comprises The powder is Particle size distribution D between 35 μm and 55 μm ５０ ; Particle size distribution D exceeding 15 μm １０ , and Particle size distribution D less than 100 μm ９０ Showing, process.

2. a mass fraction of 30% to 70% of said powder is fresh polyamide powder and a mass fraction of 70% to 30% of said powder is polyamide powder recovered in said heated enclosure at the end of a previous manufacturing process, said fresh polyamide powder having an inherent viscosity number of 0.9 dl / g to 1.4 dl / g measured according to ISO 307:2019 at 25°C; The method of claim 1.

3. The electromagnetic radiation is 25 mJ / mm ２ and wherein the laser radiation has an energy density of greater than The method according to claim 1 or 2.

4. A powder composition for additive manufacturing processes by local increase in temperature of a polyamide-based powder by electromagnetic radiation in a heated enclosure, causing local fusion of a layer of a predetermined thickness so as to form a solid layer of polyamide after cooling, said powder comprising, relative to the total weight of said composition: from 60% to 99% by weight of polyamide; - 1% to 40% by weight of an optical and / or magnetic detection additive, preferably an optical and / or magnetic detection additive selected from the group consisting of pigments comprising a spinel structure containing cations of transition metals, oxides of transition metals, sulfides of transition metals; - 0% to 5% by weight, preferably 0.1% to 4.5% by weight, of a flow agent; The present invention is characterized in that it comprises The powder is Particle size distribution D between 35 μm and 55 μm ５０ ; Particle size distribution D exceeding 15 μm １０ , and Particle size distribution D less than 100 μm ９０ 1 shows a powder composition.

5. 5. The powder composition according to claim 4, Particle size distribution D between 35 μm and 55 μm ５０ , Particle size distribution D between 15 μm and 25 μm １０ , and Particle size distribution D between 80 μm and 100 μm ９０ A powder composition showing the above.

6. obtained by dry mixing a natural polyamide powder with a polyamide powder containing a detection additive, 5. The powder composition of claim 4.

7. 0.05% to 5% by weight of an optical detection additive selected from pigments containing a spinel structure containing cations of a transition metal, and - comprising a magnetic sensing additive of 1% to 35% by weight of a transition metal oxide; 5. The powder composition of claim 4.

8. having an inherent viscosity number of 0.9 to 1.4 deciliters / g measured according to ISO 307:2019 at 25°C; 5. The powder composition of claim 4.

9. Value ΔT in the range of 30°C to 50°C = (T ｍ -T ｃ ) ｏｎｓｅｔ having 5. The powder composition of claim 4.

10. an optical detection additive, the optical detection additive comprising cobalt blue; 5. The powder composition of claim 4.

11. A three-dimensional object obtained by additive manufacturing from a composition according to any one of claims 4 to 10.

12. being colored overall blue by an optical detection additive, said optical detection additive enabling optical detection in the wavelength range comprised between 0.5 μm and 12 μm; The object of claim 11.

13. having a modulus of elasticity of 1600 MPa or more, a tensile strength of 30 MPa or more, and a modulus of elongation at break of 20% or more in a first direction and 35% or more in a second direction perpendicular to the first direction; The object of claim 11.

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