ADDITIVE MANUFACTURING METHOD, POLYMER POWDER COMPOSITION CONTAINING A DETECTION ADDITIVE, AND OBJECT OBTAINED BY SAID METHOD

Abstract

ADDITIVE MANUFACTURING PROCESS, POLYMER POWDER COMPOSITION COMPRISING A DETECTION ADDITIVE AND OBJECT OBTAINED BY SAID PROCESS A manufacturing process for a three-dimensional object, comprising a localized temperature increase of a powder by electromagnetic radiation in a heated chamber, causing the localized melting / coalescence of a layer of predetermined thickness to form, after cooling, a solid layer of polyamide, said process being characterized in that said powder comprises, by weight of the total composition: - between 60% and 99% by weight of polyamide - between 1% and 40% by weight of an optical and / or magnetic detection additive selected from the group formed by: pigments comprising a spinel structure which contains a cation of a transition metal, oxides of a transition metal,the sulfides of a transition metal - between 0% and 5% by weight of a flow agent and in that the powder has: - a D50 between 35 µm and 55 µm, - a D10 less than 20 µm and - a D90 greater than 80 µm. Figure for the abbreviation: Figure 5,

Description

Title of the invention: ADDITIVE MANUFACTURING METHOD, POLYMER POWDER COMPOSITION COMPRISING A DETECTION ADDITIVE, AND OBJECT OBTAINED BY SAID METHOD TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to the manufacture of parts from polymer materials and aims at a layer-by-layer agglomeration process, particularly by melting or sintering, of a polymer powder comprising an optical and / or magnetic detection additive. The present invention also relates to such a polymer powder supplied for this process and consumed during the process. Finally, the present invention relates to an object obtained by the process that exhibits particularly advantageous properties in the field of food production chain safety. STATE OF THE ART

[0002] Among the wide variety of additive manufacturing technologies for polymer parts now available, this application falls within the category of technologies involving the layer-by-layer agglomeration of powder to obtain a three-dimensional object. Therefore, in this document, the terms "additive manufacturing" or "3D printing" refer only to these methods. An object obtained by such a 3D printing method will be referred to as a "3D object."

[0003] In this context, the agglomeration of powders by melting, coalescence, and / or sintering is caused by radiation that melts the material to be agglomerated. For example, selective laser sintering (SLS) consists of locally densifying a material in powder form by melting it under the action of a laser. Any other source of electromagnetic radiation capable of melting the powder can also be used, for example, infrared, visible, or UV radiation. Other notable additive manufacturing methods by powder bed fusion include, in particular, laser sintering, multi-jet fusion, infrared radiation sintering, and high-speed sintering.

[0004] The use of 3D thermoplastic objects is advantageous in industrial production lines, particularly because such objects can be produced in small batches for specific uses or because they have specific structural characteristics. However, 3D thermoplastic objects are difficult to detect using foreign body detection methods commonly employed in online quality control, particularly in the food industry. Therefore, if a 3D thermoplastic object breaks, fragments can end up in the product and pose a food safety risk.

[0005] The additive manufacturing processes known in the prior art and the powders they use make it possible to produce 3D objects which have both satisfactory mechanical properties for use in industry and strong detectability by means of foreign body detection, for example by magnetic detection or by detection of an unusual color. DETAILED DESCRIPTION OF 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 present invention relates to a method for manufacturing a three-dimensional object, comprising a local temperature increase of a polyamide-based powder by electromagnetic radiation in a heated chamber, causing the localized melting of a layer of predetermined thickness to form, after cooling, a solid layer of polyamide, said method being characterized in that said powder comprises, by weight of the total composition: - between 60% and 99% by weight of polyamide; - between 1% and 40% by weight of an optical and / or magnetic detection additive selected from the group formed by: pigments comprising a spinel structure which contains a cation of a transition metal, sulfides of a transition metal; - between 0% and 5%, and preferably between 0.1% and 4.5%, by weight of a flow agent; and in what the powder presents: - a D50 particle size distribution between 35 µm and 55 µm; and - a particle size distribution Di0 of less than 20 pm and - a D90 particle size distribution greater than 80 pm.

[0008] In embodiments, a mass fraction of between 30% and 70% of said powder is fresh polyamide powder, and a mass fraction of between 70% and 30% of said powder is polyamide powder recovered from said heated chamber following a previous manufacturing process, and said fresh polyamide powder has an internal viscosity index measured according to ISO 307:2019 of between 0.9 deciliters per gram and 1.4 deciliters per gram, at 25 °C.

[0009] In some embodiments, the electromagnetic radiation causing the localized melting of a layer is laser radiation with an energy density of greater than or equal to 25 mJ / mm2. The method used in this case is preferably the selective laser sintering method, more often called SLS (abbreviated from the English "Selective Laser Sintering").

[0010] According to a second aspect, the invention relates to a powder composition for an additive manufacturing process characterized in that it comprises, by weight of the total composition: - between 60% and 99% by weight of polyamide; - between 1% and 40% by weight of a detection additive, preferably an optical detection additive and / or a magnetic detection additive, selected from the group formed by: pigments comprising a spinel structure which contains a cation of a transition metal, oxides of a transition metal, sulfides of a transition metal; - between 0% and 5%, and preferably between 0.1% and 4.5%, by weight of a flow agent; and in what the powder presents: - a D50 particle size distribution between 35 µm and 55 µm; and - a particle size distribution Di0 of less than 20 pm and - a D90 particle size distribution greater than 80 pm

[0011] In embodiments, the powder composition of the invention is obtained by dry mixing of a natural polyamide powder with a polyamide powder comprising a detection additive.

[0012] In embodiments, the powder composition of the invention comprises: - between 0.05% and 5% by weight of an optical detection additive chosen from pigments comprising a spinel structure containing a cation of a transition metal and - between 1% and 35% by weight of a magnetic detection additive among the oxides of transition metals.

[0013] Alternatively, the optical or magnetic detection additive is chosen from among the sulfides of a transition metal.

[0014] In embodiments the powder composition of the invention has an internal viscosity index measured according to ISO 307:2019 of between 0.9 deciliters per gram and 1.4 deciliters per gram.

[0015] In embodiments the powder composition of the invention has a value AT= (Tm lOonset, between 30°C and 50°C.

[0016] In embodiments the powder composition of the invention comprises an optical detection additive and said optical detection additive comprises cobalt blue.

[0017] According to a third aspect, the invention relates to a three-dimensional object obtained by additive manufacturing from a composition that is the object of the invention.

[0018] In some embodiments, the three-dimensional object is colored blue throughout by an optical detection additive. Preferably, the optical detection additive allows optical detection in a wavelength range between 0.5 pm and 12 pm.

[0019] In embodiments, the three-dimensional object has a modulus of elasticity greater than or equal to 1700 MPa, a tensile strength greater than or equal to 30 MPa, an elongation at break greater than or equal to 20% along a first orientation and greater than or equal to 35% along a second orientation perpendicular to the first. BRIEF DESCRIPTION OF THE FIGURES

[0020] Other advantages, purposes and particular features of the invention will become apparent from the following non-limiting description of at least one particular embodiment of the additive manufacturing process, of the powder composition for said process and of a three-dimensional object obtained by said process, which are the subject of the present invention, with reference to the accompanying drawings, in which:

[0021] [Fig-1] represents density curves of particle size distribution as a function of the particle size of two powder compositions according to the invention and of a natural polyamide 11 powder,

[0022] [Fig.2] represents cumulative distribution curves as a function of circularity for two powder compositions according to the invention and a natural polyamide 11 powder,

[0023] [Fig.3] represents a view captured by scanning electron microscopy of a powder composition according to the invention,

[0024] [Fig.4] represents an X-ray tomography view of a cross-section of a 3D object obtained from the additive manufacturing process from the powder composition illustrated in [Fig.3],

[0025] [Fig.5] schematically represents a cross-section of a 3D object obtained by sintering the powder composition illustrated in [Fig.3],

[0026] [Fig.6] represents a scanning electron microscope view of a powder composition for additive manufacturing,

[0027] [Fig.7] represents a view obtained by X-ray tomography of a cross-section of a 3D object obtained at the end of the sintering process of the powder composition illustrated in [Fig.6],

[0028] [Fig.8] schematically represents a cross-section of a 3D object obtained by sintering the powder composition illustrated in [Fig.6],

[0029] [Fig.9] represents a differential scanning calorimetry of a particular powder composition according to the invention,

[0030] [Fig. 10] represents a graph of the force in MPa as a function of the elongation along an xy orientation, expressed in %, obtained at the end of an elongation test on a 3D object obtained by sintering a powder composition A, according to a particular embodiment of the invention and

[0031] [Fig. 11] represents a graph of the force in MPa as a function of the elongation along an xz orientation, expressed in %, obtained at the end of an elongation test on a 3D object obtained by sintering a powder composition A, according to a particular embodiment of the invention.

[0032] DESCRIPTION OF EMBODIMENT METHODS OF THE INVENTION

[0033] The present description is given by way of non-limiting grammar, each feature of an embodiment being able to be advantageously combined with any other feature of any other embodiment.

[0034] The values ​​of the internal viscosity index of polyamide indicated in this document refer to ISO 307:2019 and a temperature of 25 °C.

[0035] The powder composition for the additive manufacturing process by sintering according to the invention comprises, by weight of the total composition: - between 60% and 99% polyamide by weight, - between 1% and 40% by weight of a detection additive, which may be an optical detection additive and / or a magnetic detection additive, and which is preferably selected from the group formed by: pigments comprising a spinel structure which contains a cation of a transition metal, oxides of a transition metal, sulfides of a transition metal; - between 0% and 5% and preferably between 0.1% and 4.5% by weight of a flow agent: and in what the powder presents: - a D50 particle size distribution between 35 µm and 55 µm, - a particle size distribution Di0 greater than 20 pm and - a D90 particle size distribution of less than 80 pm.

[0036] The powder is said to be "polyamide-based" because it mainly comprises polyamide.

[0037] The characteristics of the powder composition for additive manufacturing process by sintering, hereinafter referred to as "sintering powder", and of its components are detailed below.

[0038] The shape of the grains of the sintering powder is preferably spherical.

[0039] According to the invention, said detection additive can be selected to allow magnetic detection or optical detection, or two can be used additives, namely a first additive allowing magnetic detection and a second additive allowing optical detection, or an additive allowing both optical and magnetic detection is used. Choice of polyamide or polyamide blend

[0040] The polyamide can be chosen from any available polyamide, or mixture of polyamide, allowing the particle size characteristics of the composition of the invention to be obtained.

[0041] Preferably the polyamide is chosen from polyamides comprising one of the following monomers: PA6, PA10, PAU, PA12 and their mixtures.

[0042] Characteristics of the sintering powder and working temperature (T2)

[0043] Preferably, the sintering powder has a working temperature window between 160 °C and 210 °C. The working temperature window is the temperature range delimited by the extrapolated initial temperature of the melting peak (Tei>m or Tm>onset in °C) and the extrapolated final temperature of the crystallization peak (Tef>c or Tc>onset in °C). The difference between these two temperatures is called AT, and is expressed as follows: AT = Tei m - Tef>c or AT = (Tm - Tc)onset.

[0044] The extrapolated initial temperature of the melting peak Tm>onset and the extrapolated final temperature of the crystallization peak Tc>onset will be better understood by reading the article Polymers Applicable for Laser Sintering (LS), published by Schmid M. & Wegener K in 2016 (Additive Manufacturing: Procedia Engineering, 149, 457-464), especially with regard to [Fig.9].

[0045] Preferably, AT = (Tm - Tc)onset is between 30 °C and 50 °C. This AT is advantageous because it allows the working temperature T2 to be defined. Even more preferably, AT = (Tm - Tc)onset is between 30 °C and 35 °C.

[0046] In the case of an AT below 30 °C the polymer risks overreacting to the change of state by the supply of energy.

[0047] For an AT above 50 °C the risk is that the stable working temperature T2 cannot be defined, and consequently a general agglomeration of the powder bed and problems of coverage may occur.

[0048] Similarly, it is necessary to adapt the energy input according to the working temperature T2 chosen within this range AT= (Tm>>-Tc)onset. Excessive energy input would have the detrimental consequence of deforming the 3D printed object. Detection additive

[0049] According to an essential feature of the invention, the powder composition comprises a detection additive. This additive is advantageously a water-insoluble and non-toxic inorganic compound, preferably of the spinel type. The powder composition of the invention comprises, by weight of the total composition, between 1% and 40% by weight of a detection additive.

[0050] The detection additive may be an optical detection additive. More particularly, the powder composition of the invention may comprise, relative to the total weight of the composition, between 0.05% and 5% by weight of an optical detection additive, for example, between 0.05% and 0.5%. The latter is advantageously selected from pigments comprising a spinel structure that contains a transition metal cation. This type of pigment has the advantage of being non-toxic. In particular, the transition metal cation remains trapped in the spinel structure and cannot be solubilized under normal conditions of contact with food and beverages, nor in the event of accidental ingestion via the intestinal tract. According to a particular embodiment, the pigment is a blue pigment, preferably cobalt aluminate (CAS No. 1345-16-0), which is available under the trade name PB 28.Preferably, the optical detection additive used allows for optical detection, possibly by infrared. For example, the optical detection additive used allows for optical detection in a wavelength range from 0.5 pm to 12 pm.

[0051] The detection additive may be a magnetic detection additive. More particularly, the powder composition of the invention may comprise, relative to the total weight of the composition, between 1% and 40% by weight of a magnetic detection additive.

[0052] The magnetic detection additive is preferably chosen from among oxides containing a transition metal. For example, the magnetic detection additive is an iron oxide, such as natural or synthetic magnetite (Fe3O4). This spinel-type oxide is insoluble in water and non-toxic.

[0053] Whether as an optical detection additive or as a magnetic detection additive, one can also use an oxide of a transition metal that is not a spinel, or a sulfide of a transition metal. Magnetic detection additives must obviously be selected to exhibit specific magnetic properties that can be easily detected.

[0054] In a preferred embodiment, the powder composition of the invention comprises both between 0.05% and 5% by weight of an optical detection additive selected from pigments comprising and between 1% and 40% by weight of a magnetic detection additive from transition metal oxides. Choice of flow agent

[0055] The composition of the invention further comprises a flow agent in sufficient quantity to ensure that the composition flows freely, remains fluid and forms a uniform and homogeneous flat layer during the generative powder bed fusion (PBF) process, for example known as SLS polymer layer-by-layer sintering, LS.

[0056] The composition of the invention comprises, by weight of the total composition, between 0% and 5% by weight of a flow agent. A content between 0.1% and 4.5% by weight is preferred.

[0057] The flow agent is chosen from those commonly used in the field of polymer powder sintering, for example from: silicas, precipitated silicas, silica fumes, hydrated silicas, vitreous silicas, fumed silicas, vitreous phosphates, vitreous oxides.

[0058] Preferably the flow agent has a small contact area. Manufacturing a powder composition

[0059] According to a particular embodiment, the powder composition according to the invention is obtained by a manufacturing method which comprises a first step of mixing a so-called "natural" polyamide powder with a flow agent and at least one of the following steps: - a step of mixing the composition obtained previously with a polyamide powder composition comprising an optical detection additive; - a step of mixing the composition obtained previously with a composition containing a magnetic detection additive.

[0060] In particular embodiments, these last two mixing steps with a composition comprising a detection additive are carried out successively, it should be noted that their order can be reversed.

[0061] A natural polyamide powder is a powder composition comprising between 95% and 100% polyamide, preferably at least 99% by weight polyamide.

[0062] The polyamide powder composition comprising an optical detection additive can be obtained by reducing to powder a homogeneous liquid or solid mass comprising the polyamide and said optical additive or by solid phase polycondensation, drying and then selective grinding.

[0063] The composition comprising a magnetic detection additive can either be the magnetic additive in pure form (i.e. comprising at least 95% magnetic detection additive) or be a composition comprising a polyamide homogenized by dry mixing with a magnetic detection additive.

[0064] The mixing steps mentioned above can be carried out by dry blending or by a compounding process (known as a master batch). Compounding requires a subsequent step of selective grinding of the resulting mass and adjustment of the viscosity by solid-phase polycondensation and drying; for this reason, dry blending is preferred.

[0065] The dispersion of the flow agent requires the application of significant mixing energy to achieve good homogenization. This mixing energy is sustained susceptible to damaging detection additives. Therefore, a dry premixing of the flow agent with a natural polyamide powder is preferred in the first mixing step, prior to at least one mixing step with a composition containing a detection additive of lower intensity than the first.

[0066] In some embodiments, the mixing steps are carried out by cryo-milling, this method well known to those skilled in the art is not described in detail here.

[0067] In these other embodiments, the methods for obtaining a dry, homogeneous, and dispersed powder mixture of all the components are adapted according to the initial distributions and the final target distribution, namely:

[0068] - a D50 particle size distribution between 35 µm and 55 µm, and - a D10 particle size distribution greater than 20 µm and - a D90 particle size distribution of less than 80 pm.

[0069] Particle size distribution of the powder composition

[0070] The particle size distribution Di0 of the powder composition is greater than 17 pm. Preferably, the particle size distribution Di0 of the powder composition is greater than 20 pm.

[0071] Such a particle size distribution Di0 of the powder composition is advantageous to avoid the presence of excessive amounts of fine particles or dust which may volatilize in the air and present a health risk in case of inhalation and accumulation, irritation with the eyes and skin contact of these fine dust particles.

[0072] The D50 particle size distribution of the powder composition is between 35 µm and 55 µm. Preferably the D50 particle size distribution of the powder composition is between 38 µm and 45 µm, most preferably it is between 38 µm and 40 µm.

[0073] The applicant has found during its tests that these D50 particle size distribution intervals allow for the best performance in terms of final resolution, geometric definition of the parts obtained, as well as better coverage and good powder fluidity at temperature for the PBF powder bed process using layers of 80 pm to 120 pm.

[0074] The D90 particle size distribution of the powder composition is less than 80 µm. Preferably, the D90 particle size distribution of the powder composition is less than 75 µm.

[0075] A D90 particle size distribution of less than 80 µm, preferably less than 75 µm, is advantageous for the use of the powder in an additive manufacturing process with a layer thickness between 80 µm and 160 µm, for example, for a layer thickness of 100 µm. Preferably, the D90 is chosen to be smaller than the layer size envisaged for the additive manufacturing process.

[0076] The particle size distribution values ​​of the powder composition Di0, D50 and D9o mentioned above are determined by the static image analysis method according to ISO 13322-1:2014.

[0077] Figure 1 shows the density distribution curves as a function of particle size (expressed in micrometers, abbreviated pm) for three powders: - Curve 105 illustrates the particle size distribution of a so-called natural PAU powder, i.e., one containing at least 99% PAU, - a curve 110 illustrates the particle size distribution of a powder composition A according to the invention, in which the polyamide is a PA11 and which comprises an optical detection additive, - a curve 115 illustrates the particle size distribution of a powder composition B according to the invention, in which the polyamide is a polyamide 11 and which comprises both an optical detection additive and a magnetic detection additive.

[0078] It is emphasized that the optical detection additive and / or the magnetic detection additive are selected to obtain a particle size distribution of the powder composition as detailed above. Thus, it can be seen in [Fig. 1] that the particle size distribution of the powder compositions according to the invention, with the detection additive, remains close to that of the particle size distribution of the natural PA11 powder, with a density peak around 50 pm.

[0079] Powder composition form factors

[0080] Shape factors are dimensionless quantities used in image analysis and microscopy that numerically describe the shape of a particle, regardless of its size.

[0081] The circularity index / circ is a shape factor that is calculated as follows:

[0082] [Math.l] 4îf Zi ...... Joel p2 ' where P is the perimeter and A looks like an image of a grain of powder

[0083] Thus a sphere will have a circularity index of 1 while a mica, in parallelepiped shape, will have a circularity close to 0.

[0084] The rules and nomenclature for the description and quantitative representation of the shape and morphology of particles specified by ISO 9276-6:2008 are followed here.

[0085] Preferably, the cumulative distribution fi0 of the powder composition according to The invention is less than or equal to 0.15. Most preferably, the cumulative distribution fio of the powder composition is less than or equal to 0.10. In other words, only 10% of the powder grains 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 grains have a circularity index greater than 0.1, preferably greater than 0.15.

[0086] The cumulative distribution f50 of the powder composition is less than or equal to 0.6. Preferably, the cumulative distribution f50 of the powder composition is less than or equal to 0.55. In other words, only 50% of the powder grains 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 grains have a circularity index greater than 0.55, preferably greater than 0.6.

[0087] The cumulative distribution f90 of the powder composition is less than or equal to 0.8. Preferably, the cumulative distribution f90 of the powder composition is less than or equal to 0.75. In other words, 90% of the powder grains 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 grains have a circularity index greater than 0.75, preferably greater than 0.8.

[0088] Figure 2 shows a graphical representation of the cumulative distribution on the y-axis (expressed as a percentage), as a function of circularity on the x-axis (unitless index). Figure 2 shows: - a curve 205 illustrating the cumulative distribution of a so-called natural PA11 powder, i.e., containing at least 99% by mass of PAU, - a curve 210 illustrates the cumulative distribution of a powder composition according to the invention, in which the polyamide is a PA11 and which comprises an optical detection additive, - a curve 115 illustrates the cumulative distribution of a powder composition according to the invention, in which the polyamide is a polyamide 11 and which comprises both an optical detection additive and a magnetic detection additive.

[0089] The optical detection additive and / or the magnetic detection additive are preferably selected to obtain a cumulative distribution of the powder composition as detailed above. It can thus be seen in [Fig. 2] that the cumulative distribution of the powder compositions according to the invention, with the detection additive, remains close to that of the particle size distribution of the natural PA11 powder.

[0090] Grain morphology is important for the fluidity of the mixture and for the densification of the powder bed during successive coatings, but also for the residual porosity in the final parts obtained. Good powder sphericity Combined with a very tight distribution, i.e., a cumulative distribution like the one described above, this allows for natural densification of the powder bed through compaction and geometric arrangement of the layer. This layer is then exposed to laser energy for fusion and coalescence, promoting densification of parts with low residual porosity. Conversely, a highly heterogeneous powder with a wider distribution will tend to organize itself more chaotically and will result in less densification of the powder bed, as some of the larger grains may not melt.

[0091] To illustrate this point, Figures 3 and 6 show two views captured by scanning electron microscopy of two powders at the same magnification. Figure 3 illustrates a powder that exhibits a sphericity comparable to that of a powder composition of the invention; Figure 6 is shown for comparison.

[0092] These powders are subjected to a SLS laser sintering process at an energy density of 34 mJ / mm² (550 and 850). X-ray tomography views of the 3D object sections obtained after the sintering process are shown in Figures 4 and 7. A schematic representation of the powders 30 and 60 illustrated in Figures 3 and 6 and of 3D object sections obtained by sintering these powders are shown in Figures 5 and 8.

[0093] The powder 30 illustrated in [Fig. 3] is a powder with good homogeneity of circularity, with a grain circularity between 0.4 and 0.8, averaging 0.65. The powder 30, deposited on a previously solidified layer 505 and subjected to a SLS laser sintering process 550 at an energy density of 34 mJ / mm², yields a low and distributed residual porosity, as illustrated in section 410 in [Fig. 4], obtained by 3D object tomography, and in section 510 in [Fig. 5]. The porous parts 420, which appear black in [Fig. 4], are illustrated as white cavities in [Fig. 5]. These porous parts are less numerous and better distributed than those observed on sections of a 3D object obtained by sintering from a powder of less homogeneity of circularity of grains, represented in figures 7 and 8.

[0094] The powder 60 illustrated in [Fig.6] has a lower homogeneity than the powder 30, with a grain circularity between 0.1 and 0.8, on average equal to 0.55. The powder 60, placed on a previously solidified layer 805 and subjected to a SLS laser sintering process 550 at an energy density of 34 mJ / mm2, resulted in obtaining a 3D object with less homogeneity and greater residual porosity, shown in a tomographic cross-section 710 in [Fig.7], and in a schematic cross-section 810 in [Fig.8]. 3D object manufacturing process

[0095] It should be noted that the present application falls within the scope of technologies involving a powder bed with layer-by-layer agglomeration, with a view to obtaining a three-dimensional object. Therefore, in this document, the terms "additive manufacturing" or "3D printing" refer only to these methods. An object obtained by such a 3D printing method will be referred to as a "3D object."

[0096] The present invention relates more particularly to an additive manufacturing process by powder bed fusion (PBF), layer by layer, from a polyamide powder in a heated chamber. These methods include in particular laser sintering (LS), selective laser sintering (SLS), multi-jet fusion (MJF), infrared sintering (1RS), and high-speed sintering (HSS).

[0097] Regardless of the additive manufacturing method used, the process of the invention aims to manufacture 3D polyamide objects comprising a detection additive, from a polyamide powder composition.

[0098] The process according to the invention takes place in a closed chamber preheated to a setpoint temperature Tb. The atmosphere inside the chamber is enriched with nitrogen (or under vacuum) and depleted of oxygen in order to limit the oxidation of the polymer powder. This oxidation gradually leads to the elongation of the macromolecules constituting the polymer powder particles and represents the main aging mechanism of said powders. This elongation of the macromolecules tends to increase the internal viscosity of the polymer. Limiting the oxidation of the powders at temperature promotes the recycling of unused powder, which contributes significantly to the cost-effectiveness of the process according to the invention. Preferably, the oxygen content is less than 5% by volume, preferably less than 2%, and even more preferably less than 1%.

[0099] The holding temperature Ti is advantageously located at approximately 20 to 30 degrees around the crystallization temperature Tc of the polymer. In an advantageous embodiment, for a PA11 and / or PA12 polyamide-based powder, the preheating temperature Ti is advantageously located between approximately 140 °C and approximately 160 °C, preferably between approximately 142 °C and approximately 158 °C. In some embodiments, the heating temperature is equal to the holding temperature.

[0100] More generally, the holding temperature Tl is preferably between 150 and 185°C

[0101] The process of the invention comprises the deposition of a uniform layer of a polyamide powder bed in a preheated chamber.

[0102] Immediately after the deposition of each layer, the surface of the powder bed is rapidly heated, typically by infrared radiation, to a temperature T2 which is selected to be approximately 8% to 14% lower than the polyamide melting temperature (Tm) (i.e., 12 to 26 degrees lower than the powder's melting temperature (Tm)). This heating to a temperature T2 allows the polyamide powder to be maintained at a temperature quite close to its melting temperature, without actually reaching it. This is also referred to as the working temperature for PBF systems.

[0103] According to an advantageous embodiment, for a powder based on polyamide PA11 and / or PA6, the temperature T2 is between approximately 183 °C and approximately 204 °C.

[0104] More generally, the temperature T2 is between 168 °C and 206 °C.

[0105] Melting the powder is necessary to obtain a compact part. This melting must be transient, rapid, localized, and controlled to prevent uncontrolled flow of the liquid polymer; for this reason, it must be brief, meaning that the localized melting must be promptly followed by cooling to a temperature below the polymer's melting point Tm, towards a temperature TR at which the polymer can recrystallize from the molten state. This temperature TR can be in the vicinity of temperature T2, and is between Ti and T2.

[0106] To achieve the localized and controlled melting of a selected portion of the powder bed, electromagnetic radiation irradiates targeted areas of the polyamide powder, locally increasing the temperature and causing the polyamide grains in the targeted areas to agglomerate. Depending on the method used, the electromagnetic radiation is, for example, visible, infrared, or near-infrared laser radiation. The local temperature TL of the melting zone is preferably about 8% to 14% higher than the Tm of the polyamide (i.e., 12 to 26 degrees higher than the melting temperature Tm of the powder). A transient liquid phase is thus formed, but if TL is too high, the viscosity of the molten polymer becomes too low, and there is a risk of runoff.

[0107] By way of particular example, the temperatures Tl and T2 implemented during a sintering process according to the invention are gathered in Table 1 below and compared with the melting point Tm and the crystallization temperature Tc of the sintering powders A and B according to the invention.

[0108] [Tables] T -1- m Tc T -1- m,onset T -1- c,onset AT T! t2 Powder A: Optical additive 202 163 198 168 30 155 186 Powder B: Optical and magnetic additives 202 164 199 168 31 155 184

[0109] It is specified that: - Powder A is a powder composition according to the invention, in which the polyamide is a PA11 and which comprises an optical detection additive and - Powder B is a powder composition according to the invention, in which the polyamide is a polyamide 11 and which comprises both an optical detection additive and a magnetic detection additive.

[0110] For the determination of any interval centered on the melting temperature or the crystallization temperature, an initial temperature extrapolated from the melting peak (Tm>onset) and the final temperature extrapolated from the crystallization peak (Tc>onset) will be used more preferably, rather than the temperature values ​​corresponding to the melting and crystallization peaks, although both methods of determining these reference values ​​can be implemented without deviating from the invention.

[0111] To illustrate this point, [Fig. 9] shows a DSC (Differential Scanning Calorimetry) of a powder composition according to the invention based on PAU. This DSC shows an initial temperature rise curve 910 and a cooling curve 920. The melting and crystallization temperatures are illustrated on this graph, whether determined by identifying the corresponding peak (Tc and Tm) or by extrapolating the initial temperature for the melting peak (Tm,onset) and the final temperature for the crystallization peak (Tc>onset).

[0112] Once all the targeted areas of a powder bed layer have been scanned by the electromagnetic radiation source, a new powder bed is deposited and flattened over the previous one. It should be noted that the powder is self-supporting, meaning that it rests on the powder previously deposited during the process. In this way, a new powder bed is deposited, and the solidification of a portion of the new powder bed begins. The solidified portion of each powder bed corresponds to a layer or slice of the 3D object obtained at the end of the process.

[0113] The thickness of each slice is typically between approximately 50 pm and approximately 150 pm, preferably between approximately 70 pm and approximately 120 pm, and even more preferably between approximately 80 pm and approximately 110 pm. The deposition of each slice is followed by heating to temperature T2, as described above.

[0114] According to one embodiment of the process, the sintering of the invention is carried out by SLS and the electromagnetic radiation causing the localized melting of a layer is laser radiation with an energy density greater than or equal to 25 mJ / mm² for a working temperature T2 between 180 °C and 199 °C, for example, 188 °C. The energy density greater than or equal to 25 mJ / mm² prevents layer delamination, i.e., the separation between two successive layers of solidified polyamide.

[0115] The energy density is calculated using the simplified Morgan formula, which is expressed as follows:

[0116] [Math.2] P<2t>2t 4-F Ep ~----

[0117] where P is the power of the laser, expressed in Watts S is the spacing between scans (hatch gap), expressed in millimeters (mm); v is the laser speed, expressed in mm / second; r is the laser radius, expressed in mm

[0118] By way of example, the operating conditions of sintering processes according to the invention with different SLS systems are summarized in Table 2 below. These operating conditions are implemented on a sintering powder composition containing PAU, with a fixed layer thickness of 100 µm, at a working temperature T2 of approximately 188°C.

[0119] [Tables2] Energy density [mJ / mm²] Laser power [W] Scan speed [mm / s] Hatching gap or spacing between scans [mm] 27 70 12700 0.26 35 22 3200 0.25 29.3 14.5 3500 0.18 >35 7900 0.25

[0120] The 3D object exiting the sintering process is covered with non-agglomerated powder, this powder is removed by mechanical and / or chemical means well known to those skilled in the art (air or water jet, brushing, sandblasting, solvent phase treatment, ultrasonic bath, treatment with an HF solution, etc.) which are not detailed here.

[0121] Reuse of the polyamide powder composition that is the subject of the invention

[0122] During a process such as that described above, a portion of the powder composition for the Power Bed Fusion (PBF) additive manufacturing process according to the invention, introduced into the heating chamber, does not solidify. Advantageously, this powder is collected and sieved for reuse in a mixture with a fresh polyamide powder composition, i.e., with a powder that has not already been used in a sintering process.

[0123] Preferably, the powder composition according to the invention comprises a mass fraction of between 20% and 70% of fresh polyamide powder composition, and a mass fraction of between 80% and 30% of recovered polyamide powder. as a result of a previous manufacture. More preferably, the deposited powder bed comprises a mass fraction of between 25% and 55% of fresh polyamide powder composition, and a mass fraction of between 75% and 45% of polyamide powder recovered from a previous manufacture.

[0124] Adding fresh powder to the spent powder adds polyamide grains in good condition (not thermo-oxidized), which are not already damaged or deformed by a previous sintering process inducing thermo-oxidation, and thus maintains the internal viscosity of the mixture within a given range by lowering this viscosity at each cycle as it evolves.

[0125] Preferably, the fresh polyamide powder used has an internal viscosity index measured according to ISO 307:2019 of between 0.9 deciliters per gram and 1.4 deciliters per gram. An internal viscosity index of less than 1.4 deciliters per gram, preferably less than 1.2 deciliters per gram, makes it possible to maintain a sufficiently low internal viscosity of the polyamide powder composition, even when this composition is obtained by mixing fresh powder with already cycled powder used by a PBF powder bed process.

[0126] The method for determining the internal viscosity index of plastics and polyamides, according to ISO 307:2019, is based on determining the viscosity index of dilute solutions of polyamides in certain solvents specified in the aforementioned standard.

[0127] This viscosity plays a role in the rheology of melting and / or coalescence phenomena: the deposited particles must melt and coalesce to form a dense, non-porous mass, but without uncontrolled flow. The internal viscosity influences the mechanical properties of the part, its appearance, and the surface finish of the finished product.

[0128] For optimal use of the powder composition, it is advisable not to exceed a certain number of recycling cycles for the same powder; that is, not to recycle again a powder mixture that has undergone a high number of thermal cycles in a PBF powder bed process. Collection of the cycled powder and its sieving must precede mixing with fresh polyamide powder in order to remove aggregates of powder grains.

[0129] The number of possible cycles depends on the degree of oxidation of the recycled powder, given that the internal viscosity increases with the degree of oxidation. The inventors note that on average, the same powder can be reused in 8 to 10 recycling cycles, but this depends mainly on the duration of the powder's exposure to high temperature and the oxygen level in the chamber throughout the thermal cycle it undergoes (preheating, temperature manufacturing, and cooling), either during the entire PBF manufacturing process or during cooling to a temperature of temperature below 60 °C.

[0130] Recycling is facilitated by the fact that the fresh powder has the internal viscosity indicated above. Indeed, to manufacture good quality parts by the process according to the invention, one can use a powder whose internal viscosity index is slightly outside this range between 0.9 deciliters per gram and 1.4 deciliters per gram, but in order for the fresh powder to be recycled in the PBF process, under economically advantageous conditions and according to the technical conditions indicated above (mixed with fresh powder at a ratio of 30% to 60%), it is preferable to maintain, for the fresh powder, a continuous refreshing to 50% and the systematic sieving of the already cycled powder.

[0131] By way of example, the composition according to the invention is powder A (already described above). Fresh powder A has an internal viscosity index of 1.3 deciliters per gram. After implementing a sintering process, a new powder composition according to the invention is formed by mixing half fresh powder and half recycled powder. After one to two cycles, the powder composition has an internal viscosity index of approximately 1.7 deciliters per gram. After three to six cycles, the powder composition has an internal viscosity index of approximately 2.05 deciliters per gram.

[0132] [Tables3] Evolution of the Internal Viscosity Index (in deciliters per gram at 25°C) Powder Fresh Powder Time at temperature = Ohrs Refreshed Powder (1 to 2 cycles) Time at temperature >20hrs Refreshed Powder (3 to 6 cycles) Time at temperature >50hrs Powder A: Optical Additive 1.3 1.7 2.05

[0133] Instead of the number of cycles, one can consider the temperature time, that is, the time during which the powder composition is subjected to heating in the heated chamber. This approach can be more precise because manufacturing cycles can vary in length. The temperature times tested to arrive at the values ​​of the internal viscosity index in the table above are also indicated. This temperature time is 0 for fresh powder, greater than 20 hours for powder that has undergone 1 to 2 cycles, and greater than 50 hours for powder that has undergone 3 to 6 recycling cycles.

[0134] Magnetic detection of obtained 3D objects

[0135] The presence of an optical or magnetic detection additive in the composition of The powder of the invention allows the detection of 3D objects obtained by sintering this powder.

[0136] In the case of magnetic detection, 3D objects obtained from a powder containing a magnetic detection additive are, for example, detected by electromagnetic induction or by any other method of detecting a magnetic object. These methods, which are well known to those skilled in the art, are not described here. Optical detection of obtained 3D objects

[0137] The 3D objects obtained by additive manufacturing of a powder composition are colored in the mass, that is to say that the material constituting the 3D object is colored and that the object does not only have a color on its outer surface.

[0138] This feature allows a fragment of a broken 3D object to display on all its faces the color corresponding to the optical detection additive used. Thus, a fragment of an object colored throughout can be detected by optical detection methods when the object is broken.

[0139] Preferably, the 3D object is colored blue throughout. Since blue is an uncommon color among food products, it stands out more easily than other colors when placed in the middle of food products. In particular, infrared detection can be implemented by irradiation in a wavelength range between 0.5 pm and 12 pm. These optical detection methods, even when applied to fragments of plastic materials, are well known to those skilled in the art and will not be described in further detail here.

[0140] Mechanical properties of 3D objects obtained by sintering according to the invention

[0141] Preferably, a 3D object obtained by sintering according to the invention has a lowest tensile strength greater than or equal to 40 MPa (megapascals), preferably greater than or equal to 44 MPa.

[0142] In the case where the 3D object is obtained by sintering a powder comprising a magnetic detection additive, the 4D object preferably has a lowest tensile strength greater than or equal to 30MPa, very preferably greater than or equal to 35MPa.

[0143] Preferably, a 3D object obtained by sintering according to the invention has a lowest modulus of elasticity greater than or equal to 1600 MPa, preferably greater than or equal to 1750 MPa.

[0144] In particular, standardized specimens of 3D objects obtained from a sintering process according to the invention were tested for their tensile strength and modulus of elasticity, expressed in megapascals (MPa), and for their elongation at break, expressed as a percentage. The 3D object tested is obtained from a sintering powder composition A according to the invention comprising an optical detection additive, wherein the polyamide is PAU. The test method described implementation complies with the ISO527-l:2019 standard for the determination of tensile properties.

[0145] [Tables4] Tensile strength [MPa] Elongation at break [%] Modulus of elasticity [MPa] Powder A: Optical additive (xy orientation) 45.5 25 1794 Powder A: Optical additive (xz orientation) 44.9 40 1827

[0146] Preferably, the 3D objects obtained by the process of the invention have an elongation at break greater than or equal to 20% on a first orientation and greater than or equal to 35% on a second orientation, perpendicular to the first.

[0147] In tests carried out according to ISO527-1:2019, the results of which are presented in Table 4 above, these elongations at break were measured at 25% and 40%.

[0148] Figures 10 and 11 show the graphs corresponding to the results of the tests presented above for elongation at break. Figure 10 shows the results of the tensile elongation test along the xy orientation, and Figure 11 shows the results of the tensile elongation test along the xz orientation.

Claims

Demands

1. A method for manufacturing a three-dimensional object, comprising a local temperature increase of a polyamide-based powder by electromagnetic radiation in a heated chamber, causing the localized melting of a layer of predetermined thickness to form, after cooling, a solid layer of polyamide, said method being characterized in that said powder comprises, by weight of the total composition: - between 60% and 99% by weight of polyamide; - between 1% and 40% by weight of an optical and / or magnetic detection additive selected from the group formed by: pigments comprising a spinel structure which contains a cation of a transition metal, oxides of a transition metal, sulfides of a transition metal; - between 0% and 5% and preferably between 0.1% and 4.5% by weight of a flow agent;and in that the powder has: - a particle size distribution D50 between 35 pm and 55 pm, and - a particle size distribution Di0 greater than 20 pm and - a particle size distribution D90 less than 80 pm.;

2. A process according to claim 1, wherein a mass fraction of between 30% and 70% of said powder is fresh polyamide powder, and a mass fraction of between 70% and 30% of said powder is polyamide powder recovered from said heated chamber after a previous manufacture, and wherein said fresh polyamide powder has an internal viscosity index measured according to ISO 307:2019 of between 0.9 deciliters per gram and 1.4 deciliters per gram, at 25 °C.

3. A method according to any one of claims 1 or 2, wherein the electromagnetic radiation is laser radiation with an energy density greater than 25 mJ / mm2.

4. Powder composition for an additive manufacturing process by locally raising the temperature of a polyamide-based powder by electromagnetic radiation in a heated chamber, causing the localized melting of a layer of predetermined thickness to form, after cooling, a solid layer of polyamide characterized in that said powder comprises, on the total weight of the composition: - between 60% and 99% by weight of polyamide - between 1% and 40% by weight of a detection additive, preferably an optical detection additive and / or a magnetic detection additive, selected from the group formed by: pigments comprising a spinel structure which contains a cation of a transition metal, oxides of a transition metal, sulfides of a transition metal; - between 0% and 5% and preferably between 0.1% and 4.5% by weight of a flow agent; and in that the powder has: - a D50 particle size distribution between 35 µm and 55 µm, - a D0 particle size distribution greater than 20 µm and - a D90 particle size distribution less than 80 µm.

5. Powder composition according to claim 4, obtained by dry mixing of a natural polyamide powder with a polyamide powder comprising a detection additive.

6. Powder composition according to any one of claims 4 or 5, comprising: - between 0.05% and 5% by weight of an optical detection additive selected from pigments comprising a spinel structure which contains a cation of a transition metal and - between 1% and 35% by weight of a magnetic detection additive selected from oxides of transition metals.

7. Powder composition according to any one of claims 4 to 6, which has an internal viscosity index measured according to ISO 307:2019 of between 0.9 and 1.4 deciliters per gram, at 25 °C.

8. Powder composition according to any one of claims 4 to 7, which has an AT= (Tm-Tc)onset value, between 30 °C and 50 °C.

9. Powder composition according to any one of claims 5 to 8, comprising an optical detection additive and wherein said optical detection additive comprises cobalt blue.

10. Three-dimensional object obtained by additive manufacturing from a composition according to any one of claims 4 to 9.

11. Object according to claim 10, colored blue throughout by an optical detection additive and wherein said optical detection additive enables optical detection in a wavelength range between 0.5 pm and 12 pm.

12. Object according to any one of claims 10 or 11, which has a modulus of elasticity greater than or equal to 1600 MPa, a tensile strength greater than or equal to 30 MPa, an elongation at break greater than or equal to 20% in a first orientation and greater than or equal to 35% in a second orientation perpendicular to the first.