POLYMER-BASED BUILDING MATERIAL FOR SELECTIVE SINTERING

Description

[0001] The present invention relates to a plastic powder for use in a layer-by-layer process for producing molded articles, in which regions of a respective powder layer are selectively melted and resolidified. Furthermore, the present invention relates to a process for producing such a plastic powder, a three-dimensional object produced from the powder according to the invention, and a process and system for producing three-dimensional objects using the plastic powder.

[0002] As is known, for example, from DE 44 10 046, a process for producing a three-dimensional object can be carried out layer by layer by selective sintering using electromagnetic radiation with the aid of an electromagnetic radiation source. In such a process, a three-dimensional object is produced layer by layer – according to the principle of "additive manufacturing" – by repeatedly applying powder layers, selectively melting them (partially or completely) at the points corresponding to the cross-section of the object, and then allowing the melt to solidify. The melting of the powder layer bonds the melt to the previously melted layer.

[0003] Fig. 1shows an example of a laser sintering device with a laser beam and a deflection mirror, as is common when using a CO 2 laser, with which a process for the layer-by-layer production of a three-dimensional object can be carried out. As can be seen from Fig. 1As can be seen, the device has a container 1. This is open at the top and delimited at the bottom by a carrier 4 for carrying an object 3 to be formed. A working plane 6 is defined by the upper edge 2 of the container (or its side walls). The object is located on the upper side of the carrier 4 and is formed from a plurality of layers extending parallel to the upper side of the carrier 4 and made of a powdery building material that can be solidified by means of electromagnetic radiation. The carrier can be moved in the vertical direction, i.e. parallel to the side wall of the container 1, via a height adjustment device. This allows the position of the carrier 4 to be adjusted relative to the working plane 6.

[0004] Above the container 1 or the working plane 6, an application device 10 is provided for applying the powder material 11 to be solidified to the carrier surface 5 or a recently solidified layer. Furthermore, an irradiation device in the form of a laser 7 is arranged above the working plane 6, which emits a directed light beam 8. This is directed as a deflected beam 8' toward the working plane 6 via a deflection device 9, for example a rotating mirror. This arrangement is common for a laser sintering system with a CO2 laser. A control unit 40 enables the control of the carrier 4, the application device 10, and the deflection device 9. The elements 1 to 6, 10, and 11 are arranged within the machine frame 100.

[0005] During the production of the three-dimensional object 3, the powder material 11 is applied layer by layer to the carrier 4 or a previously solidified layer and solidified with the laser beam 8' at the locations of each powder layer corresponding to the object. After each selective solidification of a layer, the carrier is lowered by the thickness of the next powder layer to be applied.

[0006] Compared to the system described above, there are many modifications of methods and devices for producing a three-dimensional object (molded body) by selective sintering using electromagnetic radiation that can also be used. For example, instead of a laser and / or a light beam, other systems could be used to selectively deliver electromagnetic radiation, such as mask exposure systems or the like. Instead of a CO2 laser, laser diodes, which can be arranged in a line pattern, for example, can also be used.

[0007] A powder with powder particles comprising a thermoplastic polymer material is often considered as the building material. The mechanical properties of the manufactured object can be influenced by a suitable choice of polymer in the raw material. Polymers that lead to preferred mechanical material properties in the final object are described, for example, in DE 10 2008 024 281 A1 and DE 10 2008 024 288 A1. It has also been described that the mechanical properties can be further improved by using additives such as fillers, reinforcing materials, and additives, e.g. pigments. For example, carbon fibers, glass fibers, aramid fibers, carbon nanotubes, or additives that have a low aspect ratio (glass spheres, aluminum grit, etc.) or mineral additives such as titanium dioxide could be incorporated into the polymer or copolymer-containing powder.

[0008] A potential disadvantage of the known sintering processes using CO2 lasers, which operate according to the principle of "additive manufacturing" described above as an example, is the lack of flexibility with regard to the laser. To sinter plastic powder or plastic-coated particles, a CO2 laser with a wavelength of 10,600 nm is typically used. A complex mirror system is required to guide the laser beam across the build plane, and the laser must be continuously cooled. The use of fiber optic cables is not possible. Likewise, it is not possible to switch to cheaper lasers with a wavelength in the mid- or near-infrared range, in the visible light range, or in the ultraviolet range without additional measures, since such lasers do not melt plastics at all or only to an insufficient extent without suitable additives.

[0009] This problem was at least partially solved in German patent application DE 10 2004 012 683 A1 by adding absorbers to polymer-based powders. These absorbers exhibit an absorption spectrum that can absorb wavelengths in the mid- or near-infrared range, in the visible light range, or in the ultraviolet range. By exposing the absorbers to light, the heat generated in the absorber can be used to melt the polymer-based particles. However, it is known that additives mixed into laser sintering powder as a dry blend can have a negative impact on the laser sintering process (reduction of the process window, poorer z-bonding of the layers) and thus lead to poor component quality.

[0010] A disadvantage of the solution proposed in German patent application DE 10 2004 012 683 A1 is that the process tends to be unstable. In particular, the process has only a narrow build / temperature window. Another serious disadvantage is that the molded articles produced in this way exhibit inadequate mechanical properties.

[0011] WO 2005 / 085326 A1 discloses, specifically in Example 8, a plastic powder for use as a building material for the additive manufacture of a three-dimensional object, wherein the plastic powder comprises a dry mixture of polyamide 12 powder and Printex 60 particles.

[0012] US 2008 / 122141 A1 discloses, specifically in Examples 3 and 4, a plastic powder for use as a build-up material for the additive manufacturing of a three-dimensional object, wherein a dry blend mixture of polyamide 12 or polyamide 6 and 12 with carbon black of the type Raven 410 Ultra is used as the plastic powder.

[0013] The object of the present invention is to provide an improved method and / or system for producing a three-dimensional object by selectively solidifying the building material at the locations corresponding to the cross-section of the three-dimensional object in the respective layer by exposure to NIR radiation. Preferably, the aim is for this to have the largest possible construction / temperature window and good mechanical properties, particularly with regard to tensile strength, modulus of elasticity, and elongation at break. Likewise, the object of the invention is to provide a powder for use as a building material in such an improved method, as well as a molded body produced by the advantageous method.

[0014] This object is achieved by a plastic powder according to claim 1, a manufacturing method according to claim 9, a three-dimensional object according to claim 12, a manufacturing method according to claim 13 and a system according to claim 14. Further developments of the invention are defined in the subclaims.

[0015] According to a first aspect of the present invention, the plastic powder comprises a dry mixture of polymer-based particles and particles of an NIR absorber, wherein the plastic powder further comprises reflection particles with a surface that at least partially reflects the NIR radiation, wherein the NIR absorber comprises carbon black, and wherein the weight fraction of the NIR absorber in the total weight of the plastic powder is in the range of at least 0.02% and at most 0.45%. Preferably, the weight fraction of the carbon black in the total weight of polymer-based particles and carbon black particles is at least 0.07% and / or at most 0.15%, particularly preferably at least 0.08% and / or at most 0.10%.The powder according to the invention is intended to be used in particular as a building material for producing a three-dimensional object by selectively solidifying the building material at the locations corresponding to the cross-section of the three-dimensional object in the respective layer by the action of NIR radiation.

[0016] The term "solidification" refers to at least partial melting or melting followed by solidification or resolidification of the build-up material. The term "selective," in the context of solidification, refers to locally limited irradiation at areas of a continuous layer that are to be solidified, while areas of the layer that are not to be solidified are not irradiated. In contrast to this type of selective solidification, there is, for example, a technique in which the build-up material is not spread out over a complete layer, but rather only in areas where solidification is desired. In this case, irradiation can also be applied to areas that are not to be solidified.

[0017] Solidification can therefore be understood as the layer-by-layer melting and subsequent solidification of the building material by solidifying the melt at the points corresponding to the cross-section of the three-dimensional object in the respective layer by the action of NIR radiation.

[0018] The term "near-infrared (NIR) radiation" refers to electromagnetic radiation in the spectral range between visible light and mid-infrared, and in particular from 780 nm to 3000 nm. The term "NIR absorber" accordingly designates a substance or mixture of substances that at least partially absorbs NIR radiation. For the sake of readability, the following statements refer to a substance as an NIR absorber. The same applies if a mixture of substances is used as an NIR absorber. According to the invention, the NIR absorber can contain soot or it consists of soot, whereby it is preferred that the NIR absorber is soot and that other NIR absorbers are not present or are present only to a small extent. The term "to a small extent" means that the ratio of the absorption of soot to the absorption of the other NIR absorbers in at least part of the NIR range (e.g.at 980 ± 7 nm) is at least 2 or 3, preferably at least 4 or 5, more preferably at least 6 or 7 and in particular at least 8 or 9 or at least 10.

[0019] The term "dry blend" is synonymous with the term "dry mix" and, according to the invention, refers to a mixture of the polymer-based particles and the particles of the NIR absorber. Optionally, the dry mix may additionally comprise one or more additional additives, which are optionally introduced during the dry blend process.

[0020] The term "three-dimensional object" is used here synonymously with the term "molded body." The term "polymer particles" is used here synonymously with the term "polymer-based particles" and refers to particles that contain the polymer described in more detail here, preferably at least 20% by weight, more preferably at least 40% by weight, and in particular at least 60% by weight. The remaining components can essentially comprise fillers.

[0021] Preferably, the plastic powder comprises a dry mixture of polymer-based particles and particles of an NIR absorber, wherein the NIR absorber in turn comprises carbon black or is carbon black and wherein the carbon black has a primary particle diameter in the range from 15 nm to 70 nm, preferably of at least 26 nm and / or at most 58 nm and in particular of (42±16) nm.

[0022] The molded articles produced with the powder according to the invention exhibit unexpected advantages both during production and in the resulting molded articles. Careful selection of the concentration range of the specific NIR absorber disclosed here ensures that the process is stable and, at the same time, that the mechanical properties, particularly with regard to tensile strength, modulus of elasticity, and elongation at break, of the molded article produced are very good. It was shown that these advantages are only significantly achieved through the combination of the carbon black-containing or carbon black-consisting NIR absorber and the carbon black content.The experiments conducted by the inventors show that at lower carbon black concentrations, the mechanical properties, particularly with regard to tensile strength, Young's modulus, and elongation at break, are lower. At higher concentrations, the process becomes more unstable, thus impairing the product properties (smaller temperature window and processing range). With the specially adjusted proportion of carbon black relative to the polymer-based material, a balance was unexpectedly achieved between opposing effects in additive-containing plastic powders.

[0023] The preferred embodiment is a dry mixture of polymer-based particles and the particles of the NIR absorber, ie the particles of the NIR absorber have not been incorporated into the polymer-based particles, for example via a melt or via co-precipitation from a solution or in any other way.

[0024] The advantages of the invention and further preferred embodiments are explained below.

[0025] In a preferred embodiment of the invention, the carbon black has a primary particle diameter in the range from 15 nm to 70 nm, preferably at least 26 nm and / or at most 58 nm, particularly preferably the carbon black is industrial carbon black with a particle size of (42±16) nm. This measure advantageously further improves process stability. Furthermore, improvements in the fluidizability of the plastic powder and the coating quality as well as consistent coating behavior can be achieved. Coating quality is defined as a uniformly dense coating of the surface without areas of visibly lower bulk density, regardless of whether there is powder or previously melted building material beneath the coated powder layer. Constant coating behavior is defined as stable coating behavior over the duration of an entire construction job, possibly over several hours.A build job refers to the construction of a job, where a job is the compilation of positioned and parameterized components in the software.

[0026] The term "primary particle diameter" refers to the particle size of an original, non-agglomerated particle, as distinct from the diameter of an agglomerate or secondary particles. This means that an agglomerate consists of many primary particles, which represent the smallest, indivisible unit of the agglomerate. Starting from agglomerates, primary particles can be obtained by treating a sample in an ultrasonic bath. The primary particle diameter preferably refers to the average diameter. For the purposes of the present invention, the primary particle diameter can be determined according to ASTM D3849, in particular via morphological characterization using transmission electron microscopy (TEM) according to ASTM D3849.

[0027] Surprisingly, it was found within the scope of the invention that the said improvements are related to the primary particle diameter, essentially independent of the extent or degree to which the primary particles of the carbon black used were agglomerated in the original state or during the process of blending with the polymeric particles.

[0028] For the purpose of determining primary particle diameters, a LEO 912 Omega: 120 kV TEM instrument with a Proscan slow scan CCD 1024 x 1024 pixel camera and a 460 mesh copper grid with carbon film is suitable. Image analysis can be performed using Olympus Soft Imaging Solutions "analySIS." For sample preparation, 8 mg of a sample can be dispersed in 1 milliliter of isopropanol for 5 minutes in an ultrasonic bath. A few drops can be taken from this solution and dispersed again in 1 milliliter of isopropanol for 5 minutes in an ultrasonic bath. A drop of the solution can then be applied to a copper grid. Microscopic examination can be performed at various magnifications. Preferably, the examination is performed at a microscope magnification of 2,000x (including the camera's magnification, the magnification is 40,000x).Optionally, a calibration check can be performed using a line grating grid with defined line spacing and / or a qualification test based on the TEM surface.

[0029] In a preferred embodiment of the invention, the NIR absorber is homogeneously distributed in the dry mixture and / or the plastic powder. A light microscopic examination, for example, is suitable for assessing homogeneity. For the purposes of the present invention, a distribution is considered "homogeneous" in particular if, in a plurality of samples (e.g., 2, 3, 4, 5, 6, etc.), the smallest NIR absorber concentration deviates from the largest NIR absorber concentration by less than 30%, preferably less than 20%, more preferably less than 10%. The samples are preferably taken from a mixture of at least 1 g, in particular at least 5 g. This advantageously prevents location-dependent variations in the absorption of NIR radiation. Process stability is increased, resulting in a molded article with improved mechanical properties.

[0030] The powder according to the invention is obtainable by blending the polymer-based particles with the particles of the NIR absorber and, if appropriate, other additives in a single mixing step in the appropriate mixing ratio, so that the specified weight fraction is achieved. The mixing process is advantageously carried out in a single-step process by: (i) providing the polymer-based particles and the NIR absorber particles and (ii) dry mixing at least the polymer-based particles and the NIR absorber particles.

[0031] For this purpose, a container mixer from Mixaco CM150-D with a standard blade design is suitable: 1 bottom scraper and 1 dispersion blade (blades with a diameter of 400 mm), with which a two-stage mixing with 2 minutes at 516 rpm and 4 minutes at 1000 rpm can be carried out.

[0032] Alternatively, the mixing process can also be carried out with several mixing steps (multi-stage process):(i) Providing a first dry mixture (so-called masterbatch) by dry mixing at least polymer-based particles and soot particles with a first proportion of soot particles, (ii) Adding further polymer-based particles to the first dry mixture to obtain a second dry mixture with a second proportion of soot particles that is lower than the first proportion, (iii) Optionally, further separate addition steps of further polymer-based particles to the second and optionally further dry mixture(s) in order to obtain increasingly further reduced proportions of soot particles. Optionally, the dry mixture can be sieved at least once in at least one of steps (i) and (ii) and optionally the optional step (iii) and / or after completion of the addition steps, preferably through a sieve with a mesh size of 125 µm.The mixing process in one stage or in several stages is carried out with the proviso that after completion of the addition step (ii) or the addition steps (iii) the weight proportion of the carbon black in the total weight of polymer-based particles and carbon black particles is in the range of at least 0.02% and at most 0.45%, preferably at least 0.07% and / or at most 0.15%, particularly preferably at least 0.08% and / or at most 0.10%.

[0033] Surprisingly, it has been found that the homogeneity is better in the single-stage process than in the multi-stage process. The single-stage process therefore represents the preferred embodiment of the two options. The invention thus moves away from providing the plastic powder using a masterbatch process and instead proposes providing the entire quantity of plastic powder in a single process step, i.e., in a single-batch process. The advantages of the single-batch process include, in addition to a more homogeneous distribution of the NIR absorber in the powder, a simpler and more cost-effective process, since only one step is required.

[0034] In a preferred embodiment of the invention, the brightness value (L* value) of the plastic powder in the CIE L*a*b* color model, measured spectrophotometrically, is at most 75.00.

[0035] In a preferred embodiment of the invention, the carbon black is carbon black. Carbon black (EC No. 215-609-9, CAS No. 1333-86-4) is carbon black specifically produced as an industrial raw material. Carbon black is, for its intended purpose, a carbon modification with a high surface-to-volume ratio and is used primarily as a filler and as a black pigment. The carbon content in quantitative elemental analysis of the carbon black, especially the carbon black, is preferably at least 96%.

[0036] Alternatively or additionally, mixtures of plastic powder with carbon black exhibit a power consumption of less than 200 mJ, preferably less than 170 mJ, and especially less than 140 mJ, when analyzed by rheometer at aeration of 1.0 mm / s. This property(ies) results in significantly improved process stability compared to other types of carbon black.

[0037] Power consumption is preferably determined according to ASTM D 7891. A Freeman FT4 Powder Rheometer (manufacturer: Freeman Technology; software: C740) can be used for this purpose, for example. Air is blown through the bottom of a cylindrical vessel. Powder is filled into the vessel, and the power consumption of a stirrer is measured. The power consumption of the stirrer is used as a criterion at an air velocity ("aeration") of 1.0 mm / s. The vessel has the following dimensions: 50 mm x 260 ml (e.g., Split Vessel, No. 8329, C2001). The stirring tool used is one with a 48 mm diameter (e.g., Blade Assembly, C211). The filling quantity into the cylindrical vessel is 160 ml. The maximum shear rate ("tip speed") for the stirring tool used (48 mm diameter) is 20 mm / s. The aeration is set in five stages from 0.0 to 2.0 mm / s at a helical angle of 5° and a shear rate of 20 mm / s.

[0038] In principle, the present invention is not limited to specific polymer-based plastics. Suitable polymer bases can be selected from the group consisting of homopolymers, copolymers, and polyblends (also known as polymer blends). A polyblend (also referred to as a "polymer blend") is understood to be a mixture of two or more different polymers. A polyblend can be a single-phase polyblend (homogeneous polyblend) or a multiphase polyblend (heterogeneous polyblend). In a multiphase polyblend, multiple glass transitions are typically observed by differential scanning calorimetry. Furthermore, in a multiphase polyblend, multiple melting peaks corresponding to the melting points of the individual phases can be observed by differential scanning calorimetry.

[0039] The polymer can be selected from polyaryletherketone (PAEK), polyarylethersulfone (PAES), polyamides, polyesters, polyethers, polylactides (PLA), polyolefins, polystyrenes, polyphenylene sulfides, polyvinylidene fluorides, polyphenylene oxides, polyimides, polyetherimides, polycarbonates, and copolymers that include at least one of the foregoing polymers or their monomer units, as well as polymer blends of one or more of the aforementioned polymers or copolymers thereof, although the selection is not limited to the aforementioned polymers, copolymers, and polymer blends thereof. The term "polymers" can also encompass oligomers with a cyclic or ring-shaped molecular structure. An example of such an oligomer is CBT (cyclic butylene terephthalate) for the production of PBT (polybutylene terephthalate).

[0040] Suitable PAEK polymers and copolymers, for example, are selected from the group consisting of polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetherketone (PEK), polyetheretherketoneketone (PEEKK), polyetherketoneetherketoneketone (PEKEKK), polyaryletheretheretherketone (PEEEK), and copolymers including at least one of the aforementioned polymers.

[0041] Suitable polyamide polymers or copolymers can be selected from the group consisting of polyamide 6 / 6T, polyamide elastomers such as polyether block amides such as PEBAX™-based materials, polyamide 6, polyamide 66, polyamide 11, polyamide 12, polyamide 612, polyamide 610, polyamide 1010, polyamide 1012, polyamide 1212, polyamide PA6T / 66, PA4T / 46, and copolymers including at least one of the aforementioned polymers. Suitable polyester polymers or copolymers can be selected from the group consisting of polyalkylene terephthalates (e.g., PET, PBT) and their copolymers.

[0042] Suitable polyolefin polymers or copolymers can be selected from the group consisting of polyethylene and polypropylene. Suitable polystyrene polymers or copolymers can be selected from the group consisting of syndiotactic and isotactic polystyrenes. Suitable polyimide polymers or copolymers can be selected from the group consisting of polyarylamide, polybismaleimide, and especially polyetherimide.

[0043] In the context of the invention, polyamide polymers or copolymers are preferred, in particular polyamide 12, polyamide 11 and / or polyamide 1012 and / or a copolymer which includes at least one of the preceding polymers or their monomer units, and / or at least one polymer blend which comprises at least one of the said polymers or copolymers.

[0044] In one embodiment of the invention, the polymer-based particles comprise polymers or copolymers or blends of PAEK, polyamide, or polyetherimide as polymer material, wherein the PAEK is preferably PEEK, PEKK, PEK, PEEKK, PEKEKK, and / or PEEEK, and the preferred polyamide is polyamide 12 and / or polyamide 11. Furthermore, the molded articles made from these polymer materials meet the high requirements regarding mechanical stress.

[0045] The present invention is also suitable for the use of polyblends.

[0046] With regard to the grain size of the polymer-based particles, there are no restrictions beyond those usual in the field of laser sintering. Suitable average grain sizes d 50 are at least 10 µm, preferably at least 20 µm, more preferably at least 30 µm and / or at most 150 µm, preferably at most 100 or 90 µm, more preferably at most 80 µm, in particular at least 40 and / or at most 70 µm. The average grain size d 50 is preferably determined according to ISO 13320-1 (wet). A CILAS 1064, for example, can be used as an instrument.

[0047] Within the scope of the present invention, further suitable additives can in principle be added to the dry mix in order to impart advantageous properties to the plastic powder or the molded article produced therefrom, or in order to utilize certain advantageous properties of such additives in the manufacturing process. In order to utilize the advantages of the present invention to the greatest extent possible, however, it is preferred that the dry mix comprises at least one or more additives which reflect (in the weight proportion used and the particle size used) at most 70, 60 or 50%, preferably at most 40, 30 or 20%, and in particular at most 15, 10 or 5% of the NIR radiation. This ensures that the NIR radiation in the plastic powder is available to the NIR absorber for heat generation and is not reflected unused.Since diode lasers deliver significantly less energy than standard lasers, the powder can be processed in this way and the energy savings can be used to advantage.

[0048] The plastic powder comprises reflective particles with a surface that at least partially reflects NIR radiation. This measure allows the advantages associated with certain materials that reflect in the IR range to be utilized, while simultaneously exploiting the effects associated with the present invention.

[0049] Titanium dioxide, known as a white pigment, acts as one such reflective particle, for example. The use of titanium dioxide offers unexpected advantages, particularly in the context of the present invention. This is described below.

[0050] As described above, the present invention envisages the use of NIR absorbers to increase absorption in the NIR wavelength range (e.g., <1 µm). The inventors discovered that carbon black has a very high absorption capacity and the ability to efficiently convert the absorbed energy into heat. In the case of a universally applicable material, the amount of carbon black should be as low as possible to produce the brightest possible component. This makes the component more suitable for subsequent coloring, especially with light colors.

[0051] In the case of plastics, natural products with a brownish off-white color are referred to as natural colors. When using natural-colored plastics in a mixture with carbon black, the inventors discovered that the resulting components had an inhomogeneous color impression. The components appeared "spotty."

[0052] Surprisingly, it was discovered that components with a significantly more homogeneous color impression can be produced if TiO 2 is previously mixed into the natural-colored plastic. By adding titanium dioxide, components with a homogeneous color can be achieved even with the use of small amounts of carbon black. Accordingly, the reflective particles preferably comprise TiO 2 . Preferably, the reflective particles are formed essentially from TiO 2 .

[0053] Preferably, the weight proportion of the reflection particles in the total weight of the plastic powder is between 0.5% and 15.0%, preferably at least 0.5% and / or at most 5%, in particular at least 0.5% and / or at most 2%.

[0054] A further aspect of the present invention is a process for producing the plastic powder according to the invention. The production comprises at least the following steps: (i) Providing the polymer-based particles and the particles of the NIR absorber and, if applicable, further additives and (ii) Dry mixing at least the polymer-based particles and the particles of the NIR absorber and, if applicable, the further additives.

[0055] Optionally, after step (ii), the plastic powder can be packaged, preferably in an atmosphere protected from moisture.

[0056] In a preferred embodiment, the polymer-based particles are mixed with the NIR absorber particles and, if appropriate, other additives in a single-stage process in the desired mixing ratio, so that the specified weight fraction is achieved. However, the mixing process can also be carried out as described above with several mixing steps (multi-stage process). As also explained above, homogeneity is better in the single-stage process than in the multi-stage process.

[0057] The polymer-based particles can be stored together with the reflection particles, which preferably comprise TiO 2, and dry-mixed with the particles of the NIR absorber.

[0058] Conventional powders typically contain a flow aid such as fumed silica. Surprisingly, it has been found that carbon black acts as a flow aid. Since carbon black acts as a flow aid, the addition of a special flow aid to the powder is not necessary. Accordingly, in one embodiment, no additional flow aid is added to the plastic powder (i.e., only carbon black acts as a flow aid).

[0059] A further aspect of the present invention is a molded body or a three-dimensional object produced by selectively solidifying a powdered building material at the locations corresponding to the cross-section of the three-dimensional object in the respective layer by exposure to radiation, preferably by exposure to NIR radiation. A plastic powder according to the invention served as the building material.

[0060] NIR radiation is not absolutely necessary, but preferred. The powder can also be processed using other layering techniques and, unexpectedly, offers the same advantages.

[0061] In addition to the aforementioned advantages, the use of the plastic powder according to the invention as a building material results in the following effects, which represent a preferred embodiment of the invention. Accordingly, in a preferred embodiment, the molded body has at least one and preferably a combination of two or all of the following properties defined according to EN ISO 527: (i) Tensile strength of at least 40 MPa, preferably at least 48 MPa; (ii) Young's modulus of at least 1600 MPa; preferably at least 1700 MPa; (iii) Elongation at break of at least 2.5%; preferably at least 3.0%.

[0062] These values apply in particular to three-dimensional objects made of polyamide 12 that were produced using laser diodes arranged in a line pattern.

[0063] A further aspect of the present invention is a method for producing a molded body by selectively solidifying a powdered building material at the locations corresponding to the cross-section of the three-dimensional object in the respective layer by exposure to radiation, preferably NIR radiation. According to the invention, the plastic powder described above is used as the building material. This can be at least partially melted or completely melted and then resolidified by exposure to electromagnetic radiation, for example specifically in a wavelength or wavelength range within the NIR, in particular at 980±10 nm and / or 940±10 nm and / or 810±10 nm.

[0064] To make the process more economical and / or environmentally friendly, a portion of the unconsolidated building material ("used powder") left over from a previous production cycle can be reused in a subsequent cycle. For this purpose, the used powder is mixed with new powder in a predetermined ratio. In a preferred embodiment, the building material thus comprises a portion of used powder that previously remained as unconsolidated building material during the production of a molded body and a portion of new powder that has not previously been used in the production of an object. The proportion of new powder is preferably at most 70 percent by weight, in particular at most 60, 50, or even 40 percent by weight.

[0065] A further aspect of the present invention represents a system which serves to produce three-dimensional objects by selectively solidifying the powdered building material according to the invention at the locations corresponding to the cross-section of the three-dimensional object in the respective layer by exposure to radiation, preferably NIR radiation. According to the invention, the system comprises at least one radiation source designed to emit electromagnetic radiation, in particular specifically in a wavelength or wavelength range located in the NIR, a process chamber designed as an open container with a container wall, a carrier located in the process chamber, wherein the process chamber and carrier are movable relative to one another in the vertical direction, a storage container, and a coater movable in the horizontal direction.The storage container is at least partially filled with the plastic powder according to the invention as building material.

[0066] For the layer-by-layer melting of the plastic powder according to the invention, suitable process and system parameters are selected. In addition to the desired NIR absorber content, the layer thickness, laser power, and exposure speed are specifically selected.

[0067] In a preferred embodiment of the invention, the electromagnetic radiation is emitted specifically in the NIR range within a window of no more than 50 nm (λ2 - λ1 ≤ 50 nm), preferably no more than 40 nm, more preferably no more than 30 nm, and in particular no more than 20 nm. This makes it possible for the plastic powder according to the invention to comprise other substances that would interfere due to their absorption or reflection capacity in a first sub-range of the NIR range. By selecting a relatively narrow wavelength range outside the first sub-range, the disruptive influence can be reduced or prevented.

[0068] In a preferred embodiment of the invention, the radiation source emits electromagnetic radiation specifically at one or more wavelengths in the range of 500-1500 nm, in particular at one or more of the following wavelengths: 980±10 nm and / or 940±10 nm and / or 810±10 nm and / or 640±10 nm. Preferably, the radiation source emits at 980±7 nm.

[0069] In a preferred embodiment of the invention, the radiation source comprises at least one laser, preferably one or more laser diodes. The laser diodes can be arranged in lines or offset. It is also possible to arrange the laser diodes in a 2-dimensional array. This can be an edge emitter. Preferably, it is a surface emitter (VCSEL or Philips VCSEL). High build speeds can be achieved through line exposure. Furthermore, the use of laser diodes enables high efficiency and reduces energy costs.

[0070] Suitable laser diodes typically operate with a power between 0.1 and 500 watts, preferably at least 1.0 watts and / or at most 100 watts. The focus of the laser beam can have a radius between 0.05 mm and 1 mm, preferably at least 0.1 mm and / or at most 0.4 mm. The exposure speed, i.e., the speed of the laser focus relative to the build plane, is typically between 10 mm / s and 10,000 mm / s, preferably at least 300 mm / s and / or at most 5,000 mm / s.

[0071] In the context of the present invention, the terms "comprising" or "containing" and their grammatical variations have the following meanings: In one embodiment, in addition to the elements mentioned, further elements may be included. In another embodiment, essentially only the elements mentioned are included. In other words, in addition to their conventional meaning, in a particular embodiment, the terms may be synonymous with the term "consisting essentially of" or "consisting of." Fig. 1 shows an example of a conventional laser sintering device for the layer-by-layer production of a three-dimensional object. Fig. 2 shows a three-dimensional object according to the invention (right component) compared to a three-dimensional object according to a preferred embodiment of the invention (left component). The plastic powder from which the left component was constructed contained titanium dioxide. Fig. 3shows a light microscopic enlargement of the component surface of the components from Fig. 2 . A: left component of Fig. 2 ; B: right component of Fig. 2 .

[0072] The following methods are generally suitable for determining certain properties of the inventive objects and were used in the experiments described below. They represent preferred methods for characterizing certain properties of the inventive objects.

[0073] Tensile strength, Young's modulus, and elongation at break were determined in accordance with EN ISO 527. Type 1BB specimens were used. The conditioning condition has a relevant influence on the measurement results of mechanical properties such as tensile strength, Young's modulus, and elongation at break. The mechanical properties of the specimens were determined in a dry state, with testing taking place a maximum of 3 hours after unpacking the components. According to ISO 291, the preferred test climate for determining mechanical properties is a temperature of (23±2) °C and a relative humidity of (50±10)%. This test climate should be maintained during the determination of mechanical properties. According to EN ISO 527-1, the test speeds should be agreed upon between the interested parties. A test speed of 50 mm / s was used.

[0074] The process safety was determined according to the following criteria: The powder according to the invention must be fluidizable in the coating unit to ensure sufficient powder can be applied to the entire build area. A uniformly dense coating of the surface without areas of visibly lower bulk density, regardless of whether powder or a previously melted build material is present beneath the coated powder layer. Stable coating behavior over the duration of an entire build job, spanning several hours. A build job refers to the construction of a job, where a job is the compilation of positioned and parameterized components in the software. A sufficiently large temperature window to tolerate inhomogeneity in the temperature distribution within the build area.

[0075] The following reference examples and examples are provided for illustrative purposes and are not to be construed as limiting. They define further preferred embodiments of the invention. Examples Reference example 1: Mechanical properties depending on the concentration of soot

[0076] In this experiment, the mechanical properties of test specimens were investigated whose constituent material, i.e., plastic powder, differed in the weight fraction of carbon black relative to the total weight of the mixture of carbon black and polymer-based material. The carbon black used in this reference example was Monarch ®< 570. The polymer-based material was PA 2201 from EOS GmbH.

[0077] For this purpose, homogeneous mixtures were first prepared by physically mixing polymer-based particles and soot particles in the mixing ratio given in Table 1 and then used as build-up material in a selective laser sintering process on two different test benches.

[0078] In a test not described in detail here, it was confirmed that the build material according to the invention can in principle be used on a conventional laser sintering machine equipped with a CO2 laser source, such as an EOS P 396 from EOS Electro Optical Systems, with the standard settings described by the manufacturer. In the present test, a light source comprising NIR laser diodes was used instead of a CO2 laser. For further details on the hardware and suitable settings, reference is made to European patent application EP14824420.5, published as EP 3 079 912. The mechanical properties were then determined according to the described methods.

[0079] The mechanical properties were determined as described below. The test method and specimen dimensions are specified in the EN ISO 527 standard for tensile testing. The materials testing machine TC-FR005TN.A50, Dossier No.: 605922 from Zwick, with the TestExpert II V3.6 software, was suitable for this purpose. In the standardized tensile test, test results such as elastic modulus [MPa] and tensile strength [MPa] were determined.

[0080] The results are shown in Table 1 below. Tab. 1: Determination of mechanical properties: Concentration of soot [wt.%] Tensile strength [MPa] (ZYX orientation according to ISO ASTM 52921) Young's modulus [MPa] (ZYX orientation according to ISO ASTM 52921) Elongation at break [%] (ZYX direction according to ISO ASTM 52921) 0,030 23,0 1100 2,3 0,050 38,3 1724 3,04 0,060 41,4 1743 3,23 0,075 46,7 1892 3,45 0,090 49,1 1659 4,21

[0081] In series of tests it has been shown that the use of a mixture of a dry mix with approximately 0.04 to 0.45 wt.% carbon black delivers surprisingly better overall results compared to mixtures with other carbon black contents.

[0082] At lower concentrations, the mechanical properties (tensile strength, modulus of elasticity, elongation at break) are lower; at higher concentrations, the process becomes less stable (smaller temperature window / smaller processing range). Reference example 2: Improvement in mechanical properties compared to mixtures produced using a multi-stage mixing process

[0083] In this experiment, the mechanical properties of a molded article according to the invention were compared with a molded article produced using a mixture of 75 wt.% "PA 2200" and 25 wt.% "PA 2202 black," two commercially available construction materials based on the polymer type PA 12, with PA 2202 black containing carbon black. The construction material according to the invention contained 0.09 wt.% carbon black (here Monarch ®< 570) mixed with plastic particles (here polyamide type PA 12 with the trade name PA 2201 from EOS GmbH).

[0084] The specimens were prepared and tested according to Reference Example 1. The results are presented in Table 2 below. Tab. 2: Comparison of the mechanical properties of a molded body according to the invention and a molded body made from a multi-stage mixed building material. Test material Tensile strength [MPa] (XZY direction according to ISO ASTM 52921) Young's modulus [MPa] (XZY direction according to ISO ASTM 52921) Elongation at break [%] (XZY direction according to ISO ASTM 52921) PA 2201 + Monarch ®< 570 48,76 1731 15,02 PA 2200 + PA 2202 black 47,18 1604 12,72 Test material Tensile strength [MPa] (ZYX orientation according to ISO ASTM 52921) Young's modulus [MPa] (ZYX orientation according to ISO ASTM 52921) Elongation at break [%] (ZYX direction according to ISO ASTM 52921) PA 2201 + Monarch ®< 570 46,38 1726 5,47 PA 2200 + PA 2202 black 39,31 1665 3,21

[0085] The test series have shown that the use of a mixture of PA with Monarch ®< 570 is significantly more process reliable and leads to improved mechanical properties than the use of a mixture of PA 2200 and PA 2202 black. Reference example 3: Process reliability depending on the primary particle diameter of soot

[0086] Mixtures of polymer-based powder and various carbon black types were prepared. The proportion of each carbon black type in the total weight of the mixture was 0.09 percent by weight. The polymer-based particles were identical in all mixtures and, in this case, were made of polyamide 12 with the trade name PA 2201 from EOS GmbH. Various commercial products containing carbon black were used. These differed in the following properties, among others: primary particle size, BET surface area according to EN ISO 60, oil absorption number, pH value, manufacturing method, and ash content.

[0087] The mixtures were mixed homogeneously and used as build material in a laser sintering process. Process reliability was determined, and the results are presented in Table 4 below. Tab. 3: Safety of the laser sintering process when processing mixtures of polymer-based powder and different types of carbon black. Trade name carbon black Primary particle diameter [nm] (manufacturer's information, not determined according to standard.) Process reliability Average primary particle diameter (measured according to ASTM D3849) Monarch ®< 570 21 very good (46±12) nm +++ (44±14) nm Mogul L 24 very good +++ Special Black 4 25 very good (42±13) nm +++ (37±10) nm Printex ®< XE-2B 30 very good +++ Printex ®< 200 47 good ++ Printex ®< G 51 bad -- Flame soot 101 95 bad -- Arosperse 15 280 very bad ---

[0088] Compared to other types of carbon black, which differ significantly in particle size from approximately 15 to 50 nm, a significantly improved process stability is shown, which is particularly characterized by better fluidizability of the plastic powder, consistent coating behavior and better coating quality. Reference example 4: Mechanical properties depending on the type of carbon black used

[0089] In this experiment, the mechanical properties of specimens prepared according to Reference Example 1 were investigated. The mechanical properties were determined as described in Reference Example 1. The results are presented in Tables 4 and 5 below. Tab. 4: Determination of the mechanical properties in XZY direction of molded bodies made from blends of PA 2201 from EOS GmbH and various types of carbon black. Trade name carbon black Tensile strength [MPa] (XZY direction according to ISO ASTM 52921) Young's modulus [MPa] (XZY direction according to ISO ASTM 52921) Elongation at break [%] (XZY direction according to ISO ASTM 52921) Monarch ®< 570 48,76 1731 15,02 Mogul L 46,60 1614 17,19 Special Black 4 48,06 1652 12,68 Tab. 5: Determination of the mechanical properties in ZYX direction of molded bodies made from blends of PA 2201 and different types of carbon black. Trade name carbon black Tensile strength [MPa] (ZYX orientation according to ISO ASTM 52921) Young's modulus [MPa] (ZYX orientation according to ISO ASTM 52921) Elongation at break [%] (ZYX direction according to ISO ASTM 52921) Monarch ®< 570 46,38 1726 5,47 Mogul L 41,58 1722 4,24 Special Black 4 47,01 1847 5,18

[0090] The test series showed that, for the compounds with the highest process reliability, the use of a PA 2201 blend with Monarch® < 570 at the same concentration leads to even better component quality (mechanical properties: tensile strength, modulus of elasticity, elongation at break) than the use of other carbon black types. Components made from blends with Special Black 4 also performed well in terms of mechanical properties (tensile strength, modulus of elasticity, elongation at break), but exhibited poorer surface finishes.

[0091] The determination of the primary particle diameters according to TEM examination according to ASTM D3849 resulted in (46±12) nm and (44±14) nm for Monarch ®< 570 and (42±13) nm, (37±10) nm for Special Black 4. Example 5: TiO 2 -added plastic particles in a dry blend with carbon black

[0092] In this test, the optical properties of test specimens made from the same plastic powder (VESTOSINT ®< 1125 white) were compared. However, the plastic powders differed in that one contained titanium dioxide (VESTOSINT ®< 1125 white), while the other was free of titanium dioxide (PA 2201). The titanium dioxide content is estimated to be approximately 1 wt.%.

[0093] The results are in Fig. 2 and 3 shown.

[0094] In Fig. 2 and 3 It can be seen that when using the natural-colored plastic PA 2201 (i.e., without titanium dioxide) in a mixture with carbon black, the printed components have an inhomogeneous color impression. The components appear "spotty" ( Fig. 2 , component right; Fig. 3B). In tests with VESTOSINT ®< 1125 white, however, which contains the white pigment TiO 2, components with a uniform appearance, an extremely homogeneous color tone and color uniformity could be produced ( Fig. 2 , component left, Fig. 3A ).

Claims

1. A plastic powder for use as a build material for additively manufacturing a three-dimensional object by selectively solidifying the build material at the locations corresponding to the cross-section of the three-dimensional object in the respective layer by exposure to NIR radiation, wherein the plastic powder comprises a dry mixture of polymer-based particles and particles of an NIR absorber, wherein the plastic powder further comprises reflection particles having a surface at least partially reflecting the NIR radiation, wherein the NIR absorber comprises carbon black, and wherein the proportion by weight of the NIR absorber in the total weight of the plastic powder is in the range from 0.02% to 0.45%.

2. The plastic powder according to claim 1, wherein the proportion by weight of the NIR absorber in the total weight of the plastic powder is at least 0.07% and / or at most 0.15%, preferably at least 0.08% and / or at most 0.10%.

3. The plastic powder, in particular the plastic powder according to claim 1 or 2, wherein the plastic powder comprises a dry mixture of polymer-based particles and particles of an NIR absorber, and the NIR absorber comprises carbon black, wherein the carbon black has an average primary particle diameter in the range from 15 nm to 70 nm, preferably of at least 26 nm and / or at most 58 nm.

4. The plastic powder according to any one of the preceding claims, wherein, in the CIE L*a*b* color model, the brightness value (L* value) of the plastic powder, measured spectrophotometrically, is at most 75.00.

5. The plastic powder according to any one of the preceding claims, wherein the reflection particles comprise TiO2, and wherein the proportion by weight of the reflection particles in the total weight of the plastic powder is between 0.5% and 15%.

6. The plastic powder according to any one of the preceding claims, wherein the carbon black has at least one, preferably both, of the following properties: (i) it is amorphous industrial carbon black (definition according to EC No. 215-609-9, CAS No. 1333-86-4); (ii) in quantitative elemental analysis, the C content is more than 96%.

7. The plastic powder according to any one of the preceding claims, wherein, in the powder analysis of the plastic powder by means of a rheometer at aeration 1.0 mm / s, the power consumption is at most 200 mJ, preferably at most 170 mJ, in particular at most 140 mJ.

8. The plastic powder according to any one of the preceding claims, wherein the polymer-based particles comprise, as polymer material, at least one polymer which is selected from at least one polyaryletherketone (PAEK), polyarylethersulfone (PAES), polyamide, polyester, polyether, polylactide, polyolefin, polystyrene, polyphenylene sulfide, polyvinylidene fluorides, polyphenylene oxide, polyimide, polyetherimide, polycarbonate, preferably from polyamide, more preferably from polyamide 12, polyamide 11 and / or polyamide 1012, and / or at least one copolymer which includes at least one of the preceding polymers or the monomer units thereof, and / or at least one polymer blend which comprises at least one of said polymers or copolymers.

9. Manufacturing of a plastic powder according to any one of claims 1 to 8, wherein the production comprises at least the following steps: (i) providing the polymer-based particles and the particles of the NIR absorber, (ii) dry mixing at least the polymer-based particles and the particles of the NIR absorber, wherein the polymer-based particles are provided together with the reflection particles and are dry-mixed with the particles of the NIR absorber,10. Manufacturing of a plastic powder according to claim 9, wherein the mixing of the polymer-based particles and the particles of the NIR absorber takes place in one process step.

11. Manufacturing of a plastic powder according to any one of claims 9 to 11, wherein the plastic powder is provided without additional flow aids.

12. A three-dimensional object which was manufactured by selectively solidifying a powder-form build material at the locations corresponding to the cross-section of the three-dimensional object in the respective layer by exposure to radiation, in particular by exposure to NIR radiation, wherein a plastic powder according to any one of the preceding claims 1 to 8 was used as the build material.

13. A method for manufacturing a three-dimensional object by selectively solidifying a powder-form build material at the locations corresponding to the cross-section of the three-dimensional object in the respective layer by exposure to NIR radiation, wherein a plastic powder according to any one of claims 1 to 8 is used as the build material, and wherein the build material is selectively solidified by exposure to electromagnetic radiation emitted by a radiation source.

14. A system for manufacturing three-dimensional objects by selectively solidifying a powder-form build material at the locations corresponding to the cross-section of the three-dimensional object in the respective layer by exposure to NIR radiation, wherein the system comprises at least one radiation source which is configured to emit electromagnetic radiation specifically in a wavelength or wavelength range located in NIR, a carrier located in the process chamber, wherein the process chamber and the carrier are movable relative to one another in the vertical direction, a storage container and a coater which is movable in the horizontal direction, wherein the storage container is at least partially filled with a plastic powder according to any one of claims 1 to 8 as the build material.

15. The method according to claim 13 or the system according to claim 14, wherein the electromagnetic radiation is emitted specifically in the NIR range within a window of at most 50 nm, preferably at most 40 nm, further preferably at most 30 nm and in particular at most 20 nm, wherein preferably the radiation source emits electromagnetic radiation in the range from 500 nm to 1500 nm, in particular at at least one of the wavelengths 980±7 nm and / or 940±7 nm and / or 810±7 nm and / or 640±7 nm.

16. The method according to claim 15 or the system according to claim 14, wherein the radiation source comprises at least one laser, preferably at least one laser diode.