METHOD FOR PRODUCING GRAPHITED MOLDED PARTS BY 3D PRINTING

Description

[0001] The invention relates to a method for producing graphitized molded parts by means of 3D printing with filaments made of a meltable plastic.

[0002] In WO 2018 / 196965 A1, a process for the production of ceramic objects using 3D printing is described.

[0003] A carbon- or graphite-containing precursor material is mixed with an inorganic component, and a carbide-based ceramic object is produced through thermal treatment using a 3D printer. The inorganic component is a metal such as silicon, titanium, tungsten, or a metal oxide.

[0004] The precursor material also contains carbon black or graphite powder, as well as a polymer, such as polyacrylonitrile.

[0005] The precursor material prepared in this way is produced layer by layer into the desired object using 3D printing, i.e., by fused deposition modeling technology (FDM) with a 3D printer.

[0006] After printing, the object is stabilized in air at a temperature between approximately 160 °C and approximately 250 °C, followed by a carbonization process at a temperature of 1,000 °C and 1,500 °C.

[0007] Furthermore, WO 01 / 98207 A1 describes a deformable mixture and its use.

[0008] The malleable raw material comprises powdered silicon, a carbon precursor with a water-soluble, crosslinkable, and heat-curable synthetic resin, a powdered silicon filler, and a water-soluble thermoplastic binder.

[0009] The process for producing honeycomb structures includes mixing the materials to produce a deformable material mixture and subsequently forming a green body, preferably by extrusion, drying and hardening of the green body.

[0010] The green body is then heated in a nitrogen atmosphere to a temperature sufficient to carbonize the resin and sintered at a temperature sufficient to convert the green body into porous silicon carbide, preferably above 1,400 °C.

[0011] US Patent 2020 / 276760 A1 describes an additive manufacturing process for producing a semi-crystalline article using an extrusion printing process. The process includes the following steps: (i) extrusion printing of a thermoplastic polymer composition comprising a statistical PEKK copolymer and optionally one or more additives, wherein the PEKK copolymer has a T:I ratio between approximately 61:39 and 85:15, to produce an article with a weight percent crystallinity of 15% or less.

[0012] The invention is based on the objective of creating a method for producing graphitized molded parts, e.g. in the form of a honeycomb structure or another three-dimensional structure from a thermoplastic material by means of 3D printing, which is significantly simplified compared to the prior art, for use as a carrier for a catalyst, filter or similar, or another component.

[0013] The problem underlying the invention is solved by a method having the features of claim 1, for example by

[0014] Printing a 3D structure with a 3D printer using a meltable and curable plastic filament by melting the filament and applying it layer by layer until the desired structure is created.

[0015] Stabilizing the printed 3D structure made from the filament material by means of a temperature treatment for chemical or crystallographic modification of the plastic by The 3D structure is pre-stabilized at 150°C - ~180°C for a specified period, followed by a stabilization step at ~250°C ± 20°C, until the 3D structure is sufficiently dimensionally stable.

[0016] The stabilized 3D structure is then carbonized at a high temperature and finally graphitized, which takes place at a temperature of >1,800 °C, preferably at >2,000 °C.

[0017] It has surprisingly been shown that it is possible to produce a 3D object, e.g. a honeycomb structure, a figure or the like, by printing it with a 3D printer using a plastic filament without any further additives and then, after a two-stage stabilization treatment, to directly convert it into a graphitized molded part by means of a high-temperature treatment while retaining the printed structure.

[0018] In a first embodiment of the invention, the meltable filament consists of a thermoplastic polymer, such as ABS (acrylonitrile butadiene styrene copolymer) or PVA / BVOH (polyvinyl alcohol) or BVOH (butenediol vinyl alcohol copolymer).

[0019] In a further embodiment of the invention, the printed 3D structure is heated to the temperature for pre-stabilization and stabilization, respectively, using a heating ramp of 0.2 °C / min to 1 °C / min.

[0020] Finally, in accordance with the invention, the pre-stabilization and stabilization of the printed 3D structure take place after reaching the respective predetermined temperature and keeping it constant for several hours to several days, depending on the size of the 3D structure, with a temperature of 180 °C being preferred for pre-stabilization in the interest of accelerating this process.

[0021] For stabilization, it is important that the highest possible temperature is adjusted to the chosen filament material at which no melting or severe deformation occurs, so that the stabilization process can be accelerated overall.

[0022] Furthermore, it is advantageous if the printed 3D structure is stabilized by a support device during pre-stabilization and stabilization to prevent deformation, especially in the case of large parts.

[0023] A temperature-stable material, such as metal or graphite, can be advantageously used as a support device for the printed 3D structure, although it is also possible to use a molded part produced using this method as a support.

[0024] Alternatively, the printed 3D structure can be embedded in a temperature-stable powder, such as graphite dust or fine-grained table salt, as a support device.

[0025] It is understood that graphitization must take place either under vacuum or under an inert gas, such as argon, krypton, xenon or nitrogen, to prevent carbon from burning.

[0026] To initiate a purification process to remove foreign elements, such as metals, during graphitization, a halogen gas, such as chlorine, can be introduced into the furnace atmosphere.

[0027] A further extension of the invention consists in coating the three-dimensional structure with pyrolytic carbon after graphitization in order to seal the structure and make it more stable, or to use it as a support in the form of a skeleton, for example by suitable coating of the skeleton (e.g. honeycomb structure) to make it a support for a catalyst.

[0028] Furthermore, the graphitized structure can be coated with silicon carbide, silicon nitride or tantalum carbide, or converted into silicon carbide or tantalum carbide.

[0029] The graphitized 3D structure can also be impregnated with pitch or resin to achieve densification.

[0030] In a particular embodiment of the invention, in a first step a support or support structure, or a framework, is printed and stabilized on a suitable substrate, and then this support is used as a carrier, support or base for the 3D structure to be printed in a second step.

[0031] To ensure that the printed 3D structure, which is supported by the support or support structure, can be easily detached or removed, a release agent can be applied to the support or support structure after printing, at least in the areas that come into contact with it during the subsequent printing of the 3D structure.

[0032] Alternatively, the support or support structure can also be printed simultaneously with the 3D structure.

[0033] For example, a crucible or pot and a support structure in the form of a honeycomb pattern can be printed on the outside and / or inside of the crucible or pot, resulting in a more stable structure overall. The support structure can then be mechanically removed after graphitization. This can be done by scraping the crucible or pot.

[0034] The invention is explained in more detail below using an exemplary embodiment. The accompanying drawings show: Fig. 1: a schematic representation of a 3D printer; Fig. 2a: a three-dimensional structure in the form of a honeycomb after printing; Fig. 2b: the three-dimensional structure after stabilization; Fig. 2c: the three-dimensional structure after graphitization; Fig. 3: examples of three-dimensional structures produced according to the inventive method, e.g., as a support for a catalyst or a filter, a container, and a structural component (cube); Fig. 4: a typical heating curve for pre-stabilization and stabilization; Fig. 5: another example of a 3D structure in the shape of a frog; Fig. 6a: a crucible or pot with a delicate support structure on the inside in the form of a honeycomb after printing and stabilization; and Fig. 6b: the crucible or pot from which the support structure has been mechanically removed after graphitization.

[0035] For the production of molded parts, e.g. in the form of a honeycomb structure (see Fig. 2 ) or another three-dimensional structure 1 made of a meltable and curable plastic using a filament made of thermally meltable plastic are common 3D printers ( Fig. 1 ) ideally suited. Commercially available filaments can be used.

[0036] The 3D printer basically consists of a base plate 2 with spaced-apart and vertically oriented threaded supports 3, which are movable in the Y-direction (i.e., forwards and backwards). Horizontal threaded rods 4 are suspended between these supports 3 by means of threaded sleeves 5, allowing them to be moved vertically in the Z-direction. The threaded rods 4 ultimately support the actual 3D print head 6, which is movable in the X-direction (transverse direction). This head prints the 3D structure 1 by melting layers of a filament 7, which is fed to the heated print head 6. Precise X, Y, and Z positioning is usually ensured by appropriately controlled stepper motors.

[0037] With such a relatively simple 3D printer, and a suitable programmable controller, any 3D structure can be created. Examples of such 3D structures are derived from... Fig. 2 , 3 and 5 stand out.

[0038] However, if 3D structures 1 produced in this way are to be graphitized after printing, the particular problem is that the thermally meltable plastic used for printing the 3D structure 1 would remelt or at least become deformed at the high temperatures of over 2,000 °C required for graphitization. This means that the desired end product, which ultimately consists only of a graphite structure, cannot be produced in this way.

[0039] The invention comes into play here by first creating a three-dimensional structure 1 with a 3D printer using the filament 7 made of a suitable plastic by melting it and applying it layer by layer until the desired 3D structure, e.g. a honeycomb structure, a skeleton, or a framework, has been created ( Fig. 2a ABS (acrylonitrile butadiene styrene copolymer), PVA / BVOH (polyvinyl alcohol) or butenediol vinyl alcohol copolymer) are particularly suitable as plastic materials for the filament.

[0040] TPU (thermoplastic polyurethane) is also suitable, but with a higher temperature-related deformation, similar to ABS.

[0041] Subsequently, the printed 3D structure 1 is stabilized by a special temperature treatment, which alters the chemical or crystallographic structure of the plastic.

[0042] This involves pre-stabilization at 150 °C to ~< 180 °C for an extended period until the printed 3D structure 1 is dimensionally stable, followed by a stabilization step at ~< 250 °C ± 20 °C until the 3D structure is sufficiently dimensionally stable to be transported or otherwise handled without damage. A temperature of ~< 180 °C is preferred for pre-stabilization because stabilization is accelerated at this temperature without melting.

[0043] Fig. 4 shows the corresponding temperature profile for the complete stabilization process, wherein the heating of the 3D structure to the temperature for pre-stabilization and for stabilization is carried out with a heating ramp of 0.2 to 1 °C / min, preferably with 0.5 °C / min.

[0044] The actual pre-stabilization and stabilization of the printed 3D structure 1 takes place in a long-term process by maintaining a constant temperature for several hours to several days, depending on the size of the 3D structure. The result of this thermal process is a stabilized 3D structure 8, as exemplified in Fig. 2 is shown.

[0045] Furthermore, it is advantageous if the 3D structure 1 is stabilized by supports during pre-stabilization and stabilization to prevent deformation, at least at critical points. This risk of deformation is particularly high with larger printed 3D structures 1.

[0046] A possible support structure for the 3D structure can be advantageously a framework made of a temperature-stable material, such as metal or graphite, which supports the printed 3D structure 1 at several suitable points, for example on the outside, underside, or inside. A framework manufactured using this method can also be used as a support.

[0047] It is possible, as a first step, to print a support or support structure, or a framework, for example, using the described method on a suitable substrate, and then to use this support as a carrier, support or base for the 3D structure to be printed in the second step.

[0048] The support or support structure can also be printed with a higher or lower density than the 3D structure 1; at least the support or support structure should have sufficient mechanical strength, as is the case, for example, with a honeycomb or honeycomb structure or similar.

[0049] Furthermore, it must either be ensured that the material used to print the support cannot bond or glue to the material used to print the 3D structure, so that the 3D structure can be easily removed from the support or support structure after stabilization, or the support or support structure has a sufficiently low density, such as a honeycomb or honeycomb structure, so that it can be easily removed mechanically from the 3D structure without damaging it.

[0050] It is also possible, after printing the support or support structure, to apply a release agent to it, at least in the areas that may come into contact with it during the subsequent printing of the 3D structure.

[0051] Furthermore, the support or support structure, e.g., a delicate honeycomb structure, and the 3D structure can be printed simultaneously. The delicate honeycomb structure has the advantage of particularly high stability, but can be easily removed mechanically after graphitization without damaging the 3D structure.

[0052] Alternatively, the printed 3D structure 1 can be embedded in a temperature-stable powder, such as graphite dust or fine-grained table salt, as a support.

[0053] The disadvantage here is the poorer thermal conductivity of the powder, which is compensated for by the already long stabilization time.

[0054] Once stabilization is complete, the stabilized 3D structure 8 ( Fig. 2(b) The material is graphitized at a high temperature in a suitable oven under exclusion of air, maintaining high dimensional stability. This process takes place at a temperature of >1,800 °C, preferably >2,000 °C. In this process, the stabilized 3D structure is first carbonized, i.e., the plastic is converted into a carbon-containing structure or an amorphous carbon structure, which is ultimately transformed into graphite.

[0055] It is understood that the graphitization of the stabilized 3D structure 8 must take place in a suitable furnace either under vacuum or with an inert gas, such as argon, krypton, xenon, or nitrogen, in order to avoid oxidative damage. The result is a graphitized 3D structure 9 ( Fig. 2(C) ).

[0056] In order to simultaneously initiate a cleaning process to remove foreign elements, such as metals, from the graphitized 3D structure 9 during graphitization, a halogen gas, such as chlorine, can additionally be supplied to the furnace atmosphere.

[0057] An example of a graphitized 3D structure 9 in the form of a honeycomb structure is in Fig. 2c The honeycomb structure has a comparatively high mechanical stability and contains very little material; however, any other 3D structures can of course also be produced according to the inventive method, cf. Fig. 3 or Fig. 5 , in which, as an example, a frog produced using the inventive method is shown.

[0058] It is also possible to coat the graphitized 3D structure 9 with pyrolytic carbon after graphitization in order to seal the structure, make it more stable, or to use it as a support in the form of a skeleton.

[0059] Alternative coatings are also possible with silicon carbide (SiC), silicon nitride, or tantalum carbide, or it is also possible to convert the graphitized 3D structure into silicon carbide or tantalum carbide by exposing the 3D structures in the furnace to gases of the aforementioned materials, e.g. SiO or Ta 2 O 5 ).

[0060] Furthermore, the graphitized three-dimensional structure 9 can be impregnated with pitch or a resin to achieve compaction, e.g. for filtration purposes.

[0061] Since the coating and impregnation processes are carried out at high temperatures, the 3D structure must at least have been stabilized beforehand.

[0062] Out of Fig. 3 Some application examples for 3D structures manufactured according to the inventive method are shown, such as a support for a catalyst or a filter (top), a container (middle) and a structural part, e.g. a cube (bottom).

[0063] To use the graphitized 3D structure 9 as a support for a catalyst, it is only necessary to coat it with the materials required for the catalyst function in a suitable coating device. Known coating processes such as CVD (Chemical Vapor Deposition), PVD (Physical Vapor Deposition), PECVD (Plasma Enhanced Chemical Vapor Deposition), or similar methods are suitable for this purpose.

[0064] Alternatively, a wet chemical coating of the graphitized 3D structure can be carried out using a sol-gel process in order to use it as a catalyst.

[0065] Fig. 6a shows an example of a 3D structure 1 in the form of a crucible or pot 10 and an internal support structure 11 in the form of a honeycomb structure in the crucible or pot, which were printed simultaneously with the crucible or pot ( Fig. 6a It goes without saying that the 3D structure can also have a different form.

[0066] The delicate support structure 11 can be easily removed mechanically after graphitization. This can be done by scraping out the crucible or pot 10.

[0067] Fig. 6b Figure 10 shows the crucible or pot 10, from which the support structure 11 has been removed after graphitization. Traces 12 of the removed support are visible on the inner bottom of the crucible or pot 10. Bezugszeichenliste

[0068] 1 Printed 3D structure 2 Base plate 3 Threaded support 4 Threaded rod 5 Threaded sleeve 6 Print head 7 Filament 8 Stabilized 3D structure 9 Graphite-coated 3D structure 10 Crucible or pot 11 Support structure 12 Traces of removed support

Claims

1. A method for producing graphitized shaped parts by 3D printing of filaments composed of a plastic, characterized by fusing a filament (7) composed of a fusible and curable plastic and applying it layer by layer with the 3D printer until the desired structure is created, stabilizing the 3D structure (1) printed from the material of the filament (7) by a temperature treatment to chemically or crystallographically alter the plastic, through prestabilization at 150°C to ~180°C for a relatively long period, followed by a stabilization step at ~250°C ± 20°C until the printed 3D structure (1) is sufficiently dimensionally stable, and graphitizing the stabilized 3D structure (8) with or without a supporting structure at a temperature of >1800°C, preferably at >2000°C.

2. The method as claimed in claim 1, characterized in that the fusible filament (7) used is an ABS plastic (acrylonitrile-butadiene-styrene copolymer), PVA / BVOH (polyvinyl alcohol) or TPU (thermoplastic polyurethane).

3. The method as claimed in claim 1 or claim 2, characterized in that the 3D structure (1) is heated to the temperature for prestabilization and the temperature for stabilization at a heating ramp of 0.2 - 1.0°C / min in each case.

4. The method as claimed in any of claims 1 to 3, characterized in that the printed 3D structure (1) is stabilized by a support device during prestabilization and stabilization in order to avoid deformation, especially in the case of large parts.

5. The method as claimed in claim 4, characterized in that a support device used for the printed 3D structure (1) is a temperature-stable material, such as metal or graphite.

6. The method as claimed in claim 4, characterized in that the printed 3D structure (1) is embedded in a temperature-stable powder, for example graphite dust or fine-grained cooking salt.

7. The method as claimed in any of claims 1 to 6, characterized in that graphitization is effected either under reduced pressure or under an inert gas, such as argon, krypton, xenon or nitrogen.

8. The method as claimed in any of claims 1 to 7, characterized in that the furnace atmosphere is supplied with a halogen gas, such as chlorine, while the stabilized 3D structure (8) is being graphitized.

9. The method as claimed in any of claims 1 to 8, characterized in that the graphitized 3D structure (9) is coated with a pyrolytic carbon.

10. The method as claimed in any of claims 1 to 8, characterized in that the graphitized structure is coated with silicon carbide, silicon nitrite or tantalum carbide.

11. The method as claimed in any of claims 1 to 8, characterized in that the graphitized 3D structure is converted into silicon carbide or tantalum carbide.

12. The method as claimed in any of claims 1 to 8, characterized in that the graphitized 3D structure is impregnated with pitch or a resin.

13. The method as claimed in any of claims 1 to 12, characterized in that a support or supporting structure (11), or a scaffold, is first printed and stabilized on a suitable base in a first step and in that said support is then used as a carrier, prop or rest for the 3D structure (1) to be printed in a second step.

14. The method as claimed in claim 13, characterized in that the printing of the support or supporting structure (11) is followed by applying a release agent to the support or supporting structure (11) at least in the regions that come into contact with the support or supporting structure (11) when the 3D structure (1) is subsequently printed.

15. The method as claimed in any of claims 1 to 12, characterized in that a support or the supporting structure (11) and the 3D structure (1) are printed at the same time.

16. The method as claimed in any of claims 1 to 15, characterized in that simultaneous printing of a crucible or pot (10) as the 3D structure (1) and of a supporting structure (11) on the outside and / or inside of the crucible or pot (10) is carried out and in that the supporting structure (11) is mechanically removed after the 3D structure (1) has been graphitized.