METHOD FOR PRODUCING MOLDED BODY BY SINTERING

Description

[0001] The present invention relates to a method for producing a shaped body by sintering according to the preamble of claim 1. The method is particularly suitable for the powder metallurgical production of shaped bodies, but can also be used for the production of shaped bodies from ceramic material including sintering densification and strengthening.

[0002] The production of metallic molded parts using powder metallurgy is a long-established and widely used method, even in mass production. The same applies to the sinter-based production of ceramic molded parts. In particular, processes are used in which a green part is first produced from a provided feedstock consisting of a mixture of metal or ceramic powder with a binder. Alternatively, a feedstock containing only metal or ceramic powder can be arranged in a powder bed, and a green part can then be produced by selectively introducing binder into the powder bed. In both cases, after chemical debinding using a solvent, a so-called brown part is obtained, which contains only a binder component.The backbone contains a component that can no longer be removed chemically with commonly used solvents such as acetone, hexane, or ethyl acetate, but rather thermally. After thermal debinding, the resulting raw part is compressed and solidified in a sintering step. The green and brown parts regularly differ in terms of both the binder content within the material backbone and the binder composition. Green parts generally contain a higher binder content and always include binder components such as plasticizers and waxes, which can be chemically removed with a solvent. Furthermore, the green parts also contain a component of another binder that is not chemically removed but driven off by heating, i.e., thermally debound—the same binder that remains in the brown part.Green parts are therefore formed from a base material with a multi-component binder system and the metal or ceramic powder particles.

[0003] However, there are also processes in which a brown part is produced directly that contains no binder materials that can be chemically removed using solvents. These brown parts are often produced from a feedstock that has a lower proportion of binder material, and usually precisely such a material that cannot be chemically debound with the usual solvents.

[0004] The shaping process for forming the green part can be carried out in various ways. In simpler versions, this is done by simple press forming in molds, from which the parts are formed, first chemically and then thermally debound, and finally sintered. However, other shaping processes are also possible, such as MIM (Metal Injection Molding), slip casting, extrusion, or – increasingly – additive manufacturing processes like 3D printing.

[0005] While initially steel with various alloys, as well as copper and bronze, were predominantly used for the powder metallurgical production of shaped parts, the focus has recently shifted to processing metals that are significantly more reactive and react more strongly with oxygen in particular; these are referred to here as reactive metals. These reactive metals include, in particular, titanium or titanium alloys, aluminum or aluminum alloys, and magnesium or magnesium alloys. For example, in the field of medical technology, but also in other industrial sectors, various components are now manufactured from titanium or titanium alloys using powder metallurgy; these include housings for implantable insulin pumps and similar devices.

[0006] One challenge in the sinter-based production of molded parts, where the parts are densified in a single sintering step, is achieving the desired high material density and strength of the sintered part, in addition to considering material shrinkage during sintering. This is because the starting material is initially riddled with pores and sometimes even defects before sintering. These pores and defects arise from two main mechanisms: Firstly, process-related gaps, usually larger pores also known as macropores, or transverse cavities resulting from delamination, can occur in the material structure during the shaping of the green part. These gaps can arise from insufficient filling of the space enclosed by the green parts with the starting material during the shaping process, or from insufficient bonding of powder layers applied in succession.Such defects can occur in additive manufacturing processes such as 3D printing, as well as in a pressing process, but also in MIM, slip casting or extrusion.

[0007] On the other hand, micropores form - due to the process - when first the green part and then subsequently the brown part are debound: Where binder particles were initially arranged, these micropores are formed between the otherwise densely packed metal particles.

[0008] While the micropores that form during debinding can generally be closed by appropriately selecting the parameters of the sintering process, and the material can be highly densified during sintering, the defects that already arise during the shaping process of the green part, such as macropores or transverse cavities, pose a greater problem. This is because these often cannot be completely closed even in a sintering process following thermal debinding. While the aforementioned micropores cannot be closed by rearranging the powder particles, as gaps between closely packed, densely packed powder particles are unavoidable in the green part, or...Since defects such as macropores or transverse cavities are found in the brown part and are only closed during the sintering process through deformation of the powder particles and diffusion processes that connect them, they can generally be filled by rearranging the powder particles. At the locations of such defects, one or even several powder particles of the material used are "missing".

[0009] To eliminate defects that cannot be closed during the sintering process, sintered components, especially when compactness and high density (i.e., low porosity) of the finished part are crucial, undergo further post-sintering treatment steps to increase material density and close remaining macropores. Hot isostatic pressing (HIP) is frequently used for this purpose, applied to the already densified parts obtained after sintering. In this process, the sintered parts are placed in a treatment chamber and subjected to a high temperature and a high-pressure medium, typically an inert gas.Depending on the metallic or ceramic material being processed, high pressures (typically 1000 bar and higher, often up to several thousand bar) and high temperatures (typically several hundred degrees Celsius, not infrequently exceeding 1000 degrees Celsius) are required. When processing reactive metals, there are also stringent requirements for the purity of the pressing medium to prevent contamination of the components, particularly through reaction with oxygen. These process requirements, among others, make hot isostatic pressing a complex and expensive process, for which there are only a few suppliers on the market. Furthermore, it is not guaranteed that hot isostatic pressing of the sintered component can even be carried out for all materials.

[0010] On the one hand, the equipment for this process step must continue to be designed and built in such a way that it can reliably withstand the high pressures and prevent accidents, which can be devastating at these high pressures. On the other hand, generating such high pressures and temperatures is energy-intensive. Furthermore, it has been shown that the defects, which initially close under the high pressure, can often reopen after hot isostatic pressing due to the expanding gas trapped inside them when the molded part is subjected to thermal stress. This results in at least some of the pores reforming in a kind of restoring effect. Finally, the high pressure required for hot isostatic pressing, typically in the range of 1000 to 2000 bar, necessitates the use of a large quantity of the process medium, e.g.,of a gas, which – especially when the process medium has to meet the highest purity requirements – has a significant impact on the costs of the process.

[0011] Furthermore, if the defects to be sealed, such as macropores, are not enclosed within the molded part but are open to the outside, hot isostatic pressing cannot seal them. This is because the pressurized medium, such as an inert gas like argon, penetrates the open defects and keeps them open. This is particularly relevant because, in some additive manufacturing processes, such as 3D printing, delamination can occur between layers during the layer-by-layer application of material. This delamination may only become visible during debinding or even sintering. Such delamination, which represents a significant defect in the molded part, may not be apparent after sintering but may only manifest itself under mechanical stress.These defects, resulting from delamination and appearing on the surface of the molded part or extending to the surface, have a particularly high impact on the so-called alternating load, especially under dynamic stress. In the worst case, this can lead to the failure of a molded part manufactured in this way, even in critical application situations.

[0012] Furthermore, processes have been described in which the unfinished component is densified by pressurization after the forming process and before sintering. For example, EP 0 995 525 A1 describes a procedure in which, for instance, a green part is produced using MIM, this green part is debound, and then sintered. The green part, or the debound green part, is densified by pressurizing it with an inert gas, such as argon. The process disclosed therein involves pressurizing the green part with a comparatively high pressure of at least 1000 bar, and in many described experimental examples even significantly higher, which makes this procedure technically complex and expensive.

[0013] DE 10 2018 129 162 A1 describes a process in which a molded part is first formed as a green part from a mixture of a binder and ceramic granules using an additive manufacturing process. The green part is then partially debound, subsequently sintered at a significantly elevated temperature for a specific duration, and then coated. The partially debound, sintered, and coated green part is then compacted in a cold wet isostatic pressing process. The coating applied prior to the cold wet isostatic pressing is then removed, the green part is fully debound, and then sintered. A pressure to be used for the cold wet isostatic pressing process according to the teachings of this document is not disclosed.

[0014] US Patent 2017 / 088471 A1 describes a process for manufacturing ceramic components using a cold sintering process. In particular, it also describes the production of laminated composite components that can be formed from different ceramic layers. These layers can be laminated, i.e., bonded together, by applying pressure before the actual cold sintering process.

[0015] WO 2010 / 026016 A2 describes a process for manufacturing sintered ceramic molded parts. In this process, green parts are produced from a starting material containing ceramic particles and a binder by means of compression molding. Additives, such as a plasticizer, may be added to the starting material to improve its shapeability.

[0016] The invention aims to remedy this by providing a method for producing a molded body including a sintering step, which allows for the reduction or closing of defects, such as macropores or transverse cavities, even before the sintering step, thereby improving the material density obtained through sintering and thus making post-treatment, such as hot isostatic pressing, unnecessary.

[0017] This problem is solved according to the invention by a method having the features of claim 1. Advantageous further developments of the method according to the invention are specified in dependent claims 2 to 15.

[0018] An inventive method for producing a shaped body by sintering initially comprises, as is also customary in the prior art, the following steps: a. Providing a starting material containing a metal and / or ceramic powder, b. Forming a green part from the starting material by either shaping the starting material if it has a mixture of the material powder and binder material, or by introducing a binder material into a powder bed formed from the starting material, c. Chemically debinding and / or debinding the green part using a solvent to obtain a brown part, d. Thermally debinding the brown part obtained in step c., e. Compacting the brown part to form the molded body by sintering.

[0019] These process steps are, as already mentioned, also common in the prior art manufacturing processes for molded parts produced from ceramic powder using powder metallurgy or sintering. The special feature of the process according to the invention is that, after step c. and before step d., the brown part is first treated with a plasticizer, which can also be referred to as a plasticizing agent, that softens the binder material still present in the brown part that was not removed by the preceding chemical debinding and / or debinding with a solvent in step c., and furthermore, before step d., after treatment with the plasticizer, it is subjected to an isostatic pressing process by being subjected to a medium under increased pressure.

[0020] In the process according to the invention, before the final sintering step is carried out, the molded part formed from the binder-bound material powder is compacted in an isostatic pressing process. This is done, in particular, to close defects such as macropores or other flaws in the material structure that may have arisen during the forming process carried out in step b., and optionally also to close macropores that may have already formed through chemical debinding of the green part. Since the pressing process is isostatic, the pressure acts on the molded part from all sides, resulting in uniform compression without the risk of a one-dimensional change in the shape of the molded part. The pressing process is carried out, in particular, with the molded part fully formed and exposed.Due to the compaction achieved by this step, the individual particles of the powder material, i.e., the metal and / or ceramic powder, can be bonded together particularly well in the subsequent sintering step without forming a significant proportion of undesirable macropores or other defects. While compaction by isostatic pressing using a pressurized medium prior to the final sintering step is already known and described, these known methods require the application of high pressure and a complex process sequence. These disadvantages known from the prior art are avoided in the inventive method by the fact that the brown part is removed after step c.and, prior to isostatic pressing, is first treated with a plasticizer that softens the binder material still present in the brown part that was not removed by the preceding chemical debinding and / or solvent debinding in step c. The plasticizer increases the ductility of the binder material in the green part. Thus, with a comparatively low applied pressure, particularly no more than 500 bar, the material in the brown part can be densified, resulting in a particularly high material density in the molded part after sintering. Post-treatment of the molded part after sintering to increase its material density, as is frequently done in the prior art, for example, and especially by hot isostatic pressing (HIP) of the finished metal part, is therefore unnecessary.Furthermore, in molded bodies subjected to hot isostic pressing (HIP) after sintering, subsequent heat treatment or loading (especially alternating loads) can lead to the reopening of pores that were compacted and closed during the HIP process if these pores still contain gas inclusions, particularly if sintering was not performed under vacuum conditions. This effect is not observed with the process according to the invention, since the macropores are eliminated before sintering. Thus, there are no large-volume gas inclusions after sintering.

[0021] This is also because, in the isostatic pressing process according to the invention, no material bond yet exists between the powder particles in the unsintered (brown) part; only the mixture of the binder and material powder, previously softened by the plasticizer, needs to be deformed and compressed. Therefore, pressing can be carried out at a significantly lower pressure than is required in the hot isostatic pressing (HIP) process that takes place after sintering. Furthermore, raising the temperature to several hundred degrees Celsius, up to over 1000 degrees Celsius, as is common and necessary in the hot isostatic pressing (HIP) of the fully sintered part, is unnecessary. This results not only in a considerably lower energy requirement for such an isostatic pressing process according to the invention, carried out on the unsintered part, compared to the energy requirement for the hot isostatic pressing (HIP) process carried out after sintering.The equipment required for the isostatic pressing of the unsintered brown part is also considerably less complex than that required for the hot isostatic pressing (HIP) carried out after sintering, and also less complex than that required by the pressing and compaction processes known from the prior art, which are carried out before final sintering. In particular, due to the lower pressure and temperatures involved, the safety requirements for the equipment used to perform the isostatic pressing process according to the invention on the brown part treated with the plasticizer are significantly lower.

[0022] The plasticizer used according to the invention to soften the binder material in the brown part can advantageously be selected such that it is also removed and carried away before the actual sintering process, ideally in a debinding step(s) already planned. Suitable plasticizers include the usual and known plasticizers that are effective and can be used with the known binder systems and the polymers used therein; such a plasticizer can also be water, for example, if polyamides are used as binder components. Ideally, such plasticizers should evaporate at the latest at the temperatures provided for thermal debinding in the powder metallurgical manufacturing process, or be driven out of the brown part or the part to be sintered at these temperatures.It is also conceivable to remove an used plasticizer in a chemical and / or solvent-based step.

[0023] The use of a plasticizer can also be advantageous from another perspective: a plasticizer can impair the adhesive effect of the binder, increasing its stickiness. This can help to close and keep closed defects during the isostatic pressing of the brown part described here, since the binder then holds the closed defects together through its cohesive force.

[0024] The amount of plasticizer required to sufficiently soften the remaining binder material in the brown part is comparatively small, depending on the plasticizer's effectiveness. In any case, it is significantly lower than the amount of plasticizer that may have been added to the original binder system used to produce the green part, for example, to achieve the flowability necessary for thermoplastic molding. However, such originally added plasticizer is carried out in step c. and is no longer present in the resulting brown part. Accordingly, the technological properties of the brown part, particularly the binder material softened by the plasticizer, required for the subsequent isostatic pressing process can be adjusted according to requirements by selecting and dosing the appropriate plasticizer after step c.

[0025] The amount of plasticizer introduced and the resulting changes in working conditions at temperature and pressure can be adjusted by one or more treatments, as well as by selecting the appropriate plasticizer. The greatest influence on the softening of the binder material remaining in the brown part, also known as the backbone, can be achieved by using primary softeners, which can have very different chemical structures depending on the chemical composition of the polymeric binder material used.

[0026] Primary plasticizers are substances that, even in low concentrations, soften the polymer. In a polyamide system that is advantageously used as a binder material for the backbone, the amount of added plasticizer relative to the polymer content is, for example, in the range of 15 to 30 wt.%.

[0027] In a specific example, this could mean that in a green part with 12 wt% binder in a titanium-based feedstock with a polymer content of 30% in the binder, after complete extraction of the soluble components, 3.2 wt% polymer remains in the brown part. This then necessitates the uptake of 0.56 to 1.4 wt% plasticizer to make the brown part sufficiently ductile for the subsequent isostatic pressing process.

[0028] As mentioned previously, the quantity and composition of the plasticizer used in the isostatic pressing process for the brown part can be completely different from those used in a preceding forming process for the green part. Isostatic pressing of the brown part, compared to compacting the green part using the same approach, has the significant advantage that a kind of flexible, sponge-like structure forms in the plasticizer-treated brown part, which can be compacted much more effectively than a green part. Furthermore, the applied pressure is distributed much more homogeneously within the brown part softened by the plasticizer, and no local pressure pockets form after release, which would otherwise result from isolated pores.

[0029] The brown part can be treated, in particular, with a liquid plasticizer. This can be done, for example, by immersion treatment, i.e., similar to impregnation.

[0030] Incorporating the plasticizer into the brown part can be very advantageous according to the invention if the plasticizer is first dissolved in a solvent that is readily absorbed by the porous molded part structure of the brown part. Advantageously, the same solvents used in step c. for the chemical or solvent-based (pre-)debinding of the green part to form the brown part can be used here. In the case of polyamide-based binder systems, these could be, for example, solvents such as acetone, hexane, or ethyl acetate.

[0031] If the plasticizer is dispersed in the solvent at concentrations of, for example, 10 to 30% by weight, very low viscosities can be achieved, so that such a mixture, depending on the wall thickness of the brown part, can diffuse completely into the porous structure of the brown part within, for example, 30 to 60 minutes.

[0032] In the isostatic pressing process to be carried out according to the invention prior to the sintering step, a temperature higher than normal temperature can be applied, in particular to soften the binder material and to make it flowable or plastically deformable to a certain degree. For the polymer materials typically used as binders in powder metallurgy manufacturing processes or combined in binder systems, temperatures in the range of 30 °C to 200 °C are generally sufficient. Due to the typically significantly lower temperature set during the pressing process, the isostatic pressing carried out according to the invention prior to sintering, when a temperature increase is provided, can also be referred to as warm isostatic pressing (WIP), in contrast to the known hot isostatic pressing (HIP) carried out on the finished sintered molded part.

[0033] The pressure selected for the isostatic pressing process carried out according to the invention prior to the sintering step, under which the medium used to pressurize the brown part is subjected, is, as mentioned, typically significantly lower than the pressure used for hot isostatic pressing (HIP) carried out after the sintering step in the prior art, and can be in the range of > 60 bar, preferably > 80 bar, and particularly > 100 bar. Although significantly higher pressures can, in principle, be selected, especially when working at lower temperatures, these will not be used in practice, not least to avoid the need for the complex and expensive equipment required to reliably control such high pressures as those used in hot isostatic pressing.Accordingly, in the method according to the invention, pressures above 500 bar are generally not selected, and pressures below 300 bar are usually used.

[0034] In principle, any medium suitable for the molded part can be finely particulated solid particles, liquids, or gases. A liquid or, alternatively, a gas is preferred. It is important to ensure that the chosen medium, under the given pressure and temperature conditions during the pressing process, does not interact undesirably with the material system of the molded part, particularly with the metal powder in the case of a powder metallurgy-produced part, but also generally with the binder. Specifically, it must not undergo undesirable chemical reactions or form residues or deposits. The pressing process according to the invention, which is to be carried out before the sintering step, can, for example, be performed using water as the medium. However, gases, especially intergases such as argon or nitrogen, are also preferred.

[0035] Particularly when defects to be closed by the pressing process to be carried out before the sintering step, such as macropores, are exposed, i.e., have a connection through the surface of the molded part to the outside, or when, for example, defects in surface connections (delamination) observed after shaping in an additive manufacturing process such as 3D printing are to be remedied, it is advantageous and is provided in a further development of the invention that the brown part is provided with a coating, in particular a polymer coating, enclosing the outer surface of the molded part after treatment with the plasticizer and before exposure to the pressurized medium.Such a coating can be applied, for example, by dipping the brown part into a bath of liquefied coating material, such as a polymer, and the coating then solidifies. However, other application methods are also conceivable, such as spraying or application with a tool similar to those used for paints or varnishes. Encasing the part in a film or similar material, for example, by sealing it in a film or comparable encapsulation, is also possible. Such an applied coating closes existing cracks and openings on the surface of the brown part, for example, in an open pore system, so that by applying the pressurized medium, such defects can also be compressed and sealed.On the other hand, this coating also forms a protective layer around the molded part, preventing contact between the medium and the brown part if it could react with the components of the brown part or if any residues could adversely affect the sintering process. For example, the use of water as a medium is possible if such a coating is water-resistant and can be removed after the pressing process prior to sintering, without any unwanted water residue coming into contact with the brown part, becoming trapped there, or even reacting and introducing oxygen into the molded part.

[0036] The coating applied according to the aforementioned refinement can, in particular, be made of a chemically and / or solvent-based and / or thermally separable material. However, it can also be mechanically removable, for example, if a film or a comparable shell-like coating is used. If the coating is thermally separable, it can, for example, be thermally debound or removed in a single process together with the binder material (the backbone) of the binder system that holds the green part together before the sintering process. Preferably, however, the coating is made of a chemically and / or solvent-separable material, so that the coating can be removed chemically and / or by the use of a solvent after the isostatic pressing process and before the sintering step.In this way, the proportion of material, particularly organic material, which is typically reduced during subsequent thermal debinding (a process usually carried out in a single step followed by sintering), can be minimized. This avoids, or at least reduces, contamination of the molded part by incorporating, for example, carbon from the coating material. Furthermore, the plasticizer introduced into the brown part according to the invention can be removed using this method, for example, by washing it out with a solvent.

[0037] However, it is also conceivable that a coating is separated in another way, e.g. by applying a negative pressure, by means of a vacuum.

[0038] Since the coating is removed after the isostatic pressing process, pressure equalization can occur, allowing gas to escape from the compressed brown part. Ambient pressure then establishes itself there. This further suppresses any potential springback effects, in which closed pores are reopened by the continuing overpressure.

[0039] The shaping of the green or brown part in step b of the inventive method can preferably be carried out by an additive manufacturing process, such as 3D printing. However, it can also be achieved by MIM, slip casting, extrusion or by a compression molding process.

[0040] The process according to the invention can be carried out as a powder metallurgy process and, in principle, with all possible metals and metal alloys, for example, with steel, including stainless steel, bronze, copper, and the like. In particular, however, it is also very well suited for the powder metallurgical production of shaped bodies from a metal that is significantly more reactive than steel, referred to here as "reactive," such as titanium, a titanium alloy, aluminum, an aluminum alloy, magnesium, or a magnesium alloy. For this purpose, a reactive metal powder, such as titanium powder, a titanium alloy powder, magnesium powder, or a magnesium alloy powder, is used. The process according to the invention can also be applied to other reactive metals, such as so-called nickel- or cobalt-based superalloys and to the refractory metals molybdenum, tungsten, rhenium, and tantalum.In this context, "reactive metals" refers specifically to base metals, particularly those with a standard potential E° of < -1.0 V. The inventive process can also be used for the production of molded parts made of hard metals, or of a material with a metallic or ceramic matrix and embedded hard metal particles, such as tungsten or cobalt carbide. Likewise, the inventive process can also be used for the sinter-based production of ceramic molded parts, in which case green or brown parts containing a ceramic powder and binder are formed and treated with the warm isostatic pressing process disclosed herein before sintering.

[0041] By means of the warm isostatic pressing according to the invention prior to the sintering step, not only can the defects obtained during the forming process be closed, but with suitable settings of pressure and temperature, a first densification of the brown part can also be achieved, i.e. an increase in the packing density of the material powder particles in volume, thus a first reduction of the microporosity even before the actual sintering.

[0042] The inventor was able to demonstrate the effectiveness of the invention through experiments. Among other things, the inventor conducted the experiment described below, illustrated in more detail in the accompanying figures, and its results explained, and evaluated the results as described. The figures show: Fig. 1 shows a schematic representation of the inventive procedure in carrying out the process step referred to here as warm isostatic pressing in the experiment carried out, and Fig. 2 shows a bar chart comparing the determined porosity of sintered parts produced according to a process according to the invention with the determined porosity of reference parts produced identically except for the omission of warm isostatic pressing.

[0043] To carry out the experiment, a plate approximately 5 mm thick was pressed from a feedstock containing a Ti6Al4V powder and a polyamide-based binder system, and then sawn into strips or beams. These beams contained air inclusions, so-called cavities. This is shown in the representation labeled I of the Figure 1 shown where a void is marked with an L.

[0044] The resulting beams were solvent-debound in acetone. Subsequently, half of the beams (WIP) were infiltrated with a mixture of acetone and plasticizer. After drying, the treated beams were wet-coated with a mixture of acetone and an acetone-soluble polymer and dried again. After this second drying process, a polymer coating remained on the outer surfaces of the treated beams. This is shown in Figure II of the Figure 1 reproduced.

[0045] The remaining half of the bars served as a reference.

[0046] The first half of the beams coated with the polymer was subjected to a pressing process according to the invention, carried out prior to a sintering step (warm isostatic pressing, WIP). This WIP treatment was performed by pressurizing with argon at a pressure of 90 bar and initially at approximately 160°C. Subsequently, the pressure was maintained while the temperature was lowered. This WIP treatment is shown in Figure III of the Figure 1 illustrated. After this treatment, the treated beams were again debound in acetone to remove the plasticizer and the polymer coating (see Figure IV in Figure 1 ).

[0047] Subsequently, the beams that had undergone WIP treatment, as well as the untreated beams retained as a reference, were sintered in a single furnace run. This is shown in Figure V of the Figure 1Illustrated. Density was determined on the sintered components using Archimedes' principle. The measurement results were averaged, and the mean values ​​are shown in Figure 2 applied. These results show that the one associated with the as WIP The porosity of the beams treated with the above-described and inventive method, as represented by the column, is approximately 1% lower than that shown by the column described above. ref The porosity of the untreated reference beams is shown in the column labeled. It follows that the voids L initially present in both beam groups were reduced in volume or even completely closed in the WIP-treated group of beams. This is shown in the Figure 1 illustrated by the smaller depiction of the void L'.

[0048] It should be emphasized again at this point that the experiment described above and illustrated in the figures represents only one possible approach for carrying out the process according to the invention. In particular, the process is not limited to powder metallurgy manufacturing processes, and certainly not to those using the specific alloy employed in the experiment, but can equally well be carried out with ceramic material. Furthermore, the coating of the molded part prior to WIP treatment, as chosen in the experiment, is not strictly necessary. This coating is optional and will be chosen by a person skilled in the art if it is advantageous, as explained above.The decisive factor is solely the fact that a treatment step, referred to here as warm isostatic pressing, is carried out on the green part and / or brown part before the sintering step in order to obtain an increase in material density even before sintering.

Claims

1. A method for producing a moulded article by sintering, comprising the following steps a. Providing a starting material containing a metal and / or ceramic powder, b. Moulding a green part from the starting material by either moulding the starting material when it comprises a mixture of the metal and / or ceramic powder and binder material, or by introducing a binder material into a powder bed formed from the starting material, c. Chemical debinding and / or debinding by means of a solvent of the green part to obtain a brown part, d. Thermal debinding of the brown part obtained in step c, e. Compaction of the brown part to form the moulded body by sintering, characterised in that after step c. and before step d., the brown part is first treated with a plasticiser which softens the binder material still present in the brown part and not removed by the preceding chemical debinding and / or debinding by means of a solvent in step c., and furthermore, before step d., after treatment with the plasticiser, is subjected to an isostatic pressing process by exposure to a medium under an elevated pressure.

2. A method according to claim 1, characterised in that the brown part is treated with a liquid plasticiser, in particular by immersion treatment.

3. A method according to one of the preceding claims, characterised in that the isostatic pressing process is carried out at a temperature higher than room temperature, in particular at a temperature in the range from 30°C to 200°C.

4. A method according to one of the preceding claims, characterised in that the isostatic pressing process is carried out by applying a medium under a pressure of > 60 bar, preferably > 80 bar, in particular > 100 bar, but also < 500 bar, in particular < 300 bar,5. A method according to one of the preceding claims, characterised in that a gas, in particular an inert gas, or a liquid is used as the medium under increased pressure.

6. A method according to one of the preceding claims, characterised in that the brown part is provided with a coating, in particular a polymeric coating, enclosing the outer surfaces of the brown part after treatment with the plasticiser and before exposure to a medium under increased pressure.

7. A method according to claim 6, characterised in that the coating is formed from a chemically and / or solvent-based and / or thermally separable material.

8. A method according to claim 7, characterised in that the coating is formed from a material which can be removed thermally, mechanically, chemically and / or by means of a solvent, and in that after the isostatic pressing process and before step d. the coating is removed thermally, mechanically, chemically and / or with a solvent.

9. A method according to one of the preceding claims, characterised in that the plasticiser introduced into the brown part after step c. and before step d. is removed after the isostatic pressing operation has been carried out by subjecting the brown part to a medium under increased pressure and before sintering step e., in particular before step d.

10. A method according to one of the preceding claims, characterised in that the shaping of the green part in step b is carried out by an additive shaping process.

11. A method according to any one of claims 1 to 9, characterised in that the green part is formed in step b. by metal injection moulding - MIM, by slip casting, by extrusion or by a compression moulding process.

12. A method according to one of the preceding claims, characterised in that the moulded part is produced by powder metallurgy and in step a. a starting material containing metal powder is provided.

13. A method according to claim 12, characterised in that the metal powder used is a powder of a metal which is more reactive with oxygen than steel, in particular titanium powder, a titanium alloy powder, aluminium powder, an aluminium alloy powder, magnesium powder or a magnesium alloy powder.

14. A method according to one of claims 12 or 13, characterised in that the metal powder contains a hard metal powder.

15. A method according to one of claims 1 to 11, characterised in that a ceramic moulded part is produced and a starting material containing ceramic powder is provided in step a..