This invention relates to a metal injection molding feedstock based on an iron-based, metal powder with a chemical composition including the elements titanium and copper, to produced metal parts with superior yield strengths and ductility.

Background of the Invention

Metallic moldings can be produced by injection molding of thermoplastic molding materials which contain metal powders and organic binder materials. The binder materials are highly loaded with the metal powders and the resultant composite is called a feedstock. After injection molding, extruding, 3D printing or pressing the filled thermoplastic molding materials or feedstock to form a green body, the organic binder is removed and the debinded green body is sintered. For metal-containing feedstocks used with injection molders, this process is known as metal injection molding (MIM) e.g. from US 5 737683.

MIM is a versatile, low waste industrialized processed extensively used in the information and communication technology (ICT) industry for the production of various metal parts going into, for example, mobile phones and accessories, computers, tablets and earphones. MIM has also found wide adoption for the production of mechanical and functional parts such as locks and pump-parts, moving engine parts and sensor- units.

Mechanical parts in electronic devices like mobile phones, computers or laptops are regularly required to withstand strong forces. As these consumer goods have certain aesthetic requirements, they must simultaneously demonstrate good corrosion resistance under atmospheric environments. An example of such a part are hinges, as they are used in laptops or mobile phones in units to open or close the device.

Electronic devices are used numerous times a day, requiring the mechanical parts within the devices to have a superior resistance to mechanical wear in order to guarantee their maintained (continued) function. Mechanical properties such as a high yield strength and high ductility are essential for these materials to prevent failure during the device lifetime.

Furthermore, these parts are typically complex in shape, which are very difficult to machine out of the applicable wrought steel. The targeted properties like strength or hardness makes machining (for example CNC) these parts out of a block very difficult and expensive. Metal Injection molding (MIM) is an interesting alternative to machining, not only due to these technical advantages, but also due to the cost and sustainability (low waste) aspects.

In conclusion, the market has defined the need for MIM feedstocks based on a metal alloy that is corrosion resistant and wear resistant, and applicable in an industrial MIM production facility.

Typical corrosion resistant stainless steels, such as 316L or 304, used in MIM are limited in their yield strength. Conversely, those with higher yield strength and wear resistance, such as carbon steels, tend to have low elongation at break and ductility, and often lack a good corrosion resistance. Furthermore, due to the presence of carbon as an alloying element, distortion during sintering is a common problem. Achieving a high corrosion resistance and mechanical strength simultaneously is not a trivial task.

From the steel industry, e.g. US 3408 178, it has been shown that the use of a balanced concentration of alloying elements chromium, nickel, titanium and copper within an iron-based alloy, can result in an outstanding combination of mechanical and stainless properties, if the correct age-hardening is applied to result in a substantially fully martensitic condition. This steel has however, to the best of the inventors’ knowledge, not been applied in the MIM technology. This may be due to incompatibility of copper with the typical catalytic debinding step often utilized in MIM, as well as the known difficulties of titanium in the MIM sintering processes.

The utilization of a catalytic environment to perform the debinding of the green body in the MIM process, has commercial and environmental advantages. Catalytic debinding, in comparison to solvent or thermal debinding, is much faster, requiring lower uses of energy and gases. The organic waste streams from catalytic debinding, such as formaldehyde or methane, can immediately be converted and burnt to yield carbon monoxide, carbon dioxide and water. Solvent debinding requires the disposal of large amounts of contaminated organic solvents. However one disadvantage of catalytic debinding is the action the debinding atmosphere may have on reactive elements present within the metal powders. If oxidatively sensitive elements, such as copper are present, they can readily react with the acids present in the debinding atmosphere. The formation of copper nitrides during catalytic debinding using nitric acid are highly hygroscopic and can cause swelling and cracking of parts during the process. Additionally, as copper oxides readily, reducing these oxides in a subsequent sintering step may cause further fissures and cracks forming due to the large volumes of gases that need to escape. Copper has one last disadvantage, due to its low partial pressures, it may deplete from the parts during sintering when performed at high temperatures and/or reduced pressures. This may explain why compositions such as those in US 3408 178 have as yet not found wide use in MIM. The sintering process of MIM is typically performed under a reducing atmosphere to ensure a removal of all unwanted oxides and to generate pure microstructures. Hydrogen is typically used for stainless steels. Titanium unfortunately has the side effect that it readily reacts with hydrogen at temperatures over 700°C yielding brittle hydrides. These are very disadvantageous for the final alloy properties. For this reason, argon is typically used for sintering Titanium-alloys. Titanium is also very sensitive to the presence of any oxygen, which due to the above mentioned catalytic debinding step can be very critical. This additionally may explain why compositions such as those in US 3408 178 have as yet not found wide use in MIM.

Taking into account the difficulties laid out in the previous two paragraphs, finding a powder composition including titanium, particularly in combination with copper, and tuning the subsequent debinding, sintering and heat-treatment steps to reach the desired properties is non-trivial.

An object underlying the present invention is to provide a MIM feedstock that does not have the above-mentioned drawbacks. In particular, it is an object of the present invention to provide a MIM formulation for manufacturing complex parts that show mechanical properties such as a high yield strength, particularly a yield strength of 1100 MPa or more, and a high ductility, particularly an elongation at break of 3 % or more, as well as a good corrosion resistance comparable with parts made out of a block and thus can be used for the manufacturing of the above-mentioned mechanical parts for the ICT or other industries. Additionally, the metal parts shall be catalytically debindable.

Furthermore, it is an object of the present invention to provide a method for manufacturing of such complex parts.

Summary of the Invention

A first aspect of the present invention is a composition for metal injection molding, also referred to herein as “MIM feedstock”, comprising

(a) from 40 to 70 % by volume of a metal powder, based on the total volume of the composition, in which the metal powder is an alloy comprising:

(a1) from 0.5 to 2.0 % by weight of titanium,

(a2) from 11.0 to 12.5 % by weight of chromium,

(a3) from 7.5 to 12 % by weight of nickel,

(a4) from 0 to 0.5 % by weight of niobium,

(a5) from 0 to 2.5 % by weight of copper,

(a6) from 0.1 to 1.5 % by weight of molybdenum, and (a7) from 69 to 80.9 % by weight of iron, all based on the total weight of the metal powder; and (b) from 30 to 60 % by volume, based on the total volume of the composition, of a binder comprising:

(b1) from 40 to 97.5 % by weight of at least one polyoxymethylene,

(b2) from 2 to 35 % by weight of at least one polyolefin,

(b3) from 0 to 20 % by weight, preferably from 0.5 to 20 % by weight, of at least one further polymer, and

(b4) from 0 to 5% by weight, preferably from 0.05 to 3 % by weight, of at least one dispersant, all based on the total weight of the binder.

When considering the metal powder (MP) composition embodied in this invention, more specifically the presence of titanium and copper, it is surprising that the metal powder can be sintered to achieve sintered parts with high densities following the catalytic debinding MIM method. Furthermore, after a chosen heat-treatment, that the final parts exhibit mechanical properties and corrosion behaviour known for forged and cast parts using a similar metal chemistry.

A second embodiment of the present invention is the use of the MIM feedstocks described herein in a metal injection molding process or in an additive manufacturing process to form metal parts.

A third embodiment of the present invention is a process for manufacturing a metal part by metal injection molding comprising (I) providing a composition as described herein;

(II) injection molding the composition to form a green body;

(III) catalytically debinding the green body with an acid to form a brown body;

(IV) sintering the brown body under a non-oxidative atmosphere, atmospheric or reduced pressure, and a temperature of from 1 340 to 1 450 °C to form a sintered part;

(V) optionally heat-treating the sintered parts with a solution annealing and/or precipitation-hardening process.

A fourth embodiment of the present invention is a process for process for manufacturing a metal part by additive manufacturing comprising

(I) providing a composition as describe herein;

(II) subjecting the composition to a fused filament fabrication step to form a green body;

(III) catalyti cally debinding the green body with an acid to form a brown body; (IV) sintering the brown body under a non-oxidative atmosphere, at atmospheric or reduced pressure, and a maximum temperature of from 1 340 to 1 450°C to form a sintered part; (V) optionally heat-treating the sintered parts with a solution annealing and/or precipitation-hardening process.

The correct choice of sinter and heat-treatment profile allows for the effective implementation of this alloy into a MIM-process to deliver these exceptional mechanical properties.

Detailed Description of the Invention MIM Feedstock

The composition according to the invention, also more specifically referred to herein as “MIM feedstock” or “feedstock”, comprises from 40 to 70 % by volume of the metal powder as describe above and from 30 to 60 % by volume of a binder, based on the total volume of the mixture. It has to be notated that the term “MIM feedstock” does not intend to restrict its use to MIM but the composition may also be used in Additive Manufacturing and other potential applications.

Preferably, the feedstock comprises or essentially consists of from 45 to 65 % by volume of the metal powder and from 35 to 55 % by volume of the binder, based on the total volume of the feedstock.

Particularly preferably, the feedstock comprises from 50 to 64 % by volume of a metal powder and from 36 to 50 % by volume of a binder, based on the total volume of the feedstock.

Preferably the feedstock essentially consists of the metal powder and the binder, where the % by volume of the metal powder and the binder add up to 100 %.

The mixture may be prepared by any method known to the skilled person. Preferably the feedstock is produced by melting the binder and mixing in the metal powder. For example, the binder may be melted in a twin-screw extruder at temperatures of preferably from 150 to 220 °C, in particular of from 170 to 200 °C. The metal powder is subsequently metered in the required amount into the melt stream of the binder at temperatures in the same range. Alternatively, the binder may be melted in a sigma- kneader extruder at temperatures of preferably from 150 to 220 °C, in particular of from 170 to 200 °C. The metal powder is subsequently metered in the required amount into the melt stream of the binder at temperatures in the same range.

A particularly preferred apparatus for metering the metal powder comprises as essential element a transport screw which is located in a heatable metal cylinder and transports the metal powder into the melt of the binder. The above described process has the advantage over mixing of the components at room temperature and subsequent extrusion with an increase in temperature that decomposition of polyoxymethylene (POM) used as binder as a result of the high shear forces occurring in this variant is largely avoided.

The further components of the feedstock are presented in more detail below.

Metal Powder

According to the present invention, the MIM feedstock comprises from 40 to 70 % by volume of the metal powder. In a preferred embodiment, the MIM feedstock comprises from 45 to 65% by volume of the metal powder and particularly preferably from 53 to 64 % by volume of the metal powder, based on the total volume of the MIM feedstock.

The metal powder consists of a pulverant metal alloy material comprising or essentially consisting of:

(a1) from 0.5 to 2.0 wt% of titanium,

(a2) from 11.0 to 12.5 wt% of chromium,

(a3) from 7.5 to 12 wt% of nickel,

(a4) from 0 to 0.5 wt% of niobium,

(a5) from 0 to 2.5 wt% of copper,

(a6) from 0.1 to 1.5 wt% of molybdenum,

(a7) from 69 to 80.9 wt% of iron.

As used herein, “essentially” means that there may be minor amounts of other components, in particular, unavoidable impurities that do not significantly influence the chemical and mechanical properties of the alloy. The terms “wt%” and “% by weight” are used herein synonymously.

Chromium, Cr, contributes to the corrosion resistance of the metal. Cr is a ferrite stabilizer, and should, at least for that reason, not be too high in content. The amount of Cr in the metal may be from 11.0 to 12.5 % by weight, preferably from 11.5 to 12.4 % by weight, most preferably from 11.7 to 12.3 % by weight.

Niobium, Nb, is a strong carbide former, thereby improving the corrosion resistance and hardness of the alloy. The metal powder may have a niobium content of 0 to 0.5 % by weight. In a first preferred embodiment the metal powder may have a Nb content of from 0.05 to 0.1 % by weight, most preferably from 0.1 to 0.35 % by weight. In another preferred embodiment the alloy is free of Nb.

Copper, Cu, is an austenite stabilizer, thereby improving the alloys corrosion resistance. Simultaneously, it can form precipitates which can improve the material hardness. The metal powder may have a Cu content of 0 to 2.5 % by weight. In a first preferred embodiment the metal powder may have a Cu content of from from 1 to 2.4 % by weight, preferably from 1.8 to 2.3 % by weight. In another preferred embodiment the alloy is free of Cu.

Molybdenum, Mo, both stabilizes the formation of ferrite and is a carbide former. Both can improve the materials hardness. The amount of Mo in the metal may be from 0.1 to 1.5 % by weight. In a first preferred embodiment the metal powder may have a Mo content of from 0.12 to 0.5 % by weight, preferably 0.15 to 0.4 % by weight. In this case at least one of Cu and Nb, preferably both, Cu and Nb, should be present. In a second preferred embodiment Mo may be present in an amount of from 0.5 to 1.3 % by weight, particularly 0.7 to 1.2 % by weight. In this case at least one of Cu and Nb may be omitted.

Titanium, Ti, contributes to the strength of the metal by forming carbides and/or nitrides. Additionally, Ti stabilizes ferrite leading to improved hardness. The amount of Ti in the metal may be from 0.5 to 2 % by weight in the metal. In a first preferred embodiment Ti may be present in an amount of from 0.5 to 1.5 % by weight, preferably from 1.0 to 1.8 % by weight. In this case at least one of Cu and Nb, preferably both, Cu and Nb, should be present. In a second preferred embodiment Ti may be present in an amount of from 0.9 to 1.8 % by weight, most preferably from 1 to 1.7 % by weight. In this case at least one of Cu and Nb may be omitted.

Nickel, Ni, is used as the main austenite stabilizer. The amount of Ni in the metal may be from 7.5 to 12 % by weight. Preferred amounts of Ni in the metal powder may be from 8 to 11.5 % by weight, particularly 8.5 to 11% by weight.

Iron, Fe, is used as balance in an amount of from 69 to 80.9 % by weight, i.e. it sums up with the other components to 100 % by weight. Furthermore, the metal may contain minor amounts of unavoidable impurities that do not significantly influence the chemical and mechanical properties of the alloy.

In a preferred embodiment the metal powder comprises a pulverant metal alloy material comprising or essentially consisting of (a1) from 1.0 to 2.0 % by weight of titanium,

(a2) from 11.0 to 12.5 % by weight of chromium,

(a3) from 7.5 to 12 % by weight of nickel,

(a4) 0 % by weight of niobium,

(a5) 0 % by weight of copper,

(a6) from 0.5 to 1.5 % by weight of molybdenum, and

(a7) from 72 to 80 % by weight of iron, all based on the total weight of the metal powder. In another preferred embodiment the metal powder comprises a pulverant metal alloy material comprising or essentially consisting of The composition according to claim 1, wherein the alloy comprises (a1) from 0.5 to 1.5 % by weight of titanium,

(a2) from 11.0 to 12.5 % by weight of chromium,

(a3) from 7.5 to 12 % by weight of nickel,

(a4) from 0.15 to 0.5 % by weight of niobium,

(a5) from 1.5 to 2.5 % by weight of copper, (a6) from 0.1 to 0.5 % by weight of molybdenum, and

(a7) from 70.5 to 79.25 % by weight of iron, all based on the total weight of the metal powder.

These metals are alloyed within one powder. The use of a single, fully alloyed or pre- alloyed powder is preferred. This ensures the homogeneous distribution of elements throughout the final metal part microstructure, ensuring an optimal mechanical performance and corrosion resistance.

For the preparation of the metal powder, the inorganic material has to be pulverized. To pulverize the inorganic material, any method known to the person skilled in the art may be used. For example, the inorganic material may be ground. The grinding for example may take place in a classifier mill, in a hammer mill or in a ball mill.

Metal powders for use in MIM feedstocks may also be atomized using gas or water atomization. Atomization of these powders are typically performed under inert atmospheres such as Argon and Nitrogen. Argon is often preferred as nitrogen has the disadvantage of being incorporated into the powder microstructure. Resulting nitrides may be disadvantageous for the final sinter part performance. Beneficial for the use in both MIM is a spherical powder with a particle size distribution generally from 0.005 to 100 pm, preferably from 0.1 to 70 pm, particularly preferably from 0.2 to 30 pm, most preferably from 0.3 to 20 pm measured by laser diffraction.

The mean particle size (diameter) of the powder is preferably below 100 pm, more preferably below 50 pm, most preferably below 20 pm. Powders may be sieved or classified after atomization to reach the desired particle size distributions. Plasma- treatments of the powders may also be utilized to improve the powder sphericity and to remove contaminants.

Binder

According to the present invention, the feedstock comprises from 30 to 60 % by volume of the binder. In a preferred embodiment, the mixture comprises from 35 to 55 % by volume of the binder and particularly preferably from 36 to 50 % by volume of the binder, based on the total volume of the feedstock.

According to the present invention, the binder comprises or essentially consists of (b1) from 40 to 97.5 % by weight of at least one polyoxymethylene (POM), (b2) from 2 to 35 % by weight of at least one polyolefin (PO), (b3) either no further polymer (FP) or from 0.5 to 20 % by weight of at least one further polymer (FP), and (b4) either no dispersant or from 0 to 5% by weight of at least one dispersant, each based on the total weight of the binder, where the % by weight of (b1), (b2), (b3) and (b4) add up to 100 %.

In a preferred embodiment, the binder comprises or essentially consists of (b1) from 62 to 94.95 % by weight of at least one polyoxymethylene (POM), (b2) from 3 to 20 % by weight of at least one polyolefin (PO), (b3) either no further polymer (FP) or from 2 to 15% by weight of at least one further polymer (FP) and (b4) from 0.05 to 3 % by weight of at least one dispersant, each based on the total weight of the binder, where the % by weight of components (b1), (b2), (b3), and (b4) usually add up to 100%.

Particularly preferably, the binder comprises or essentially consists of (b1) from 83 to 92.9 % by weight of at least one polyoxymethylene (POM), (b2) from 4 to 15 % by weight of at least one polyolefin (PO) and (b3) from 3 to 10 % by weight of at least one further polymer (FP) and (b4) from 0.1 to 2 % by weight of at least one dispersant, each based on the total weight of the binder (B), where the % by weight of components (b1), (b2), (b3) and (b4) add up to 100 %.

According to the present invention, the POM differs from the PO, PO differs from the FP, the FP differs from the dispersant and the dispersant differs from the POM. However, POM, PO, FP and the dispersant may comprise identical building units and, for example, differ in a further building unit and/or differ in the molecular weight.

The components (b1) POM, (b2) PE, (b3) FP, and (b4) dispersant of the binder are described in more detail below.

Polyoxymethylene

According to the present invention, the binder comprises from 40 to 97.5 % by weight of Polyoxymethylene (also referred to herein as “POM”). In a preferred embodiment, the binder comprises from 62 to 94.95 % by weight of POM and particularly preferably from 83 to 92.9 % by weight of POM, based on the total amount of the binder. At least one POM may be used in the binder. “At least one POM” within the present invention means precisely one POM and also mixtures of two or more POMs.

For the purpose of the present invention, the term “polyoxymethylene” or “POM” encompasses both, POM itself, i. e. polyoxymethylene homopolymers, and polyoxymethylene copolymers and polyoxymethylene terpolymers.

POM homopolymers usually are prepared by polymerization of a monomer selected from a formaldehyde source.

The term “formaldehyde source" relates to substances which can liberate formaldehyde under the reaction conditions of the preparation of POM.

The formaldehyde sources are advantageously selected from the group of cyclic or linear formals, in particular from the group consisting of formaldehyde and 1,3,5- trioxane. 1,3,5-trioxane is particularly preferred.

POM copolymers are known per se and are commercially available. They are usually prepared by polymerization of trioxane as main monomer. In addition, comonomers are concomitantly used. The main monomers are preferably selected from among trioxane and other cyclic or linear formals or other formaldehyde sources.

The expression “main monomers" is intended to indicate that the proportion of these monomers in the total amount of monomers, i. e. the sum of main monomers and comonomers, is greater than the proportion of the comonomers in the total amount of monomers.

Quite generally, the POM according to the present invention has at least 50 mol % of repeating units -CH2O- in the main polymer chain. Suitable polyoxymethylene (POM) copolymers are in particular those which comprise the repeating units -CH2O- and from 0.01 to 20 mol %, in particular from 0.1 to 10 mol % and very particularly preferably from 0.5 to 6 mol % of repeating units of the formula (I), wherein

R ¹ to R ⁴ are each independently of one another selected from the group consisting of H, Ci C4 alkyl and halogen-substituted Ci C4 alkyl; R ⁵ is selected from the group consisting of a chemical bond, a (-CR ^5a R ^5b -) group and a (-CR ^5a R ^5b O-) group,

R ^5a and R ^5b are each independently of one another selected from the group consisting of H and unsubstituted or at least monosubstituted Ci C4 alkyl, wherein the substituents are selected from the group consisting of F, Cl, Br, OH and Ci C4 alkyl; and n is 0, 1, 2 or 3.

If n is 0, then R ⁵ is a chemical bond between the adjacent carbon atom and the oxygen atom. If R ⁵ is a (-CR ^5a R ^5b O-) group, then the oxygen atom (O) of the (-CR ^5a R ^5b O-) group is bound to another carbon atom (C) of formula (I) and not to the oxygen atom (O) of formula (I). In other words, formula (I) does not comprise peroxide compounds. The same holds true for formula (II).

Within the context of the present invention, definitions such as Ci C4 alkyl, as for example defined above for the radicals R ¹ to R ⁴ in formula (I), mean that this substituent (radical) is an alkyl radical with a carbon atom number from 1 to 4. The alkyl radical may be linear or branched and also optionally cyclic. Alkyl radicals which have both a cyclic component and also a linear component likewise fall under this definition. Examples of alkyl radicals are methyl, ethyl, n-propyl, iso-propyl, butyl, iso butyl, sec butyl and tert butyl.

In the context of the present invention, definitions, such as halogen-substituted C1-C4- alkyls, as for example defined above for the radicals R ¹ to R ⁴ in formula (I), mean that the Ci C4 alkyl is substituted by at least one halogen. Halogens are F (fluorine), Cl (chlorine), Br (bromine) and I (iodine).

The repeating units of formula (I) can advantageously be introduced into the POM copolymers by ring-opening of cyclic ethers as first comonomers. Preference is given to first comonomers of the general formula (II), wherein R ¹ to R ⁵ and n have the meanings as defined above for the general formula (I).

As first comonomers mention may be made for example of ethylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, 1,3-butylene oxide, 1,3-dioxane, 1,3-dioxolane and 1,3-dioxepane as cyclic ethers and also linear oligoformals or polyformals such as polydioxolane or polydioxepane. 1,3 dioxolane and 1,3 dioxepane are particularly preferred first comonomers, very particular preferred is 1,3 dioxolane as first comonomer.

POM polymers which can be obtained by reaction of a formaldehyde source together with the first comonomer and a second comonomer are likewise suitable. The addition of the second comonomer makes it possible to prepare, in particular, POM terpolymers.

The second comonomer is preferably selected from the group consisting of a compound of formula (III) and a compound of formula (IV), wherein Z is selected from the group consisting of a chemical bond, an (-0-) group and an (-0 R ⁶ 0-) group, wherein R ⁶ is selected from the group consisting of unsubstituted Ci Cs alkanediyl and C3 Cs cycloalkanediyl. The Ci Cs alkanediyl is a hydrocarbon having two free valences and a carbon atom number of from 1 to 8. The Ci Cs alkanediyl according to the present invention can be branched or unbranched.

A C ₃ Cs cycloalkanediyl is a cyclic hydrocarbon having two free valences and a carbon atom number of from 3 to 8. Hydrocarbons having two free valences, a cyclic and also a linear component, and a carbon atom number of from 3 to 8 likewise fall under this definition.

Preferred examples of the second comonomer (b1c) are ethylene diglycidyl, diglycidyl ether and diethers prepared from glycidyl compounds and formaldehyde, dioxane or trioxane in a molar ratio of 2 : 1 and likewise diethers prepared from 2 mol of a glycidyl compound and 1 mol of an aliphatic diol having from 2 to 8 carbon atoms, for example the diglycidyl ether of ethylene glycol, 1,4 butanediol, 1,3 butanediol, 1,3 cyclobutanediol, 1,2 propanediol and 1,4 cyclohexanediol.

In a preferred embodiment component (b1) is a polyoxymethylene (POM) copolymer which is prepared by polymerization of from at least 50 mol-% of a formaldehyde source, from 0.01 to 20 mol-% of at least one first comonomer (bib) and from 0 to 20 mol-% of at least one second comonomer (b1c).

In a particularly preferred embodiment the POM may be a POM copolymer which is prepared by polymerization of from 80 to 99.98 mol-%, preferably from 88 to 99 mol-% of a formaldehyde source, from 0.1 to 10 mol-%, preferably from 0.5 to 6 mol-% of at least one first comonomer and from 0.1 to 10 mol-%, preferably from 0,5 to 6 mol-% of at least one second comonomer.

In a further preferred embodiment the POM may be a POM copolymer which is prepared by polymerization of from at least 50 mol-% of a formaldehyde source, from 0.01 to 20 mol-% of at least one first comonomer of the general formula (II) and from 0 to 20 mol-% of at least one second comonomer selected from the group consisting of a compound of formula (III) and a compound of formula (IV).

In a preferred embodiment of the present invention at least some of the OH-end groups of the POM are capped. Methods for capping OH-end groups are known to the skilled person. For example, the OH-end groups can be capped by etherification or esterification.

Preferred POM copolymers have melting points of at least 150°C and weight average molecular weights MW in the range from 5000 g/mol to 300000 g/mol, preferably from 6000 g/mol to 150000 g/mol, particularly preferably in the range from 7000 g/mol to 100 000 g/mol.

Particular preference is given to POM copolymers having a polydispersity (Mw/Mn) of from 2 to 15, preferably from 2.5 to 12, particularly preferably from 3 to 9.

The measurement of the weight average molecular weight (Mw) and the number average molecular weight (Mn) is generally carried out by gel permeation chromatography (GPC). GPC is also known as sized exclusion chromatography (SEC).

Methods for the preparation of POM are known to those skilled in the art.

Polyolefin

According to the present invention, the binder comprises from 2 to 35 % by weight of polyolefin (also referred to herein as “PE”). In a preferred embodiment, the binder comprises from 3 to 20 % by weight of the polyolefin and particularly preferably from 4 to 15 % by weight of the polyolefin, based on the total amount of the binder. According to the present invention, the polyolefin is at least one polyolefin. “At least one polyolefin” within the present invention means precisely one polyolefin and also mixtures of two or more polyolefins.

Polyolefins are known per se and are commercially available. They are usually prepared by polymerization of C2 Cs alkene monomers, preferably by polymerization of C2 C4 alkene monomers.

Within the context of the present invention, C ₂ Cs alkene means unsubstituted or at least monosubstituted hydrocarbons having 2 to 8 carbon atoms and at least one carbon-carbon double bond (C=C double bond). “At least one carbon-carbon double bond” means precisely one carbon-carbon double bond and also two or more carbon- carbon double bonds.

In other words, C2 Cs alkene means that the hydrocarbons having 2 to 8 carbon atoms are unsaturated. The hydrocarbons may be branched or unbranched. Examples for C2 Cs alkenes with one C=C-double bond are ethene, propene, 1 -butene, 2-butene, 2 methyl-propene (= isobutylene), 1-pentene, 2-pentene, 2-methyl-1 -butene, 3-methyl-1- butene, 1 -hexene, 2-hexene, 3-hexene and 4-methyl-1-pentene. Examples for C2 Cs alkenes having two or more C-C-double bonds are allene, 1,3-butadiene, 1,4- pentadiene, 1,3 pentadiene, 2-methyl-1, 3-butadiene (= isoprene).

If the C ₂ Cs alkenes have one C-C double bond, the polyolefins prepared from those monomers are linear. If more than one double bond is present in the C ₂ - Cs alkenes, the polyolefins prepared from those monomers may be crosslinked. Linear polyolefins are preferred.

It is also possible to use polyolefin copolymers, which are prepared by using different C ₂ Cs alkene monomers during the preparation of the polyolefins.

Preferably, the polyolefins are selected from the group consisting of polymethylpentene, poly-1 -butene, polyisobutylene, polyethylene and polypropylene. Particular preference is given to polyethylene and polypropylene and also their copolymers as are known to those skilled in the art and are commercially available.

The polyolefins may be prepared by any polymerization process known to the skilled person, preferably by free radical polymerization, for example by emulsion, bead, solution or bulk polymerization. Possible initiators may be, depending on the monomers and the type of polymerization, free radical initiators such as peroxy compounds and azo compounds with the amounts of initiator generally being in the range from 0.001 to 0.5% by weight, based on the monomers. Further Polymer

The binder may comprise from 0.5 to 20 % by weight of a further polymer. In a preferred embodiment, the binder comprises from 2 to 15 % by weight of the further polymer and particularly preferably from 3 to 10 % by weight of the further polymer, based on the total amount of the binder.

The further polymer according to the present invention is at least one further polymer. “At least one further polymer” within the present invention means precisely one further polymer and also mixtures of two or more further polymers.

As already stated above, the at least one further polymer differs from the polyoxymethylene, the polyolefin and the dispersant describe below. According to the present invention, the at least one further polymer is preferably selected from the group consisting of a polyether, a polyurethane, a polyepoxide, a polyamide, a vinyl aromatic polymer, a poly(vinyl ester), a poly(vinyl ether), a poly(alkyl(meth)acrylate) and copolymers thereof. Preferably, the further polymer is selected from the group consisting of a poly(C2-C6 alkylene oxide), an aliphatic polyurethane, an aliphatic uncrosslinked epoxide, an aliphatic polyamide, a vinyl aromatic polymer, a poly(vinyl ester) of an aliphatic C Cs carboxylic acid, a poly(vinyl ether) of a C Cs alkyl vinyl ether, a poly(alkyl(meth)acrylate) of a Ci-e alkyl and copolymers thereof.

Preferred further polymers are described in more detail below.

Polyethers comprise repeating units of formula (V). wherein

R ^{1 1} to R ¹⁴ are each independently of one another selected from the group consisting of H, Ci C4 alkyl and halogen-substituted Ci C4 alkyl;

R ¹⁵ is selected from the group consisting of a chemical bond, a (-CR ^15a R ^15b -) group and a (-CR ^15a R ^15b O-) group, wherein

R ^15a and R ^15b are each independently of one another selected from the group consisting of H and unsubstituted or at least monosubstituted C1 C4 alkyl, wherein the substituents are selected from the group consisting of F, Cl, Br, OH and Ci C4 alkyl; and n is 0, 1, 2 or 3.

If n is 0, then R ¹⁵ is a chemical bond between the adjacent carbon atom and the oxygen atom. If R ¹⁵ is a (-CR ^15a R ^15b O-) group, then the oxygen atom (O) of the (-CR ^15a R ^15b O-) group is bound to another carbon atom (C) of formula (V) and not to the oxygen atom (O) of formula (V). In other words, formula (V) does not comprise peroxide compounds. The same holds true for formula (VI).

Typical polyethers as well as their preparation are known to the skilled person.

A preferred polyether according to the present invention is, for example, a poly(alkylene glycol), also known as a poly(alkylene oxide).

Polyalkylene oxides and their preparation are known to the skilled person. They are usually synthesized by interaction of water and a bi- or polyvalent alcohol with cyclic ethers, i. e. alkylene oxides, of the general formula (VI). The reaction is catalyzed by an acidic or basic catalyst. The reaction is a so called ring-opening polymerization of the cyclic ether of the general formula (VI). wherein

R ¹¹ to R ¹⁵ have the same meanings as defined above for formula (V).

A preferred poly(alkylene oxide) according to the present invention is derived from monomers of the general formula (VI) having 2 to 6 carbon atoms in the ring. In other words, preferably, the poly(alkylene oxide) is a poly(C2-C6 alkylene oxide). Particular preference is given to a poly(alkylene oxide) derived from monomers selected from the group consisting of 1,3 dioxolane, 1,3 dioxepane and tetrahydrofuran (lUPAC-name: oxolane). In other words, particularly preferably, the poly(alkylene oxide) is selected from the group consisting of poly-1,3 dioxolane, poly-1,3 dioxepane and polytetrahydrofuran.

In one embodiment, the poly(alkylene oxide) can comprise OH-end groups. In another embodiment, at least some of the OH-end groups of the poly(alkylene oxide) can be capped. Methods for capping OH-end groups are known to the skilled person. For example, the OH-end groups can be capped by etherification or esterification.

The weight average molecular weight of the poly(alkylene oxide) is preferably in the range of from 1 000 to 100000 g/mol, particular preferably from 1 200 to 80 000 g/mol and more preferably in the range of from 1 500 to 50000 g/mol.

A polyurethane is a polymer having carbamate units. Polyurethanes as well as their preparation is known to the skilled person.

Within the present invention, aliphatic polyurethanes are preferred. They can, for example, be prepared by polyaddition of aliphatic polyisocyanates and aliphatic polyhydroxy compounds. Among the polyisocyanates, diisocyanates of the general formula (VII) are preferred

OCN-R ⁷ -NCO (VII), wherein R ⁷ is a substituted or unsubstituted C1-C20 alkanediyl or C4-C20 cycloalkanediyl, wherein the substituents are selected from the group consisting of F, Cl, Br and Ci C ₆ alkyl.

Preferably R ⁷ is a substituted or unsubstituted C2 C12 alkandiyl or C ₆ C15 cycloalkanediyl.

The Ci C20 alkanediyl is a hydrocarbon having two free valences and a carbon atom number of from 1 to 20. The Ci C20 alkyanediyl according to the present invention can be branched or unbranched. Wthin the context of the present invention, definitions such as C4-C20 cycloalkanediyl means C4 C20 cycloalkanediyle. A C4 C20 cycloalkanediyl is a cyclic hydrocarbon having two free valences and a carbon atom number of from 4 to 20. Hydrocarbons having two free valences, a cyclic and also a linear component and a carbon atom number of from 4 to 20 likewise fall under this definition.

Preferred diisocyanates are selected from the group consisting of hexamethylene diisocyanate, 2,2,4 trimethyl hexamethylene diisocyanate, 2,4,4 trimethyl hexamethylene diisocyanate, 1,2-diisocyanatomethyl cyclohexane, 1,4 diisocyanatomethyl cyclohexane and isophoron diisocyanate (lUPAC-name: 5-iso- cyanato 1 (isocyanatomethyl) 1,3,3 trimethyl-cyclohexane).

The diisocyanates may also be used in oligomeric, for example dimeric or trimeric form. Instead of the polyisocyanates, it is also possible to use conventional blocked polyisocyanates which are obtained from the stated isocyanates, for example by an addition reaction of phenol or caprolactam.

Suitable polyhydroxy compounds for the preparation of aliphatic polyurethanes are, for example, polyesters, polyethers, polyesteramides or polyacetales or mixtures thereof.

Suitable chain extenders for the preparation of the polyurethanes are low molecular weight polyols, in particular diols and polyamines, in particular diamines or water.

The polyurethanes are preferably thermoplastic and therefore preferably essentially uncrosslinked, i. e. they can be melted repeatedly without significant signs of decomposition. Their reduced specific viscosities are as a rule from 0.5 to 3 dl/g, preferably from 1 to 2 dl/g measured at 30°C in dimethylformamide.

A polyepoxide comprises at least two epoxide groups. The epoxide groups are also known as glycidyl or oxirane groups. ”At least two epoxide groups” mean precisely two epoxide groups and also three or more epoxide groups.

Polyepoxides and their preparation is known to the person skilled in the art. For example, polyepoxides are prepared by the reaction of epichlorhydrine (lUPAC-name: chlormethyloxirane) and a diol, a polyol or a dicarboxylic acid. Polyepoxides prepared in this way are polyethers having epoxide end groups.

Another possibility to prepare polyepoxides is the reaction of glycidyl(meth)acrylate (lUPAC-name: oxiran-2-ylmethyl-2-methylprop-2-enoate) with polyolefins or polyacrylates. This results in polyolefins or polyacrylates having epoxy end groups.

Preferably, aliphatic uncrosslinked polyepoxides are used. Copolymers of epichlorhydrine and 2,2 bis (4-hydroxyphenyl)-propane (bisphenol A) are particularly preferred.

Component (b3) (the at least one further polymer (FP)) can also comprise a polyamide. Aliphatic polyamides are preferred.

The intrinsic viscosity of suitable polyamides is generally from 150 to 350 ml/g, preferably from 180 to 275 ml/g. Intrinsic viscosity is determined here from a 0.5 % by weight solution of the polyamide in 96 % by weight sulfuric acid at 25 °C in accordance with ISO 307.

Preferred polyamides are semicrystalline or amorphous polyamides.

Examples of polyamides suitable as component (b3) are those that derive from lactams having from 7 to 13 ring members. Other suitable polyamides are those obtained through reaction of dicarboxylic acids with diamines.

Examples that may be mentioned of polyamides that derive from lactams are polyamides that derive from polycaprolactam, from polycaprylolactam, and/or from polylaurolactam.

If polyamides are used that are obtainable from dicarboxylic acids and diamines, dicarboxylic acids that can be used are alkanedicarboxylic acids having from 6 to 14 carbon atoms, preferably from 6 to 10 carbon atoms. Aromatic dicarboxylic acids are also suitable.

Examples that may be mentioned here as dicarboxylic acids are adipic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, and also terephthalic acid and/or isophthalic acid.

Examples of suitable diamines are alkanediamines, having from 4 to 14 carbon atoms, in particular alkanediamines having from 6 to 8 carbon atoms, and also aromatic diamines, for example m-xylylenediamine, di(4-aminophenyl)methane, di(4- aminocyclohexyl)methane, 2,2-di(4-aminophenyl)propane, 2,2-di(4-aminocyclohexyl)- propane, and 1,5-diamino-2-methylpentane.

Other suitable polyamides are those obtainable through copolymerization of two or more of the monomers mentioned above and mentioned below, and mixtures of a plurality of polyamides in any desired mixing ratio.

Preferred polyamides are polyhexamethyleneadipamide, polyhexamethylene- sebacamide, and polycaprolactam, and also nylon 6/6,6, in particular having a proportion of from 75 to 95 % by weight of caprolactam units.

Particular preference is given to mixtures of nylon 6 with other polyamides, in particular with nylon 6/6,6 (PA 6/66), particular preference being given to mixtures of from 80 to 50 % by weight of PA 6 and from 20 to 50 % by weight of PA 6/66, where the PA 6/66 comprises from 75 to 95 % by weight of caprolactam units, based on the total weight of the PA 6/66 in the mixture. The following, non-exclusive list comprises the abovementioned polyamides, and other suitable polyamides, and also the monomers comprised.

AB polymers:

PA 4 Pyrrolidone PA 6 e-Caprolactam PA 7 Ethanolactam PA 8 Caprylolactam PA 9 9-Aminopelargonic acid PA 11 11-Aminoundecanoic acid PA 12 Laurolactam

AA/BB polymers:

PA 46 Tetramethylenediamine, adipic acid PA 66 Hexamethylenediamine, adipic acid PA 69 Hexamethlyenediamine, azelaic acid PA 610 Hexamethylenediamine, sebacic acid PA 612 Hexamethylenediamine, decanedicarboxylic acid PA 613 Hexamethylenediamine, undecanedicarboxylic acid PA 1212 1.12-Dodecanediamine, decanedicarboxylic acid PA 1313 1.13-Diaminotridecane, undecanedicarboxylic acid PA 6T Hexamethylenediamine, terephthalic acid PA MXD6 m-Xylylenediamine, adipic acid

PA 6I Hexamethylenediamine, isophthalic acid PA 6-3-T Trimethylhexamethylenediamine, terephthalic acid PA 6/6T (see PA 6 and PA 6T) PA 6/66 (see PA 6 and PA 66) PA 6/12 (see PA 6 and PA 12) PA 66/6/610 (see PA 66, PA 6 and PA 610)

(see PA 6I and PA 6T)

Diaminodicyclohexylmethane, adipic acid Diaminodicyclohexylmethane, laurolactam as PA 6I/6T + diaminodicyclohexylmethane Laurolactam, dimethyldiaminodicyclohexylmethane, isophthalic acid Laurolactam, dimethyldiaminodicyclohexylmethane, terephthalic acid Phenylenediamine, terephthalic acid

Preferred polyamides are PA 6, PA 66 and PA PACM 6. Vinyl aromatic polymers are polyolefins having unsubstituted or at least monosubstituted styrene as monomer unit. Suitable substituents are, for example, C1 C6 alkyls, F, Cl, Br and OH. Preferred vinyl aromatic polymers are selected from the group consisting of polystyrene, poly-a methylstyrene and copolymers thereof with up to 30 % by weight of comonomers selected from the group consisting of acrylic esters, acrylonitrile and methacrylonitrile.

Vinyl aromatic polymers are commercially available and known to the person skilled in the art. The preparation of these polymers is also known to the person skilled in the art.

Preferably, the vinyl aromatic polymers are prepared by free radical polymerization, for example by emulsion, bead, solution or bulk polymerization. Possible initiators are, depending on the monomers and the type of polymerization, free radical initiators such as peroxide compounds and azo compounds with the amounts of initiator generally being in the range from 0.001 to 0.5 % by weight, based on the monomers.

Poly(vinyl esters) and their preparation are known to the skilled person. Poly(vinyl esters) are preferably prepared by polymerization of vinyl esters. In a preferred embodiment of the present invention, the vinyl esters are vinyl esters of aliphatic C1 C6 carboxylic acids. Preferred monomers are vinyl acetate and vinyl propionate. These monomers form poly(vinyl acetate) and poly(vinyl propionate) polymers.

Poly(vinyl ethers) are prepared by polymerization of vinyl ether monomers. Poly(vinyl ethers) and their preparation are known to the skilled person. In a preferred embodiment, the vinyl ethers are vinyl ethers of aliphatic C1 C8 alkyl ethers. Preferred monomers are methyl vinyl ether and ethyl vinyl ether, forming poly(methyl vinyl ether) and poly(ethyl vinyl ether) during the polymerization.

Preferably, the poly(vinyl ethers) are prepared by free radical polymerization, for example by emulsion, bead, solution, suspension or bulk polymerization. Possible initiators are, depending on the monomers and the type of polymerization, free radical initiators such as peroxide compounds and azo compounds with the amounts of initiator generally being in the range from 0.001 to 0.5 % by weight, based on the monomers.

Poly(alkyl(meth)acrylate) within the present invention comprises poly(alkyl acrylate), poly(alkyl methacrylates) and copolymers thereof. Poly(alkyl(meth)acrylate) comprises units derived from monomers of formula (VIII), wherein

R ⁸ is selected from the group consisting of H and C1 C8 alkyl and R ⁹ is a radical of formula (IX) wherein R ¹⁰ is a C ¹ C ¹⁴ alkyl.

Preferably, R ⁸ is selected from the group consisting of H and CrC4-alkyl, particularly preferably R ⁸ is H or methyl. Preferably, R ¹⁰ is a CrCe-alkyl, particularly preferably,

R10 is methyl or ethyl.

If R ⁸ in formula (VIII) is H and R ⁹ is a radical of formula (IX) and R ¹⁰ in formula (IX) is methyl, then the monomer of formula (VIII) is methyl acrylate.

If R ⁸ in formula (VIII) is H and R ⁹ is a radical of formula (IX) and R ¹⁰ in formula (IX) is ethyl, the monomer of formula (VIII) is ethyl acrylate.

If R ⁸ in formula (VIII) is methyl and R ⁹ is a radical of formula (IX), then the monomers of formula (VI) are methacrylic esters. Poly(alkyl(meth)acrylates) comprise as monomers preferably 40 to 100 % by weight of methacrylic esters, particularly preferably 70 to 100 % by weight of methacrylic esters and more preferably from 80 to 100 % by weight of methacrylic esters, each based on the total amount of the poly(alkyl(meth)acrylates). In another preferred embodiment, the poly(alkyl(meth)acrylates) comprise as monomers from 20 to 100 % by weight of methyl acrylate, ethyl acrylate or a mixture thereof, preferably from 40 to 100 % by weight of methyl acrylate, ethyl acrylate or a mixture thereof and particularly preferably from 50 to 100 % by weight of methyl acrylate, ethyl acrylate or mixtures of thereof, each based on the total weight of the poly(alkyl(meth)acrylate).

The person skilled in the art knows that the monomers described above for the preparation of the components (b1), (b2) and (b3) can undergo changes in their structure during the polymerization reaction. Consequently, the building units of the polymers are not the same as the monomers from which they are derived. However, the person skilled in the art knows which monomers correspond to which building unit of the polymers. Under the conditions of compounding or processing by injection molding, virtually no transacetalization occurs between component (b1), the polyoxymethylene (POM), and component (b3), the at least one further polymer (FP), i. e. virtually no exchange of comonomer units takes place.

In another embodiment of the present invention that is a particular useful for fused filament fabrication processes the at least one further polymer (FP) is selected from the group consisting of a polyether, a polyurethane, a polyepoxide, a polyamide, a vinyl aromatic polymer, a poly(vinyl ester), a poly(vinyl ether), a poly(alkyl (meth)acrylate) and copolymers thereof.

Dispersant In one embodiment of the present invention, the mixture may comprise from 0 to 5% by volume of a dispersant. In a preferred embodiment, the mixture comprises from 0.05 to 3% by volume of the dispersant and particularly preferably from 0.1 to 2% by volume of the dispersant, each based on the total volume of the mixture. As the dispersant one or more dispersants may be used.

Useful dispersants are generally known in the art. Examples are oligomeric polyethylene oxide having a low molecular weight of from 200 to 600 g/mol stearic acid, stearamides, hydroxystearic acids, fatty alcohols, fatty alcohol sulfonates and block copolymers of ethylene oxide and propylene oxide and also, particularly preferably, fatty acid esters.

Metal part manufacturing process Injection Molding

Metal Injection Molding of the MIM feedstock may be carried out following the steps:

(I) providing a composition as describe herein;

(II) injection molding the composition to form a green body; (III) catalytically debinding the green body with an acid to form a brown body;

(IV) sintering the brown body under a non-oxidative atmosphere, atmospheric or reduced pressure, and a temperature of from 1 340 to 1 450 °C to form a sintered part;

(V) heat-treating the sintered parts with a solution annealing and/or precipitation hardening process. Process step (II) is typically performed using conventional screw or plunger injection molding machines and processes known in the art. Preferably, mold temperatures of from 80°C to 140 °C and screw temperatures of from 160 to 200 °C and injection pressures of from 500 to 2000 bar are utilized. Through this process, a three- dimensional green body is produced.

Additive Manufacturing

Besides MIM, additive manufacturing methods including fused filament fabrication based on filament, rod/bar and granulate extrusions may be employed to prepare the green bodies from the feedstocks described above.

The manufacturing of metal parts using additive manufacturing may be carried out following the steps

(I) providing a composition as described above;

(II) subjecting the composition to a fused filament fabrication step to form a green body;

(III) catalytically debinding the green body with an acid to form a brown body;

(IV) sintering the brown body under a non-oxidative atmosphere, at atmospheric or reduced pressure, and a maximum temperature of from 1 340 to 1450°C to form a sintered part;

(V) optionally heat-treating the sintered parts with a solution annealing and/or precipitation-hardening process.

The use of such feedstocks in additive manufacturing according to process step II is generally described in WO 2016/012486, which is incorporated herein by reference.

Debinding

Process step (II) is followed by a process step (III) in which at least part of the binder is removed from the three-dimensional green body. The at least partial removal of the binder in a reactive atmosphere is also called catalytic debinding. The terms “process step (III)” and “catalytic debinding” for the purpose of the present invention are synonymous and are used interchangeably throughout the present invention.

To remove at least part of the binder in process step (III), the three-dimensional green body is preferably treated with a gaseous acid comprising atmosphere. Appropriate processes are described, for example, in US 2009/0288739 and US 5 145900. This process step (III) is, according to the invention, preferably carried out at temperatures below the melting temperature of the binder. In general, the process step (III) is carried out at a temperature in the range of from 20 to 150°C and particularly preferably of from 100 to 140°C. Preferably, process step (III) is carried out for a period of from 0.1 to 24h, particularly preferably of from 0.5 to 12h.

The treatment time required depends on the treatment temperature and the concentration of the acid in the treatment atmosphere and also on the size of the three- dimensional object.

Suitable acids for process step (III) of the present invention are, for example, inorganic acids which are either gaseous at room temperature or can be vaporized at the treatment temperature or below. Examples are hydrogen halides and nitric acid. Hydrogen halides are hydrogen fluoride, hydrogen chloride, hydrogen bromide and hydrogen iodide. Suitable organic acids are those, which have a boiling point at atmosphere pressure of less than 130 °C, e. g. formic acid, acetic acid or trifluoroacetic acid and mixtures thereof. Acids with boiling points above 130 °C, for example methanesulfonic acid, can also be utilized in process step (III) when dosed as a mixture with a lower boiling acid and/or water. Preferred acids for process step (III) are nitric acid, a 10 % by weight solution of oxalic acid in water or a mixture of 50 % by volume of methanesulfonic acid in water.

Furthermore, BF3 and its adducts with inorganic ethers can be used as acids.

If a carrier gas is used, the carrier gas is generally passed through the acid and loaded with the acid beforehand. The carrier gas, which has been loaded in this way with the acid, is then brought to the temperature at which process step (III) is carried out. This temperature is advantageously higher than the loading temperature in order to avoid condensation of the acids. Preferably the temperature at which process step (iv) is carried out is at least 1 °C, particularly preferably at least 5 °C and most preferably at least 10 °C higher than the loading temperature.

Preference is given to mixing the acid into the carrier gas by means of a metering device and heating the gas mixture to such a temperature that the acid can no longer condense. Preferably the temperature is at least 1°C, particularly preferably at least 5°C and most preferably at least 10°C higher than the sublimation and/or vaporization temperature of the acid and/or the carrier gas.

The carrier gas in general is any gas that is inert under the reaction conditions of the catalytic debinding step. A preferred carrier gas according to the present invention is nitrogen. The binder removal can also be carried out under reduced pressure.

The catalytic debinding is preferably continued until the polyoxymethylene (POM) of the binder has been removed to an extent of at least 80 % by weight, preferably at least 90 % by weight, particularly preferably at least 95 % by weight, based on the total weight of the POM. This may be checked, for example, with the height of the weight decrease.

It is known to the skilled person that at the temperatures of the catalytic debinding step, the metal powder comprised in the three-dimensional green body may undergo chemical and/or physical reactions. In particular, the particles of the metal powder may fuse together, undergo solid state phase transitions and/or chemical reactions with the acidic atmosphere or carrier gas.

The same holds true for the binder. During the catalytic debinding step the composition of the binder may change.

Sintering

Process step (III) may be followed by a process step (IV) in which the three- dimensional brown body is sintered. Process step (IV) is also called “sintering”. The terms “process step (IV)” and “sintering” for the purpose of the present invention are synonymous and are used interchangeably throughout the present invention.

After the sintering, the three-dimensional object is a three-dimensional sintered body. The three-dimensional sintered body comprises the consolidated form of the initial metal powder and is essentially free of the binder.

“Essentially free of the binder” according to the present invention means that the three- dimensional sintered body comprises less than 5 % by volume, preferably less than 2 % by volume, particularly preferably less than 0.5 % by volume and most preferably less than 0.01 % by volume of the binder.

It is known to the skilled person that during the sintering process the metal powder is sintered together to give a sintered inorganic powder. Furthermore, during the sintering process the metal powder can undergo chemical and/or physical reactions. Consequently, the metal powder comprised in the three-dimensional brown body usually differs from the sintered inorganic powder comprised in the three-dimensional sintered body.

In one embodiment of the present invention, after process step (III) and before process step (IV), the three-dimensional brown body obtained in process step (III) may be heated for preferably 0.1 to 12 h, particularly preferably from 0.3 to 6 h, at a temperature of preferably from 250 to 700 °C, particularly preferably from 250 to 600 °C to remove the residual binder completely. In one embodiment of the present invention, process step (IV) is defined by the following temperature profile: i. heating to a temperature of from 550 to 650 °C at a rate of from 2 to 7 °C/min, ii. holding at the temperature from 550 to 650 °C for 0.5 to 1.5 h, iii. heating to a temperature of from 1 340 to 1 450 °C at a rate of 2 to 7 °C/min, iv. holding at the temperature of from 1 340 to 1 450 °C for 0.5 h to 1.5 h, and v. cooling down to ambient temperature at a rate of from 5 to 15 °C/min.

Process step (IV) is preferably performed using an atmosphere of hydrogen or argon and atmospheric pressures. The use of reduced pressures is also possible.

Post T reatment

Finally, the target properties are best met when performing an optional final process step (V) including a heat treatment of the metal parts received after process step (IV).

Preferable heat treatments include a solution annealing and/or a precipitation hardening step.

The solution annealing step may preferably be performed in a non-oxidative atmosphere, such as but not limited to an argon or nitrogen atmosphere. The solution annealing step may preferably be performed at a temperature of from 800 and 1 200 °C for 15 to 60 min.

The precipitation-hardening step may preferably be performed between 450 and 550°C for between 30 min to 5 hours, particularly in but not limited to nitrogen, argon or air.

In one embodiment of the present invention, process step (V) is defined by the following temperature profile and conditions:

(i) a solution annealing step performed in argon or nitrogen at a temperature of from 800 to 1 200 °C for 15 min to 60 min, and

(ii) a precipitation-hardening step performed in air, argon or nitrogen at a temperature of from 450 to 550 °C for 10 min to 2 h.

By using the aspects of the inventions metal parts having a density of 7.3 g/cm ³ more may be produced. Furthermore, metal parts having a tensile strength of 1100 MPa or more, preferably 1200 MPa or more, more preferably 1300 MPa or more, most preferably 1400 MPa or more may be received. Finally, the metal parts according to the invention may have a ductility (elongation at break) of 3 % or more, preferably 4 % or more, more preferably 5 % or more, most preferably 6 % or more. All percent, ppm or comparable values refer to the weight with respect to the total weight of the respective composition except where otherwise indicated. All cited documents are incorporated herein by reference. The following examples shall further illustrate the present invention without restricting the scope of this invention.

Examples The melt flow rate (MFR) was measured according to ISO 1133-1 using 190°C and 21.6 kg.

The tensile strength, yield strength and elongation to break was measure according to ISO 6892-1:2019 B.

The corrosion resistance was measured by a salt-spray mist test according to ASTM B117 using a Weiss Umwelttechnik SC/KWT 450 salt-spray mist chamber.

The particles sizes of the metal powder were determined by static light scattering measurements performed on a Beckman Coulter LS 13320. Particle sizes are given in mircrometer unless otherwise specified.

The chemical composition of the metal powders was determined using induction- coupled plasma - optical electron spectroscopy performed on an Agilent 5110.

Gas atomized metal powders MP1 to MP3 were procured with properties given in Table 1 (amounts in wt%).

Table 1

These powders were converted into MIM feedstocks by blending in a sigma-kneader with 7-8 wt% POM, 0.7-0.8 wt% PE and 0.25-0.3 wt% additives. A metal powder loading of 64 vol% was targeted. Resultant properties of the feedstock are listed in Table 2. Table 2

The MIM feedstocks were injection molded using an injection pressure between 900 and 1000 bar, a pack pressure between 800 and 100 bar, a nozzle temperature of around 190°C, a mold temperature of around 125 °C and screw speeds of between 8 and 12 m/min to make a green body shape.

These green bodies were sintered in a Molybdenum furnace under either Argon or Nitrogen atmosphere. The following temperature profile was utilized:

I heating from room temperature to 600°C at 5 °C/min, and ii waiting at 600 °C for 1 hour, iii heating from 600 °C to the given maximum temperature at 5 °C/min, iv waiting at the given maximum temperature for 1 h, and v furnace cooling to ambient temperature.

Various maximum temperatures in steps iii and iv were tested. The results are shown in Table 3. Table 3

* OSF = Oversizing Factor

Table 3 shows that sintered part densities over 7.3 g/cm ³ can be achieved for the three feedstocks. Higher densities are typically reached when using hydrogen as a sintering atmosphere. The metal parts produced by sintering at 1400 °C under either argon or nitrogen from Feedstocks 2 and 3 where further heat treated. Details to the various heat treatment conditions are provided in Table 4a. Results from mechanical testing of these heat- treated parts are shown in table 4b, In Table 5 salt-spray mist test (SST) are also given for certain conditions.

Table 4a

Table 4b

From Table 4 it can be seen that higher yield strengths are reached for heat-treated samples originally sintered under hydrogen. Feedstock 2 has marginally higher yield strengths than Feedstock 3.

Table 5

From Table 5 it can be seen that hydrogen sintering preferably yields corrosion resistance parts.