权利要求：
3. claims
1. A filament comprising a core material (CM) coated with a layer of shell material (SM), wherein the core material (CM) comprises the components a) to c) a) 30 to 80% by volume, based on the total volume of the core material (CM) of at least one inorganic powder (IP), b) 20 to 70% by volume, based on the total volume of the core material (CM) of at least one binder (B) comprising component b1) b1) at least one polymer (P) c) 0 to 20% by volume, based on the total volume of the core material (CM) of at least one additive (A), wherein the at least one polymer (P) is a polyoxymethylene (POM) homopolymer, a polyoxymethylene (POM) copolymer or polyoxymethylene (POM) terpolymer and wherein at least some of the OH-end groups of the polyoxymethylene (POM) homopolymer are capped, and the shell material(SM) comprises the components d) to f) d) 75 to 100% by volume, based on the total volume of the shell material (SM) of at least one thermoplastic polymer (TP) e) 0 to 20% by volume, based on the total volume of the shell material (SM) of the at least one inorganic powder (IP), f) 0 to 25% by volume, based on the total weight of the shell material (SM) of the at least one additive (A), wherein the thickness of the layer of shell material is 0.05 to 0.5 mm.
2. The filament according to claim 1, wherein the binder (B) i) comprises from 50 to 96% by weight or the at least one polymer (P), based on the total weight of the binder.
4. 3. The filament according to any of claims 1 or 2, wherein the binder (B) in the core material (CM) further comprises components b2) and/or b3) b2) at least one polyolefin (PO), b3) at least one further polymer (FP), in case component b) is a polyoxymethylene (POM). 4. The filament according to claim 3, wherein the binder (B) comprises 2 to 35% by weight of component b2), based on the total weight of the binder (B), and/or from 2 to 40% by weight of component b3), based on the total weight of the binder (B).
5. The filament according to any of claims 1 to 4, wherein i) the diameter of the filament is 1.5 to 3.5 mm, preferably 2.0 to 3.1 mm, more preferably 2.6 to 3.0 mm, and/or ii) the diameter of the core material is 1.5 to 3.0 mm, preferably 1.9 to 2.7 mm, more preferably 2.2 to 2.7 mm, and /or iii) the thickness of the layer of shell material (SM) is 0.09 to 0.3 mm, preferably 0.1 to 0.25 mm, and/or iv) the particle size of the inorganic powder (IP) is from 0.1 to 80 µm, preferably from 0.5 to 50 µm, more preferably from 0.1 to 30 µm, and/or v) the inorganic powder (IP) is a powder of at least one inorganic material selected from the group consisting of a metal, a metal alloy and a ceramic material, and/or vi) the at least one thermoplastic polymer (TP) of the shell material (SM) is selected from the group of polyoxymethylene (POM), polyolefins (PE) such as polypropylene or polyethylene, polyurethanes (PU), polyamides (PA), polyethers (PETH), polycarbonates (PC), and/or polyesters (PES) such as polylactic acid and blends thereof.
6. The filament according to any one of claims 1 to 5, wherein the polymer (P) in component (b1) is a polyoxymethylene (POM) copolymer which is prepared by polymerization of - from at least 50 mol-% of a formaldehyde source (b1a), - from 0.01 to 20 mol-% of at least one first comonomer (b1b) of the general formula (II) wherein R1 to R4 are each independently of one another selected from the group consisting of H, C1-C4-alkyl and halogen-substituted C1-C4-alkyl; R5 is selected from the group consisting of a chemical bond, a (¨ CR5a R5b¨) group and a (-CR5a R5bO¨) group, wherein R5a and R5b 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, CI, Br, OH and C1-C4-alkyl; n is 0, 1, 2 or 3; and - from 0 to 20 mol-% of at least one second comonomer (b1c) 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 (- O-) group and an (-O-R6-O-) group, wherein R6 is selected from the group consisting of unsubstituted C1-C8-alkylene and C3-C8-cycloalkylene.
7. The filament according to claim 3, wherein the further polymer (FP) is at least one further polymer (FP) 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.
8. A process for the preparation of a filament according to any of the claims 1 to wherein a core material (CM) is coated with a layer of a shell material (SM) by co- extrusion of the core material (CM) with the shell material (SM).
9. A process for preparation of a three-dimensional green body, by a fused filament fabrication process, comprising at least the steps a), b), c), a) providing the filament according to any of claims 1 to 7 on a spool to a noozle, b) heating the filament to a temperature (T M), c) depositing of the heated filament obtained in step b) in a build chamber using a layer based additive technique in order to form the three dimensional green-body.
10. The process according to claim 9, wherein the temperature (T M) in step b) is 140 to 240°C.
11. The process according to claim 9 or 10, wherein step c) is followed by a step d) in which at least a part of the binder (B) and/or at least a part of the shell material (SM) is removed from the three-dimensional green body in order to form a three- dimensional brown body.
12. The process according to claim 11, wherein in step d) i) the binder (B) and/or the shell material (SM) is removed by acidic treatment and/or ii) the binder (B) and/or the shell material (SM) is removed at a temperature below the melting point of the binder (B) and/or the shell material (SM).
13. The process according to any of claims 11 or 12, wherein step d) is followed by a step e), in which the three-dimensional brown body is sintered to form a three- dimensional sintered body.
14. A three-dimensional green-body, prepared by the process according to claim 9 or 10.