WE CLAIM:
1. A method for manufacturing a 3D item (1) by means of 3D printing,
wherein the method comprises the step of:
depositing, during a printing stage, a 3D printable material (201) to provide 3D printed material (202), wherein the 3D printable material (201) comprises a core-shell filament (320), the core-shell filament (320) comprising a core (321) and a shell (322), the core (321) comprising a core material (1321) having one or more of a core glass temperature Tgl and a core melting temperature Tml, and the shell (322) comprising a shell material (1322) having one or more of a shell glass temperature Tg2 and a shell melting temperature Tm2, wherein one or more of the shell glass temperature Tg2 and the shell melting temperature Tm2 is lower than one or more of the core glass temperature Tgl and the core melting temperature Tml, and
wherein the method further comprises the step of: heating, during a finishing stage, the 3D printed material (202) to a temperature equal to or higher than one or more of the shell glass temperature Tg2 and the shell melting temperature Tm2, and equal to or lower than one or more of the core glass temperature Tgl and the core melting temperature Tml.
2. The method according to claim 1, wherein one or more of (a) the core glass temperature Tgl and shell glass temperature Tg2, (b) the core melting temperature Tml and the shell melting temperature Tm2, (c) the core melting temperature Tml and the shell glass temperature Tg2, and (d) the core glass temperature Tgl and the shell melting temperature Tm2, differ with at least 40 °C.
3. The method according to any one of the preceding claims, wherein the core material (1321) comprises one or more materials selected from the group consisting of polycarbonate, polysulfone, polyphenyleen sulfide, high Tg polycarbonate, polybutylene terephthalate, crystalline polyethylene terephthalate, polyethylene naphthalate, polyether ether ketone, and wherein the shell material (1322)

comprises one or more materials selected from the group consisting of amorphous polyethylene terephthalate, acrylonitrile butadiene styrene, poly(methyl methacrylate), polystyrene, styrene methylmethacrylate, methylmethacrylate acrylonitrile butadiene styrene, styrenic block copolymer, and thermoplastic polyurethane.
4. The method according to any one of the preceding claims wherein the core-shell filament (320) has a core diameter (dl) selected from the range of 100 - 3000 um, wherein the shell thickness (d2) is selected from the range of 100 - 2000 um, and wherein the core diameter (dl) is larger than the shell thickness (d2).
5. The method according to any one of the preceding claims, wherein the heating step comprises one or more of (i) providing a hot gas to the 3D printed material (202), (ii) providing IR radiation to the 3D printed material (202), and (iii) heating the 3D printed material (202) in a heating chamber.
6. The method according to any one of the preceding claims, wherein the 3D printed material (202) forms a surface (205) of the 3D item (1), and wherein during the finishing stage the 3D printed material (202) is heated until at least a part of the surface (205) has a predetermined average surface roughness (Ra) equal to or lower than 5 um for an area of at least 25 mm2.
7. The method according to any one of the preceding claims, wherein during the printing stage pressure is applied to the core-shell filament (320) to provide a deposited core-shell filament having a deformed core (321) with a first dimension (hi) and a second dimension (wl) perpendicular to each other and perpendicular to a longitudinal axis (A) of the core-shell filament (320) that have a ratio (hl/wl) of less than 1.
8. A 3D item (1) comprising a surface (205) formed from a 3D printed material (202), the 3D printed material (202) comprising a plurality of core-shell

filaments (320), each core-shell filament (320) comprising a core (321) and a shell (322), the core (321) comprising a core material (1321) having one or more of a core glass temperature Tgl and a core melting temperature Tml, and the shell (322) comprising a shell material (1322) having one or more of a shell glass temperature Tg2 and a shell melting temperature Tm2,
wherein one or more of the shell glass temperature Tg2 and the shell melting temperature Tm2 is lower than one or more of the core glass temperature Tgl and the core melting temperature Tml, and
wherein at least part of the surface (205) has an average surface roughness (Ra) equal to or lower than 5 um for an area of at least 25 mm2.
9. The 3D item (1) according to claim 8, wherein the core material (1321) comprises one or more materials selected from the group consisting of high Tg polycarbonate (PC), polysulfone, and polyether ether ketone, and wherein the shell material (1322) comprises one or more materials selected from the group consisting of amorphous polyethylene terephthalate and acrylonitrile butadiene styrene.
10. The 3D item (1) according to any one of the preceding claims 8-9, wherein one or more of the core-shell filaments (320) have a deformed core (321) with a first dimension (hi) and a second dimension (wl) perpendicular to each other and perpendicular to a longitudinal axis (A) of the core-shell filament (320) that have a ratio (hl/wl) of less than 1, and wherein adjacent cores have a core-core distance (d3) selected from the range of at maximum 100 um.
11. A lighting device (1000) comprising the 3D item (1) according to any one of the preceding claims 8-10, wherein the 3D item (1) is configured as one or more of at least part of a lighting device housing, a wall of a lighting chamber, and an optical element.
12. A 3D printer (500) for providing a 3D printable material (201) to a substrate (1550), the 3D printable material (201) comprising a core-shell filament (320), the

core-shell filament (320) comprising a core (321) and a shell (322), the core (321) comprising a core material (1321) having one or more of a core glass temperature Tgl and a core melting temperature Tml, and the shell (322) comprising a shell material (1322) having one or more of a shell glass temperature Tg2 and a shell melting temperature Tm2, wherein one or more of the shell glass temperature Tg2 and the shell melting temperature Tm2 is lower than one or more of the core glass temperature Tgl and the core melting temperature Tml,
wherein the 3D printer comprises:
a printer head (501) comprising a printer nozzle (502),
a 3D printable material providing device (575) configured to provide the 3D printable material (201) to the printer head (501),
a heating system (700), and
a control system (C), and
wherein the control system (C) is configured to execute the method according to any one of claims 1-8.
13. A computer program product comprising instructions which, when the computer program product is executed by the 3D printer according to claim 12, cause the 3D printer to carry out the method according to any one of claims 1-7.