| CPC B29C 64/336 (2017.08) [B29C 64/118 (2017.08); B33Y 10/00 (2014.12); B33Y 70/10 (2020.01); B33Y 80/00 (2014.12); F21V 7/24 (2018.02); B29L 2031/747 (2013.01)] | 7 Claims |

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1. A method for producing a 3D item by means of fused deposition modelling, the method comprising a 3D printing stage, wherein the 3D printing stage comprises layer-wise depositing 3D printable material to provide the 3D item comprising 3D printed material on a receiver item, wherein:
during at least part of the 3D printing stage the 3D printable material comprises core material and 3D printable shell material; wherein the 3D item comprises a core-shell layer of the 3D printed material, wherein the core-shell layer comprises (i) a core comprising core material and (ii) a shell comprising 3D printed shell material; wherein the shell at least partly encloses the core;
the core material is one or more of (i) reflective, (ii) transmissive, (iii) absorbing, and (iv) luminescent for light having a wavelength in the visible wavelength range;
the 3D printable shell material is transmissive for the wavelength in the visible wavelength range;
the core-shell layer has a layer width (W) and a layer height (H) and a first aspect ratio AR1, wherein AR1=W/H, wherein AR1>1;
a cross-sectional plane of the core-shell layer, wherein the cross-sectional plane comprises the layer width and the layer height (H), and the core provide a core cross-section, wherein the core cross-section has a first dimension (A1), which is the longest dimension of the core cross-section, and a second dimension (A2), perpendicular to the first dimension, providing a second aspect ratio AR2, wherein AR2=A1/A2, wherein 2≤AR2≤10,000; and wherein AR2/AR1≥1.5,
wherein the core comprises two or more strip-like elements, wherein each of the two or more strip-like elements has an element length individually selected from the range of at least the layer width, wherein LE≥3*A1, and wherein the spacing distance between two neighboring strip-like elements S≤10*LE.
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