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 AR 1 , wherein ARi=W/H, wherein ARi>1; - the core-shell layer has a cross-sectional plane , wherein the cross-sectional plane comprises the layer width (W) and the layer height (H), and the core has a core cross-section , wherein the core cross-section has a first dimension (A 1 ), which is the longest dimension of the core cross-section , and a second dimension (A 2 ), perpendicular to the first dimension (A 1 ), providing a second aspect ratio AR 2 , wherein AR 2 =A 1 /A 2 , wherein 2≤AR 2 ≤10,000; and wherein AR 2 /AR 1 ≥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 (L E ) individually selected from the range of at least the layer width (W), wherein L E ≥3*A 1 , and wherein the spacing distance (S) between two neighboring strip-like elements S≤10*L E . - 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 AR 1 , wherein ARi=W/H, wherein ARi>1; - the core-shell layer has a cross-sectional plane , wherein the cross-sectional plane comprises the layer width (W) and the layer height (H), and the core has a core cross-section , wherein the core cross-section has a first dimension (A 1 ), which is the longest dimension of the core cross-section , and a second dimension (A 2 ), perpendicular to the first dimension (A 1 ), providing a second aspect ratio AR 2 , wherein AR 2 =A 1 /A 2 , wherein 2≤AR 2 ≤10,000; and wherein AR 2 /AR 1 ≥1.5, <EOS>