The invention provides a method for producing a 3D item by means of fused deposition modelling, the method comprising a 3D printing stage comprising layer-wise depositing an extrudate comprising 3D printable material, to provide the 3D item comprising 3D printed material, wherein the 3D item comprises layers of 3D printed material, wherein the method further comprises controlling a first temperature T.sub.1 of the 3D printable material within a first temperature range, wherein the 3D printable material comprises a thermoplastic host material and a dopant material in the range of 1-20 vol. %, the dopant material comprising polymeric flake-like particles having a metal coating, wherein the 3D printable material has an optical property that irreversibly changes from a low-temperature optical property to a high-temperature optical property when increasing a temperature of the 3D printable material over a change temperature T.sub.c, the optical property being selected from the group consisting of reflection, transmission, luminescence, absorption, and color, wherein the change temperature T.sub.c is within the first temperature range, wherein during at least a first part of the 3D printing stage the first temperature T.sub.1 is below the change temperature T.sub.c, and wherein during at least a second part of the 3D printing stage the first temperature T.sub.1 is above the change temperature T.sub.c.