Method and device for in situ marking a workpiece in a thermal forming process

A method for in-situ marking a workpiece in a thermal forming process of a polymer material, using a device comprising a surface directed towards the workpiece, wherein a number of heating elements is distributed behind the surface for a local heating of a workpiece surface portion, wherein each of the heating elements comprises a solid material having a surface structure and a heating structure, wherein the device further comprises a controller connected to the individually controllable heating elements, wherein the method comprises the step to energize predetermined heating elements through control signals from the controller for creating a predetermined marking through altering the surface portion of the workpiece comprising an altering taken from the group encompassing darkening, burning, foaming of the surface portion of the workpiece associated to said predetermined heating elements or creation of altered, darker or lighter molecules in the surface portion of the workpiece compared to adjacent surface portions of the workpiece.