Mold machining and injection molding of diffractive microstructures

Diffractive microstructures are used for many applications due to their unique optical functionalities, e.g. as security features on banknotes or documents. By developing more complex microstructures which almost cannot be copied, the protection against counterfeiting can be improved. This paper introduces a modified diamond turning process to machine such kind of functional microstructures. A fast-tool-servo assisted diamond turning process is presented, which enables machining of holograms consisting of diffractive microstructures with an overlaying pattern. The resulting structure is capable of shaping incident light into a defined intensity distribution. Such holograms could be used as security tags or be embossed in plastic packaging of valuable products. In order to make this technology accessible to mass production, replication of the microstructures by injection molding is essential. To investigate the replicability by injection molding, a blazed structure was diamond turned into a mold, identical to the basic structure of the holograms. Three optical polymers (polymethylmethacrylate PMMA, cyclic olefin copolymer COC, cyclic olefin polymer COP) were used for injection molding experiments to investigate the filling behavior of the mold and the replicated quality of the diffractive microstructure.