Evaluation of ultra-precision milling strategies for micro lens array mould inserts for the replication by injection-compression moulding

摘要

Micro lens arrays (MLA) are broadly used in a multitude of optical applications, whereas increasingly extended areas need to be structured. Furthermore, growing quantities of microoptical components require micro- and nano-replication techniques, such as injection moulding and injection compression moulding. Therefore, ultra-precision milling strategies for manufacturing of mould inserts have been evaluated regarding applicability, surface quality and processing time. Different milling strategies were investigated regarding processing time and resulting surface quality. Single flute diamond milling tools were used to mill the mould cavities in nickel-phosphorus (NiP) coating deposited on tooling steel. The experiments were conducted on a 5-axis ultra-precision milling machine and qualitative results regarding observed effects of the different strategies were evaluated. The achieved surface qualities were analysed and quantified using white light interferometry. Methods of ring, radial or spiral milling strategies showed different failure modes and shortcomings such as stepping, pin residues or long processing times. Using a method, where a specifically designed milling tool was immersed in a single step into the bulk material, best results were achieved and the processing time for each lens was reduced to a minimum. The quality of the MLA could be further increased by immersing the milling tool in a defined angle. It was observed, that the centering and radius of the diamond of the milling tool was a major factor. Conclusively, the results show that high quality tooling in combination with the applied milling strategy enables significant improvements in quality and costs.