INJECTION MOLDING OF MICRO-FEATURES

A method for creating regular arrays of surface features in the 10µm to 200µm range over large areas in metallic materials enables the use of treated surfaces on injection molds. This surface control is used for manufacturing products with controllable optical, tribological, heat transfer, and surface tension properties. However, the flow, filling, and solidification behavior of molten polymer at the micro scale is not well understood. Therefore, the purpose of this study is to understand the formation of micro-structure on a polymer surface and derive a range of robust solutions for injection molding of these micro-featured surfaces. This study uses critical inputs of the molding process, mold geometry, and working materials, and determines a range of feasible solutions to recreate these micro structures without filling or de-molding defects. Five different molding parameters hypothesized as most influential in molding microstructures were varied and the resultant micro-features characterized using 3D surface profiling white light interferometry and scanning electron microscopy. Regression responses were determined and optimal value ranges for the key input parameters were identified and independent tests run to verify the results. The five parameters tested are: injection pressure, injection velocity, mold temperature, melt temperature and shape of micro structures. These tests were carried out in both HDPE and EPDM/PP alloy materials using different mold insert materials and coatings. The tests first conducted on a flat surface were repeated on a tube surface to identify if change in geometry of component changes the formation of micro-structures. The results obtained from these tests gave us a clear indication of how each of the above parameters controls the formation of molded microfeatures, and how parameters interact. In this case mold temperature was found to be the most critical factor, with a sensitivity of only a few degrees greatly improving the micro feature formation. The biggest concern was uniformity of micro features across the molded surface. Based on this understanding we were also able to simulate macroscopic mold filling conditions and verify homogeneous filling across the molded surface. These experimental results are used to drive a predictive tool for process planning of complete undamaged feature formation at the micro scale and uniformity at the macro scale. It was also found that formation of structures at micro level is different from formation of structures at macro level