Analysis of Sink Marks for Plastic Parts Molded in Steel and Aluminum Alloy Molds

Recently developed aluminum alloys show significant potential as injection mold materials for their ability to cool plastic parts faster than steel. These alloys maintain more uniform mold temperatures that can have significant effects in reducing post-molding shrinkage. Commercially available software can be used to predict the global shrinkage in a part. However, none of the currently available software predicts localized sink mark formation. In the present work, temperature and pressure histories from a three-dimensional molding analysis using C-Mold™ are used to determine the initial conditions for a sequentially coupled thermal and structural finite element analysis using ANSYS™. The thermal conductivity, density and specific heat of the polymer are input as temperature dependent properties. The polymer is modeled as a temperature dependent elastic material. Correlations made between numerical and experimental data for sink mark depths in parts molded in P-20 steel and QE-7™ aluminum alloy molds validate the use of the sink mark simulation method. Numerical comparison of sink mark depths for parts molded in aluminum alloy and steel molds show that aluminum alloys reduce sink mark depths in molded parts.