A study of localized shrinkage in injection molding with high thermal conductivity molds

Recently developed aluminum alloys show significant potential as injection mold materials to cool plastic parts faster than steel. These alloys maintain more uniform mold temperatures, which will reduce the differential post-molding shrinkage. Although commercially available software can be used to estimate the global shrinkage in a part, none of the currently available software predicts localized shrinkage values. In the present work, temperature and pressure histories are generated from a three-dimensional injection molding analysis using flow analysis software. These histories are then used to determine the initial conditions for a sequentially coupled thermal and structural finite element analysis (FEA). A significant difference between the approach presented in this paper and previous approaches is that the mold geometry is explicitly modeled in the FEA to take into account mold material dependent heat transfer characteristics which is critical in studying sink mark formation. Temperature-dependent thermal and structural properties of the polymer (Phillips Marlex HGL 120-01 Polypropylene) are used in the FEA. The polypropylene is modeled as a perfectly elastic material. Two custom molds made with P-20 steel and QE-7 aluminum alloy were fabricated to compare the numerical results to experimental data. Correlations made between numerical and experimental data for six parts selected at random from 20 parts from each mold validated the sink mark simulation method. Numerical studies of sink mark depths for 24 test cases at a constant packing pressure of 10.34 MPa and a constant coolant temperature of 8°C indicate that aluminum alloy molds reduce sink mark depths by about 15% when compared to P-20 steel molds.