Prediction of dimensional changes and residual stresses in injection molded plastic parts

A new model of solidification of liquid amorphous polymers between cooled parallel plates predicts the measured in-plane and through-thickness shrinkage of injection-molded plaques. The model uses a reference strain field to account for the melt that is forced or removed from the cavity during the packing phase to fill the space created by material contraction during solidification. The model can also account for material freeze-off effects in which the cavity pressure is controlled by the solidification process and the through-thickness shrinkage is significantly bigger than the in-plane shrinkage. The model predicts that packing pressure significantly effects part shrinkage, warpage and distribution of residual stresses. This reference-strain approach to modeling solidification can also be used to predict local surface defects such as sink marks and global dimensions of injection-molded plastic parts. By accounting for sprue freeze-off, and by using the measured cavity pressure as a time-dependent boundary condition, the model correctly predicted the measured plaque shrinkages in a controlled test