Prediction and validation of the specific volume for inline warpage control in injection molding

The material-inherent specific volume behavior of polymers is highly important for producing dimensionally accurate injection molded parts since the shrinkage and the shrinkage-induced warpage are attributable to the specific volume behavior. Thereby, the prevalent process parameters melt temperature and cavity pressure significantly influence the prevailing specific volume. Many empirical models describe this pressure-specific volume-temperature (pvT) behavior. To minimize part warpage, a segmented dynamic mold temperature control has been developed which aims to homogenize the specific volume of a platen-shaped geometry. The mold temperature control is thereby controlled by a model predictive control concept, which predicts the local specific volume using an empirical pvT-model. A modified Tait-pvT-model is used for the prediction, which considers the effect of the cooling rate on the specific volume. To achieve homogenization of the local specific volume, the pvT-model is validated regarding its prediction accuracy by comparing the predicted specific volumes with specific volume measurements of molded test specimens using processing points with varying cooling rates. For the predictions of the final specific volumes, two approaches regarding the selection of the cooling rate were used. The results show that the selection of the cooling rate within the modified Tait model has a significant impact on the calculated final specific volume.