Measurement of specific volume of polymers under simulated injection molding processes

Abstract The pressure-specific volume-temperature (pvT) diagram is well established in science and industry for plastics injection molding purposes. However, the exact specific volume during the process cannot be measured and the help of a pvT model is necessary. The pvT model is based on the data measured under standard conditions and might not reflect the specific volume at process level. In order to know the exact variation of the specific volume, a pvT testing device was used to simulate the injection molding process. Four parameters (starting temperature, holding pressure, holding-end temperature and cooling rate) corresponding to injection temperature, holding pressure, holding time and cooling time were considered. The semi-crystalline polypropylene (PP) and the amorphous acrylonitrile-butadiene-styrene (ABS) were employed. The design of experiments (DoE) method was applied to analyze the influences of these parameters on the changes of the specific volume during the simulated injection molding cycles. The results demonstrated that the specific volume especially at the phase transition and shrinkage stages is significantly influenced by the complicated injection molding process. The starting temperature has a great effect on the specific volume but has little effect on the shrinkage. The specific volume decreases non-linearly with decreasing cooling rate during the shrinkage stage.