Influence of Pressure on Volume, Temperature and Crystallization of Thermoplastics during Polymer Processing

The main shrinkage and dimensional stability impact factors are processing temperature and pressure. For that reason, the pvT-behaviour is often used to estimate plastic part shrinkage. As there are still differences between predicted and actual part dimensions, a closer look was taken at these influencing factors. During processing, temperature changes occur due to heat conduction to the mold walls, and the pressure varies in the filling stage and holding time along the flow path. However, these pressure gradients are typically not taken into account in plastics processing. Hence, the influence of pressure was examined in detail and further investigations on pvT-behavior of polymers were performed. For that purpose, fundamental examinations of the behavior of amorphous thermoplastics during cooling and compression were made. These include analysis in the different phases and at different compression speeds as well as variation of the pressure-temperature-cycles and their succession. It was found that the specific volume is not defined by one value of p and T, but is dependent on the pathway of the process. That applies for both thermal expansion and compressibility. The adiabatic compression heating was also examined! and a numeric solution was found to easily adapt the pvT-results on the base of compressibility measurements. These findings are discussed by means of the free volume theory. Additionally, a choice of these investigations was performed with semi-crystalline thermoplastics. It was found, that compression heating is even more important for these polymers because it can superpose crystallization heating at appropriate parameters. Furthermore, it can be seen that the density, morphology and lamellae thickness is influenced by pressure. In the outlook of this paper, the impact of these results on the injection molding process is discussed in detail.