Influence of process management on the increase of dynamic solubility limits in injection foam molding

The foam injection molding process provides a multitude of advantages both during production and throughout the life cycle of foamed components. Yet still there are many uncertainties concerning the conditions inside the cylinder during gas injection. For a better understanding of these processes, the solubility of gases in polymers is crucial. In current literature, almost exclusively static solubility is investigated. It is questionable whether results from static solubility measurements are relevant for highly dynamic processes like injection molding. Additionally, it is unknown whether the limits detected in static measurements also count for dynamic systems. In this work, a novel measurement method based on the bulk modulus of gas containing polymer melts, which provides in-line determination of dynamic solubility limits, was applied. Using a talc filled polypropylene in a MuCell®-process the question was whether the solvable amount of gas can be influenced and increased by a larger gas injection time to dosing time ratio (Rt=ti/td). Starting at a gas concentration that was initially beyond the solubility limit the ratio Rt was increased stepwise, while process pressure and gas content were kept constant. The bulk modulus method in combination with transmissive ultrasonic measurements proves that the dynamic solubility can be increased above 1.6 wt% nitrogen at 140 bar. In addition, compression work and injection work were monitored and compared to the respective states of solution. These findings show that the influence of material on solubility limits might only be relevant at long dosing and gas injection times as an equilibrium cannot develop in short times typical for injection molding. This approach is useful when large amounts of gas should be dissolved in the melt in order to achieve high density reductions. It opens new perspectives for injection foam molding.