Development of measurement method for determination of dynamic solubility limits in injection foam molding

There is still uncertainty concerning processes and conditions inside the cylinder during gas injection in injection foam molding. Apart from static solubility and its measurement, determination of dynamic solubility is of great interest for the operator in order not to overload the melt. Our development of a novel measurement method makes the detection of the ultimate limit of solvable amount of gas at certain process conditions prior to production possible, using the compression modulus of the gas-polymer-solution in a MuCell®-process. The method is based on the fact that the compressibility of a polymer-gas-solution beyond its solubility limit (i.e. under presence of a second, discrete gas phase) drops sharply. Based on three different versions of a polypropylene homopolymer dynamic solubility measurements at different pressures were carried out. Depending on material formulation and process conditions dynamic solubility limits between 0.6 wt% and 3.6 wt% nitrogen were measured. For verification of the obtained results, ultrasonic measurements were conducted simultaneously. Excellent agreement between the two methods was observed. For the whole test procedure, a measurement flange was used. However, it could be shown that the compression modulus can also be determined without any additional equipment. This simple yet innovative method yields new insights in terms of process limits and conditions inside the barrel. Future developments should include the fully-automatic self-adjustment of the machine.