Theoretical Background and Automation Approach for a Novel Measurement Method for Determining Dynamic Solubility Limits of Supercritical Fluids in Injection Foam Molding

In terms of sustainability, injection foam molding gains in importance. Yet still many questions throughout the whole process remain unanswered. Especially, the conditions inside the cylinder during gas injection are fraught with uncertainty. Our development of a new methodology for the determination of dynamic solubility in injection foam molding helps to answer the most basic question regarding gas loading: Using the bulk modulus of the polymer–gas mixture, the dynamic solubility limit can be detected. In a first series of tests, the methodology was verified with simultaneous ultrasonic measurements—an excellent agreement was observed. In this work, a second, more thorough test series is presented. Using the bulk modulus methodology on different polypropylene grades, dynamic solubility limits between 0.62 and 2.56 wt% nitrogen at pressures between 80 and 200 bar and a temperature of 230°C were determined directly on an injection molding machine. The detailed theoretical background for this technique, as well as a mathematical automation approach, is provided. This innovative yet simple method yields novel insights on process limits and provides the possibility of an a priori machine setup or a fully automatic self‐adjustment of the machine.