A design tool for core-back timing in foam injection molding

A visual observation experiment mimicking the pressure history in foam injection molding was utilized to infer the bubbles formation in core-back operations. The different stages of bubble nucleation, dissolution, and re-nucleation, corresponding to injection, packing, and core-back opening, respectively, were studied. A reduction of the number density of re-nucleated bubbles with respect to first nucleation was observed. In order to understand this observation, the evolution of the blowing agent concentration around the dissolved bubbles was analyzed. A characteristic time was introduced, required to achieve blowing agent uniformity or, equivalently, required to cancel the mass transport-mediated memory of the system. A numerical model, assuming Fickian behavior, was derived and validated with the experimental results. The study allowed for the development of a predictive tool to design core-back operations in foam injection molding.