Analysis of Shrinkage Development of a Semicrystalline Polymer during Injection Molding

The phenomenon of shrinkage in injection molding is particularly relevant for the processing of semicrystalline polymers. Nevertheless, if compared with the considerable effort spent by researchers to investigate the evolution of shrinkage and thermal stresses in amorphous polymers, the research work devoted to shrinkage of semicrystalline polymers is rather limited in the literature. In this work, the influence of holding pressure and time, and geometric constraints, on the shrinkage of a semicrystalline polymer was explored. Adopting a technique based oil strain gauges, the time at which shrinkage started inside the mold was measured as a function of the holding pressure. Experimental results were compared with predictions for shrinkage obtained by a code developed at the University of Salerno, which takes into account crystallization kinetics and the effect of crystallinity on material properties. In particular, a solidification criterion based on the degree of crystallinity was identified.