As-molded shrinkage on industrial polypropylene injection molded parts: experiments and analysis

The phenomenon of shrinkage in injection molding is particularly relevant in semi-crystalline polymers. Despite of this, if compared with the considerable effort spent by researchers to investigate (both experimentally and theoretically) the evolution of shrinkage and thermal stresses in amorphous polymers, the relevant research work is scarce for semicrystalline polymers. This is probably due to the fact that the phenomenon is strictly related to the evolution of crystallinity, which is per se quite difficult to be predicted. In fact, modern approaches to the phenomenon of dimensional accuracy in injection molding link the evolution of shrinkage from the moment of first solidification to a force balance between restraining and constraining forces which sets in inside the mold before ejection. Such as approach needs a complete understanding of what happens inside the cavity during the molding cycle. In this work, with reference to a simple rectangular cavity, the complete evolution of shrinkage from the instant of first solidification inside the mold to some minutes after demolding was followed for a typical multiphase industrial polymer, i.e., an industrial polypropylene-ethylene-propylene rubber (iPP-EPR) copolymer with a small percentage of talc. The influence of holding pressure and time, and geometric constraints on linear shrinkage was explored. Experimental results were compared with predictions for shrinkage obtained by a code developed at the University of Salerno, which takes into account crystallization kinetics.