Tuning the properties of injection molding polypropylene parts by in-mold annealing

Abstract

A way to improve sustainability of plastic products is to obtain a mono-material part, tuning the final properties by the transformation process. In the injection molding process, mold temperature cycle is recognized as a key parameter in determining the final characteristics of the part. In this work, injection molding tests coupled with in-mold annealing were conducted on an isotactic polypropylene. The polymer is injected in the cavity at high temperature (140°C and 160°C) and this temperature is kept for 5 minutes (annealing step); afterwards, the mold temperature is cooled down at room temperature. The samples are characterized by optical and electronic microscopy; furthermore, atomic force microscopy coupled with HarmoniX tool allows to characterize simultaneously the local morphology and the elastic modulus. Final morphology distribution along the sample thickness is significantly influenced by the adopted in-mold annealing conditions, in particular, an oriented zone forms close to the cavity surface, a spherulitic part in the core, and a transition zone in between. The extension of the oriented layer decreases in the presence of annealing. On its turn, morphology greatly affects mechanical properties: the oriented layer shows the highest values of elastic modulus, the transition zone shows the minimum value of elastic modulus; the elastic modulus of the spherulitic layers depends on spherulite dimensions. X-ray WAXS analyses were performed on the surface of the sample along the flow path to investigate the orientation and the crystalline phases that form during the process and to relate them with morphology.