Fast temperature evolution on the mold surface: Analysis and simulation

Abstract

Injection molding is one of the most widespread processes in the polymeric manufacturing field, because it allows to have a good reproducibility of the molded objects in a very short cycle time. The possibility to control mold temperature during the process, so as to have high temperature during filling and low temperature during cooling, is considered very attractive for several reasons.To this purpose, fast mold surface heating is proposed in this paper. A mold temperature rise of about 100°C in 1-2 seconds was obtained by a thin electric heater layered on the mold surface. A fast cooling phase and, as a consequence, a fast cycle time is not lost because the heating layer thickness is small.Injection molding tests were performed with a very accurately characterized iPP as far as rheology, quiescent crystallization, effect of flow on nucleation and spherulitic growth rates and spherulitic/fibrillar transition.A series of tests with different heating powers (steady mold wall temperatures) held constant from about 2s before the contact of the polymer on the cavity surface to the end of filling are considered in this work. The effect of the heating powers on the skin, the shear layer and spherulitic layer thicknesses is identified and discussed.A simple model of the morphology evolution during the process, accounting of the effect of molecular stretch on nucleation density and growth rate of the isotactic polypropylene (iPP) adopted for the experiments, was adopted to describe the experiments The model predictions for final morphologies distributions reproduce main characteristics of the experimental results.