Study of the Influence of Surface Mold Deposits on the Demolding Stage of the Injection Molding Process of Thermoplastics

Abstract

Due to increasing expectings from the market, the aspect of molded parts has to be improved. Some of the defects observed such as scratches on these parts is related to the demolding stage. To limit this, we investigated the influence on demolding forces using various surface deposits on the mold surface, mainly PVD and PACVD deposits : Chromium nitrium (CrN), Titane nitrium (TiN), Diamond like Carbon (DLC), glassy deposit (SiOx), Chromium and polished steel on an cube-shaped insert in an instrumented mold (with force sensors). Injection campaign was led on three polymers which differ in terms of nature : an amorphous polymer (polycarbonate), a semi-crystalline one (polybutylene terephatalate) and one mix of copolymers (styrene acrylonitrile/ acrylonitrile butadiene styrene). We studied the evolution of these forces through the demolding stage. This allowed us to evaluate the work energy necessary to eject the part from the insert, and to correlate those data to shrinkage of the polymer part, adhesion between polymer and mold surface and friction coefficient between those surfaces during the demolding stage. We also measured the influence the surface temperature of the part just before the demolding stage thanks to an infrared camera to investigate the thermal influence of these deposits in the injection process. Our results show an influence of deposits on demolding forces which is strongly dependent on nature of the polymer (of course) but also on its chemical nature. They also have a slight influence on temperature of the part even if they are only a few microns thick. We therefore developped a method to evaluate surface deposits and their impact on demolding forces, in terms of adhesion polymer/treament and friction.