INFLUENCE OF MOLD SURFACE COATINGS IN INJECTION MOLDING. APPLICATION TO THE EJECTION STAGE.

Abstract

Mold surface coatings are used in thermoplastics injection molding to increase lifespan of the mold before maintenance. Development of coating processes such as PVD (Phase Vapor Deposit) and PACVD (Plasma Assisted Chemical Vapor Deposit) allowing thin coatings manufacturing gives access to new application fields. Investigated coatings were Chromium nitrium (CrN), Titane nitrium (TiN), Diamond like Carbon (DLC), glassy deposit (SiOx), Chromium and polished steel. This work intended to study the impact of those coatings on ejection stage in terms of unsticking the part from the mold surface and generation of scratches. We studied coatings nature and their processings which influence their roughness. Injection campaign was led on an cube-shaped insert in an instrumented mold (with force sensors) on three polymers which differ in nature: an amorphous polymer (polycarbonate), a semi-crystalline one (polybutylene terephatalate) and a blend of copolymers (styrene acrylonitrile/ acrylonitrile butadiene styrene). We studied the evolution of these forces throughout the demolding stage. This allowed us to evaluate the work energy necessary to eject the part from the insert. We correlated those data to shrinkage of the polymer part, adhesion between polymer and mold surface and friction coefficient between those surfaces during the demolding stage. Surface energies of the polymers as well as those of the coatings were measured, and their evolutions with temperature were used to take into account this adhesion. Study of demolding forces showed a role of the coatings depending on the polymer and its nature, and roughness of the coating. We noted that ejection consisted of two stages: unsticking of the part and dynamic friction. Amorphous polymers are mainly affected by the first step, related to the adhesion at polymer/mold interface. PBT, due to a higher shrinkage, is very sensitive to dynamic friction.