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.

Synthèse bibliographique : micro-texturation et microinjection de thermoplastiques

“…outil soit ex situ à partir d'équipements spécifiques de laboratoire [75], soit in situ dans le moule en équipant un éjecteur d'un capteur de force [76]. Peter Jones [77], dans un procédé d'injection classique, propose de prendre en compte le coefficient de friction, les propriétés mécaniques du polymère, le retrait et les dimensions de la pièce pour calculer la force nécessaire à l'éjection de la pièce, F é , telle que :…”

A Structured Review and Classification of Demolding Issues and Proven Solutions

“…The absence of a unique selection of parameter levels to optimize demolding behaviour for the surface treatment and polymers investigated was highlighted. Charmeau et al (2008) investigated the effect of coatings on the mold surface in order to reduce the occurrence of weldlines in polymer parts and concluded that the contact thermal resistance associated with the coating was the dominant parameter. Ejection forces were evaluated and the impact was then studied through ejection work and friction coefficients.…”

Influence of Injection and Cavity Pressure on the Demoulding Force in Micro-Injection Moulding

“…A study of F* in regards to applied mould surface treatments showed that the efficiency of the coatings depended on the polymer used, its shrinkage nature, and the roughness of the coated surface [24]. Griffiths et al reported defects on PC moulded parts (Fig.…”