The effect of temperature and surface roughness on coefficient of friction between thermoplastics and molding surfaces

Abstract

Key issues in the design of injection moulds are the design of the impression and the gating system. Other functional systems require particular attention in precision moulds, namely cooling, guidance and ejection. In the ejection phase of injection molding process the molding parts are mechanically forced to separate from the molding surfaces, this aspect being more relevant when deep cores are involved. The performance of the ejection system depends on factors that must be considered in the design stage of the tool, namely, the draft angles, the surface finish, and the properties of the molding material at the ejection temperature and the dimensioning of actuation devices (e.g., hydraulic or pneumatic cylinders). Furthermore, the knowledge of the friction properties of the mating pair: mould surface (typically steel) and part (plastics), is important to optimize the ejection system solution. The coefficient of friction between plastic/steel at the ejection stage depends on variables, namely the surface texture of the core and the temperature at ejection. This paper assesses the results of static coefficient of friction of three thermoplastics. The results were obtained with a prototype apparatus that reproduces the conditions occurring during the ejection phase. Focus is made on the effect of the roughness and the temperature at ejection.