Study of ejection forces in injection moulding of thin-walled tubular mouldings

Abstract

The development and manufacturing of ejection systems in injection moulds can be a complex task, particularly when the parts have long ejection paths. During the injection of tubular- or box shape parts, the shrinkage is constrained by the mould. The mouldings shrink against the core, inserts or pins, thus, during ejection, it will be necessary to overcome the frictional forces resulting from the shrinkage. Understanding the ejection forces could be a useful contribution to improve ejection systems designs and guarantee the structural integrity of the mouldings. This information could be even more useful in the processing of thin-walled parts which are more prone to damage during ejection. Frictional forces could be decreased by using specific coatings in the mould core and by optimization of the processing parameters. This study describes the use of DLC and WS2 coatings and the effect of mould temperature in the ejection forces of tubular mouldings in poly(lactic acid) (PLA) and polystyrene (PS). The studies were based on tubular cup shaped mouldings (60 mm diameter, 60 mm length, 0.5 mm thickness and 0.35 º draft angle).