A two-stage collapsible core for injection moulded plastic parts with internal undercuts

The internal grooved (undercut) features of plastic injection moulded parts are formed by collapsible cores. While collapsible cores have been designed for pipe sizes smaller than 90 mm in diameter, those for larger pipes are custom designed. The objective of this research was to fully automate the design procedure which, for uPVC pipe fittings is laborious and error-prone at present. This paper discusses the design of the four key geometrical features of a two-stage, lever-actuated collapsible core, namely, the number of segments, the path of the retracting segments during demoulding, the minimum angle of taper of the grooved undercut to permit the segments to retract during de-moulding and the prevention of interference between moving components. After these key geometrical features are established, it is then possible to fully automate the design of two-stage, lever-actuated collapsible cores in a CAD system.