The development of a mathematical model and computer program for simulating the injection moulding of thermosetting elastomer materials

A mathematical model for the simulation of the injection moulding of thermosetting elastomers has been developed. The model uses suitably reduced forms of the fundamental equations of continuity, momentum and energy as a basis, with a constitutive equation to describe how the elastomer viscosity varies with local flow conditions. A cure model is used to calculate cure levels during the injection phase, and the time taken for the final moulded component to reach a specified minimum cure level during the subsequent cure cycle. Moulds are defined by splitting the various elastomer flowpaths into a network of end to end connected geometric entities of simple cross section, for instance circular, rectangular and annular. The moulds elements are discretised using a finite difference mesh and the equations which comprise the model are cast into a suitable finite difference form for solution. Solution of the continuity and momentum equations involves numerical integration using the trapezoidal rule and the energy equation is solved using a fully implicit Crank Nicholson method, since this gives unconditional stability. The model also allows for a wall slip boundary condition. The flow model has been experimentally validated by simulating an extrusion rheometer and comparing predicted capillary pressure drops with measured ones. It has also been validated by comparing real injection moulding pressure drops with corresponding predictions. The cure simulation has been validated by comparing predicted cure times with measured cure times taken during the injection moulding trials. The effect of the variation of material properties, heat transfer coefficient and finite difference mesh geometric parameters on simulated results have been assessed. The effect of wall slip on simulated injection results has been investigated.