Numerical simulation of the plastics moulding process using a thin gap numerical model

Abstract

The injection moulding process is one of the most popular means of mass producing components to very small tolerances. Before the advent of the affordable computer, the methods used in injection mould design were empirical in nature. Computational fluid dynamics offers the mould designer a tool to predict and verify the mouldability of a part before any metal is cut on the actual mould. This thesis describes the development of a thin gap numerical model that can be used by mould designers. The thin gap assumption enables the Navier-Stokes equations to be simplified by the Hele-Shaw approximation enabling the flow variables to be solved more efficiently, without a significant loss of accuracy. Finite element solutions of the thin gap model are well documented and are well suited to solving problems with complex geometries but in general finite element methods require more memory and are slower in execution than finite-difference and finite volume methods. A personal computer based program utilising the finite volume method for solving the Hele-Shaw flow problem is developed in this thesis, which simulates the filling stage of the plastics injection moulding process. The basic model was first developed on a two dimensional constant thickness Cartesian coordinate domain using a constant tiscosity. This Newtonian model was then extended with the Volume-of-Fluid method to include the moving boundary or free front. A suitable material model was chosen to predict the viscosities of various grades of plastic at different temperatures and pressures. Once this model produced accurate results it was extended to include generalised curvilinear coordinates in order to solve problems on complex three dimensional geometries. The program is tested and verified extensively with numerical experiments and experimental data from previous published work and excellent results are obtained. The final product of this thesis is a computer program which simulates the moving boundary during the filling stage of the injection moulding process. This work contributes to the development and understanding of finite volume and finite-difference methods in the simulation of plastic injection moulding.