Paper to the 3rd International Conference of the Polymer Processing Society, Stuttgart, paper 05-7, 11th-15th April 1987.

S F Bush

Summary

Arguably the main problems in injection moulding reside in post-moulding distortion, manifested as shrinkage, warping and sinking. A major contribution to these problems is the existence of non-equilibrium conformations or shapes of the polymer chains in the post-mould state. As is well-known, the degree of this non-equilibrium condition varies throughout the moulding, usually being greatest in the wall regions closest to the gate. This variation arises essentially from the many large differences in strain rate experienced by a chain flowing into and within a mould, coupled with the fact that production rates of cooling do not, in general, allow chains time to fully relax while still in the fluid state.

During the injection process, the conformation of the chains, particularly their degree of entanglement, determines, with the temperature, the local viscosity. Both viscosity and conformation change with time and from point to point. In fact chain shape (and therefore viscosity) is the product of local conditions inherited from upstream. Mathematical models of injection moulding, however, customarily treat viscosity, where it is not taken as constant, as an empirical function of the local variables usually a principal strain rate and temperature.

By contrast the approach adopted by this paper is to employ a formulation which treats chain conformation and viscosity as dependent variables to be calculated within the computation along with the usual microscopic variables, namely bulk velocities and temperature. The chain shape is characterised by variables whose local rates of change are functions of the shape variables themselves, the velocity gradients and temperature. The chain shape in a finite region of the mould at any instant is then obtained as the resultant of the local rate of change and values of the chain shape convected into the region from upstream. The local viscosity and elasticity are then obtained. In this way we both obtain a prediction of molecular orientation and account directly for the viscosity and elasticity memory.

The results of this formulation are presented as simulations of three-dimensional injection mouldings of a variety of basic shapes. The simulation model is part of a design system which includes a graphical interpreter of mould cavity detail. Velocities, temperatures, pressure and molecular orientation are displayed by the post-processor as colour-graphic contours at various stages of mould-filling.

See also the section on Polymer Morphology & Fibre Reinforcement Mechanisms.