Simulation of injection molding using solid elements.

Three-dimensional solid elements offer several advantages over two-dimensional elements in the simulation of the injection moulding process. However, software packages supporting 3D solid elements are not commercially available, so DuPont has developed software, with limited features, to demonstrate the feasibility of 3D elements in coupled flow and warpage analysis. A J van der Lelij of DuPont's European Technical Centre in Geneva describes the results of the experiments and concludes that suppliers of flow analysis software should extend their element libraries to include 3D elements, to give users more modelling freedom.

Software to simulate the filling phase of the injection moulding process has been available for some considerable time, with more recent additions of programs which allow simulation of the packing phase, of part cooling, of part warpage, and the simulation of gas-assisted injection moulding.

Most commercial software (such as Moldflow, C-Mold and Ideas for Plastics) uses finite element models, with basic input of two-dimensional shell elements for the part and beam elements for the runner system. These elements permit modelling of temperature distribution over the thickness of the part, computed with melt temperature and shear heating effect as heat input, and conduction and convection to the mould walls as heat output. In these calculations, the mould walls may be at non-uniform temperatures, based on analysis with boundary elements, for example, describing the cavity. In most packages, however, flow analysis is based on symmetrical temperature distribution over the thickness (as in Moldflow); this is acceptable, since mould wall temperatures generally have little effect on the filling pattern.

DuPont's efforts to simulate warpage behaviour of glass-fibre reinforced parts started in 1990, before commercially available software had recognised these materials, and was based on computed fibre-orientation (direction of flow) from Moldflow and thermal stress analysis with Ansys. The two programs are linked with a 'home-made" interface known as Restran. In this approach, differential shrinkages are measured on moulded parts, in the direction of flow and across it, in the plane of the shell. Corrections for non-uniform temperature distribution at ejection can be applied, as well as for non-uniform wall thickness and for percentage orientation. Most results are reasonable to good (75 per cent), with occasional poor (25 per cent) results.

When commercial software packages started (around 1992 - 93) to include warpage simulation for glass-fibre reinforced materials, DuPont tested their prediction capabilities. The results with this commercial software were no better than those obtained by DuPont with its own software. One important reason for poor results is imperfect simulation of the so-called 'corner effect' which refers to the change of …