Exercise in injection moulding: Predicting the non-uniform shrinkage from PVT data

Injection moulding is a widely spread technique for producing plastic parts of many kinds, for example bowls, chairs, coverings for mobile phones etc. The basic principle of injection moulding is to inject molten plastic into a closed, cooled mould (i. e. tool), where it solidifies to give the product.

The product is recovered by opening the mould to release it. The quality of the product is highly dependent on tool design, process parameters such as pressure and temperature and which type of polymer that is used. Here, a plastic bar with four indentions (in the form of parallel lines) is manufactured using two types of commercial plastics (Polypropylene (PP) and Polycarbonate (PC)).

Pressure transducers measure the pressure in the mould during the injection and the solidification. The temperature is measured by inserting a thermometer in the plastic melt. The difference in dimensions between the mould and the cooled plastic bars (i.e. the shrinkage or expansion) is measured. Five types of polymer bars for different temperatures and pressures are manufactured for both PP and PC.

The aim is to understand the relation between the process parameters (temperature and pressure - e.g. PVT data) and the dimension of the final product. To obtain this, the solidification of the plastic should be modeled, constructing a small program, using for example Matlab. The theory described in the next section allow the construction of a small Matlab program to make a modeling of the solidification and shrinkage/expansion and relate these results to the measurements.