Within injection moulding systems the nozzle and gate regions force the polymer melt from large cross-sectional areas into smaller ones. As this occurs the polymer melt must extend as well as shear as it is forced into a smaller section. The added work done in extending the polymer in these regions results in a contraction pressure loss. To predict this pressure loss the extensional properties of the material must be gathered. True extensional properties of molten polymers are exceptionally difficult to obtain at high extension rates. Through the use of approximate contraction analysis techniques it is possible to obtain approximate extensional viscosity data from the flow of polymer through a simple contraction. Repeating this experiment at various temperatures, a temperature dependent extensional viscosity model can be determined. Extensional and shear viscosities are determined for acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), polypropylene (PP) with and without talc filler, and polybutylene-terephthalate (PBT) of various glass fibre content of which interesting results are observed in the effect of additional fibre content on the extensional viscosity. The factors that affect the measurement of the extensional viscosity using this technique are investigated in this thesis. The approximate contraction analysis methods were then applied to a numerical shear analysis and provided an improved simulation of the injection moulding process. Finally the approximate methods were applied to the more typical geometries faced in injection moulding to provide a method for predicting the entrance pressure in these geometries.