In the processing of polypropylene via a melt, the extruder exerts an important influence on product quality. Trials were carried out on production extruders with the aim of improving melt quality. As an aid to evaluating the screws, a computer model was developed which could simulate extruders. This model is described in the first part of this paper. In the second part, the computer model is tested using experimental data taken from a range of different extruders. The comparisons illustrate the excellent correlation and hence validity of the model. On the basis of these results, recommendations are given for the optimization of single-screw extruders.