Numerical simulation of the extrusion process of polystyrene in a intermeshing co-rotating twin screw extruder was carried out to compute the residence time distribution(RTD)of the material particles and other flow field parameters in various processing conditions．The computed RTD was compared witll the on-line measurement results

and a fairly good agreement was reached between them．Moreover

based on the numerical results

a definite relationship was determined for the influence of the screw rotation speed and feed rate on the residence time of the polymer melt．Numeric simulation results also predicted the profile of the flow field parameters such as temperature

filling ratio

pressure and viscosity along the axis of the screw under various processing conditions．The computer simulation method helps to investigate the internal flow state and residence time of the polymeric melt in a twin screw extruder and therefore helps to optimize the production．