A three-dimensional finite element model was implemented for the solution of mass and momentum conservation equations in the kneading disc section of an intermeshing co-rotating twin-screw extruder. The polymer melt was modeled with a Carreau constitutive equation. The particle tracking technique was used to obtain residence times and analyze distributive mixing for different length to diameter ratios (L/D) of kneading discs. Previous studies from Kalyon et al. (1) and Cheng and Manas-Zloczower (2) have shown that the mixing performance of intermeshing corotating twin-screw extruders is highly dependent on the combination of screw configuration and operating conditions. The complexity of the geometry and the transient character of the flow demand powerful computational tools to characterize the flow and to develop a prediction tool for the analysis of relative performance between different configurations. Difficulties arise in the particle tracking technique because of the time discretization and the presence of moving boundaries. Results show the importance of particle history on the evaluation of the relative performance of different configurations of kneading blocks and suggest a reevaluation of the use of average flow characteristics for the analysis of mixing. Results also confirm the importance of an accurate description of the geometry and clearances in order to obtain information about relative mixing performance. (C) 2004 Society of Plastics Engineers.