A theoretical and experimental study of the behavior of a viscous non-Newtonian polymer melt in a List/Buss Kneader is presented. The modelling study for viscous non-Newtonian behavior considers the dynamics of flow including the influences of screw oscillations and interrupted screw flights. Calculations are made using different methods for various individual Kneader elements which consider (i) pumping capabilities, (ii) leakage flows through the slices in interrupted flights, (iii) pressure distributions and flow fluxes. Predictions of pressure profiles, fill factors, and positions of starvation in modular machines are made. An experimental study which uses polypropylene considers the distributions of polymer in modular screws with different screw configurations. The experimental results are compared to the flow simulation.