A viscosity model showing a direct correlation between molecular weight distributions and shear thinning behavior of polymeric melts was used in 3D numerical simulations of an extrusion process. The equipment analyzed was a single screw extruder with an annular die attached. The material of choice in the simulations was polystyrene in a range of molecular weights and degrees of polydispersity. The influence of material parameters (molecular weight and polydispersity) on the system operating point, power consumption and residence time distribution was analyzed. The results were generalized and a window of processing conditions for resins with different properties was analyzed. The effect of blending homologous polymers with different molecular parameters on material processability was also presented. The model can be extended to other materials and various processing equipment.