Abstract: In the present study, the flow and mixing characters of the typical screw elements in the reciprocating pin single-screw extruder were studied using a Polyflow finite clement package. Based on the statistical treatment of the flow trajectories for a large number of the mass points which were located at the entrance of the screw clement，the dynamic mixing process was analyzed.Furthermore, the distribution index and the segregation scale were applied to evaluate the distribution mixing and the dispersivc mixing, respectively.At last，the effects of the operating conditions, material viscosity and the pins on the mixing performances of the extruder were also analyzed. Some of the numerical results were com-
pared with previous results.

Key words: reciprocating pin single-screw extruder, distributive mixing, dispersive mixing