Recently, several investigators have challenged the simplifying assumption of using a stationary screw with a rotating barrel when analyzing the flow in single screw extruders. The investigators claim that there are significant differences between the realistic and simplified cases, such as a 12 percent difference in throughput [1]. This paper presents an analytical derivation for the flow in a laid out system and one which includes the curvature of the screw. An experimental set-zip was built which allows the rotation of the barrel with a stationary screw and the rotation of the screw with a stationary barrel, and a three dimensional flow simulation program was used to model the flow in the extruder using the two conditions. The analytical, experimental and numerical analyses show that it is justified to use the stationary screw assumption when modeling the flow in single scr ew extruders.