In the co-rotating twin screw extruder with fully intermeshing screws, there is an interaction of one screw flight with the flow channel in the adjoining screw. Therefore, it is not necessary to provide anti-rotational mechanisms in the barrel walls. If material sticks to the screw surface in this apparatus, the adjacent screw crest will wipe the material from the companion screw flank as the two screws intermesh; and the adjacent screw will, thus, transport the extrudate forward. This unique screw to screw inter-relationship results in a specific geometric screw flank profile which must be maintained in order to minimize the build-up of residual material on the screw surfaces.