A mixing section for an extrusion screw has an inlet end and an outlet end as well as alternating wiping lands and barrier lands. The wiping lands have a greater helix angle than the barrier lands. The wiping lands and the barrier lands define inlet channels which narrow toward the outlet end and outlet channels which widen toward the outlet end. A helical pattern of mixing channels is cut into the wiping lands and the barrier lands. The mixing channels may be oriented generally at approximately right angles to the wiping lands and the barrier lands. A portion of the extrudate encounters the inside wall surfaces of the mixing channels and changes direction which improves the mixing of the extrudate.