A mixing section for an extrusion screw comprises a shaft having at least one set of spaced-apart cuspidate shaft teeth, with the shaft teeth extending radially outwardly from a root of the shaft. Each of the shaft teeth comprises a flank face and a rake face opposite the flank face, with the tooth transitioning between the flank face and the rake face at an outermost cusp. Some embodiments have one or more longitudinally extending, longitudinally spaced series of annular sets of circumferentially spaced shaft teeth. Some embodiments have one or more helically extending series of spaced-apart cuspidate shaft teeth. The mixing section may be received in a mixing section barrel to form a mixing portion of a screw extruder.