an extruder machine for processing thermoplastic materials such as low density polyethylene , the extruder incorporating a body having a bore therethrough with a rotor or screw journalled therein . the rotor has several successive mixing and pumping sections . the mixing sections on the rotor are multiple flighted defining primary and secondary channels having their discharge and entrances respectively blocked or restricted by annular dam arrangements . the rotor includes a plurality of adjustable dams , arranged across the channels , permitting a variation in the degree of homogenization which may be performed upon the extrudite . the dams may be radially displaceable to vary the distance between the radially outer surface of the dam and the surface of the bore .