a vented , multi - staged rotary screw extruder is taught for extruding thermoplastic materials . the screw has a final pumping - section of the screw that includes two or more channel depths which provides exit - pressure and flow stability . the channel - depth of the first segment of the final pumping - section is deeper than the channel depth of the second segment of the final pumping - section . the axial length of the first segment is preferably about two - thirds of the final pumping - section length . the deeper first channel - depth acts as a reservoir resulting in smaller fluctuations in axial final fill - length as the input flow to the total pumping - section varies . the second channel - depth in the remaining approximate one - third of the total pumping - section is preferably sized so as to optimize pressure development .