a rotary screw extruder and a method are provided to both devolatilize and dry plasticized resinous hygroscopic materials during extrusion thereof into a finished product of indefinite length . the extruder includes a screw having three successive pumping sections and a pair of vent sections . each vent section separates two of the successive pumping sections . the first pumping section meters plasticized material throughflow . the intermediate pumping section has partially full channels that permit a continuous , helical passage to interconnect the two vent sections . the partially full channels establish a free surface which promotes devolatilization and drying . the third pumping section pressurizes the devolatilized and dried plasticized material for extrusion through a conventional die orifice . the devolatilizing and drying method begins by controlling the flow rate of plasticized material into a gas removal zone . the gas removal zone is provided with a flow capacity exceeding the controlled flow rate of plasticized material so that a helical , free surface exists throughout the gas removal zone . each end of the gas removal zone is connected to a reduced pressure source . the plasticized material is worked as it goes through the gas removal zone to heat the material , to promote drying and devolatilization thereof , and to maintain a uniform flow rate .