Vented extruder

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.