Abstract: The temperature of an extruder screw having an axial bore therein is controlled by providing a closed temperature control system within the screw which is in heat transfer relationship with a second temperature control system through which is circulated a cooling medium from an outside source. The quantity of outside cooling medium may be monitored to control the closed temperature control system. The apparatus for carrying out this process comprises a cooling medium retained within the screw and a heat exchanger extending into the cooling medium. A second cooling medium from an outside source is circulated through the heat exchanger so that heat is transmitted from the closed circuit cooling medium to the open circuit cooling medium.