method and apparatus for automatically controlling the operation of a rotary extruder , including using a pressure transducer communicating with the annular space between the barrel and rotor to monitor the hydraulic pressure of the melted polymer , using a microprocessor - based controller to compare the measured pressure with a predetermined set point , and thereafter selectively controlling the speed of a feed drive motor or melt drive motor to maintain the melt pressure within a desired range while holding the extruder rotor speed constant .