A plastic extruder system (10) having a barrel (16) and a shell (18) with heat exchanger elements (20) surrounding the barrel (16). Two thermocouples (24, 26) are provided, one (24) for sensing the temperature of the inner surface of the barrel (16) and the other (26) for sensing the temperature of the shell (18). A system controller (12), into which a desired barrel setpoint temperature can be set and stored, receives and stores the independent temperature signals from the thermocouples (24, 26) and controls the heat exchanger elements (20) to automatically maintain the temperature of the extruder barrel (16) close to the desired barrel setpoint temperature.