a plastics extruder in operative association with a control arrangement is disclosed . the plastics extruder includes a screw extruder , a screen downstream of the screw extruder , a throttle located between the screw extruder and the screen , a moulding tool downstream of the screen , and a melt index measuring device . the measuring device includes a metering pump whose rotational speed is controllable and which is provided with a rotational speed indicator , a measuring nozzle disposed in a sample flow channel , and pressure and temperature sensors connected to the sample flow channel directly upstream of the inlet of the measuring nozzle . the sample flow channel is adapted to be located downstream of the screen pack and upstream of the moulding tool . for the purpose of regulating the viscosity of the plastics material melted in the screw extruder , the throttle cross - section is variable by a servomotor connected to the output of a programmable two - coordinate controller . the characteristic of the controller provides a desired value output and can be predetermined as a temperature - pressure characteristic when the metering pump is operated with constant delivery , or as a temperature - delivery characteristic when the metering pump is operated with a constant pressure drop via the measuring nozzle while the temperature sensor is connected to one of two actual value inputs of the two coordinate controller . either the pressure sensor or the rotational speed indicator is connected to the second actual value input .