Method of and apparatus for controlling the viscosity of molten plastics material which is to be moulded

A control arrangement for a plastics extruder having a casing, which includes a screw extruder, a screen downstream of the screw extruder, a throttle having a cross section located between the screw extruder and the screen wherein said throttle is constructed as an axial annular gap throttle comprising a hollow cylinder and a pin piston axially displaceable therein by a servomotor means, a moulding tool downstream of the screen, and a melt index measuring device disposed in close thermal contact on a casing of the extruder, between said screen and said moulding tool;

• said melt index measuring device comprising a metering pump whose rotational speed is controllable and which is provided with a rotational speed indicator, a sample flow channel, a measuring nozzle disposed in the 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 being located downstream of the screen and upstream of the moulding tool, wherein the control arrangement comprises said servometer means for varying said cross section of said throttle to regulate the viscosity of a plastics material melted in said screw extruder, said servomotor means being connected to the output of a programmable two-coordinate controller whose characteristic provides a desired value output and can be determined 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 said measuring nozzle, and a temperature sensor connected to one of two actual value inputs of said two-coordinate controller, and wherein at least one of said pressure sensor and said rotational speed indicator is connected to actual value input.