Method of operating a screw extruder and screw extruders for carrying out said methods

DETAILED DESCRIPTION The invention is based on recognition of the fact that the product properties resulting during the fusion process in a screw extruder are largely determined by the energy introduced.
In order to achieve product properties which remain constant, it is desirable to keep the input energy constant.
With regard to the thermal energy supplied, this can be effected by a control of the housing temperature.
The mechanical introduction of energy through friction, on the other hand, is conventionally effected without control in that a fixed extruder speed is set or the extruder speed is controlled in accordance with other criteria --as explained above--but not with the object of maintaining the input energy constant.
In relatively large screw extruders, in particular, however, the proportion of mechanical energy introduced is dominant.
Starting from this, it is the object of the invention to provide a control for the mechanical introduction of energy in order to keep this constant so as to achieve as uniform a product quality as possible.
According to the invention, this problem is solved by a method of operating a screw extruder in which the speed of rotation of the drive motor of the screw extruder is controlled by controlling the speed of rotation in dependence on the specific energy input in such a manner that this is kept substantially constant.
Preferably,a control depending on the product temperature is superimposed on the energy control in the form of a cascade control.
Another object of the invention is to provide a screw extruder for carrying out the above-described method.
Accordingly, the present invention also provides a screw extruder having a housing in which at least one screw is mounted, a drive motor having a variable speed for continuously driving the screw, feed devices for supplying basic products, additives and the like, and a control device for controlling the speed of rotation of the screw, wherein the drive motor is controlled by a device for measuring the driving power of the drive motor, at least one device for determining the amount of product supplied per unit of time (throughput) and means for feeding a signal corresponding to the quotient of driving power and throughput as an actual value to an input of the said control device, said control device having an output connected to the drive motor to maintain the specific energy input constant