Method for extruding plastics

Claim 1

In a method of forming smooth uniform annular extrusions of plastic material in which molten plastic material is continuously passed from an extruder screw under pressure through the annulus formed between an inner mandrel member and a surrounding outer die member, the inner mandrel and the outer die member both being maintained stationary at the die outlet, the improvement which comprises continuously shearing the molten plastic in a shear zone upstream of the die outlet and downstream of the extruder screw in a direction transverse to the direction of flow of molten plastic through said shear zone while simultaneously continuously pumping said molten plastic through said shear zone independently of said extruder screw, said shearing being operable to increase the temperature of the molten plastic as it passes through said shear zone and said pumping being operable to increase the pressure of said molten plastic as it passes through said shear zone and to decrease the pressure in said extruder, said shearing and pumping being accomplished by passing said molten plastic through the annulus between a hollow shear ring mounted in said outer die member and said inner mandrel member while rotating said ring relative said mandrel.