An extruder for degassing
An extruder for degassing a thermoplastic plastics material melt comprises at least one screw mounted for rotation in a barrel. The barrel includes a first feed aperture for the introduction of the melt and a degassing port downstream thereof connectable to means for degassing the melt. The at least one screw is mounted on a shaft and has a helical flight. In the region of the port, the flight is replaced by alternating disc members and spacer members mounted on the shaft. The diameter or major dimension of sheet production extruder corresponds substantially to the combined diameter of the screw core and its flight whilst the diameter of the spacer members corresponds to that of the screw core. The disc members have profiled downstream faces and the thickness of each disc member and each spacer member lies in the range of 0.1 to 0.5 times the diameter of the screw.
A twin-screw degassing extruder is described in U.S. Pat. No. 4,423,960. In such arrangement, individual screw sections are mounted on a spindle. The screw sections are prevented from rotation relative to one another by key members which engage in a groove in the spindle.
The internal diameter of the barrel of the twin screw extruder disclosed in this prior document is enlarged in the degassing region in order to obtain a greater free volume and hence to achieve better degassing of the plastics material melt. These structural features do give good degassing results. On the other hand, the enlargement of the barrel in the degassing region is a very cost-intensive feature in the production of the barrels. This is because it is necessary to produce barrels and screw sections having enlarged diameters in the degassing region, which diameters differ from the respective diameters of the barrel and screw in the input region of the extruders.