The screw sections have helical flights
In such known prior twin screw extruder, the screw sections have helical flights disposed around their peripheries in the region below the degassing port. These flights occupy a large volume of the interior of the barrel. The conveying operation of an extruder is, in general, achieved by the adhesion of the melt to the interior wall of the barrel with substantially simultaneously the melt being scraped from the wall by the helical flights provided on the rotating screw. It is generally true to say that the better the material adheres to the interior wall of the barrel, the better the conveying effect of the helical flights of the screw.
However, since a considerable portion of the internal surface of the barrel, in which the material is being conveyed, is missing due to the provision of the degassing port, the conveyance of material is inadequate in the degassing region of such known degassing extruders. The material remains in the screw threads as a block or undergoes laminar flow without performing a rolling movement against the internal wall of the barrel if, as is necessary, part of the internal wall is absent as a result of the provision of the degassing port.
The present invention seeks to provide a single screw extruder which achieves enhanced degassing without incurring excessively high costs in the construction and manufacture of the extruder. The invention also seeks to provide an extruder in which the degassing, even of plastics materials which are difficult to degas, such as highly viscous plastics materials or even elastomeric materials, is enhanced.