DE2417865

1471115 Extruding cellular polymers GENERAL ENG CO (RADCLIFFE) Ltd 4 April 1974 [11 April 1973] 17490/73 Heading B5A Cellular plastics or rubber material is extruded Fig. 1, by at least one non-positive displacement screw extruder 10 delivering through a feed pipe 12 to a positive displacement means 13, which meters the material and delivers it at the required pressure to an extrusion die 14, the extruder 10 comprising a vent V through which a fluid blowing medium is introduced into the material, and a pressure sensing means (23), Fig. 5 (not shown) located on the extruder between its outlet and the vent V. The vent V, operates at partially filled flights of the screw. The apparatus separates the compounding function of the screw from the function of generating pressure for extruding through the die. The positive displacement means may comprise a gear pump, or an intermeshing positive displacement multi-screw pump. The non-positive screw extruder may comprise a single screw, or a plurality of single, or non- positive multiple screws which separately or serially feed a single die. The extruder or extruders, and the gear pump or pumps may be geared together as at 25, and more than one gear pump may be geared together. The pressure sensing means may feed signal to the extruder drive to maintain constant the delivery pressure to the positive displacement means. In the Fig. 2 embodiment (not shown) the screw extruder delivers, via a manifold, to four gear pumps each supplying a separate die. In the Fig. 3 modification of the Fig. 2 embodiment, each of the gear pumps 13 supplies the same tubular film extrusion die at spaced apart peripheral locations. In the Fig. 4 embodiment (not shown), each of two extruders delivers to a gear pump, which supply the same die head with different materials at the same, or different rates. The Figs. 2-4 embodiments may be modified by the provision of a die extruding multiple layers or, as in Fig. 6, by each extruder and displacement means feeding sequential dies from which successive coatings of different materials are extruded on to each other. Alternatively one extruder may, via a manifold and separate gear pumps, feed all of the dies to provide successive coatings of the same material. A single die may be fed from more than one extruder and displacement means, to apply multicoloured coatings, e.g. to a wire, Fig. 7 (not shown). The extruded material may comprise natural or synthetic materials, or polyvinyl chloride, polyethyle