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6 temperatures (approximately 10 ̊C/18 ̊F). The combination of down gauging and melt temperature reduction results in material savings and a higher out- put, as cooling rate often limits output (See Breaker Plates, Screen Packs and Gear Pumps in Section 4.). Gear Pump To the Die Breakerplate andScreenpack Extruder Figure 3. Gear Pump ➤ EXTRUSION PROCESSES Although a single screw or a twin screw extruder may be used to produce the products described in this section, most commonly single screw machines are used as they are simpler and cheaper. For un-plasticized polyvinyl chloride (UPVC), however, twin screw machines are widely used as they can both compound and then shape the material. Tube or Pipe Tube or pipe is extruded from an annular die, usually mounted as an in-line extension (Figure 4.). Within this extension is a mandrel which forms the inner surface of the tube. Low air pressure is usually applied through the mandrel. The tube is held to shape by this internal air pressure as it leaves the die. The molten tube proceeds to a sizing arrangement that sets its dimen- sions and then to a water-bath or a series of air-cooling rings that fix its dimensions. The sizing equipment, used to control the external diameter of the tube, is normally a water-cooled cylinder (that is, a “sizing die”), which is positioned close to the extruder die. For small-diameter tube or pipe an arrangement of sizing plates or rings, located in the cooling bath, are used instead. Internal air pressure forces the molten tube outward against the inner surface of the sizing die, plates or rings. With larger tubes or pipes the circular profile can be maintained by applying a partial vacuum through fine holes in the inner surface of the sizing die. For optimal control of OD and roundness vacuum sizing tanks are often used, which combine accurate sizing with controlled product cooling. AfterPDF Image | Dynisco Extrusion Processors Handbook
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