Rotating flight extruder

There are different components that make up the extruder each with their specific important function. All components have to operate efficiently otherwise the extruder s operation is inefficient. A very important and essential parameter in the extruder is the plasticator s pumping process. It is the interaction between the rotating flights of the screw and the stationary barrel wall. For the plastic material to be conveyed, its friction must be low at the screw surface but high at the barrel wall. If this basic criterion is not met, the plastic will usually rotate with the screw and not move in the axial output direction. 

The centerline distance can be made considerably shorter in double-flighted, bilobal screws (p = 2). in this case the smallest centerline distance is CJT) = O.sV2 (= 0.707) and the maximum channel depth 0.293 D. Thus, the channel depth and channel volume can be made substantially larger, which results in improved throughput capability. These considerations have led several twin screw extruder manufacturers to switch from triple-flighted co-rotating extruders to double-flighted extruders. 

Plasticator A very important component in a melting process is the plasticator with its usual specialty designed screw and barrel used that is used in different machines (extruders, injection molding, blow molding, etc.). If the proper screw design is not used products may not meet or maximize their performance and meet their cost requirements. The hard steel shaft screws have helical flights, which rotates within a barrel to mechanically process and advance (pump) the plastic. There are general purposes and dedicated screws used. The type of screw used is dependent on the plastic material to be processed. 

Fig. 20. Photographs taken through a transparent barrel section in a twin-screw extruder showing the presence of bubbles at an extraction pressure of 8 Torr (MacKenzie, 1979). The polymeric solution is heptane-poly(dimethyl siloxane). (a) Screw rotational speed is 15 min . Note how bubbles are dispersed on pushing side of flight. Flow is from right to left, (b) Stationary screw. Note how the bubbles shown in (a) coalesce when the screw is stopped. 

All single-screw extruders have several common characteristics, as shown in Figs. 1.1 and 1.2. The main sections of the extruder include the barrel, a screw that fits inside the barrel, a motor-drive system for rotating the screw, and a control system for the barrel heaters and motor speed. Many innovations on the construction of these components have been developed by machine suppliers over the years. A hopper is attached to the barrel at the entrance end of the screw and the resin is either gravity-fed (flood-fed) into the feed section of the screw or metered (starve-fed) through the hopper to the screw flights. The resin can be in either a solid particle form or molten. If the resin feedstock is in the solid form, typically pellets (or powders), the extruder screw must first convey the pellets away from the feed opening, melt the resin, and then pump and pressurize it for a down-... 

Campbell, G.A. and Spalding, M.A., Numerical Analysis of the Melting Process for Barrier-Flighted Single-Screw Extruders Using Screw Rotation Physics, SPE ANTEC Tech. Papers, 56, 418 (2010)... 

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