Corotating Screws

Type Single Screw Corotating Screw Low-Speed High-Speed Type Type Counterrotating Twin Screw... 

Figure 1. (A) Twin screw configurations. (B) Movement principle of corotating screw.

In contrast, corotating screws form V-shaped wedge areas that have four to five times more volume. This enables material to be transferred from one screw to the other, resulting in a renewal of material layers and surfaces. The result is a higher degree of mixing. 

The necessary shear velocity required to wipe the boundary layers is proportionately lower because of the low relative velocity. Also, the material is drawn into the roller gap and is squeezed onto the surface. In corotating screws, one crest edge wipes the flanks of the other screw with a tangentially oriented, constant relative velocity. There is a higher relative velocity in this arrangement, and hence there is a sufficiently high shear velocity available to wipe the boundary layers. The calender effect does not occur however, a more efficient and uniform self-cleaning action is achieved. 

Several types of multiple-screw machines are available, including intermeshing corotating screws (in which the screws rotate in the same direction, and the flight of one screw moves inside the channel of the other), intermeshing counterrotating screws (in which the screws rotate in opposite directions), and... 

Gonsider Figure 6.11 it identifies the cross-sections of two intermeshing corotating screw shafts, each of which has maximum screw radius Rs and a centerline distance Cl. We may define the important characteristic quantity... 

Twin screw extruders can be operated with either counter- or corotating screws. Additionally, the screws can be fully intermeshing, partially intermeshing, or nonintermeshing. Figure 7-10 illustrates the various cases. 

Figure 12.30 Schematic of corotating screws defining location of high and low pressure regions.

FIGURE 8.7 Corotating screw extruder, (a) self-wiping intermeshing screw elements, (b) selfwiping intermeshing kneading block elements. 

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