Backward pumping screw

Figure 2.16 "Backward pumping screw" with left/right hand thread...

Pressure build-up zones are located in the extruder in the discharge zone and upstream of each backward pumping screw element, e.g., mixing elements, re-conveying, or neutral kneading blocks. 

The pitch direction, i. e., the direction of the angle of rotation of the thread. If the thread of the element runs counter to the direction of rotation of the screw shafts, one obtains a conveying element. If the thread runs synchronously with the screw shaft rotation direction, the screw element will have a backward-pumping effect (Fig. 12.4). 

TME elements (Fig. 12.27) are formed from pitch-less rings with the same outer diameters as the screw diameter and with holes milled around their circumference. The holes can be located to form positively conveying, backward pumping, or neutral conveying elements. The toothed rings are mounted on one shaft and partnered with rings on the opposite shaft whose outer diameters correspond to the core diameter of a screw element. 

Figure6.15 (a) Forward pumping (right-handed) and (b) backward pumping (left-handed) screws.

Single screw I/D = 20 to 35 the screw turns as if to unscrew itself backwards out of the barrel. Shear rate >100 1/s in the screw channels between 100 and 1000 1/s in most die lips and > 1000 1/s in flight-to wall clearances. Volumetric pumps with feed rate determined by the bulk density of the feed. Recommended rpm given previously. Output [kg/h] = 1000 (D/16). ... 

A reciprocating-screw machine is an exunder system that has the advantages of screw plastification and ram injection in the same unit. In this design, as the screw rotates, it is pushed backwards by the molten polymer that collects in front of the screw. At a predetermined point, rotation stops and the nozzle moves in the forward position, acting as a ram to force the melt into the mold. It remains forward until the mold gates freeze, then pumps back to repeat the cycle while the mold opens. 

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