Backward-Pumping Elements

Restrictive elements are also discs that close the entire cross-section of the barrel bore. They are used only for special applications and are hardly ever used to separate process zones. They are described in Section 12.3.5. 

Although kneading element, including those with narrow, modified kneading discs, are predominantly self-wiping, they will shear the polymer in the shearing fields in the tip area and between the kneading discs. 

Mixing elements on the other hand are normally designed to minimize the amount of dispersive, shear-intensive mixing. This comes at the cost of the self-wiping function and the wiping of the barrel wall. Mixing elements are only partially self-wiping and sometimes not at all. 

Mixing elements are based on positive conveying, neutral, or backward-pumping elements. 

Conveying mixing elements operate most effectively in the filled extruder zones, i.e., they should operate against pressure. This guarantees that the product flows back through the gaps in the flights and mixes with the product in the screw channel upstream. 

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Figure 12.22 Operating principle of backward-pumping elements...

Igel elements (Fig. 12.26), which were also developed very early on, aptly named after their resemblance to a hedgehog. In these elements, a basic conveying element is overlapped with a backward-pumping element with the same pitch and flight depth. Very sharp edged ( spines ) are formed to divide the melt flow. 

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. 

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