Screw Elements and How They Work

Conveying elements take the product to be metered - pellets, powder, ground product, melt, etc. - into the extruder and transport it downstream to the first processing section where it may be compressed. Then, the product is transported through processing sections, 

Conveying elements are supplied with different pitches and lengths. They almost always wipe the barrel with small clearances to prevent leakage flows over the tips. This means that only single- and double-flighted elements can be used in extruders with a D()/D, ratio of more than 1.36. 

Conveying elements are designed with identically shaped and located face sections. This makes it easier to fit the elements onto the screw shaft to form a continuous profile with no offset angle with respect to the previous element. Double-flighted conveying elements are normally used. Their length is usually equal to a whole pitch or half a pitch, i. e., a complete profile thread or half a profile thread (Fig. 12.11). 

A cut perpendicular to the axle of a self-wiping screw element shows the same free cross-section at any point independent of the screw pitch or pitch direction. If this needs to be enlarged to improve product intake, the contour of the intermeshing profile may have to be abandoned in some cases. These undercut or double undercut elements (Fig. 12.12) offer 15 to 30% increase in the product volume, but increase the risk for product building up on the un-cleaned flanks of the elements. 

Standard profile Shear edge profile Box profile 

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