Screw and Other Paste Extruders

Wet mass rheology heavily impacts performance through controlling both developed pressures and extrusion through the die, as in the case of paste extrusion (see Paste Extrusion and Screw and Other Paste Extruders subsections). In addition, the developed pressure in the roller nips behaves in a similar fashion to roll presses (see Roll Presses subsection). 

In the past twin and other multi-screw extruders were developed to correct the problems that existed with the single-screw extruder. Later the single-screw designs with material developments practically eliminated all their original problems. 

Shear flow n. The flow caused by the relative parallel or concentric motion of the surfaces confining a liquid, as in an extruder screw or caused by a pressure drop, in the direction of flow, from the entrance of a flow passage to its exit, as in a die. Sometimes the two basic driving modes coexist, as in the metering sections of most extrusion screws and in wire-covering dies. In the direction crosswise to any laminar flow, successive layers slide past each other in shear. 

Most plastics e.g. polyolefins and polystyrenes and their derivatives such as ABS (acrylonitrile-butadiene-styrene) and SAN (styrene-acrylonitrile) are supplied by the manufacturers in ready-to-use form with most of the above-mentioned stabilizers or simply need to be additionally stabilized with other additives, e.g. antistatic agents and HALS stabilizers, as required. On the other hand, in the case of other materials (e.g. PVC) it is the end user who adds the additives, pigments or preparations. This is normally done on fluid or high-speed mixers, although in the past gravity mixers or tumble mixers were also used. The mixture is then homogenized on mixing rolls, kneaders, planetary extruders or twin-screw kneaders and further processed. 

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