Twin versus Single Screw Extruder

Screw elements for co-rotating twin screw extruder 

Modular screw elements for a counter-rotating extruder are shown in Fig. 10.2. 

Screw elements for counterrotating twin screw extruder 

Often used with modular design of screw and barrel-great flexibility 

Moduiar design of screw and barrei is rareiy used-iess fiexibiiity 

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Figure 9.12 Droplet radius versus wt% dispersed polystyrene (PS) in a blend with polypropylene (PP) at an average shear rate of 10 sec in a single twin screw extruder at T = 200°C. The viscosities of pure PS and PP at these conditions are about 5 x 10 and 2 X 10 Pa-s, respectively both melts are modestly shear thinning (power-law slopes —1/3). The interfacial tension T was measured to be 4.9 dyn/cm. The predicted Taylor limit, a = 0.5 T/rfsy, is shown. (From Elmendorp and van der Vegt 1986, reprinted with permission from the Society of Plastics Engineers.)...

Thiele (18) stressed the utility of continuous-single and twin-screw extruders compared with Banbury batch type mixers on the basis of the working volume, that is, small working mass in the extruder channels in continuous process equipment versus a large amount of blended material per cycle in batch type mixers. With a relatively smaller working volume in an extruder, there is more enhanced intimate mixing on the microscopic scale in a shorter period of residence time. 

If an extrusion process requires a substantial amount of cooling, this is usually a strong indication of improper process design. This could mean improper screw design, excessive length-to-diameter ratio, or incorrect choice of extruder, e.g., single screw versus twin screw extruder. 

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