Co-rotating System and Counter-rotating System of The Twin Screw

In a co-rotating system (Fig. 10.8a), the screw transports material around up to the point where the screws intermesh. 

Because of the two opposing and equal velocity gradients acting at this point, material is taken over by the other screw 

with the least amount of material passing through the gap between the two screws. This system offers a higher degree 

of self-cleaning with greater control over residence time distribution. The majority of the material follows an 8-shaped

pattern along the entire length of the barrel.

Figure 10.8. (a) Co-rotating system and (b) counter-rotating system

In a counter-rotating system (Fig. 10.8b), the material is carried by the screw flights in such a way that all the material 

is forced to the center where the two screws meet, forming a material build-up. A small amount of material will pass 

through the gap between the two screws and will rotate out with the parting screw flights. Because the two screws are 

intermeshed, not all material will pass through. A basic advantage of the counter-rotating system is that the material 

that does pass through the nip of the two screws is subjected to an extremely high degree of shear. Figure 10.9 shows 

different configurations of intermeshing screws in a twin-screw extruder.

Figure 10.9. Twin-screw extruder with varying configuration of intermeshing