In order to create a design with greater operating flexibility and with greater operational control, twinscrew
machines were developed. Twin-screw extruders can be co-rotating, counter-rotating,
intermeshing or non-intermeshing in terms of basic designs. Co-rotating, intermeshing screw designs
have dominated that scene as far as these extruders are concerned. This is because of relative ease
of design and manufacture compared to counter-rotating designs.
The screw design of twin-screw extruders can dramatically affect operating efficiency and machine
capability. Screw components in the feed section of the barrel can be single, double or even triple
flight arrangements. With more flights intertwined on the shaft, the conveying capacity of the screw
is reduced but the residence time distribution (RTD) is lower. This promotes a first-in, first-out movement
of the extrudate. However, double- and triple flighted screws produce more shear across the screw
channel and therefore, improved processing uniformity.