Extrusion Equipment Design And Selection: Part 3 - Twin-Screw Extruders

Twin-Screw Extruders

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.