Twin-Screw Extruders

1 Modular Co-rotating Twin-Screw Extruders

Modular co-rotating twin-screw extruders have become the most important commercial machine for continuous mixing of compounds based on thermoplastics. They also are the most studied and understood of the equipment developed in the period after World War II .

The machine contains two shafts mounted with various screw and mixing element modules each having a specific function. These are usually right-handed and left-handed screw elements, kneading disc blocks, distributive mixing elements, rotor and polygonal elements, and valves.

Modular co-rotating twin-screw extruders are widely used for blending of polymers and incorporation of fillers, pigments, oils, and other additives into the polymeric base. Moreover, they are used for removal of liquids (devolatilization), for reactive extrusion, polymerization, and grafting .

Modular co-rotating twin-screw extruders are suitable mixing equipment for TPO, TPV, and SEBS compounds. Each compound requires specific process conditions. Typically, TPV requires high torque and long residence time. Low bulk density materials in highly filled TPO and SEBS-based formulations require improving feed intake, such as feed enhanced technology (FET). This high-torque extrusion technology increases the feed intake/feed zone throughput capacity for difficult-to-feed materials by improving the conveying efficiency through an increase in the coefficient of friction between the feedstock and barrel wall. This design of twin-screw extruder is also capable of providing good homogenization of the reactants and sufficiently large free volume for precise adjustment of residence time as needed for the reaction .

2 Modular Counter-Rotating Twin-Screw Extruders

Modular counter-rotating twin-screw machines with modular construction are used for mixing, blending, devolatilization, and reactive extrusion. Two classes of these machines have been commercialized, namely tangential screw and intermeshing screw machines . The intermeshing screw machines have been used for difficult mixing problems involving pigments, fillers, and lubricants that are difficult to disperse. The machines with tangential screws provide the processed material with a milder stress history and consequently are much less able to break up difficult particle agglomerates than the intermeshing screw machines. Therefore, they found their major applications in devolatilization and reactive extrusion .