What are the advantages of twin­-screw extrusion over conventional compounding processes?

Presently, the conventional process is to use co-rotating twin-screw extruders for compounding. However, this was not the scenario about 10 years ago, especially in India. The modern twin-screw extruders can be designed in such a way that all the processed materials experience the same amount of work. The technical term for this is 'narrow residence time distribution’.

Conventional compounding processes using single-screw extruders coupled with dispersive kneaders, co-kneaders and other batch-type mixers or continuous mixers achieve mixing or kneading using large gaps in the working members similar to a kitchen mixer. In such a situation, polymer molecules experience high or low intensity work for a small or large amount of time  at random.  Therefore, heat and shear sensitive material cannot be processed.

Importantly, an optimal amount of work can never be done, since mixing would need a minimum amount of work. To ensure this, larger amount of work will have to be done so that all parts experience the minimum needed work. This aspect is measured as specific energy, which is the total work (energy) done per unit mass of material. The first or second generation co-rotating twin­-screw extruders were no better than the efficiency of the kneaders. Even today, state-of-the-art twin-screw extruders from other reputed manufacturers are not designed to be efficient.

The new generation extruders such as STEER's OMEGA series extruders operate by transferring energy equal to the total enthalpy of the process (useful work required for achieving the stated objective such as melting and mixing) and are therefore, extremely energy efficient. This energy efficiency has another desirable outcome. The output of the OMEGA extruder is enhanced considerably for the same motor power rating.