Twin screw feeders are generally used where extreme flushing of powders can occur or the complete opposite with cohesive powders like pigments or those with a high resin content, which require a large entrainment area to enhance flow. The one main advantage of twin screws is the fact the faces run together and not edge to edge such that a self cleaning effect is the result. Also the pulsing effect is reduced at the outlet, particularly important with sluggish powders.
Solid screws take the form of a continuous Archimedean spiral with a shaft running throughout the screw. The vast majority of applications use this design principle. Even with the flavour powders usually associated with the snack industry solid screws are used, the only additional feature given to the screw are breaker bars set on to the screw in the trough area to break up any dead area that may form in the trough.
On occasions wire screws can be used, which take the form of a helix spring manufactured from square section material. This type can only be employed for very difficult fibrous materials, which need a larger entrainment area in the screw, i.e. minimum blade width and no centre shaft. It should be noted that wire screws have minimum surface area, which does not support serious build up of cohesive powders.
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