Where the material’s handling characteristics are too challenging for one screw, two screws operating side by side often provide the needed solution whether for floodable or hard to flow materials.
The basic twin screw design employs a pair of milled, concave-profile screws set 90o out of phase with each other so they fully intermesh. This arrangement forms an endless series of sealed pockets that contain and advance the material to discharge, effectively producing positive-displacement-type feeding. And because of the intermeshed screws’ self-wiping action, material build-up is minimized, and complete discharge is assured.
Variants of the basic twin screw approach include the twin auger, twin spiral screw, and double spiral screw, each optimized to a different handling concern.
The twin auger replaces the solid milled screws with closed-flight augers, and the twin spiral variation employs helical elements mounted to relatively large-diameter central shafts to handle freely to poorly flowing powders, fibers or flakes.
The double spiral screw design uses non-intermeshing helical screws and reduced central shaft diameter to handle pellets, granules and flakes without binding or material degradation.