Mixing effects of constituting elements of mixing screws in single and twin screw extruders

In extrusion, mixing of solids and melts has always been problematic, leadi ng to diverse models describing the melting process. It is found that for f oods based on cereals, only a few are valid, due to the simultaneous presen ce of water and high viscous non-Newtonian material. Mixing trials are summ arised for single and twin screw extruders, with particulate solids of diff erent particle sizes like, maize grits, wheat flour, sucrose crystals and g lucose syrup. Special mixing-heads for single screw extruders like, pineapp le-heads, and slotted flight sections for counter-rotating twin screw extru ders are investigated. The effects of screw geometry on mixing has been mea sured using co-rotating twin screw extruder modular elements like: single l ead elements, mixing paddles and reversed pitch elements in a translucent m odel extruder by means of a study of flow phenomena and residence time dist ribution (RTD) measurements. Mixing effects are reported and their influenc es on viscous dissipation, residence time, curve spread and stagnancy are e xplained. (C) 1999 Elsevier Science S.A. All rights reserved.