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The Materials Processed in Extruders

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The materials processed in cooker-extruders are particulate moist solids or high viscosity dough-like fluids. As the screw rotates, the flights drag the material towards the exit. The flow channel, described above, is delimited by two solid surfaces, namely, the screw and the barrel. Friction with the moving material occurs on both surfaces. Ideally, the friction at the barrel surface should be the strongest of the two, in order for internal shear to occur. Were the friction weak at the barrel surface, e.g. as a result of lubrication, and strong at the screw surface, the material would stick to the screw and turn with it, without shear and without forward movement. Grooving helps reduce slippage at the barrel surface.

Screw configuration is such that the flow area along the flow channel is progressively reduced. Consequently, the material is progressively compressed as it moves down the barrel. ‘Compression ratio’ is the ratio of the cross-section area of the flow channel at the feed end to that at the exit end. Reduction of the flow area can be achieved by several types of screw configurations (Harper, 1987). The most common are the progressively decreasing screw pitch and the progressively increasing root (core) diameter (Figure 15.2). Screw configurations corresponding to compression ratios between 2 and 4 are common. The pressure developed in a cooking extruder can be in the order of a few MPa.


Figure 15.2


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