Effects Of Processing Parameters

The process parameters for extrusion generally include screw speed, feed moisture, feed rate, and barrel temperature. Changes in these processing parameters will change the motor torque, die pressure, and product temperature. Visual product characteristics such as expansion ratio, specific length (length of extrudate per unit mass), and bulk density also will change.

Increases in screw speed at constant feed rate will cause a quick initial increase in die pressure and motor torque, followed by a sharp decrease to the new equilibrium value (3). The initial quick increase in die pressure and motor torque is due to the effort of the motor drive to overcome the inertia of the screw shaft and the momentary increase in the output. Because of a rapid decrease in the degree of fill due to screw speed increase, both torque and die pressure fall sharply after the initial increase (4,5). The specific energy input increases with increasing screw speed at constant feed rate; therefore, the product temperature increases. Extrudates are usually lower in expansion ratio, higher in specific length, and lower in bulk density with increasing screw speed (4,5).

Increasing feed moisture lowers the torque, specific mechanical energy input, and product temperature (3). Die pressure drops initially due to a lower dough viscosity caused by moisture increase. However, as the product temperature gradually becomes lower, the dough viscosity rises, causing die pressure to recover. Final equilibrium die pressure will depend on the relative effect of feed moisture and product temperature on the dough's viscosity. Extrudates are generally lower in expansion ratio and specific length, but higher in bulk density with a higher feed moisture (6).

An increase in feed rate causes rapid increases in motor torque and die pressure and a slower decrease in product temperature (3,7). Increases in feed rate while maintaining constant screw speed increase the degree of fill in the extruder barrel (8) and, thus, raise motor torque and die pressure (9). Since the specific energy input is decreased with increasing feed rate, the product temperature decreases. Increasing feed rate usually increases both expansion ratio and specific length, resulting in a lower bulk density (10).

Decreasing the barrel temperature causes the product temperature to decrease. A lower product temperature corresponds to a lower dough temperature or a higher dough viscosity and, therefore, a higher motor torque and die pressure (3). The physical characteristics of extrudates from lower barrel temperatures are usually similar to those from higher feed moistures.

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