Twin Screw Extrusion

Single screw extruders are practical for simple, low cost extrusion operations. The demand for more diverse food quality products that require a expanded range of cooking applications, ingredients, and better control of processing variables has led to a increased use of twin screw extruders (Harper, 1989). Twin screw extruders are generally classified in two ways; by the degree of intermeshing between the two screws and by the direction in which they rotate. The screws are either rotating in the same direction (co-rotating) or in opposite directions (counter-rotating). Twin screw extruders are more complex and more expensive than single screw extruders, but they allow the user more control over extrusion parameters such as residence time and shear (Schüler, 1986).

Twin screw extruders can also be classified by the path of flow in the extruder channel. The flow can move either across or lengthwise in the channel or both depending upon the screw configuration as shown in Figure 3 (Schüler, 1986). In a lengthwise open extruder, the material is conveyed from inlet to outlet moving from the channel of one screw to the channel of another. In a crosswise open extruder, there is an area common to both screws consisting of a path across the flights. Material may be exchanged from one flight to the other flight in a direction perpendicular to the screw channel (Schuler, 1986; Harper, 1989). Whether a screw configuration is lengthwise or crosswise opened or closed, will effect the conveying conditions, mixing action, and the pressure build-up capacity of the screw system (Schuler, 1986). As shown in Figure 3, co-rotating intermeshing screws can not be lengthwise and crosswise closed, and partially intermeshing co-rotating screws can not be lengthwise open and crosswise closed. In contrast, a counter-rotating intermeshing lengthwise open and crosswise closed system does not exist either (Schuler, 1986; Harper, 1989).

The action of the intermeshing counter-rotating screw and the intermeshing co-rotating screw differ significantly. In the counter rotating system, the screws roll off each other in the wedge area, whereas the co-rotating screws are always in contact with each other creating a natural wiping action (Schuler, 1986). Counter-rotating fully intermeshing twin screw extruders have a positive displacement characteristic, but they require reduced screw speeds to lower the separating forces that cause wear on the screw by the calendaring effect at the nip between the screws (Harper, 1989). These types of extruders are most commonly used in the plastic industry for their ability to process low viscous materials requiring low screw speeds and long residence times. Counter-rotating fully intermeshing twin screw extruders are not widely used in the food industry, but are utilized in the production of gum, jelly and licorice confectionery products which have plastic-like characteristics (Dziezak, 1989).

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