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Figure 3. The various types of twin screw mechanisms (Ziminski and Eise, 1980).

For the counter-rotating twin screw extruder, non-conjugated screws are useful for mixing sections while conjugated screws and used where pumping is needed (Martelli, 1983). Co-rotating twin screw extruders are the most popular extruders used in the food industry. The advantages of this design are its pumping efficiency, residence time control, self-cleaning mechanisms and uniformity of processing (Schüler, 1986). Intermeshing co-rotating screws convey four to five times more material than intermeshing counter-rotating screws due to the interaction of the two screws that move the material from one screw to the other, resulting in a well mixed product (Hartley, 1984). Hartley (1984) discussed the advantages of the co-rotating screws. The intermeshing co-rotating screw configuration allows for one crest edge to wipe the other screw channel. This occurs at high speeds and allows for a efficient self-cleaning. Hartley (1984) reported that co-rotating screws have a translational motion in which one crest edge wipes a screw flank tangentially with a constant velocity. This can occur at a relatively high speeds and without producing a calendar effect. Therefore, the screws achieve a morre efficient and uniform self cleaning. Kneading discs are also used to aid in conveying the material forward to increase pressure at the die or conveying in reverse to reduce pressure by slowing down the passage of material (Hartley, 1984). Kneading discs also improve the mixing capabilities of the extruder barrel (Schüler, 1986).

There are three different types of screw designs for the fully intermeshing co-rotating twin screw extruders; the single lobe, two lobe and the three lobe. The three lobe design has a low volumetric capacity and is used to obtain high average shear rates and high energy input. The two lobe system is more common to the food industry. It has high conveying rates and relatively low average shear rates which result in a "gentler" treatment of the ingredients (Schüler, 1986). Single lobe designs have very low shear rates and are used when extruding materials that do not have free-flowing characteristics.

As the products flows down the channel, it moves from screw to screw in a "figure eight-like" motion. This results in better heat transfer between the product and the barrel, and better mixing capabilities and temperature control (Hartley, 1984; Schüler, 1986).

The most important feature to the success of the co-rotating twin screw extruder is the sealing profile formed by the intermeshing of the screws. It is this profile that makes the twin screw so unique. Self-cleaning characteristics directly result from the sealing profile, simply because it allows the crest of one screw to wipe the flanks of the other screw at a constant velocity. This provides a uniform shear velocity high enough to effect a wiping action for the bounding layers (Schüler, 1986).

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