Single screw extruder, improvement mixing

Single-screw extruders have been widely used for blend preparation however, they do not offer sufficiently high stress levels to improve mixing thus, special designs of screws have been developed such as those with mixing heads or barrier zones that increase residence time and enhance blend mixing. 

These dependencies are shown in Fig. 9.2. Evidently, simple shear is inefficient for generation of a large inteifacial area (for smaller deformatiOTis, R < 10, the inefficiency is not as dramatic). The inefficiency can be reduced if a large deformation is obtained not at once, but in a series of smaller deformation steps. The best formula is stretch-twist-fold, the second best is stretch-and-fold, and the third is stretch-and-randomize. Thus, to improve the mixing efficiency of the processing units that operate in the shear mode (like in a single-screw extruder), the flow should be interrupted and the flow lines randomized. For uniform shear strains of randomly oriented elements, the interfacial area increases linearly with strain, and when the process is repeated, the total increase of the interfacial area becomes proportional to the square of strain (y ), etc. ... 

When single-screw extruders are starve fed (Fig. 5.12d), plastic particles do not immediately fill the screw channel. As a result, the first few channels of the feed zone lack the pressure required to compact the polymer particles. Particle conveyance in the imfilled channels is not as steady as transport with filled channels. Consequently, metered feeding is seldom used with single-screw extruders. Such feeding can be used to reduce the motor load, limit temperature rises, add several components through the same hopper, improve mixing in singlescrew extruders, control flow into vented barrel extruders, and feed low-bulk-density materials. 

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