The twin-screw extruder is a kind of multi-screw extruder, which is developed on the basis of a single-screw extruder. In the barrel of the twin-screw extruder, two screws are placed side by side. Therefore, it is called twin-screw extruder. According to the relative position of the screw, it can be divided into meshing type and non-engaging type. The meshing type can be divided into partial meshing type and full meshing type. According to the direction of rotation of the screw, it can be divided into two types: the same direction rotation and the reverse rotation type. It can be divided into inward and outward. The nature of the co-rotating twin-screw pressure zone is different, and the material is rotated by the screw in the inner cavity of the sleeve to generate a high pressure zone and a low pressure zone. Obviously, the material will flow from the high pressure zone to the low pressure zone in two directions: first, the wear of the screw with the screw is reduced, and the material is transported, sheared, mixed, and heated by the casing of the barrel. Under the action of high pressure, it matures and is finally extruded outside the cylinder.
 Reverse-rotating twin-screw extruders generally use two screws of identical dimensions but with opposite threads. The difference between the inward rotation and the outward rotation is that the position of the pressure zone is different. The pressure generated by the inward rotation of the twin screw is high and low. When the material passes through the twin screw, extremely high pressure is generated at the inlet, which makes feeding difficult. At present, this inward reverse rotation type is rarely used; the pressure generated by the twin screw outward rotation is upper and lower, which is favorable for feeding. However, compared with the co-rotation, the C -shaped material flow formed by the material in the screw cannot be moved from one screw to the other, the mixing degree of the material is significantly reduced, and the self-cleaning ability is not rotated in the same direction. The twin screw is effective and stable.
The twin-screw extruder has the advantages of strong adaptability, slip transport and self-cleaning, but its structure is complex, the investment cost is high, and the corresponding maintenance and operation costs are also high. Therefore, twin-screw extruders are generally used in the production of high value-added aquatic and pet food, such as squid, turtle and juvenile fish, because these products are sold on the market at a price sufficient to return twin-screw technology. The cost of manufacturing the product; other special aquatic feeds such as particulate aquatic feed ( 0.8 to 1.5 mm in diameter ), high-fat aquatic materials and feeds with small production volumes but often changing formulations are also produced by twin-screw extruders.
The twin-screw extruder has a relatively late development in China. For the twin-screw extruder of medium-sized and above, due to the large variation range of raw material characteristics, the screw speed needs to be adjusted within a large range, because its working principle is different from that of single The screw extrusion extruder has a large difference in the specific structure, especially the arrangement of the barrel, the screw, the thrust bearing and the gear box is complicated, resulting in an increase in equipment cost. The left and right C -shaped material streams are formed along the inner wall of the sleeve , which is the main flow of the material; the other is the reverse flow formed by the gap of the screw engaging portion. The reason for the countercurrent is that the left screw pulls the material into the meshing gap, and the right screw pulls the material out of the gap. As a result, the material advances in a "∞" shape, changing the flow direction. This not only contributes to the mixing and homogenization of the material, but also causes grinding ( ie shearing ) and rolling between the slots of the screw, resulting in a calendering effect, which is much smaller than the reverse screw calendering effect. .
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