Twin-screw extrusion puffing machine is a multi-screw extrusion puffing machine, is developed on the basis of the single-screw extrusion puffing machine, in the barrel of the twin-screw extrusion puffing machine, side by side placement of two screws, so called twin-screw extrusion puffing machine. According to the relative position of the screw can be divided into meshing type and non-meshing type, meshing type can be divided into partial meshing type and full meshing type; according to the direction of rotation of the screw can be divided into the same rotation and reverse rotation two types, reverse rotation can be divided into inward and outward two.
Isotropic twin-screw pressure zones are different in nature, the material in the inner cavity of the sleeve by the rotation of the screw, resulting in a high-pressure zone and low-pressure zone. Obviously, the material will flow in two directions from the high-pressure area to the low-pressure area: one is with the direction of screw rotation along the inner wall of the sleeve to form the left and right two C-shaped material flow, which is the mainstream of the material; the other is through the gap between the screw engagement part to form the reverse flow. The reason for the reverse flow is that the left screw pulls the material into the gap, and the right screw pulls the material out of the gap, resulting in a "∞" shape, changing the direction of material flow. This not only helps the mixing and homogenization of materials, but also makes the screw groove between the grinding (i.e., shear) and rolling effect, the calendering effect, this effect is much smaller compared to the reverse screw calendering effect. Of course, the calendering effect is small, the material wear and tear on the screw is also reduced, the material is so through the conveying, shear, mixing and barrel shell heating, at high temperatures, high pressure under the action of maturation, and finally extruded outside the barrel.
Reverse rotation twin-screw extrusion puffing machine generally uses two screws of exactly the same size, but the opposite direction of the thread. The difference between inward rotation and outward rotation is the location of the pressure zone is different, twin-screw inward rotation of the pressure generated by the upper high and lower low, the material through the twin-screw, the inlet will generate extremely high pressure, resulting in feeding difficulties, the current inward reverse rotation type is rarely used; twin-screw outward rotation of the pressure generated by the upper low and lower high, which is conducive to feeding. However, compared with the same rotation, the C-shaped material flow formed in the screw cannot be moved from one screw to the other, and the degree of material mixing is significantly reduced, and its self-cleaning ability is not as effective and stable as that of the twin screw with the same rotation.
Reverse rotation twin-screw because of the pressure difference between the upper and lower, resulting in the screw to the sides of the separation force F, the screw under the action of F pressed to the barrel, accelerating the barrel and screw wear, and the higher the speed, the greater the F, the more serious the wear, thus limiting the speed of the screw; and isotropic twin-screw does not exist to separate the two screw force, so less wear, high-speed operation, and can achieve a high yield, so isotropic rotation Twin screw is widely used.
The source of heat required by the material, in addition to the same part as the single screw, most of it comes from the meshing gap; shear, extrusion and mixing by the meshing threads, generating heat and homogenizing it. The size of the gap has a great impact on the quality of puffing, the gap is small, shear force is large, but the amount of material passed through the reduction; gap is large, the amount of material passed through the increase, but the shear force is reduced. Twin-screw forced conveying and self-cleaning properties, so that the material in the barrel of the short residence time and uniform; twin-screw good mixing properties so that the material gets the heat in time to homogenize, accelerate the degree of maturation of the material, reduce the fluctuations in material temperature, improve the yield and quality of puffed products.
Twin-screw extrusion puffing machine has the advantages of strong adaptability, slip conveying and self-cleaning, but its structure is complex, high investment costs, the corresponding maintenance and operating costs are also higher. Thus, twin-screw extrusion puffers are generally used in the production of those aquatic and pet feeds with high added value, such as the production of eel, snapper and juvenile fish feed, because these products are sold in the market at a price sufficient to return the cost of manufacturing products with twin-screw technology; in addition, some special aquatic feeds such as particulate aquatic feeds (0.8 to 1.5 mm in diameter), high-fat aquatic and small production volume but The recipe often changes the feed also need to use twin-screw extrusion puffing machine for production.
Twin-screw extrusion puffing machine in the domestic development is relatively late, for more than medium-sized twin-screw extrusion puffing machine, due to its raw material characteristics change range, the need to adjust the screw speed in a wide range, due to its working principle is different from the single-screw extrusion puffing machine, there are major differences in the specific structure, especially the barrel, screw, thrust bearing and gear box arrangement is more complex, resulting in increased equipment costs.
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