Difference between single screw and twin screw
Single Screw Extruder
The screw of single-screw extruder is composed of a shaft connecting screw units of various structures. The whole screw consists of three sections: feeding section, kneading section and melting and homogenizing section. After the material enters the barrel from the feeding port, it undergoes solid transportation, melting and homogenizing process in the screw, so that the material is transformed from a loose state to a continuous plastic dough.
In the single-screw extrusion chamber, the material is basically closely around the screw, in the form of a spiral continuous band, when the screw rotates, the material moves forward along the screw like a nut, but when the friction between the material and the screw is greater than the friction between the material and the barrel, the material will be co-turned with the screw, and this can not be realized to the forward extrusion of the material and the conveying effect. When the moisture and oil content of the material is higher, this trend is more obvious. In order to avoid these problems, most of the single-screw extrusion extruder now adopts segmented, single and double screw, pressure ring and kneading ring staggered arrangement of the combination of screw and inner wall grooving barrel to adapt to the changing conditions of the material in the cavity.
Twin-screw extrusion puffer
Twin-screw extrusion puffer is a kind of multi-screw extrusion puffer, which is developed on the basis of single-screw extrusion puffer. In the barrel of twin-screw extrusion puffer, two screws are placed side by side, so it is called twin-screw extrusion puffer. According to the relative position of the screws can be divided into meshing and non-meshing type, meshing type can be divided into partially meshing type and fully meshing type; according to the screw's rotating direction can be divided into the same direction of rotation and reverse rotation of two types, reverse rotation can be divided into two kinds of inward and outward.
Co-rotating twin-screw pressure zones are different in nature, the material in the inner cavity of the sleeve by the screw's rotating action, resulting in high-pressure and low-pressure zones. Obviously, the material will flow along 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 screw engagement part of the gap to form a countercurrent. The reason for the countercurrent is that the left screw pulls the material into the gap, and the right screw pulls the material out of the gap, the result is that the material is "∞" shape forward, changing the direction of the material flow. This not only contributes to the mixing and homogenization of materials, but also makes the screw teeth between the grooves to produce grinding (i.e., shear) and rolling effect, the calendering effect, this effect is much smaller compared with the reverse screw calendering effect. Of course, the calendering effect is small, the material on the screw wear and tear is also reduced, the material is so through the transportation, shear, mixing and barrel shell heating, in the high temperature, high pressure to achieve maturity, and finally extruded out of the barrel.
Reverse rotation twin-screw extrusion puffer generally uses two screws of exactly the same size, but with opposite thread directions. The difference between inward and outward rotation lies in the different locations of the pressure zone, the twin-screw inward rotation produces pressure for the upper high and lower low, the material through the twin-screw, in the entrance will produce very high pressure, resulting in feeding difficulties, this inward reverse rotation is rarely used; twin-screw outward rotation produces pressure for the upper low and high, is conducive to the feeding of the material. But the reverse rotation and the same rotation compared to the material in the screw to form a C-shaped material flow can not be moved from one screw to another screw, the material produced by the degree of mixing is significantly reduced, and its self-cleaning ability is not the same as the rotation of the twin-screw effective and stable.
Reverse-rotating twin-screw due to the pressure difference between the top and bottom, resulting in the screw to both sides of the offset separation force F, screw under the action of F pressure to the barrel, accelerating the wear of the barrel and screw, and the higher the rotational speed, the greater the F, the more serious the wear and tear, thus limiting the speed of the screw; and isotropic rotating twin-screw does not exist in the separation of the two screws of the force, so the wear and tear is smaller, can be high-speed operation, and can achieve a very high yield, so the isotropic rotating twin-screw is more widely used. Therefore, co-rotating twin-screw is widely used.
The source of heat required by the material, in addition to the same part of the single screw, most from the meshing gap; by the meshing threads of the shear, extrusion and mixing, heat generation, and heat homogenization. The size of the gap has a great impact on the quality of puffing, the gap is small, the shear force is large, but the amount of material through the reduction; gap is large, the amount of material through the increase, but the shear force is reduced. Twin-screw forced conveying and self-cleaning characteristics, so that the material in the barrel of the short stay and uniform; twin-screw good mixing performance so that the material to get the heat in time to homogenize the material to speed up the degree of maturity, reduce the fluctuation of the material temperature to improve the yield and quality of puffed products.
Twin-screw extrusion puffing machine has the advantages of adaptability, slip conveying and self-cleaning, but its structure is complex, high investment costs, the corresponding maintenance and operating costs are also higher. Therefore, twin-screw extruder is generally used in the production of those aquatic and pet feeds with high value-added; in addition, some special aquatic feeds such as particulate aquatic feed, high-fat aquatic feeds and feeds with small production volume but often changeable formulas need to use twin-screw extruder for production.
