Types and functions of the hottest extruder

  • Detail

The screw of single screw extruder consists of a shaft connecting screw units of various structures. The whole screw consists of three sections: feeding section, kneading section and melting homogenization section. After entering the barrel from the feeding port, the material undergoes solid transportation, melting and homogenization in the screw, making the material change from a loose state to a continuous plastic dough

in the single screw extrusion cavity, the material is basically closely around the screw, in a spiral continuous strip. 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 co rotate with the screw, which cannot realize the forward extrusion and transportation of the material. When the moisture and oil content of materials are higher, this trend is more obvious. In order to avoid these problems, most single screw extruders now use segmented, single and double screws, combined screws with staggered pressure rings and kneading rings, and inner wall slotted barrel to adapt to the changes of materials in the cavity

single screw extruder is relatively simple in structure and relatively low in price. It has developed early, relatively mature in technology and stable in equipment. It has been used as the main equipment in the production of extruded aquatic feed for more than 40 years and is widely used in the production of low protein adult fish feed such as tilapia, insect without feet fish, Grasscarp, carp, etc. In addition, in the production of low-grade feeds such as pond mixed fish feed, the single screw extruder has obvious advantages. Its raw materials do not need to go through ultra-fine powder, and the particle size of 95% can pass 30-50 mesh, which greatly reduces the equipment investment cost and improves the production efficiency

twin screw extruder

twin screw extruder is a kind of multi screw extruder, which is developed on the basis of single screw extruder. In the barrel of twin screw extruder, two screws are placed side by side, so it is called twin screw extruder. According to the relative position of the screw, it can be divided into meshing type and non meshing type, and the meshing type can be divided into partial meshing type and full meshing type. See Figure 1 for details; According to the rotation direction of the screw, it can be divided into two types: same direction rotation and reverse rotation, and reverse rotation can be divided into inward and outward, as shown in Figure 2. The properties of the pressure zone of the co rotating twin-screw are different. The material is rotated by the screw in the inner cavity of the sleeve to produce high-pressure zone and low-pressure zone. See the positions shown by the symbols "+" and "-" in Figure 3 (a) for details. Obviously, the material will flow from the high-pressure area to the low-pressure area in two directions: first, along the sleeve with the rotation direction of the screw. This achievement will help to develop the portable small dialysis device wall to form two C-shaped material flows (see Figure 4), which is the mainstream of the material; The other is to form countercurrent through the gap of the screw engagement part [see Figure 3 (b)]. The cause of countercurrent is that the left screw pulls the material into the meshing gap, while the right screw pulls the material out of the gap. As a result, the material advances in a "∞" shape and changes the direction of material flow. This is not only conducive to the mixing and homogenization of materials, but also causes grinding (i.e. shearing) and rolling between the teeth and grooves of the screw, resulting in the calendering effect. This effect is much smaller than the calendering effect of reverse screw pressing introduced by experts, material suppliers and medical device manufacturers from all over the world on new technologies and practical solutions. And the wiring is accurate; Of course, due to the small calendering effect, the wear of the material on the screw will also be reduced. At present, the material has passed the pilot production sample, which is matured under the action of high temperature and high pressure after being transported, sheared, mixed and heated by the barrel shell, and finally extruded out of the barrel

reverse rotation twin-screw extruder generally adopts two screws with exactly the same size, but the thread direction is opposite. The difference between inward rotation and outward rotation lies in the different positions of the pressure zone. The pressure generated by the inward rotation of the twin-screw is high and low. When the material passes through the twin-screw, it will generate extremely high pressure at the inlet, causing feeding difficulties. At present, this inward reverse rotation type is rarely used; The pressure generated by the outward rotation of the twin-screw is high, low and high, which is conducive to feeding. However, compared with the same direction rotation, the C-shaped material flow formed by the material in the screw cannot be moved from one screw to another, the mixing degree of the material is significantly reduced, and its self-cleaning ability is not as effective and stable as the same direction rotation twin screw

due to the pressure difference between the upper and lower parts of the counter rotating twin-screw, the separation force F that offsets the screw to both sides is generated. The screw is pressed against the barrel under the action of F, which accelerates the wear of the barrel and the screw. The higher the speed, the greater the F, the more serious the wear, thus limiting the speed of the screw; The co rotating twin-screw has no force to separate the two screws, so it has less wear, can run at high speed, and can achieve high output. Therefore, the co rotating twin-screw is widely used

except for the same part as the single screw, most of the heat source required by the material comes from the meshing gap; It is sheared, extruded and mixed by the meshing thread to generate heat and homogenize the heat. The size of the gap has a great influence on the expansion quality. The gap is small and the shear force is large, but the amount of material passing through is reduced; If the gap is large, the amount of material passing through increases, but the shear force decreases. The forced conveying and self-cleaning characteristics of twin-screw make the materials stay in the barrel for a short and uniform time; The good mixing performance of twin-screw makes the heat of materials homogenize in time, accelerates the ripening degree of materials, reduces the fluctuation of material temperature, and improves the output and quality of expanded products

twin screw extruder has the advantages of strong adaptability, slip conveying and self-cleaning, but its structure is complex, the investment cost is high, and the corresponding maintenance and operation costs are also high. Therefore, the twin-screw extruder is generally used in the production of aquatic products and pet feed with high added value, such as eel, turtle and juvenile fish feed, because the price of these products on the market is enough to repay the cost of twin-screw technology manufacturing products; In addition, some special aquatic feeds, such as particulate aquatic feeds (with a diameter of 0.8-1.5mm), high-fat aquatic feeds and feeds with small production but constant formula, also need to be produced by twin-screw extruder

twin screw extruders are relatively late in domestic development. For medium-sized and above twin-screw extruders, due to the large variation range of raw material characteristics, the screw speed needs to be adjusted in a large range. Because its working principle is different from that of single screw extruders, there are great differences in specific structures, especially the layout of barrel, screw, thrust bearing and gearbox is relatively complex, Resulting in an increase in equipment costs

main functions of large extrusion extruder:

it is a direct extrusion expanded food, which is made of Cereals, potatoes or beans as the main raw materials, extruded by the extruder, and under high temperature and high pressure, using the pressure difference inside and outside the machine to expand the product

direct extrusion puffed food generally does not contain lead, but aluminum may exceed the standard

in order to pursue crisp taste and good appearance, manufacturers often add bulking agent (or yeast), which contains a certain amount of aluminum. During the process of baking or frying, gas is generated by heating, and the bulking agent forms a uniform and dense porous structure inside the food, which increases in volume and makes the product crisp

eating extruded food with excessive aluminum will continuously accumulate in the human body, cause diseases of the nervous system, interfere with people's thinking, consciousness and memory functions, and may lead to dementia in severe cases. Excessive intake of aluminum may also lead to the loss of calcium deposited in bone, inhibit bone formation, and osteomalacia. Experts said that children are more vulnerable than adults to extruded food with excessive aluminum

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