At present, most feed mills in China use ring mold granulation to produce aquatic feed. The hard pellet feed produced by ring die granulation sinks into the bottom of the water for 10-20 minutes, which is broken, and the feeding ability is poor, and some of it remains in the water environment, which is seriously polluted by water; while the extruded bulk feed passes through high temperature and high pressure, which is a kind of low pollution. High-quality environmentally-friendly feed with low waste and high conversion rate. Therefore, the aquaculture material has gradually replaced the hard granules into the mainstream of aquaculture feed. The production process of aquatic expanded feed generally includes raw material pulverization, mixing, secondary micropowder, extrusion puffing, drying, cooling and other processes. Extrusion and puffing plays an important role in the whole process, and affects feed quality and production efficiency. Big. To this end, this paper discusses the working characteristics and rational selection of single and twin-screw aquatic feed extrusion extruders, which provides a reference for the correct application of aquatic feed extrusion extruders, so that extrusion technology can play a role in production. It can increase production and reduce consumption, and improve the nutrient digestibility and energy utilization rate during feeding.
1 Structural characteristics and working principle of extrusion extruder
The extrusion extruder is mainly composed of a barrel and a screw which is rotated and expanded in the extruder barrel, and has two types: a single screw extrusion extruder and a twin screw extrusion extruder. During the extrusion process, the material enters the cavity from the feed port and is pushed, mixed, compressed and sheared as it is pushed forward by the screw. Due to the driving force, friction and shearing force and external heating, the material is heated by pressure, reaches a high temperature and high pressure state, and matures into a modified paste. When the material is extruded from the front die hole, the temperature and pressure suddenly drop to the normal temperature and normal pressure state, so that the moisture in the material is rapidly vaporized and evaporated, and the volume rapidly expands to become a puff.
1.1 Single screw extrusion extruder
The screw of the single-screw extruder is composed of a shaft connecting the screw units of various structures.The entire screw consists of three sections: the feed section, the twist section and the melt homogenization section.After the material enters the barrel from the feed port, it undergoes a solid conveying, melting and homogenization process in the screw to convert the material from a loose state to a continuous plastic dough shape.
In the single-screw extrusion chamber, the material is substantially tightly wrapped around the screw, in the form of a spiral continuous strip. When the screw rotates, the material moves forward along the spiral like a nut, but when the friction between the material and the screw is greater than the material When the friction with the barrel, the material will co-rotate with the screw, which can not achieve the forward extrusion and conveying of the material.This trend becomes more pronounced as the moisture and oil content of the material is higher. In order to avoid these problems, most of the single-screw extruders now use segmented, single and double helix, combined screw and inner wall slotting cylinders with staggered pressure ring and kneading ring to adapt to changes in materials in the machine cavity. Happening.
The structure of single-screw extruder has relatively simple structure and relatively low price. Its development is relatively early, the technology is relatively mature, and the equipment is stable. It has been used as the main equipment in the production of expanded aquatic feed for more than 40 years and is widely used in Luo. Non-fish, squid, grass carp, squid and other low-protein adult fish feed production. In addition, in the production of low-grade feed such as pond mixed fish, the single-screw extruder has obvious advantages. The raw material does not need to pass ultra-fine powder, and the particle size of 95% can pass 30-50 mesh, which greatly reduces the investment cost of equipment. Increased production efficiency.
1.2 Twin-screw extruder
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-intermeshing type. The meshing type can be divided into partial meshing type and full meshing type, as shown in Fig. 1; according to the direction of rotation of the screw, it can be divided into two directions: the same direction rotation and the reverse rotation type. Class, reverse rotation can be divided into inward and outward, as shown in Figure 2. 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, as shown in the symbol “+” and “-” in Fig. 3(a). . Obviously, the material will flow from the high pressure zone to the low pressure zone in two directions: one is to form two left and right C-shaped material flows along the inner wall of the sleeve with the direction of rotation of the screw (see Figure 4), which is the mainstream of the material; A backflow is formed by the gap of the meshing portion of the screw [see Fig. 3(b)]. 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, and the rolling effect occurs, which is much smaller than the reverse screw rolling effect. . Of course, the calendering effect is small, and the wear of the material on the screw is also reduced. The material is conveyed, sheared, mixed, and heated by the outer casing of the barrel, and matured under the action of high temperature and high pressure, and finally extruded outside the cylinder.
Reverse-rotating twin-screw extruders generally use two screws of exactly the same size 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 counter-rotating twin-screw has a separation force F that shifts the screw to both sides due to the pressure difference between the upper and lower sides. The screw is pressed against the barrel under the action of F, which accelerates the wear of the barrel and the screw, and the higher the rotational speed, the more F Large, the more severe the wear, thus limiting the speed of the screw; while the twin-screw in the same direction does not have the force to separate the two screws, so the wear is small, the speed can be operated at a high speed, and the high output can be achieved, so the twin-screw is rotated in the same direction. More widely used.
The source of heat required for the material, except for the same portion as the single screw, is mostly from the meshing gap; the shearing, squeezing and mixing of the meshed threads generates heat and homogenizes the heat.The size of the gap has a great influence on the quality of the puffing, the gap is small, the shearing force is large, but the amount of material passing through is reduced; the gap is large, the amount of material passing through is increased, but the shearing force is reduced. The characteristics of twin-screw forced conveying and self-cleaning make the residence time of the material in the barrel short and uniform; the good mixing performance of the twin screw makes the heat obtained by the material timely homogenize, accelerates the maturity of the material, and reduces the fluctuation of the material temperature. Increased production and quality of expanded products.
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 feed, because these products are sold on the market at a price sufficient to return the twin-screw technology. The cost of manufacturing the product; other special aquatic feeds such as micro-aquatic feed (0.8-1.5 mm in diameter), high-fat aquatic products and feeds with small production but often changing formulas are also produced by twin-screw extruder.
The twin-screw extruder has a relatively late development in China. For the twin-screw extruder, the medium-sized twin-screw extruder has a large range of raw material characteristics, which requires the screw speed to be adjusted within a large range. 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. Under the same production capacity, the comparison results of the co-rotating twin-screw extruder and the single-screw extruder are shown in Table 1.