At present, the aquaculture extruder has gained popularity in the field of aquatic feed. As a high-end product with high feed intake rate, high conversion rate and more environmental protection requirements, the expanded aquatic feed has been more and more accepted by the market. With the further improvement of the expansion technology, many manufacturers have changed the small-grain aquaculture feed originally processed by the crushing process into a direct-use extruder. The molds for producing small-grain aquatic feeds are generally 1.2mm, 1.0mm and 0.8mm, and some even The manufacturer requires processing of expanded pellets with a die diameter of 0.6 mm.
The advantage of using small particles processed by the crushing process is that the production is relatively convenient and the cost is relatively low, but the high cost of the formula, the large amount of powder, the poor stability of the water and the many materials returned to the machine make the people more inclined to use the extruder directly. produce. The small particles are directly expanded by using a puffing machine, and have many advantages such as low formulation cost, low powder content, good water stability, and beautiful appearance, but the production is relatively troublesome and the processing cost is high, especially the processing die aperture is 0.6. When the expanded pellets of mm are more demanding due to lower productivity and stricter process configuration, the cost is higher.
In this paper, the mold is 1.0mm, and the extruder uses a single-screw extruder to produce small-diameter granulated puffed aquatic feed as an example. The process configuration requirements, process parameters and related precautions in the production process are briefly introduced.
1 Requirements for fineness before puffing of raw materials Processing small-sized puffed aquatic feed has extremely strict requirements on the fineness of crushing of raw materials. Theoretically speaking, when the pulverization fineness of the raw material reaches 100% and passes through the 60-mesh sieve, it can be ensured that the extruder having a die hole of 1.0 mm can be normally produced. However, it has been proved that when such a crushing fineness is produced, after the extruder is operated for a period of time, the die hole starts to clog, the expansion coefficient of the particles gradually becomes larger, and the expanded particles become longer. When more die holes are blocked, and the speed of the cutter reaches the limit, the length of the expanded pellets will not be able to meet the requirements.
Therefore, in order to be able to stably produce for a long time, it is recommended that the pulverization fineness of the raw material reaches 100% over 80 mesh sieve, the minimum is 98%, and the maximum particle size of the sieve material must not exceed 0.3 mm. Under such fineness, the raw material can ensure stable production of the extruder for a long time, and at the same time, the raw material is relatively smooth and beautiful, and the appearance of the expanded particles is relatively smooth and beautiful.
Table 3-1 Comparison of advantages and disadvantages of processing small particles by crushing process and direct expansion process
2 The equipment before the expansion of the chemical section and the fineness of the raw material before the expansion of the raw material are not only related to the crushing equipment, but also involve the screening equipment, that is, the fineness of the final raw material is jointly guaranteed by the crushing equipment and the screening equipment.
2.1 Requirements for crushing equipment and control The choice of crushing equipment is the key to the entire process design. The pulverization fineness of the raw material is required to reach 80 mesh. At present, a vertical ultrafine pulverizer is generally selected, and the current hammer pulverizer cannot meet the requirements. Moreover, the raw materials are required to achieve such fineness, and a strict handling requirement is required for the used superfine pulverizer.
Taking the micro-pulverizer as an example, it is necessary to control the fineness of the raw material by controlling the rotation speed of the classification wheel on the ultrafine pulverizer. For the general fish material formula, when it is required to achieve such a fineness, the frequency converter of the classifying wheel is generally controlled at about 40 Hz; at the same time, the damper opening degree of the fan is reduced, and the general door opening degree is about 20%. Under such a condition, the feeding amount is gradually increased, and the load rate of the main motor is increased. When the main motor load reaches 90% or more, the equipment is stably produced. Under such a working condition, the fineness of the material from the pulverizer can generally reach 80 mesh or more.
2.2 Requirements and control equipment of sieving equipment After pulverization, there will be a small amount of large particles mixed with impurities, and there are some impurities in the whole process. In addition, during the pulverization process, the fibers in the material are relatively small in bulk density. Some of the fibers are not comminuted very well. Some of these impurities and fibers can be "melted" during the extrusion process of the extruder, but some of them will block the die hole before reaching the die hole. Some fibers will hang on the cutter even if they pass through the die hole, thus affecting The cutting of the puffed product by the cutter causes the appearance of the puffed particles to be cut to decrease.
Therefore, it is necessary to set up a good sifting equipment in the process before the expansion of the chemical section. It is currently a good practice to arrange a high square screen after comminuting the equipment and to configure an ultra-segmented stage screen in front of the chamber to be expanded.
The sieve mesh of the high square sieve is generally between 40 and 60 mesh. For formulas with low oil content and good fluidity in the formula, a 60-mesh sieve can be selected. For formulas with high oil content and poor material flowability, a 40-mesh sieve can be selected.
The setting of the high square sieve can clean the impurities in the material well, but the fibers in the material can not be cleaned well. At the same time, there is a long process flow between the high-square screening and the puffing warehouse. The small material addition is generally set in this process. In this process, the addition of small materials and equipment maintenance will also be carried out. Bringing debris to the material. Therefore, due to the above two reasons, it is necessary to add a super-segmented grade sieve in front of the warehouse to be expanded, and a super-subdivided sieve screen generally selects a 60-mesh sieve.
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3 Requirements of raw material formula 3.1 Requirements of starch content in raw material formula In order to better understand the relationship between starch content, die hole and floating characteristics of common freshwater floating aquatic feed, we obtained some data through two sets of comparative experiments. Flour is used as the raw material for starch in this test.
The first set of experiments was to use the same formulation to process floating aquafeeds using templates of different pore sizes with the same process parameters, with a starch content of 16% and a fat content of 3%. The main purpose of this group of experiments is to understand the effect of different pore sizes on the floatation when the starch content of ordinary freshwater formula is less than 20%. The data is shown in Table 3-2.
The second set of experiments was to use different formulas to process floating aquafeeds using the same specification of small pore size templates with the same process parameters. Understand the minimum amount of starch in the formulation of a 100% floating floating feed under the same specification of a small aperture template. The data is shown in Table 3-3.
From the above two sets of comparative experiments, the following conclusions can be drawn: 1 For the formulation of ordinary floating aquaculture feed with low oil content using large pore size template, the starch content can be <20%; 2 processing small pore size common floating fish formula requirements The starch content must be ≥ 20%.
3.2 Effects of wheat, corn and flour on puffing For the starch-containing raw materials in the formula, the author thinks that it is better to use flour. I have done a test in which different starch-containing materials are used to process small particles of 1.0 mm. When the other raw materials are unchanged, the starch-containing raw materials are added with 26% corn, wheat and flour, respectively. Such different formulations were produced using a single-screw extruder. Under the premise of consistent processing parameters, it was found that the floating rates of the three were not as good as wheat, wheat was not as good as flour, and the floating rate of flour was over 98%. Corn is only about 80%. At the same time, it was found that when the corn was processed into a starch raw material, more fiber paste was formed on the blade, which caused the slit to be irregular and easy to pull the surface of the particle.
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