In order to make feed nutrients of extruded sinking fish feed be absorbed by fishes to the greatest extent, extruded sinking fish feed made by fish feed extruder should have good stability. On the other hand, because the digestive tract of fishes is short, the ripening degree of extruded sinking fish feed has great influence on the growth of fishes. So the ripening degree is an important factor for identifying feed quality. Sinking characteristic of sinking fish feed in another important factor. Good quality sinking fish feeds can all sink in the water and the sinking speed is controlled strictly.
I. Extruded Sinking Fish Feed Processing Technology of Fish Feed Extruder
1.1 Process method of extruded sinking fish feed by fish feed extruder
The main manufacture equipment of processing extruded sinking fish feed is fish feed extruder. The proportional prepared feed ingredients such as starch and ground raw materials are mixed with a certain amount of water and steam thoroughly and then they are fed into the extrusion chamber. Under the pushing action of extrusion screw, the temperature rises and the pressure of extrusion chamber is increased, so raw materials can be ripened. When the raw materials of high temperature and under high pressure are extruded from the die mold, the pressure is decreased in instantaneous and under the influence of pressure difference, the moisture content in raw materials is vaporized rapidly to make raw materials inflate. At the same time, the extruded fish feeds are cut by the knife of fish feed extruder to a appropriate size.
1.2 Specifications of extruded sinking fish feed made by fish feed extruder
The quality extruded sinking fish feed made by fish feed extruder must meet the following two requirements:
● The sinking rate should reach 98% or greater.
The test method of sinking rate is to choose one hundred of dried extruded fish feed pellets randomly and then put them in to 3.5% salt water with temperature of 20 ℃ and record the number of sinking fish feed within 30 seconds
● The time of stability in the water is up to two hours or more. The test method of the stability time is to choose one hundred of dried extruded fish feed pellets randomly and put them in the water to start timing until there are more than five fish feed pellets begin to disperse and can not be picked up. This time period is called the stability time of extruded sinking fish feed. The stability of extrude sinking fish feed is related to the dietary habit of fishes.
In the view of environment, the longer the stability time of extruded fish feed in the water, the more advantageous to eat for fishes, which can prevent feed disperse and cause the water pollution.
1.3 Expansion degree of sinking fish feed pellets
The expansion degree of floating fish feed pellets is 1.5-2.1 times of their original size; while sinking fish feed pellets almost do not expand, the expansion degree is 1.0-1.3, namely, the size of finished sinking fish feed pellets is a little larger than the size of die mold hole.
II. How to Make Sinking Fish Feed Pellets Efficiently by Fish Feed Extruder
The major methods for improving sinking fish feed processing efficiency of fish feed extruder are as follows.
● Feed formula of sinking fish feed pellets
When designing sinking fish feed formula, the starch must be contained and the content should be low, generally between 10% and 20%, and 15% is the most suitable proportion. Because the starch can influence water resistence of fish feed pellets and if there is little starch, there will be more fine and so affect the appearance of fish feed pellets; in addition, the fat content of sinking fish feed formula should be higher than floating fish feed pellets, about 5-8% in common; and plant raw material are more easier to extrude than animal raw materials.
● Die mold of fish feed extruder
The aperture ratio of die mold for making sinking fish feed pellets is 40% higher than that for making floating fish feed pellets, because when making sinking fish feed pellets, the pressure difference of extrusion chamber is smaller, so the extrusion rate is lower. Besides, the length-diameter ratio of die mold for making sinking fish feed pellets should be larger than that for making floating fish feed pellets, about 1:1.8 in common.
Die Mold of Fish Feed Extruder
● Screw specifications
Making sinking fish feed pellets by fish feed extruder requires that the residence time of raw materials in the extrusion chamber should be short. The residence time for making floating fish feed pellets is 30-35 seconds and for making sinking fish feed pellets is 20-30 seconds when using the same fish feed extruder. The shearing force should be small and the discharge temperature is low relatively. Therefore the screw for making sinking fish feed pellets can not be faulted through screw and the pressure relief opening should be open as far as possible. Furthermore, the part of screw in the direction of discharge port should be steep to make raw materials be discharged quickly.
● Control of moisture content
You should add more water to raw materials in the conditioner when making sinking fish feed pellets than that for making floating fish feed pellets. The moisture content of raw materials for making floating fish feed discharged just from conditioner is about 25%; for making sinking fish feed is about 28%. You can add 2-3% water to raw materials in the extrusion chamber too, which is more favorable for making sinking fish feed pellet.
● Control of conditioning temperature
Generally speaking, there is no big difference of conditioning temperature when making sinking fish feed pellets and floating fish feed pellets, and the conditioning temperature is all above 90℃. High conditioning temperature is good for ripening starch. The ripening rate for making sinking fish feed is 70-85% and for making floating fish feed is 80-95%. However due to the different formula design, the conditioning time for some sinking feed formula can not reach 90℃ and only 60-80℃. Under this situation, water adding volume in the conditioner can be decreased and at the same time the water adding volume can be increased in the extrudion chamber. Then the conditioning temperature can be increased.
● Regulate rotation speed of main engine
The rotation speed of main shaft for making sinking fish feed pellets is 250-400 r/min( this is for double-screw fish feed extruder with speed control device); single screw fish feed extruder does not have speed control device, the rotation speed of its main shaft is about 350 r/min. The higher the rotation speed, the higher the shear force, so there will be more mechanical energy for making sinking fish feed.
● Control of fish feed pellets drying machine
Many people think that the higher the temperature of fish feed pellet drying machine, the better drying effect. But when drying sinking fish feed, it is a different story. We must pay more attention on the second ripening of components of sinking fish feed pellets. That is to say, if the temperature of sinking fish feed extruded just from discharge port is 80℃ which is lower than the temperature 120℃ of drying machine, the starch in the sinking fish feed pellets will be ripened for the second time, and this will cause the extruded sinking fish feed pellets to become floating fish feed pellets after drying; furthermore, if the temperature of fish feed pellet drying machine is too high, the moisture content of dried fish feed pellets will uneven,(moisture content of pellet surface and moisture content inside of feed pellets). Thus when drying sinking fish feed pellets, it requires low temperature and slow drying speed. The drying temperature is controlled within 100℃, the best is within 90℃, and the belt speed is as slow as possible(40-60minutes).