Factors of affecting the length of pellets in feed pelletizer machinery

1 The influence of the parameters of the granulator itself

      Suppose the output of the granulator is Q (kg/h), the inner diameter of the ring die is D (mm), the effective width of the ring die is W (mm), the linear speed of the ring die is V (m/s), and the speed of the ring die is n (R/min), the hole diameter of the ring die is d (mm), the opening rate of the ring die is ψ, the number of openings is N (pieces), the length of the produced particles when they are not cut is L (mm), and the particle density Is ρ (kg/ m3)

1.1 The impact of granulator output

      It can be seen from the formula that the particle length L is directly proportional to the output Q. With other parameters unchanged, the higher the output, the longer the particles, and vice versa. Therefore, in order to meet the specific pellet length requirements, adjusting the output of the pellet mill is a possible method of choice. In actual feed production, feed manufacturers do the same, and sometimes even sacrifice a lot of output to meet the pellet length requirements. , This issue will be mentioned later in the discussion.

1.2 The influence of ring die linear speed or rotation speed

      It can be seen from the formula that the particle length L is inversely proportional to the ring die linear velocity V or the ring die speed n. Under the condition that the feed rate, that is, the output Q, remains unchanged, the faster the ring die speed, the more instantaneously extruded particles Shorter, the longer it is. For granulators of different sizes, the speed of the ring die varies greatly. Generally, the speed of small granulators is high, and the speed of large granulators is low, but the linear speed of the ring die is controlled within an appropriate range, generally 6 m/s~9m/s to meet the needs of granulation. When the linear velocity of the ring die is low, the quality of the pellets produced is high, but the pellets produced may be longer than required. At the same time, too small linear velocity will affect the output; when the linear velocity of the ring die is high, it is beneficial to the output. However, the pellets produced may be shorter than required, and the quality of pellets will become worse. This requires feed manufacturers to choose a suitable ring die line speed according to different feed varieties. The method can be adopted Change the transmission ratio of the granulator transmission system to achieve.

1.3 The influence of ring die diameter and opening rate

      The ring die aperture d does not directly affect the particle length L. It and the number of openings N together have an effect on the length L, that is, the size of the ring die aperture ratio ψ, because the aperture ratio ψ is proportional to the square of the ring die aperture d The product of the number of openings N. The greater the opening rate of the ring die, the shorter the particles produced, and vice versa. The opening rate of the ring die is different when it corresponds to different ring die diameters. For example, a ring die with a diameter of ф1.8mm has an opening rate of about 25%, and a ring die with a diameter of ф5mm has an opening rate. About 38%, generally the larger the pore size, the higher the opening rate. In actual production, especially when producing small diameter pellets, such as producing ф1.8mm shrimp feed, some users will complain that the pellets produced are too long, because the corresponding ring die opening rate is low when the diameter is small One of the solutions is to sacrifice a part of the output, or to increase the linear speed of the ring die discussed in the previous section. Another method is to adjust the cutter, etc. Discussed in the following content.

1.4 The influence of the inner diameter of the ring die, the effective width of the ring die and the particle density

      For a granulator, the inner diameter D of the ring die and the effective width W of the ring die are relatively fixed parameters, which generally do not change, and it is not easy for users to change them, so their influence is not discussed here. The density of the pellets is related to the raw materials for pelleting, and also related to the compression ratio of the ring die. The larger the compression ratio, the stronger the pellets produced and the greater the density. But for general products, the density of the pellets is not much different. Therefore, the particle density is not a major influencing parameter, so I won’t discuss it too much here. In short, for the same granulation material, the greater the density of the produced particles, the shorter the particle length.

2. Effect of cutter

2.1 The influence of the number of cutters

      The number of cutters is usually determined by the number of press rollers of the granulator. Generally, the granulator has 2~3 press rollers. Each press roller is equipped with a cutter. If each cutter is used, the ring Every time the die rotates one round, a certain amount of incoming material is extruded and then cut by the cutter 2 or 3 times accordingly. Therefore, if the output is constant, the granulator with three cutters is better than the granulator with two cutters. The granules produced by the mechanism are shorter, which means that the three-roll granulator is more conducive to controlling the length of the granule than the two-roll granulator. This is also true in actual use, such as when producing small-sized aquatic materials. Due to the short length requirement, a three-roller granulator is usually used and three cutters are used for cutting.

2.2 The influence of cutter structure and adjustment

      Cutters are generally divided into hard blade type and thin blade type. Hard blades have good wear resistance and poor toughness, and are suitable for large-diameter livestock and poultry materials; thin blades have good toughness and poor wear resistance, and are suitable for aquatic materials with small diameters. When adjusting, the hard blade should generally be adjusted so that the blade is about 5mm away from the outer surface of the ring die. If the distance is too small, the powder will increase, and the blade may be damaged. If it is too large, the length of the particles will not be consistent, and long particles may appear. As the distance increases, the bending moment of the knife edge on the particles increases, and the particles may break off the surface of the ring die. Because of its elasticity, the thin blade can be adjusted to a position close to the outer surface of the ring die, so that the cut particles are neat and consistent, which is especially suitable for small-diameter aquatic materials such as shrimp pellets. In actual production, the adjustment of the cutter is very flexible, and one, two or three cutters can be used for cutting as needed. Another situation is that in theory, when two or three cutters are used at the same time, the cutting edge should be adjusted to a position equal to the distance from the outer surface of the ring die, so that the cut particles are neat, but the actual situation is not In this way, because the pellet mill may have defects in distribution when feeding, the amount of feed distributed to each pressure roller cannot be guaranteed to be completely consistent, so that the length of the pellets extruded in each extrusion zone is different , Some extruded particles in the extrusion zone are longer, and some extruded in the extrusion zone are shorter, but they are basically the same in an extrusion zone. In this case, each cutter should be adjusted separately Position, so that the cutter at the press roll with less feed distribution is closer to the surface of the ring die, and the cutter at the press roll with more feed distribution is farther away from the surface of the ring die. The length of the particles produced on the above is the same.

3. Influence of ring die hole structure

      There are two commonly used ring die hole structures, straight cylindrical holes (Figure a) and stepped cylindrical holes (Figure b).

      The length of the particles produced by the straight cylindrical hole is more uniform (as shown in Figure a), while the particles produced by the stepped cylindrical hole often have the phenomenon of individual long particles (as shown in Figure b). Its formation mechanism is In this way: the reason for the stepped cylindrical hole is that the actual granulation requires relatively small compression, and the ring die cannot be made too thin in thickness due to the need for strength, and can only be used to pass through the original cylindrical hole. The method of expanding the hole to achieve the purpose not only guarantees the compression ratio of the ring die, but also meets the strength requirements. However, the consequence of such a compromise is that the cutter is cutting because the cutting point is away from the particle bending point. The distance increases. In the case that the cutter is not sharp enough, the particles may break from the junction of the stepped holes and become longer than ordinary particles. This situation is even worse when the outer stepped holes are drilled deeper. obvious. The solution is to change the feed formula, use a larger compression ratio as much as possible, do not use step holes or make it as short as possible; or use a sharper cutter, and try to be as close as possible to the ring die; or adjust the feed moisture to be larger to make the extrusion The brittleness of the particles is reduced, and the texture becomes soft and not easy to break.

4 .The influence of the outer surface structure of the pressure roller

　　The structure of the outer surface of the pressure roller mainly includes tooth groove type, tooth groove type with sealing edge and honeycomb type. The tooth groove type pressure roller has good coil material performance and is widely used in livestock and poultry feed plants. However, because the feed slides in the tooth groove, the pressure roller and the ring die wear unevenly, and wear on the two ends of the pressure roller and the ring die. For a long time, the two ends of the ring die are difficult to discharge, and the produced particles are shorter than the middle part of the ring die. The tooth groove type pressure roller with sealing edge is mainly suitable for the production of aquatic material. The aquatic material is easy to slide when squeezed. Because the tooth groove is sealed on both sides, it is not easy to slide to both sides when the feed is squeezed. It is more uniform, and the wear of the pressure roller and the ring die is also more uniform, so that the length of the particles produced is more consistent. The advantages of the honeycomb press roll are that the ring die is uniformly worn and the length of the produced particles is relatively consistent, but the performance of the coil is poor, which affects the output of the pelletizer, and it is not as common as the tooth groove type in actual production.

5 .The influence of the screening system after granulation

      The granulated granules must be sieved to remove unnecessary large particles, small particles and powders to obtain qualified products with uniform length. Feed mills generally use rotary sizing screens or vibrating sizing screens for particle screening. The grading screen is generally equipped with two upper and lower screens, the upper screen is the longer particles or large impurities, the lower screen is the finished material, and the lower screen is the smaller length Therefore, for an existing grading screen, how to configure the screen reasonably is the key to affecting the uniformity of the length of the finished material. If the feed manufacturer requires a higher uniformity of the length of the finished material, it must remove more large particles and small particles, and get less finished materials. On the contrary, more finished materials are obtained, which depends on each production. The manufacturer controls it by himself.