The granulator is the key equipment of the feed production process, and the ring mold is the heart part of the granulator's work and one of the most wearable parts of the granulator. Studying the cause of the ring die failure, improving the use conditions of the ring die, and extending the service life of the ring die have a great impact on improving product quality and output, reducing energy consumption, and reducing production costs.
01. Working principle
The ring die is driven by the motor through the speed reducer. The pressure roller installed in the ring die does not revolve, but it rotates due to friction with the rotating ring die (by compacting the material). The conditioned material that enters the press chamber is divided between the press rollers by the spreader, pinched and squeezed by the press rollers, and continuously extruded through the ring die holes to form columnar particles and follow the ring die The circle rotates and is cut into pellets of a certain length by a cutter fixed outside the ring die. The linear speed of the ring die and the pressure roller is the same at any contact point, and all the pressure is used for granulation.
In the normal working process, the ring die always has friction with the material. As the amount of production material increases, the ring die gradually wears out and eventually causes failure. Therefore, one of the indicators to measure the working performance of the ring die is the production volume. However, in the actual production process, most ring dies failed before reaching the theoretical production capacity.
02. Analysis of failure causes-abrasive wear
There are many causes of wear, including normal wear and abnormal wear. The reasons for normal wear and tear mainly include the formulation of materials, crushing particle size, and the quality of powders. Under normal wear, the ring die wears evenly in the axial direction, resulting in a larger die hole and thinner wall thickness. The main cause of abnormal wear: the pressure roller is adjusted too tightly, and the gap with the ring die is small, and they wear each other; the angle of the spreader is not good, resulting in uneven distribution of materials and part of the first wear; the mold falls into the metal and wears out. In this case, the ring die often exhibits irregular wear and is mostly drum-shaped.
1. Raw material size
The crushing fineness of raw materials should be moderate and uniform, because the crushing fineness of raw materials determines the surface area of ??the feed composition. The finer the particle size, the larger the surface area, the faster the material absorbs moisture in the steam, which is beneficial for conditioning and particle formation. If the particle size of the raw material is too coarse, it will increase the wear of the die, and cause a decrease in productivity and an increase in energy consumption. It is generally required that the raw materials pass through the 8-mesh sieve surface after crushing, and the content on the 25-mesh sieve should not exceed 35%. For materials with high crude fiber content, add a certain amount of grease. During the granulation process, the friction between the material and the ring die can be reduced, which is beneficial to the material passing through the ring die, and the appearance of the particles is smooth after forming.
2. Miscellaneous raw materials
Too much sand and iron impurities in the material will accelerate the wear of the die, so the cleaning of raw materials is very important. At present, most feed mills pay more attention to the removal of iron impurities in raw materials, because iron materials will cause strong damage to the die, the roller and the equipment. However, it does not pay attention to the removal of sand and gravel impurities. This point should be brought to the attention of granulator users.
3. The addition of steam
Adding steam to the material can soften the material, split the oil-containing cell tissue in the material, and make the oil content in a free state, which plays a lubricating role in the granulation process, thereby reducing the wear of the material on the die and increasing the output of the granulator . Generally, the pressure of adding steam should be between 0.2 and 0.4MPa. The pressure level varies with the type of material being pressed. For materials with higher protein content, the steam pressure should be lower. For materials with more fibers, the steam pressure can be slightly higher. Higher. Adding steam should make the moisture content of the material reach 16% ~ 17%, and the temperature after material conditioning is the best when it is 78 ~ 87 ~ C.
4. The gap between the ring die and the pressure roller
If the gap between the ring die and the pressure roller is too small, it will accelerate the wear on the surface of the ring die and the pressure roller, especially during the idling operation when the granulator is started and stopped. Taking into account factors such as the roundness error of the ring die and the pressing roller surface and the coaxiality error between the working surface and the mounting surface, the gap between the ring die and the pressing roller is generally 0.2 to 0.4 mm, and small diameter particles are suppressed Take a small value when feeding or using a new ring die.
5. Mounting angle of spreader
If the installation angle of the spreader is incorrect, the material distribution between the ring die and the pressing roller will be uneven. Therefore, when entering the extrusion, the extrusion stress and frictional force of the ring die and the pressing roller are non-uniform in the axial direction, which eventually results in uneven distribution of the ring die and the pressing roller.
03. Analysis of failure causes-fatigue damage
The ring die of the granulator is a porous ring-shaped part, and the working conditions are bad. In the process of use, it can withstand the squeezing force of the pressure roller and the friction force of the material for a long time, so that it produces bending stress and contact pressure stress. But these stresses are not factors that affect the life of the ring die, and the main failure mode still comes from fatigue failure. Therefore, appropriately increasing the width, thickness and inner diameter of the ring die can improve the bending resistance and productivity of the ring die. At the same time increasing the inner diameter and the effective extrusion length can significantly reduce the contact pressure stress of the ring die.
In order to effectively improve the stress conditions of the ring die, in addition to changing the ring die's external dimensions, it is also possible to adopt appropriate materials and heat treatment techniques, and design the ring die surface opening ratio.
The ring die is usually made of carbon (alloy) steel or stainless steel material through forging, cutting, drilling, heat treatment and other processes. When drilling a ring die, a multi-hole gun drill should be used to ensure the quality of each hole. During heat treatment, vacuum quenching equipment should be used to avoid the surface oxidation decarburization that often occurs in ordinary equipment, thereby obtaining higher hardness.