The reasons for wearing of ring die for ring die pellet mill

Pellet mill is the key equipment for pelletizing technology, and the ring die is the heart part of pelletizing, which are one of the easiest wearing part of pellet mill. Researching the reasons for the failure of ring die and improving the service condition has great influence to the production quantity and quality, to reduce the power consumption and production cost etc.

In theory, a common ring die and a pair of common compression rollers could only produce about 2000t and 500 t pellets separately. Practically, the well managed manufacture could only produce over 1000t and 250t. While for the manufacture with bad management, it only produce 500-600t and 80-100t, then the new ring die and compression rollers need to be replaced. From the above, we come to realize that lengthening the service life of ring die bring considerable benefits to the manufacture.


 


1. Working principle
The ring die is rotating under the driving of the motor through speed reducer, and the compression roller dose not in revolution in the ring die, but it rotates under the friction force with the rotating ring die. When different powdery materials enter into the compression zone between the active rotating ring die and driving rotating compression rollers, the materials has certain looseness with the big or small inside holes. At the same time, the materials are driven and compressed by the rotating ring die. When the pressure is up to the standard, the material is up to the setting density. The high density materials enter into the crushing zone from the compressing zone. Due to the relative mechanical interlocking between the particles, the contacting surface between the particles is increasing. Due to the materials own features and the cohesiveness after conditioning, it ensures that the materials in the ring die for a period of pressure maintaining. When the density and durability is up to a standard, the pellets are compressed out from the ring die holes, and cut into the required length.

During the normal operation of ring die, friction force always exist the between ring die and materials. As the increasing of the material production, the ring die is wearing and loses effectiveness at last. So the standard of measuring the ability of ring die is production amount. During the practical producing, most ring die lose their effectiveness before the theoretical production ability. Because the ring die is high is price, it leads to a certain loss for the users. This article is going to analysis the reasons for the loss of effectiveness of ring die and give some advice to the manufacturing and using of ring die.

2. The reason for the loss of effectiveness
From the situation of losing effectiveness of ring die for pellet mill, it can be classified into three types. The first one is that the inner wall of small discharging hole is wearing with increasing, and the pellets produced by the pellet mill are out of the limited scope. The second type is the inner surface of the holes is sags and crests which blocked the flowing of the materials and the production is lower or for stopping using. The third is the wearing diameter is enlarged and the thickness is lower. At the same time, the inner wall of ring die hole is wearing, that the ring die surface between the holes is thinner, so the density of structure is lowered. When the diameter is beyond the limited scope (before the first situation happed), the cracking is emerged on the most dangerous section and enlarges increasingly, until the ring die loss efficiency out of the beyond the scope. The reason for the three situations of losing efficiency is basically concluded with the wearing at first, and the second is fatigue.


 
1) Abrasive wear
There are many reasons for abrasive wear, it classified into the normal wearing and abnormal wearing. The reason for normal wearing mainly contains formula of feedstock, particle size of crushing and the conditioning quality of powder materials. Under the normal wearing, the ring die is easy to wear with axial uniformly, which enlarges the ring die hole and make the wall thinner. The reason for abnormal wearing mainly includes: too tightness of compression roller and the small clearance with ring die which against each other; the angel of spreader is not proper, and it lead to the uniform spreading of feedstock and part of it is wearing at first; the iron scraps are entered into the chamber with wearing. Under this condition, the ring die are easy to wear with irregular, most of it is like a drum.

a. The particle size of raw materials 
The fine degree of crushing materials should be uniform that decides the surface area of the feeding construction. The smaller the particle size, the larger the surface area is. The feedstock absorbs the stream more rapidly, which are benefit to the pelletizing and conditioning. If the particle size is bigger, it will increase the wearing of compression roller and lead to the lowering of production efficiency with larger power consumption. The material is usually required to pass through the screen with 8 meshes, and the ratio of 25 meshes is less than 35%. For the crude fiber content materials, it is proper to add some grease during pelletizing, to reduce the friction force between the feedstock and ring die that are beneficial to pass through the ring die, so the pellets has smooth surface after pelletizing.

b. The impurities contained in the raw materials
There is too much sand or iron impurities in the feedstock, which can increase the wearing die compression rollers. So it is important to remove the impurities in the raw materials. At present, most manufactures pay more attention to remove the impurities. Because the iron scraps destroy the compression roller, ring die and the whole equipment badly. But they make little of removing the sand. This point should be noticed by the user either.  

c. Adding steam 
Adding steam to the materials can soften the materials and make the oil contained cell tissue split which make the oil content is in free states. And the tissues have lubrication function during pelletizing process, to decrease the wearing between the feedstock and the compression roller. So the production of pellet mill is increased. In general, the pressure is about 0.2-0.4MPa. The pressure changes along with the changing of types of materials. The high protein content materials need lower pressure; while the fiber content materials need higher pressure of steam. The moisture content of feedstock is between 16%-17%, the temperature is about 78-87 after conditioning.

d. The clearance between the ring die and the compression roller
If the clearance between the ring die and roll is too small, it will increase the wearing of the surface of ring die and roller. Especially during the idle running for start up and stop the machine, the wearing force is much stronger. Thinking about the roundness error of the ring die and roller surface and axiality errors of the working surface with fixing surface, the clearance between the roller and ring die is usually about 0.2-0.4mm. When the small diameter feed pellet are compressed or a new ring die is used, the small figures should be adopted.

e. The fixing angel of spreader
If the angle of spreader is not proper, it will lead to the uniform spreading of materials between the ring die and compression roller. Therefore, stress between the ring die and rollers and friction force is not uniform axially, it finally lead to the uniform spreading of wearing of ring die and compression roller.

2) Break with fatigue
The ring die for pellet mill is a ring shape spare parts with many holes and it works in a poor condition which undertakes long time pressure of compression roller and the friction force of feedstock during operation. It is easy to produce bending stress and contact pressure. But it is not the pressure that affect the ring die service time, it mainly loss efficiency by the fatigue. In the thesis of “Discussion about the hardness of ring die during pelletizing”, the author Huang Chuan hai analysis the stress of ring die during the pelletizing and thought that ring die is under the alternating force during pellet making and the character of the alternating force is. It shows that the alternating force is recycled without symmetry which usually made the ring die undertake fatigue break. The result is identical to the result of loss efficiency of ring die during production. It is proper to increase the width, thickness and bore size of ring die on this basis, which can improve the bending resistance ability of ring die and production efficiency. At the same time, with the increasing of bore size and actual pressing length, the contact pressure of ring die could be reduced evidently. In order to improve the stress condition of ring die efficiently, it can adopt the proper texture and heat treatment technology, as well as design the porosity of ring die etc, except changing the size of ring die. The ring die usually make of carbon steel or stainless steel after the process of forging, cutting, drilling and heat treatment etc. When drilling the hole of ring die, it is necessary to use the porous gun drill to ensure the quantity of die holes. The vacuum quenching equipment should be used for heat treatment to avoid of carbon removing for the surface oxidation which usually found in the common equipment. So the hardness of ring die is increased.

The porosity of ring die surface affect the production of pellet mill and hardness of ring die directly. The higher the porosity is, the higher the production of pellet mill is, and the hardness of ring die is lower; on the contrary, the hardness of ring die is improved and the production of pellet mill is decreased. Due to the difference between the texture and size of ring die, now it is difficult to found the proper porosity ratio to coordinate with the production and service life. For the ring die with diameter of 2-12mm, the porosity ratio is usually between 20-30%. The smaller the diameter is, the porosity ratio is smaller. On the contrary, the porosity ratio is larger. The manufacture can adopt the way of testing in turn to confirm the porosity ratio on the basis of material for ring die, outer structure and size, to ensure the enough hardness of ring die. So the ring die are prevent from breaking to shorten its lifespan. 

As for the design of outer structure of ring die, it is proper to reduce the spread of grooves along the diameter of ring die surface and reduce the depth of the grooves. The grooves along the diameter direction are easy to produce the concentration of stress. Under the long time alternating stress, it is at this section that produce slight crack with broadening. At last, the ring die is break up.

 


3. Preservation of ring die for pellet mill 
1) After the ring die of the pellet mill has been stored for six months, the oily stuff in the ring die should be replaced. For the long time of preservation, the stuffed materials are become stiff and the machine can not be used for making pellet with blockage.

2) The ring die should always be place at dry, clean and airy place. If the machine is not use for long time, the surface of machine should be coated with waste oil machinery to avoid of moisture corrosion in the air. The production warehouse are usually has many materials for production, so remember not to store the ring die here. The materials are easy to absorb the moisture content without dropping off. If the ring die are preserved in this place, it will speed up the corrosion process and affect the service life.

3) If the ring die is going to be unpacked for reserve, the production materials should be compressed out completely before stopping with the oily materials, to ensure that each holes of ring die can discharge next time. If the ring die are stored without oily materials, the ring die are corroded after long time of preservation. That is because the moisture content in the production materials speeds up the corrosion which gives rise to the rough surface of the ring die and affects the discharging process. Sometimes the ring die is out of use and increase the loss. So it is very important for using and preservation for ring die.

From the above analysis, there are mainly two reasons for losing efficiency of ring die during production. The first is abrasive wearing, and the second is break with fatigue. In addition, the preservation of ring die for pellet mill is also very important with regular maintaining. It is not practical to avoid of loss efficiency of ring die completely, as long as the condition for manufacturing, producing and using is improved, the ring die service span will be lengthened with cost saving.

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