Pellet mill is the key equipment for the feed production processing, and ring die is the heart and most wearing part of the pellet mill. Research the failure causes of the ring die, improve the working condition of the ring die can improve the quality of products, reduce the energy consumption (energy consumption of pelleting takes 30%-35% of total workshop energy consumption) and reduce the production cost (ring die wearing takes more than 25%-30% of the total costs of the workshop), which are of great influence.

1.  Working Principle (figure 1) 
Ring die is driven by the motor through reducer rotation, rollers installed in the ring die do not revolve, but will rotate with the friction of the ring die (by pressing the material). Mixed materials get into the press chamber and being separated evenly between the rollers, being pressed through the ring die holes continuously and shaped to columnar pellets and rotate with ring die, pellets are cut to certainly length by a cutter installed outside the ring die. Linear velocity for each connecting points between ring die and rollers are same, all of the pressures are used to make pellets.

During the normal working of the ring die, friction between materials exists all the time. Along with the increasing of the material, ring die would wear little by little and failure finally. This text analysis the ring die failure causes, and give advices to the manufacturing and service conditions of ring die.

Pelleting Chamber of Feed Pellet Mill

2. Failure Causes Analysis

Failure causes of ring die can be divided into three kinds:

First:

After working for a period of time, ring die hole inner wall would be worn and the diameter will increase, diameters of pellets made by this kind of ring die are over the specified value and lose efficacy;

Second:

Inner surface of worn ring die are irregularity, materials can't flow fluently, feed output amount reduces;

Third:

If ring die is worn, the inside diameter increase, wall thickness reduce, holes worn, too, thickness of wall reduces, the structure strength reduce, before diameter of holes increase over the specified value(first kind), crack appear on the most dangerous section and increase till the ring die lose efficacy.

Conclude the 3 above kinds of failure causes, abrasive wear is the first causes, second is fatigue rupture.

2.1 Abrasive wear

Causes of wear are various, they can be divided into normal wear and abnormal wear. Normal wear is mainly causes of material formula, fineness of pulverization, tempering quality of powder and etc.. Mainly causes for abnormal wear are: rollers too tight, gap between ring die is small, wear each other; wrong angle of the spreader leading to the nonuniform of material wear; metals get into the ring die and wear. Wear under these conditions are generally irregular, in shape of waist drum.

2.1.1 Raw material granularity

Fineness of raw material should be moderation and uniform, surface area is decided by the fineness of pulverized raw material, the finer the particle size, the larger the surface area, the faster the moisture absorption in the material, to the benefit of tempering and shaping of pellet. Coarse material will increase the wear of ring die, and reduce the production, increase the energy cost. Generally the materials should get through the 8 mesh screen after pulverized, or no more than 35% on 25 mesh screen. As for materials with high content of coarse fiber, adding some grease while pelleting can decrease the friction between materials and ring die, materials easy to get through ring die and pellets are smooth in surface.

2.1.2 Impurity content in materials

Too much of sand and iron impurity contents in the material will quicken the wear of ring die, so it is important to clean the impurity. At present, most of the feed plant attach importance to iron impurity cleaning, because iron impurity would cause violent damage to the ring die, roller and even equipment, but ignore the sand impurity, this should be brought to the attention of the granulator user.

2.1.3 Steam addition

Adding steam to the material can soften the material and making the oily cells tissue in the material division, the oil was free state, and lubricate in the process of pelleting, thus lighten the material abrasive action to the ring die, improve the pellet mill yield. Generally the pressure of steam should between 0.2-0.4MPa, the pressure changes with the pelleting materials, for materials with high protein content, the steam should be less, while for higher fiber content materials, the steam should be higher. After adding steam, moisture content of materials should become 16%-17%, temperature of the material after tempered is better at 78-87℃.

2.1.4 Gap between ring die and rollers

The gap between the ring die and the roller is too small will quicken the wear of the ring die and the surface of the roller, especially at the moment of pellet mill start, stop and free running. Considering the ring die and roller surface deviation from circular form and coaxial error of working surface and install surface, gap between ring die and roller is generally 0.2-0.4mm, smallest value are obtained when pelleting the small diameter pellets or using new ring die.

Rollers

2.1.5 Spreader installation angle

Wrong installation angle of spreader will lead to unevenness of material between ring die and rollers. Therefore, while pelleting, crushing stress and friction of ring die and rollers become unevenness along the shaft direction, end up with unevenness wear of ring die and rollers.

2.2 Fatigue rupture

Pellet mill ring die is a multi-hole ring-shaped work pieces, works under bad conditions, bearing the extrusion force of roller and friction of material for long terms, produce bending stress and contact compressive stress. These stress are not the element that influence the service life of ring die, the main form of failure is fatigue rupture. Huang Chuanhai analysis the ring die stress conditions while working in “Discussion of Feed Pellet Mill Ring Die Strength”, in the article, Huang propositions that the ring die is subjected to alternating action during the granulation process, and its alternating stress cyclic behavior is:

It shows that the alternating stress is not symmetric cycle, ring die is usually fatigue rupture, which is agree to the failure result in practical using. On this base, come up with the opinion that increasing the thickness, width and inner diameter properly can increase the resistance to bending and productivity of ring die. Increase the inner diameter and valid pelleting length can obviously reduce the connection pressure stress of ring die.

In order to improve the stress condition of ring die, except for changing the boundary dimension, we can also apply the suitable material and heat treatment techniques, and design the aperture ration on ring die surface.

Ring die is generally made of carbon (alloy) steel or stainless steel by processes of forging and pressing, cutting, drill, and heat treatment. Ring die drill should use multihole gun-drill to ensure the holes’ quality. Heat treatment should adopt vacuum hardening equipment, which would avoid decarburization of surface oxidize that usually appears in general equipment, and become higher hardness.

Ring Die

Aperture ratio of ring die directly influence the yield of pellet mill and strength of ring die. The larger the aperture ratio, the larger yield the pellet mill, the lower strength of the ring die; otherwise, the ring die strength increase, the pellet mill yield reduce. Because of the difference of material selection and ring die size, it is hard at present to get proper aperture ratio for ring die to harmonize its strength with service life. As for ring dies of hole diameter 2-12mm, the general aperture ratio is 20-30%. The smaller the hole, the smaller the aperture ratio, conversely, the larger aperture ratio. Manufacturers can decide the aperture ratio according to the ring die material, outline structure and size, take successive approximation test method, make sure the ring die is strong enough, prevent fracture to reduce its service life.

When design the outline structure of ring die, the ring die should be to minimize the radial distribution of the number of grooves, and reduce its depth. Because the radial distribution of the groove is the part where most likely to produce stress concentration. In the long-term alternating stress, this part is the first easy to produce micro-cracks, and then gradually expand, eventually causing ring die fracture.

The above analysis shows that in the production process of ring dies, there are two main causes for failure, one is abrasive wear, and second is fatigue rupture. To fundamentally avoid the failure of the ring die is impractical, only to improve the ring die manufacturing, production and use of conditions can effectively extend the ring die service life and save costs.