The major reasons for ring die failure of pellet machine-2

From the point of actual failure of ring die in pellet machine, it can be divided into three kinds. First: after the ring die has working for a period of time, the wall of the discharging hole wears and the aperture increases. As a result, the diameter of feed pellets produced exceeds the specified value; second: after the wall of ring die wears, the inner surface is uneven so that the feed flow is blocked and the discharging amount declines; third: the wear of ring die wall increases the inner diameter, the wall thickness decreases as well. At the same time, the wall of the discharging hole wears, making the wall thickness between each discharging hole thinning, thus the structural strength declines. Before the diameter of the discharging hole exceeding the specified value, a crack would appear on one of the most dangerous section first and constantly expanding, until the crack extends to a larger range and lead to the ring die failure. To sum up, the substantial causes of the above three kinds of failure phenomenon are the abrasive wear and the fatigue damage.



(1) Abrasive wear
There are many reasons for abrasive wear, which can be divided into normal wear and abnormal wear. Normal wear is mainly caused by the material formula, granularity, conditioning quality of powder, etc. In normal wear, the ring die wears evenly in axial direction, die holes get large and the thickness of the wall gets thinning. The main reason for abnormal wear: the roller is too tight, and clearance between the ring die is small so that the roller and ring die would wear with each other; the angle of the feeder is poor, resulting in uneven distribution of the material and part of the ring die wears; metal fallen into the machine and get the ring die wears. In this case, the wear of ring die is irregular, most of which is drum-shaped.

(2) Fatigue damage
The ring die in pellet machine is the porous ring-shaped work piece with harsh working conditions. In the use of ring die, the bending stress and contact stress would be produced due to the long time bearing of the extrusion force from the roller and the friction from materials. But the stress is not the factors affecting the service life of ring die, the main failure is still caused by the fatigue damage.
To effectively improve the stress condition of the ring die, in addition to change the dimensions of the ring die, appropriate material selection, heat treatment technology as well as the design of ring die surface opening rate can be adopted.

The above analysis shows that there are two main failure reasons of the ring die in the process of production, one is the abrasive wear, the other is the fatigue damage. To prevent ring die failure fundamentally is impractical. However, we can improve the manufacturing, production and use condition of ring die so as to prolong the service life and cost savings in biomass pellet production.
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