FBA Issue 5: November / December 2005
Maintaining your pellet die
by KOH Hui Meng
DECREASING throughput starts slowly as a slight drop in on-line capacity or tonnes per hour of production. When that happens, a plant manager will find that a standard feed formulation batch will take longer to run than before. Subsequently, he will find production costs escalating as operating time is lengthened to make up for the lower production rate.
Sometimes, this problem can be traced back to poor die functioning. Replacing the die means loss time for the plant as well as the possibility of losing the initial investment of the die.
Avoiding die hole rollover
The leading cause of declining die throughput is the die face condition known as rollover. Rollover is the deformation of the die hole that restricts the die hole entrance. Simply put, the greater the amount of material that passes through the holes, the lesser the quantity of feed that can enter the die. This condition worsens when the die runs for a long period of time.
The major cause of rollover is excessive tightening of the roller shells against the die face. Pellet mill operators may adjust rollers by using the feed-tag method--placing a tag between the roller and the die face, or the skip-touch method--adjusting the roller until the die face just turns the shell.
Both these methods pose problems. First, the feed tag method often leaves too large a gap between the roller and the die face, and a larger gap is less efficient. Pelleting efficiency, however, is improved by reducing the angle at which feed is pelleted and adjusting the rollers as close as possible to the die face. The right roll adjustment is also important.
Second, in the skip-touch adjustment method, a problem arises when the roller touches the die face. The contact between the both metal faces initiates plasticisation and peening of the die face, which encourages easy breakage of the die. When a die begins to rollover and the operator senses that the production rate is slowing down, there is a tendency to tighten the rollers even more, which further compounds the problem.
The key is, from the onset, not to allow over-tightening of the rollers. For maximum pelleting efficiency and protection from rollover in most situations, roller adjustment is best accomplished by adjusting the rollers so that they skip-touch the die face at a gap of about 0.125-0.25 mm. Also, rollers should be adjusted periodically, at least twice a week on the same die during single-shift operations.
How does a production manager correct a rollover problem? As is the case in most pellet die problems, prevention is the best solution. Personnel must first be trained to recognise the condition and adjust the rollers properly. Often, the die can be saved through reworking procedures, which would include counter-sinking the die holes.
In some cases, however, poor roller adjustment is not the cause of the problem. Instead, this might be related to the die material and the pattern of the die holes. A good indicator of a misapplied hole pattern is when die face rollover becomes a chronic, repetitive problem. In this case, the plant operator will need to revert to the manufacturer for a reassessment of the hole pattern or die material.
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