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A new pellet mill that produces a low formation rate is a common complaint in the first weeks of operation, and it is rarely a defective machine. A new die has to be run in, the roller gap has to be set, the moisture has to be tuned to the new die, and the operator has to learn the machine’s behaviour. A mill that forms poorly on day one often forms well after the die is broken in and the settings are corrected, and the operator who changes the die or the machine before the break-in is finished usually makes the problem worse.

This page covers the causes of a low formation rate on a new pellet mill, how to tell a break-in problem from a real fault, and how to correct each cause. It is written for the operator and the commissioning engineer who are starting a new machine rather than for the buyer who is comparing models.

The short answer

A new pellet mill forms pellets poorly for one of five reasons: the die is not yet run in, the moisture at the die is outside the workable range for the new die, the roller gap is not set correctly, the feedstock is not prepared to the specification the die was selected for, or the die temperature has not reached the operating range.

The practical rule: run the die in with a prepared material before production, set the roller gap on every roller, hold the moisture at the die, and do not change the die or the machine until the break-in is complete and the settings have been checked. A low formation rate in the first days is a normal break-in symptom, and a low formation rate that continues after the break-in is a real fault.

A low formation rate is not the same as a low density. A low formation rate means that a high proportion of the material is not forming into pellets and is coming out as fines or dust, while a low density means that the pellets that do form are light. The two can occur together, but they are measured differently and they point to different checks.

Why a new die forms poorly at first

A new die is not the same as a worn die, and the difference is in the hole surface and in the compression.

A new die has hole surfaces that are rougher and entry edges that are less polished than a run-in die, and the material does not slide through them as easily. The result is a higher resistance, a lower throughput, and a pellet that is less dense until the die is polished by the material passing through it.

The die temperature also takes time to reach the operating range, because the die body has to heat up through the friction of the material, and a cold die does not soften the lignin enough to bind the pellet. A new die that is fed production material at full rate before it is warm produces soft, dusty pellets and a low formation rate, and the operator often reads this as a machine fault.

The break-in is the period in which the die is polished, the temperature is established, and the settings are tuned to the material. The length of the break-in depends on the die, the material, and the loading procedure, and a properly loaded die usually breaks in within the first hours to the first day of running. A die that still forms poorly after several days of proper loading is not a break-in problem.

Causes of a low formation rate on a new pellet mill

The die is not run in. The most common cause of a low formation rate on a new mill is a die that has not been broken in. The die should be loaded with a prepared material before production, at a low feed rate, until the die is warm and the pellets form steadily. Feeding production material at full rate into a cold, new die is the most common reason a new die blinds in the first hours.

The moisture at the die is outside the range. A new die has a different workable moisture range from a run-in die, and the moisture that worked on the old die may be wrong for the new one. A moisture that is too high makes the material slip and the pellet soft, and a moisture that is too low makes the pellet brittle and dusty. The moisture should be measured at the die inlet, not at the dryer outlet, and the target should be established in the break-in rather than taken from the old die’s setting.

The roller gap is not set correctly. A roller gap that is too wide leaves the material unpressed and the pellet light, and a gap that is too tight overloads the motor and blinds the die. The gap should be set on every roller to the manufacturer’s specification for the new die, and it should be checked again after the first hours of running, because the new parts settle under the initial load.

The feedstock is not prepared to the specification. A new die is selected for a specific feedstock, moisture, and particle size, and a feedstock that is coarser, wetter, or more abrasive than the specification produces a low formation rate. The particle size should be matched to the die hole diameter, and the contamination should be removed before the die, because the stones and the metal damage the new die and the formation.

The die temperature has not reached the range. A new die takes longer to reach the operating temperature than a run-in die, and a die that is below the range produces soft pellets. The die temperature should be monitored, and the feed rate should be adjusted to bring the temperature into the range before the output is judged.

How to tell a break-in problem from a real fault

The break-in problem improves as the die is polished and the settings are corrected, and a real fault does not improve with time. The distinction can be made by comparing the formation rate on the first day with the formation rate after the break-in and the settings correction.

The formation rate should climb steadily during the break-in, and a rate that rises from a low start is the normal pattern. A rate that is flat or falling is not a break-in symptom, and a rate that stays low after the break-in is complete is a real fault.

A low formation rate that persists after the five causes have been checked is a real fault, and the feeder and the roller gap should be checked first, because they are the most common causes, and the die and the drive should be inspected if the first two are correct.

How to correct the low formation rate

Run the die in properly. Load the die with a prepared material at a low feed rate, bring the die up to temperature, and only then change to the production material and bring the feed up in steps. The loading material is normally a dry material of the same family as the production material, prepared so that it slides through the die holes, and mixed with a small amount of oil or water where the manufacturer recommends it. The loading continues until the die is producing pellets steadily at the operating temperature and the current has stabilized, and only then is the feed changed to the production material and brought up in steps.

Set the moisture at the die. Measure the moisture at the die inlet and adjust it in the conditioning stage rather than in the hopper. The target should be established in the break-in and recorded as the baseline for the new die.

Set the roller gap on every roller. Set the gap to the manufacturer’s specification, check it on every roller rather than one, and re-check it after the first hours of running, because the new parts settle.

Match the feedstock to the die specification. Confirm the particle size, the moisture, and the contamination against the specification the die was selected for. Grind the material if it is too coarse, and remove the stones and the metal before the die.

Bring the die temperature into the range. Adjust the feed rate to bring the die temperature into the operating range before the output is judged, and monitor the temperature against the record from the break-in.

What to check before blaming the machine

Before the machine is blamed, confirm the following. The die has been run in, not just started. The moisture at the die has been measured and set. The roller gap has been set on every roller and re-checked after the first hours. The feedstock has been matched to the die specification. The die temperature has reached the operating range. The operating record has been kept, so that the formation rate on day one can be compared with the formation rate after the break-in.

What to specify before buying a line

Before the line is specified, the break-in procedure should be addressed in the quotation and in the commissioning plan. The quotation should state the die specification matched to the feedstock, the moisture range, the roller gap, the loading material recommendation, and the acceptance test on the actual material. A line that is specified without the break-in procedure is a line that can form poorly in the first weeks and be blamed on the machine.

If you are commissioning a new pellet mill and the formation rate is low, send the die specification, the feedstock, the moisture at the die, the roller gap setting, the loading procedure, and the formation record, and the Shandong Changsheng Machinery team can review the break-in and the settings and propose the correction and the acceptance test.

Bottom line

A new pellet mill that forms pellets poorly is usually a break-in problem, not a defective machine. The die has to be run in with a prepared material, the moisture at the die has to be set for the new die, the roller gap has to be set on every roller, the feedstock has to be matched to the die specification, and the die temperature has to reach the operating range before the output is judged. The formation rate should climb steadily during the break-in, a rate that is flat or falling is not a break-in symptom, and a rate that stays low after the break-in is a real fault. Confirm all five causes before blaming the machine, keep the operating record so that the break-in can be compared with the steady state, and specify the break-in procedure and the acceptance test in the quotation so that a new line is judged on the material and the settings rather than on the first day’s output.