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A pellet that crumbles when squeezed is a durability failure, and it is the failure that shows up first in the hand and last in the specification. The pellet looks formed at the die, it survives the cooler, and then it breaks in the fingers, in the bag, or at the customer, and by the time the complaint arrives the material has already been produced and shipped. The squeeze test is a rough but honest signal: a pellet that crumbles under light finger pressure is a pellet that will not survive the conveying, the storage, or the customer’s handling.

This page is about the durability of the finished pellet and the squeeze check, while the moisture, the fines, the density, and the die running-in are covered on their own pages with their own checks. It is written for the operator and the quality engineer who have to decide whether the pellet is weak and what to correct.

The short answer

A pellet that crumbles when squeezed comes from one of five causes: the moisture at the die is outside the workable range, the die compression ratio is too low for the material, the cooling is too fast or too slow, the fines and the weak pellet are not being removed, or the material itself binds poorly.

The practical rule: confirm the durability with a defined test rather than the fingers alone, check the moisture at the die first, then the die specification, then the cooling and the screening, and only then the material. A pellet that crumbles at the mill is a fault, and a pellet that crumbles after storage is a fault that may have started at the cooling or at the bagging rather than at the die.

How to run the squeeze check

The squeeze check is a field signal rather than a specification, and it is worth running the same way every time so that the readings are comparable.

Hold the pellet between the thumb and forefinger at the middle, and apply a light pressure without crushing it against a hard surface. The pressure is the one that would dent a firm pellet without breaking it, and it is a judgement rather than a measured force. Do it on several pellets from the bag rather than on one, because a single pellet can be unrepresentative, and note whether the pellet breaks, dents, or resists.

The hand tests one pellet in one direction, while the durability test tumbles a sample and measures the proportion that survives, so a pellet that passes the hand can still fail the tumbler. The squeeze check tells the operator to look, and the defined durability test tells the buyer whether the pellet meets the specification.

The durability is measured by a defined tumbler method on a sample that has been cooled and rested, and the target is set by the market the pellet is sold into and by the customer’s acceptance specification rather than by a general figure. The specific limit and the method should be confirmed against the destination standard before the pellet is judged.

The five causes and how to tell them apart

Cause	What points to it	What to check
Moisture outside the workable range	The pellet is soft and deforms, or it is brittle and dusty	The moisture at the die inlet, not at the dryer outlet
Die compression ratio too low for the material	The pellet is formed but light, and the surface is rough	The die specification and its definition against the material
Cooling too fast or too slow	The pellet cracks, or it deforms after the cooler	The cooling air, the residence time, and the pellet temperature at the bag
Fines and weak pellet not removed	The bag contains dust, and the durability is low	The screen condition and the fines recirculation
The material binds poorly	The pellet is weak even when the moisture and the die are correct	The lignin, the ash, the fiber, and the blend

Moisture at the die

The moisture at the die is the strongest single influence on the durability, and it works in both directions.

A moisture that is too high lowers the friction and the die temperature, the lignin does not soften enough, and the pellet is formed soft and crumbles. A moisture that is too low leaves the material brittle, and the pellet cracks rather than compacts. The workable range depends on the material, the die, and the machine, and it should be confirmed in a trial rather than taken from a general figure.

The moisture should be measured at the die inlet, not at the dryer outlet, because the material changes between the two points, and the target should be recorded on the run sheet so that the next run can be compared with the last.

Die compression ratio and the die specification

The die compression ratio sets how much resistance the material meets, and the resistance is what compacts the pellet.

A die with a compression ratio that is too low for the material does not generate enough pressure, and the pellet is formed light and crumbles. A die with a ratio that is too high can produce a dense but brittle pellet, and it can overload the motor, so the correction is a matched ratio rather than the highest ratio.

The compression ratio definition varies between suppliers, and the effective working length should be compared rather than the ratio number alone, because two dies with the same nominal ratio can behave differently if the definition behind the number is different.

Cooling and the pellet temperature at the bag

The cooling affects the durability through the strength of the pellet and through the fines.

A pellet that is cooled too fast develops a hard shell and a soft core, and it cracks, and a pellet that is cooled too slowly stays warm and soft, and it deforms in the screen and the conveyor. The cooling should bring the pellet to close to ambient before the bagging, and the cooling rate and the residence time should be matched to the pellet diameter and the throughput.

The pellet temperature at the bag is the field check, and a pellet that is still warm in the bag is a pellet that is being bagged too early, regardless of what the cooler outlet reading says.

Screening and the fines

The fines are the small particles that break away from the pellet, and a high fines content means that the pellet is weak somewhere in the chain.

The screen removes the fines that are already present, but a screen that is blinded or undersized lets them into the product, and a high fines content in the bag is a sign that the pellet is crumbling before it reaches the customer. The screen should be sized for the throughput and cleaned on schedule, and the fines that are removed should be returned to the mill where the die and the material allow it, with the recirculation ratio confirmed in trial.

The material and the binding

Some materials bind less readily than others, and the material sets the boundary of what the die and the process can achieve without a binder.

A material with a low lignin, a high ash, or a fibrous structure produces a weaker pellet than a clean softwood at the same die and the same moisture, and a blend with a poorly binding component can lower the durability of the whole batch. The material should be tested for the lignin, the ash, the fiber, and the blend ratio, and the die and the process should be set for the material rather than for a general wood.

Where the material cannot reach the durability without an additive, a binder or a blend can be considered, but the effect depends on the material and the dosage, and the combination should be established by testing rather than assumed.

How to diagnose a pellet that crumbles

The diagnosis moves from the finished pellet back through the line, rather than from the die forward.

The first step is to confirm the durability with a defined test on a cooled and rested sample, and to record the result against the specification. The squeeze check is the field signal, and the durability test is the record.

The second step is to check the moisture at the die, not at the dryer outlet, and to compare it with the workable range for the material and the die.

The third step is to check the die specification against the material, including the compression ratio, the effective working length, and the hole diameter.

The fourth step is to check the cooling and the pellet temperature at the bag, and the screen condition and the fines content in the bag.

The fifth step is to check the material, including the lignin, the ash, the fiber, and the blend ratio, and to compare the current batch with a batch that produced a good pellet.

What to correct and in what order

The correction should be made one variable at a time, with the durability measured after each change, so that the effect of the change can be seen.

Correct the moisture at the die first, because it is the strongest single influence and the easiest to adjust. Correct the die specification second, if the moisture is in range and the pellet is still weak. Correct the cooling and the screening third, if the pellet is acceptable at the die but crumbles after the cooler. Correct the material or the binder last, if the first three are correct and the pellet is still weak.

Changing the moisture, the die, and the cooling at the same time makes the result impossible to attribute, and the next run starts from an unknown baseline.

What to specify before buying a line

Before the line is specified, the durability requirement should be stated in the quotation and in the acceptance test, together with the material, the moisture, the pellet diameter, the test method, and the sample point. A durability limit without the method and the sample point is not a specification, and a line that is specified without the durability requirement can be built and still fail the market.

If you are producing pellets that crumble when squeezed and need to trace the cause through the moisture, the die, the cooling, the screening, and the material, send the durability result with the method, the moisture at the die, the die specification, the cooling and screening arrangement, and the material data, and the Shandong Changsheng Machinery team can review the line configuration and propose the correction and the acceptance test.

Bottom line

A pellet that crumbles when squeezed is a durability failure, and the squeeze check is a field signal rather than a specification. Run the squeeze check the same way every time, on several pellets from the bag, and confirm the result with a defined tumbler test on a cooled and rested sample, because the hand tests one pellet in one direction and the tumbler tests the whole sample. Then check the moisture at the die, the die compression ratio against the material, the cooling and the pellet temperature at the bag, the screen and the fines, and the material and the binder, in that order, and correct one variable at a time with the durability measured after each change. State the durability requirement with the method and the sample point in the quotation and in the acceptance test, and trial the pellet on the actual material before shipment, so that the pellet that reaches the customer is the pellet that was tested rather than the pellet that was hoped for.