Tracing Pellet Quality Problems: Moisture, Fines and Inconsistent Shape

Read the symptom first

Pellet problems present as symptoms, and each symptom narrows the search. Trying to fix the extruder first is the most common wasted effort, because most pellet defects originate upstream.

Bubbles, voids and porous pellets

The pellet has internal holes, or its surface is pitted.

Most likely cause: moisture or volatiles. Water in the melt turns to steam and forms voids as the pellet cools. Volatiles from ink, adhesive or degraded polymer do the same.

Check in this order:

1. Was the material dry enough before extrusion? Compare the moisture leaving the dryer against

what the extruder was specified to accept.

1. Is the material reaching the dryer as a mat? Film that mats has a dry outside and a wet

inside; airflow is not reaching the material.

1. Is degassing working? Vacuum level, vent condition and whether venting is adequate for the

actual volatile load.

1. Is melt temperature excessive? Too much heat generates volatiles that were not in the feed.
2. Is residence time too long? Polymer held at temperature degrades and produces its own gases.

Only after these should the die and cutting be examined.

Fines and dust

Fines are small fragments produced instead of whole pellets.

Most likely causes:

• Blunt or incorrectly set cutting blades. A dull cut shears and crushes rather than slices.
• Wrong blade-to-die clearance. This is a setting, and it drifts with wear.
• Die holes partially blocked or worn. Uneven flow produces strands or droplets that cut badly.
• Water temperature wrong at the die face (for die-face cutting). Too cold and the pellet is

brittle when cut; too hot and it smears.

• A worn screen downstream, which lets previously captured contamination through and disrupts

the cut.

Fines are also a yield loss, not only a quality issue: material that leaves as dust does not leave as product.

Inconsistent pellet size or shape

Pellets vary noticeably from one another.

Most likely causes:

• Unstable feed bulk density. This is the leading cause for film and fibre. Surging feed

produces surging melt pressure, and pressure variation appears directly as size variation.

• Unstable melt pressure from a partially blinded screen or a filtration concept that is at its

limit.

• Inconsistent melt temperature, often from variable feed moisture.
• Worn die or worn cutter hub, which changes the geometry the pellet is formed from.
• Cutting speed not matched to throughput.

If the feed is low bulk density film and there is no compaction stage, this is the first place to look.

Surface defects, discolouration and odour

Most likely causes:

• Thermal degradation from excessive temperature or excessive residence time.
• Insufficient degassing, so volatiles remain in the melt.
• Contamination that survived filtration — ink, adhesive or foreign polymer.
• Cross-contamination from a previous run, where the line was not purged adequately between

materials.

Discolouration that appears only at the start of a run usually indicates purge and changeover rather than a process fault.

A general order of investigation

1. Confirm the feed. Material type, contamination, moisture content and bulk density.
2. Check the mechanical dewatering and drying stages against the moisture target.
3. Check degassing — vacuum, vent condition, and whether it is needed at all.
4. Check filtration — screen condition and pressure drop.
5. Check melt temperature and residence time.
6. Check the die and cutting arrangement — blade condition, clearance, water temperature and

cut speed.

1. Check for purge residue if the problem appeared after a material change.

Steps 1 to 3 account for the majority of pellet quality complaints, which is why the order runs upstream to downstream rather than the reverse.

Related: How to Reduce Moisture After Film Washing · Screen Changers and Melt Filtration · Service & Maintenance in the FAQ