Troubleshooting · Defect guide
Black specks and burnt particles
From the pellets, or from the machine?
Black specks have two main sources: contamination that arrives with the material, and melt that hangs up in dead spots inside the machine until it burns and breaks loose. This guide shows how to tell the two apart with checks ordered from cheapest to most expensive, and is honest that additives only address part of the problem.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 22 October 2026 · about 7 min read
Which kind of speck is it?
Black specks are small black or dark-brown particles embedded in the part or at its surface. They are most visible in white, clear and light-coloured parts. The key sign is a random position: they do not appear in the same place on every shot.
- Specks from degraded melt: black or brown carbon flakes, often worst at start-up, after a break, or after a material change.
- Contamination specks: foreign polymer, dust, metal or paper fragments with irregular shape and colour, typically from recycled pellets or poorly stored regrind.
- Burn marks at the end of fill are a different defect. They sit in the same place every shot, where the cavity fills last, and come from trapped air compressed until it overheats. The fix is venting and injection speed, not the subject of this guide.
Not sure which defect you have? Start from the troubleshooting overview.
Mechanism: melt hangs up and burns
Most of the melt passes through the barrel and leaves as a part within a few shots. But wherever flow is slow or there is a crevice, such as scratches on the screw flights, the gap around the check ring, a poorly seated nozzle thread or a dead corner in the hot runner, some material stays for a very long time. Continuous heat degrades it from yellow to brown to carbon baked onto the metal. Once that layer is thick enough, shear or a temperature change breaks off small flakes that travel with fresh melt and end up as black specks in the part.
The practical takeaway: specks formed in the machine are a time-and-hang-up problem. The longer melt sits hot, the more it degrades. Formulation can slow degradation, but deposits that are already baked on must be removed mechanically.
Four groups of causes
1. Material
Recycled pellets or regrind carrying contamination, material that has been through several heat histories and is starting to degrade, and regrind made from parts that already had specks, which feeds the problem back into the machine.
2. Storage and handling
Hoppers, dryers, conveying lines and blenders that are not cleaned and hold dust or old pellets, bags left open in dusty areas, and granulators with leftovers from the previous job.
3. Process
Barrel or hot-runner temperatures above the resin supplier's recommended range, long residence time because the shot is small relative to the barrel, stopping the machine without lowering temperatures, and too little purging when changing material or colour.
4. Machine and mould
Scratches or wear on the screw and barrel wall, a worn check ring, a nozzle that is not seated tightly and leaves a crevice, and dead corners in the hot runner. All of these collect burnt deposits. See Injection molding for the machine basics.
Step-by-step checks, cheapest first
- Inspect pellets on a white tray. Spread virgin resin, masterbatch and regrind separately under good light. If specks are already in the pellets, the problem starts before the machine.
- Purge into air and inspect the melt. Clean pellets but specks in the purge mean they form in the barrel or nozzle.
- Log when the specks appear. Heavy at start-up, after breaks or after colour changes and then fading points to degradation during stops. A steady rate all shift points to a permanent dead spot.
- Compare shot size with barrel capacity. A common industry guideline is a shot of roughly 20–80% of barrel capacity. A very small shot in a large barrel means long residence time; consider moving the mould to a smaller machine.
- Check every temperature zone, including nozzle and hot runner, against the resin supplier's range, and measure the actual temperature of purged melt, because the display may not match the melt.
- Inspect hoppers, dryers and conveying lines for dust, old pellets and residue.
- Pull the screw, check ring, nozzle and hot runner. The most expensive step because the machine stops. Baked-on deposits or wear call for mechanical cleaning and replacement of worn parts.
Cause → check → fix table
| Cause | Check | Fix |
|---|---|---|
| Specks arrive with pellets or regrind | Inspect each material on a white tray | Notify the supplier, quarantine affected lots, never regrind parts that have specks |
| Dust and old material in handling | Inspect hopper, dryer, lines and granulator | Set a cleaning schedule; keep bags and bins closed |
| Melt degrades during stops | Many specks at start-up, then fewer | Lower temperatures during long breaks; purge before stopping and after restarting |
| Long residence time in the barrel | Compare shot size with barrel capacity | Move the mould to a better-sized machine |
| Temperature too high | Measure the temperature of purged melt | Bring it into the resin supplier's recommended range |
| Too little purging at material or colour change | Specks or old colour after a job change | Standardise the purge procedure; use a purging compound suited to the resin |
| Scratched or worn screw and barrel | Pull the screw and inspect the flights | Clean and polish, repair or replace the screw |
| Crevices at check ring or nozzle | Inspect for deposits around the check ring and nozzle thread | Clean, replace worn parts, seat them tightly |
| Dead spots in the hot runner | Barrel purge is clean but parts still have specks | Clean the hot runner; check heaters and thermocouples |
The 20–80% of barrel capacity figure is a general industry guideline; follow the machine and resin suppliers' recommendations as well.
How much can additives help?
The honest answer: only with the heat-degradation part. Deposits baked into the machine and contamination in the pellets are fixed by cleaning and material control. These are the options that actually exist, including ones ACS does not sell:
- A suitable antioxidant system slows degradation of the melt during processing. It helps where residence time is long, regrind share is high, or recycled pellets have seen several heat histories, but it does not remove material that has already burnt. See Recycled PCR / rPP compounds.
- Purging compounds push out old melt and loose deposits at material or colour changes. Several specialist makers offer them; they are not on ACS's product list.
- Melt filtration when recycled pellets are made: screens in a screen changer catch contamination before pelletising. That is an upstream fix, not one at the moulding machine. See From flakes to pellets.
What additives cannot fix: carbon baked into scratches, the check ring, the nozzle or the hot runner; contamination that arrives with the pellets; and dirty material handling.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
FAQ
Black speck questions we hear often
We get lots of black specks after start-up or a break, then they fade. Why?
While the machine stands, melt left in the barrel, nozzle and hot runner stays hot without moving, so it degrades and forms burnt deposits. When moulding restarts, those deposits break loose and show up as specks in the first parts. Lower barrel temperatures during long breaks, purge before stopping and after restarting, and reject the first parts. If specks keep coming after the process has settled, there is a dead spot in the machine where melt hangs up permanently, and it needs to be stripped and inspected.
How do we tell whether specks come from the pellets or from the machine?
Spread the virgin resin, masterbatch and regrind separately on a white tray and inspect them under good light. If pellets already carry specks or burnt particles, the problem starts with the material or its handling. If the pellets are clean, purge into air and inspect the purged melt. Specks in the purge mean they form in the machine. If both pellets and purge are clean but parts still have specks, check the hot runner and the mould.
We used a purging compound and the specks are still there. What next?
A purging compound pushes out old melt and loose deposits, but carbon baked onto scratches in the screw, a worn check ring or a poorly seated nozzle thread will not come off by purging alone. If specks drop for a while after purging and then return, pull the screw, check ring, nozzle and hot runner, clean them mechanically and replace worn parts.
Our recycled pellets already contain black specks. Can we fix that at the moulding machine?
Hardly. Those specks are contamination or material that burnt before it reached you, so the fix is upstream: sorting, washing and melt filtration when the recycled pellets are made. Ask the supplier how they check each lot for specks and keep a retained sample of every lot for comparison. If the speck level is more than the part can accept, reduce the recycled share in parts where surface appearance matters.
Specks still there after purging?
Send photos of the part, the pellets on a white tray and the purged melt on LINE. Our technical team will help you work out whether the cause is material, process or the machine itself.