Burn marks show up as brown, black or silver streaks on a molded part, usually at the last place the melt fills or at a rib corner. They are one of the few defects that scrap a part outright: you cannot polish them out, and on a white or food-contact part they are an automatic reject.
This is a shop-floor guide, not a textbook summary. The numbers below are the ranges we actually run on 180T to 2500T machines at our Foshan plant.
What a burn mark actually is
Two different things get called “burn marks”, and they need different fixes.
1. Diesel effect (compression burn)
Trapped air in the cavity gets compressed by the incoming melt faster than it can escape. Compression heats the air to several hundred degrees and it scorches the plastic at that spot. You see a localized brown or black patch, almost always at the end of fill, a blind rib bottom, or a thick-to-thin transition.
Tell-tale sign: the mark is always in the same place, on every shot, and it gets worse as you raise injection speed.
2. Thermal degradation (material burn)
The melt itself overheats and breaks down. You get broad brown streaks or a general yellowing, often with a acrid smell at the nozzle and occasional black specks. Unlike the diesel effect, this moves around and follows the melt temperature, not the fill pattern.
Diagnosis table
| Symptom | Most likely cause | First thing to change |
|---|---|---|
| Dark mark at end of fill, fixed location | Trapped air, no venting | Drop injection speed 20-30% at that stage; add vent 0.02-0.03 mm |
| Dark mark at rib bottom or boss | Deep blind pocket | Add ejector pin or venting insert in the pocket |
| Brown streaks all over, smell at nozzle | Melt too hot / long residence | Lower barrel temp 10-20 C; check barrel-to-shot ratio |
| Black specks, intermittent | Degraded material in dead spot | Check nozzle and barrel for dead spots, purge |
| Silver streaks near gate then darkening | Moisture + heat (hydrolysis) | Dry the resin; PA/PC must be dried properly |
| Mark appears only at high speed | Diesel effect confirmed | Profile the screw: fast to 90%, then slow the last 10% |
The fixes, in the order we try them
Step 1 – Profile the injection speed
The instinct is to slow the whole shot down. That works, but you pay for it in cycle time and in short shots on thin walls. Better: keep 85-90% of the fill fast, then drop to 20-30% speed for the last 10-15% of stroke. Air gets time to escape through the parting line while the cavity is nearly full.
Step 2 – Vent where the air actually is
Venting land depth matters more than vent width. For PP and HDPE, 0.03 mm is safe; for PA and PC you can go to 0.015-0.02 mm before you start flashing. A vent that is too deep flashes, a vent that is too shallow does nothing. If the burn is at a rib bottom, the fix is usually an ejector pin in that pocket – the pin clearance does the venting.
Step 3 – Check holding pressure and time
Over-packing closes off the vents before the air has left. Reduce holding pressure in 5% steps and shorten hold time until the part is dimensionally stable but not compressed solid. On a 3 mm wall PP part we typically run hold at 40-60% of injection pressure.
Step 4 – Melt and mould temperature
Lower melt temperature reduces both degradation and the amount of gas generated. But too low and you get weld lines and short shots. Practical range we run:
| Material | Melt temp (C) | Mould temp (C) | Vent depth (mm) |
|---|---|---|---|
| HDPE | 200-240 | 20-50 | 0.03 |
| PP | 210-250 | 20-60 | 0.03 |
| ABS | 210-250 | 50-70 | 0.025 |
| PC | 280-310 | 80-110 | 0.015-0.02 |
| PA6 (nylon) | 240-270 | 70-90 | 0.015 |
Step 5 – Dry the material
Nylon and PC hydrolyse. If you see silver streaking that turns into brown at the far end of fill, the problem is moisture, not temperature. PA6 needs 4 hours at 80 C; PC needs 3-4 hours at 120 C. A drying hopper that has lost its heater is a very common cause of sudden burn marks in humid months – and in Foshan, that is most of the year.
When it is the tool, not the process
If you have slowed the shot, vented properly, dropped the temperature and still get a mark in the same spot, the cavity geometry is the problem. Options, cheapest first:
- Move the gate so the weld/burn land falls in a non-cosmetic area
- Add a venting insert or a porous steel insert in the pocket
- Reduce the rib thickness – a rib thicker than 60% of the wall creates a hot spot and a gas trap
- Open the parting line locally by 0.02 mm around the trap
On a new tool we will usually ask for venting to be machined at the first trial rather than after, because adding it later means welding and re-polishing the cavity.
What to agree with your molder up front
Burn marks are cosmetic defects, and cosmetic defects need a defined acceptance limit or you will argue about them forever. Before production:
- Get a limit sample signed off – a physical part with the maximum acceptable mark
- State the viewing distance and lighting (we use 500 mm, 30 seconds, daylight-type light)
- Say whether the surface is cosmetic A, B or C – internal parts rarely need A
Related reading: injection moulding process guide, cost per part calculator, and clamp tonnage calculation.
Related guide: knit lines and weld lines in injection molding.
Need a part quoted, or want a second opinion on a defect you are seeing? Send us the part photo and your current parameters – we will tell you which of the five steps above is most likely your problem.
