Troubleshooting · Defect guide
Burn marks and dieseling
Air with nowhere to go turns into heat
Brown-to-black scorch marks at the end of fill or in blind corners rarely come from bad resin. They come from air in the cavity that the advancing melt corners with no way out and compresses until it is hot enough to scorch the melt surface. This guide covers how to tell burn marks from black specks, the mechanism, four groups of causes, checks from cheapest to most expensive, and an honest answer: additives have almost nothing to do with it.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 22 October 2026 · about 7 min read
Confirm it is a trapped-air burn
A trapped-air burn is easy to recognise: a yellow-brown to black scorch in the same place on every shot, and that place is the last point the melt reaches, such as the far end of the part, the bottom of a deep rib or boss, a blind corner, or where two flow fronts meet. Often the same spot also shows a slightly incomplete or rounded edge.
- Trapped-air burn: fixed location at the end of flow or in a blind corner; slowing the final injection stage makes it fade.
- Black specks: dark dots or flakes at random locations, from degraded material in the machine or contamination. See Black specks.
- Burning at the gate: shear heating or overheated melt, a different mechanism (see the FAQ below).
- Short shot at the same spot: often appears together with the burn, because trapped air both resists filling and scorches the surface. See Short shots.
Mechanism: dieseling
Before injection the cavity is full of air. As melt flows in, that air must escape through vents or the parting line. If the escape route is blocked, too small, or missing at the last point to fill, the air is herded into one spot and compressed very quickly. Rapid compression drives the gas temperature up sharply, the same principle a diesel engine uses to ignite fuel. That heat scorches the melt surface brown or black. The faster you inject, the faster the air is compressed and the hotter it gets.
The practical takeaway: a burn mark is an air-escape problem. There are two main fixes: give the air a way out (vents), or push it more slowly near the end so it has time to escape. The first fixes the root cause; the second is a workaround until the mould is corrected.
Four groups of causes
1. Part design
Blind pockets where the melt itself closes off the air's escape route: deep rib bottoms, tall bosses, or a thin area surrounded by thicker walls so the melt races around and seals it before filling it. Places where two flow fronts meet can also trap air; see Weld lines.
2. Mould
The most common cause. No vent at the last point to fill, vents clogged with deposits from volatiles or wax, parting faces that have been ground or repaired until the venting disappeared, and deep cores or corners where the only exit for the air is the path the melt has already sealed.
3. Process
Final-stage injection speed that is too high, so air is compressed faster than it can leave. High melt temperature, which makes the surface scorch more easily. Clamp force far above what the part needs, which can squeeze the parting-line venting tighter.
4. Material
Very easy-flowing grades fill the cavity so fast that air cannot keep up. Volatiles or moisture add more gas that must escape. Some lubricants and additives outgas during moulding and leave deposits that block vents sooner. For background on how melt flows, see Injection molding.
Step-by-step checks, cheapest first
- Check the location over several shots. If it is in the same place every time, it is air. If it moves around, look at black specks instead.
- Slow the final injection stage. Split the speed profile so it slows only as the melt nears the burnt spot. If the mark fades, trapped air is confirmed.
- Clean the vents and parting faces. Wipe out deposits and mould again. If the problem disappears for a while and then returns, put vent cleaning on a fixed schedule.
- Run a short-shot series. Mould progressively fuller shots to see how the flow front advances and where the last point to fill is. That is where a vent must be.
- Check melt temperature and residence time. If they are above the resin supplier's recommended range, bring them down so the melt surface tolerates the hot air better.
- Add a vent at the last point to fill. This is mould work. Vent depth must match the resin's viscosity; use the depth recommended by your resin supplier or mould maker.
- Use special venting or vacuum. For deep corners where a normal vent is impossible: porous or vented inserts and cores, or evacuating the cavity before injection. The most expensive step, used when the others are not enough.
Cause → check → fix table
| Cause | Check | Fix |
|---|---|---|
| Final-stage injection speed too high | Slow only the final stage and see if the mark fades | Use a speed profile that slows near the burnt spot |
| Vents clogged with deposits | Inspect vents and parting faces; mould again after cleaning | Clean on a fixed schedule |
| No vent at the last point to fill | Short-shot series to find the last point to fill | Add a vent at that point |
| Blind pocket, deep rib or boss | Burn at the bottom of a deep feature where a normal vent is impossible | Vented or porous insert, or vacuum venting |
| Air trapped where flow fronts meet | Short shots to see where two fronts converge | Vent at the meeting point or move the gate |
| Melt too hot or held too long | Measure purged melt temperature; compare shot size with barrel capacity | Bring into the resin supplier's recommended range |
| Clamp force far higher than needed | Compare set clamp force with what the part requires | Reduce within the flash-free window |
| Volatiles from resin or additives | Vents clog unusually fast with oily or powdery deposits | Dry as the resin supplier specifies, lower temperature, clean more often |
How much can additives help?
The honest answer is almost not at all. A trapped-air burn is about the air's escape route and injection speed, and no additive makes air leave a mould faster. These are the only material factors involved, including ones ACS does not sell:
- Outgassing that builds up in vents. Lubricants, waxes or some additives used in excess can volatilise during moulding and deposit in the vents. Reducing them, or choosing a masterbatch that releases little volatile matter, can stretch the cleaning interval, but it is not the main fix.
- The resin's antioxidant system helps the melt withstand processing heat, but it does not stop scorching from very hot compressed air at one spot.
- A stiffer-flowing grade may slow the flow front as it reaches the last point to fill, but often produces short shots instead. Fix the venting first.
What additives cannot fix: missing vents, clogged vents, blind pockets with no air path, and excessive final-stage injection speed.
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
Burn mark questions we hear often
What is the difference between burn marks and black specks?
Burn marks from dieseling appear in the same place on every shot: the last point to fill or a blind corner, where air gets trapped. Black specks come from degraded material hanging up in the barrel, nozzle or hot runner, or from contaminated feed, so their location is random. The quickest way to tell them apart is to slow the final stage of injection. If the marks clearly fade, it is trapped air; if nothing changes, look at black specks instead.
Slowing injection removes the burn, but now the part short-shots. What now?
Use a multi-stage speed profile: inject fast early so the melt fills before it cools, then slow down only near the end, where the flow front approaches the trapped air. If you still have to choose between burning and short-shotting, the air has no way out, and the fix is in the mould: clean the vents, add a vent at the last point to fill, or use vacuum venting.
How deep should mould vents be?
There is no single number for every resin. The right depth depends on melt viscosity: easy-flowing resins need shallower vents. Too deep and melt flows into the vent as flash; too shallow and the air cannot escape in time. Use the depth recommended by your resin supplier or mould maker. Just as important is cleaning vents on a schedule, because deposits from volatiles gradually block them.
The part burns at the gate, not at the end of fill. Is that the same problem?
No. Burning at the gate usually comes from shear heating when melt is forced through an undersized gate at high speed, or from melt that is too hot or sits in the barrel too long. Check gate size, early-stage injection speed, melt temperature and residence time. Burning at the end of fill or in blind corners is the trapped-air problem described on this page.
Still burning after you slowed injection?
Send photos from several shots, the burn location and your machine settings on LINE. Our technical team will help you separate trapped air, mould and material causes.