A burn mark is one of the few molding defects that looks exactly like what it is: a scorched, brown-to-black discoloration, usually at the end of fill, in a blind pocket, at the tip of a rib, or right where two flow fronts closed on each other. And it’s one of the most commonly misdiagnosed, because the word “burn” sends people straight to the barrel temperature.
The mechanism tells you why that’s the wrong first move. A mold cavity is full of air before the shot. When melt rushes in, that air has to leave — and if it can’t, the incoming plastic traps and compresses it. Compress a gas fast enough and it heats violently; this is the same adiabatic compression that fires a diesel engine, which is why the industry calls it dieseling. The trapped pocket gets hot enough to scorch and carbonize the resin around it, leaving a burn. The plastic didn’t get too hot in the barrel. The air got too hot in the cavity.
That reframing is the whole article: burns are a venting and speed problem far more often than a melt-temperature problem.
Where gas gets trapped
Burns show up in predictable places — wherever air has nowhere to go as the melt closes in:
- Last-to-fill areas. The end of the flow path, where all the displaced air ends up.
- Blind pockets, deep ribs, and boss tips. Dead ends with no vent — the air is cornered.
- Where two flow fronts meet. The fronts close on the air between them; this is why a burned weld line is a classic.
- Thin sections at the end of a long flow, where the melt races in and outruns the air’s escape route.
- Anywhere the vents are blocked. Vents clog with resin residue, gas deposits, and mold release over a run — a tool that vented fine last month may not today.
The cause tree
| Cause | What’s happening | Fix |
|---|---|---|
| Inadequate or blocked venting | Air has no escape path (or the vent has plugged with residue) | Add vents at the burn location; clean existing vents |
| Injection speed too high | Melt outruns the air; no time for it to escape before compression | Reduce injection speed, especially at end of fill |
| Injection pressure too high | Drives compression harder | Reduce injection pressure |
| Screw speed / back pressure too high | Shears and degrades melt, adding volatiles and gas | Reduce screw RPM and back pressure |
| Clamp tonnage too high | Excess clamp crushes vents closed | Set clamp to what the part needs, not machine maximum |
| Melt or mold temp too high | Degradation adds gas and lowers the scorch threshold | Lower melt and mold temperature |
| Wet resin / volatiles | Moisture and additives generate gas in the barrel | Dry to spec; check additive load |
| Dirty mold surface | Residue and excess mold release burn and gas off | Clean the mold face; use release sparingly |
The pattern worth internalizing: the top three are venting, speed, and clamp — all about whether the air can get out — not the heat setting people reach for first.
Vent depth: the line between burns and flash
Venting has a two-sided failure that makes it a real engineering decision rather than “cut it deeper.”
- Too shallow and the gas can’t escape — burns, short shots, and poor fill.
- Too deep and plastic flows into the vent — flash.
There is a workable window between them, and it depends on the resin: a stiff, high-viscosity material tolerates a deeper vent without flashing, while an easy-flowing, low-viscosity resin will push into a vent that a thicker material wouldn’t. Vent depth is therefore a material-specific spec, not a universal number — take it from the resin supplier’s guidance and the toolmaker rather than from habit. (Any figure someone quotes from memory should be treated as a starting point to verify.)
Two supporting points matter as much as depth:
- The vent has to lead somewhere. A vent land that doesn’t open into a relief channel and out of the mold just moves the trap a few millimeters.
- Vents need maintenance. They plug over a production run. Cleaning vents belongs on the preventive-maintenance schedule, not on the emergency list after burns appear.
And a related trap: over-clamping crushes vents shut. Running maximum tonnage “to be safe” can convert a properly vented tool into an unvented one — which is how a plant trades flash for burns without realizing they caused it.
Slow it down at the end of fill
The other high-leverage lever is speed, and specifically speed where the gas is. Air escapes at a finite rate; if the melt front arrives faster than the vents can bleed the cavity, you compress what’s left.
Because burns are usually at the end of fill, the effective fix is often a multi-stage injection profile: fill fast through the bulk of the cavity to keep the melt hot and avoid short shots and weld lines, then slow the final segment as the melt approaches the last-to-fill area so the remaining air has time to escape. This is a better answer than slowing the entire shot, which costs cycle time and can introduce flow marks and cold weld lines.
A practical sequence
- Map the burn. Is it at end of fill, in a blind pocket, at a rib tip, or on a weld line? The location names the trapped-gas site.
- Inspect the vents at that location — present? blocked? leading out of the mold?
- Check clamp tonnage. Are you crushing the vents by running maximum clamp?
- Reduce injection speed at the end of fill (stage the profile) before touching barrel heats.
- Rule out gas sources: wet resin, excessive screw speed and back pressure, degradation, dirty mold face, over-applied release.
- Only then consider melt/mold temperature — and treat it as a contributor to degradation, not the primary cause.
FAQs
What actually causes burn marks in injection molding?
Trapped air, not an overheated barrel. The cavity is full of air before the shot, and if that air can’t escape as the melt fills, it gets compressed rapidly — and compressing a gas fast heats it violently, the same adiabatic effect that fires a diesel engine. That superheated pocket scorches and carbonizes the resin around it. So the cause is almost always inadequate or blocked venting, excessive injection speed, or vents crushed shut by too much clamp — the melt temperature is usually a contributor at most.
Why does turning down the barrel temperature not fix my burns?
Because the heat doing the damage isn’t in the barrel — it’s in the trapped air pocket inside the cavity. Lowering melt temperature slightly reduces degradation and raises the margin before scorching, but it does nothing about air that has no escape path. If the vent at the burn location is missing, plugged, or crushed by excessive clamp, the compression will keep superheating that pocket regardless of your barrel setpoints. Vent it and slow the end of fill first.
How deep should a mold vent be?
Deep enough to let gas out, shallow enough that plastic won’t flow into it — and where that window sits depends on the resin’s viscosity. Stiff, high-viscosity materials tolerate deeper vents; easy-flowing, low-viscosity resins will flash into a vent that a thicker material wouldn’t. That makes vent depth a material-specific specification from the resin supplier and toolmaker, not a universal number. Just as important: the vent must lead into a relief channel and out of the mold, and vents need periodic cleaning because they plug with residue during production.
Can too much clamp tonnage cause burn marks?
Yes, and it’s a commonly missed cause. Running maximum clamp “to be safe” can crush the vents closed, turning a properly vented tool into an unvented one — so the plant trades flash for burns and short shots without connecting the two. The correct approach is to set clamp tonnage to what the part’s projected area and material actually require, with sensible margin, rather than defaulting to the machine’s rating. If burns appeared after someone increased clamp, that’s your first suspect.