Burn Marks in Injection Molding — How Do You Identify the Root Cause, Where Do They Occur, and What Process Changes Eliminate Them?

Last updated: 2026-09-03 · Reading time: about 6 min

A burn mark — the diesel effect — is a black or brown scorch where trapped air compresses so fast it heats past the resin’s degradation point and ignites the melt. It repeats at the same spot every shot, so find the air trap and give it an escape: vents, a slower fill, or vacuum.
See also related article.

Key facts

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  • Trapped air compresses adiabatically to 50–100 bar and 300–500 °C — above the resin’s degradation temperature — and burns the flow front into a localized 2–10 mm defect that weakens the part.
  • Four places trap air: the end of fill, weld lines, blind ribs and deep pockets — plus a fifth look-alike, resin degradation from a melt temperature that is simply too high.
  • Fix the air escape — clean or add vents (0.01–0.05 mm deep, 5–10 mm wide), slow the injection speed, add overflow wells — and as a last resort vacuum venting, which eliminates 95–100%.

    • Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. See also related guide.

    Where do burn marks occur — and what causes each one?

    The location tells you the cause. Four spots account for almost every diesel-effect burn.

    1. At the end of fill. The most common spot. The melt pushes air ahead of it into the last void; if the vents are too small, too few or blocked by residue, the air compresses to 50–100 bar and 300–500 °C and ignites the flow front, leaving a carbonized black or dark-brown mark. Add vents (0.02–0.05 mm deep, 5–10 mm wide), clean the blocked ones, or slow the fill so the air has time to escape.
    2. At weld lines. When two flow fronts meet, the air between them is trapped and ignites as they converge. The burn comes with a weak weld, because degraded chains do not bond across it. Add a vent at the weld, an overflow well past it to carry the burned melt away, or move the gate to relocate the weld to a vented spot.
    3. In blind ribs and deep pockets. A rib that stops short of the part edge is a dead end, and a narrow rib (typically 50% of the wall) cannot be reached by the parting-line vent. Add a vent pin or slot at the rib end, extend the rib to the edge so the air escapes, or reduce the rib depth so it fills before the air compresses.
    4. From resin degradation, not air. If the melt runs above the degradation temperature, the resin burns in the barrel or nozzle and injects as random black specks 1–3 mm across. Drop the melt temperature 10–20 °C, cut the residence time, purge the barrel and nozzle, and check the heater bands for a hot spot.

    What process changes eliminate burn marks?

    Five levers, in the order you would try them. The last one removes the air entirely.

    Process change How it works Effectiveness
    Reduce injection speed A slower fill gives air time to escape through the vents Eliminates 60–80% of air-trap burns
    Add or clean vents Vents provide an escape path for trapped air Eliminates 80–100% of end-of-fill burns
    Reduce melt temperature Keeps the resin below its degradation point Fixes degradation burns; may cause short shots
    Add overflow wells Wells collect the burned melt past the weld Fixes weld-line burns; adds trimming cost
    Vacuum venting A pump evacuates the cavity before injection Eliminates 95–100% of all burn marks

    Vacuum venting is the fix of last resort — and the most complete. A pump pulls the cavity down to 10–50 mbar before injection, so there is no air left to trap, compress or ignite. It works regardless of geometry or resin, improves weld-line strength, and lowers the injection pressure the melt has to fight. At $5,000–15,000 for the pump, tank and controls, it earns its place only on molds whose burns survive venting and process changes.

    “A burn mark is the mold telling you where the air has nowhere to go. In the DFM we run the fill and mark every last-to-fill corner, weld line and blind rib, then put a vent there before we cut steel — because a vent added on the screen is free, and a vent burned into the tool after T1 costs a weld and a re-trial.”

    — Project Engineer, aCatMould

    Frequently asked questions

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • How do you tell a burn mark from a contamination black spot?

    A burn mark is a sharp-edged black or dark-brown scorch at a fixed location — end of fill, weld line or rib end — that repeats on every shot, because the air traps in the same place every time.

    A black spot is a random speck that moves around shot to shot, from contaminated resin, degraded material or a dirty barrel. Burns are fixed by improving the air escape; spots are fixed by cleaning the barrel and nozzle and screening the resin. Same spot every shot means a burn; random means a spot.

    What is the maximum vent depth — and what if the vent is too deep?

    The vent must let air out but hold melt back. Typical maxima by viscosity: PP and PE 0.03–0.05 mm, ABS and PS 0.02–0.04 mm, PC and PMMA 0.03–0.05 mm, PA and POM 0.01–0.03 mm, TPU and TPE 0.05–0.08 mm. The groove is 5–10 mm wide, cut on the parting line.

    Too deep and melt leaks in as flash you have to trim at $0.01–0.05 a part; too shallow (below 0.01 mm) and the air cannot escape, so the burn stays. Clean the vents every 5,000–10,000 shots — release residue, degraded resin and dust plug them and the burn returns.

    Can mold-flow analysis prevent burn marks — and what does it show?

    Yes. Mold-flow analysis predicts the air-trap locations before the mold is built: it traces the flow front, flags the last-to-fill areas, and reports the trap severity — the trapped-air volume, the compression ratio and the expected temperature rise.

    That lets you move the gate, add vents at the predicted traps, place overflow wells at the welds, or adjust the geometry — a vent rib, or extending a blind rib to the edge. It runs $500–2,000 for a mid-size part at about 80–90% accuracy; the misses come from vent blockage or resin-batch variation the model does not see.

    Key takeaways

    1. A burn mark is trapped air compressed to 300–500 °C that ignites the melt — it repeats at the same spot every shot (end of fill, weld line, blind rib), unlike a random contamination speck.
    2. Fix the air escape: clean or add vents 0.01–0.05 mm deep and 5–10 mm wide, slow the injection speed, add overflow wells at welds; vacuum venting to 10–50 mbar kills 95–100% when the rest fail.
    3. Mold-flow analysis predicts the traps at about 80–90% accuracy before steel is cut, so the vents go in on the screen instead of after T1.

    Burn marks are almost never a mystery — they are a map of where the air cannot leave. Read the location, add the escape, and verify on the next shot. When burns survive venting and process changes, vacuum venting removes the air itself. If a part keeps scorching, a DFM review finds the traps and puts the vents in before the mold is cut.

    Sources
    Plastics Technology — burn marks, venting and molding troubleshooting
    Beaumont Technologies — mold filling and air-trap analysis
    Autodesk Moldflow — mold-flow simulation and air-trap prediction
    MoldMaking Technology — venting and mold design practice
    Society of Plastics Engineers — injection molding defects and process

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet