What Causes Burn Marks on Molded Parts?

Last updated: 2026-09-01 · Reading time: about 4 min

A burn mark is the cavity leaving a signature: a dark or scorched patch that says something overheated right there — usually air, sometimes the melt itself. The position of the mark names the cause: burns at the last-filled corner point at trapped and compressed gas, burns near the gate point at degraded or overheated resin, and silvery streaks that look like burns point at moisture. Fix the cause at its location, and the mark leaves with it.

Key facts

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  • The classic corner burn is compressed air igniting: gas with no exit heats past the resin’s degradation point in the instant before the melt arrives, scorching both itself and the part surface it touches.
  • Gate-area burns have a different parent — material that spent too long hot: barrel temperature, residence time, screw speed and back pressure each push the resin toward degradation before it ever reaches the cavity.
  • Speed profiles matter: a front that runs full speed into a corner compresses trapped gas harder than one that eases off — which is why the same tool can burn on aggressive settings and stay clean on a shaped profile.

    • Runner system — The channel network that delivers melt from the machine nozzle to each cavity. Cold runner: solidifies between shots, produces runner waste. Hot runner: stays molten, eliminates runner waste but adds $15k–$40k to mold cost. See also related guide.

    Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. See also related guide.

    Where the burn sits tells you what caused it

    Read the map before touching a setting. A dark patch at the fill’s end — the far corner, the tip of a rib, the last place to fill — is the trapped-gas signature: air compressed until it crossed the degradation line. That is a venting conversation first, and the venting page covers the geometry end of it.
    A dull or browned zone near the gate, where material is freshest, points upstream instead: resin that degraded in the barrel or the hot runner — temperature set too high, residence stretched by a small shot in a big barrel, shear heat from an aggressive screw speed.
    And the lookalike worth catching early: silvery streaks radiating along flow lines are moisture flashing to steam, not burning at all — the drying page owns that one, and treating it with slower injection wastes a shift. One symptom, three causes, three different owners — which is why the location, not the color, is the first fact on record.

    How do you make the marks go away?

    For corner burns, give the air a way out and a softer arrival: vents restored or added at the last-filled region — grooves clog over thousands of shots, so a burn that grows slowly over a production life is often just a dirty vent, a first stop on the maintenance list — plus a speed profile that decelerates into the end of fill so the trapped gas compresses gently instead of detonating.
    For gate-area burns, cool the resin’s history: barrel and melt temperatures verified against the datasheet rather than habit, residence time shortened where the shot is small relative to the barrel, screw speed and back pressure eased to trade shear heat away. For the moisture lookalike, dry to spec and the streaks vanish without touching a pressure. The order is deliberate: geometry and conditioning before process surgery, because a vent fix survives a machine change and a temperature band-aid does not.
    The trial record then confirms which cause actually owned the mark — one clean shot after the fix, at production speed.

    Which burn-mark checks belong in the record?

  • Mark location photographed and named — corner, gate area or streak along flow; the position is the diagnosis.
  • End-of-fill venting inspected — grooves checked against the venting map, cleaned or deepened where clogged or missing.
  • Speed profile reviewed — deceleration into the end of fill, not a flat aggressive curve into a corner.
  • Thermal history verified — melt temperature, residence time, screw speed and back pressure against the datasheet window.
  • Moisture ruled in or out — silvery streaks checked against drying records before they are treated as burns.
  • The cure confirmed and written down — one clean shot at production settings, recorded in the process documentation.

    • Frequently asked questions

    Are burn marks only a cosmetic problem?

    Not always.
    Scorching is degradation — the material in and around a burn has lost molecular weight and with it strength, so a burn on a structural surface marks a local weak zone, not just a blemish. On a hidden face of a bracket that never bends, a faint burn may truly be cosmetic; on a load path or a sealing face, it is a defect that happens to be visible. Grade the face before deciding to live with it.

    Why do burns appear only on fast shots?

    Because speed is the ignition system. A fast front reaches the corner with more energy and compresses the trapped gas harder — the same air that eases out on a slow fill detonates on a quick one. That sensitivity is diagnostic: if slowing down removes the burn, the venting was marginal all along, and the honest fix is both — restore the vent and shape the profile — rather than locking the process into a speed that costs cycle time forever.

    Can degraded material just be reprocessed?

    Carefully, and sparingly: scorched resin carries decomposition products that darken and embrittle every blend it enters, so burnt sprues and parts should follow the regrind rules with low blending ratios at best — or leave the stream. The better habit is preventing the burn in the first place, since the material cost of burns compounds quietly through every regrind decision that follows.

    Key takeaways

    1. Position diagnoses the burn: end-of-fill marks mean trapped gas, gate-area marks mean degraded resin, silver streaks mean moisture.
    2. Fix geometry and conditioning first — vents and drying survive machine changes; temperature band-aids do not.
    3. Burns can be structural, not just cosmetic — degraded material is weak material, so grade the face before accepting the mark.

    If burns are marking your parts, send a photo with the part oriented as it sits in the tool. You will get the cause named by position and the fix ranked — venting, profile, thermal history — by a named tooling engineer, before the next batch runs.

    Sources
    Plastics Industry Association — industry resources on injection molding practice
    Autodesk Moldflow — injection molding simulation, reference for air-trap prediction
    HASCO — moldmaking components, reference for venting elements

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