Last updated: 2026-09-02 · 5 min read

Burn Marks in Injection Molding — What Causes the Black or Brown Discoloration at the End of Fill and How Is It Fixed?

Burn marks (also called diesel effect) are black or brown discoloration at the end of fill or in areas where air is trapped — the cause is compressed air igniting the molten plastic, similar to a diesel engine.
The fix is to improve venting (add or deepen vents to 0.02–0.05mm depth), reduce injection speed, or relocate the gate so the air has an escape path before the melt front reaches the trap point. Burn marks are a fill-phase defect — they appear at the last point to fill, where the air has no escape route and is compressed to the point of ignition.

Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. See also related guide.

Key Facts

  • Burn marks occur when trapped air is compressed to 200–400°C — above the auto-ignition temperature of most resins (PP at 350°C, ABS at 400°C, PC at 550°C). The compressed air ignites the melt, creating a localized burn that appears as black or brown discoloration.
  • Vent depth is critical: 0.02mm for PP/PE, 0.03mm for ABS/PS, 0.015mm for PC/PA — vents that are too shallow do not allow air to escape; vents that are too deep allow flash. The vent depth must be matched to the resin’s viscosity.
  • Burn marks are different from gas traps: a burn mark is a surface defect (discoloration); a gas trap is a void or bubble inside the part where the air was compressed but did not ignite. Both indicate inadequate venting, but the fix is the same — add or improve vents at the trap location.

    • Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. The related guide covers adjacent topics.

    Burn Mark Causes and Solutions

    Cause Solution
    Insufficient venting Add vents or deepen existing vents to 0.02–0.05mm; add vent wells at the end of fill
    Injection speed too high Reduce injection speed so the air has time to escape through the vents before the melt front reaches the trap
    Gate location causing air trap Relocate the gate so the fill pattern pushes air toward the vents rather than trapping it
    Blocked or clogged vents Clean vents (mold maintenance); add vent area proportional to the volume of air to be evacuated
    Excessive melt temperature Reduce barrel temperature to lower the melt temperature — the auto-ignition threshold is closer at higher melt temperatures

    Vent Depth by Resin

  • PP, PE (low viscosity): vent depth 0.02mm — the low viscosity melt flows easily into shallow vents, so the vent must be shallow enough to prevent flash but deep enough to allow air escape.
  • ABS, PS, HIPS (medium viscosity): vent depth 0.03mm — the medium viscosity allows slightly deeper vents without flash risk.
  • PC, PA, POM (high viscosity): vent depth 0.015mm — the high viscosity melt requires shallower vents to prevent flash; these resins also require more vent area because the air escapes more slowly.

    • What This Means for Your Quotation

    The mold design should include venting analysis — identifying the last-to-fill areas and specifying the vent depth and location for the production resin. The quotation should document the venting strategy so the buyer understands how the mold will prevent burn marks. When the venting is inadequate, the mold requires rework (additional vents or gate relocation) that delays the program and adds cost.

    Frequently Asked Questions

    Can burn marks be eliminated by adding vacuum vents?

    Yes — vacuum vents (vacuum system) actively evacuate air from the cavity before the melt front reaches the trap point. Vacuum systems are effective for deep draws and complex geometries where passive vents are insufficient. The vacuum system adds $2,000–5,000 to the mold cost but eliminates burn marks and gas traps in applications where venting alone is inadequate.

    How do you distinguish burn marks from material degradation?

    Burn marks from trapped air appear at the last-to-fill area and are localized (small, discrete spots). Material degradation appears throughout the part as uniform discoloration — the cause is excessive barrel temperature, excessive residence time in the barrel, or contaminated material. If the discoloration is localized at the end of fill, it is a burn mark (venting issue); if it is uniform across the part, it is material degradation (process or material issue).

    Do vented pins or porous metal inserts help with burn marks?

    Yes — vented ejector pins and porous metal inserts (sintered metal) provide localized venting at trap points where conventional parting-line vents are not effective. Porous metal allows air to escape through the microscopic pores but prevents the melt from flashing. These solutions are effective for specific trap points but add cost ($200–500 per pin or insert) and require periodic cleaning to maintain the venting effectiveness.

    Key Takeaways

    1. Burn marks are caused by trapped air igniting the melt at the end of fill — the fix is to improve venting (add or deepen vents), reduce injection speed, or relocate the gate.
    2. Vent depth must match the resin viscosity: 0.02mm for PP, 0.03mm for ABS, 0.015mm for PC — too shallow traps air, too deep causes flash.
    3. The mold design should include venting analysis identifying the last-to-fill areas and specifying vent depth and location — so burn marks are prevented by design, not by trial-and-error mold rework.

    Send aCatMould the part drawing with the resin specification, and the mold design includes a venting analysis with vent depth and location for the production resin — so burn marks are prevented by design, not by trial-and-error rework.

    Sources
    Plastics Technology — burn marks and diesel effect in injection molding
    SPE — mold venting design guidelines
    MoldMaking Technology — troubleshooting burn marks and gas traps

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