Last updated: 2026-09-01 · 5 min read

What Is the Diesel Effect in Injection Molding — and How Do You Eliminate the Burn Marks It Causes?

The diesel effect is an adiabatic-compression phenomenon where trapped air at the end of fill is compressed so rapidly that its temperature exceeds the auto-ignition point of the resin or its volatile additives. The result is a localized burn mark — typically brown or black discoloration, charred material, or a small crater — at the last-to-fill region of the cavity.
The fix is not to reduce mold temperature or injection speed (which may create other defects) but to provide the trapped air with an escape path through vent design, parting-line relief, or ejector-pin clearance.

Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. See also related guide.

Key Facts

  • The diesel effect is named after the diesel engine, which ignites fuel by compression alone (no spark plug). In injection molding, air trapped in a blind pocket or at the end of fill is compressed to 1/10–1/20 of its original volume in milliseconds. The adiabatic temperature rise can exceed 400 °C — enough to char most thermoplastics.
  • The burn mark appears at the last-to-fill location — typically at the end of flow path, in a blind rib, behind a boss, or at an ejector-pin pocket. The location tells you where the air is trapped; the geometry tells you why there is no escape route.
  • Increasing injection speed makes the diesel effect worse — faster compression means higher peak temperature. Decreasing injection speed may reduce the burn but can introduce short shots or flow marks. The correct fix is venting, not speed reduction.

    • Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. The related guide covers adjacent topics.

    The Physics of Adiabatic Compression

    When the advancing melt front reaches the end of the cavity, the remaining air has nowhere to go. If the cavity has no vent at that location, the air is compressed by the injection pressure (700–2,000 bar) into a tiny volume. The compression happens in 10–50 milliseconds — too fast for heat to dissipate to the mold steel. This is adiabatic compression.

    The temperature rise follows the adiabatic gas law: T₂ = T₁ × (P₂/P₁)^((γ-1)/γ), where γ = 1.4 for air. Starting at room temperature (25 °C = 298 K) and compressing from atmospheric pressure (1 bar) to 500 bar: T₂ = 298 × (500/1)^(0.286) ≈ 298 × 5.9 ≈ 1,758 K (1,485 °C). Even at a lower compression ratio of 50:1 (50 bar), the temperature reaches approximately 850 °C — well above the auto-ignition temperature of most thermoplastics (350–500 °C).

    Compression Ratio Peak Temperature Effect on Resin
    10:1 (10 bar) ~570 °C discoloration (yellowing)
    50:1 (50 bar) ~850 °C charring, brown burn mark
    200:1 (200 bar) ~1,200 °C severe char, crater, material degradation

    Where the Diesel Effect Appears

  • End of flow path — the last region to fill, where the advancing melt front traps air against the cavity wall. The burn mark is a brown or black spot at the furthest point from the gate. If the part has multiple gates, the burn appears at the weld line (where two flow fronts meet and trap air between them).
  • Blind ribs and bosses — a rib that does not extend to the parting line creates a dead-end pocket where air is trapped as the melt fills around it. The burn mark appears at the root of the rib or at the base of the boss.
  • Ejector-pin pockets — if the ejector pin does not extend flush with the cavity surface, a small air pocket remains behind the pin. The compression of this air pocket produces a burn mark on the part surface at the pin location.
  • Deep-draw features — a deep pocket (such as a battery compartment or a connector recess) traps a large volume of air. Without a vent at the bottom of the pocket, the air is compressed as the melt fills from the top.

    • How to Eliminate the Diesel Effect

    1

    Add vents at the last-to-fill location

    Identify the last-to-fill region using mold-flow simulation or by observing the short-shot fill pattern at T1. Place a vent (0.02–0.05 mm deep for most resins, 3–5 mm wide) at the exact location where the burn mark appears. The vent allows air to escape rather than compress. If the vent is too shallow, it clogs with residue; if too deep, it creates flash.

    2

    Vent blind features (ribs, bosses, pockets)

    Any blind feature that creates a dead-end air pocket needs its own vent. For ribs: extend the rib insert to the parting line and add a vent at the insert shut-off. For bosses: add a vent pin (a small ejector pin with a flat-ground tip that sits 0.02 mm below the cavity surface) at the base of the boss. For deep pockets: add a vent insert or a porous-metal vent plug at the bottom.

    3

    Flush ejector pins and eliminate air pockets

    Ejector pins must sit flush with the cavity surface (±0.01 mm). A pin that sits 0.05 mm below the surface creates an air pocket that compresses and burns. Grind the pin face to match the cavity contour. For cosmetic surfaces, use sleeve ejectors or blade ejectors that seal better than round pins.

    4

    Adjust process parameters as a secondary measure

    After venting is confirmed, fine-tune injection speed: a slower injection speed reduces the compression rate and the peak temperature, but only if the air has an escape path. Reducing speed without venting merely delays the compression — the air is still trapped, just compressed more slowly. Reduce injection speed only as a complement to venting, not as a substitute.

    Frequently Asked Questions

    Is the diesel effect the same as a gas burn?

    The diesel effect is a specific type of gas burn caused by adiabatic compression of trapped air. Not all gas burns are diesel-effect burns — some are caused by volatile gasses released by the resin itself (e.g., moisture in PET, monomer in POM, or flame-retardant decomposition products).
    The diagnostic test: if the burn appears only at the last-to-fill location and is accompanied by a charred surface, it is a diesel effect. If the burn appears at random locations or along flow paths, it is a volatile-gas burn requiring different countermeasures (better drying, lower melt temperature, or a different resin grade).

    Can vacuum venting eliminate the diesel effect?

    Vacuum venting (drawing a vacuum on the cavity before injection) removes most of the air from the cavity, reducing the volume available for compression. It is effective for high-precision parts where even small burn marks are unacceptable (optical lenses, medical devices). However, vacuum venting adds cycle time (2–5 seconds per shot for the vacuum draw) and requires a sealed mold with O-ring gaskets at every shut-off surface. For most production molds, properly designed passive vents are sufficient.

    Why does the burn mark appear only on some shots and not others?

    Intermittent burn marks indicate that the vent is partially effective — it allows some air to escape but not enough on every shot. Variations in injection speed, melt temperature, and viscosity shift the fill pattern slightly, changing the amount of air trapped and the compression ratio. A vent that is partially clogged with mold deposit (released volatiles, lubricant residue) becomes less effective over time. Regular vent cleaning (every 5,000–10,000 shots for engineering resins) restores the vent opening and eliminates the intermittent burns.

    Key Takeaways

    1. The diesel effect is adiabatic compression of trapped air — the air temperature exceeds the resin’s auto-ignition point, producing a burn mark at the last-to-fill location. The root cause is insufficient venting, not excessive injection speed or mold temperature.
    2. The fix is always venting: a 0.02–0.05 mm deep vent at the exact location where the burn appears. Every blind feature (rib, boss, pocket) needs its own vent. Process parameter adjustments (slower speed, lower pressure) are secondary to venting — they cannot fix a cavity with no air escape.
    3. Vent maintenance is ongoing — vents clog with mold deposit over time. A burn mark that appears after 10,000 shots on a mold that previously ran clean is almost always a clogged vent, not a process change.

    A mold that arrives at T1 with burn marks at the end of fill is telling the tooling engineer that the vent design was incomplete. The fix is straightforward — identify the last-to-fill location, add or deepen a vent at that exact spot, and verify that the burn disappears at the target injection speed. A mold that arrives at T1 without burn marks but develops them after 20,000 shots is telling the maintenance team that the vent cleaning schedule needs to be tightened.

    Sources
    Plastics Technology — troubleshooting burn marks
    Beaumont Technologies — mold venting
    Tooling & Production — injection molding defects

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