Burn Marks in Injection Molding: Causes and How to Prevent Them

A burn mark is a scorched, discolored spot—usually brown or black—on a molded part, commonly found at the last areas to fill or in tight corners. Trapped gas is a frequent culprit, but material degradation, excessive residence time, hot-runner issues, and process settings can produce identical symptoms. Location and timing provide the strongest clues for root cause verification.

The Mechanism

Burn marks—sometimes called gas burns or dieseling—typically appear at the end of fill, deep inside ribs, or in sharp corners. As plastic rushes into the cavity, it displaces air. If that air cannot escape through mold vents, it is compressed rapidly. This high pressure generates heat intense enough to scorch the advancing plastic.

This mechanism tightly links burn marks to mold venting. It also explains why burns frequently appear alongside short shots in hard-to-fill areas: both result from plastic and trapped air competing for the same constrained space.

Evaluating the Root Cause

Different mechanisms require different corrections. A capable molder diagnoses the specific cause before altering the process.

Trapped Air and Inadequate Venting: Air compresses and scorches. This often repeats consistently at end-of-fill locations. Injection Speed: High fill speeds compress trapped air more violently, exacerbating the diesel effect. Excessive Melt Temperature: Overheated resin degrades and discolors, creating marks that mimic gas burns but aren’t strictly location-dependent. Residence Time: Material sitting too long in a hot barrel degrades before it ever enters the cavity. Gate Location and Fill Pattern: Poor flow dynamics can trap air in corners without venting access.

What Burns Tell the Buyer

Burn marks are highly diagnosable defects that indicate specific process or tooling conditions:

First, they map where air is trapped. Consistent burns in the same location point to fill sequence and vent condition. However, material-history effects must still be ruled out.

Second, they often accompany filling problems. Burns and short shots in the same zone share a root cause. While slowing injection speed might lessen trapped-gas burns, the supplier should verify venting capability before declaring speed reduction the permanent fix.

Third, they signal material overheating. Burns caused by thermal degradation—rather than trapped air—point directly to melt temperature or residence-time problems.

Why a Clean Tool Starts Burning

Two tooling mechanisms explain burn marks that suddenly appear months into a production run.

First, vents fill in over time. According to MoldMaking Technology, the buildup of non-aqueous volatiles from resin additives etches the steel around vents. This residue cannot simply be wiped away; it requires diamond polishing. A vent that slowly closes with residue stops functioning long before the visual change is apparent.

Second, vents are often intentionally cut shallow initially. Moldmakers typically set vents at the low end of the recommended depth range to avoid flashing the tool, expecting the molder to dial in the final depth during trial. If that optimization never occurred, the mold operates with marginal venting from day one.

When burns appear mid-program, ask when the vents were last measured against their baseline depth and whether they were fully optimized during qualification.

Buyer Action Plan

Note the location and timing. Cavity dependence, startup behavior, and relation to idle time help separate venting issues from material degradation.

Do not assume the material is simply “too hot.” Most burns stem from trapped air. The fix is usually improved venting and adjusted fill speed, not just lowering the melt temperature.

Request a controlled comparison. Require marked sample photos, cavity history, process condition logs, vent inspection reports, and side-by-side before/after trial results.

Watch for the lighter version. When trapped air compresses but does not reach scorching temperatures, it produces a silvery surface patch rather than a char mark. See the gas marks review for this pattern.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, diagnose production problems remotely, or certify suppliers. Confirm defect causes and corrective actions with your supplier against your specific part, tool, and process.