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. It often sits at the last areas to fill or in tight corners. Trapped gas is a common cause. Material degradation, long residence time, hot-runner issues, and process settings can look the same. Location and timing are the best root-cause clues.

The Mechanism

Burn marks, sometimes called gas burns or dieseling, typically show up at the end of fill, deep in ribs, or in sharp corners. Plastic entering the cavity displaces air. If that air cannot leave through mold vents, it compresses fast. The heat is high enough to scorch the advancing melt.

That is why burns and mold venting travel together. It also explains why burns often sit with short shots in hard-to-fill zones: plastic and trapped air are fighting for the same space.

Evaluating the Root Cause

Different mechanisms need different corrections. A capable molder names the cause before changing the process.

Trapped air and inadequate venting: Air compresses and scorches. The mark often repeats at the same end-of-fill location. Injection speed: High fill speed compresses trapped air harder and worsens the diesel effect. Excessive melt temperature: Overheated resin degrades and discolors. Those marks can look like gas burns but are not as tightly tied to one location. Residence time: Material that sits too long in a hot barrel degrades before it reaches the cavity. Gate location and fill pattern: Poor flow can trap air in corners that have no vent path.

What Burns Tell the Buyer

Burn marks point to specific process or tooling conditions.

First, they map where air is trapped. Consistent burns in one place point to fill sequence and vent condition. Still rule out material-history effects.

Second, they often ride with filling problems. Burns and short shots in the same zone usually share a root cause. Slowing injection may reduce trapped-gas burns, but the supplier should verify venting before treating speed reduction as the permanent fix.

Third, they can signal material overheating. Burns from thermal degradation, not trapped air, point to melt temperature or residence time.

Why a Clean Tool Starts Burning

Two tooling mechanisms explain burns that appear months into production.

First, vents fill in over time. Per MoldMaking Technology, non-aqueous volatiles from resin additives build up and etch the steel around vents. That residue does not wipe off. It needs diamond polishing. A vent can stop working long before the visual change is obvious.

Second, vents are often cut shallow on purpose at tool build. Moldmakers typically start at the low end of recommended depth to avoid flash, expecting the molder to dial depth during trial. If that optimization never happened, the mold ran with marginal venting from day one.

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

Buyer Action Plan

Note location and timing. Cavity dependence, startup behavior, and idle-time links help separate venting from material degradation.

Do not assume the material is simply “too hot.” Most burns come from trapped air. The usual fix is better venting and adjusted fill speed, not only lower melt temperature.

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

Watch for the lighter version. When trapped air compresses but does not scorch, you get a silvery surface patch instead of a char mark. See the gas marks review.

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.