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, often found at the last areas to fill or in tight corners. Trapped gas is an important pattern to verify, while material degradation, residence time, hot-runner condition, and process settings can produce similar evidence. Location and timing provide useful clues, but the cause still needs verification. This guide is part of the injection molding defects section.

What a Burn Mark Is

Burn marks (sometimes called gas burns or dieseling) appear as discoloration or scorching, typically at the end of fill, in deep ribs, or in corners where air can get trapped. One common mechanism is the “diesel effect”: as plastic rushes into the cavity, it pushes air ahead of it. If that air has nowhere to escape, it can be compressed to a high pressure and temperature and scorch the plastic. When this mechanism is responsible, the burn often appears near the trapped-gas location.

That’s why burn marks and venting are so closely linked, and why burns so often show up in the same hard-to-fill spots as short shots: both are symptoms of plastic and air competing for the same space at the end of fill.

Why Burn Marks Happen

Source	What’s happening
Trapped air / inadequate venting	Air can’t escape, compresses, and may scorch; an important pattern to verify at end-of-fill locations
Injection speed too high	Fast fill compresses trapped air more violently
Excessive melt temperature	Overheated resin degrades and discolors
Long residence time	Material sitting too long in a hot barrel degrades
Gate location / fill pattern	Poor flow can corner air where it can’t vent

Trapped air with inadequate venting is a common mechanism, especially when burns repeat at end-of-fill locations. Material degradation, excessive residence time, hot-runner behavior, and process conditions can produce similar discoloration. The supplier should verify the mechanism before assigning a tooling or process correction.

What Burns Tell a Buyer

Burn marks are useful precisely because they’re diagnosable:

• They may mark where air is trapped. A consistent burn in the same location makes fill sequence and vent condition important checks, but degradation and material-history effects should also be reviewed.
• They can accompany filling problems. Burns and short shots in the same area create a pattern worth reviewing together. Higher speed may worsen trapped-gas burns, but venting should not be declared the root cause until the supplier verifies it.
• They can signal material overheating. Burns from degradation (rather than trapped air) point to melt temperature or residence-time issues in the process.

A capable molder reads which of these is happening and responds accordingly—improving venting, reducing injection speed near the end of fill, or correcting temperature—rather than simply pushing harder. How a supplier diagnoses and resolves burns is part of the process discipline covered in the supplier capability checklist.

What a Buyer Should Do

• Note the location and timing. Repeat location, cavity dependence, startup behavior, and relation to idle time help separate venting, degradation, and equipment patterns.
• Don’t assume it’s just “too hot.” Most burns are trapped air, not melt temperature—so the fix is often venting and fill speed, not simply cooling the melt.
• Connect burns and short shots as a verification question. If both appear in the same area, ask whether fill sequence and trapped gas connect them, and request evidence rather than assuming a shared root cause.
• Request a controlled comparison. Ask for marked sample photos, cavity and shot history, process conditions, vent inspection findings, and before/after trial results.
• Watch for the lighter version. When air is trapped but compression falls short of scorching, the result is a silvery surface patch instead of a char mark—see the gas marks buyer review for that pattern and the venting questions that go with it.
• Consider geometry and gating. Deep ribs and corners are natural air traps; how the part is gated affects where air gets cornered, linking to gate design.

This is an independent buyer resource and does not replace supplier engineering review. Whether a burn is a venting, process, or material issue depends on the specific tool and part, so confirm the cause and remedy with your molder.

Buyer FAQs

What causes burn marks in injection molding?

Trapped air is one common cause. As plastic fills the cavity, it pushes air ahead of it; if that air cannot escape, it may compress and heat enough to scorch the plastic. Other causes include excessive material temperature or residence time, degradation, hot-runner conditions, and process settings. Location, cavity dependence, and timing help the supplier separate these patterns.

Are burn marks caused by the plastic being too hot?

Not usually in the way the name suggests. While excessive melt temperature and long residence time can degrade and discolor material, most burn marks come from trapped air being compressed and scorching at the end of fill—not from the melt itself being too hot. That’s why the typical fix is better venting and adjusted fill speed rather than simply lowering the melt temperature.

How are burn marks fixed?

The correction depends on the verified cause. If trapped gas is confirmed, vent cleaning or modification and a validated fill profile may help. If degradation is responsible, material heat history and residence time require review. Ask for before/after trial evidence and a repeatable production window rather than accepting a single improved sample.

Why do burn marks and short shots appear together?

They can appear together when trapped gas and end-of-fill resistance are involved: gas may resist filling and may also heat enough to discolor the part. They can also arise from different mechanisms. Ask the supplier to map the defects to fill sequence, inspect venting, review material heat history, and demonstrate the correction through a controlled trial.