Can burn marks be caused by something other than gas trapping?

Yes. Excessive injection speed, barrel overheating, and material degradation also cause discoloration. However, gas trapping is the most common root cause, especially in ribs and bosses.

Should I lower injection speed to fix burn marks?

Lowering speed reduces gas compression but may cause short shots. A better approach is to add venting first, then optimize speed.

How do I find where gas is trapping in my mold?

Use mold flow simulation to predict gas trap locations. In existing molds, look for burn marks on parts — they indicate the exact gas trap points.

Troubleshooting Guide6 min read

Why Injection Mold Burn Marks Occur (And How Gas Venting Fixes Them)

Key Takeaway: Burn marks and short shots usually mean trapped gas. Diagnose by locating defects on the part, then fix by adding parting-line vents, gas-release pins, or porous inserts at the gas trap points.

Identifying the Root Cause

Burn marks and short shots share a common root cause: gas trapped in the mold cavity. When the melt front pushes air into a dead end, two things can happen. If the gas compresses enough, it ignites the resin — creating burn marks. If gas pressure exceeds injection pressure, the cavity cannot fill — causing short shots.

Before modifying the mold, confirm that the defect is gas-related:

Location consistency — gas defects appear in the same spot on every part.
Speed sensitivity — defects worsen at higher injection speeds.
Pattern match — marks appear at rib tips, boss bases, or end-of-fill areas.

Step-by-Step Diagnostic Process

Follow this sequence to isolate and fix gas-related defects. The process applies to both new and existing molds.

Step 1: Map defects — Mark the exact location of burn marks or unfilled areas on 5–10 consecutive parts. Consistent locations confirm gas trapping.
Step 2: Check existing vents — Inspect parting-line vents for resin deposits or wear. Clogged vents are the most common cause of "sudden" burn marks in established molds.
Step 3: Match to flow path — Compare defect locations to the expected fill pattern. Gas traps at last-to-fill positions are normal; gas traps mid-fill suggest flow imbalance.
Step 4: Select the fix — Use the table below to choose the right venting solution.

Venting Solutions by Defect Location

Defect Location	Recommended Fix	Component
Parting line area	Add or deepen parting-line vents	Standard vent machining
Rib tips	Add porous metal vent inserts at rib base	Cavity Inserts
Boss bases	Use gas-release center pins	Gas Release Units
Deep cavity walls	Add vacuum-assisted venting	Vacuum Venting
Cosmetic surfaces	Use porous sintered metal inserts	Porous Metal Guide

Process Adjustments (Temporary Fixes)

While mold modifications are being implemented, these process changes can reduce defect severity. As noted by injection molding troubleshooting references, process changes alone rarely eliminate gas defects permanently.

Reduce injection speed by 10–20% to give gas more time to escape.
Lower melt temperature by 5–10°C to reduce gas generation.
Add a brief hold-pressure delay to allow gas to migrate to vent locations.
Switch to a lower gas-generating resin grade if available.

Prevention in New Mold Design

Gas trapping is predictable. Moldflow and similar mold flow simulation software can identify gas trap locations before the mold is cut. Design venting into the mold from the start:

Place parting-line vents at every predicted last-to-fill location.
Spec gas-release center pins for all boss features deeper than 2× wall thickness.
Include porous vent inserts in blind ribs and cosmetic areas.