Injection molding defects
Burn marks are dark brown or black discolorations, usually at the last place the cavity fills, deep ribs, or around trapped air pockets. This guide explains the diesel effect from compressed air, how to tell venting problems from gate shear burns, and a practical fix order before you accept scrap.
Defect troubleshooting
Burn marks are thermal damage on the part surface or at the flow front, not ordinary colorant variation.
On molded parts, burn marks usually look like charred, brown, or black spots at the end of the flow path, in blind ribs, around bosses, or near parting-line traps. Industry references describe the most common mechanism as trapped air in the cavity: as the melt advances, gas with nowhere to escape is compressed adiabatically, heats sharply, and can degrade the polymer at the flow front. That phenomenon is often called the diesel effect because it parallels compression heating in a diesel engine.
Burn marks are not always isolated. The same trapped air that overheats can resist the melt front, so short fill at corners or weld-line weakness may appear on the same shots. Gate-area blackening without an end-of-fill pattern points more toward excessive shear or residence time at the gate than toward vent starvation at the last fill.
For new programs, vent paths and last-to-fill regions should be reviewed in CAD and mold design before steel is cut. NetProto returns DFM feedback with instant quotes so teams can discuss rib ends, parting-line relief, and cosmetic risk on a hardened steel production tool, depending on part size and complexity, while changes are still inexpensive.
Root causes
Use location and fill history to decide whether venting, clamp force, fill profile, gate shear, or material condition is the primary lever.
| Likely driver | What you see | First lever to try |
|---|---|---|
| Insufficient or blocked vents at last-to-fill regions, ribs, or parting-line traps | Burn at farthest fill, rib tips, or pockets; may pair with incomplete fill at the same corner | Clean vent lands and inserts; add or deepen vent channels at end-of-fill per mold design practice; confirm air can reach parting line or ejector relief |
| Clamp force high enough to pinch parting-line vents closed | Burns appear near parting surface after tonnage increase; sudden onset on a stable mold | Reduce clamp to the minimum that prevents flash on that resin and geometry; re-verify vent gap after tonnage change |
| Fill rate too aggressive in the final portion of the shot | Burn worsens when injection speed is high; eases when the last stage of fill is slowed | Profile fill so air has time to escape before the last segment compresses trapped gas; balance against short fill risk |
| Excessive shear at the gate or hot-runner tip | Blackening concentrated at the gate, not only at end-of-fill; may track with high screw speed or small gate land | Lower injection speed through gate entry; review gate style and runner loss; verify hot-runner tip alignment |
| Melt overheated or held too long in the barrel | General discoloration or streaks across the part; odor or specks on the same lot | Reduce melt temperature and residence time within supplier guidance; purge after downtime; confirm lot and regrind limits |
| Moisture or contamination driving degradation at the flow front | Burn with splay-like streaks on hygroscopic grades; lot change coincides with onset | Verify drying and material handling; do not only chase venting if degradation is material-driven |
Fix strategy
Slowing fill can mask diesel-effect burns temporarily, but recurring end-of-fill char usually means air cannot leave the cavity fast enough.
Troubleshooting
Record burn location relative to gate and last fill before you change vent steel or accept slower cycles.
Location
Mark whether discoloration is at end-of-fill, inside ribs, at the gate only, or along a weld. End-of-fill and rib-tip burns strongly suggest trapped air. Gate-only burns suggest shear or residence time.
Mold bench
With the mold cold and safe, check vent lands for plastic buildup, closed-off inserts, or crushed vents. Confirm ejector and core clearances still allow air to reach open vent paths.
Machine
If burns sit near the parting line, reduce clamp incrementally while watching for flash. Slow the last segment of fill and note whether burns and short fill at corners change together.
Material
Confirm dryer status, lot identity, and whether melt temperature or screw speed increased recently. Purge and sample if degradation is suspected before blaming venting alone.
Permanent fix
When conservative process settings still char the same region, schedule vent additions or trap relief in the next mold revision. Customer-owned hardened steel tools can be modified, but mold modifications are scoped and quoted separately from production.
Prevention
Most diesel-effect burns are preventable when last-to-fill air paths are designed with the part, not added only after first shots fail.
In CAD review, flag blind ribs, deep pockets, and isolated bosses that will trap air; plan vent direction to parting line or pin vents.
Avoid sharp dead corners with no vent path in the mold design; use gradual rib bases when cosmetics and strength allow.
Document a fill profile that slows before the cavity closes out, especially on thin-wall or fast-fill programs.
Set clamp from flash trials on the actual resin, not maximum machine tonnage, so parting-line vents stay open.
Upload CAD for DFM feedback before tooling release on qualified programs so rib ends and vent strategy are flagged early.
Keep fill-only sample photos at startup so operators can separate new burns from contamination specks or drag marks.
Tooling context
Burn marks are a venting and process-window issue. For general molded dimension bands tied to surface finish class, use the published commercial tolerance table rather than inventing burn-specific limits in a troubleshooting guide.
FAQ
The most common cause is trapped air compressed as the cavity fills, heating enough to degrade the polymer at the flow front (the diesel effect). Other causes include blocked vents, excessive clamp force pinching vents, aggressive final fill speed, gate shear overheating, and melt degradation from temperature or moisture.
They often show at the last place the cavity fills, at rib tips, in deep pockets, or near parting-line air traps. Gate-centered blackening without an end-of-fill pattern more often points to shear at the gate than to vent starvation at the farthest fill.
Slowing fill, especially near the end of the shot, can give trapped air time to escape and often reduces diesel-effect burns. It is a temporary lever if vents are inadequate. Very slow fill can also cause short shot if pack is not rebalanced.
Burn marks from trapped air tend to sit at flow-related locations and track with fill profile or vent condition. Random black specks scattered on the surface may be degraded material, contamination, or wear debris and need a material and screw audit, not only vent cleaning.
Upload CAD for an instant quote and DFM feedback. Engineers can review rib ends, likely last-to-fill regions, and cosmetic surfaces while the hardened steel tool is still in planning, depending on part size and complexity. There is no minimum order quantity once the tool is running.
Ready to quote
Bring CAD and sample photos into an instant quote so DFM review and vent strategy align with your cosmetic targets. One hardened steel tool is built for the life of the part on qualified programs, depending on part size and complexity.
