Burn marks, silver streaks and stubborn short shots usually have one root cause: air that had nowhere to go. When melt fills a cavity in a fraction of a second, the air ahead of the flow front is compressed and heated so fast that it scorches the plastic or blocks filling entirely. Venting is the cheapest fix in the whole process — no steel changes to the part geometry, no different material — yet it is the most commonly under-engineered feature on prototype molds.
Why trapped air actually burns plastic
Air is a poor conductor of heat, but compress it hard enough and it behaves like a diesel engine. At typical injection pressures the flow front can compress a trapped air pocket to several MPa in milliseconds, and the temperature spike can exceed the decomposition point of ABS, PC or PA. The result is the classic dark scorch at the end of fill, often on a parting line or at the blind end of a rib. Beyond the cosmetic damage, decomposed polymer outgasses corrosive byproducts that pit the mold steel over time.
Trapped gas is not always air. Moisture that survived a lazy drying cycle turns to steam, and some resins (especially flame-retardant grades and regrd blends) release volatile gas during fill. Those gases are produced by the melt itself, so vents at the end of fill help but a dryer helps more.
The numbers that matter: vent depth and land
A vent is a shallow groove cut at the parting surface, and two dimensions control everything:
- Vent depth — the gap between the parting surfaces at the vent. Too deep and you flash; too shallow and the vent seals with residue after a few shots. Typical values: 0.010–0.015 mm for rigid PC, 0.015–0.025 mm for ABS and PC blends, 0.020–0.030 mm for PP, PA and POM (these are less viscous and flash more easily — start at the low end).
- Vent land — the flat length before the vent deepens into a relief channel. Usually 0.75–1.5 mm. If the land is too long, gas can't escape fast enough and the vent acts as a plug; too short and the flash builds immediately.
Behind the land, open the relief channel to 0.3–0.5 mm deep so the gas actually has somewhere to dump. On a well-designed parting surface, vents 8–12 mm wide spaced every 25–40 mm around the end of fill are a reasonable starting pattern.
Where to put vents
Follow the last place to fill
Vents belong where the flow front arrives last. On a balanced part that is often the far corner or the blind end of a deep rib. If you are not sure where that is, watch the first shots: the burn mark itself is a map. It marks the exact spot where gas got trapped, so put a vent there and the scorch usually disappears within a shot or two.
Ribs and bosses
Deep ribs fill last and trap air at their tips. Parting-line vents can't reach inside the cavity there, so use vented ejector pins — standard pins with two or three ground flats (0.02 mm deep, 2–3 mm long land) that bleed gas straight out the back of the mold. They cost slightly more than a plain pin and are the single most effective fix for rib-end burn. Remember they need periodic cleaning; a sealed vented pin is worse than none because it hides the problem.
Weld lines
Where two flow fronts meet, the trapped air and cold fronts create a weak weld line. A vent near the confluence improves both appearance and weld-line strength — often by 20–30% in break tests, though the exact gain depends on material and geometry.
Vacuum venting for prototype molds
Prototype molds cut quickly in aluminum are especially vent-sensitive: fewer shots to "wear in" a seal, and small cavity blocks mean less room for a full vent pattern. A vacuum venting system pulls the cavity down to roughly 20–40 kPa absolute before injection, removing most of the gas before it can be compressed. For hard-to-fill parts — thin walls under 1 mm, long flow paths, high-viscosity PC — vacuum venting on a prototype tool frequently turns a problem part into a smooth first-shot success.
It adds cost and plumbing, so our typical decision rule: if the part needs three or more venting iterations on a conventional tool, or the flow length ratio exceeds about 250:1, spec vacuum venting from the start.
A troubleshooting sequence that works
- Confirm the material was dried to spec (ABS typically 2–4 h at 80 °C; PC 3–4 h at 120 °C). Steam streaks look like air burns and waste venting effort.
- Slow the fill speed 20–30% on the affected shots. If the burn fades, it is flow-front gas, and venting will fix it.
- Add or deepen vents at the burn location. Work in 0.005 mm depth steps — chasing flash by over-deepening a vent costs more to repair than a second trial.
- If rib and boss ends still scorch, switch to vented pins.
- Only then consider process tricks like profiled injection speed or vacuum venting.
Design-side notes that reduce venting pain
A few plastic injection DFM choices make venting easier before a mold is ever cut: keep nominal wall thickness uniform so the flow front moves predictably; avoid blind pockets deeper than 3× wall where gas has no escape route; and give ribs a draft of at least 0.5° so the last-filled tip is not a knife-edge that seals shut. If your team is weighing prototype tooling options, our soft and production tooling service cuts vents into aluminum molds the same week, and first shots usually tell you within a day where the gas lives.
Venting is the rare mold problem where the fix is measured in hundredths of a millimeter and costs a machining hour, not a redesign. Get the depth, land and placement right, and most burn marks and short shots disappear on the next shot.
