Venting at Diameter Transitions in Stepped Core Pins
Key Takeaway: Step transitions create unique gas traps that parting line vents cannot reach. Gas release stepped core pins with SVC channels vent air directly from the step face along the shaft to atmosphere. Specify SVC channel orientation toward the step transition face for optimal venting.
How Air Traps Form at Steps
When resin flows from the narrow section to the wide section of a stepped hole, the sudden diameter increase creates a flow separation zone at the step face (the shoulder). The resin expands outward to fill the larger diameter, but the inner surface at the step shoulder sees delayed contact. Air pockets form in this delay zone.
The problem is amplified by:
- Large step ratios — Greater diameter change means larger expansion zone and bigger air pockets
- Deep steps — More distance from the parting line means less chance of air escaping naturally
- Fast injection speed — Higher velocity creates stronger flow separation at the transition
Standard vs Gas Release Stepped Pins
| Feature | Standard Stepped | Gas Release Stepped |
|---|---|---|
| Step geometry | Same (one-step or two-step) | Same + vent channel |
| Air trap at step | No venting — trapped | SVC/flat-sided vent past the step |
| Burn mark risk | High (at step face) | Low (air evacuated) |
| Short shot risk | Moderate (incomplete fill at step) | Low (air removed) |
| Cost premium | Baseline | +15–25% |
| Available models | One-step, two-step | Gas release one-step |
Vent Channel Design at Step Transitions
- Channel type: SVC (half-moon) preferred over flat-sided for stepped pins — provides consistent vent area across the diameter change
- Channel orientation: Orient toward the step face (the shoulder surface where air traps form)
- Channel depth: 0.02–0.05 mm at the narrowest point to prevent resin flash while allowing air flow
- Continuous path: The vent channel must run continuously from the step zone past the plate bore to connect with a parting line vent or dedicated vent slot
When to Specify Gas Release Stepped Pins
- Burns or short shots consistently appear at the step transition surface
- Step transition is more than 5 mm from the nearest parting line vent
- Step ratio exceeds 1.5:1 (larger diameter ≥ 1.5× smaller diameter)
- Processing resins with high gas content (POM, PA6, flame-retardant grades)
According to ScienceDirect's mold venting reference, venting at geometric discontinuities (step transitions, rib intersections) is essential for eliminating localized gas traps that parting line vents cannot address.
Frequently Asked Questions
Why does air trap at step transitions?+
The sudden diameter increase creates a flow separation zone at the step face. Air pockets form in this zone and cannot escape upstream (resin blocks) or downstream (resin flowing past).
Can I vent step transitions with parting line vents alone?+
Only if the step is within 2–3 mm of the parting line. For deeper features, gas release stepped core pins with shaft-mounted vent channels are the only reliable solution.
Which vent type works best for stepped pin transitions?+
SVC (Semi-circular Vent Channel) is preferred — provides consistent vent area across the diameter change. Orient the channel toward the step face.
Gas Release Stepped Core Pins
Need Vented Stepped Core Pins?
Specify step dimensions, step ratio, and vent channel orientation — we manufacture gas release stepped pins with verified vent geometry.
✓ SVC and flat-sided options✓ MOQ 1 piece✓ Custom step configurations