Gas Venting Calculation for Deep-Rib Molds
Key Takeaway: For deep ribs (depth:width > 5:1), calculate required venting area as: 1 mm² per 50 cm³ cavity volume for standard resins, or 1.5 mm² per 50 cm³ for gas-sensitive resins (PA+GF, POM). Select gas release pin count accordingly.
Why Deep Ribs Trap Gas
In injection molding, the resin front advances through the cavity and pushes air ahead of it. In deep, narrow ribs, the air has no escape path — it compresses until it either: (1) dissolves into the resin (slow, causes bubbles), or (2) ignites from adiabatic compression (>400°C), creating diesel/burn marks. This phenomenon is well-documented by the Society of Plastics Engineers.
Venting Area Calculation
| Parameter | Standard Resins | Gas-Sensitive Resins |
|---|---|---|
| Venting area per 50 cm³ | 1.0 mm² | 1.5 mm² |
| Typical resins | ABS, PC, PP, PE, HIPS | PA+GF, POM, PPS, PBT+GF |
| Pin type (ø5mm, flat-cut) | ~1.2 mm² per pin | ~1.2 mm² per pin |
| Pins needed per 100 cm³ | ~2 pins | ~3 pins |
Example: Dashboard Rib Panel
A dashboard support panel with 24 ribs (20mm deep × 1.5mm wide), total cavity volume 180 cm³, material PA66+GF30:
- Required venting area: 180 ÷ 50 × 1.5 = 5.4 mm²
- Using ø5mm flat-cut gas release pins (1.2 mm² each): 5.4 ÷ 1.2 = 5 pins minimum
- Placement: one pin at each of the 5 deepest rib ends (furthest from gate)