Venting Design for Injection Molds: Complete Guide to Gas Evacuation
Why Venting is Critical
When molten plastic enters the cavity, it displaces air that must be evacuated. If air cannot escape, it becomes compressed and heated, causing burn marks, short shots, and weld lines. Proper venting also reduces injection pressure requirements by 5-15%.
Vent Depth by Material
| Material | Max Vent Depth (mm) | Flow |
|---|---|---|
| PP, PE | 0.015-0.030 | Low viscosity |
| Nylon (PA) | 0.010-0.020 | Low when dry |
| ABS, HIPS, PS | 0.020-0.040 | Medium |
| PC, PMMA | 0.020-0.050 | High viscosity |
| POM | 0.010-0.020 | Low-Medium |
| PVC Rigid | 0.020-0.040 | High viscosity |
| LCP | 0.008-0.015 | Very low |
Vent Placement
- At the end of fill (last point to fill)
- Along parting line at 25-50mm intervals
- Around ejector pins (use as vents)
- At weld line locations
- At deep blind pockets (core vent pins)
Dimensions
- Land length: 0.5-1.5mm
- Relief groove: 0.5-1.0mm deep
- Relief width: 3-10mm connected to atmosphere
- Vent width: 5-15mm per vent, multiple recommended
Vacuum Venting
For high-performance applications (thin-wall, micro-molding, optical), active vacuum venting draws air before and during injection. Systems achieve 10-40 torr and reduce fill pressure by 5-10%.
Troubleshooting
Burn Marks (Dieseling)
Brown-black marks near end of fill. Trapped air compressed to 200-300°C+. Solution: add vents at burn locations.
Short Shots
Incomplete fill from trapped air back-pressure. Add core vents or porous steel inserts.
Vent Flash
Thin plastic fins protruding from vents. Vent too deep. Reduce depth, shorten land to 0.5mm max.
Porous Steel Inserts
Inserts with 7-20 um interconnected pores provide micro-venting where conventional vents are impossible. Cost: $100-300 per insert; life: 100,000-500,000 cycles.