Ejector System Design for Injection Molds: Pins, Sleeves, Strippers and Best Practices
Function and Importance
The ejector system pushes the solidified plastic part out of the mold after opening. A well-designed system ensures clean, undamaged ejection with minimal cycle time. Ejection accounts for 5-10% of the cycle but causes 20-30% of part defects if poorly designed.
Types of Ejector Systems
| Type | Mechanism | Pros | Cons |
|---|---|---|---|
| Ejector Pins | Pins push from behind | Simple, low cost | Leaves marks |
| Sleeve Ejectors | Tubular around cores | Even ejection | More expensive |
| Stripper Plate | Full plate pushes surface | No marks, even force | Higher cost |
| Blade Ejectors | Flat rectangular pins | Good for thin ribs | Wear faster |
| Air Ejection | Compressed air | No marks, fast | Limited to shallow cavities |
Ejector Pin Design
Pin Diameter
- Minimum: 2mm (smaller is fragile)
- Standard: 3mm, 4mm, 5mm, 6mm, 8mm, 10mm
- Spacing: 25-50mm on flat surfaces
- Total contact area: 1-3% of projected part area
Placement
- Place pins on ribs, bosses, and vertical features (stiffest areas)
- Avoid cosmetic surfaces
- Even distribution to prevent tilting
- Place near deep cores creating release resistance
Common Ejection Problems
Part Sticking
Causes: Insufficient draft, wrong polish direction. Solution: Increase draft, add cavity-side ejector pins, textured finish on mold surface.
Pin Push Marks
Causes: Pins too large, part too thin. Solution: Reduce pin diameter, increase pin count, limit pin extension to 0.01-0.05mm.
Distortion During Ejection
Causes: Uneven force, part too hot. Solution: More pins for even distribution, increase cooling time, use stripper plate.