Gate Design in Injection Molding: Types, Placement, Sizing and Optimization
The Role of Gates
The gate is the entrance through which molten plastic enters the mold cavity from the runner system. Gate design directly affects fill pattern, packing efficiency, part appearance, and dimensional quality.
Common Gate Types
| Gate Type | Advantages | Disadvantages | Best For |
|---|---|---|---|
| Edge (Side) Gate | Simple, low cost | Visible gate mark | General purpose |
| Pin (Pinpoint) Gate | Small gate mark | Pressure drop | Multi-cavity, small parts |
| Submarine (Tunnel) Gate | Fully self-degating | Tunnel wear | High-volume automation |
| Fan Gate | Wide fill front | Large gate mark | Large flat parts |
| Diaphragm Gate | Uniform round fill | Complex machining | Cylindrical parts |
| Valve Gate (Hot Runner) | No gate vestige | High cost | Cosmetic parts |
Gate Sizing Formulas
Edge Gate Depth
d = t x 0.5 to 0.8 (t = wall thickness). Example: 2.5mm wall -> gate depth 1.25-2.0mm
Edge Gate Width
w = d x 2 to 5 (width = 2-5x depth)
Pin Gate Diameter
D = t x 0.3 to 0.5 (typically 0.5-1.5mm)
Gate cross-section should be 1-3% of runner cross-section for normal flow materials.
Gate Placement Rules
- Place gate at the thickest section of the part
- Position gate for shortest flow path
- Orient gate so flow does not hit cores directly
- Place gate to minimize visible weld lines
- For aesthetic surfaces, locate gate on non-visible side
- Avoid gating into thin sections that freeze before packing
Gate Freeze Time
Gate freeze time determines how long packing pressure can be applied. For edge gates, freeze time is proportional to gate thickness squared. A 1.5mm gate freezes roughly 2.25x faster than a 2.25mm gate. Design gates thick enough to stay open during the entire pack-hold phase.