How to Specify Ejector Pin Tolerances — h5, h6, h7 Selection Guide for Injection Molds
Ejector pin tolerance determines the clearance between the pin and its bore in the mold plate. Too much clearance allows molten plastic to flash into the gap, creating a raised ring around the pin mark. Too little clearance causes the pin to seize in the bore (galling), which can break the pin or damage the bore surface. The correct tolerance is a balance between flash prevention and free sliding movement.
The ISO 286 tolerance system defines the standard grades used for ejector pins: h5, h6, and h7. Each grade specifies a maximum diameter deviation below the nominal dimension. The smaller the number, the tighter the tolerance — and the higher the cost.
Understanding Tolerance Grades
| Grade | Ø4 mm Range | Tolerance Band | Cost Impact | Typical Use |
|---|---|---|---|---|
| h5 | 4.000 – 3.995 mm | 5 μm | +15–25% | Thin-wall, precision |
| h6 | 4.000 – 3.992 mm | 8 μm | Baseline | Standard engineering |
| h7 | 4.000 – 3.988 mm | 12 μm | −5–10% | Commodity, low-precision |
The tolerance band doubles approximately every two grades: h5 is roughly half the band of h7. This means h5 pins require more precise grinding, higher-quality measurement, and tighter process control — hence the cost premium.
Clearance Calculation: Pin + Bore
The functional clearance is determined by the combination of pin tolerance and bore tolerance. Standard practice is to pair:
- h5 pin + H6 bore → 0.000–0.013 mm clearance (tightest practical fit)
- h6 pin + H7 bore → 0.000–0.020 mm clearance (standard fit)
- h7 pin + H8 bore → 0.000–0.030 mm clearance (loose fit)
These clearance values are for room temperature. At operating temperature, the mold plate (around the bore) is hotter than the ejector plate (around the pin base), creating differential thermal expansion. The bore grows slightly while the pin shank remains near ambient. This thermal expansion increases clearance by approximately 0.003–0.008 mm depending on mold temperature. Therefore, clearances that feel tight at room temperature are usually fine at operating temperature.
Selection by Resin Type
The primary selection criterion is the resin's viscosity and flash sensitivity:
| Resin Category | Examples | Flash Thickness | Pin Tolerance |
|---|---|---|---|
| Low viscosity, flash-sensitive | PA6, PA66 (unfilled), POM | Flash at >10 μm gap | h5 + H6 bore |
| Medium viscosity, moderate flash | ABS, PC, PC/ABS, PBT | Flash at >20 μm gap | h6 + H7 bore |
| High viscosity, flash-tolerant | PP, PE, HIPS, TPE | Flash at >30 μm gap | h7 + H8 bore |
| Glass/mineral filled | PA66-GF30, PBT-GF15 | Fillers seal the gap | h6 + H7 bore |
Glass-filled resins are an interesting case: the glass fibers actually help seal the pin-bore gap because they bridge across the clearance. This means glass-filled resins are less flash-sensitive than their unfilled counterparts — you can use h6 even where unfilled PA66 would need h5.
Selection by Pin Diameter
The absolute tolerance band scales with diameter per ISO IT grade formulas. Smaller pins have proportionally tighter bands:
| Diameter | h5 Band | h6 Band | h7 Band |
|---|---|---|---|
| Ø1 mm | 4 μm | 6 μm | 10 μm |
| Ø3 mm | 4 μm | 8 μm | 10 μm |
| Ø6 mm | 5 μm | 8 μm | 12 μm |
| Ø10 mm | 6 μm | 9 μm | 15 μm |
| Ø20 mm | 7 μm | 13 μm | 21 μm |
For Ø1–2 mm pins, even h7 provides a tight 10 μm band. In practice, most suppliers grind small pins to h6 by default because the precision required is within standard grinding capabilities. Specifying h5 for small pins adds cost but provides marginal additional benefit.
Common Mistakes to Avoid
- Over-specifying h5 everywhere — h5 costs 15–25% more than h6 and is unnecessary for 80% of applications. Use it only where flash prevention justifies the premium.
- Mismatching pin and bore grades — An h5 pin in an H8 bore provides no benefit — the loose bore negates the tight pin tolerance. Always match pairs: h5/H6, h6/H7, h7/H8.
- Ignoring thermal expansion — A fit that is too tight at room temperature will seize at operating temperature if the bore material expands less than expected. Always verify clearance at operating temperature.
- Measuring pins after handling — Body heat from handling expands the pin by 1–2 μm. Always measure pins at thermal equilibrium on a surface plate, not directly from your hands.