Understanding the ISO Tolerance System
The h7 designation comes from the ISO 286-1 tolerance system, which defines limits and fits for cylindrical parts. The lowercase "h" indicates a shaft (external dimension) with a maximum at nominal and a minimum below nominal. The number "7" indicates the tolerance grade — IT7, which allows approximately 10-25 μm of variation depending on the nominal diameter.
For injection mold dowel pins, h7 is the standard tolerance because it balances two competing requirements: tight enough for precision alignment, loose enough for hand assembly and disassembly during mold maintenance.
h7 Tolerance Values by Pin Diameter
| Nominal Diameter | h7 Tolerance Range | Actual Pin OD | Paired H7 Bore |
|---|---|---|---|
| 6mm | 0 to -12 μm | 5.988 to 6.000mm | 6.000 to 6.012mm |
| 8mm | 0 to -15 μm | 7.985 to 8.000mm | 8.000 to 8.015mm |
| 10mm | 0 to -15 μm | 9.985 to 10.000mm | 10.000 to 10.015mm |
| 12mm | 0 to -18 μm | 11.982 to 12.000mm | 12.000 to 12.018mm |
| 16mm | 0 to -18 μm | 15.982 to 16.000mm | 16.000 to 16.018mm |
| 20mm | 0 to -21 μm | 19.979 to 20.000mm | 20.000 to 20.021mm |
Why h7/H7 Is the Correct Fit for Mold Alignment
The h7 pin in an H7 bore creates what ISO 286-1 classifies as a "transition fit" — the fit can be either a slight interference or a slight clearance depending on where the actual dimensions fall within the tolerance bands:
- Maximum clearance: Pin at minimum (e.g., 9.985mm) + Bore at maximum (e.g., 10.015mm) = 0.030mm clearance. The pin slides in freely by hand.
- Zero clearance: Pin at maximum (10.000mm) + Bore at minimum (10.000mm) = 0.000mm. The pin fits snugly with light hand pressure.
- Maximum interference: Not possible with h7/H7 — the pin maximum equals the bore minimum, so interference cannot occur. This is a deliberate design choice to ensure the pin can always be assembled and disassembled by hand.
This contrasts with an m6/H7 press-fit used for permanently installed pins (like spring guide pins), where the pin is deliberately oversized and requires an arbor press to install.
Impact on Part Quality
Mold plate alignment directly affects the molded part's wall thickness uniformity. With h7/H7 dowel pins, the maximum plate misalignment is limited to the maximum clearance in the fit:
- With h7/H7 dowel pins: Maximum misalignment ≈ 0.015-0.030mm. Wall thickness variation: ±0.015mm (excellent for most applications).
- With h9/D10 loose-fit pins: Maximum misalignment ≈ 0.10-0.15mm. Wall thickness variation: ±0.10mm (unacceptable for precision parts).
- Without dowel pins (bolts only): Maximum misalignment = bolt clearance = 0.5-1.0mm. Wall thickness variation: ±0.5mm (unacceptable for any application).
Why 52100 Bearing Steel for Dowel Pins
Our dowel pins use 52100 bearing steel (SUJ2 per JIS) hardened to HRC 60-64. This material choice serves two purposes:
- Dimensional stability: 52100 at HRC 60+ does not deform under the clamping loads applied by mold bolts. Softer materials (S45C at HRC 25) can compress at the contact surface, gradually increasing the clearance and reducing alignment precision.
- Wear resistance: During mold assembly and disassembly, the pin slides in and out of the bore thousands of times over the mold's lifetime. Soft pins develop wear steps that create additional clearance. 52100 pins maintain their dimensional accuracy for 10,000+ assembly cycles. See ASTM A295 (High-Carbon Anti-Friction Bearing Steel) for material specifications.