The Tolerance Chain
Mold alignment precision is determined by a chain of tolerances. No single component determines the outcome — it's the sum of all errors:
1. Shaft-Bore Fit (H7/g6)
The ISO 286-1 standard H7/g6 fit for a Ø25 mm pin creates 0.013–0.034 mm diametral clearance. This means the pin can shift ±0.017 mm within the bushing bore — this is the theoretical maximum misalignment from the fit alone.
2. Pin Straightness
A leader pin with 0.005 mm/100 mm straightness error adds angular misalignment that increases with engagement length. For a 100 mm long pin, the tip can deviate 0.005 mm from the theoretical centerline.
3. Bore Concentricity
If the bushing bore is not concentric with the plate reference surface, the entire pin-bushing system shifts by the concentricity error. Typical plate boring achieves 0.010 mm concentricity.
Total Error Budget
For a typical Ø25 × 100 mm leader pin system:
Total mismatch = 0.017 + 0.005 + 0.010 = 0.032 mm (worst case)
This is acceptable for most injection molds. For precision molds requiring < 0.015 mm, use H6/g5 fit with ball cage bushings (reduces the fit contribution to ±0.005 mm).