MIM is an excellent process for producing small precision gears (modules 0.15-1.0 mm) that would be expensive to machine and cannot be stamped. Here are the specific design rules.
MIM gear design limits:| Gear Parameter | Minimum | Recommended | Constraint |
|---|---|---|---|
| Module (metric) | 0.15 mm | ≥ 0.25 mm | Below 0.15 mm: tooth strength inadequate |
| Number of teeth | 6 | ≥ 10 | Below 6: undercut in mold profile |
| Pressure angle | 20° standard | 20° or 25° | 14.5° possible but less common |
| Face width | 0.5 mm | ≥ 1.0 mm | Below 0.5 mm: alignment affected by shrinkage variation |
| Outer diameter | 2 mm | ≥ 5 mm | Below 2 mm: handling and inspection difficult |
| Bore diameter (molded) | 0.3 mm | ≥ 0.6 mm | Core pin strength |
| Tooth-to-tooth pitch error | ±0.02-0.05 mm | ±0.03 mm | Affected by shrinkage uniformity |
Gear tooth profiles are particularly sensitive to shrinkage variation because a small percentage error translates directly into mesh clearance changes. The mold cavity for a MIM gear must be oversized by the shrink factor (typically 1.16-1.20x), but the gear tooth profile (involute curve) is also scaled proportionally.
Key tolerance specifications for MIM gears:| Feature | Tolerance | Impact on Performance |
|---|---|---|
| Bore diameter | H7 or ±0.02 mm (post-sintering reaming) | Shaft fit — the single most critical gear dimension |
| Outside diameter | ±0.05-0.10 mm | Center distance in gear pair |
| Tooth thickness | ±0.02-0.05 mm | Backlash |
| Runout (bore to OD) | 0.05-0.10 mm | Noise and vibration |
| Option | Tolerance Improvement | Cost Increase |
|---|---|---|
| As-sintered | Baseline | Baseline |
| Bore reaming | Bore to H7 (±0.01 mm) | +$0.05-0.15/part |
| Tooth coining | Tooth thickness to ±0.01 mm | +$0.10-0.30/part |
| Full CNC finishing | All gear features to IT7 | +$0.50-2.00/part |
Design gears for MIM with module ≥ 0.15 mm (0.25 mm recommended), 20° pressure angle, and at least 10 teeth. The bore is the most critical feature — mold it undersize and ream to H7 after sintering. The tooth involute profile is scaled by the shrink factor in the mold design. For micro-gears (module < 0.5 mm), MIM is often the only economical production process.