Flatness is one of the most challenging dimensions to control in MIM, because the sintering process involves 14-20% shrinkage and the part's own weight can cause sagging during heating.
Typical as-sintered flatness by part geometry:| Part Type | Flatness (per 25 mm span) | Per 50 mm span |
|---|---|---|
| Symmetrical part, uniform wall | ±0.05-0.10 mm | ±0.10-0.20 mm |
| Asymmetrical part, uniform wall | ±0.08-0.15 mm | ±0.15-0.30 mm |
| Part with variable wall (2:1 ratio) | ±0.10-0.20 mm | ±0.20-0.40 mm |
| Large flat plane (>30 mm) without ribs | ±0.10-0.25 mm | ±0.20-0.50 mm |
| After coining operation | ±0.025-0.05 mm | ±0.05-0.10 mm |
| After surface grinding | ±0.005-0.015 mm | ±0.01-0.025 mm |
| Factor | Impact | Mitigation |
|---|---|---|
| Wall thickness variation | Primary cause — differential shrinkage | Design for uniform walls (<2:1 ratio) |
| Part geometry asymmetry | Uneven shrinkage forces | Add ribs or symmetrical features |
| Sintering support (setter) design | Gravity sag during sintering | Use shaped setters that support flat surfaces |
| Heating rate | Thermal gradient causes bowing | Slow ramp rate in sintering cycle |
| Part placement in furnace | Hot/cold zones across the part | Standardized tray layout, documented in control plan |
For a 30 mm × 30 mm flat surface on a symmetrical part with uniform wall thickness: ±0.10-0.15 mm as-sintered is typical. For tighter flatness, add a coining operation (±0.03-0.05 mm) or surface grinding (±0.01 mm) after sintering.
Design tip for flat surfaces: Adding ribs (0.5-0.8 mm thick, 2-3 mm tall) to the underside of flat surfaces dramatically improves as-sintered flatness by stiffening the part against gravity sag during sintering — at minimal additional cost since the ribs are molded in the same shot.