After debinding, the MIM part enters the "brown" state — a porous structure of metal particles held together only by residual binder (5-10%) and weak interparticle forces. It is the most fragile state in the entire MIM process.
Quantifying brown part strength:| Material | Green Strength (after molding) | Brown Strength (after debinding) | Comparison |
|---|---|---|---|
| 316L (POM binder, catalytic debind) | 15-25 MPa (flexural) | 3-8 MPa | About the strength of a hard chalk |
| 316L (wax-polymer, solvent debind) | 10-18 MPa | 2-5 MPa | Weaker — can crumble under finger pressure |
| Fe-2Ni (POM binder) | 18-28 MPa | 4-10 MPa | Slightly stronger than 316L |
| Ti6Al4V (POM binder) | 12-20 MPa | 2-5 MPa | Similar to 316L — handle with same care |
- Brown parts must be transferred to the sintering furnace on the same tray — avoid re-gripping after placement
- Parts with thin walls (< 0.5 mm) or long slender features (> 10:1 aspect ratio) are at highest risk
- Tumbling vibratory feeding is not possible in the brown state — use vacuum pick-and-place or manual tray loading
- Drops from bench height (0.8 m) will fracture most brown parts — 100% scrap
| Feature | Effect on Brown Part Strength | Design Recommendation |
|---|---|---|
| Thin wall (< 0.5 mm) | Very fragile — crack during handling | Thicken to ≥ 0.5 mm or add handling tabs |
| Sharp internal corners | Stress concentration → crack at R < 0.2 mm | Use R ≥ 0.3 mm |
| Large flat unsupported area | Sags under its own weight during transfer | Add stiffening ribs (≥ 0.5 mm thick) |
| Long slender protrusion (> 5 mm unsupported) | Breaks off during handling | Support with gusset or redesign |
| Asymmetric geometry | Twists during handling, creating internal stress | Balance the geometry where possible |
A typical MIM brown part has about 3-8 MPa flexural strength — comparable to chalk or dry clay. It will break if dropped from bench height, crack if pinched too hard, and cannot be tumbled or vibratory-fed. This is why automation in MIM lines uses vacuum pick-and-place rather than vibratory bowl feeders for brown parts.
The green part (before debinding) is significantly stronger (15-25 MPa) and can be handled and inspected with normal care. The transition from green to brown is the critical handling point.