Developing a new MIM material formulation requires finding the optimal balance between powder content (for sintering) and binder content (for molding flow). Here is the standard methodology.
Step 1: Determine CPVC (Critical Powder Volume Concentration)CPVC is the highest powder loading at which the binder still fills all interstitial spaces. Above CPVC, the feedstock becomes too viscous to mold. Below CPVC by 3-5 vol% is the optimal processing range.
| Test Method | What It Measures | How to Interpret |
|---|---|---|
| Oil absorption (ASTM D281) | Amount of liquid needed to reach pasty consistency | Estimate starting binder content |
| Torque rheometry | Torque required to mix powder + binder at increasing powder loading | CPVC = inflection point where torque spikes |
| Capillary rheometry | Viscosity vs powder loading | Optimal = 3-5 vol% below CPVC |
| Loading Level | Relative to CPVC | Expected Behavior |
|---|---|---|
| Loading A | CPVC - 7 vol% | Low viscosity, easy molding, high shrinkage (>20%) |
| Loading B | CPVC - 5 vol% | Good balance — optimal molding and sintering |
| Loading C | CPVC - 3 vol% | Moderate viscosity, acceptable for simple parts |
| Loading D | CPVC (theoretical) | Very high viscosity — may not mold successfully |
For each loading level:
- Measure MFI (target: able to mold within ±5% of target)
- Mold test bars and measure green density
- Sinter under standard conditions
- Measure sintered density, shrinkage, and mechanical properties
The optimal loading typically falls at CPVC minus 3-5 vol%, where:
- Feedstock flows easily into the mold
- Sintered density exceeds 96% of theoretical
- Shrinkage is predictable (±0.3% batch to batch)
The optimal binder content is determined through a systematic test: prepare feedstocks at 3-5 different powder loading levels around the estimated CPVC (critical powder volume concentration). Test each for molding behavior (MFI, viscosity), sintering response (density, shrinkage), and mechanical properties. The optimal loading is typically 3-5 vol% below CPVC — high enough for good sintering, low enough for reliable molding.