MFI (Melt Flow Index) is measured per ASTM D1238: preheat 4-6 g of feedstock in the MFI tester barrel at 160-200°C (binder-dependent), apply a 2.16-10 kg load, and collect the extruded material over 15-60 second intervals. Weigh the collected material and calculate: MFI (g/10min) = (average weight per cut × 600) / cut interval in seconds. Target is ±5% of nominal. If MFI is too high (low viscosity), increase powder loading by 1 vol%. If MFI is too low (high viscosity), decrease powder loading by 1 vol%.
MFI test conditions for common MIM feedstock:
| Binder System | Test Temperature | Load | Expected MFI Range |
|---|---|---|---|
| POM-based | 180-200°C | 2.16 kg | 800-1500 g/10min |
| Wax-polymer | 160-180°C | 5.0 kg | 200-600 g/10min |
| Water-soluble (PEG) | 150-170°C | 2.16 kg | 500-1200 g/10min |
| MFI vs Target | Meaning | Likely Cause | Corrective Action |
|---|---|---|---|
| > +10% | Feedstock too fluid | Low powder loading, or high binder content | Check binder wt% by TGA; add 1 vol% powder |
| +5 to +10% | Noticeable drift | PSD shift toward finer, or compounding temperature high | Monitor next batch; check powder PSD |
| ±5% | Acceptable — in spec | — | Continue production |
| -5 to -10% | Noticeable drift | PSD shift toward coarser, or compounding temperature low | Monitor; check powder PSD |
| < -10% | Feedstock too viscous | High powder loading, or binder degradation | Check binder content; reduce 1 vol% powder |
- Test every feedstock batch before release to production
- Record MFI on a control chart with upper and lower control limits at ±5% of nominal
- If a batch falls outside ±5%, investigate and adjust before using that batch
- Archive a retained sample from every batch (minimum 6 months) for future reference