Viscosity is the measure of a fluid's resistance to flow. For MIM feedstock, it determines how easily the material fills the mold cavity — and it changes dramatically with temperature and shear rate.
How viscosity affects MIM molding:| Viscosity Level | Effect on Molding | Typical Cause |
|---|---|---|
| Too low | Flash at parting line, inconsistent shot weight, binder-powder separation | High temperature, low MFI reading, excessive binder content |
| Optimal | Complete fill, consistent shot weight, minimal flash | Proper feedstock formulation + correct molding parameters |
| Too high | Short shots, high injection pressure required, mold wear | Low temperature, high powder loading, degraded binder |
MIM feedstock viscosity is measured using a capillary rheometer — not the MFI tester (which gives only one data point). A capillary rheometer measures viscosity across a range of shear rates, producing a flow curve.
| Measurement | MFI Tester | Capillary Rheometer |
|---|---|---|
| Data output | One number (g/10min) | Viscosity vs shear rate curve |
| Shear rate | Fixed (low, ~10-100 s⁻¹) | Variable (10-10,000 s⁻¹ — covers molding range) |
| What it tells you | Quick consistency check | Full flow behavior at molding conditions |
MIM feedstock is "shear-thinning" — its viscosity decreases as the shear rate increases. At the low shear rate of the MFI test (10-100 s⁻¹), the viscosity is high. At the high shear rate inside the mold gate (10,000-100,000 s⁻¹), the viscosity drops dramatically — which is what allows the material to fill thin cavities.
Quick Q: What is MIM feedstock viscosity?Viscosity is the resistance of MIM feedstock to flow — it determines how easily the material fills the mold cavity. It is highly dependent on temperature (higher temperature = lower viscosity) and shear rate (MIM feedstock is shear-thinning — viscosity drops at higher flow speeds). Viscosity is measured with a capillary rheometer across the full range of shear rates encountered in molding. MFI testing provides a quick consistency check but does not capture the full rheological behavior.