Melt flow index is the first number most buyers look at on a resin data sheet, and the most common one used to match an equivalent grade. It’s useful, but only if you read it the right way.
What MFI measures
MFI is the weight of molten resin, in grams, pushed through a small standard die in 10 minutes at a set temperature and load (ASTM D1238 / ISO 1133). You’ll also see it called MFR (melt flow rate) or MI (melt index, mostly for polyethylene). MVR is the same idea measured by volume, in cm³/10 min.
A higher MFI means the resin flows more easily, usually because its molecules are shorter. A lower MFI means a stiffer melt with longer molecules, which generally brings more toughness, better stress-crack resistance and more melt strength.
Test conditions differ by resin
MFI numbers are only comparable at the same temperature and load. Common conditions:
| Resin | Typical test condition |
|---|---|
| Polyethylene (HDPE, LDPE, LLDPE) | 190°C / 2.16 kg (HLMI for low-MI HDPE: 190°C / 21.6 kg) |
| Polypropylene | 230°C / 2.16 kg |
| Polystyrene (GPPS, HIPS) | 200°C / 5 kg |
| ABS | 220°C / 10 kg |
| Polycarbonate | 300°C / 1.2 kg |
| Acetal (POM) | 190°C / 2.16 kg |
| PLA | 210°C / 2.16 kg (commonly) |
Always confirm the condition printed on the data sheet; some producers use others.
Typical MFI by process (polyolefins)
| Process | Typical MFI range (g/10 min) |
|---|---|
| Pressure pipe, large blow-molded parts (HDPE) | Under 0.1–0.5, often specified by HLMI |
| Blown film | ~0.05–2 |
| Sheet and profile extrusion | ~0.5–5 |
| Bottle blow molding | ~0.2–2 |
| General injection molding | ~5–30 |
| Thin-wall injection molding | ~30–100+ |
General guidance only. Your part, tooling and equipment set the real window.
Using MFI to match an equivalent
- Compare at the same test condition. An MFI of 10 at 2.16 kg and 10 at 5 kg are different materials.
- Stay in a tight band. Within about ±10–20% is a common starting point; film and pipe usually need tighter.
- Don’t match on MFI alone. Density, comonomer or copolymer type, additives, filler and certifications matter just as much.
- Use your COAs, not just the data sheet. The actual values on your incoming lots show what your process is tuned to.
Common mistakes
- Comparing MFI measured at different loads or temperatures.
- Assuming two grades with the same MFI will process the same. Flow at high shear rates can differ.
- Testing wet nylon, PET or PC, which reads artificially high.
Running polyethylene? See HMW-HDPE, LLDPE and polyethylene. Or send us the grade you run now →