MFI (Melt Flow Index)
Also called: Melt flow index, melt flow rate, MFR, melt index
Melt flow index (MFI) measures how easily a molten plastic flows, in g/10 min, and is the most quoted specification for recycled granules.
Melt flow index (MFI), also called melt flow rate (MFR), is the weight of molten plastic in grams that flows through a standard die in 10 minutes under a set temperature and load. It is measured to ASTM D1238 or ISO 1133 and reported in g/10 min. Higher MFI means the material flows more easily, which generally indicates lower molecular weight.
Test conditions
- PP is usually tested at 230 C / 2.16 kg.
- Polyethylenes such as HDPE and LDPE are usually tested at 190 C / 2.16 kg; some pipe and blow grades are quoted at higher loads.
- An MFI value is only comparable when the temperature and load are stated.
Why it matters when buying recycled plastic
- Low MFI suits pipe extrusion and blow moulding, where melt strength matters.
- Higher MFI suits thin-wall injection moulding, where the melt must fill long flow paths.
- Recycled lots vary, so ask for MFI per lot, not one figure for all supply, and agree an acceptable range.
- Learn how to test it in our guide to checking MFI.