What Is MFI (Melt Flow Index) and How Do You Read It?

MFI (Melt Flow Index) measures, in grams, the amount of polymer that flows through a standard die in 10 minutes under a specific temperature and weight. ASTM D1238 or ISO 1133 are generally used as the test standard.

The result is an indirect indicator of the material's flowability in the molten state — MFI is inversely proportional to molecular weight: as molecular weight rises, MFI falls and the material becomes more viscous (slower-flowing).

Low MFI (more viscous, high molecular weight): generally provides higher impact strength and mechanical strength. Preferred for slow, continuous-flow processes such as film and pipe extrusion.

High MFI (more fluid, low molecular weight): fills the mold quickly, shortening cycle time. Preferred for processes requiring rapid filling, such as injection molding.

The right choice depends on your production method — there is no universally 'good/bad' MFI, only the range suited to your process.

An MFI value alone is not meaningful — the temperature and weight at which it was measured must also be stated (e.g. '230°C/2.16kg'). The typical test condition for PP is 230°C/2.16 kg, and for PE it is 190°C/2.16 kg.

Two MFI values measured under different conditions cannot be compared directly. In our spec tables, MFI is always given together with the test condition; when comparing, look at values measured under the same condition.