Melt Flow Index (MFI)

The ease with which a thermoplastic polymer melt flows is measured by the Melt Flow Index (MFI). It is defined as the amount of polymer, measured in grams, that flows in ten mins through a capillary with a particular diameter and length under pressure from different gravimetric weights at different temperatures.

Last Updated: July 13th, 2023 First Published :



The value of MFI is typically correlated by polymer processors with the grade of polymer that they must select for various operations, and most frequently, this number is not accompanied by the units because it is assumed to be g/10min. Similarly, the MFI measurement test conditions are typically specified in kilograms instead of other units.

The MFI is influenced by various factors such as the molecular weight, molecular weight distribution, and viscosity of the polymer. Generally, materials with higher molecular weights or narrower molecular weight distributions have lower MFI values, indicating lower flowability.

A high melt flow rate corresponds to low molecular weight; it is an indirect indicator of molecular weight. The melt flow rate also serves as a gauge for how well a material’s melt flows under pressure. Although it should be remembered that the viscosity of any such substance depends on the applied force, the melt flow rate is inversely related to melt viscosity under the test conditions. The degree to which the molecular weight distribution is broad is frequently assessed using ratios between two melt flow rate estimates for the same material at various gravimetric weights.

The polyethylene melt flow rate is typically measured at 190 °C, while the polypropylene melt flow rate is measured at 230 °C. The melt index of the material should be high enough for the molten polymer to easily form into the required item, yet low enough for the mechanical strength of the finished product to be adequate for usage.

Video 01: Melt Flow Index

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