MFI (MFR) is the extruded mass of polymer per unit time through a standard die under specified temperature and piston load, typically reported in g/10 min.
Melt Flow Index
MFI (MFR) is the extruded mass of polymer per unit time through a standard die under specified temperature and piston load, typically reported in g/10 min.
Formula
MFR = m / t
m is mass of extrudate collected over time interval t under standard die, temperature, and nominal load. Results are normalized to grams per 10 minutes for reporting.
The melt flow index (MFI), also called melt mass-flow rate (MFR), is a single-point rheological QC test for thermoplastics. Molten polymer is extruded through a standard capillary die (2.095 mm diameter for Procedure A/B) while a defined piston load produces a nominal shear stress regime.
Test temperature and load are chosen per material grade—for example polyethylene is often tested at 190 °C with 2.16 kg load, while other resins use 230 °C or higher loads (5 kg, 10 kg, 21.6 kg) to bring flow into measurable range.
MFI inversely correlates with average molecular weight only qualitatively; broad polydispersity or long-chain branching can produce similar MFI with different processability. It should complement intrinsic viscosity, capillary rheometry, or dynamic shear rheology for process design.
Moisture-sensitive polymers must be dried before testing; trapped moisture hydrolyzes chains or causes foaming, biasing MFI high. Barrel residence time and piston insertion depth must follow standards to avoid thermal degradation artifacts.
Processors use MFI for lot-to-lot incoming inspection and adjusting extrusion blow molding or injection molding parameters, but final part properties still depend on shear history, cooling rate, and orientation.
Related calculator
Estimate apparent melt viscosity from melt flow index using melt density and an empirical mass-flow correlation.
Open calculator →