MFI and MFR are the same measurement under two names: melt flow index and melt flow rate both report how much molten thermoplastic a standard die extrudes in 10 minutes at a set temperature and load. Melt flow rate is the name today’s ISO and ASTM test standards use; melt flow index is the older name for the same method and is still in use.
An MFI on one datasheet and an MFR on another can be compared directly if both were measured at the same temperature and load. Check the unit first: in polyethylene literature MFR can also mean melt flow ratio, a ratio of two flow rates with no unit.
What MFI and MFR Measure
MFI and MFR measure how much molten resin a weighted piston pushes through a standard die at a fixed temperature, expressed as the mass extruded in 10 minutes.
The resin is melted in the heated barrel of an extrusion plastometer. Lengths of extrudate are cut over timed intervals, weighed, and the mass scaled to 10 minutes. The result is quoted in g/10 min, which is the same number in dg/min.
A higher value means the melt flows more easily, and within one polymer type it usually means a lower molar mass.
The test is used mainly for quality control, to confirm that lots of a grade flow alike. Its shear rates are far below those in extrusion or moulding, so a single value does not always correlate with how a resin behaves in processing.
MVR, the melt volume-flow rate, comes from the same test reported as a volume, in cm³/10 min. It converts to MFR through the density of the melt at the test temperature, which is lower than the solid density a datasheet lists.
MFR (g/10 min) = MVR (cm³/10 min) × melt density at the test temperature (g/cm³)
A melt-flow value on a datasheet or certificate of analysis has no meaning without its temperature and load. Polypropylene normally has one permitted set of conditions under ISO 1133, but polyethylene has four permitted loads at the same temperature. Two figures taken at different loads never belong in the same column of a comparison, whatever each sheet calls them.
The Difference Between MFI and MFR
There is no difference in the measurement: IUPAC’s polymer glossary has melt flow index as its entry and lists melt flow rate and MFR as synonyms. What differs is which documents use which name, and the letters MFR carry a second meaning.
Name and Standard
The current ISO and ASTM test standards use melt flow rate. ISO 1133-1:2022 covers the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastics, and ASTM D1238-26 is the test method for melt flow rates of thermoplastics by extrusion plastometer.
Melt flow index is the method’s historical name, and it is still in use. For polyethylene tested to ASTM D1238, melt index names the result at 190 °C under 2.16 kg.
The name does not tell you which standard a figure was measured to; the test method printed beside the value identifies the standard.
By definition MFI is a mass per 10 minutes. Usage is looser than the definition, so read the unit: g/10 min is a mass rate and cm³/10 min a volume rate.
A value measured to ASTM D1238 can be set beside one measured to ISO 1133 at the same temperature and load, because the differences between the two methods are small. When a value sits close to a specification limit, have it measured to the standard the limit was written to.
When MFR Means Melt Flow Ratio
MFR means melt flow ratio when the figure compares the results at two loads: the flow rate at a high load divided by the flow rate at a low load. This use appears in polyethylene literature, patents in particular, where the ratio is typically the 21.6 kg result over the 2.16 kg result, written I21/I2.
The unit separates the two. A melt flow rate always carries a unit, g/10 min or dg/min, while the ratio is a plain number with two loads named.
The test standards call this figure the flow rate ratio, abbreviated FRR in ASTM D1238.