What Is Melt Flow Index (MFI)? Definition

Melt flow index (MFI) is a measure of how easily a molten thermoplastic flows: the mass of polymer, in grams, pushed through a standard die in 10 minutes at a set temperature and load. It is measured to ASTM D1238 or ISO 1133 and reported in g/10 min; a higher MFI means an easier-flowing, lower-viscosity melt.

In short: MFI is a quick, single-point flow test that tells a molder whether a resin grade is thin and easy-flowing or thick and stiff.

What Is Melt Flow Index in Plastics?

The test uses an extrusion plastometer: a heated barrel with a standardized die of about 2 mm (0.08 in) bore at its base. A few grams of resin are loaded, preheated, and then a dead weight is placed on the piston — 2.16 kg (4.76 lb) is a common load. The extrudate that flows out over a timed interval is weighed and scaled to grams per 10 minutes.

Temperature and load are fixed by material, so an MFI value is meaningless without its test condition. Polyethylene is commonly measured at 190 °C (374 °F) and polypropylene at 230 °C (446 °F); a datasheet entry reads, for example, "MFI 12 g/10 min (230 °C/2.16 kg)."

Melt flow rate is an indirect measure of molecular weight: a high MFI corresponds to lower molecular weight and lower melt viscosity; a low MFI to longer chains and a stiffer melt. The practical goal is a melt index high enough that the plastic forms easily, but low enough that the finished part keeps the mechanical strength it needs.

ASTM itself cautions that the result is not a fundamental polymer property. It is an empirical number measured under fixed laboratory conditions that may differ substantially from those in a production molding machine, which is why ASTM positions it mainly as a quality-control and grade-comparison test.

MFI vs MFR vs MVR

The names are often used interchangeably, but there is one real distinction.

Term	What it reports	Unit	Notes
MFI (melt flow index)	Mass of melt extruded in 10 min	g/10 min	The most widely used name for the test
MFR (melt flow rate)	The same measurement	g/10 min	The name used in the title of ASTM D1238
MVR (melt volume rate)	Volume of melt extruded in 10 min	cm³/10 min	Measured on the same type of instrument; reported by volume instead of mass
Flow rate ratio (FRR)	MFR at one load ÷ MFR at another	none	A rough indicator of molecular-weight distribution breadth

How to Use Melt Flow Index When Choosing a Resin

Compare like with like. Only compare MFI values from the same standard, temperature and load, and only within the same polymer family.
Match flow to the part. Long flow lengths and thin walls favor a higher-MFI grade; load-bearing parts often favor a lower-MFI grade, trading some flow for strength.
Use it as a lot check. ASTM positions MFI as a test of flow uniformity, so a lot that reads differently from the last one is worth investigating before it goes into the press.
Do not use it to predict fill on its own. Gate location, wall thickness and processing temperatures matter as much as the resin's MFI.

Resin grade selection is reviewed alongside wall thickness and gating in the DFM review that opens every plastic injection molding project at NICE Rapid.

Frequently Asked Questions