What Is Melt Flow Index (MFI)? Definition

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

• MFI (melt flow index) — mass of melt extruded in 10 min; unit g/10 min; the most widely used name.
• MFR (melt flow rate) — the same measurement; unit g/10 min; the name used in the title of ASTM D1238.
• MVR (melt volume rate) — volume of melt extruded in 10 min; unit cm³/10 min; measured on the same instrument but reported by volume.
• Flow rate ratio (FRR) — MFR at one load ÷ MFR at another; no unit; 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.