Key takeaways
- Matching a grade to a process. Injection moulding wants high MFI (10–60 g/10 min for thin-wall); blow film wants low MFI (~1–2); raffia wants low MFI (~3).
- Tracking lot-to-lot consistency — if two lots arrive with MFIs that differ by more than ~10%, the producer reactor was running differently.
- Quick-screening counterfeit material. A grade pretending to be PE100 with an MFI of 5 is not PE100.
Melt Flow Index — sometimes called Melt Flow Rate, sometimes MI, sometimes just "flow" — is reported on every polyolefin certificate of analysis. It is also the single number that buyers most commonly use as a stand-in for "is this the right grade." That use is incomplete.
MFI is measured by ASTM D1238 / ISO 1133. A controlled mass of pellets is melted in a heated barrel, then forced through a standardised die under a standardised weight (2.16 kg for most polymers; 5 or 21.6 kg for higher-density ones). The number reported is how many grams flow out in ten minutes.
A high MFI means the polymer flows easily — which is to say, the average molecular chain is shorter. A low MFI means the chains are long and tangled and the melt is viscous. That is the entire physical meaning of the number.
- Matching a grade to a process. Injection moulding wants high MFI (10–60 g/10 min for thin-wall); blow film wants low MFI (~1–2); raffia wants low MFI (~3).
- Tracking lot-to-lot consistency — if two lots arrive with MFIs that differ by more than ~10%, the producer reactor was running differently.
- Quick-screening counterfeit material. A grade pretending to be PE100 with an MFI of 5 is not PE100.
MFI tells you nothing about molecular weight distribution. Two grades can have the same MFI and behave completely differently in a real process — one might be narrow-distribution, the other broad-distribution. Narrow distributions give better mechanical properties and worse processability; broad distributions are the opposite.
MFI also tells you nothing about long-chain branching, additives, density, or comonomer content. Two LLDPEs with identical MFI of 1.0 g/10 min may differ in tear strength by 30% if one is C4 and the other is C8.
Specify a window, not a point. "MFI 0.9–1.1 at 190 °C / 2.16 kg" is a usable spec. "MFI ~1.0" is not, because every producer rounds differently. Pair the MFI window with a density target and (for polyethylene) a comonomer requirement, and you have closed the door on most substitution mistakes.
When you see MFI on the polymer CoA, check it against the order spec to two decimal places. The number on the CoA is the number the producer’s lab measured — it is not a marketing figure.