Key takeaways
- MFI (ASTM D1238 / ISO 1133) reports how many grams of melt flow through a standard die in 10 minutes — high MFI means shorter chains and easy flow, low MFI means long, viscous chains.
- It is ideal for matching a grade to a process, tracking lot-to-lot consistency (>~10% drift signals a different reactor run), and quick-screening counterfeit material.
- MFI says nothing about molecular-weight distribution, long-chain branching, additives, density or comonomer — two resins with identical MFI can behave very differently.
- Specify a window with conditions — e.g. "MFI 0.9–1.1 at 190 °C / 2.16 kg" — not a point, and pair it with a density target and (for PE) a comonomer requirement.
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.
Frequently asked
Questions on the desk
What is Melt Flow Index (MFI)?
MFI — also called Melt Flow Rate or MI — is measured by ASTM D1238 / ISO 1133: a controlled mass of pellets is melted and forced through a standard die under a standard weight (2.16 kg for most polymers), and the grams that flow out in ten minutes is the reported number. It is a proxy for average chain length.
What does a high or low MFI mean?
A high MFI means the polymer flows easily because the average 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.
What MFI do I need for my process?
Match MFI to the process: injection moulding wants high MFI (10–60 g/10 min for thin-wall), blow film wants low MFI (~1–2), and raffia wants low MFI (~3). The wrong window scraps a run.
Is MFI enough to specify a polymer grade?
No. MFI ignores molecular-weight distribution, long-chain branching, additives, density and comonomer — two LLDPEs with the same MFI can differ in tear strength by 30% if one is C4 and the other C8. Always pair an MFI window with density and comonomer.
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