Technical Know-How · Aug 18, 2026
One number appears on every polymer data sheet, yet it quietly controls injection pressure, cycle time, wall thickness limits and mechanical properties: the Melt Flow Index (MFI), or Melt Flow Rate (MFR).
Understanding how to read it will save you from specifying grades that your process cannot run — and help you evaluate equivalent-grade offers from different suppliers.
MFI is measured by heating a polymer to a set temperature (e.g. 190°C for PE, 230°C for PP) and pushing it through a fixed die under a fixed load (most commonly 2.16 kg). The result, in g/10 min, is a rough inverse measure of molecular weight: lower MFI → higher molecular weight → stronger but stiffer melt flow; higher MFI → lower molecular weight → easier flow but weaker parts.
MFI is meaningless without its test conditions. Compare only values measured under the same temperature and load. On our data sheets, every MFI value is labeled, e.g. "MFR 190°C / 2.16 kg". HDPE grades sometimes also report a high-load value (21.6 kg) — the ratio between the two is a useful indication of molecular weight distribution.
Within one material family, raising MFI almost always means sacrificing impact strength, ESCR and long-term creep resistance. That is why "high-flow" is not automatically "better" — it is only better if your part geometry demands it. A responsible supplier will challenge your MFI request if the application needs toughness.
When a supplier offers an equivalent grade, the first check is MFI under identical conditions, followed by density. Within ±20% of MFI and ±0.005 g/cm³ of density, an equivalent is usually processable — but always confirm application-specific properties (film dart impact, ESCR, HDT) with the producer's data sheet before committing a production lot.
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Three polyethylene families dominate flexible packaging. Understand density, MFI and film performance to specify the right film-grade resin — and the right supplier.
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One number on every data sheet controls molding pressure, cycle time and mechanical properties. Here is how to read MFI/MFR like a sourcing engineer.
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