What Does Melt Flow Index (MFI) Tell You?

Last updated: 2026-09-01 · Reading time: about 4 min

Melt flow index (MFI) measures how easily a molten resin flows: the grams extruded through a standard orifice in ten minutes under a fixed temperature and load.
A higher MFI means the resin flows more easily — better for thin walls — at the cost of some impact toughness; a lower MFI means a stiffer, tougher melt that needs bigger gates, runners and injection pressure. It is the first number to check on a datasheet and the fastest way to catch a batch that is not what the label says.

Key facts

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  • MFI is measured under fixed conditions and quoted with them — for example, at 230 °C and 2.16 kg — because the same resin moves differently at different test settings.
  • High MFI helps thin walls, long flow lengths and small gates; low MFI carries higher molecular weight, so it holds impact strength and stiffness better.
  • MFI compares grades of the same resin family; it does not translate across families, and it does not predict how a resin behaves at the high shear rates inside a real tool.

    • Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. See also related guide.

    Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. See also related guide.

    Why does MFI matter before the tool is cut?

    Because the datasheet value quietly sizes the tool. A grade with low MFI pushes every flow-path decision in the harder direction: wider runners, larger gates, higher injection pressures, thicker minimum walls — and a thin-wall design that looks fine on screen may simply not fill with the grade selected. A high-MFI grade makes those decisions easier but gives back less toughness and can flash through gaps that a stiffer melt would never enter.
    Neither end is better; the mistake is choosing the grade after the geometry is frozen. When the grade is fixed at DFM together with the wall and tolerance strategy, the MFI value on the datasheet becomes a design input instead of a late surprise.

    How do the two ends compare in practice?

    Property on the floor High MFI Low MFI
    Filling thin walls Easier; longer flow at lower pressure. Hard; needs bigger gates, hotter melt or thicker walls.
    Impact toughness Lower — shorter chains sacrifice some of it. Higher — higher molecular weight carries it.
    Flash behavior Flashes through parting-line gaps sooner. Forgiving on worn or loosely built tools.
    Batch consistency checks Any MFI: a drifting value between lots is the early warning. Same — the number exists to be tracked, not ignored.

    What should you actually do with the MFI number?

  • Read the test conditions with the value — a grade quoted at a different temperature or load is a different number; compare like with like.
  • Fix the grade before the DFM freeze — MFI interacts with wall thickness, gate size and runner design; changing the grade later reopens all of them.
  • Use it as a batch fingerprint — when parts drift from lot to lot, the incoming MFI value is the first record to pull; it separates a resin shift from a process shift in minutes.
  • Do not compare families by MFI — a PP and a PC with the same MFI are not interchangeable; the index only ranks within a family under the same test conditions.

    • Frequently asked questions

    Is a higher MFI grade always the easier choice?

    Easier to fill, yes — but it gives back some impact strength and flashes sooner.
    If the part is thin-walled and non-structural, the trade usually pays; if it takes loads or drops, the tougher low-MFI grade deserves the extra process effort.

    Why did my new material lot mold differently?

    Check the MFI on the incoming lot against the certificate. Resin suppliers allow a range, and a lot near one end of it fills, packs and flashes differently from a lot near the other end — the process sheet then needs the small shift that keeps the parts consistent.

    Does MFI tell me the part’s final properties?

    No. It describes the melt, not the finished part. Mechanical properties come from the grade’s tested data — which is why a full datasheet review, not a single index, sits behind the material decision at DFM.

    Key takeaways

    1. MFI is the flow fingerprint of the melt — higher flows easier with less toughness, lower is tougher but hungrier for pressure and section.
    2. Fix the grade, and its MFI, before the tool design freezes; the number sizes gates, runners and minimum walls.
    3. Track MFI between lots as a batch fingerprint, and compare values only within a family at identical test conditions.

    If your part has thin walls, long flow paths or tight windows, send us the 3D model, the resin grade and its datasheet before the deposit. You will get the fill-feasibility read against the grade’s actual MFI, the gate and runner plan that follows from it, and the T1 evidence list — from a named tooling engineer, before any steel is ordered.

    Sources
    MatWeb — material property database for grade-level melt flow data
    Plastics Industry Association — industry resources on plastics processing practice
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions, International Organization for Standardization

    Sources: Plastics Technology · Plastics Technology Online · Prospector — Resin Datasheets · MatWeb — Material Property Database