Every bag of polymer pellets carries a number that quietly governs whether the resin will run cleanly on your machine: its melt flow index, or MFI. It looks like a small technical detail on a datasheet, yet it separates a grade that fills a thin-walled mould in seconds from one that holds its shape in a large blow-moulded drum. For anyone buying polyethylene, polypropylene or PVC, reading MFI correctly is the difference between a smooth production run and a warehouse of off-spec resin. This explainer decodes what the number means, how it is measured, and how to use it when specifying a grade.
The short version
MFI (also called melt flow rate, MFR) is the weight of molten plastic, in grams, that flows through a standard die in 10 minutes at a set temperature and load. High MFI = thin, easy-flowing melt — good for filling intricate injection moulds. Low MFI = thick, strong melt — good for blow moulding, pipe and film. It is a processing fingerprint, not a quality grade, and it is only meaningful when the test conditions are quoted with it.
What the number actually measures
Melt flow index is a measure of the ease of flow of a molten thermoplastic. In the test, pellets are melted in a heated barrel and pushed through a precise die by a weighted piston; the machine records how many grams emerge over ten minutes. That figure — for example 2 g/10 min or 20 g/10 min — is the MFI. Because flow is the inverse of viscosity, a higher MFI means lower melt viscosity and easier flow, while a lower MFI means a stiffer, more viscous melt that resists flow. Crucially, MFI is an indirect proxy for average molecular weight: longer polymer chains tangle more and flow less, so high-molecular-weight grades show low MFI.
"MFI is a fingerprint of how a resin behaves when it is molten — not a verdict on whether it is good or bad."
How it is tested: ASTM D1238 and ISO 1133
Two standards govern the measurement, and they are close cousins. ASTM D1238 is used mainly in North America and uses the term melt flow index; ISO 1133 is the international standard, common in Europe, and uses the term melt flow rate. Both push the melt through a die of about 2.095 mm under a standard load — most often 2.16 kg — at a polymer-specific temperature: typically 190°C for polyethylene and 230°C for polypropylene. Because the result changes with both temperature and load, an MFI figure is meaningless on its own — it must always be read together with the conditions, written as, for instance, "190°C/2.16 kg." Our laboratory teams verify these values against supplier certificates before release, part of our quality assurance routine.
High flow or low flow? Matching MFI to the process
There is no universally "better" MFI; the right value is dictated by how the resin will be shaped. Injection moulding needs higher-flow grades so the melt races into thin, complex cavities before it freezes. Blow moulding, pipe and sheet extrusion need lower-flow grades, because a stiff melt has the melt strength to hold a parison or hold its wall thickness without sagging. Film sits in between, balancing flow for output against strength for bubble stability. Pick a grade whose MFI is tuned to the wrong process and you fight the material all day — flash and short shots at one extreme, sag and weak parts at the other.
Typical MFI ranges by polymer and process
The table below gives indicative ranges drawn from published industry sources. They are general reference points for orientation, not specification limits — always work to the grade datasheet and the conditions stated on it.
| Polymer & process | Test condition | Typical MFI (approx.) | Why |
|---|---|---|---|
| HDPE — blow moulding | 190°C / 2.16 kg | 0.1–1 g/10 min | High melt strength for parisons |
| HDPE — injection moulding | 190°C / 2.16 kg | 4–30 g/10 min | Fast mould filling |
| PP — film & extrusion | 230°C / 2.16 kg | 2–12 g/10 min | Output vs. melt stability |
| PP — injection moulding | 230°C / 2.16 kg | 30–70 g/10 min | Thin-wall, fast-cycle parts |
| LDPE — film | 190°C / 2.16 kg | ~0.3–2 g/10 min | Bubble stability for blown film |
Because conditions differ by polymer, you cannot compare an HDPE MFI measured at 190°C directly against a PP value at 230°C — only like-for-like conditions are comparable.
Beyond a single number: the MFR ratio and its limits
One figure cannot capture everything. Measure MFI at a high load and again at a low load, and the ratio between them — the flow rate ratio — hints at the breadth of the molecular weight distribution: a broader distribution shear-thins more, processing well at high speed but often trading away some impact strength. MFI also says little on its own about density, additives or how a grade behaves under the high shear rates of real machinery, where rheology is more complex than a slow gravity-driven test. That is why experienced buyers treat MFI as the first filter, then confirm density, comonomer and additive package against the full datasheet — the kind of judgement our global sourcing desk applies to every polymer enquiry across the industrial products sector.
Frequently asked questions
What is melt flow index (MFI) in simple terms?
Melt flow index is a measure of how easily a molten plastic flows. It is the weight in grams of polymer that is pushed through a standard die in 10 minutes at a set temperature and load. A higher number means the melt flows more easily (lower viscosity); a lower number means a stiffer, more viscous melt. It is a quick, standardised way to compare grades and to check that an incoming batch matches its specification.
What is the difference between MFI and MFR?
They describe the same measurement. MFI (melt flow index) is the older term used mainly with the American standard ASTM D1238, while MFR (melt flow rate) is the term used in the international standard ISO 1133. Both report grams of polymer extruded through a standardised die per 10 minutes under defined temperature and load, so the values are directly comparable when the test conditions match.
Does a higher MFI mean better quality plastic?
No. MFI is not a quality score — it is a processing characteristic. A high-MFI grade flows easily and fills complex injection moulds quickly, but generally has lower melt strength and impact resistance. A low-MFI grade is tougher and holds its shape in blow moulding and extrusion but is harder to push through a mould. The right value depends entirely on the process and end use, not on a higher or lower figure being inherently better.
What test conditions are used to measure MFI?
The polymer is melted in a heated barrel at a material-specific temperature and forced through a die of about 2.095 mm by a standard weight, most commonly 2.16 kg. Polyethylene is typically tested at 190°C and polypropylene at 230°C. Because the value changes with temperature and load, an MFI figure is only meaningful when the test conditions are stated alongside it.
How do I use MFI to choose a polymer grade?
Start from the process. Injection moulding favours higher-flow grades so the melt fills thin, intricate cavities; film, sheet and pipe extrusion and blow moulding favour lower-flow grades for melt strength and dimensional stability. Match the grade's stated MFI and test conditions to your machine and part, then confirm the value on the supplier's certificate of analysis before committing to a large order.
Sourcing polymers with Arian Holding
Arian Holding trades the full polyolefin and engineering-resin ladder — HDPE, LDPE, LLDPE, PP, PVC and PET — alongside the petrochemical feedstocks that make them. Our trade desk helps buyers translate a process requirement into a precise, certified order: the right grade and MFI, density and additive package, matched to your line and backed by a certificate of analysis. Explore the range in our Polymers catalogue, then request a quote with your target MFI, volume and application.
Sources: ASTM D1238 and ISO 1133 melt-flow test standards; Infinita Lab — MFI testing (ASTM D1238 & ISO 1133); Instron — ISO 1133 melt mass-flow rate; SpecialChem — melt flow index in polymers; Melt flow index overview; EUPE — MFI of polymers and grade table. Figures are indicative and provided for general information only; always confirm against the current standard edition and the grade datasheet — this is not engineering or trading advice.