Reading Polymer
Melt Flow Index
One small number on a resin datasheet — the melt flow index — tells you more about how a polymer will process than almost anything else on the page. Here is what it measures, how it is tested, and how to read it when specifying a grade.
Open the technical datasheet for any grade of polyethylene or polypropylene and one figure sits near the top of almost every one: the melt flow index. It looks deceptively simple — a single number in grams per ten minutes — yet it quietly decides whether a resin will fill an injection mould cleanly, hold a stable film bubble, or blow into a bottle without sagging. Read it correctly and you specify the right material first time; misread it and you order a technically valid resin that will not run on your line. This explainer unpacks what melt flow index is, how it is measured, and how buyers use it, as part of the Polymers range within our Products & Trading sector.
What melt flow index measures
Melt flow index (MFI) — also written as melt flow rate (MFR) — is the mass of molten polymer, in grams, that flows through a standard die in 10 minutes under a fixed temperature and a fixed weight. That is the whole test: heat the resin, push it through a calibrated capillary with a known load, and weigh what comes out. The result is an indirect measure of the polymer's melt viscosity, and through it, of its molecular weight.
The relationship is inverse. A high MFI means the melt flows easily — lower viscosity, shorter polymer chains, lower molecular weight. A low MFI means the melt is stiff and slow — higher viscosity, longer chains, higher molecular weight. So a resin quoted at 20 g/10 min is far runnier when molten than one at 0.3 g/10 min. Crucially, MFI is not a grade of quality; it is a processing fingerprint that has to match the shaping method you intend to use.
How the test works: ASTM D1238 and ISO 1133
Two standards govern the measurement, and they describe essentially the same instrument — a heated barrel, a piston loaded with a weight, and a die measuring 2.095 mm in diameter by 8 mm long. ASTM D1238 is the long-standing North American method and the origin of the term "melt flow index." ISO 1133 is the international method and prefers the term "melt mass-flow rate." The two differ in small procedural details — preheat time, the amount of resin charged, the exact starting position of the piston — but both report grams per 10 minutes, and for most commercial grades the numbers are directly comparable so long as the temperature and load match.
That last point is where buyers get caught. An MFI value is meaningless without its test condition. The same resin gives a very different number at 190 °C under 2.16 kg than at 230 °C under 5 kg. Polyethylenes such as HDPE and LDPE are conventionally run at 190 °C / 2.16 kg; polypropylene, which melts hotter, is run at 230 °C / 2.16 kg. Higher loads (5 kg or 21.6 kg) are used for very high-molecular-weight or very stiff resins that barely move under the standard weight.
Melt flow index is not a score to be maximised — it is a match to be made. The best resin is the one whose flow suits the process, not the one with the biggest number.
Typical values by process
Because MFI tracks how freely a melt moves, each shaping process has a comfort band. Injection moulding wants the melt to race into a cold, complex mould before it freezes, so it favours higher MFI. Film extrusion and blow moulding, by contrast, need the molten polymer to hold its own shape — as a rising film bubble or a hanging parison — so they demand lower MFI and the higher melt strength that comes with it. The table below shows indicative ranges; exact figures vary by producer and grade.
| Process | Typical MFI band | Why |
|---|---|---|
| Injection moulding | ~4–30 g/10 min (up to higher for thin-wall) | Runny melt fills complex, cooling moulds fast |
| Film & sheet extrusion | ~0.5–3 g/10 min | Melt strength keeps the bubble/web stable |
| Blow moulding | ~0.1–1 g/10 min | High molecular weight resists parison sag |
| Pipe & profile extrusion | ~0.2–1 g/10 min | Low flow gives stiffness and top-load strength |
As a rough polymer-family guide, HDPE commonly spans roughly 0.1–30 g/10 min depending on end use (blow-moulding grades cluster at the low end), LDPE around 0.1–20 g/10 min, and polypropylene anywhere from about 0.5 to 50 g/10 min across the grade spectrum — with specialist meltblown PP for filtration media running far higher still. When you compare a PE or PP grade against your process band, you are really asking whether its flow behaviour fits.
The limits of a single number
MFI is powerful precisely because it is simple, but that simplicity is also its blind spot. It is a single-point measurement taken at very low shear, whereas real processing happens at high shear across a range of rates. Two resins can carry an identical MFI and still behave differently on the line because their molecular weight distribution — the mix of short and long chains — is different. A broad distribution shear-thins more, flowing easily under the high shear of injection moulding while retaining melt strength at rest.
To see that, engineers use the MFR ratio: the flow measured at a high load (say 21.6 kg) divided by the flow at a low load (say 2.16 or 5 kg), at the same temperature. A larger ratio signals a broader molecular weight distribution and more shear-thinning — generally better high-shear processability but often lower impact strength. For demanding applications, reading MFI and MFR ratio together tells a far fuller story than either alone, which is why datasheet figures should always be checked against real trials under independent quality assurance.
Why it matters for sourcing
For a trading and supply partner, melt flow index is the common language that connects a customer's process to a producer's grade slate. Matching the correct MFI band — and confirming it batch to batch — is what prevents a converter from taking delivery of a resin that is technically on-spec yet wrong for the line. Polymers also sit downstream of the petrochemical feedstock chain, so grade availability and pricing move with that market. Arian Holding sources HDPE, LDPE, PP, PVC and PET against exact customer specifications, with global sourcing, lab verification under quality assurance, and the supply chain & logistics reach to deliver consistent grades across the region. To match a grade to your process or request current availability, request a quote and our polymers desk will respond.
Frequently asked questions
What does melt flow index actually measure?
Melt flow index (MFI), also called melt flow rate (MFR), is the mass in grams of molten polymer that flows through a standard die in 10 minutes under a set temperature and load. It is an inverse proxy for molecular weight and viscosity: a high MFI means a thinner, easier-flowing melt and lower molecular weight; a low MFI means a stiffer, higher-viscosity melt and higher molecular weight.
What is the difference between MFI and MFR?
They describe the same measurement. MFI (melt flow index) is the older term associated with ASTM D1238, common in North America, while MFR (melt mass-flow rate) is the ISO 1133 term used internationally. Both report grams per 10 minutes; just confirm the standard, temperature and load are the same before comparing two numbers.
What test conditions are used for HDPE and polypropylene?
Polyethylene grades such as HDPE and LDPE are typically tested at 190 °C with a 2.16 kg load, while polypropylene is tested at 230 °C with a 2.16 kg load. Higher loads such as 5 kg or 21.6 kg are used for very high molecular weight or very stiff resins. Always quote the condition alongside the value.
Does a higher melt flow index mean better quality?
No. MFI is not a quality grade, it is a processing characteristic. The right value depends on the process: injection moulding favours higher MFI for fast mould filling, while film extrusion and blow moulding need lower MFI for melt strength. A resin is correct when its MFI matches the application, not because the number is high or low.
Why do two resins with the same MFI behave differently?
Melt flow index is a single-point measurement at low shear. Two resins can share an MFI yet differ in molecular weight distribution, which governs shear-thinning and melt strength. The MFR ratio, obtained by dividing the flow at a high load by the flow at a low load, indicates how broad that distribution is and predicts real processing behaviour more completely than MFI alone.
Sources: ASTM D1238 & ISO 1133 (melt mass-flow / melt volume-flow of thermoplastics); Instron — ISO 1133 melt flow testing; Infinita Lab — MFI testing guide; HDPE melt flow index guide; Asaclean — MFR in plastic processing. Technical figures are indicative industry ranges provided for general information, not engineering or trading advice; confirm current specifications against the applicable ASTM or ISO standard and supplier datasheet for your application.