Melt flow index is the mass in grams of molten polymer extruded through a standard die in ten minutes, under a specified temperature and load. It is reported in g/10 min.
That is the whole definition. Everything else is consequence.
What it actually tells you
MFI is a single-point viscosity measurement. It is not a fundamental material property – it is the answer to one specific question asked under one specific set of conditions.
But it is a very useful question, because melt flow index correlates inversely with molecular weight:
High MFI = the polymer flows easily = lower molecular weight = shorter chains Low MFI = the polymer resists flowing = higher molecular weight = longer chains
And molecular weight drives almost everything else you care about:
| Low MFI (high molecular weight) | High MFI (low molecular weight) | |
| Melt viscosity | High | Low |
| Melt strength | High | Low |
| Impact strength | Better | Worse |
| Environmental stress crack resistance | Better | Worse |
| Ease of filling a thin-wall mould | Harder | Easier |
| Typical use | Pipe, blow moulding, film | Thin-wall injection, fibre |
This is why you cannot simply pick “a good grade”. A pipe resin and a thin-wall container resin are the same polymer at opposite ends of the same trade-off – strength against processability.
The formula
The tester extrudes polymer through the die and a cut length of extrudate is collected over a timed interval and weighed.
MFI (g/10 min) = (mass of extrudate in grams ÷ collection time in seconds) × 600
The 600 converts to a ten-minute basis: 10 minutes = 600 seconds.
Worked example. A cut of extrudate weighs 0.42 g and was collected over 30 seconds:
MFI = (0.42 ÷ 30) × 600 = 8.4 g/10 min
Several cuts are taken and averaged, with the first and any inconsistent ones discarded.
Test conditions – the part people leave off
An MFI number without its conditions is meaningless. Different polymers are tested at different temperatures and loads, and the same polymer tested under a different load gives a completely different number.
| Polymer | Temperature | Load | Written as |
| Polyethylene (general) | 190 °C | 2.16 kg | MFI @ 190/2.16 |
| Polyethylene (higher load) | 190 °C | 5.0 kg | MFI @ 190/5 |
| HDPE pipe grade | 190 °C | 5.0 kg | MFI @ 190/5 |
| Polypropylene | 230 °C | 2.16 kg | MFI @ 230/2.16 |
| Polystyrene | 200 °C | 5.0 kg | MFI @ 200/5 |
| ABS | 220 °C | 10 kg | MFI @ 220/10 |
| Polycarbonate | 300 °C | 1.2 kg | MFI @ 300/1.2 |
Always write the conditions next to the number. “MFI 8” on a data sheet is not information. “MFI 8 g/10 min @ 190 °C / 2.16 kg” is.
The governing test methods are ASTM D1238 and ISO 1133. They are broadly equivalent for routine work; confirm which one a data sheet used before comparing numbers across suppliers.
MFR, MVR and FRR – three terms people mix up
MFR – melt flow rate. The same thing as MFI, measured by mass. The two terms are used interchangeably; MFR is the more current name.
MVR – melt volume rate. The same test measured by volume rather than mass, reported in cm³/10 min. Converting between them needs the melt density:
MFR = MVR × melt density
MVR is often preferred because it is measured by piston displacement rather than by cutting and weighing, which removes an operator variable.
FRR – flow rate ratio. The MFI measured at a high load divided by the MFI at a low load, for instance 190/21.6 divided by 190/2.16. This is a rough indicator of molecular weight distribution: a high FRR means a broad distribution.
FRR is genuinely useful and rarely asked for. Two resins can have identical MFI and behave completely differently on your line because one has a broad distribution and the other a narrow one. Broad-distribution resins shear-thin more, which usually means easier processing at high shear.
Typical MFI values by polymer
Working ranges, not specifications – always use the actual grade data sheet.
| Application | Polymer | Typical MFI (g/10 min) | Conditions |
| HDPE pipe (PE 80 / PE 100) | HDPE | 0.2 – 1.1 | 190/5 |
| HDPE blow moulding | HDPE | 0.2 – 0.8 | 190/2.16 |
| HDPE blown film | HDPE | 0.03 – 0.1 | 190/2.16 |
| HDPE injection moulding | HDPE | 4 – 20 | 190/2.16 |
| LDPE blown film | LDPE | 0.3 – 2 | 190/2.16 |
| LLDPE blown film | LLDPE | 0.8 – 2 | 190/2.16 |
| PP injection moulding, general | PP | 10 – 35 | 230/2.16 |
| PP thin-wall injection | PP | 35 – 100+ | 230/2.16 |
| PP raffia / tape | PP | 2 – 4 | 230/2.16 |
| PP blown / cast film | PP | 2 – 8 | 230/2.16 |
Notice the range within a single polymer. HDPE spans from around 0.03 for film to 20 or more for injection moulding – a difference of nearly three orders of magnitude in the same material family.
Why this matters when you buy masterbatch
Here is the connection almost nobody makes at the point of purchase.
A masterbatch is mostly carrier resin, and that carrier has its own MFI. When you dose masterbatch into your polymer, you are blending two different polymers, and the result is a compound whose flow behaviour is neither of theirs.
Three practical consequences:
1. The carrier should be equal to or higher in MFI than the host polymer. A carrier that flows more easily than the material around it melts early, spreads, and distributes pigment. A stiffer carrier resists distribution and can survive as a discrete unmixed domain – which in a thin film is a visible fish-eye and a weak point. This is counter-intuitive to many buyers, who assume the carrier should “match”.
2. At high dosages the carrier moves your compound’s MFI measurably. At 1% it barely registers. At 6% – a UV-grade black for pipe, a high-opacity white for film – it is a real shift. IS 4984 for HDPE pipe explicitly limits the finished pipe’s melt flow rate to within ±30% of the base resin’s, so a mismatched carrier at pipe dosage can push you outside a standard you have to meet. We cover that in carbon black content in HDPE pipes.
3. A carrier far higher in MFI than the host weakens the part. Very high MFI means short chains and poor mechanical properties. Dose 8% of a very high-MFI carrier into a structural part and you have diluted its strength with something weaker.
So the right question to a masterbatch supplier is not “what is your dosage?” but “what is the carrier polymer and its MFI, and how does that sit against my resin?” Very few buyers ask it. It is on any competent technical data sheet.
Limitations worth knowing
MFI is a single-point measurement taken at very low shear rate. Real processing happens at shear rates orders of magnitude higher, where the polymer’s behaviour can differ substantially – particularly for broad-distribution and highly branched resins that shear-thin strongly.
So MFI is an excellent quality-control and comparison tool, and a poor predictor of processing behaviour on its own. For a full picture you need a rheology curve. For checking that this lot matches the last one, MFI is fast, cheap and entirely adequate – which is why every plant has a tester.
FAQs
What is the formula for melt flow index? MFI = (mass of extrudate in grams ÷ collection time in seconds) × 600. The 600 converts the result to grams per ten minutes.
What is the unit of melt flow index? Grams per 10 minutes (g/10 min). Melt volume rate, the volumetric equivalent, is reported in cm³/10 min.
What is the melt flow index of HDPE? It depends entirely on the grade. HDPE blown film grades run around 0.03 to 0.1 g/10 min, pipe grades around 0.2 to 1.1 at 190/5, and injection moulding grades from about 4 to 20 at 190/2.16.
Does high MFI mean better or worse plastic? Neither – it means different. High MFI flows easily and fills thin sections well but has lower impact strength and stress crack resistance. Low MFI is stronger and tougher but harder to process.
What is the difference between MFI and MFR? None in substance. Melt flow rate is the current name for the same measurement; melt flow index is the older term. Both are measured by mass, in g/10 min.
What is the difference between MFR and MVR? MFR is measured by mass, MVR by volume. MFR = MVR × melt density. MVR removes the cut-and-weigh step, so it is less operator-dependent.
Why does masterbatch carrier MFI matter? Because the carrier is a polymer being blended into yours. It should be equal to or higher in MFI than your resin so it disperses properly, and at high dosages it measurably shifts your compound’s flow – which matters where a standard limits it, as IS 4984 does for HDPE pipe.
Which test standard covers melt flow index? ASTM D1238 and ISO 1133. Confirm which was used before comparing numbers from different suppliers.