Two batches of the same polymer can behave completely differently on your production line — one flows smoothly, the other clogs or fills poorly. The number that predicts this is the melt flow index. Ignore it, and you’re guessing whether a material will run well before you’ve even loaded the hopper.
For anyone in R&D, procurement, or processing, the melt flow index is one of the fastest ways to check whether a plastic grade fits your process. It links directly to how the polymer flows when melted.
In this guide, you’ll learn what melt flow index means, how the test works, what high and low MFI values tell you, and why the number matters when you select plastics and masterbatch. By the end, you’ll read an MFI spec with confidence.
What Is Melt Flow Index?
Melt flow index (MFI), also called melt flow rate (MFR), is a measure of how easily a molten plastic flows under a set temperature and load. It’s expressed in grams of polymer that flow through a standard die in 10 minutes (g/10 min). A higher number means the material flows more easily.
MFI is essentially an indicator of a polymer’s molecular weight and viscosity. Longer polymer chains resist flow, so they give a lower MFI. Shorter chains flow more freely, giving a higher MFI.
Because it’s quick, cheap, and repeatable, MFI is one of the most widely used quality-control checks in the plastics industry. It won’t tell you everything about a material, but it gives a fast, consistent signal of flow behaviour.
How Is the Melt Flow Index Test Done?
The MFI test uses a device called a melt flow indexer (or extrusion plastometer). The polymer is heated in a barrel to a specified temperature, then pushed through a die of standard diameter by a weight resting on a piston.
The material that extrudes over a set time is collected and weighed. The result is scaled to grams per 10 minutes. Test conditions — temperature and load — are defined by standards such as ASTM D1238 and ISO 1133, and they vary by polymer type.
This is why an MFI value only makes sense alongside its test conditions. An MFI figure for polypropylene tested at one temperature and load can’t be compared directly with a value measured under different conditions.
High MFI vs Low MFI: What’s the Difference?
The MFI number tells you about flow, and flow connects to strength. Understanding the trade-off helps you pick the right grade.
A high MFI means the polymer flows easily. That suits processes needing thin walls and fast fill, like injection moulding of complex parts. The trade-off is generally shorter chains and somewhat lower mechanical strength.
A low MFI means the polymer is more viscous and flows less readily. That points to longer chains and typically better mechanical strength and toughness. It suits processes like blow moulding and extrusion, where melt strength matters.
Melt Flow Index Comparison Table
|
Aspect |
High MFI |
Low MFI |
|
Flow |
Easy, fast |
Slow, viscous |
|
Molecular weight |
Lower |
Higher |
|
Mechanical strength |
Generally lower |
Generally higher |
|
Best-suited process |
Injection moulding, thin walls |
Blow moulding, extrusion |
| Typical trade-off | Fill ease over strength |
Strength over flow |
Why Melt Flow Index Matters for Manufacturers
MFI is a practical decision tool, not just a lab number. It helps you match a material to a process before production, avoiding costly trial runs with the wrong grade.
For processing, it predicts flow. An injection moulder filling a thin, complex part usually wants a higher MFI so the cavity fills fully — reducing short shots. An extrusion or blow-moulding line often wants a lower MFI for melt strength and part toughness.
For quality control, MFI catches inconsistency. If incoming batches show MFI drift, the polymer’s molecular weight has shifted, and processing may change. Checking MFI on delivery is a fast way to keep your line stable.
MFI also matters when selecting masterbatch. The carrier resin’s flow should be compatible with your base polymer so the additive or colour disperses evenly. A mismatch can cause poor dispersion, streaking, or processing issues.
How MFI Guides Masterbatch and Compound Selection
When you add a masterbatch or use a compound, you’re blending materials that each have their own flow behaviour. Matching MFI across them keeps the melt uniform.
If a masterbatch carrier flows very differently from your base resin, you can get uneven dispersion of pigment or additive. That shows up as colour streaks, weak spots, or inconsistent parts. Choosing a carrier with a compatible melt flow index avoids this.
For custom compounds, MFI is one of the properties formulators tune to hit both flow and strength targets. Sharing your process type and target MFI with your supplier helps them recommend the right grade the first time.
Typical MFI Ranges by Process
Different processes favour different flow behaviour, and MFI ranges reflect that. Injection moulding of thin-walled or intricate parts generally calls for higher-MFI grades so the melt fills every corner before it sets. Fast fill reduces short shots and shortens cycle time.
Extrusion and blow moulding usually work better with lower-MFI grades. Here the melt must hold its shape as it leaves the die or inflates, so melt strength matters more than easy flow. A grade that’s too free-flowing can sag or collapse.
Film and fibre applications sit across the range depending on the exact process. The practical takeaway is simple: define your process first, then choose an MFI band that supports it rather than fighting it.
Limitations of Melt Flow Index
MFI is useful, but it isn’t the whole story. It measures flow at a single low shear rate, while real processing often happens at much higher shear. Two materials with the same MFI can still behave differently on the line.
It also says nothing directly about properties like impact strength, clarity, or long-term durability. Treat MFI as a fast first filter, not a complete material specification.
For critical applications, pair MFI with other data — molecular weight distribution, mechanical tests, and trial runs. Used alongside these, MFI remains one of the quickest and most valuable checks in your toolkit.
Conclusion
The melt flow index is a small number with big influence. It signals how a polymer flows, hints at its molecular weight and strength, and predicts how it will behave in your process. High MFI favors easy fill; low MFI favours melt strength.
Use it to match material to process, catch batch inconsistency, and select compatible masterbatches and compounds. When flow and strength are balanced for your application, you get fewer defects and more consistent output.
Not sure which MFI grade or masterbatch suits your process? Talk to our technical and R&D team at Surya Compounds & Masterbatches for a recommendation matched to your product and machine.
Frequently Asked Questions
Q: What does melt flow index tell you?
A: Melt flow index tells you how easily a molten plastic flows under set temperature and load. A higher value means easier flow and generally lower molecular weight and strength. A lower value means more viscous flow and generally higher molecular weight and mechanical strength.
Q: What is the unit of melt flow index?
A: Melt flow index is expressed in grams per 10 minutes (g/10 min). It measures how many grams of molten polymer flow through a standard die in ten minutes under a specified temperature and load, following standards such as ASTM D1238 or ISO 1133.
Q: Is high MFI better than low MFI?
A: Neither is universally better. High MFI flows easily and suits injection moulding of thin, complex parts. Low MFI is more viscous, offers better melt strength, and suits blow moulding and extrusion. The right choice depends on your process and strength requirements.
Q: What standards are used to test melt flow index?
A: The two main standards are ASTM D1238 and ISO 1133. They define the test temperature, applied load, and die dimensions for each polymer type. Because conditions differ by material, an MFI value should always be quoted with its test conditions.
Q: Why does melt flow index matter when choosing masterbatch?
A: The masterbatch carrier resin should have a melt flow index compatible with your base polymer. If flows differ too much, pigment or additive may disperse unevenly, causing streaks, weak spots, or processing problems. Matching MFI helps ensure uniform dispersion and consistent parts.
