When comparing resin data sheets side by
side, it's easy to skip over the melt flow line. It seems like a small detail.
It's not. When choosing a grade for a new part or finding out if a new
supplier's lot will work in your machine, MFI is often the first spec to know
whether a resin will flow on your machine. In this article, I'll explain what
MFI means, how it relates to processing, and how to read it without getting
fooled.
What Is Melt Flow Index (MFI)?
Melt flow index (MFI) is the mass of
molten polymer, in grams, that flows through a standard die in 10 minutes under
a set temperature and load. A higher MFI means the melt flows more easily. MFI
and melt flow rate (MFR) are the same measurement, defined in ASTM D1238 and
ISO 1133.
Why MFI Matters in Plastic Processing
MFI is inversely related to melt
viscosity, and lower value usually indicates higher molecular weight than its
values, thus, it is closely related to the processing method. As Imubit points
out, higher MFI values contribute to molds filling quickly while lower values
provide the melt strength needed for film bubbles and blow molding parisons.
Good polymer grade selection starts here, because
MFI also works as a quality check. ASTM D1238 testing is used for incoming
material acceptance, and Intertek notes that degraded material generally flows
more because its molecular weight has dropped.
Key Factors to Consider When Using MFI for Polymer Grade
Selection
MFI is only useful when you read it in
context. These five factors are what I would check before approving any grade.
Skipping one is how a spec that looks right on paper fails on the line.
1. Match the MFI Range to Your Process
Injection molding HDPE often runs about 4
to 30 g/10 min, while blow molding HDPE sits near 0.1 to 1.0. Polypropylene for
thin-wall injection molding is often above 40 g/10 min.
2. Understand Molecular Weight and Viscosity
Within one polymer family, lower MFI
means higher molecular weight, which adds toughness but slows flow. Watch
one-sided specs. A maximum-only limit can let in material that will not fill
thin walls, and a minimum-only limit can let in weaker, low-molecular-weight
material.
3. Check the Test Conditions
PE is normally tested at 190°C and PP at
230°C, usually with 2.16 kg. Infinita Lab points out that the same polymer
gives a very different value under different conditions. Look for the full
format, such as 190°C/2.16 kg.
4. Compare Within the Same Polymer Type
Reported typical ranges are wide: LDPE
0.1 to 30, HDPE 0.1 to 3, and PP 1 to 50 g/10 min. They also vary by
manufacturer and additives. A PP and a PE with the same number are not
equivalent.
5. Verify Supplier Data and Batch Consistency
Ask for the data sheet or certificate of
analysis, and set a tolerance window. Infinita Lab suggests about 15 to 25% of
the target as practical. Pellets stored warm, damp, or in sunlight can drift,
so storage matters too.
If you source through a B2B polymer
marketplace India buyers use, confirm the MFI and its test
conditions with the seller before ordering. Polymers Bazaar is one example. It
describes itself as a B2B digital marketplace connecting verified polymer
buyers and sellers across West Bengal, where buyers can send one RFQ to
multiple verified sellers or compare listed prices. Put the MFI range and test
conditions in that RFQ.
Practical Examples
These are illustrative scenarios, not
real cases.
●
Bottle line: A blow molder receives an HDPE lot at 4 g/10 min against a 0.5 target.
The parison would likely sag, since blow grades usually sit near 0.1 to 1.0.
●
Cap molder: A PP lot reads far below the spec window. Cavities may not fill, and
the safer fix is usually a grade in the right window.
Common Mistakes
●
Mixing test conditions: Values from different temperatures or loads cannot be compared.
●
Assuming higher is better: A higher MFI can mean lower molecular weight and weaker impact
resistance.
●
Relying on one number: MFR uses a single shear rate, so for sensitive jobs ask for the flow
rate ratio too.
●
Ignoring storage: Oxidation, moisture, and UV can raise MFR before the resin reaches
your machine.
Key Takeaways
●
Choose MFI by process, not by
habit.
●
Read every value with its
temperature and load.
●
Compare only within the same
polymer type.
●
Use a tolerance window, not a
one-sided limit.
●
Confirm batch data with the
supplier.
Conclusion
MFI will not lead you to a grade, but it
highlights them quickly. When read with the test conditions and your process in
mind, supplier data sheets become much easier to compare. We hope this article leads you to approach
this with a better idea of what matters, and save you some trial and error.
FAQs
What does a high MFI mean?
It means lower melt viscosity, easier flow, and
usually lower molecular weight.
Is MFI the same as MFR?
Yes, MFI is the ASTM term and MFR is the ISO 1133 term
for the same test.
What unit is MFI measured in?
Grams per 10 minutes (g/10 min) under ASTM D1238 or
ISO 1133.
Which processes use low MFI grades?
Blow molding, film, and extrusion usually use lower
MFI, while injection molding uses higher.
Can MFI change after processing or
storage?
Yes, degradation and poor storage can raise it.