What is MFI, and why does its test condition decide a granule purchase?

When you ask for a granule quote, the first number exchanged is usually the MFI, the melt flow index. It tells you how freely the material flows when molten, and in practice decides whether a grade suits your production line.

What does MFI measure?

A set amount of granule is pushed through a standard die in a heated instrument under a set weight, and the mass that comes out in ten minutes is reported in grams per 10 minutes (g/10 min). A higher MFI therefore means a more fluid melt; a lower MFI means a thicker melt of higher molecular weight.

The key point: MFI means nothing without its test condition

The same material gives one value at 190 °C with a 2.16 kg load and a completely different value at 230 °C with 5 kg. A supplier who writes only “MFI = 5” has not said anything yet. Polypropylene is normally tested at 230 °C with 2.16 kg, polyethylene at 190 °C (2.16 or 5 kg). A complete datasheet entry shows:

• the value and unit (g/10 min);
• the test temperature (190 °C or 230 °C) and the load (2.16 kg or 5 kg);
• the test method (ASTM D1238 or ISO 1133);
• the batch number and production date on the bag label and on the datasheet of that shipment.

How does MFI relate to your process?

• Pipe and profile (extrusion): usually a thicker melt with a low MFI, so the melt does not sag under its own weight and the wall stays even.
• Film: behaviour on the film line depends on MFI together with the grade type, not on one number.
• Injection moulding: thin, complex parts need a more fluid melt to fill the mould.

A practical rule: match the MFI to your own line, not to the price. A slightly cheaper grade whose MFI does not suit your machine can cost more in scrap and downtime.

Why MFI alone is not enough

MFI is one dimension of a material. Also ask for density, ash or filler percentage and, for mechanical work, impact strength. Two grades can share an MFI and behave completely differently on your machine.

Questions

Is a high or low MFI better?

Neither. Pipe extrusion usually suits a lower MFI, thin injection parts a higher one.

Why do two suppliers quote different MFI for one grade?

Usually the test condition differs. Compare temperature and load first, then the number.

Does MFI stay constant across production?

Within a uniform batch, yes. Differences between batches are what the batch number on the datasheet lets you trace.

What if the supplier gives no datasheet?

Without the datasheet of that shipment, an MFI figure is only a verbal claim. An MFI test can be run on a sample of the same shipment to check it.

For help choosing a grade for your line, contact the Epic Polymer specialists: +98 919 212 4919 · WhatsApp. Weekly base prices (Persian): granule price page.