MFI of PP: What Melt Flow Index Values Mean for Your Process

MFI, or melt flow index, is the single number most moulders check first on a bag of PP granules. It tells you how easily the melted polymer flows, and that decides which process and which part a grade is suited to.

What MFI measures

In an MFI test, a small amount of granules is melted in a heated barrel and pushed through a standard small die by a fixed weight. The amount that flows out in ten minutes, in grams, is the melt flow index. For polypropylene the test is normally run at 230 °C with a 2.16 kg weight, under ISO 1133 or ASTM D1238, so the result is written as g/10 min at 230 °C/2.16 kg.

A higher MFI means the polymer flows more easily. A lower MFI means it is thicker in the melt, which usually goes with longer polymer chains and better toughness.

Rough MFI ranges by process

Every line is different, so treat these as a starting point rather than a rule:

• Extrusion, sheet and blow moulding: low MFI, often below about 3 g/10 min, so the melt holds its shape.
• Raffia and tape: commonly around 2 to 4 g/10 min. See our guide to PP raffia granules.
• General injection moulding: a broad middle band, often around 8 to 25 g/10 min, for crates, buckets, furniture and housewares.
• Thin-wall and fast-cycle moulding: higher MFI, often 30 g/10 min and above, so the melt fills thin sections quickly.

Why MFI matters on the shop floor

• Too low for your mould: short shots, incomplete filling and higher injection pressure.
• Too high for your part: flash, and often lower impact strength.
• Changing from batch to batch: the machine settings that worked yesterday stop working today. For many moulders, a steady MFI matters more than the exact number.

MFI of PP vs PPCP

Both PP homopolymer and PPCP are available across a wide range of MFI values. For the same MFI, the copolymer will usually be tougher, which is why it is chosen for parts that get dropped or knocked.

MFI in recycled PP

Reprocessing can raise the MFI of polypropylene, and recycled material comes from many sources, so its MFI varies more than virgin PP. That is why recycled granules should be tested batch by batch rather than judged from a single data sheet.

How we test MFI

Every batch of our recycled PP and PPCP granules is tested in our own laboratory for melt flow index, density, moisture, contamination, elongation and Izod impact strength before it is dispatched. See how we test, or tell us the MFI your process needs.