How to check MFI of recycled plastic granules

Melt flow index (MFI, also called melt flow rate) is the single most useful number on a recycled granule test report. It tells you how easily the melted plastic flows, which decides whether it will run on your machine.

How MFI is measured

A small sample is melted in a heated barrel and pushed through a standard die by a fixed weight. The amount that flows out in 10 minutes, in grams, is the MFI. The standards are ASTM D1238 and ISO 1133. Conditions matter, so always ask which ones were used:

• PP: usually 230 °C with a 2.16 kg load.
• PE (HDPE, LDPE, LLDPE): usually 190 °C with a 2.16 kg load. Some pipe grades are also reported at 5 kg.

Which MFI range for which process?

• Pipe and large blow moulding: low MFI, so the melt holds its shape.
• Film and raffia: low to medium MFI.
• General injection moulding: medium MFI.
• Thin-wall injection moulding: high MFI, so the melt fills fast.

Your machine supplier or current virgin grade datasheet gives the best target. Use it as your starting range.

Why recycled MFI varies

Recycled granules come from mixed sources, and each melt cycle shortens polymer chains, so MFI usually drifts upward. That is why a good supplier blends lots and reports MFI per lot.

How to agree MFI with a supplier

1. Give a target value and a tolerance, for example "MFI 8 ± 2 at 230 °C / 2.16 kg".
2. Ask for a test report with every lot, not just with the first sample.
3. Test the sample yourself, or at a local lab, before confirming bulk.
4. Keep a retained sample from each delivery in case of disputes.

When you post a requirement on PlasticSetu, add your MFI range. The desk shares it with suppliers so quotes come back for material that will actually run.