What is MFI (melt flow index) and why does it matter for recycled polymers?

If there is one number that matters most when buying or selling recycled polymer granules, it is the MFI. More than colour, more than source material, more than packaging — the melt flow index determines whether the material will run in your machine or cause costly downtime. Understanding it is essential for every polymer buyer.

What exactly is MFI?

Melt Flow Index (MFI) — also called Melt Flow Rate (MFR) or Melt Index (MI) — is a standardised measure of how easily a thermoplastic polymer flows when it is melted. Technically, it is defined as the mass of polymer (in grams) that flows through a standardised die over exactly 10 minutes, under a specified temperature and load.

In simple terms: it tells you how runny the melted plastic is. A high MFI means the melt flows easily — like warm honey. A low MFI means the melt is thick and viscous — more like cold caramel. Both extremes have their uses, but each process requires a specific range.

How is MFI measured?

The test apparatus is called an MFI tester or melt flow indexer. A small sample of the polymer (around 4–5 grams) is loaded into a heated cylindrical barrel. After a set preheat time, a standardised weight is applied via a piston. The molten polymer is forced through a die (typically 2.095 mm diameter). A timed sample is collected and weighed. The result — expressed in g/10min — is the MFI.

Test conditions vary by polymer: polyethylene (HDPE, LDPE, LLDPE) is tested at 190°C with a 2.16 kg load. Polypropylene (PP) is tested at 230°C with a 2.16 kg load. Standards: ASTM D1238 or ISO 1133.

MFI and molecular weight: the relationship

MFI is inversely related to molecular weight. Higher molecular weight polymer chains are longer and more entangled — they resist flow, giving a lower MFI and a stronger, tougher material. Lower molecular weight chains flow more freely — giving a higher MFI and easier processing, but at the cost of some mechanical strength.

This is why pipe-grade HDPE has a very low MFI (0.3–1.0 g/10min) — the long chain structure gives the pipe its burst pressure resistance. And why injection-moulding grades have high MFI (10–30 g/10min) — the material needs to fill complex mould cavities quickly.

MFI ranges by process

Pipe extrusion (HDPE)

Pipe extrusion requires MFI in the range of 0.3–2.0 g/10min. The low MFI provides the melt strength needed to maintain consistent wall thickness. Too high an MFI and the pipe walls sag or thin unevenly.

Blow moulding (HDPE)

Blow moulding of bottles and containers typically uses HDPE at MFI 0.3–2.5 g/10min. The process requires good melt strength so the parison does not stretch and thin before inflation.

Film extrusion (LDPE/LLDPE)

LDPE film grades typically run at MFI 0.5–4.0 g/10min. Stretch film and agricultural film require specific MFI ranges for bubble stability during blown film extrusion.

Injection moulding (PP/HDPE)

Injection moulding needs high-flow material — PP typically 4–30 g/10min, HDPE 6–20 g/10min. The higher MFI allows the melt to rapidly fill long, complex flow paths before the material cools.

Why MFI is especially critical for recycled polymers

Virgin polymer MFI is tightly controlled at the point of manufacture. With recycled polymer, MFI can vary significantly between batches if the supplier is not diligent about feedstock sorting and processing controls.

When polymers are mechanically recycled — shredded, washed, extruded — the heat and shear of the process can cause some molecular weight reduction, which shifts MFI upward. If the feedstock is mixed, the MFI will be unpredictable. This is why feedstock segregation and per-batch MFI testing are non-negotiable for quality recycled granule suppliers.

How OPAC POLYMERS controls MFI

At OPAC POLYMERS, MFI verification is performed on every batch before dispatch — not on selected samples. Feedstock is sorted by grade and source before processing. The washing and drying line removes moisture — a critical step, since wet material can show a falsely elevated MFI in testing. Our sourcing network — both domestic and international — gives us access to feedstock quality that lets us hit target MFI ranges consistently. When you receive OPAC POLYMERS material, you should not need to adjust your process parameters from batch to batch.