Technical guide by Plasticalia Chemical
Melt flow index (MFI): what it is and how to choose a grade
The melt flow index is the first figure to look at when choosing a polyethylene or a polypropylene. We explain what it measures, how to read it and what value you need for each process.
What the melt flow index is
The melt flow index (Melt Flow Index, MFI, or Melt Flow Rate, MFR) measures how many grams of molten plastic flow in 10 minutes through a standard die, at a given temperature and under a given weight. It is expressed in g/10 min and measured according to ISO 1133 or ASTM D1238.
In practice, it is a measure of how easily the molten plastic flows: the higher the number, the more fluid the material is and the more easily it fills a mould; the lower the number, the more viscous it is, with longer chains and, usually, greater mechanical strength.
Conditions matter: always compare like with like
A melt flow index can only be compared with another one measured under the same conditions:
- Polypropylene: 230 °C and 2.16 kg.
- Polyethylene (HDPE, LDPE, LLDPE, MDPE): 190 °C and 2.16 kg. For very viscous grades (blow moulding, high-density film) it is also given at 5 kg or 21.6 kg, because at 2.16 kg they barely flow.
- Polystyrene: 200 °C and 5 kg. It is sometimes given by volume (MVR, in cm³/10 min) instead of by weight.
An HDPE with 0.3 g/10 min (2.16 kg) and another with 10 g/10 min (21.6 kg) may be similar materials: do not compare them without checking the load.
What melt flow index you need for each process
| Process | Polypropylene (230 °C / 2.16 kg) | Polyethylene (190 °C / 2.16 kg) |
|---|---|---|
| Blow moulding of bottles and jerrycans | 1.5–3 | 0.1–0.5 (HDPE) |
| Blown film | — | 0.3–2 (LDPE, LLDPE) |
| Raffia, fibre and ropes | 2–5 | 0.5–1 (HDPE monofilament) |
| Sheet and thermoforming | 2–4 | — |
| General injection moulding | 8–25 | 4–20 (HDPE) |
| Thin-wall, fast-cycle injection moulding | 35–80 | 20–60 |
| Nonwovens (spunbond) | 25–40 | — |
Indicative ranges. Every machine and every part has its own particularities: talk to us before changing grade.
What each end of the scale gains and loses
- High flow: fills complex and thin-wall moulds, allows shorter cycles and lower temperatures. In return, the part is usually less resistant to impact and cracking.
- Low flow: greater mechanical strength, better melt strength (essential in blow moulding and film) and more toughness. In return, it needs more pressure and temperature.
A practical tip
The melt flow index that appears in the name of some grades is only indicative: the real value is the one on the technical data sheet and on each batch's certificate. If your process is sensitive, always ask for the certificate of analysis.
In our catalogue you will find the melt flow index of each grade on its data sheet. If you are not sure which one to choose, tell us about your process and your part and we will recommend the most suitable one.