Melt flow index: the figure that sends a material to injection or to blow moulding
On the data sheet of a plastic pellet, one line matters more than the others to the workshop: the melt flow index. Two polyethylenes with the same name can have very different indices, and serve neither the same parts nor the same machines.
By Tecmom editorial team · 2 min read
A simple measurement
The melt flow index, or MFI, is measured with a very simple instrument. The material is heated in a barrel, pushed by a piston carrying a standard weight, and what comes out of a small die in ten minutes is weighed. The result is expressed in grams per 10 minutes.
The conditions are set by ISO 1133: for example 190 °C under 2.16 kg for polyethylene, 230 °C under 2.16 kg for polypropylene. An index only means something together with its test conditions.
Runny or viscous, short or long
A high index means a fluid material that flows easily. It is generally made of shorter molecular chains. A low index means a viscous material with longer chains, harder to make flow but stronger in the molten state.
The index is therefore a shortcut: it gives an indirect idea of chain length, and through it of properties such as impact or cracking resistance.
Injection or blow moulding
In injection moulding, the material must quickly fill thin walls and fine details, such as the thread of a cap: fairly fluid grades are chosen. In extrusion blow moulding, it is the opposite: the tube of hot material must hang under the die without stretching under its own weight before the mould closes. It needs good melt strength, so a low index.
That is why the same supplier offers one high-density polyethylene for injection and another for blow moulding. See our article on extrusion and injection blow moulding.
The limits of a single figure
The index is measured at a low flow rate, far from the speeds inside an injection press. Two materials with the same index can behave differently in the mould. It is used to sort and check lots, not to predict everything.
PET, for its part, is characterised instead by its intrinsic viscosity, expressed in decilitres per gram; bottle grades sit at around 0.7 to 0.85 dl/g. See our article on PET.
The melt flow index does not say everything about a material. It says the essential thing to start with: which machine it is meant for.
Glossary
- Melt flow index
- The mass of molten material flowing in ten minutes under standard conditions, in g/10 min.
- Viscosity
- A fluid's resistance to flow.
- Grade
- A variant of a material, suited to a process or use.
- Intrinsic viscosity
- A measure of PET chain length, in dl/g.