MFI – Melt Flow Index Measurement
What is the melt flow index?
The melt flow index (MFI) of a thermoplastic is a quantity that quantifies the quantity of material that will be extracted from an extruder at a given temperature and under a given applied load.
How is the melt index measured?
Melt index is determined by MFI analysis with a plastometer, by measuring the amount of melt that flows through a weight extrusion plastomer. It depends on the load (force) applied to the fade, but also on the temperature.
This method is quite simple to implement and is often used for quality control of incoming raw materials or production batches.
Mass or volume determination?
The melt index is calculated by mass (MFR) or by volume (MVR). To measure the MFR, weighings of extruded material leaving the die are carried out at regular time intervals.
To measure the MVR, it is the measurement of the displacement speed of the piston which is used to quantify the melt index.
The MFR and the MVR are related by the density of the material under the temperature conditions of the measurement.
Limits of the MFI
Melt index meters often use low shear conditions. However, the hot fluidity of polymers often depends on stresses and flow speeds which can vary greatly depending on the industrial processes used. The fluidity index is still a very good indicator and can also be used in a comparative approach.