MFI is short for melt flow index. It expresses, as a mass, how much molten plastic flows through a standard orifice in a given time at a given temperature and under a given load. The higher the value, the more easily the melt flows; the lower it is, the heavier and more viscous the melt behaves.
In film production, MFI is one of the key criteria in choosing a raw material. The polyethylene grades chosen for blown film have different flow properties from those used in injection moulding. The flow value affects how the extruder behaves, the stability of the bubble and the mechanical properties of the film. The right value is determined by the raw material producer’s technical data sheet and the film to be produced; there is no single correct value.