The melt flow index, or MFI, also called melt flow rate (MFR), measures how easily a molten plastic flows. It is the weight of polymer, in grams, that flows through a standard die in ten minutes under a set temperature and load, tested to ISO 1133 or ASTM D1238.
A low MFI means a thick, viscous melt with high melt strength, suited to blow moulding, pipe and blown film. A high MFI means an easy-flowing melt suited to injection moulding. Polyethylene is normally tested at 190 degrees Celsius and polypropylene at 230 degrees Celsius, with a 2.16 kg load. For recycled granules, a stable MFI from batch to batch is as important as the value itself. It is the first figure converters look for on a technical data sheet.
