Melt flow index (MFI) is a measure of how easily the melt of a thermoplastic polymer flows. More specifically, it is the mass of polymer (in grams) that flows in 60 seconds through a tube of specified diameter and length by application of prescribed pressures at prescribed temperatures. The specific method for this testing is described in ASTM standard D1238 and ISO standard 1133. Melt flow rate is inversely proportional to viscosity of the melt under the conditions of the test. Ratios between two melt flow rate values for one material at different pressures are often used as a measure of the broadness of the polymer's molecular weight distribution. Melt flow rate is very commonly used for polyolefins, with polyethylene being measured at 190°C and polypropylene at 230°C.
Something to consider in selecting the proper material grade is to choose one with a melt flow index high enough to easily form the part, yet low enough to provide sufficient mechanical strength for the part's intended use.