Melt mass flow rate, or MFR (or indeed melt flow index as it is sometimes referred to), is a one number value which indicates the viscosity of a polymer in the melt phase. The higher the number, the lower the viscosity and correspondingly, the lower the umber the higher the viscosity. The viscosity, or resistance to flow, is also indicative of the weight average molecular weight of a polymer and so this one number gives a rapid indication of two very important variables. This simple test is also widely used as a QC test and many companies accept or reject incoming polymer by comparing the value they measure with that on their certificate of conformity. The test is standardised in ISO1133-1:2011 which stipulates a range of temperature and mass combinations for suggested resins.
