Melt mass flow rate (MFR) index is determined with viscosity measurement in molten state and it represents polymer quantity given in grams, that flows from a standard capillary tube with a standard weight at temperature and time conditions in accordance with ISO 1133.
Melt volume flow rate (MVR) index is determined in the same way as for the MFR, but in this case the MVR is expressed as the polymer quantity which is extruded from standard capillary tube that subjects to standard weight (ISO 1133) given in cm3.
MFR and MVR are two different ways of expressing the same material property; this is a characteristic linked to molecular weight in the sense that an increase in molecular weight leads to viscosity increase of polymers and a decrease of the MFR or MVR.
Assuming that other structural characteristics do not submit to significant changes, an increase of MFR-MVR will improve material processability but other properties will decrease their characteristics such as impact resistance.