The melt flow index is a standardized test method to determine the flow rate of plastics at a defined temperature and constant force. The test method is standardized in the common test standards ISO 1133 and ASTM D 1238. A thermoplast material is melted in a barrel. The application of a weight causes a constant pressure on the polymer melt. At the lower end of the test barrel is a standardized capillary through which the plastic melt is extruded.
Measured is the mass or melt volume rate (MVR), which flows out of the barrel in a defined time unit and is given in cm3 / 10min.
When the melt density is known, the melt flow rate (MFR) can be determined from the melt volume rate (MVR).
A weighing of the extruded strand is not necessary in this measurement method.
The determination of the melt index is very often used in quality control of plastics.
Determination of the MFR by Method A
When determining the MFR according to method A, the extruded strand of material is cut off at constant time intervals. Using a laboratory scale, the strand sections are weighed and the resulting mass per unit time in g/10min. is specified.
Determination of MVR according to Method B
When determining the MVR according to Method B, the extruded volume of the polymer melt is determined and in cm3/10min. specified. The measured melt density allows conversion of the MVR measured value into the MFR measured value.