The melt flow index (MFI) is a measure of the ease of flow of the melt of a thermoplastic polymer. It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied via prescribed alternative gravimetric weights for alternative prescribed temperatures.Polymer processors usually correlate the value of MFI with the polymer grade that they have to choose for different processes, and most often this value is not accompanied by the units, because it is taken for granted to be g/10min. Similarly, the test load conditions of MFI measurement is normally expressed in kilograms rather than any other units. The method is described in the similar standards ASTM D1238 and ISO 1133.

   

Originally called Melt Flow Index or Melt Index (typically for polyethylene, but applied to a variety of materials), the standard designation today is Melt Mass-Flow Rate or MFR, which is a mass flow expressed in g/10min. An alternative quantity is the volume flow expressed in cm3/10min, called Melt Volume-Flow Rate or MVR. MVR multiplied by the melt density (i.e. density of the material in the melted state) gives MFR.

Melt flow rate is very commonly used for polyolefins, polyethylene being measured at 190 °C and polypropylene at 230 °C. The plastics engineer should choose a material with a melt index high enough that the molten polymer can be easily formed into the article intended, but low enough that the mechanical strength of the final article will be sufficient for its use.

Melt Flow Index is a very valuable test for quality assurance. 

• Raw material certification
   
• Comparison of "good" and "bad" samples
   
• Batch to batch comparisons
   
• Grade differentiation of materials
   
• Determination of the nature of material degradation (processing, transportation, storage, and drying conditions)
   

Refer the product: Melt Flow Index Tester