MFI is one of the key parameters that should be controlled since it is often critical in determining how the polymer will be processed and molded.

Because the melt flow index is inversely proportional to the molecular weight, it also serves as an indicator of the molecular weight. If the melt flow index increases, the softening temperature decreases, consequently causing the hardness to decrease as well. As a general rule, the plastic with a higher yield resistance has a higher molecular weight than that of one with less resistance.

MFI measurement is carried out in accordance with ASTM D1238 and TSE 1323 as follows;

A small amount (approximately 4-5 grams) of polymer sample is taken in the specially designed MFI apparatus. The device mold with an opening of approximately 2mm is inserted into the apparatus. The material is properly packed inside the barrel to avoid the formation of air pockets. A plunger, which acts as the medium causing the extrusion of the molten polymer, is mounted.

The sample is preheated for a given period of time. After preheating, a certain weight is applied to the plunger. The weights set out in the standard are 2.16kg, 5kg, etc. The weight applies force to the molten polymer and the polymer begins to flow from the mold. The polymer is weighed accurately after the desired time. MFI is expressed as grams of polymer for every 10 minutes of testing.

For MFI, the applied temperature and load vary depending on the polymer type.