Melt Flow Index - Measurement

Measurement

The procedure for determining MFI is as follows:

1. A small amount of the polymer sample (around 4 to 5 grams) is taken in the specially designed MFI apparatus which is nothing but a miniature extruder. The apparatus consists of a small die inserted into the extruder, with the diameter of the die generally being around 2 mm.
2. The material is packed properly inside the extruder barrel to avoid formation of air pockets.
3. A piston is introduced which acts as the medium that causes extrusion of the molten polymer.
4. The sample is preheated for a specified amount of time: 5 min at 190°C for polyethylene and 6 min at 230°C for polypropylene.
5. After the preheating a specified weight is introduced onto the piston. Examples of standard weights are 2.16 kg, 5 kg, etc.
6. The weight exerts a force on the molten polymer and it immediately starts flowing through the die.
7. A sample of the melt is taken after desired period of time and is weighed accurately.
8. MFI is expressed as grams of polymer/10 minutes of flow time.

Synonyms of Melt Flow Index are Melt Flow Rate and Melt Index. More commonly used are their abbreviations: MFI, MFR and MI.

Confusingly, MFR may also indicate "melt flow ratio", the ratio between two melt flow rates at different gravimetric weights. More accurately, this should be reported as FRR (flow rate ratio), or simply flow ratio. FRR is commonly used as an indication of the way in which rheological behavior is influenced by the molecular mass distribution of the material.