Having the Melt Flow Index of a material does not provide a real picture of how the polymer will actually process, but it can be a helpful indicator in some cases. This post will outline an example of how the data can be of use to a technician.
Question: MFI is not supposed to be an accurate measurement of viscosity, so should we stop testing it?
My Response: The melt flow index is a low pressure & low shear rate test with a single data-point as a result ( grams extrudate per 10 minutes). Injection molding is a very dynamic process which typically involves very high shear rates and shear stresses. The only real value MFI data provides is for relative use in material comparison for material selection.
Example: Let’s suppose you are making good parts with one lot of material which has a MFI of 40 g/10min. If you are running out of material and have to change lots and you can choose between one grade with a 45 MFI and another which has a 25 MFI. You might choose the 45 MFI material because it has the highest likelihood of molding similar to the 40 MFI lot.