I received this question last week…
I am trying to find out why the injection pressure of PP is lower than the injection pressure of PC in terms of its MFI. But the MFI for PC is at a different temperature and for PP is at a different temperature.
Essentially, Melt Flow Index (MFI) is not an objective test. MFI uses different specifications for testing different materials. The pressures and shear rates are different than those encountered in production, and the temperatures are typically not the same either.
The benefit to this test is that it is easy for anyone to conduct, and it provides a single number result.
The most effective use of the MFI test, I have encountered, is to compare different batches of the same material grade for material scheduling.
Objective testing such as capillary rheometry or spiral flow are much more involved, but can provide much more comprehensive results.
I discuss this in more detail in these posts: