Rheology MFI MVR Differences

MFI/MVR = g/cc = melt density.

The MFI (or MVR) is just an easy test method to compare polymers. It represent only one point on the viscosity vs shear rate curve of a polymer, without even giving you the viscosity and the shear rate, at a given temperature.

You can have two polymers having a similar MFI but having two completely different viscosity vs shear rate behavior. This is because their viscosity vs shear rate curves cross each other near the shear rate at which the MFI is conducted.

Since there is a large shear rate (and temperature) difference in the different parts of the extrusion process (along the screw, melt pump, die...) you should have the complete viscosity vs shear rate curve of your polymer if you really want to understand what is happening regarding the polymer flow in the extruder.

I suggest that you find a lab in your area that can measure the complete viscosity vs shear rate curve of your polymer with a good rotational rheometer or even a capilary rheometer. Any engineering university with a program on polymer processing or research center on polymer material should have that.

If you want to know more abour rheology, company like TA Instruments (who makes and sells rheometer) give good introduction courses.