Wipidedia refers to MI wrongly. It says, "Synonyms of Melt Flow Index are Melt Flow Rate and Melt Index. Actually, more commonly used are their abbreviations: MFI, MFR and MI." I believe MI was originally used exclusively for PE. But, I could be wrong.
If you'll read the ASTM D 1238 testing specification you'll be hard pressed to find the term MI used at all. I knew what you meant when you asked your question. And I gave you the best possible answer, under the circumstances. Mainly, the relationship between two different dilute solution vistosities.
As stated before by my colleagues, the PVC resin in it's neat form is compounded w/ various ingredients; fillers, stabilizers, modifiers, and more...each of which alter the flow properties of the blend. Added to this the PVC is heat and shear sensitive. As such, the viscosity characteristics of a PVC compound are changing as the flow measurement is being measured. The material cross-links, the viscosity builds, end result is non flow at all and burning.
As Skip pointed out..."K-value is a rating for PVC resin only, not compound. Since PVC resin alone would be difficult to melt and test via MI, a series of identical formulations, differing only in the K-value or intrinsic viscosity of the resin would be the only way I can think of to directly correlate MI to K value." Even so, care must be taken to insure all testing is parallel/equal in heat histories, ram loading, orifice size, and test timing.
The K value of PVC resins is given in the manufacturer's literature. It is might also be given as IV or RV by some resin manufacturers. These terms can be related to one-another by this chart:
56 / 0.66
57 / 0.68
58 / 0.70
59 / 0.72
Over the short span (above) this scale is linear.
y = 0.02x - 0.46
R^2 = 1
Good luck w/ your investigation.