The mw distribution morphology and to some extent mol wt is also altered hence the MFI changes.. Dr. Khandwe

Every time you process a material you heat it (obviously). This is called the heat history of the material. This heat history affects the molecular weight (MW) of the material, the more it is heated the more of the affect.

Depending upon your application, the 40% reuse of regrind exceeds a manufacturer's specification, which is usually 15%. Therefore, you are introducing 40% of a material with a different MFI than the original virgin.

With this said, MFI comes in a very wide spec; i.e., 10 - 18 for one virgin PC grade. Utilizing regrind properly may still have a combined MFI fall within that spec. The best thing to do in setting-up your process is to determine the proper RV (relative viscosity) of the material for the item being molded. Doing this will establish part of the proper process that when a different lot of material is introduced it will not have a profound affect on the set-up and, therefore, no alteration of the injection stage would be necessary to compensate for varying MFI. This all presumes that you didn't change the MW with excessive heat during processing.

The following is an example that you could encounter while molding PC. You use the manufacturers melt range of the spec to set-up the process. Thinking that PC is an amorphous material you figure that the more heat you give it the higher the MFI becomes. Heating PC from lower range to a certain point will prove this but going beyond this point (still within the spec of the material) could reverse this and now the material starts to become more viscous. This is regardless of whether you are molding 100% virgin or a virgin/regrind mix. Adding heat can change the MW of the material which this will affect the MFI.

Just to reiterate what has already been said: regrind has lower molecular weight (mw)due to heat history, scission, etc. Lower mw = shorter polymer chains = less resistance to flow (ie. lower viscosity).