a ratio compares the amounts measured in the same unit and uses the conjunction to
There were 51 women to 49 men is a ratio.
Mix four parts gasoline to one part oil.
A rate expresses a faction and often uses the conjuction "per"
51% (ie 51/100) of the group were women
the fuel mix is one fifth oil.
he was getting 50 miles per gallon.
now the original question as you're comparing dimensinally distinct measures it must be a rate.
however melt flow index seems to be an industry term that impplies that it is a rate already so saying
Plastic materials with melt flow index of 10gr/10min will be demanded in bulk by the producers.