Melt flow index
The melt flow index refers to the amount of thermoplastic material extruded in a given time under specific conditions.
This index only applies, of course, to thermoplastics.
When a thermoplastic (semi-crystalline) melts, the crystalline part passes from a solid state to a liquid state during the fourth transition, and its melt flow index will therefore depend on its degree of crystallinity.
The greater the crystallinity of a thermoplastic, the easier it will be to mould (this is the case for polypropylene, for example). On the other hand, PEEK remains viscous because its crystallinity is 30% with 70% of the structure being amorphous.