Meaning
Temporal and thermal variation alters the measured extrusion rate of a molten polymer through a standardized capillary die under specified conditions of load and temperature. The occurrence of melt flow index drift reflects underlying molecular weight alterations resulting from polymer chain cleavage or radical crosslinking during compounding, storage, or moulding. This metric governs process consistency, moldability windows, and dimensional stability in injection, blow moulding, and film extrusion operations.
It ceases to apply to thermosetting resins undergoing irreversible chemical cure or to raw elastomers that do not display measurable thermoplastic capillary flow.
Molecular Mechanism
Processing shear and elevated barrel temperatures trigger mechanical and thermal degradation of macromolecular chains. In polypropylene, chain scission dominates, breaking backbone chains to form shorter polymers that push melt flow values upward. Polyethylene grades, conversely, frequently undergo radical crosslinking reactions that build high-molecular-weight branched structures, causing the melt flow index to decline over consecutive passes.
Uncontrolled hydrolytic degradation in polyesters and polyamides drops molecular mass dramatically within seconds, producing surging melt flow rates. Additive consumption, particularly of primary and secondary processing stabilizers, accelerates these kinetic shifts once protective stabilizer reserves run out.
Tooling Instability
Fluctuations in flow behaviour disrupt the delicate fluid dynamics required to fill multi-cavity injection tools evenly. An unexpected increase in melt flow index causes flash at tool parting lines, core pin deflection, and overpacking near gate regions. A downward drift results in short shots, cold welds, incomplete surface replication, and high injection pressure alarms.
Viscosity changes shift the screw recovery time and the transition from velocity-controlled filling to pressure-controlled packing, introducing part-to-part weight variability. Extrusion operations suffer melt fracture, surging head pressure, and severe wall thickness variations across downstream calibration tanks.
Quality Verification
Receiving protocols verify incoming resin lots against procurement specifications using standard capillary plastometers under ISO 1133 or ASTM D1238 test regimes. Datasheets state nominal melt flow numbers with strict allowable tolerances, typically plus or minus ten to twenty percent. Processing high fractions of regrind widens this variance, as accumulated thermal history progressively shifts the baseline value.
Moulders run routine purge sampling during continuous production to ensure regrind additions do not push resin parameters outside qualified processing windows. Stable production demands that melt flow measurements remain locked to guarantee uniform cooling, consistent shrinkage, and predictable mechanical durability.
