Meaning
Melt flow rate defines the mass of a thermoplastic polymer passing through a standard capillary under a fixed load and temperature over ten minutes. Thermoplastic processors use melt flow rate to gauge polymer chain length distribution and predict resin behavior during extrusion or injection. Lower melt flow rate figures indicate higher molecular weights alongside elevated melt viscosities.
High density polyethylene extrusion grades typically maintain melt flow rate values below one gram per ten minutes to ensure parison sag resistance during blow molding.
Melt Viscosity
Molecular weight dictates melt viscosity because longer polymer chains entangle more intensely under shear stress. Melt flow rate tests apply low shear rates that fail to replicate the high shear conditions encountered inside injection molding nozzles. Shear thinning behavior causes polymer melts to flow more readily as injection speed increases.
Resins sharing identical melt flow rate numbers can exhibit different processing behaviors if molecular weight distributions diverge.
Thermal Degradation
Processing temperatures accelerate polymer chain scission when residence times inside the barrel exceed recommended limits. Excessive barrel heat lowers molecular weights and shifts melt flow rate upward during production runs. Moisture contamination triggers hydrolytic degradation in condensation polymers like polyamides and polyesters.
Degraded resin reduces mechanical properties in molded components and increases scrap rates significantly.
Resin Economy
Virgin polymer prices depend on melt flow rate consistency because suppliers guarantee narrow specification windows for demanding applications. Regrind incorporation alters melt flow rate values due to previous thermal history and mechanical shear exposure. Compounders blend high melt flow rate scrap with virgin pellets to balance processability against final part impact strength.
Continuous melt flow rate monitoring prevents costly downtime caused by unexpected viscosity shifts on the shop floor.
