Melt Viscosity and Apparent Melt Viscosity - ASTM D3835
The ratio of shear stress to shear rate at a given shear stress or shear rate.
The melt viscosity of a material exhibiting non-Newtonian behavior (ratio of shear stress to shear rate is not constant at a specified temperature and pressure). Apparent viscosity values can be corrected for comparison with melt viscosity values.
Testing is performed using a capillary rheometer. A plastic charge is heated in the rheometer barrel and is forced through a die with a plunger. The plunger is pushed either by a constant force or at constant rate depending on the equipment. Measurements are taken once the system has reached steady-state operation.
Melt viscosity is sensitive to molecular weight, melt stability, fillers, and additives.