ISO 11443:2021 specifies methods for determining the fluidity of plastics melts subjected to shear stresses at rates and temperatures approximating to those arising in plastics processing. Testing plastics melts in accordance with these methods is of great importance since the fluidity of plastics melts is generally not dependent solely on temperature, but also on other parameters; in particular shear rate and shear stress.

The methods described in this document are useful for determining melt viscosities from 10 Pa∙s to 10⁷ Pa∙s, depending on the measurement range of the pressure and/or force transducer and the mechanical and physical characteristics of the rheometer. The shear rates occurring in extrusion rheometers range from 1 s⁻¹ to 10⁶ s⁻¹.

Elongational effects at the die entrance cause extrudate swelling at the die exit. Methods for assessing extrudate swelling have also been included.

The rheological techniques described are not limited to the characterization of wall-adhering thermoplastics melts only; for example, thermoplastics exhibiting "slip" effects^[1][2] and thermosetting plastics can be included. However, the methods used for determining the shear rate and shear viscosity are invalid for materials which are not wall-adhering. Nevertheless, this document can be used to characterize the rheological behaviour of such fluids for a given geometry.