Capillary rheometers are used to determine shear and extensional
properties of the fluid under different shear rates and
temperatures which usually occur during HME. The pressure drop
at the die entry region which is required for the calculation
of extensional viscosity and corrected shear viscosity may be
obtained by twomethods viz. Bagleymethod and Cogswellmethod.
In the Bagley method, pressure drop across dies with different
length/diameter (L/D) ratio are determined and then pressure drop
is extrapolated to calculate pressure drop across zero length die.
Extrapolation errors and nonlinearity of data are limitations of this
method. In Cogswell’s method, a die of the same diameter as the
long die but effectively zero length (L/D ratio