Fibers are treated as Newtonian fluids. In elongational flow of Newtonian fluids, the elongational viscosity (or sometimes called Trouton viscosity) is related to the zero shear viscosity by a factor of 3. Because this approach is applied to the computation of melt and dry spun fibers, only the zero shear viscosity is described.
In melt spinning, the fiber is considered to be liquid until its temperature
falls below the solidification temperature 
. For the liquid state an exponential approach is used, see
Equation 22–35.
 | (22–35) |
where 
, 
, 
, and 
are user-specified constants.
Below this temperature the value given in the material dialog box for the
fiber polymer material is used. Typically, a high value like 
is used for the fiber viscosity to simulate a solid fiber.
This value may depend on your polymer and the range of viscosity values in your
simulation. You can use every profile available in the materials dialog box
except UDF’s to describe temperature dependency of the viscosity of the
solidified fiber.
The fiber model uses a blending interval for the temperature 
to provide a smooth transition of the viscosity between liquid
and solid state of the fiber. The viscosity in this blending interval is
computed as
 | (22–36) |
Important: The chosen values of the blending interval may influence the results. Values for the blending interval should be adapted to the rheological data of the polymer.
In dry spinning, the following approach is used to consider the effect of solvent on the zero shear viscosity:
 | (22–37) |
In this equation, 
is considered to be the degree of polymerization as it is used
by Ohzawa [487]. 
, 
, 
, 
, 
, and 
are user-specified constants.