The PTT and Giesekus models recommended for extrusion are also recommended for fiber spinning. The Oldroyd-B and Maxwell models are also good choices, especially for highly strain-hardening materials. Note that the White-Metzner model is not well-suited for fiber spinning because it is not able to properly model the elongational behavior of polymer melts. If qualitative information on the macromolecular behavior is required, it can also be interesting to consider using the DCPP model.
Both single- and multi-mode models are acceptable for a 2D model, but a single-mode model is strongly recommended for a 3D model. For a 2D model, three modes are recommended, with the relaxation times no more than one decade apart.
The flow involves a typical shear rate 
in the die and a typical elongation rate 
in the fiber itself. Two or more orders of magnitude may exist
between these quantities. It is worth mentioning that the actual deformation in
shear evolves linearly with the shear rate, while the actual deformation in
elongation evolves exponentially with the elongation rate. Hence, although numerical
values for the typical shear rate in the die can be larger than the numerical values
of the elongation rate in the fiber, the actual corresponding deformation will be
significantly larger in the fiber. Consequently, you should focus on the elongation
character more carefully.
For a single-mode model, select a relaxation time of about 
. For a multi-mode model, select one relaxation time <
and one > 
, with no more than one decade between relaxation times.
For a strongly strain-hardening material (for example, LDPE), use a low strain rate with the Maxwell or Oldroyd-B model, a low value of ε (typically 10-3 to 10-2) with the PTT model, or a low value of α with the Giesekus model (typically 10-3 to 10-2). For strain-thinning or moderate strain-hardening materials (for example, LLDPE or HDPE), use the PTT or Giesekus model with a higher value—typically about 0.1 or more—for ε or α. Also, for strain hardening materials, the DCPP model can be used with a large enough value of q (number of arms). Currently, the Leonov model and the “simplified viscoelastic model" are not recommended for fiber spinning simulations.
If data on elongational viscosity are available, they should be used. If the resulting values for the nonlinear parameters are not in agreement with the expected behavior of the melt, they can be fixed as noted in Assigning a Value to a Parameter.
In the automatic fitting procedure, it is preferable to consider the data in the range of angular frequencies and shear rates of interest, typically one decade above and one below. If data extrapolation is necessary, it should be done over no more than one decade. Also, use appropriate weighting factors (see Weighting Measured Data) if some data are more reliable than others.
The shear viscosity curve for the model may differ from measurements, but this can generally be disregarded, since elongation is the main component of the flow.