Simulating the Impact of Nozzle Hole Locations and Diameters on Film Thickness in Spinning Process

In the film spinning process, pressure applied on the cellulose solution in an extruder forces the polymer through a narrow slit. Subsequently, a rotating chill roll stretches the emerging film in an air gap. As a desired result, the film becomes thinner, but as an undesired side effect the width of the film also decreases. Furthermore, the thickness variation in the lateral direction is another perturbation. To improve the control over the film properties, we experimented with two distribution plates having different locations and sizes of the holes through which the dope entered the nozzle. As a support for the experimental study, we also conduct a finite element simulation for both the viscous flow of the polymer through the nozzle and the mechanical balance in the film passing through the air gap. For the flow in the nozzle, we apply the cross model for a description of the dependence of the viscosity on the shear rate, employing experimentally obtained values for the parameters in the model. The connection between the flow in the nozzle and the sheet stretching in the air gap is the velocity profile in the slit.

Because the thickness dimension of the film is much smaller than the film width and stretching length, we can approximate the situation in the air gap with a 2D membrane model, where the velocity and stress assume constant values in the thickness direction. In addition, we model the convective and radiative heat transfer in the air gap. We carry out the simulation in the COMSOL Multiphysics software comparing the simulated thickness profiles generated by the two different plates with each other as well as with the experimentally obtained measurements.