Coat-hanger dies are commonly used in polymer processing for the production of sheets and films. Both geometrical and material quality of products is governed by the uniformity of flow rate and residence time distribution in the transverse direction at die exit. Up to now, various analytic design equations for coat-hanger dies have been developed under the assumption that the flows in manifold and slot are one-dimensional and have no interactions. The coat-hanger dies to be considered in this study are designed according to the selected one-dimensional design equation that can be applied to manifold of various cross-sectional shape. For the numerical simulation of non-Newtonian fluid flow in the die, three-dimensional and two-dimensional finite element codes are developed respectively along with corresponding models.
In order to determine design conditions under which one-dimensional design is valid, the flows in the coat-hanger dies is simulated using three-dimensional finite element code. In this simulation the fluid property, which is a power-law index of power-law fluid, is set to be the same as assumed in one-dimensional design. Changing slot thickness has negligible effect on flow rate distribution and residence time distribution. In respect of flow rate distribution, there is an optimum manifold angle for each power-law index. Optimum manifold angle increases from 10˚ to 20˚ as the power-law index of fluid decreases from 1.0 to 0.4. Uniformity of residence time distribution is found to improve consistently with increasing manifold angle.
The effect of fluid property is also considered from the simulation of the flows of varying fluid property in the same die. Decreasing power-law index of fluid drives the flow to die center while residence time distribution shows little change with varying fluid property. There are two additional important design parameters, the length of land and the geometry of die inlet, that is not taken into account in one-dimensional analysis. If the ratio of the width of die inlet to the total width of die is small, the ratio is 0.1 in this study, the effect of the inlet geometry on the flow rate and residence time distribution is negligible. In our design, land of uniform thickness is adopted. If any modification is not imposed on the land section, land of uniform thickness does not contribute for improvement of flow rate uniformity.
Finally, two-dimensional simulation, which has been widely used for its relatively lower load of computation, is compared with three-dimensional simulation to investigate the problems embedded in two-dimensional model. There is a significant difference between the two models in flow rate distribution and total pressure drop. Particularly, pressure distribution obtained from the two different models shows that two-dimensional model is inadequate for the analysis of flow in regions of manifold and inlet.
