Flow Behavior of Herschel-Bulkley Fluid in a Slot Die

To achieve uniform thickness of coated films using non-Newtonian fluids, we are often required well-designed geometries of flow channel in a slot die. This is the case especially for fluids with the properties of both shear thinning and yield stress which is non-negligible in a range of low shear rate such as in an intermittent die coating. In the present work, flow in the slot die has been studied using Herschel-Bulkley fluid expressing both shear thinning and yield stress. The Herschel-Bulkley fluid includes power-law and Bingham fluid models comprehensively. This fluid model is applicable to many real fluids. We derive simplified equations for the outflow distribution as functions of power-law index, yield stress and the geometry parameters of the die. Furthermore, we propose a useful designing method for predicting the optimum geometry so as to guarantee uniformity of outflow from the die slot, based on the HerschelBulkley fluid. The value of this method is confirmed by comparing it with experimental results and with previous theoretical results for the power-law and Bingham fluid models.