Experimental and theoretical study of the manifold flow in a curtain coater

Abstract

Curtain coating is a contactless process with the potential for increased quality and productivity for coaters. Nevertheless, this demanding process requires a perfectly stable curtain to obtain good coverage. Problems can originate from the internal flow when the fluid goes through cavities and slots before reaching the inclined plane. Three-dimensional computational fluid dynamics simulations were performed in the manifold of a laboratory curtain coater to analyze the phenomena taking place in the first cavity, determine the causes of defects, and propose potential solutions. Reynolds number is a relevant parameter for Newtonian and non-Newtonian fluids and increasing it leads to disturbances in the manifold. The power law index also significantly affects flow uniformity, because its decrease leads to perturbations. Finally, yield shear stress has no effect on vortex formation for coating colors. To maintain vortex-free operation, the Reynolds number at the inlet must remain below a critical value (equal to 20 with the studied geometry), whatever the fluid. Geometrical changes were simulated with a higher radius of the inlet pipe or an end-fed manifold, resulting in improved flow uniformity. Simulation results were validated using flow visualization experiments with tracers using a transparent replica of the coater.