Experimental Studies of Air Entrainment in a Blade Coating System - Part II: Minimization of Air Entrainment, 1998 Coating Conference Proceedings

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The experimental evidence in Part I demonstrates that the formation of large-scale air entrainment can cause coating defects. The objective in Part II is to investigate the optimum operating conditions to minimize the detrimental effects of air entrainment. Due to the existence of the generated vortices in the coating pond [1-4], however, the flow instabilities induced by air entrainment cannot be readily reduced in the conventional pressurized pond coaters.

In this experiment, the principle component is a newly developed prototype of a pressurized coater head where the operation parameters can be optimized to eliminate the generation of vortices. A vortex-free coater head with a flow return chamber not only eliminates the hydrodynamic instabilities but also removes the possibility of entrapment or accumulation of the entrained air bubbles in the pond. This minimizes the adverse air entrainment effects, leading to substantial improvement in coating uniformity. Results to date indicate that with vortex-free coaters operated at sufficient pond pressure is the key to minimizing or eliminating air entrainment. The effects of the backing roll speed, supply flow rate, blade load, overflow gap, and the outlet gap on coating characteristics have been systematically evaluated and reported. Also, the advantages of a vortex-free coater head, particularly with respect to elimination of adverse effects of air entrainment, are presented based on both experimental results and analysis.