Investigation of the Exit Flow From a Beveled Blade Coater with Implications for Weeping, 2000 Coating Conference Proceedings

M. A. Johnson, D. W. Bousfield--In the blade coating of paper webs, blade deposits known as "weeps", often limit the speed of operation, lead to visual defects in the coated sheet, and are linked to problems during subsequent calendering and printing. In this paper, the flow field at the exit of the blade is analyzed with computational fluid dynamics. An equation is proposed to predict the sub-ambient fluid pressure at the downstream tip of the blade. Pressure drops at the blade tip are calculated for a variety of coating formulations based on the high-shear viscosity, critical machine speed for the onset of visible weeping, coat weight, and an estimate of the coating liquid surface tension. The results for two sets of experimental data are compared. The calculated pressures may be sufficient to cause the formation, growth, and collapse of vapor or entrapped air bubbles in the exit flow region, especially if these pressures are combined with a local diverging channel, filter cake formation, or roughness of the web and blade. The equation proposed within this paper may be helpful to predict blade coater operational limits. In addition, a new understanding of one of the mechanisms that may lead to weeping is proposed.