A Critical Condition for Overfill-to-Starve Transition of Doctor-Bladed Liquids in Gravure Coatings

Abstract

The present study measured free surface profiles of doctor-bladed fluids on engraved roll surface in order to quantify the volumes of fluid retained in tri-helical grooves. The fluid overfilled the grooves to cover the entire surface of the roll at low roll speeds; whereas, it exhibits a concave surface profile in each groove at high speeds, i.e. a starved configuration with a partial filling of the groove. The overfill-to-starve transition was observed with decreasing the static liquid pressure and the depth-to-width aspect ratio of grooves, or increasing the blade thickness and the fluid viscosity. The dimensionless volumes of residue on the roll obeyed a master curve with respect to a balance between the pressure-driven and viscous drag flows. An empirical equation was newly derived based on the one-dimensional lubrication approximation, which allowed us to predict an optimum condition in stable gravure coating operations.