Abstract
The mechanism on how a liquid bridge between a moving roller and a scaled-up gravure cell breaks during the coating process was studied experimentally. A three-dimensional flow visualization technique was employed to observe the fluid motion from two viewing angles with two CCD cameras. It was found that the liquid bridge goes through a three-stage movement before it breaks up. The first two stages are two-dimensional in nature and occur at a very short time. At the third stage, the liquid bridge advances to the coating gap out of the gravure cell and the three-dimensional fluid motion was observed. The mechanism was influenced by capillary number, coating gap and the geometry of the gravure cell. Values of pickout were also found to depend on these parameters. 2D simulation on the emptying process based on a commercial software package FLOW3D was also discussed. A bead vacuum system was added to the tensioned web slot coating operation (TWSC). Flow visualization was applied to monitor the coating flow behavior. It was found that coating defect would disappear under vacuum and the whole process from air entrainment to good film was observed. The coating window found to depend on several operating parameters, such as liquid viscosity, surface tension and substrate tension. The most critical parameter is the liquid viscosity. The effect of vacuum would increase with liquid surface tension and would decrease with substrate tension. The study was carried out both experimentally and theoretically; the lubrication approximation and membrane approximation were applied to describe the force balance between the coating bead and the substrate, the force balance can predict the coating gap, which is difficult to measure experimentally. Landau-Levich equation was applied to predict the thickness of wet film. The wet thickness which predicted by theory were smaller then the experiment findings, but the agreement is qualitatively satisfactory.