Slot coating flows: Feasibility, quality

In slot coating, liquid emerges from a feed slot into the coating bead, the liquid bridge in the gap between the slot die-lips, web, and upstream and downstream menisci. The web passes through the bead and leaves it with a liquid layer on its surface. A slight vacuum applied at the upstream meniscus produces a pressure drop across the bead that helps to offset forces that would otherwise destroy it. The gas beyond the downstream meniscus is usually ambient air.;Viscous free surface flows in slot coating were analyzed by theory and experiment. With a viscocapillary model based on lubrication theory coupled with Landau-Levich film flow approximation, the effect of gap shape on coating windows and the transient response of the flow to ongoing disturbances were analyzed. With Navier-Stokes theory, solved by Galerkin's method with finite element basis functions, the evolution of steady states as operating and design parameters change was analyzed by first order continuation. Turning points in the solution paths revealed operating limits that confine the range of parameters within which the slot coating bead is sustained. Outside of these limits, the slot coating bead cannot be maintained. The effect of lip shape on stability and frequency response of the two-dimensional flow was evaluated.;Flow visualization, with a precision transparent glass roll, revealed that the slot coating flow is beautiful and rich with structure. Recirculations were rampant throughout the bead when coating at high speed and low flow rate. The experiments revealed that ribbing is linked to air fingers that form at the upstream or downstream meniscus. With a new slot die apparatus equipped with interchangeable lip shapes, the effect of lip shape on the vacuum versus speed and flow rate versus speed planes of the coating window were found. The results agree well with the theoretical predictions. The experiments also revealed hysteresis of operability limits.;Altogether, the theoretical and experimental results show how lip shape alters the slot coating flow and its stability and sensitivity of the final layer thickness to ongoing operating disturbances.