Two-layer slot coating

Two-layer slot coating is a method of depositing two miscible liquid layers onto a substrate. In slot coating, liquid is pumped through a feed slot in an applicator die onto a substrate translating in its own plane past the die. Two layers of liquid can be deposited onto the substrate simultaneously by employing an applicator die with a pair of feed slots. Doing so, however, is generally more difficult than coating successive single layers in two coating/drying operations in tandem. Coating is successful only when the bead, the liquid contained between the die and substrate, is stable and can be achieved only within a range of operating parameters, e.g. the vacuum pressure, substrate speed, gap, coated thickness and viscosity.; In this research, the predictions of perfectly steady two-dimensional operating states were made by solving the Navier-Stokes system by the Galerkin/finite element method and were accompanied by (1) arc-length parameter continuation of the ratio of upper to lower-layer liquid viscosity; and (2) evaluation of the consequences of ever-present three-dimensional perturbations, by linear stability analysis, and of ongoing small forced disturbances, by frequency response analysis.; The pressure distribution in the liquid must be accommodated by the pressure difference across the bead. It was found that, for the flowrates tested, only over a limited range of viscosity ratio of about 0.27 < m < 2.2 was the bead stable as it tried to locate outside of its range under the upstream die lip. The range of viscosity ratio over which the bead was stable was lowered to m = 0.17 by reducing the pressure at the upstream meniscus. At this limit, an unstable “ribbing” mode was found on the downstream meniscus; an instability noted by Cohen in his flow visualizations. The range of viscosity ratio over which the bead was stable was raised m = 8 by manipulation of the die lip geometry. No unstable mode was found in this range. Heightened and reduced sensitivities were noted with respect to ongoing disturbances of the substrate speed, slot gap and liquid delivery rates.