Coatable splice process in film coating: A fluid mechanical stability problem



In slide coating of photographic films, liquid layers flow down an inclined plane, across a small gap under a pressure differential (vacuum), and onto a fast-moving substrate. Process interruptions to allow splices of adjoining rolls of substrate to pass through the coater result in significant waste. Coating over splices fails due to air entrainment and/or two free-surface flow instabilities during the necessary gap widening: (1) standing waves (ribbing) or rupture of the liquid bridge when vacuum is kept high or (2) lateral contraction when vacuum is lowered. The problem of coating over splices is solved here by reducing it to a fluid-mechanical stability problem. First, air entrainment was eliminated by serrating the splice trailing edge and lateral contraction by prewetting the substrate with liquid streams. Avoiding rupture of the liquid bridge then reduces the problem to finding the vacuum at which the bridge becomes unstable to small disturbances. This “critical” vacuum is found by finite-element stability analysis of the quasi-steady 2-D free-surface flow to small 2-D and 3-D disturbances, direct tracking of critical points, and verification by solving the transient 2-D skip-out process. Three modes of failure are predicted if the vacuum is not sufficiently reduced with gap widening. Implementation of a computed vacuum strategy in a production line gave sufficient latitude for coating over imprecise splices.