All coating processes involve some sort of startup period in which the coating material undergoes large deformations before achieving steady conditions. A good characterization of the startup process is often important for reducing waste and ensuring that the process operates within desired limits. A similar understanding of the transient response of coating flows to a variety of perturbations is also desirable so that a breakdown of the coating bead and non-uniformities in coating can be avoided. FLOW-3D can conveniently be used to simulate these processes in both two and three dimensions.
Multi-layer slide coating is a process by which many layers of differing coating materials are laid upon a web simultaneously. It is commonly used in the making of photographic film. The layers typically each have different fluid properties, though they are most often miscible, so interfacial tension between the layers is small. Of particular interest is the location and stability of the static contact line on the die face and the dynamic contact line where the liquid first meets the moving web. This position is affected by the rate of liquid flow, the web speed and the amount of vacuum in the air space upstream of the dynamic contact. In the three-layer example shown, the vacuum is 1.5 mbar. Also of importance is the maintenance of sharp interlayers between each of the fluids. FLOW-3D is able to simulate the transient behavior of the process during startup because it is a fully transient, three-dimensional flow model.