In curtain coating, liquid flows from either a slot or slide die and is allowed to fall under gravity onto a horizontally-moving substrate. Curtain coating can involve a single layer of fluid or multiple fluid layers, and is used in the production of photographic film, specialty paper and packaging.
The physical properties of the liquid and the speed of the substrate with respect to the speed of the flow from the slot determines the thickness of the coating, as well as the stability at the contact line, where the liquid first makes contact with the substrate. An unstable contact line can result in either puddling or air entrainment under the coating which can produce a non-uniform coating thickness and other defects. FLOW-3D can be used to investigate the stability of a curtain coating by simulating the process with different process parameters such as flow rate, flow width, and substrate speed, as well as changes to the liquid’s physical properties like viscosity, surface tension, and adhesion force.
The fluid layers typically each have distinct 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 surface. This position is affected by the rate of liquid flow, the speed of the moving substrate, and the amount of vacuum in the air space upstream of the dynamic contact. Also of importance is the maintenance of sharp interlayers between each of the fluids. FLOW-3D is a fully transient, three-dimensional flow model, and so is able to simulate the transient behavior of the process during startup.
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 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.