FLOW-3D has been used by coating researchers in industry and academia for machine design research, process window development, and optimization. Understanding coating physics at the micron scale can be difficult due to the complex nature of coating fluid rheology and its interaction with substrates and dies. FLOW-3D provides high fidelity modeling for understanding the effects of surface tension, wall adhesion, solute transport, density driven flows, and phase change.
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.
For multi-layer curtain coating processes, liquid is ejected from a slot die and then allowed to fall under gravity to the substrate, as shown in the simulation below. 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.
Dip coating is the immersion of a substrate into a tank containing coating material, removing the piece from the tank, and allowing it to drain. Transient coating problems such as this can be solved simply and efficiently using FLOW-3D because the motion of fluid within a stationary mesh is determined (not the motion of a fluid following mesh). This 3D simulation shows a dip coating process with concomitant evaporation. A wet film is deposited by withdrawal of a small discrete substrate from a solution. The model additionally accounts for the evaporation of the solvent. This is relevant in the case of volatile solvent, for which evaporation overlaps with fluid mechanics during film deposition. The residue model provides the unique capability to calculate the profile of the coated dry film. The correct evaluation of the “edge effects” allows engineers to analyze the influence of process parameters or fluid properties on the final thin film geometry and homogeneity. Modeling courtesy of Roche Diagnostics.
A presentation on the dip coating process is available in our 2013 Conference Proceedings, “Model of dip coating with concomitant evaporation,” by Dr. Julien Boeuf of Roche Diagnostics GmbH.
Roll coating processes are common to a range of industries including those dealing with textiles, adhesives, and sealants. FLOW-3D gives process engineers and scientists the ability to assess various material properties and coating regimes to identify sources of defects and optimize roll coating process parameters.
Forward, Reverse and Meniscus Roll Coating
In this example, velocity streamlines are plotted for the forward (top), reverse (middle), and starved (bottom) operating regimes common in roll coating processes. FLOW-3D gives researchers the ability to analyze factors such as roll speeds and material properties and their effect on the stability of the dynamic contact line as well as contributions to defects such as air entrainment, ribbing, and non-uniform edge profiles.
In the forward roll coating simulation shown below, FLOW-3D accurately captures the onset of a ribbing instability as it relates to increased roll speeds, as described in Lee, et al . The model implements one-fluid VOF, surface tension and viscosity to capture the complex nature of such instabilities which can be seen in production.
In the simulation , FLOW-3D captures a cascade defect in a forward roll coating process. Due to increased roll speed of the web roller on the top, the dynamic contact line becomes unstable, allowing air to be entrained into the coating fluid.
Gravure coating transfers fluid from an engraved cylinder, called a gravure roll, onto a moving substrate. The gravure roll is patterned with small wells or cells that have been engraved into its surface. The engraved cylinder rotates through a well of fluid. The thickness of the fluid application to the gravure roll is controlled by a doctor blade. The cup-like shape of each cell captures and holds the fluid in place as the cylinder turns past the doctor blade. The pattern, depth, and shape of the cells determine the weight and appearance of the coating on the substrate. The FLOW-3D simulation shown below looks at the effect of cell depth on deposition. The model compares two cell depths: 30 microns and 53.3 microns. The 30-micron cell depth allows for a much more uniform deposition, which will transfer to the resultant coating.
Slot Die Coating
FLOW-3D is used in industry research and design of slot die coating processes. In slot die coating, fluid is forced out of a slot onto a rapidly moving substrate which is positioned very close to the slot. Sometimes multiple slots are used to create layered coatings of several materials. Many industries employ slot die coating machines because of their relative simplicity. Another benefit of slot die coating is a high rate of coating uniformity, even in coating thicknesses measured in nanometers.
The FLOW-3D simulation shown to the right, courtesy of 3M, shows the fluid residence time inside the internal cavity of a slot die. The design of the slot die is very important to the success of the coating process, and is specific to the rheology of the coating fluid.