Panel coating is a unique application of fluid coating technology that provides capability and functionality to a discrete substrate. But what is important to understand and what tradeoffs exist?
Unlike continuous coating application against a moving substrate, we are trying to develop steady state control over an intermittent process. The fluid needs to exit the slot die only when we want it to and stop flowing before a mess is made. In intermittent coating there are a number of variables to consider.
Like continuous coating, the math and physics involved to control flow and predict behavior for two dimensional laminar flow is the same. The trick is to develop quality coating immediately and stop the flow perfectly. So this head and tail development is more critical in rigid panel coating than it is in continuous coating.
Many fluids can be coated onto rigid panels, so what are the key factors to be aware of in the fluid chemistry, surface energy and rheology to predict quality performance? One key factor is viscoelasticity of the fluid to be coated. In addition to the viscosity, elasticity can cause die swell, edge bead or ribbing. The external effects of fluid flow and viscoelasticity typically require some trial and error because predicting flow outside the slot die has fewer boundary conditions for simulation. Owning or having access to research and development level panel coaters are very helpful with new fluids or substrates.
There are two main techniques for coating rigid panels – proximity and curtain coating. Both utilize slot dies, but each has a different set of criteria to develop successful performance.
In proximity coating, the gap maintained between the slot die lip face and the rigid panel is 1-2 times the wet coating thickness. So, the lower the wet coat weight, the closer the slot die gets to the substrate. For rigid panels that have variation in surface caliper, this small gap can lead to variation in coating or damage to the substrate.
For very light coat weights (less than 1 mil or 25.4 microns), curtain coating should be considered. For curtain coating, the critical calculation of success is the Weber number. No heavy math is required, just a simple understanding of the fluid and the equipment setup.
We = ρQV/σ
We = Weber number
ρ = density
Q = volumetric flow rate
V = impingement velocity
σ = surface tension
This simple equation helps us determine the success of coating based off a couple of easy to find numbers from the fluid and process. Success is typically predicted when the Weber number is greater than 2. One thing that you will notice with this equation is that the volumetric flow rate needs to be high and therefore the line speed needs to be high for low coat weights. This may limit the equipment capability if the coat weight is low because of the acceleration required to enter the coating curtain.
One of the big differences between proximity and curtain coating is the area coated. In proximity coating there is typically a dry border left on the substrate. With curtain coating, there will be overcoat that needs to be dealt with.
How about the equipment utilized? First of all, the slot die is going to be positioned vertically. Why does this matter? Air. Evil air. When the fluid initially fills the manifold of the slot die, the air in the system wants to rise to the highest point. For a vertically oriented slot die, this means that the air will travel to the back of the manifold or the joint of attachment for the fluid delivery pipe or tube. If the air has no release mechanism, then the air will slowly release over time, causing coating defects. Ideally, a purge valve will be placed either in the back line of the manifold or the fluid delivery tube, whatever the highest point in the system is. Once the slot die is filled and purged, the air will release and then operate correctly. Without a purge mechanism, intermittent coating will fail forever, due to pressure variations.
Intermittent coating is also subject to start and stop phenomenon. To relieve the pressure jolts that would occur with a valve with back pressure releasing a fluid into the slot die manifold and creating a heavy starting edge bead, a recirculation loop should be provided back to the day tank. Maintaining pressure within the entire circuit is critical to performance. The head and tail coated on the panel are a function of the stability of the pressure in the fluid delivery system in combination with the valve control and mechanical movement of the equipment.
After the equipment is set-up correctly, the coating of the rigid panel can still have issues. These issues can include the variation in the substrate, control of the travel and potential environmental factors. Let’s break these down a little.
Especially in glass substrates, the surface flatness can vary tremendously. If the final product requires micron level control, and the equipment is precisely manufactured to deliver micron level performance, the product may still fail if the substrate variation exceeds the coating specification. Make sure that the substrate flatness meets or exceeds the flatness requirement for coating caliper control both cross web and down web.
Another key factor in the coating of rigid panels is how to properly handle the substrate. Should the slot die move over the panel or should the panel move under the slot die? Can the panel be held stable with a vacuum table, or will the suction cause damage to the substrate? These factors need to be considered to properly design the equipment. Consider the stability and weight of the equipment traveling during the process. Also consider the ability to maintain precision as a heavy and stable piece of equipment traverses the coating area.
Environmental factors come into play for both proximity and curtain coating, but more so in curtain coating. Because of the larger gap created in curtain coating we have to be concerned with air in the room disrupting the curtain. With proximity coating, room air won’t cause a fluid disturbance to the same extent as curtain coating, but mechanical vibration and environmental debris can cause coating defects. Make sure the fluid and substrate are clean before coating.
Rigid panel coating is becoming more and more common place. Understanding the variables to consider will help your company attack this multi-variable process with the right tools for success. If you work through the questions presented here, you will have a deeper understanding of how to successfully coat panels and limit coating defects.