- October 18, 2013, Mark Miller
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On many coating projects that I have been involved in, the coating defect that the team was trying to eliminate was apparent on the web, but where the defect originated was not known. This leads the process and product team down different paths of experimentation and analysis to understand the root cause of the defect. Sometimes this approach takes a couple hours, and sometimes it takes a couple months. Either way, time could be reduced with a better theoretical understanding of the flow dynamics.
So what techniques can be used to better understand fluid flow and defect analysis? Flow simulation software and lab-scale visualization techniques can provide great insight into the world of fluid dynamics. I have used these techniques in the past to reduce troubleshooting time and use of raw materials, reduce defects, and improve equipment design. We can see what the final product looks like off the winder. We can watch the fluid being coated on the web and after the curing step. What we cannot see is what is going on at the interface of the fluid and the substrate, or how the fluid is reacting as it travels from the tank to the pump to the coating head.
One technique that has been successfully used to see these difficult regions of the coating system is photo-visualization. Photo-visualization of a coating process requires high speed cameras, proper lighting, and light manipulation through mirrors and optics to develop an image that captures the fluid and defect of interest.
As an example, I have seen a fluid be coated against a quartz backing roll, where the camera is located inside the roll and captures an image of the fluid as it exits a slot die. Even without this unique design, placing optics where people can stand in a typical coating arrangement can bring better understanding of the flow dynamics involved at the fluid substrate interface. This perspective can answer questions that cannot be answered with internal flow computer programs or the naked eye.
Working back from what we can see to what requires a more complex arrangement of light management takes some time, but in the end, may be the only way to truly know what is going on at the coating head/substrate interface. Working back from this point into the coating head requires theoretical visualization.
Computer models of the manifold geometry, materials of construction, rheological information, and proper fluid models are critical to prediction of the flow behavior inside a coating head.
Taking the information obtained through designed experiments that systematically break down the process and product variables, combining them with visualization techniques, and closing the loop with theoretical internal flow data can provide the manufacturing engineer with the complete picture of fluid interactions. Interactions with piping, distribution, coating, surface reaction, and drying all lead to potential defects. Breaking each unit operation down to see what is happening leads to solutions with data instead of inference.
If you are interested in discussing this concept further, contact Mark D. Miller, founder and CEO of Coating Tech Service, LLC (www.coatingtechservce.com) at firstname.lastname@example.org or 612-605-6019.