By Dr. Edward D. Cohen
A: The basic reason for current deficiencies due to design and operating parameters is that all coaters were originally designed and built to exacting performance values for all hardware elements. These specifications would indicate the range of conditions than can be obtained from each element and the precision and variation required. For example, the web-handling system used to transport the substrate would specify the range of line speeds and tension levels and the variation allowed. The hardware would then be designed to meet these values. If the dryer was designed for aqueous solvents, it is not capable of drying organic solvents. As long as the operating conditions stay within design limits, the coater should function effectively.
The original coater may have been designed to run at 200-400 fpm, and it performs satisfactorily within this range. But then, the new product must be run at 140 fpm because of drying limitations. The panel board will often have a line-speed control setpoint that will permit these values to be set, and thus the coater will run at this line speed. However, because of being outside of the design limits, there will be poor control of the line speed, which can result in poor coatweight uniformity and drying variability. This can be a significant factor in coating uniformity for pre-metered coating methods, such as slot-die, cascade and extrusion coating, which require a very uniform line speed to obtain minimum variability.
Control of tension control, dryer air
Tension control is another area where current capability is important. Currently, there is an increase in the number of products with thin webs being coated, and coaters designed for thick webs may not have the capability needed for these new thin films.
If a new product needs minimum contamination, the exiting dryer air and ambient air could contain a high level of particulates that could hurt product quality. Another consideration is that coaters can function for many years, and so the ability to obtain original-design conditions often deteriorates due to wear and tear.
To prevent these problems, the overall coating system should be characterized to determine current capability, and then hardware improvements made where needed. Often normal maintenance and parts replacement will sustain or improve coating capability.
References
E.D. Cohen, “Web Coating Defects: Role of the Coater Module,” Converting Quarterly, 2012 Q4, page 22.
R. Wagner, Jr., and E.B. Water, Solvent-Based Coating Technology, Multilayer Flexible Packaging, Elsevier, 2nd ed., 2016.
J. Greener, G. Pearson, and M. Cakmak, Coating & Solidification, in Roll-to-Roll Manufacturing: Process Elements and Recent Advances, John Wiley & Sons, 2018.