- October 01, 2003, Edward D. Cohen, Edward D. Cohen Consulting Inc.
A successful coating process is relatively easy to define by stating the desired performance requirements and the ability to meet safety and environmental standards. However, requirements such as cost, profitability, product quality, and performance are becoming more difficult to achieve for several reasons:
The coater is unable to produce improved quality.
New coated structures are more complex — multiple layers, thin coatings, light gauge substrates.
High costs due to poor coater yield and capacity.
Technology deficiencies in the coater.
Slow product development cycle time.
As a result, an ongoing technical effort is needed to improve the process so it meets current and future business and technical goals. For this effort to be a success, here are some of the technical factors you need to consider.
Basic Operating Principles
A successful coating operation requires an understanding of some basic principles. The coating process is composed of several different process technologies, all of which need to function as one unit for effective coating operation. All of the coating technologies now have both an extensive practical and theoretical basis, permitting effective utilization of the equipment and development of new products and processes.
Table I gives the major technologies and the technical expertise needed for each area.
All of these technologies need to function effectively and interact with each other so the entire coater functions as needed. There is no one technology that can be focused on to the exclusion of the others — they all are important. A process can be limited by its weakest link, and if not corrected, it will limit the process, even if the other elements are improved.
Therefore, it is essential to analyze all of the operations to identify the rate-limiting steps and eliminate them.
One example is the interaction of the coating applicator and the dryer. All coating applicators have an optimum set of coating conditions — line speed, viscosity, percent solids, etc. — for defect-free film that results in a solvent load to be removed in the dryer.
The dryer also has optimum conditions for product performance and quality, which may not produce the drying capacity needed to remove the solvent load. When this occurs, conditions have to be modified because a wet film exiting the dryer is unacceptable. The resulting change in process conditions to dry the film can lead to reduced quality and poor economics.
Often the limitations are not in the major technologies but are in the other technologies such as web tracking capability, coating weight control, viscosity control, and solution mixing quality, which often are neglected.
Another basic principle: For optimum quality and productivity, each step must not produce defects or be out of specification. The subsequent process steps will not cure defects introduced in a previous step. The coating process is serial in nature in that it starts with raw materials, which are converted to the final product in the subsequent steps.
A base roll with defective gauge bands and poor planarity will not be changed in the coater and produce acceptable product. The coating applicator will not remove contaminants from a solution. A poor coating weight profile will not change in the dryer.
A continuous improvement program is required for ongoing success. There are many operations in the coater, and it is unrealistic to change and upgrade them in a short time. In addition, future product requirements may require unanticipated technologies.
Accurately Identify Needs
Technical efforts need to be focused on the right items to ensure success. Because of the process complexity, a cursory review can lead to efforts being made on minimal-return projects or projects with high glamour and minimal value.
The identification process needs to locate the limiting process steps, accurately identify product losses, and determine technical obstacles. There are a variety of statistical process control tools, Pareto diagrams, process capability, and cause and effect diagrams available for this effort.
After needs are established, they should be prioritized and an action plan implemented.
Utilize Appropriate Hardware
Coating line hardware should be matched to product performance needs. Coating lines come in a wide variety of configurations from small, narrow, low-volume research and development coaters to wide, high speed, capacity manufacturing coaters.
While all coating lines have the same basic modules, they are each a custom machine built to the client's specification. The final cost depends on the exact specifications and tends to increase with the desired quality and productivity levels. Therefore, it is essential to specify and use a machine that can meet your needs.
Either under-specifying or over-specifying can lead to problems. Building a low-cost machine when a higher cost and quality is needed will produce an inferior product.
Safety and environmental requirements — current and future — must be an integral part of the hardware effort.
There are many different coating applicators, and no one method is suitable for all requirements. Each coating method has an optimum range of wet coating thickness, line speed, viscosity, etc., in which it will produce defect-free film. Outside of this range, product can be coated, but it will have a higher defect level.
Therefore, it is essential to use the correct coating method, which will be in the optimum range for the product coated.
Strengthen Secondary Systems
The coater's secondary systems also need to be strengthened for the coating line to operate efficiently. A quality control system to ensure the product meets customer needs is required. The system can be either elaborate — using computers and software systems — or simple — using hand plotting of quality control charts — depending on the need.
An essential part of a quality control system is process measurement technology. Instruments for on-line measurement of coating weight, defect inspection systems, and coating solution viscosity are available and should be considered. It is more effective to detect out-of-limits product immediately after the applicator as opposed to winding a long roll and testing it off line.
Obviously, a system to ensure that raw materials are specified correctly and quality level is maintained is essential. Often the available substrate webs cannot meet the wetting and adhesion needs of a product. Surface treatment systems — flame treatment, electrical discharge, and plasma — can be installed on-line to correct these deficiencies. Substrate cleaning systems also can be added to remove contaminants.
Trained personnel are essential to operating a coater successfully. Operators must be familiar with all of the coater core technologies, coater operation, and product testing. Educational programs on the coating process should be conducted routinely so operators have a good knowledge base from which to make decisions.
Operations personnel also should be in communication with customers so they are aware of the customer needs and problems with the product. Product development personnel also need to be trained in the capabilities of the coater so products they develop can be manufactured.
Considering these technical factors will help you meet your goal of improving your coating process.
Edward D. Cohen is a technical consultant in all aspects of the web coating process. His expertise is in the coating and drying of thin films, coating process development and scale-up, polyester base development, film defect mechanisms, and defect characterization techniques. He has more than 40 years of experience in coating research and manufacturing technology and has extensive publications in the field including two books. Currently he is technical consultant for the Association of Industrial Metallizers, Coaters and Laminators (AIMCAL). He can be reached at 480/836-9452; Cohened146@aol.com
This article, along with future articles by other authors, is provided as a cooperative effort between PFFC and AIMCAL. Authors contribute to AIMCAL's technical and education offerings, which include the association's Fall Technical Conference, Summer School, and Ask AIMCAL.