Coating Matters | Stop Defects Before They Start
- Published: May 27, 2014, By Mark Miller
One of the best ways to not have defects in fluid coating isn’t a trick with a pump or a tip with a coating head. Reducing defects works best by practicing preventative medicine. Defect prevention comes from a handful of considerations at the beginning of a product development cycle.
Process Fundamentals
Each component in the roll-to-roll coating process (coating head, fluid delivery system, web control, and roll consistency) provides an area of opportunity for improved coating performance or reduced capability. Each of these items is a multiplier that can take an existing coating defect and magnify the result. With proper design and precision machining, the process equipment can eliminate some defects that occur on the standard machine.
Having the proper equipment can only take you so far. Understanding the underlying theory that drives the defects is critical to developing an approach to solve the problem. Theory needs to be combined with empirical evidence to develop a solid understanding of the coating process for your specific product. Computer models and simulations can reduce the empirical evidence data gathering time associated with process development. Understanding fundamentals of coating and how they can be applied to your raw materials will prevent defects and allow for improved troubleshooting.
Product Requirements
The final product requirements provide the performance criteria that measures success. These requirements are adjusted by varying the functional areas of coating technology (raw materials, solution chemistry, process equipment, curing, and web handling). Working backwards from these product requirements leads to the product requirements that define success:
- Product (number of layers, number of coatings, placement of coatings, dry coat weight, uniformity required, coat width, coating temperature, product yield, quality)
- Solution (percent solids, solvent type, viscosity, surface tension, curing requirements)
- Substrate (thickness, roll length, surface energy, thickness uniformity, quality)
Analytical Techniques
Always make sure that quality can be measured and agreed to by the final customer. Once these performance criteria are set, then we need to make sure they can be measured precisely and accurately for the product release criteria. These test methods can be destructive or non-destructive, off-line or on-line.
What is right for your product is a combination of required line up time, capability of the system versus cost, and operator inspection training. As an example, coat weight truly should be a weight of the coated sample as a standard test method. However, if an on-line gauge can capture the coat weight within the specification required by the customer, then destructive off-line testing should be limited to a checking test instead of an every-roll test.
Right Tools
Using the appropriate coating method for the product is critical to reducing defects that may be inherent in certain systems. There are multiple coating techniques and many roll-to-roll coating equipment designs that provide the flexibility to coat your product in the proper fashion for defect reduction. Defining the coating window for a specific product design takes time on the line with the equipment, but the key is to understand the limits of a given coating system. The product should operate within a large coating window that allows for some variation in equipment setup and substrate or solution variation.
Each coating method provides for a range of viscosities, line speeds, uniformity, cost, and coating thickness. For example, if a coating needed to be coated in a clean room environment, with two layers simultaneously, with a tight uniformity tolerance, slot die would be one of the only options. Coating with a different system would require multiple passes on the coating line magnifying coating defects.
Operational Procedures
Once the proper equipment, substrates, solutions, and product performance criteria are set, training operators and providing easy-to-follow operational procedures are required. Recipe development in software packages are a great solution for reduction in operator error. As is providing a feedback system for operators to assess and report variations in coating due to substrate, solution, or equipment performance. The more automated these systems become, the more they will be followed.
The ultimate in this line of thinking is to utilize a computer input and output program that feeds a Statistical Process Control (SPC) system. SPC allows engineers and operators to understand a change by having a non-biased chart of the inputs that affect product performance.
Defect prevention is a function of the application of the items listed above in a program at the site level. Defects need to be reduced on an ongoing basis as fluids are changed and substrates are altered. Assuming that a defect will cure itself is impractical. If coating used to be good, and now it’s not, means that something changed. If not the equipment, then the process technique used or the raw materials supplied.
Practice defect prevention, and avoid bringing in a specialist later on to correct the coating issue.
Mark D. Miller, author of PFFC's Coating Matters column, is a fluid coating expert with experience and knowledge in the converting industry accumulated since 1996. Mark holds a Bachelor's degree in Chemical Engineering from the Univ. of Wisconsin-Madison and a Master's degree in Polymer Science & Engineering from Lehigh Univ. and a Juris Doctor from Hamline Univ. Mark is a technical consultant and CEO of Coating Tech Service LLC. He has worked in web coating technologies and chemical manufacturing operations and is a certified Six Sigma Black Belt trained in both DMAIC and DFSS disciplines. Coating Tech Service provides process troubleshooting and project management for precision coated products. Mark has extensive process knowledge in high precision coating applications including thin film photo voltaic, Li-Ion battery, and optical systems technology. Mark has been integral to new developments and technology that minimize product waste and improve process scalability.
