Coating thickness has a major impact on a product’s performance, quality, cost, and aesthetics. Carestream Contract Manufacturing understands the challenges in achieving the coating thickness specifications required by its customers and chooses proper coating methods and measurement techniques during the early design stages to save significant time and cost in the product scale-up of high-precision applications.
Determining Coating Thickness in a High-Precision Roll-to-Roll Process
Successfully controlling thickness can be difficult when conducting high-precision roll-to-roll coating. Cross-web and down-web coating thicknesses can fluctuate across millions of square meters of coated material. The best techniques for controlling coating thickness vary according to the unique properties of the substrate, the coating, and the method of its application. For example, in order to prevent the coated thickness from increasing over time in a non-aqueous, pan-fed gravure coating application, a solvent replenishment system is required to keep the solid concentration of the coating solution constant.
There are a number of destructive and non-destructive methods for measuring coating thickness. To determine the appropriate measurement approach, Carestream considers a plethora of factors such as the properties of the coating and the substrate as well as the end use of the coated product. Destructive techniques such as the cross-sectioning of film, use of a profilometer, chemical stripping, and other methods can be helpful, but may cause damage to coated materials and have long testing times.
Non-destructive methods that do not alter the materials being measured include magnetic, capacitive, and radiographic testing, as well as eddy-current testing and optical methods such as spectral reflectance and ellipsometry. Depending on the optical properties of both the coating and the substrate, effectively conducting on-line measurement can be a challenge. Carestream has begun to incorporate some sophisticated coating thickness measurement equipment on its pilot coaters to aid in product and process development.
Advantages of Premetered Coating Methods
One of the primary advantages of premetered coating methods such as curtain, slot die, or slide-bead is that the down-web coating thickness is independent of coating solution properties such as viscosity. These methods are well suited for high-precision manufacturing of transparent conductive films, flexible displays, and medical imaging media.
For premetered coating methods, the down-web coating thickness is determined by the volumetric flow rate of coating solution per unit width and the web speed. These parameters can be controlled with a high degree of accuracy, thereby sometimes eliminating the need for on-line, down-web thickness measurement. With the proper expertise, premetered coating methods can achieve coating thickness uniformities within 1-2 percent of aim in both the down-web and cross-web directions. Such expertise includes in-depth knowledge of best practices for web speed control and design of coating solution delivery systems, coating dies, and the coating station itself, as well as understanding key process/product interactions.
In conclusion, successfully controlling coating thickness can be complicated due to the large number of potential product and process parameters that must be controlled, depending on the coating method under consideration. Controlling coating thickness to meet high-precision application requirements requires a coating partner with broad experience and deep technical knowledge. Carestream understands the integral relationship between coating thickness, coating method, and product and process parameters to maximize product quality and decrease cost and product development cycle time.