- Part 1: Conditions of the Reverse Roll Coater
- Part 2: Two Basic Approaches to Reverse Roll Coating
- Part 3: Factors that Influence Defects in Metered Roll Coating
- Part 4: Position of the Wetting Line Affects Roll Coating Thickness
- Part 5: Controlling the Position of the Wetting Line in Reverse Roll Coating
- Part 6: Affects of Non Newtonian Flow and Viscoelasticity on Reverse Roll Coating
- Part 7: Using Hydraulic Force to control the Capillary Number and Metering Roll Ratio
- Part 8: Mechanical Design of the Reverse Roll Coater
Controlling Ribbing and Cascading in Metered Roll Coating
Part 1 and Part 2 of this series we discussed how reverse roll coating 1) can be used to apply a broad range of coating viscosities, 2) can apply these coatings over a broad range of thicknesses, and 3) can function over an equally broad speed range. Attempts to define limits are only generalizations, because there is an interdependency among all these factors, and there are different standards of acceptability that apply in every situation. One unavoidable truth is that reverse roll coating does have a lower limit with regard to film thickness due to the increasing roll that mechanical inaccuracies play as the metering gap dimension is decreased. This seems to have been pushed as far as possible by the narrow web, magnetic tape industry, where metered films under .001″ are applied with good uniformity, but with extreme care.
However, within the mechanical limits of a well made reverse roll coater, there lies an optimum set of conditions which will provide the smoothest, most uniform film for a given coating application. Otherwise, two well known defect conditions may appear in metered roll coating. One condition is usually defined as “ribbing”, although “record grooving”, “corduroy” and simply, “lines” are also used to describe this condition. The other defect is most often described as “cascading”, although “waves at the seashore” or “seashoring” are particularly descriptive terms sometimes used. Ribbing lines are oriented down web, and cascading occurs across web.
In recent years, the factors influencing these defects, both mechanical and coating related, have been the subject of several outstanding studies which have been very helpful in understanding the reverse roll coating process.
These studies have applied the laws of fluid mechanics, sometimes with the help of very powerful computers and sophisticated computer modeling techniques, to the reverse roll mechanism. Pilot coater studies were done to confirm the validity of the data. In some cases, cameras and strobe lighting were used to document results. Several factors have been seen as significant in these studies. One of these is the existence of the dynamic wetting line. The position of this wetting line affects the thickness of the metered coating and the ribbing and cascading phenomena.