- 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
Reverse Roll Coaters must be of Uncompromised Precision
In Part 7 of this series on reverse roll coating we discuss the use of hydraulic force to control Metering Roll Ratio and Capillary Number. The mechanical design of the Reverse Roll Coater must be uncompromised when it comes to precision. Rolls must be made of high strength steel or cast iron of sufficient diameter and wall thickness to provide substantial resistance to the gap separating forces encountered in coating. Casting thin films requires extreme concentricity of rolls and bearings and precise gap setting mechanisms. Metering roll doctoring is critical. The backing roll must be in excellent condition.
Drives must provide a uniform velocity to all rolls throughout the entire process speed range. The substrate moving through the coater must be isolated from upstream and downstream sources of tension and speed instability. Metering and applicator roll speeds should be infinitely adjustable through the entire speed range.
Operating procedure should be kept in mind. Many adjustments are required to bring the coating into specifications, both from a film thickness and optical quality standpoint. Adjustments then, as well as a clear field of vision, must be carefully provided for in the design. Instrumentation must be provided to aid in the process of establishing and maintaining the proper coater parameters.
The reverse roll coater is indeed versatile. However, producing acceptable results is far from automatic. Renewed interest in academic circles in this long utilized coating method has provided useful insight into the reverse roll mechanism. Powerful computers have been utilized to predict fluid flow through modeling techniques. Experiments have validated theory. As a result, those of us in the industry are now in a better position to understand how to manipulate our machinery to produce the often elusive results we seek. We better understand why our time honored responses work and why they sometimes do not. We can now understand and perhaps be more prone to experiment with counterintuitive solutions which may be most beneficial.