- 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
In Part 6 we discussed the affects of non newtonian flow and viscoelasticity on reverse roll coating. One other factor affected by both the capillary number and the metering roll ratio is the roll separating force exerted by the coating as it passes through the converging geometry of the metering gap. See Figure 7.
This hydraulic force is proportional to the capillary number so it increases with both viscosity and applicator roll speed. At some point, this force can cause the rolls to separate, more so in the center than at the ends, causing an unacceptable variation in film thickness in the cross-web direction. There may be mechanical solutions to this problem up to a certain extent, but there are practical limitations. Interestingly, the roll separating force is inversely proportional to the metering roll ratio. In fact, it has been documented that this force can even become negative if the ratio is high enough.