Part 7: Using Hydraulic Force to control the Capillary Number and Metering Roll Ratio

This entry is part 7 of 8 in the series Reverse Roll Coating

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.

Using Hydraulic Force to control Capillary Number and Metering Roll Ratio Figure 7

Figure 7

Part 8 of the Reverse Roll Coating Series discusses the mechanical design of the reverse roll coater.

Series Navigation<< Part 6: Affects of Non Newtonian Flow and Viscoelasticity on Reverse Roll CoatingPart 8: Mechanical Design of the Reverse Roll Coater >>