Part 5: Controlling the Position of the Wetting Line in Reverse Roll Coating

This entry is part 5 of 8 in the series Reverse Roll Coating

Metering Roll Ratio vs. Capillary Number to Increase/Decrease the Wetting Line

In Part 4 of this series we discussed how the position of this wetting line affects the ribbing and cascading phenomena in reverse roll coating. The most important factor in controlling the position of the wetting line is the metering roll ratio. The second most important factor influencing the position of the dynamic wetting line is the capillary number of the coating. The capillary number is defined as:

Viscosity x Roll Speed
Surface Tension

Increasing the capillary number generally has the same effect as increasing the metering roll ratio, that is, it stabilizes the coating to ribbing. The wetting line moves from the outlet toward the center of the metering gap as the capillary number increases due to the shape of the curve the meniscus can maintain. As the capillary number increases, the liquid supports a more strongly shaped curve that can locate closer to the center of the gap. Still higher capillary numbers can cause the wetting line to locate upstream of the gap, causing the onset of cascade. Since capillary number is directly proportional to applicator roll speed, which itself is proportional to line speed, the implications of this can readily be seen.

Using Capillary Number to control the Wetting Line in reverse roll coating Figure 6

Figure 6

In general, a higher capillary number shifts the optimum metering roll speed ratio to a lower value and vice versa. However, it has been observed that at extremely low capillary numbers, the coating becomes stable to ribbing at a lower metering roll ratio. The concept of eliminating ribbing by lowering the metering roll speed is counterintuitive, but correct. At a higher capillary number, the more conventional adjustment of increasing the metering roll ratio is correct. This phenomena is best illustrated by the following Stability Diagram in Figure 6.

Operating under the set of conditions represented by ” A ” on the diagram (low capillary number, low speed ratio) would produce a ribbed coating. The usual response, increasing metering roll speed, would shift point ” A ” in a straight line to the right, and no improvement would be seen. However, lowering metering roll speed would soon yield operating conditions in the stable area.

Contrast this with the more conventional speed adjustment required to shift point ” B ” into the stable area. Also, note how changing the applicator roll speed (strongly associated with production speed) can shift the point in or out of the stable area.

The metering gap dimension (along with other factors) can alter the size, shape, or position of the stable region, and generally reduces its area, making it more difficult to achieve acceptable results. In fact, under certain conditions no stable region exists.

Also consider that the demarcation between ribbing and uniform coating may be rather vague. In fact, it may be that ribbing always exists, but if the amplitude is small enough, the coating appears acceptably smooth to the eye.

Part 6 of the Reverse Roll Coating Series discusses the affects of non newtonian flow and viscoelasticity on reverse roll coating.

Series Navigation<< Part 4: Position of the Wetting Line Affects Roll Coating ThicknessPart 6: Affects of Non Newtonian Flow and Viscoelasticity on Reverse Roll Coating >>

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