Part 4: Position of the Wetting Line Affects Roll Coating Thickness

This entry is part 4 of 8 in the series Reverse Roll Coating

How the Wetting Line Affects Ribbing and Cascading in Roll Coating Thickness

In Part 3 of this series we discussed how to control defects in metered roll coating to prevent ribbing and cascading. In Part 4 we discuss how the position of this wetting line affects the thickness of the metered coating and the ribbing and cascading phenomena.

The “wetting line” as illustrated in Figure 5 can be described as the point at which the coating pulls away from the metering roll as it departs the metering gap and travels along with the applicator roll. When the wetting line is located at or near the center of the gap, the metered film thickness is at its minimum (for a given gap setting) and the coating appears the smoothest. If the wetting line is positioned to the outlet or metered film side of the gap, generally the ribbing phenomena is more pronounced. This appears to be related to the shape of the “meniscus” that occurs at the wetting line, and with instabilities arising from forces affecting this shape. The coating thickness can also be expected to increase slightly.

Reverse Roll Coating Thickness Figure 5

Figure 5

On the other hand, if the dynamic wetting line is positioned on the inlet side of the gap, the onset of cascade occurs, and the coating becomes substantially thicker in an oscillating manner. This appears to be due to an instability in the position of the wetting line as it moves back and forth on the inlet side. As the wetting line moves further from the center, the thicker the metered film becomes, until the thickness equals the gap dimension. This causes the film to reattach to the roll near the center of the gap and the metered film becomes thin again temporarily. However, the forces causing the wetting line to locate on the inlet side of the gap are still at work, and the entire process repeats itself, resulting in the cascade condition.

Ratio of the Metering Roll Speed to the Applicator Roll Speed

The most important factor in controlling the position of the wetting line, and therefore, the appearance of the coating, is the ratio of the metering roll speed to the applicator roll speed. The effect of this speed ratio on ribbing or cascade is widely known ~ speed up the metering roll to eliminate ribbing, slow it down to eliminate cascade. Interestingly, The position of the wetting line is accurately predicted as a function of the metering roll ratio in the mathematical reverse roll coater models that were created in the studies cited.

Part 5 of the Reverse Roll Coating Series discusses how the capillary number also affects the position of the wetting line.

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