Visualization Study of Flow Stability in Reverse Roll Coating

Reverse roll coating is widely used to coat a thin liquid layer onto a moving substrate. A metered liquid layer is created within the gap between a pair of co-rotating rolls. To avoid roll damage due to roll run-out, substrate caliper change or splice passage, when the gap between the rolls is small, one of the rolls will have a compliant polymer cover. The existence of a deformable cover in the gap between a metal roll and a rubber roll creates an elastrohydrodynamic flow field in that region. As the liquid passes through the coverging-diverging section within the gap, it generates pressure, and this pressure can deform the elastic roll surface, which in turn alters the geometry of the gap and the flow. Therefore, the uniformity of coating is affected differently than what is observed when only rigid rolls are used. In this study, visualizations of the flow between a reverse deformable roll and solid stainless steel roll are done to determine how the uniformity of coating in the high roll speed region is affected by operating parameters, namely, the speed ratio between the rolls, roll cover properties and liquid properties. The wavelength of ribbing is investigated to verify the effect of speed ratio, wet coating thickness and viscosity. For these experiments, the roll coating apparatus is used with 4 inch diameter rolls installed one above the other. A high-resolution camera is used for visualizations of the flow, and the ribbing wavelength is measured from photographed images. The uniformity of coating and its dependence on capillary number is evaluated. A numerical simulation of the flow between metal and rubber rolls enabling prediction of the ribbing wavelength is also performed.