Many industrial voens are designed using air bars and air foils that supply drying gas against the backside of the coated substrate. See for example Shu. The purpose of these devices is to heat the substrate in order to cause evaporation of a carrier solvent and to provided substrate flotation. Typically, the air bars and foils are installed in one standard width in a given oven, while the coated substrate may take on a variety of widths depending on the product. If the air bars or foils are wider than the substrate, high velocity or turbulent air flow can result at the edge of the coated film. When the coating is susceptible to mottle, drying patterns can result at the edge which do not occur in the center. This paper will present some experimental data demonstrating this excess air foil effect on mottle. It will also describe smoke tests performed to better understand the phenomena and some simple theories to explore the mechanism behind the effect.
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