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Comparison of vertical and horizontal slot coatings was carried out experimentally using aqueous glycerin solutions with varying viscosity on a laboratory‐scale slot coater. The maximum coating speeds achieved in vertical slot coating were found to be higher than those of horizontal slot coating for low‐viscosity solutions. The coating defects observed below the minimum wet thickness changed from ribbing to air entrainment at a critical solution viscosity of around μ = 75 mPa s. Above this critical viscosity, a lower minimum wet thickness was obtained for the horizontal coating but not for vertical coating. The difference between vertical and horizontal coating was mainly the length of the coating bead. The dimensionless minimum wet thickness was correlated as a function of Reynolds number. There existed three regions of minimum wet thickness depending on Reynolds number. Initially, the dimensionless wet thickness increased in the low Reynolds number region (Region I), then reached a plateau in the intermediate Re region (Region II), finally dropped off sharply in the high Re region (Region III). The boundaries of these three regions were identified by two critical Reynolds numbers, i.e., Re = 1 and 20. Only Regions I and II of the wet thickness could be found for horizontal coating, while mainly Regions II and III were obtained for vertical coating, depending on the viscosity of the coating solution. POLYM. ENG. SCI., 47:1927–1936, 2007. © 2007 Society of Plastics Engineers

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