Design principles of integrated vacuum slot arrangement

Abstract Coating on a substrate with a slot‐die forms alternate wet and dry stripes as the substrate speed is increased when the upstream pressure equals the downstream pressure. The coating thickness is thus nonuniform across the width of the substrate. Applying vacuum upstream of the feed slot allows uniform coating at higher speeds. Different methods to apply vacuum were invented over five decades. The latest method, an integrated vacuum slot arrangement, is novel and compact. Vacuum is applied through a second slot, called vacuum slot, in the die placed upstream of the feed slot. The design principles of the arrangement are elucidated by modeling air flow in the arrangement, which is similar to liquid flow in the feed slot, but in the opposite direction. Well‐accepted equations that have been derived by rigorous order of magnitude analysis to describe liquid flow have been used to model air flow. Sensitivity of the performance of the arrangement with respect to geometry is described. © 2007 American Institute of Chemical Engineers AIChE J, 2007