Flows of a viscoelastic fluid in slot die coating were experimentally and theoretically investigated. A viscoelastic fluid having a constant viscosity and a relaxation time (Boger fluid) as well as Newtonian fluid were used as test liquids in a wide range of Weissenberg numbers (0 < Wi < 42). We observed experimentally that the Boger fluid reduced the widthwise non-uniformity of the flow inside a slot die as flow rates were increased. However, this advantage disappeared in the higher flow rate region. This was not observed in the case of Newtonian fluids. In order to depict this behavior, a one-dimensional flow model was presented, where cavity flow and slot flow were combined by means of a planar entrance flow. The pressure drop induced by the planar entrance flow was assumed to characterize the flow from a cavity to a slot. Moreover, an excess pressure drop due to the viscoelasticity was empirically estimated, where the high deformation rates attained in the entrance flow from the cavity to the slot caused the increase of the excess pressure drop. The prediction by this model qualitatively agreed with the experimental results and showed utility of the proposed simple model.
