Minimum wet thickness in extrusion slot coating

Abstract

An experimental study has been carried out to determine the minimum wet thickness of extrusion slot coating. It was found that there exists a critical capillary number Ca* for Newtonian solutions within the range of experimental conditions: below Ca*, the minimum wet thickness depends strongly on the coating speed and the fluid viscosity, or the capillary number Ca. Above Ca*, the minimum wet thickness is independent of the capillary number and is a function of the coating gap only. Comparing with slide coating, we have found that the minimum wet thickness for both coating methods are very close if Ca < Ca*. As Ca > Ca*, the minimum wet thickness for extrusion slot coating will become constant as the capillary number increases and the dimensionless minimum wet thickness will reach a constant value, but the minimum wet thickness for slide coating will increase as the capillary number increases. The experimentally determined minimum wet thickness for extrusion slot coating has been compared with the theoretical predictions of Ruschak, and of Higgins and Scriven. The effect of bead vacuum was also investigated, it was found that increasing bead vacuum can effectively reduce the minimum wet thickness particularly if the fluid viscosity is low and the coating gap is narrow.