Flow in tensioned-web-over-slot die coating: Effect of die lip design

Tensioned-web-over-slot die (TWOSD) coating is one of the most successful high-speed liquid coating process. It deploys elastohydrodynamic interaction to control the distance between the moving substrate and the coating die lip surface in order to be able to coat an ultra-thin liquid layer. However, flow instabilities that come from the gas–liquid interface and microvortices inside the flow may lead to coating defects. Therefore, the range of operating conditions of uniform coating is limited. The operating window of the process is a strong function of the geometry of the die. However, this relationship and, in general, the fundamental mechanisms of the elastohydrodynamic interaction are not known. In this study, we analyze TWOSD coating flow by solving the Navier–Stokes equation coupled with thin cylindrical shell equation using the finite element method. The boundaries that define the regions in the parameter space of uniform coating are automatically computed by a direct tracking method, based on multi-parameter continuation. The results show that the coating window of the process can be enlarged by designing the appropriate lip geometry.