Optimization of slot-coating processes: minimizing the amplitude of film-thickness oscillation

Slot coating is one of the most common coating methods for high-precision coatings. The flow limits of steady-state operation have been extensively analyzed in the literature. However, even the best-designed slot-coating operations are subjected to small oscillations on process conditions, such as flow rate, vacuum pressure and gap fluctuations. These time-dependent events lead to thickness variations on the deposited liquid layer that may be unacceptable for product performance. Therefore, the design of slot-coating processes, e.g. the specification of process conditions and die geometry, has to take into account not only the behavior of steady-state flow but also how the flow responds to small periodic disturbances. The process parameters should be such that the film-thickness oscillation is minimized. In this study, coating-thickness variations related to an ongoing oscillation of the coating gap is studied for different process parameters and frequency of the perturbation by solving the transient Navier-Stokes equations with appropriate boundary conditions for free-surface flow. The amplitude of the deposited film-thickness oscillation is used as the objective function of a bound-constrained optimization algorithm. The results show that at a fixed web speed and wet thickness, the film-thickness amplitude may be reduced by a factor of 4 by adjusting other process parameters.