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.