A Numerical Study on Improvement of Coating Uniformity by Controlling the Pressure at the Exit of the Slot Die Nozzle

Abstract

In this study, numerical analysis was carried out to investigate the effects of slot die nozzle exit rear pressure on coating uniformity improvement in coating nozzles which are widely used in industry. Coating uniformity in coatings of viscosity materials such as inks and surface coating solutions is closely related to the quality of the product. Especially, coating uniformity is an important performance indicator for slot die coatings, which are used primarily in the production of optical or chemical products such as displays, touch screens, solar panels, and batteries. In general, the average thickness of a thin film in a slot die coating is determined by the supply flow rate and the moving speed of the plate. However, due to various parameters, thickness irregularities and coating defects due to pores occur locally. Therefore, many studies have recently been performed to solve the local defect of the slot die coating. In order to improve the coating uniformity, this study applied the vacuum pressure condition to the slot die nozzle rear end and numerical study on the coating uniformity according to the vacuum pressure was carried out. The numerical analysis proceeded to 2-D, unsteady condition and the VOF method. A commercial numerical simulation software (STAR-CCM+ V.12.06) was used. It was simulated that the coating liquid was supplied onto the moving plate surface. The moving plate speed was fixed 0.01m/s. The nozzle geometry simulates a slot die nozzle. And the coating raw material supply flow rate was considered for two conditions (0.005, 0.006m/s). Two vacuum pressure conditions (−3, −300Pa) were applied to the rear end of the nozzle. The viscosity of the viscous material was fixed at 2 Pa.s. The analysis results were analyzed through the uniformity of the material supplied to the surface. As a result of analysis, uniformity of coating was lowered regardless of the velocity when the vacuum pressure condition was −3.0 Pa, and uniformity was lowered as the velocity increased. However, it was confirmed that uniformity of the coating was increased at all velocity condition when the vacuum pressure condition was −300 Pa. This is because as the flat plate moves, the reverse pressure is applied to the downstream of the coating solution, thereby improving the uniformity of the coating liquid flowing through the flat plate and preventing the inflow of air or the like. Through this study, it was confirmed that the coating uniformity can be improved by controlling the vacuum pressure at the rear end of the nozzle.