A flow visualization technique was employed to observe the start-up of slot die coating. The effects of five variables, i.e., prewetting on the die surface, fluid viscosity, slot gap, coating gap, and die lip length on the start-up time were investigated. Prewetting on the die surface can effectively reduce the start-up time. Any variable that enlarges the steady-state coating bead would increase the start-up time. Therefore, increasing the fluid viscosity, coating gap, slot gap and decreasing the die lip length can be helpful in reducing the start-up time. It was found that there exist four distinct modes for the establishment of steady-state coating bead. The four distinct modes can be distinguished by a Reynolds number, which is defined as the ratio of the inertial force of the coating solution impinging on the moving web to the viscous force needed to pull the coating solution on the web. Comparison of the experimental observation with the theoretical prediction based on the commercial package Flow3D was also performed; the four modes can be found in the numerical simulations, the start-up time computed to reach steady state is shorter. POLYM. ENG. SCI., 49:1158-1167, 2009. (C) 2009 Society of Plastics Engineers
