An experimental study was carried out to investigate the fluid mechanics of solution casting. Newtonian glycerol solutions were used as test fluids and were casted on the polyethylene terephthalate films. The focus of this study is on the evaluation of the operating window, i.e., a region for stable and uniform processing. Different types of defects such as stable or unstable poolings, vibrating edges, and air entrainment outside the operating windows were observed. The effects of two different start-up approaches on the operating window were studied. One of the key operating parameters is the maximum casting velocity Vmax for stable operation. The fluid viscosity is the most critical parameter on Vmax. It was found that if the gap between the slot die exit and the moving film substrate is smaller than 600 m, Vmax might go down and then go up as the fluid viscosity increases. On the other hand, if the gap is larger than 600 m, Vmax will decrease as the fluid viscosity increases. Competition of two different types of defects, i.e., unstable pooling and air entrainment, can decide Vmax, as evidenced by the observation on the fluid motion and dynamic contact angles of different cases. The importance of different forces on Vmax was also analyzed. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013