An experimental study was carried out to investigate the effects of polymer additives on the operating windows of slot coating. Two polymer additives-carboxlmethyl-cellolose (CMC) and polyacrylamide (PAA)-were added to Newtonian glycerin solutions as test fluids. The flow experiment was perfomed on a lab coater. A flow visualization technique was employed to observe the shapes of coating beads. It was found that three types of defects appear at the lower bound of the operating window, i.e., ribbing, air entrainment, and break lines coupled with dripping. Adding a small amount of polymer additives has little effect on the operating window if the defect is ribbing, but it will expand the air entrainment region. The coating speed can be increased significantly if the defect is break lines coupled with chipping. (C) 2004 Society of Plastics Engineers.