Abstract
The fluid mechanics of narrow stripe coating for low-viscosity Newtonian solutions were investigated. It was found that the coating solution will expand laterally after emanating from the slot die channel at low coating speeds. As coating speed increases, the stripe width will contract until it is close to the slot channel width, then coating failure would appear. Three slot channel widths were tested, coating failure such as ribbing appears at high speeds for two larger slot channel widths. However, for the smallest channel width, the critical speed for coating failure to appear is much higher and the coating failure is not ribbing but the narrow stripe breaks periodically. The effects of each parameter on the coating width variation were examined and a universal correlation which involves modified Reynolds and Bond number was established to predict the coating width. A flow visualization technique was applied to observe the mechanism on how a narrow stripe breaks at high coating speeds. 2-D and 3-D numerical simulations on coating flows were examined for a better understanding on the stability of narrow stripe coating. For certain operating conditions, the downstream contact line of the coating bead may climb on the die shoulder, which may cause vortices near the downstream region. Since most of the coating solutions in industry are volatile, vortices in the flow fields should be avoided to prevent the coating solutions from degradation. Otherwise, some coating defects such as bead-break in stripe coating or rivulet in conventional slot-die coating would take place. To avoid the contact lines from climbing on the die shoulder, many factors such as viscosity of the coating solutions, film thickness, and die geometry were tested with software package FLOW-3D. Among all the factors, designing the dies by adopting smaller angles at the die edge or by increasing the contact angles of the coating solutions on the die can effectively pin the positions of the downstream contact lines at the die edge, and suppress the formations of vortices as well. Besides, the stability of the coating beads was also analyzed. With appropriate adjustments, higher coating speeds can be reached by the use of the newly-designed slot die.