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The effect of bead vacuum on the minimum wet thickness obtained on horizontal and vertical slot die coatings was experimentally examined using a flow visualization technique. The commercial software package FLOW3D was used to simulate the two-dimensional coating flow. Macroscopic material balances were applied to estimate the effect of bead vacuum. Applying vacuum on the coating bead was found to reduce the minimum wet thickness and increase the maximum coating speed, with the effect on horizontal slot die coating being more pronounced. Visual observations revealed that with increasing vacuum, the upstream meniscus against the moving web was pulled back significantly for a low-viscosity solution, and the dynamic contact angle was reduced for a high-viscosity solution. The observed upstream wetting length for a low-viscosity solution subject to bead vacuum was consistent with the prediction using macroscopic balances. The sizes of coating beads predicted by FLOW3D were generally larger than those observed from flow visualization. Coating failures caused by high bead vacuum could be predicted with the aid of numerical simulations.

The Effect of Bead Vacuum on Slot Die Coating