Ribbing Instability in Forward Deformable Roll Coating, 1994 Coating Conference Proceedings

M. S. Carvalho, T. J. Anderson, L. E. Scriven

Ribbing, corduroy and pin-striping all refer to nonuniform coating whose thickness profile is wavy in the the transverse direction and more-or-less steady in the coating direction. This flow instability occurs if the roll speed is too high or if the liquid is too viscous. It can be associated with defects in roll coating, rod coating and many other applications that involve a film split between a pair of rolls. Understanding of this phenomenon can lead to a better control of the roll coating process and thus to a faster production rate (line speed).

Previous authors have reported conditions for the onset of ribbing and how rib spacing varies with flow conditions between rigid rolls. In practice, one of a pair of rolls is often covered by a layer of elastomer or other resilient material that can deform under the hydrodynamic pressure that develops in the gap. Ribbing from rigid gaps and, for the first time, from positive, deformable gaps is compared here. The experiments were made in a bench-scale roll coater with rolls 20.3 cm in diameter and 30.5 cm in length. The clearance between the rolls varied from 50 (pm) to 750 (pm). In order to scale up the critical conditions found in the experiments, the results were presented in dimensionless form. A relatively simple new detection techique was used to find the critical conditions (capillary numbers) at apparent onset of ribbing and, for the first time, at the onset of unsteady, or wandering, ribbing. It also yielded ribbing amplitude versus roll-speed. The results indicated that a roll coater with a deformable gap can be operated faster than one with a rigid gap, without producing ribbing – but that operating faster can also lead to longer wave-length ribbing that is more difficult to level.