Coating with deformable rolls: An experimental investigation of the ribbing instability

Results from a systematic experimental investigation of deformable roll coating, operating in forward mode, are presented, with particular emphasis as to how the governing parameters of the coater (peripheral roll speed and gap setting) and the physical properties of the associated deformable roll covering (thickness, hardness and material type) influence the onset of the ribbing instability. It is found that for positive gap settings, the critical roll speed at which ribs are first observed can be increased by employing a larger gap or using a thicker covering of compliant material. For negative gap settings, explored here for the first time, ribbing is observed to be ever present over the operating range considered. However, for large negative gaps, a reduction in roll speed and the use of a thinner compliant covering are all found to produce ribs of shorter wavelength.