Meaning
A surface defect characterized by periodic ridges or lines oriented in the direction of substrate travel indicates an instability in the fluid meniscus. Ribbing occurs when the capillary forces at the coating head overcome the viscous forces that should keep the film flat. This pattern creates a variation in coating thickness that can ruin the performance of the product.
It is a common problem in roller coating and slot die application.
Surface Instability
Operating a coating line above a certain speed often triggers the ribbing defect as the meniscus becomes unstable. The distance between the ridges is determined by the gap between the die and the web. If the gap is too large, the fluid cannot maintain a stable bridge and the pattern forms.
Pattern Defect
Identifying this issue early is necessary because it is often invisible to the naked eye at high production speeds. Laser scanners and high speed cameras are used to detect the thickness variations in real time. If ribbing is detected, the operator must adjust the pump pressure or the web tension to stabilize the flow.
Coating Uniformity
Achieving a uniform layer is impossible once the ribbing instability has started. In battery manufacturing, these ridges lead to uneven lithium ion distribution and hotspots during charging. The cost of calling a run early is high, but the cost of shipping ribbed material is the total loss of the batch.
Ribbing is suppressed by increasing the viscosity of the coating fluid or decreasing the coating gap.
