The world has gone patch coating crazy! Batteries, pharmaceuticals, even adhesives. The interest in reducing waste and providing functional coating that can be used right off the coating station is very high. With this increased interest in intermittent coating, we also have to consider the technical challenges.

Every coating process has a start and stop. In patch coating, this occurs more often than at the beginning and end of a roll. Of course the steady-state continuous flow in between starting and stopping has been discussed at length and has its own issues to deal with. But what special issues are associated with the start and stop of a coating head? For this discussion, we will concentrate on slot die patch coating. Any shape can be coated-as long as it is a rectangle!

In the start-up and stop flow analysis, the challenge is to reduce waste and defects associated with the transition from fluid flow to not. The considerations of start-up include wetting of the fluid on the substrate, pump control of fluid dynamics, and physical position of the coating equipment to the substrate.

The considerations of stopping are similar to start-up, but need to consider fluid reaction to mechanical and rheologic behavior. The resultant “tail” of the coated patch are a function of the timing between the valve for fluid control, mechanical movement of the coating head, and wettability of the substrate by the fluid.

There is not just one way to coat patches of fluid onto a substrate, but as long as you are aware of the coating fundamentals, you will be successful in your intermittent coating operation.