- Air pulled in at dynamic wetting line
UNSTABLE CROSSWEB BANDS – this variable crossweb band pattern is associated with oscillation or instability in the coating bead. (one example of air issues)
Air that is introduced into the fluid prior to the coating head is entrained. I remember the term by thinking about the fact that the bubble is already on the “train track” before it enters the coating head. You are probably already thinking this, but it is easier to keep air out than to remove it once it is in the system.
So how does air enter the system prior to the coating head? One way is through excessive agitation of the liquids in the mixing tank. This can be reduced through proper mixing techniques and preparing batches ahead of time to allow settling prior to the coating run.
It is also possible to place a nitrogen blanket over the fluid to remove air. If the liquid moves from one vessel into another, before the coating head, both vessels need to have air reduction measures.
The fluid moves, right? So when the fluid passes a seal, make sure the seal does not leak and cause air entrainment. Lastly, make sure the fluid moves uphill as much as possible. Any bends in the pipe/tubing will allow air to settle. This curved settlement area for the air will slowly leak into the coating head, showing up as a random and difficult to diagnose bubble problem. If a curve in the tube or pipe is necessary, a relief valve will help.
Temperature and pressure also can be your friend. If the process and materials allow, you can heat the fluid up and pull a vacuum. These process conditions will help reduce bubble formation prior to the fluid traveling to the coating head. Ultrasonic measures also have been utilized to reduce bubbles, but these have had mixed results.