Wouldn’t life be great if complex things could be reduced to simple functions? In the world of coating, if you look deep enough, you can find some of these useful nuggets.
In self-metered coating, the liquid flow can be studied as a collection of bulk flow and flow along surfaces. At the surface, there are forces fighting over whether the fluid should move forward, sit still, or swirl in place. But why is this important?
Let’s look at the final coated product and work our way back to the equation. Let’s assume you are coating an emulsion adhesive and there is a pesky streak on the coated web. Where did it come from? There are many potential factors, including dirt, gels, bubbles, a nick in the flow surface, or one of many equipment issues.
To decide where to start, you would want to eliminate the obvious first and work your way to the more complicated. Stop the coating line, check the manifold for particles stuck in the gaps and loading of filters in the fluid flow lines. Inspect the flow surfaces and clean the system. Now that these are eliminated, what can we look at if the streak persists?
One area to consider is the geometry of flow within the slot die. The flow channels of a slot die can be simplified and studied like an air duct. When the air duct is a rectangular shape, like the exit of the slot die onto a substrate, the following equation applies:
τw = wall sheer stress
μ = viscosity
V = volumetric flow rate
b = slot gap
When wall sheer stress is greater than the shear stress through the system, then particles will choose to move forward instead of sit on the flow surface. The bulk shear stress for most fluids is approximately 1 N/m2. This means that agglomeration of particles will not occur and streaks will not form if the wall shear stress is greater than this level.
This equation typically requires computer simulation to verify, but the power of the equation is simple – if the volumetric flow rate is too low, viscosity too low, or gap too large, the wall sheer stress may not be enough to clean the system.
If you have consistent streaks in the coating, consider increasing the flow rate, viscosity, or narrowing the gap. If the equation is balanced, the contamination will be less likely to occur, and splitting and cleaning the die will not be as critical a step between product changes.
But remember: these factors need to be balanced with the other needs of the flow control. Slot die operation is a balance between variables and control factors that can develop an excellent thin film or one full of coating defects. Pressure drop, fluid velocity, volumetric flow rate, and shear effects need to be in the proper coating window for optimized operation.