Coating, Casting, and Curtains
- Published: January 30, 2015, By Mark Miller
If you'd like to hear from Mark Miller's own lips rather than read his blog, titled "Coating, Casting, and Curtains," click on his podcast below:
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When a slot die is coating a fluid, does it care which way it is pointed? No, it does not. So why are different terms utilized when the same piece of equipment is pointing up in the air, down towards the ground, or horizontally towards the wall? It has more to do with physics and fluid flow than the direction of coating. There are advantages and disadvantages in the direction you point your coating equipment, so let’s walk through each one here. But remember, the equipment doesn’t change, just the direction does.
Coating. In direct coating, the slot die coating head is pointing in a horizontal arrangement towards the substrate. The fluid forms a barrier between the metal face of the slot die lips and the substrate that is accepting the fluid being delivered. The viscous forces that control fluid flow at the exit point of the slot die control the resulting coated product performance and appearance.
The advantages include close proximity gap control and ease of setup and operation. The fluid will coat the width prescribed by the coating head and mechanical control of the coating bead (or meniscus) can be dictated by the lip face geometry, attack angle and offset of the upper and lower bodies of the slot die.
The disadvantages include the close proximity of the slot die to the substrate, not allowing for large variation in the substrate or very low coat weights. Lower coat weights require moving closer and closer to the substrate, possibly causing tear outs or damaging the equipment.
Curtain. In curtain coating, the slot die is positioned above the substrate from quite a distance. Actually, the way the physics of fluid flow works, there is a minimum distance that the slot die needs to be away from the substrate so gravity can do it’s work. The speed of the substrate also comes into play, as there is a minimum coat speed to form a solid coating or else the fluid puddles and does not spread evenly.
V = U * h > Vmin = 2σ/ρVcurtain
Vcurtain = (2gh)0.5 free fall estimation
h = height of slot die from substrate
g = gravity
U = substrate speed
σ = surface energy
ρ = density
V = volumetric flow through the slot die
The advantages of curtain coating include the ability to coat from a distance, allowing substrates with larger variation to pass undisturbed past the coating head. In addition, because of the minimum speed required, curtain coating is typically completed at higher line speeds than typical coating.
The main disadvantages include the neck-in that occurs when dropping fluid from a distance and the sensitivity to disturbances in air or tension. The neck-in can be overcome through the use of side rails that carry the fluid to the substrate.
Casting. When you have learned enough about coating sideways and coating downwards, you might as well stick the slot die up in the air and coat like a teapot. Casting fluid down a slot die allows for a controlled curtain coating and lay down of multiple layers at once. Previously utilized in photographic film manufacturing, casting (or slide coating as it is more commonly called) is a multilayer coating technique.
The advantages of slide coating are similar to curtain coating with the added advantage of cascading multiple chemistries onto a substrate at once.
The main disadvantages mirror curtain coating with the added issue of requiring a chemist to develop an understanding of what viscosities will layer without mixing. To have the fluids stay un-mixed, the surface energy of the top layer needs to be lower than the surface energy of the bottom layer.
Well, which ever way you want to point your coating head, consider the pros and cons and develop a technique that works best for you. Coating, casting and curtains all have there place in the converting industry, but remember that the tooling doesn’t change, just the way you point the tool does.
