Choosing the Right Coating Methodology
As web coating technologies have become more advanced, choosing the right methodology requires more sophisticated analysis. To get you started on the process, we’ve created an overview of web coating and the different factors to consider.
How does the web coating process work?
In the converting process, web coating replaces a boundary layer gas at a solid surface with a layer of liquid, then solidifies the liquid layer by drying or curing.
It’s a complicated and delicate balance of fluid mechanics and interfacial phenomena.
Each web coating machine will have the same basic features:
1 – substrate unwind
2 – solution delivery
3 – coating applicator
4 – drying/curing oven
Then at the other end of the machine
5 – material rewind
Multiple converting functions can take place within an integrated system. For example, integrate a coater with a drying station at the beginning of the point of the process, and then additional converting stations for die cutting, laser cutting, lamination and/or pouching at the end.
There are essentially two methods of delivering web coating, and within those delivery methods are the coating methods themselves.
- Mechanical Metering
Mechanical metering requires a flooded surface and a mechanical device to wipe away excess and set the thickness of coating. The following coating methods are used with mechanical metering:
Reverse roll coating is considered a roll to roll coating methodology for wet coatings. Its unique feature is the two reverse running nips: An application roller runs in one direction, the support roller in the other.
Coating is applied in excess to the surface of the application roller, and the coating is then regulated at the metering nip.
With comma roll coating, the coating solution is either dispensed directly to the substrate and metered via the comma roll; or the coating solution is metered by the comma roll onto an applicator roll then applied to the substrate.
Coating fluid is dispensed to the substrate, which then passes through a knife and roller. The knife meters the coating to the required specs.
A roller applies the coating fluid to the substrate, and then a Meyer bar meters out the correct amount.
- Volumetric Metering
Volumetric metering allows for the delivery of an exact volume of material to the substrate, which gives you the ability to control the thickness.
With slot die coating, the coating fluid is precisely delivered to the slot die head, then applied directly to the substrate.
With gravure, the coating fluid is delivered onto a roller with patterns engraved into the face of the roll. Excess material is removed with a doctor blade. The volume of fluid in the engraved patterns is transferred to the substrate ensuring the precise amount of coating is applied.
Check out our post for more on slot die coating.
Choosing the right coating methodology
There are several key coating specs you’ll need to consider as you’re choosing from the coating methodologies we’ve just listed.
Viscosity Range – This is a measure of the thickness of the material. Fluid viscosity is measured in centipoises (cps). The more viscous the material, the less runny it will be.
Shear Level – The amount of shearing force that’s put onto the fluid when it’s being coated. If you take a plate with wet glue on it; sandwich it with another plate; then slide one plate but keep the other steady — that’s shearing.
Minimum and Maximum Wet Thickness – Depending on the end product, the thickness may be very thick like heavy-duty adhesives or very thin like window tinting. Each application method has an inherent range of wet thicknesses that work best.
Uniformity – This is the range of how uniform the coating is from edge to edge. If you’re trying to make a clear uniform coating, less variation in overall thickness of material is to your advantage.
Download Delta ModTech’s coating methods comparison chart to see how these methods align with your specs. Naturally, costs and timelines will be the ultimate determining factor, but this comparison chart can get you started in the right direction.