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Thoughts on Coating Technique


If you learned how to coat mesh with liquid direct emulsion from an old book, or by listening to other novices (and a few experienced screenmakers) on social media, you are probably having problems. And the bad news is that you may not even know you have problems, because it seems that everyone you know has the same outcome. This has led you to believe that the norm is to have the same experiences as many others. But, it does not have to be that way. Some of the signs of poor coating practices are…

  • Making multiple strokes to get the necessary opacity
  • Having to resort to a print/flash/print technique
  • Blocking out pinholes in your stencil
  • Experiencing jagged edges on your image rather than sharp edges
  • Having your stencil breakdown prematurely on press

Now the good news! You can improve your coating method and all of these problems can be eradicated from your life.

Most of the best screenmakers that I know coat their mesh with the correct Emulsion Over Mesh Ratio (EMOR) in mind. They are more concerned with printed image quality and are coating for a specific stencil thickness. This also allows them to make a stencil that does not require multiple strokes to achieve opacity.

So, how do they coat? They make what is considered a base coat which completely fills the mesh openings, but without the necessary build up. They allow this base coat to dry.

Once dry, they measure any EMOR that is present with an electronic thickness gauge. They then start building up the thickness on the bottom of the stencil (opposite the squeegee side) with thin coats that dry very quickly. This also promotes a better Rz factor (smoothness of the stencil) that increases the resolution of the image edge.

When they are close to the correct EOMR, they make a face coat on the squeegee side of the stencil to create what I coined as an Anti-Friction Coating in 1983, which allows for the squeegee to move fast across the surface as it shears the ink from the top of the mesh openings. As well, this Anti-Friction Coating decreases the “chattering” of the mesh caused by a squeegee that “drags” across the surface of the stencil. This creates two additional benefits. It improves the resolution of the ink flow at the image edge, and it decreases the opportunity for mesh movement which is reflected by a slight shadow in the printed image on the mesh that is not correctly tensioned.

Yes, it seemingly takes longer to prepare a stencil that works well, it is time well spent if one wants and needs to improve the quality of their printed images. When one benefits cost, the care that goes into the stencil not only creates benefits, it has a tremendous return on investment.

Choosing a Coating Method

There are many ways in which one can do things. And, for the sake of argument, there is no incorrect way if you the way you choose meets your goal.

For example, if you were in Los Angeles and wanted to go to New York, who is to say that going west would be a bad decision for a way to get to New York. It would probably be a more scenic route and you would have many adventures along the way. Or you could go north up into Canada and then head east. Or you could go south and turn around and head north when you arrived at Ushuaia. That would be a very scenic route as well. The problem with all of these and more routes that you might take is that they take up a lot of time and the cost is much higher than using a more direct route.

I have seen screenmakers take many different routes to accomplish the goal of producing a simple stencil. And, for them, their route produced no issues for them. They were happy with the route they chose and continue along their way over many years.

However, the most successful screenmakers take the shortest route to achieve the prescribed goal of producing an outstanding stencil – and taken together as a majority, they are doing what is known as the Best Practice way.

Drying the Emulsion

Continuing with our Best Practice method, let’s discuss the drying of the emulsion. Just as in the coating technique there are many ways to dry the emulsion.

Moisture Content

If you do not allow the emulsion to dry thoroughly before making the exposure, the resulting exposure will take more time as the sensitizer cannot properly crosslink with the alcohol in the emulsion there is moisture within the emulsion. Your exposure, if set on a completely dry stencil (with 4 to 6% moisture level), is 45 seconds, and you attempt to expose a stencil for 45 seconds that has a 20 percent moisture level, the stencil will not be fully exposed. The detail in the stencil will suffer and you risk having the stencil delaminate from the mesh during development or worse yet, on the press during the print run.

The solution is to invest in a moisture meter so that you can know the moisture level prior to exposure. This will assure that you will take stencils that are properly dried to the exposure unit and that your stencils will be properly exposed.

Temperature

All emulsions have a sensitizer that is sensitive to UV energy, but also subject to cross hardening when exposed to temperatures above approximately 104˚F (40˚C). You should never allow the emulsion to be heated past the manufacturers suggested temperatures as this will pre-harden the emulsion and cause the stencil to not expose or develop properly.

Screen Alignment During Drying

There are three ways to align the screen when drying the emulsion. Horizontally with the squeegee side up, horizontally with the squeegee side down, and even vertical. If your only goal is to dry the emulsion any alignment will allow the emulsion to dry.

However, when drying a stencil made with a direct, liquid emulsion the Best Practice way would be horizontal with the squeegee side up. This allows the wet emulsion to move down to the bottom of the mesh and produce a better Emulsion Over Mesh Ratio (EOMR) on the bottom of the mesh, which in turns provides for a shoulder to stop the flow of ink and form a higher resolution edge definition in the final print.

If you dry the direct, liquid emulsion in a vertical alignment, you risk having the emulsion move down the mesh while it is drying. This can produce a slightly thicker thickness of emulsion which increases the EOMR and produces a different thickness of ink. While not a great concern on spot colors with large areas, fine details and halftones would suffer in resolution.

When using a capillary stencil, the Best Practice method would be to dry the screen horizontally with the squeegee side down as the emulsion on the capillary film is not a liquid and the slightly moist emulsion next to the mesh can move down and around the mesh threads to produce a better bond.

In your explanation of the coating method I gave, you will note that the base coat was dried before any additional coats were made. This changed the scenario of how the emulsion would flow in the drying process.

When you coat a mesh using direct, liquid emulsion the liquid is smooth on the surface. However, as it dries, the emulsion flows downward. As the water evaporates from the emulsion the surface is no longer smooth. If you were to dry a stencil coated with direct liquid emulsion and dry the emulsion with the squeegee side down, you would have a smooth surface on the squeegee side, but the emulsion on the print side, which was the top of the drying stencil you would note that the surface is rough. Without a making a final face coat on the print side, your EOMR is mostly on the squeegee side and the resulting printed image would suffer in edge definition.

There is a lot more to the coating technique and this article is only to provide a few thoughts on the subject. There are other articles on this site to help you better understand the coating process.

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