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![]() Expose Yourself Continued: Coating to Suit
One thing, however, will always remain true: The coating must completely enclose the mesh --both sides. Neither substrate nor squeegee should ever come into direct contact with bare mesh. The acronym-loving screenprinting world has even developed a special term that refers to the thickness of the emulsion covering, EOM or Emulsion Over Mesh.
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Two things determine EOM: the emulsion itself and the coating process. Let’s take a closer look at some of the key factors in the coating process.
Putting On Your Coats Most of the emulsion coating has to end up on the print side because the stencil has to be thicker on that side of the screen. Screenprinters refer to it as forming a gasket between the screen and the substrate. That's a useful idea because a sharp imprint depends on this seal being perfect. Though perfect, it is quite temporary, being formed and released only as the squeegee passes over the opposite side of the screen. To create an effective gasket, more of the stencil has to be positioned on the print side of the screen. This is easily arranged as you coat your screens. Simply apply the last pass of the scoop coater to the squeegee side of the screen and you'll automatically force most of the emulsion through to the print side. When the screen has been exposed and washed out, the resulting stencil will have open areas that correspond to the printable areas of the artwork. The squeegee pushes ink into these openings, and the seal formed between the substrate and the screen limits it to only those areas. At least, that's what happens when the seal is perfect. If it is less than perfect, ink can leak under the edges and produce a fuzzy-edged or blurred imprint.
Stencils Have Depth Too
One popular method of increasing the depth of a stencil is to apply successive coats of emulsion, one on top of the other. Each additional coat will marginally increase the overall thickness of the emulsion layer. This practice is referred to as wet-on-wet coating
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But there are other reasons for applying more than one coat. Multiple passes with the coating trough are often used to make sure that all openings in the print area of the mesh are completely filled with emulsion. A single pass of the coating trough may leave behind voids caused by air bubbles. No matter how carefully you mix or pour emulsions, air can become trapped in them. As the coater moves up the screen, the trapped air finds its way out in the form of bubbles in the openings of the mesh. After a few seconds, these bubbles pop, leaving voids. A second coat will fill them in. Think of these subsequent coats as cheap insurance.
Tip:
Developing A Standard Coating Practice A lot depends on the emulsion you're using because the emulsion, at least in part, determines your coating practices. Large shops that do a wide range of printing may use several different types of emulsion depending on the mesh and the substrate to be printed. For small to medium shops, this is impractical because emulsion has a shelf life of a few months at best and it gets expensive to have half-used containers of several different types sitting around. Fortunately, printing operations working with a more limited range of substrates can generally make do with one type of emulsion, even it they have to occasionally change coating methods to adapt it to the job at hand. Sign shops printing with air-dry inks will most often work with a fairly narrow range of mesh counts usually falling somewhere between 200 threads per inch up to the low 300s. The advantage for a shop working within such limits is that a single emulsion should prove adequate for all their needs. This also makes it fairly simple to develop a standard coating method. Coating practices may vary from shop to shop, depending on the mesh and type of emulsion being used, but within the shop itself consistency is the rule. Many screenprinters maintain a general practice of coating 2 + 2, that is two wet-on-wet coats to the print side of the screen, then rotating the screen and applying two coats on the squeegee side. Others prefer a 2 + 3 coating sequence, which means the final three coats are applied to the squeegee side of the screen. The number and sequence of the coats depend on the emulsion and the type of mesh used, but the general rule is to apply no more emulsion than you need to do the job. Apart from the number of coats, other factors in the coating process can affect the thickness of the emulsion coating: how fast you pull the coating trough up the screen, how firmly you hold it against the mesh, and with some coating troughs, whether you use the rounded or sharp edge. The coatings produced by the two sides vary markedly in thickness, and some screenprinters become quite passionate about which edge is the proper one to use. Some prefer the rounded edge for everything but a process called face coating, where additional coats of emulsion are applied to a screen already coated and dried to build up the thickness of the emulsion and provide a super smooth surface. Others use the sharp edge almost exclusively, preferring the thinner emulsion layers it lays down. There is considerable difference of opinion on the matter, but almost everyone agrees that sticking to a standard coating procedure helps control at least one important variable. If special circumstances make alterations to the coating process necessary, you're in a lot better position to make those changes if you already know exactly how your original screens were coated. The first step in establishing your own coating procedure is to check with the emulsion manufacturer for information on how to produce the best results. And, as always, be sure to make test prints before starting your production runs. Next time we will take a closer look at the emulsions themselves and give you some hints about what to consider when choosing an emulsion to use in your shop.
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