Expose Yourself Continued: Coating to Suit
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Expose Yourself Continued: Coating to Suit

When we talk about direct emulsion stencils, the inevitable question arises: How thick a coating of emulsion do you need to get good results? The complete answer to that question depends on a number of factors.

By Bill Stephens

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. Letís take a closer look at some of the key factors in the coating process.

    Putting On Your Coats
    >From the very first time you pick up a scoop coater you will notice that when you apply liquid emulsion to one side of the screen, the pressure tends to force it right through to the opposite side of the mesh. This doesn't mean you can get away with making a single pass. At least one coating should be applied directly to both print side and squeegee side of the screen to insure that both the emulsion layers are as smooth and uniform as possible. Regardless of how many coats you apply, or the sequence in which you apply them, the final coating will always be made on the squeegee side of the screen.

    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
    The stencil also plays an important, if secondary, part in regulating the ink deposit. Unlike the original artwork, which is two-dimensional, a screen stencil will have a third dimension -- depth
    . Each opening is actually enclosed by a wall of emulsion only a few microns high. During a print stroke, these walls form open-topped ink containers a few microns deep. The substrate becomes the bottom and the ink is held in place by the seal between the print side of the screen and the substrate. The combined depth of emulsion coating and the mesh that supports it determines the total ink deposit. Of the two, mesh will have a far greater effect because the thread diameter of the mesh is responsible for most of the depth of those containers. The portion of the ink deposit due to the mesh will never vary. However, the portion determined by the stencil can either be increased or decreased depending on how many of coats emulsion you apply or the type of emulsion used.

    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.

    The number of emulsion coats can also be taken to extremes. If two coats per side are good, four coats are not necessarily better. You can easily build up a layer of emulsion so thick that it will be almost impossible to dry or expose properly. Even if you do manage to expose them, coatings that are too thick also produce poor printing results. Some ink will stick to the inside of those walls of emulsion -- those containers we referred to earlier -- rather like dry paint building up on the inside of a paint can. As ink builds up on the stencil, the openings become smaller. This problem shows up most clearly when printing fine details or thin lines.

    Developing A Standard Coating Practice
    Because so much depends on the emulsion coating being neither too thick nor too thin, it is important to develop a standard coating practice and make sure that everyone in your shop sticks to it. This will not only help to insure consistent print results, but will enable you to do effective troubleshooting should problems arise and to make changes, when specialized jobs dictate a different coating method.

    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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