Halftone Stencils: Getting Film Quality with Direct Emulsions
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Halftone Stencils: Getting Film Quality with Direct Emulsions

Film stencils are widely praised for their ability to accurately reproduce fine detail such as the thousands upon thousands of tiny dots that make up a halftone. The key to success lies on the print side of the screen, where the smooth surface of a film stencil provides a tight seal with the surface of the substrate. This seal sharply restricts ink flow, which results in sharply defined prints.

By Bill Stephens

It is all but impossible to match the smoothness and manufactured-in uniformity of this surface with a direct liquid emulsion.

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  • Direct emulsions, as their name implies, are applied directly to the screen. The smoothness and evenness of the resulting stencil depends on a number of variables like the quality of the emulsion itself and the skill of the person handling the coating trough. Direct emulsions also take much longer to dry than films. At best they are more than half water, all of which has to evaporate before the stencil can be exposed. As water leaves, the emulsion coating contracts, leaving behind a rough surface that closely corresponds to the peaks and valleys of the surface of the mesh itself. Getting from this point to a surface that comes close to the smoothness of a film stencil takes time and a bit of skill.

    Still, many printers choose to stick with direct emulsions for their halftone work because it's a stencil making process they know well. In addition, direct emulsion stencils are more durable, making them the first choice for longer print runs. Unlike a film stencil, a direct emulsion completely encloses the mesh threads, giving it a superior grip. Direct emulsions also allow the printer to tailor-make the stencil to fit the job. The thickness of a film stencil is built in at the factory. While films come in a variety of thicknesses, if those predetermined specifications don't match your requirements, you're pretty much out of luck. But because direct emulsion stencils are built up on the screen itself, one coat at a time, it is possible to make tiny incremental adjustments to stencil thickness of as little as a single micron. This ability gives the printer almost perfect control over ink deposit.

    The direct emulsion coatings that come closest to making stencils with film-like uniformity are applied by automatic coating machines. To put one on your shop floor, expect to pay about what you'd pay for a new Cadillac. For that reason smaller shops and part-time screenprinters, like most sign businesses, prefer to rely on a pair of experienced hands on the coating trough. That's not altogether bad news. With some specialized coating techniques you can produce pretty decent halftone stencils manually.

    Choosing a direct emulsion
    Stencil quality isn't entirely a matter of technique. It also depends on the quality of the emulsion itself. For critical work it would be inadvisable to use anything but a top quality emulsion from a reliable manufacturer and follow the instructions on the technical data sheets exactly. There is also the question of suitability. Some emulsions are designed for use with solvent-based inks, others with water-based inks. A few can handle both. Water resistance can also be important even when you're using a solvent based ink -- if you're working in a humid environment, for example.

    Emulsions are evaluated in terms of their ability to reproduce fine detail (resolution) and their ability to accurately reproduce the edges of the artwork as it appears in the positive (definition). The term bridging characteristics describes the emulsion's ability to span the open spaces between mesh threads. A good stencil follows the edges that appear in the positive exactly, even when those edges cut directly across a mesh opening. Poor stencils tend to take the longer zigzag route around the mesh opening because they dare not stray very far from the support of the mesh threads. This creates a saw-toothed edge. In a halftone stencil, this means misshapen dots, and any change in dot size from the original means a change in the tone in the printed image.

    Emulsions are typically rated in terms of viscosity and solids content. Viscosity refers to a liquid's ability to flow. Low viscosity emulsions, for example, flow very easily, which can help fill in the tiny mesh openings that typically occur in the higher thread count meshes normally used for halftone work. Still, that may matter less than solids content when it comes to selecting an emulsion for halftone printing. Emulsions that have a higher solids content generally do better at bridging mesh openings.

    They also produce stencils with a smoother surface. Even at best an emulsion will contain at least 50% water. This means that as this particular emulsion dries, half its volume is going to evaporate into thin air. This causes the coating to contract, shrinking away so that the knuckles where threads overlap stand out and depressions in the emulsion surface mark the mesh openings. The difference between these hills and valleys is expressed in a measurement called Rz value. [See Printing Halftones (Part Two): Two Critical Factors in Halftone Stencils] An emulsion with a higher solids content minimizes these differences. It shrinks less because it contained less water in the first place.

    For all of the above reasons, good quality dual-cure emulsions are currently probably the most popular type for halftone work. Pure photopolymers tend to expose work very quickly, but for halftone work you may get better results with an emulsion that has a wider exposure latitude.

    Coating Sequences: Wet on Wet
    The closer your direct emulsion coating comes to matching the smoothness and evenness of a film stencil, the better job it is likely to do of printing halftones. The quality of the emulsion itself is one factor, but a lot also depends on the coating process.

    There are two distinctly different types of coating methods. The first is known as wet-on-wet. This involves coating the screen with generous amounts of emulsion in the hope of filling in the openings in the mesh as completely as possible. Most printers prefer to use a coating trough with a rounded edge for this. Some coating troughs come with two different edges, a rounded edge and a sharp edge. This is the time to put that rounded edge to work.

    A coating tough should be clean and its edges free of dents and nicks. Make sure itís well filled. You donít want to run out of emulsion halfway up the screen. Once the trough has been filled, the emulsion should be applied without delay. Emulsion left sitting around open is a ready target for any bit of dust that might be settling out of the air. In a low humidity environment open emulsion can begin to form a skin in surprisingly little time, so keep your emulsion in a container until you need it, and after you apply it, wash out your coating trough promptly.

    Once the emulsion meets the mesh, it's time to slow down. Emulsion is fairly viscous, and it needs time to flow into and fill the mesh openings. Opt for a slow steady application speed. Zipping a scoop coater up the screen too quickly can trap air bubbles in the emulsion, and when they pop, they leave open mesh behind.

    Coat the print side of the screen first. The screen should be firmly supported. A coating trough pressed firmly against the mesh can cause a poorly supported screen to wobble, or even fall over.

    Begin your coating pass at the bottom of the screen. Tilt the trough so that the emulsion flows against the mesh, and trying to keep both the pressure even and the trough level, draw it slowly up the screen. Then repeat the process. Apply a second coating directly on top of the first. This second coating fills in any gaps in the first one.

    The next step is to turn the screen and apply several coats to the squeegee side of the screen. There are a number of possible sequences. One that works well is two passes on the print side of the screen followed by three on the squeegee side. Most of the stencil's thickness comes from these wet-on-wet coatings, so the easiest way to produce a thinner stencil is to eliminate one or more of these passes. In any case, the squeegee side is always coated last, because this forces plenty of emulsion through to the print side. This is where the stencil must extend above the threads of the mesh threads, the critical point where it contacts the surface of the substrate.

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    Direct emulsions should be dried in complete darkness or under safelight conditions. The ideal drying position is horizontal with print side down. This helps preserve the evenness of the coating and helps keep most of the emulsion on the print side of the screen, which is where you want it. A pair of strategically placed sawhorses can be used to support all but the very largest screens.

    Direct emulsions must be dried completely before they are exposed. Any lingering dampness within the emulsion will interfere with the chemical reaction that takes place during exposure. In addition to creating an imperfect stencil, this can also result in a screen that is difficult or impossible to reclaim. It is also very important that coated screens are dried slowly. Ill-conceived attempts to speed up the process can trap moisture inside the coating. The emulsion remains damp even though it feels dry to the touch.

    Wet on dry
    Even when this initial coating of emulsion is completely dry, the surface may still be too rough for halftone work. The problem becomes how to transform this into something that resembles the almost perfect smoothness of a film stencil.

    The solution is to put more coats on top of already dry emulsion, the process known as face coating. Face coats are applied using a sharp-edged coating trough, because they don't have to deposit a lot of emulsion on the screen. It only has to fill in the tiny depressions that have formed over the mesh openings as the original emulsion coating dried. The sharp edge of the coater works best. It deposits just the right amount of emulsion to fills in the low points and scrapes the excess away from the high points where mesh threads overlap.

    Face coats are applied only to the print side of the screen. That's where a smooth surface is critical. More than one face coating may be needed, and it is important to allow each to dry before the next coating is applied. In Printing Halftones (Part Two): Two Critical Factors in Halftone Stencils we pointed out that halftones require the thinnest possible stencils, so how do we justify the added thickness from all of this face coating? It's a trade-off. We trade a slight gain in thickness for a significant improvement in the smoothness or Rz value of the stencil surface. In fact, according to manufacturers' data a face coat may add not much more than about a micron to the thickness of the stencil.

    Care and handling direct emulsions
    Another set of variables affects emulsion performance. These relate to handling and storage. If a container of emulsion has been left sitting on a loading dock for an extended period too long at sub zero temperatures don't even bother trying to put it on a screen. Age makes a difference. Pure photopolymer emulsions come pre-sensitized from the manufacturer. These are the champions of longevity and may be good for a year or more. But all emulsions have a finite shelf life. With diazo-sensitized emulsions, including dual-cure varieties, once the sensitizer goes in the clock really starts ticking. The product information sheets emulsion manufacturers provide will give you an approximate shelf life, but if the emulsion has been sitting around for a couple of months, you would be well advised to avoid using it for your most critical work.

    Exposing halftone stencils
    Most of what follows applies equally to film and direct emulsions. When the screen is dry, it's ready to expose. The positive is attached. A couple of pieces of transparent tape will hold it in position on the screen. The screen is placed in the exposure unit equipped with a rubber blanket and a vacuum pump. The vacuum is switched on forcing the blanket tight. Because of its flexibility it wraps tightly around both the flat mesh and the decidedly unflat frame. This forces the positive tightly against the emulsion coating allowing for the most perfect possible translation of artwork to screen.

    The only practical option for exposing halftone screens is a powerful single point light source with a high UV output. Ideally, that output perfectly matches the sensitivity range of the emulsion. This information can be found in the manufacturer's product data sheets. The quality of the exposure depends on the total amount of UV radiation that hits the emulsion. Timing exposure is only a way of controlling this critical quantity. The most accurate exposures are made possible by exposure lights equipped with integrators. These devices measure radiation and shut off the exposure light once the screen has received a predetermined amount.

    Even with an integrator-equipped exposure unit it is advisable to shooting test screens and use an exposure calculator. For one thing, you may not want a perfect exposure. Sometimes perfect exposures fill in detail that an underexposed screen might well hold. Of course none of that detail will do you any good, because underexposed emulsion is soft and the screen may not last out the print run. The solution is to wash out the screen, dry it, and put it back under the exposure light one more time. This little trick is known as post exposures and emulsions subjected to the treatment will continue to harden as long as it retains some unexposed sensitizer. Because of the rapid exposure times of pure photopolymers, this practice is pretty well limited to diazo-sensitized emulsions.

    Washing out
    When washing out your exposed halftone screens use a gentle stream of water. Do not make the mistake of using a pressure washer. Even a blasting spray from an ordinary garden hose can easily tear away soft emulsion and hundreds of tiny halftone dots can end up down the drain. Keep the pressure down, but use lots of water. It takes a lot of washing to make sure those open areas are completely open. As for temperature, the manufacturer may have a recommendation, but it's hard to go wrong by keeping the cold tap turned up.

    If some parts of the image don't open up, it's almost impossible to fix it in the washout tank. Solving the problem means shooting a new screen. First you need to dry the screen you're working on and look at it under at least 30X magnification. That gives you a close up look at the edges of all those little dots where most halftone problems show up. If the dots are crisp and sharp, the screen can be blocked out and put on the press. If they're not, it's time to start tracing the problem by working backward through the process, beginning with exposure, then moving on to coating, mesh and ultimately the positive.

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