Printing Halftones (Part III): Making Halftone Stencils with Photoreactive Film - The Online Magazine for the Sign Trade.
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Printing Halftones (Part III): Making Halftone Stencils with Photoreactive Film

Despite the many advantages of using films for halftone work, liquid direct emulsions continue to be very popular and also have a few advantages of their own. Discover the many advantages and disadvantages of halftone stencils.

By Bill Stephens

Halftone stencils are produced only from photoreactive emulsions, these emulsions become hardened and water-resistant when exposed to ultraviolet light. The hardening allows screenprinting stencils to withstand the abrasion that takes place during the printing process.

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  • Halftone stencils undergo a considerable soaking during the washing out process as well, so water-resistance also comes in handy. Washing out takes place immediately after exposure and it requires drenching the screen with generous quantities of water to wash away unexposed areas of emulsion. Unexposed emulsion remains soft and dissolves on contact with water. Every stencil contains at least some unexposed emulsion that has been shielded from the UV by the opaque areas of the film positive. Once this emulsion has been washed away, it leaves behind openings in the stencil through which ink will pass when the screen is printed.

    Screenprinting emulsions are usually categorized by their photo-sensitizers. Diazo is probably the most familiar. Diazo emulsions consist of diazo sensitizer added to a base of polyvinyl acetate/polyvinyl glycol. There are pure photopolymer emulsions as well. In these, the entire emulsion is light sensitive and no additional sensitizer has to be added. Pure photopolymers require the shortest exposure times of any screenprinting emulsion. Dual-cure emulsions are a hybrid of the other two types. In dual-cures, a diazo sensitizer is added to a base, which contains some photoreactive elements. To some extent they also combine the virtues of the other two.

    Screenprinting emulsions can come in both liquid and dry form. The dry form is called film. In this case, the pre-sensitized photoreactive emulsion has been spread out on a clear polyester-backing sheet. Since it is applied under factory conditions, the thickness and evenness of the emulsion coating can be very precisely controlled, a precision that’s almost impossible to match with a liquid direct emulsion. This is one of the reasons film stencils are noted for producing remarkably sharp imprints, a quality which makes them a natural choice for precision work like printing halftones. Even printers who use direct emulsions for most of their other work often switch to film for highly detailed jobs.

    Films are also easy to apply and can be ready to go onto the press much sooner than direct emulsions. Films are applied to wet mesh, which means they can go directly onto screens straight from the degreasing or reclaiming process. They also dry more rapidly because -unlike direct emulsions- the films themselves contain no moisture. The only water that has to be evaporated is what the mesh holds. Liquid emulsions, on the other hand, have to be applied to dry mesh and the emulsions themselves are at least 50% water. Coated screens also have to be dried fairly slowly to avoid trapping moisture within the emulsion.

    Films come in two different types, direct and indirect. Direct films resemble direct emulsions in that they are applied to the screen first, then exposed. Indirect films are exposed first and then applied to the mesh. Although less popular than either direct emulsions or direct films, indirect films offer one key advantage: since they are exposed off the screen, the mesh can't interfere with the exposure, which means no moiré problems.

    Films offer several advantages for halftone printing, in Printing Halftones (Part Two) Two Critical Factors in Halftone Stencils, we learned about the importance of controlling stencil thickness. Since films essentially have their thickness built-in at the time of manufacture, controlling stencil thickness is mostly a matter of buying the right film. Films are available in thicknesses that range anywhere from 10 to 1000 microns.

    The thickness most commonly used for halftone work is usually 20 microns or under. It should be noted that final stencil thickness will be somewhat less than film thickness since film is slightly absorbed into the mesh during application. Furthermore, application methods will also have a major impact on stencil thickness.

    The second critical advantage films have for halftone printing is their remarkably low Rz values. Rz is the measurement of the stencil's surface. The smoother the surface, the lower the Rz value. Note that this applies only to the surface on the print side of the screen. Of course, that is the surface that comes into contact with the substrate and a smoother surface generally improves the quality of the seal between the stencil and the substrate. A good seal keeps the ink constrained within the limits formed by the stencil opening. It is a very brief seal that endures only during the momentary contact between stencil and substrate as the squeegee passes over the screen. In halftone printing, we have to reproduce very tiny dots and reproduce them accurately, which means the quality of that seal is extremely important.

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    Films do have two strikes against them. Firstly, they are not as rugged as direct emulsions. Generally, this means they are the second choice for longer print runs. Secondly, their bond to the mesh tends to be relatively weak, which means the stencil stands an increased risk of delamination, or coming off the mesh. Indirect films are especially vulnerable, which means they are pretty much limited to shorter print runs.

    Some of these delamination problems can be traced back to improper application. Skipping the degreasing step, for example, is a sure way to ask for trouble. Degreasing is an essential first step whether you are using films, or direct emulsions, but it seems to be especially important with film.

    Monofilament threads, especially new ones, tend to be rather smooth, difficult for film to hold onto. Abrading the mesh, or roughening up those threads a little can give the film more of a surface to grip. Abrading is usually done by hand with a commercial product designed especially for this purpose. Look in your suppliers' catalogs under abraders. Some are combined with degreasers that allow you to get two jobs done in one step and using only one product.

    Applying indirect film
    Film application methods vary from manufacturer to manufacturer, thus it is crucial to always follow the instructions provided with your film. However, the steps outlined below should give you a general idea of what's involved.

    Indirect films are exposed with the backing sheet closest to the light source (the backing sheet is the shiny side, the emulsion side is the duller of the two). This means that the backing sheet, not the emulsion, goes against the emulsion side of the film positive. This is exactly opposite to the placement of the positive when exposing direct films and direct emulsions, where emulsion on the screen should be in direct contact with the emulsion on the positive.

    Correct exposure times for various types of exposure units will be listed in the film manufacturer's specifications. However, exposure calculators should always be used, since the UV output of most exposure units will vary over time. Many printers who use indirect film use step-wedge calculators. For indirect film stencils, as for any kind of halftone work, you need the services of a sufficiently powerful single-point exposure unit with a high-output UV bulb.

    After the film is exposed, the next step is to develop it. The usual developer is a solution of hydrogen peroxide, although manufacturers sometimes have their own developing chemicals. Always use manufacturer’s products when available. Water temperature and timing are important as well, you don’t want to over develop the film. Follow the manufacturer’s recommendations and be very careful, as indirect film emulsion is very soft at this stage. Use a gentle spray. Rely on quantity of water, rather than force. After the unexposed emulsion has been washed away, you will be left with the stencil attached to the clear polyester-backing sheet. The next step is to apply that stencil to your stretched, degreased screen. Your working area should be equipped with a hard, flat, preferably waterproof, surface small enough to fit inside the frame of the screen. One-quarter-inch-thick plate glass works well.

    Place the film, backing sheet down, on top of the plate. Wet the mesh in the screen. Place screen print side down over the waiting film and lower it until it comes into contact with the emulsion. Once it has settled into the emulsion, pat the surface with clean, unprinted newsprint using light, but firm, pressure to force the stencil into the mesh. Remember that the emulsion is very soft at this point, so easy does it. Replace newsprint sheets as they become saturated and make sure you work the entire surface.

    When most of the water has been removed, set the screen aside to dry. Once it is dry, apply blockout (screen filler) to open non-printing areas. When the blockout dries your screen is ready for the press. Note that indirect films cannot be used with water-based inks; solvent-based or UV inks are fine.

    Direct or capillary film
    Remember, direct films are photosensitive and should be applied under safelight conditions. Begin with clean, degreased mesh, a good rule for any type of stencil application. Since direct film is exposed on the screen, colored mesh should be used to minimize light scatter. Direct film can be applied to screens emerging sopping wet from the degreasing tank. Using a wetting agent (sometimes called a surfactant) insures the screen retains the maximum amount of water. This is important because water is one of the key factors in promoting good bonding between mesh and film, so the more water the screen holds the better. The direct film itself is dry so there is only the moisture in the screen to soften the emulsion enough to help it bond to the mesh.

    Direct films are frequently referred to as capillary films, because they are literally drawn up into the mesh through capillary action. Once you bring the wet mesh into contact with the emulsion side of the film the rest is pretty much automatic. There are two ways of bringing the two together. With small screens, place the film emulsion side up on a flat surface like a tabletop or sheet of plate glass. The damp screen is positioned print-side-down and lowered onto the emulsion. A little pressure may be necessary to complete the bonding process. A light window-cleaning type squeegee can be used on the squeegee side of the mesh to get rid of some of the excess water. Set the screen aside for a few minutes to dry then carefully peel away the backing sheet and allow the emulsion to continue drying. Removing the backing sheet will allow moisture to evaporate more readily and speed up the drying process. For larger screens, it is easier just to unroll the film onto the mesh. Start with the film cut to size and rolled up, with the emulsion side outward. Some manufacturers suggest supporting large pieces of film by rolling them around a rigid hollow plastic tube. Wet the mesh and position the screen with the print side facing you, the screen should be leaned against a firm support, such as a wall. Then you simply start at the top of the frame, bring the film into contact with the wet mesh, and slowly allow it to unroll down the screen. If some areas are not adhering properly, use a spray bottle to rewet those spots. Allow the screen to dry a few minutes and carefully peel away the backing sheet. The backing sheet must be removed and the stencil should be completely dry before exposure takes place.

    Despite the many advantages of using films for halftone work, liquid direct emulsions continue to be very popular, they also have a few advantages of their own. Next time, we’ll cover direct emulsions, inks, and squeegees as we wrap up this series on printing halftones.

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