Excellence in stencilmaking yields screens that stand up to production wear and produce detailed, high-quality graphics. Find out how to regulate the instabilities in the process that can lead to less than desirable results.
By Ross Balfour
Correct exposure is of paramount importance in optimizing stencil performance, no matter which type of stencil system you plan to use. Producing a screen-printing stencil, even for use with the fine mesh counts designed for printing halftones, involves exposing a coating that is very thick in comparison to those used for other photographic or imaging processes. Because of this, depth of cure through the stencil becomes a real issue.
Poor through-cure, or underexposure, will cause one or more of the following problems: loss of detail during processing, excessive pinholes, scum leaking into and then blocking image areas, premature stencil breakdown during printing or cleanup, and last but not least, difficult or impossible reclaim. Remember, we’re talking about expensive screen mesh.
Overexposure, in comparison, causes detail to shrink on the screen, with eventual loss of parts of the image altogether. This effect is usually most severe and easily noticeable with halftones.
A minimum vacuum pressure of 20 in. Hg in the exposure frame is required to ensure good enough contact between film positives and the screen during exposure. This prevents the undercutting of the image, and subsequent loss of detail, that occurs when light leaks under the positive. A good light source fitted with a metal-halide bulb is recommended to produce optimum results. The output spectrum of such a bulb is a good match for the maximum sensitivity of most stencil materials. It’s also important to have the placement of the lamp—and the reflector design—optimized so as to ensure even coverage of the entire image area during exposure.
Even coverage is essential for accurate reproduction of the image, as well as stencil durability. If coverage is very uneven, then the exposure latitude of the stencil material may be exceeded and areas of the screen may be either under or overexposed, or sometimes even both on the same screen. In this respect, dual-cure emulsions possess the widest exposure latitude, although being very similar overall to diazo products in optimum exposure time. Photopolymer emulsions, because they expose in a fraction of the time and have inherently much less latitude, really do require more even exposure intensity in order to produce consistent results.
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