This primer describes the importance of using high-quality lights and art in the stencilmaking process.
The essence of screen printing is a stencil that blocks the flow of ink to non-image areas of the print and is open in image areas, allowing ink to flow freely through the mesh. The integrity of the stencil material and the uncompromised openness of the image areas are equally essential. In addition, there should be a clear demarcation between image and non-image areas. These attributes originate with the artwork.
Artwork refers to any imaged media placed between a light source and photostencil film or emulsion during exposure. Most photographic stencils are contact-exposed in a vacuum frame, so the dimensions of the artwork are the same as those of the stencil and the final print. Imagesetter-exposed silver film (right-reading photographic positives), laser toner (usually on vellum), inkjet on inkjet film, thermal images, and knife-cut masking film are examples of commercial media used for contact exposure.
Photostencils can never be better than the artwork used to generate them, nor can the final print be better than the stencil. If the dark areas of the artwork lack sufficient density to block light during exposure, or if they contain pinholes or thin spots, stencil material will harden in what should be the open image areas of the stencil, and ink will not flow readily through the image areas of the stencil during printing.
Similarly, if clear areas of the artwork are, at best, translucent rather than transparent, or contain artifacts such as dirt or dust, light energy will be filtered or blocked before it reaches the stencil material; non-image areas of the stencil won’t be fully cured or crosslinked and may break down during the printing run. Finally, if the delineation of image and non-image areas is pixilated, fuzzy, or lacking in acutance (edge sharpness or definition), so will the image edges on the final print.
This article was provided by Don Marsden, Ulano Corp., Brooklyn, NY.
• For imagesetter-exposed silver film, any text or other information should be on the emulsion side of the positive and appear as you will want it to look on the final print; the emulsion side of the positive is placed against the stencil material during exposure.
• Projection exposure requires silver film positives that are considerably smaller than the final size of the print. Such positives must be of very good quality, as projection by its nature enlarges imperfections. Computer-to-screen systems don’t use artwork as we’ve defined it here. These systems either apply an image directly to the stencil material via inkjet deposition or with a digital-light-processing system to expose a screen pixel by pixel (digital direct exposure).
• The dark and clear areas of artwork can be measured on a densitometer. The opacity of the dark areas is referred to as D-max (maximum density) followed by the numeric densitometer reading. D-min refers to the minimum density, which is a measure of the clarity of the artwork. D-max 4.0 is ideal for stencilmaking, as it allows full exposure of non-image areas even if the artwork medium is cloudy or fogged.
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