Learn how to make other types of artwork fit the screen-printing mold.
These proofs show halftone dots the way they will appear on the film separations. The physical dot gain you can expect on press is represented by optical dot gain (higher color density) on the proof. Since most proofing systems are designed for offset printing, the standard optical gain is typically about 20%, which means the colors on the proof print will appear 20% darker than on the actual printed piece. Overlay and laminant proofing materials work for offset litho because the dot-gain profile is uniform. Since it is uniform, we can use an optical approximation to represent the actual physical dot gain likely to occur from screen printing.
Screen printers often get into trouble by matching their process-color ink densities to the color printed on the proofing material. This is usually done by matching the color strength of each of the solid CMYK values on the proofing media. In the end, the final screen-printed graphic may end up being 40% darker than the original image because of the 20% proofing adjustment made to the file plus an additional 20% physical gain that naturally occurs on press.
On halftones with line counts above 65 lines/in., the accuracy of our proofs begins to falter due to changing dot-gain profiles. Dot gain when screen printing at finer line counts results in a loss of highlight and shadow information, and more gain in the quarter-tone, mid-tone, and three-quarter-tone areas than the proof can show.
The applications most likely to face these circumstances include fine-line halftones used on compact discs, fine-art prints, and other highly detailed applications that are viewed at close distances. If you attempt to use a conventional analog proof for such images, the highlights and shadows will print too light, and the middle values of the image will be too dark. The overall visual effect will be a very high contrast image with major shifts in neutral colors, such as gray, beige, tan, and brown.
If you have an accurate profile of the gain that occurs on press, you may be able to adjust the proofing system to represent this profile. But even then, many proofing devices lay down the images with a continuous, fine inkjet spray or dye sublimation. Neither of these proof types show the halftone dot structure, which limits their usefulness to screen printers.
<P>The only real solution is to produce the proof image on a screen printing press. If the job in question will be a high-volume run, this is a very viable solution. It's the only way to be assured that the printed piece will meet client expectations.
<P>To provide our customers with the printed graphics they expect, we have to take control of file preparation, even when they (or their designers) provide the artwork. And one of the first areas we need to focus on is educating our production and sales personnel about what we expect to see in customers' digital files and why.
<P>We can help our customers produce more useful files in number of ways. For example, we can establish our own internal specifications or guidelines and submit them in writing to our clients. We also can offer workshops and seminars on file preparation and explain why screen printing is different from other printing processes. Finally, after we've done our magic and produced the job, we can provide personal feedback to the designers or agencies that created the graphic files, pointing out the problems we discovered and recommending how they can optimize the images for screen printing in the future.
<P>Mostly, however, we remain on our own when it comes to fine-tuning digital artwork for screen printing. So we need to develop standard procedures for checking and correcting (also called preflighting) the files, which will help us uncover problem areas quickly and streamline file preparation. All files should go through this procedure as soon as they arrive--not days or weeks later. This way, if there is an unresolvable issue with the file, we or the customer may be able to correct the problem without sacrificing the job deadline. <P><B>About the author</B>
<P>Mark A. Coudray is president of Coudray Graphic Technologies, San Luis Obispo, CA. He has served as a director of the Screenprinting and Graphic Imaging Association International (SGIA) and as chairman of the Academy of Screenprinting Technology. Coudray has authored more than 70 papers and articles over the last 20 years, and he received the SGIA's Swormstedt Award in 1992 and 1994. He covers electronic prepress issues bimonthly in Screen Printing magazine. He can be reached via e-mail at firstname.lastname@example.org or email@example.com.
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