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Preparing Digital Files for Screen Printing

(October 2002) posted on Wed Oct 09, 2002

Learn how to make other types of artwork fit the screen-printing mold.

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By Mark A. Coudray

Now imagine a customer provided the same image for reproduction as a 12 x 18-in., 85-line/in. screen-printed poster. The image would need to be expanded to three times its original size, which would drop the resolution to 1/3 the original value or 100 dpi. At 100 dpi, the maximum line count you print while maintaining image accuracy is 50 lines/ in., not the 85 lines/in. requested by the customer.


For screen printers, the solution is to interpolate the image. This means relying on software to boost the apparent resolution so that the image contains enough information to be enlarged and support printing at a higher line count. But interpolation really only provides an approximation of the image at a higher resolution, which is a compromise rather than a solution. Fortunately, a new option to correct this problem has recently made its appearance.


Color-separation hurdles


With files we receive in CMYK form, all color-separation parameters, including screen angles, dot gain, gray balance, and more, are already established. If the file adheres to lithographic parameters, which is likely the case, you can probably use it as is, provided the line count of the screen-printed graphic is 50 lines/in. or lower. For applications such as large-format and P-O-P display work, lines counts in this range are frequently adequate because the image is viewed at a distance and the dots are large enough to maintain the tonal-range and dot-gain profiles used in litho.


But we begin to run into problems when the line count increases to 65 lines/in. or beyond. Even if we use fine mesh counts and print UV ink, the extreme highlight and shadow dots will become increasing difficult to hold. The dot-gain profile transforms from a traditional, uniform litho profile (Figure 1A)  to the more typical screen-print profile (Figure 1B) as the line count increases. The higher the line count, the greater the influence on the dot-gain profile and the bigger the difference in the printed file's appearance.


To minimize the problems, always ask for both the CMYK and RGB versions of the file. Regardless of how the CMYK file was produced (whether it was created in Photoshop or captured with a high-end scanner), the separation parameters are not easily changed without damaging the color information in the file.



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