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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.


By Mark A. Coudray

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Almost all color-separation programs will discard digital color information when converting from the larger RGB color gamut to the more compressed CMYK gamut. And during this conversion, ink color settings, total ink limit, dot gain, gray balance, and UCR/GCR parameters are also established. But they are established based on standard web offset printing (SWOP) values, and screen printing inks do not match SWOP color parameters. So these values are almost useless.

 

However, if we can get the RGB file, we can establish the correct separation parameters for your particular application before the conversion to CMYK. This allows us to maintain color continuity with other CMYK pieces created through lithography or other printing methods <B>(Figures 2A-C)</B>.

 

If we cannot obtain the original RGB file, we may be able to convert the CMYK file back to RGB, then apply the correct parameters for screen printing. But the final image will not be as accurate as the original RGB image. The problem is that because black is added to the image during the RGB-to-CMYK conversion, reconverting to RGB will tend to produce too much black, which will be carried over and increased even more when the image is again reconverted to CMYK for the final separations.

 

Screen printing also requires specific screen angles, line counts, and dot shapes unlike those used for other printing methods. These screen angle sets are one of the most commonly overlooked sources of moir&eacute;. For coarse halftones of 50 lines/in. or below, conventional litho angles (Y = 0 °; C = 15°; M = 75°; K = 45°) will work if the mesh is a plain weave of 355 thread/in. or more. The dots are large enough that the angular interference of the mesh with the halftone structure is not too much of a problem.

 

But as the line count gets higher (smaller dots), we must rotate the entire angle set away from the traditional litho angles, primarily because the angles for the yellow and black dots correspond exactly with the weave of the screen mesh and are likely to cause moir&eacute;. In most cases, these angles are corrected at the imagesetting stage on the RIP when separations are output to film.

 


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