Here’s a trick I once saw at a business printing vendor who was doing large format poster printing for a client of mine. The job in question was a process color poster with a heavy coverage PMS border around a central, full-color image. After having problems with mechanical ghosting, the custom printing service tilted the entire poster on the press sheet. I’d never seen this before; nor have I seen it since. And I’m sure it made for challenges on the trimming equipment.
Let’s see what we can learn from the experience at this large format printing vendor.
What is mechanical ghosting?
Ghosting is a problem printing companies face in which faint images from one part of a press sheet show up elsewhere on the sheet. In other cases, mechanical ghosting can involve light or dark areas of ink coverage that are inconsistent with their surroundings (and that don’t reflect what is actually in the art files).
Various design elements precipitate this kind of ghosting, but printing large format posters (or even small book covers) with an image surrounded by a PMS border can cause particularly challenging ghosting headaches. This is because the press deposits a lot of ink at the top of the border (the width of the poster), then very little along the sides of the border (a narrow strip running the length of the poster along both sides), then a lot of ink at the bottom of the border. (This is called ink starvation: i.e., printing a lot of ink, then very little ink, then a lot of ink). The result is an inconsistent ink deposit, with an overinked top and bottom border edge, and sides that are too light.
Evening out the ink flow is a solution.
Some online printing companies add “ghost bars” or “take-off bars.” These are extra blocks of solid ink coverage outside the live image area, added by the printing companies in prepress and platemaking, in order to make the press lay down more ink along the sides of the poster. These “take-off bars,” as the name suggests, are then trimmed away during the finishing stages of production, after presswork is complete.
Another option, if there is not enough room on the press sheet for take-off bars, is to screen the top and bottom of the border slightly. Without the screen, the horizontal part of the border at the top and bottom of the large format posters would be darker than the sides. Screening back these portions of the design in anticipation of mechanical ghosting can even out the ink density all the way around the border.
This solution was employed by one client to avoid mechanical ghosting on their magazine covers. This company screened back (i.e., lightened) the top and bottom of the yellow border around the cover of their magazine, and the mechanical ghosting actually equalized the ink on the border so it was the same all the way around the central image. Granted, I would think the reader’s eye would be more forgiving with yellow ink than with a darker color, but for this magazine and its custom printing vendor this solution worked.
Cocking the image
The custom printing vendor I worked with employed a different solution. Since only one image of the poster printing run fit on each press sheet, with an ample non-printing area, the business printing service was able to tilt the poster slightly on the sheet (maybe 40 degrees). As this “cocked” poster traveled through the press, the tilting actually evened out the amount of ink deposited by the press rollers. There was no single portion of the poster, when tilted in this way, that required that much more ink than any other area. Granted, the custom printing vendor did also add take-off (or ghost) bars to further equalize the ink lay down, but I think the tilting made all the difference. With all the printing companies I have worked with since this poster printing run, I have never seen this trick repeated.
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on Monday, August 1st, 2011 at 1:57 pm and is filed under Large-Format Printing, Poster Printing, Printing.
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