Offset Printing: Respect the Limits of Offset Printing

Here are some things to keep in mind while you’re designing any custom printing project for printing press services. It is unbelievably easy to forget them.

Variation in Colors

If you plan to use a color build on a number of pages within signature work, such as a print book, keep in mind that there will be color variation across the press sheet and particularly from press signature to press signature.

Let’s say that you have a background area screen that will appear on all pages within a section of the book. If you have other four-color images, or heavy-coverage solid colors, within the signature, your book printer may need to adjust the inks on press to keep the photos true to color throughout the press run. In this case, you may find a color shift within the area screens if you lay one page of the print book beside another.

To sidestep this issue, consider adding a separate PMS ink for the background instead of building the background color with process inks. A PMS color will remain exactly the same throughout the press run, whether or not the book printer needs to adjust the process inks. This will cost a little more, but if you’re already printing your job on a five- or six-color press, this added amount should be minimal (perhaps a few hundred dollars). And it’s money well spent.

Crossover Alignment

Commercial printing is an art as well as a science. As noted above, there are variations in both printing and finishing. It is important to remember that finishing equipment is not 100 percent exact. If you extend type or a graphic from one page to another in multi-page signature work (such as a print book), the left-hand page (called the “verso”) portion of the image may not align precisely with the right-hand page (called the “recto”) portion of the image.

There are a few ways around this problem (or, rather, limitation of offset lithography). If you position the crossover image within the center spread of the signature (the two pages that are side by side on the press sheet), your image can cross over from the left-hand page to the right-hand page without any misalignment (after all, they’re side by side on the sheet, unlike all other pages in the press form).

Another solution, which is less effective, is to avoid placing images that cross over diagonally from one page to another in a signature, and avoid having thin lines cross over from the left-hand to the right-hand pages. Thicker crossover images are a little more forgiving than thinner ones, just as horizontal crossovers are more forgiving than diagonal ones. (This really has more to do with the limitations of the human eye.) And as noted in the preceding section, the colors on the left-hand page won’t exactly match those on the right-hand page unless you’re using a PMS (rather than a process color build).

Total Area Coverage

Presumably, if you’re designing a four-color product, you can specify 100 percent ink coverage for each of the process colors (cyan, magenta, yellow, and black). In reality, this is not wise. If you’re printing on newsprint, you’ll get a sticky mess of paper and ink. Sheetfed printing on gloss stock will be slightly more forgiving, but the mass of ink will never completely dry.

In offset custom printing, only so much ink can either sit up on top of a coated press sheet or seep into the fibers of an uncoated press sheet without compromising either the paper or the printing process. For sheetfed printing on gloss or dull stock, you can usually get away with 340 percent total area coverage (the sum of all process inks: for example c100m80y60k100). For newsprint, you would want to lower the total area coverage to approximately 240 percent (for example c60m80y20k80), or less.

These percentages are just starting points. Ask your commercial printing vendor what the target percentages should be for the paper you’re using and for his particular printing press. It is quite possible to get deep, rich colors on press without adding excessive amounts of ink.

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