Digital Printing

Some of the rules change when you step outside the world of old-fashioned offset printing. As you've seen in some of the preceding sections, the behavior of ink on paper has a lot to do with the appearance of your final job. And the limitations of the ink-and-paper relationship have an inescapable effect on what you can print successfully. Digital printing is subject to restraints too. It's just that the restraints are slightly different from those of the offset world.

In their earliest iterations, digital printing devices were not much more than glorified laser printers. The toner-based engines were beefed up to print faster and bigger, but they were still prone to have all the characteristics of laser printers. Color consistency between impressions was quite problematic, innocent environmental influences such as humidity were mortal enemies of registration, and halftone reproduction on high-speed, black-and-white machines was, to be charitable, miserable. While digital printing has always offered advantages such as relatively affordable short runs and the ability to print variable data, it was initially deemed appropriate only for basic direct mail or text-only pieces because of the superior quality of offset printing.

Digital Printing Advantages

The gap between digital and conventional offset is closing, and the premium digital print offerings from vendors such as Xeikon, Kodak, Xerox, and HP Indigo are beginning to rival the appearance of offset printing while still offering the additional appeal of customization and short runs.

Short Runs

Cranking up an offset press requires a large minimum number of impressions because of all the attendant hardware. Plates have to be burned and positioned on the press, and a fair amount of time is devoted to getting the multiple inks up to optimal behavior and registration. The time and materials that are expended to reach that optimal behavior are referred to as makeready. Toner-based digital printing offers some advantages here. There are no plates to burn, so the minimum run for a digital job can be 200 or less, rather than 10,000. This may lower the threshold for color printing for many jobs.

Variable Data

We've all received those compelling personal letters: "Dear [Your Name Here]." Such customization is the most basic form of variable data publishing (VDP), but it's just a hint of what can be done with VDP. Since each impression of a document on a toner-based digital press can be different, images and other artwork can be customized for very targeted direct-mail pieces. Preparing such a piece requires quite a bit of planning, and the VDP software used by the print service provider is rather expensive. However, you may find the added expense and complexity worthwhile, since it's been shown that customized direct mail pieces elicit much higher response than generic mailings. If you have used the mail-merge features available in a word processing program, or the Data Merge features offered in PageMaker and InDesign, you have done basic variable data publishing. The complexities of setting up VDP work are outside the scope of this book.

Digital Printing Issues

Although offset printing benefits from at least 100 years of refinement, digital printing is a relatively recent undertaking. Digital printing solves some problems, such as the need for short runs. But it introduces new challenges, such as the behavior of toner on paper and paper size limitations. Large areas of uniform color are still problematic on most toner-based systems. For example, a brochure cover that is printed with a full-bleed build of C100-Y80 will look mottled compared to the same piece printed on a conventional offset press. For now, it's just one of those things that you have to anticipate. Design around the limitation by using a collage of images or smaller areas of color in which any mottling will not be glaringly apparent (Figure 2.21).

Figure 2.21. A screen build that covers large areas uniformly on a conventional offset press (left) may appear mottled when printed on a toner-based digital press (right).

However, you may find it surprising that rich blacks are not necessary on most toner-based presses. Not only is the dense black nature of toner sufficient for complete coverage, adding three other colors can interfere with the toner's adhesion to paper, and will actually make things worse.

Most toner-based printing lacks the inherent shine we're accustomed to seeing in printed pieces, even on coated stock. This is due to the nature of the toners themselves. The HP Indigo presses use a slurry of toner in a carrier (called HP ElectroInk), so their output often more closely resembles traditional offset printing. But all of the toner-based output can be coated, either via extra imaging units, or through coating stations attached to the press. Standalone coating equipment can also be used, but this requires that the printed pieces be moved to a separate coating machine and fed through it.

There are other differences between digital, toner-based printing and conventional offset printing. For example, most digital printing processes do not require trapping since toner is usually placed onto a carrier, and all colors are transferred to the paper in one impression, which often eliminates misregistration. Many digital presses use stochastic screening rather than conventional halftone dots and angles. These differences contribute to digital printing being a viable alternative to offset.

Registration

On most toner-based digital presses, toner for all four process colors is accumulated on a carrier that is held by the strange miracle of electrostatic force, then deposited as a single transfer to the paper. As a result, registration is somewhat easier to maintain than on offset presses, which apply each color separately from individual inking units. Sophisticated internal monitoring in these modern presses also ensures consistency. These devices demand tight environmental controls. Slight changes in humidity and temperature can play havoc with output, so toner-based digital presses are usually sequestered in specially constructed rooms that are engineered to maintain a constant environment.

Spot Colors on Toner-Based Digital Presses

Currently, spot-color offerings are limited on toner-based digital presses. Although HP Indigo and Xeikon presses do offer additional units for available spot colors (which include several metallics and fluorescents), not every Pantone color is available for these presses. However, this limitation is not as dire as it sounds. Even though the primary toner colors are called cyan, magenta, yellow, and black on these devices, the pigments are not identical to those used in offset process colors. In some ways, this is actually good news. The toners are often more vibrant than standard process inks, so they can simulate a wider range of Pantone colors without resorting to actual spot colors. At this writing, Pantone provides a digital chip book certified for two Xerox digital presses, the iGen3™ and DocuColor™ 8000.

Paper Requirements and Limitations

Most digital presses are sheet-fed and have much smaller mouths than their offset brethren, which generally limits their output to approximately tabloid sizes. Roll-fed devices such as the Xeikon presses, however, cut the paper only after printing. And while the imaging width is about 19 inches, the total length of a piece can be banner-sized.

While there is a wide range of paper certified for toner-based digital presses, stock choice isn't unlimited. Consult your print service provider for samples of supported stock before you get your heart set on a particular paper that is subsequently not deemed appropriate. The complicated paper paths in such devices preclude the use of extremely thin (or extremely heavy) stock. And the high heat of fusing the toner to the substrate can cause curling or waving. In the interior of a digitally printed piece, this may not be so noticeable, but covers may require the extra step of lamination to keep them from curling.

Cracking and Flaking

Toner is fused to the surface of paper rather than being partially absorbed as conventional offset ink is. Consequently, it's sitting on top of the paper like a coat of inflexible paint and is prone to cracking during any folding or creasing processes. Consider this as you design for toner-based digital output, and avoid large instances of crossover art if possible. A rule here and there or the occasional line of headline text shouldn't be a problem. But the more toner encrusting the paper along the fold, the uglier the outcome can be.

Resolution and Screen Ruling

For conventional offset presses, platesetters are often used to digitally image plates. Data-driven lasers expose the photosensitive surface of a plate, which is then developed and mounted on a press. Imagesetters are used to expose film, which is then used to create an image on a printing plate (see Chapter one, "Life Cycle of a Print Job," for more information on imagesetters and platesetters). Whereas imagesetters and platesetters achieve resolution of 24003600 dpi, most toner-based electrostatic systems fall between 6001200 dpi. Consequently, while these systems are capable of fairly high line screens (150200 lpi and higher, depending on the vendor), be cautious about using infinitesimal line weights or type that has extremely fine serifs. It's a good idea to ask your print service provider to provide specifications so you know the limitations of their printing process before you go too far in your design.