Even if you are an old hand in the printing and binding businesses, some aspects of digital printing might catch you off guard. Consider the paper situation: While the digital-printing process is completely different from the offset process, identical papers are often used, and expected to perform in the same manner. This expectation can lead to imaging disasters. Even more insidious are the digital-print problems that go undetected until the postpress stage.

UNIQUE CHALLENGES

Before we look at specific paper pitfalls, two unique digital-printing challenges must be considered:

Press model and manufacturer

The digital process comprises a dizzying array of technology. Therefore, virtually all of the paper-stress factors mentioned in this article are generalities. One of the best ways to avoid problems is to network with other digital-press users to pool your paper knowledge. Most digital-press vendors have user groups that meet regularly to discuss such topics.

Communicating with the press manufacturer is also important. Most publish lists of papers that have been “approved” or “certified” for use on their machinery. In the best cases, this list reflects a careful process of communication and testing between paper mill and press manufacturer. The presence of a paper brand on such a list does not guarantee it will run successfully, but it does represent a commitment on the part of the press manufacturer to make the paper work. This is important in an industry where time between service calls is measured in hours, and less-than-competent repair technicians would prefer to blame stock rather than fix the machine.

A digital-print shop should not take these lists as gospel, but they do make a good starting point (see “How to be a digital-paper pioneer,” p. 36). Any conventional shop entering the digital on-demand arena should not assume that current house papers will be successful on the new digital presses. Likewise, established digital printers must be prepared to re-qualify papers whenever a new model press is installed, even if the new machine comes from a trusted manufacturer.

Lack of adjustment

The second major challenge is the lack of adjustments available to the digital-press operator to compensate for problems. Those with a conventional background will be frustrated to discover that most settings on digital presses are fixed by the manufacturer. Running problem paper requires closer attention to external factors. Once the job is running, opportunities for problem correction are minimal; therefore, problem avoidance is essential.

STRESSED STOCK

Today's high-speed digital presses subject paper to three major stresses that are not present in the offset process, and that may impact the bindery.

Tortuous travel

Virtually all digital presses subject paper to a circuitous route of starts and stops, sharp turns, twists and about-faces, most of which are accomplished without grippers or other mechanical devices to keep the paper on track. This makes grain direction crucial. Most sheetfed digital presses will grip paper along the 11- or 12-inch edge, whether printing one- or two-up. Proper grain direction is perpendicular to the direction of travel, meaning long grain for 8½ × 11-inch paper, and short grain for 11 × 17-inch stock. Ignoring this rule will likely result in additional spoilage, downtime from paper jams and registration problems.

This is a classic example of the potential for conflict between the digital pressroom and the bindery. For instance, when letterfolding an 8½ × 11-inch sheet, it is best to run grain long for the digital press, but grain short for the folder. If a heavier stock is required, problems will be exacerbated in both departments.

Here's a more complex example: A 5½ × 8½-inch book is run two- or four-up, and then perfect-bound. Running the ideal grain direction for the digital press will result in wavy pages because the paper grain wasn't run parallel to the spine.

Each shop must determine its own priorities in resolving these conflicts. Ultimately, careful job planning should determine grain direction. Using proven papers will help minimize problems when binding considerations necessitate running the wrong grain.

High heat

Papers in toner-based presses are exposed to warping, drying and blistering temperatures that approach the ignition point during the image-fusing process. If the page is printed on both sides, it endures this process twice. Therefore, control of moisture content, both in the paper and the ambient pressroom atmosphere, is critical. In fact, the only difference between some paper mills' digitally branded cut-sheet papers and their conventional, fullsize counterparts is moisture content imparted to the paper in the manufacturing process. Climate control and acclimation by the print shop is even more important.

Another quirk of digital fusing is the application of fuser oil or agent, generally a highly viscous, silicone-based liquid. This is applied to the paper to ensure that toner bonds to the stock, rather than to the rollers. The normally invisible liquid can cause streaking on coated stocks or on heavy coverage. These streaks may not disappear when the oil evaporates. Thus, one of digital printing's chief advantages — eliminating ink-drying delays — may actually be negated by the need to allow fuser agent to evaporate. Unlike ink, fuser oil will not cause marking or rubbing in the bindery, but its slippery properties may increase feeding problems, particularly in the small, friction-feed equipment often found in quick-print environments.

Electrostatic charge

Since electrical charging is at the heart of laser/toner technology, normal static-control measures, such as sprays and tinsel, are inapplicable in parts of the digital pressroom. Instead, control of ambient pressroom temperature and humidity are key to success. Many commercial operations build separate digital areas to allow for such controls, as well as to avoid harmful particulate matter like spray powder. Unfortunately, the paper is rarely treated with the same respect.

When this article was written, the outside temperature (and in the trailers of paper merchants' trucks) was -5°F — meaning that paper delivered on the same day to a controlled digital environment will suffer a temperature shock of approximately 80°F. Temperature and moisture content have strong impacts on electrostatic charge. Using unacclimated paper may result in voids or deletions, where the electrostatic charge varied and the image did not properly transfer to the page. Higher-than-usual jam rates may also occur.

A ream of paper may acclimate overnight, while a skid of paper could take up to a month. This must be considered in a true on-demand environment, where paper may be delivered minutes before a job is run. Storage conditions should also be analyzed. A climate-controlled digital pressroom may be insufficient if paper is kept in a warehouse subjected to temperature extremes.

BINDERY SNAFUS

Many papers appear to run acceptably on the digital press, when in reality the previously mentioned factors have created problems that won't surface until the job hits the bindery. Here are the four most common symptoms:

Wrinkling, excessive waving

Obviously, wrinkled output is unacceptable. Excessive wave or curl to the paper must also be closely monitored, since it may result in wrinkling once the pages are passed through some bindery equipment, particularly those using rollers, such as folders.

Cracking

Although it's not unique to digital, cracking on the fold is much more problematic in this application, since the image rests on top of the paper rather than being absorbed into it.

Flaking

Incompatible paper can cause flaking problems on toner-based presses. Flaking occurs when the fusing process fails to bond toner to paper, and the image rubs off in flakes. Flaking may be obvious, or very subtle. Simply running your fingernail across a solid image area will quickly determine if flaking is going to be a problem.

There is rarely anything that can be done to solve this problem, other than switching papers. Jobs produced on a monochrome press have a higher incidence of flaking than process color output; this can be easily illustrated by running a gloss-coated cover on a black-and-white digital press.

Rubbing

Rubbing or scuffing occurs when the printed sheet is subjected to pressure, and is a very common problem on book covers. Some presses are especially susceptible to this problem, and require specially treated papers to image successfully. In offset, this problem is best mitigated through the use of varnishes or coatings, whereas digital prints have had to rely on the much more expensive laminating process.

Interestingly, when combined with the right papers, some color digital presses actually exhibit superior rub resistance, eliminating the need for additional protection. This is an example of how careful testing can not only alleviate problems, but also reveal a competitive advantage.

In the on-demand environment, however, inline finishing is more commonly employed, and some jobs do not visit the bindery at all. This in no way exempts the job from these pitfalls. Work that looks beautiful off the digital press may experience some of the previously described problems during the shipping or mailing process.

BRAVE NEW WORLDS

Successful digital printers are carving out new applications and markets. Since everyone is a pioneer, a culture of willingness, experimentation and testing will open up many opportunities. Projects that others refuse to produce may not be the headaches they appear, but rather, opportunities in disguise. In such instances, customers are often willing to be more flexible on paper specifications — and price — if it means success for their jobs.

Many of the early adopters of digital-press technology were prepress vendors. They had the skills needed to tweak digital files for optimal appearance on a digital press. This resulted in some beautiful press sheets. Unfortunately, many of these same organizations had no experience with papers or with the bindery process, and have since passed from the scene. Commercial printers, however, possess the skills needed to make the digital-printing process functional from start to finish.

How to be a digital-paper pioneer

You have your digital press. You are familiar with a narrow range of paper that runs well — and you've found out the hard way about a few that won't run at all. Now your customer is specifying a stock that you've never tried on your digital press. What to do?

First, apply common sense. No amount of experimentation is going to run 22-pt. railroad board or 9-lb. onionskin through your digital press. Neither is a digital press a good place to use up that stock you've had open on a skid in your warehouse for the last two years. If the paper in question falls within the general specifications of your press (or for the more adventurous, just outside these specs), then further testing is in order.

Analyze job parameters

Next, analyze the job parameters. How heavy is the coverage? Is the run length long or short? Small jobs can often “limp” to the finish. If your only experience with a particular paper is on very short runs, proceed cautiously when accepting longer-run jobs. Many digital-paper problems are cumulative and only become apparent when a job has been underway for some time.

It may also be worthwhile to contact your press manufacturer. Are there any known problems with the paper in question? Or, has the manufacturer not gotten around to testing it yet? Most manufacturers are eager to demonstrate that their presses can handle the widest possible range of papers. Ask your paper merchant to send the press manufacturer samples. If the press vendor is uncooperative, perhaps it is time to look for a different digital press.

Also consider the paper mill from which the stock originates. Is it a reputable company? Perhaps you have used its papers for offset printing with success. Is the paper marked as “laser compatible?” This usually refers to office printers, not digital presses, but it does indicate that the mill recognizes the digital market, and is attempting to understand its special needs.

Finally, learn the client's reasons for specifying the particular stock. If problems arise, you will know what papers can be substituted without compromising the integrity of the order.

[时间：2003-06-30  作者：Bisenet  来源：Bisenet]