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Sep 1, 2001 12:00 AM
As digital-printing technology matures and its cost points recede, printers have begun teaming their conventional offset presses with new digital presses. Their aim: to offer a unique value-added service that can also turn a profit. Transactional printing, newsletters, labels, plastic cards, retail flyers, direct mail and human-resource booklets are the more common applications that benefit from the economy of static, lithographic color combined with variable, digital black and white.
Combining an older technology with a new one, however, presents technical challenges. In the marriage of offset and digital, printers need to realize that stock, ink or environmental conditions that may be ideal for one technology could wreak havoc on the other. The two processes can be made compatible, but only by controlling printing variables. Following are the most crucial determinants of “hybrid” success.
The one factor that demands the most consideration when combining offset with electrophotographic digital printing is ink. Considering that its monetary value — and impact on the profitability of a job — pales in comparison to paper, it's a factor printers may be compelled to overlook. In this application, however, its characteristics directly impact product quality.
“Ink is the least expensive component, but the most important,” says Frank Kanonik, national program manager for Xerox Corp.'s (Rochester, NY) Marketing Partnership Program, and PIA Digital Printing Council technical advisor. “Many lithographic inks cannot stand up to the heat of an electrophotographic digital press.” That's because offset inks often contain high levels of wax and resin to increase rub resistance. But once the job goes through a xerographic digital press, it is subjected to temperatures up to 400ÞF — which fuses the toner to the stock but can soften the wax and resin in the typical offset ink, resulting in ghosting and offsetting, and turning yellows opaque. Metallic and fluorescent inks also are problematic in digital printing. In the first instance, flakes from the metallic ink can build up on the press' fuser rollers.
For combination litho/digital jobs, Kanonik suggests using a laser-compatible ink, which can withstand such high temperatures. To determine an ink's laser compatibility, GATF Lab Services (Sewickley, PA) offers a “blocking” test that simulates the pressure and heat inside a digital printing press. In it, a clamp holds two samples of ink and paper under 25 lbs. of pressure at 400ÞF for 30 minutes. If the sheets stick, the ink is not laser-compatible. For more information, call (800) 910-4283.
To be laser-compatible, these inks commonly have low, or no, wax and resin content, which consequently lowers rub resistance. Printers have a few options for applications demanding a sturdier finish. You may apply a varnish or aqueous coating on the job to seal in the ink and toner; Kanonik notes, however, that this can actually soften the ink. Or, you can send the job back through the offset press and flood coat it. Printers will need to experiment to figure out the right formula.
Westerfield-Bonte (Louisville, KY), a $1.4 million printer specializing in short-run publishing and legal and financial printing, takes a unique approach to increasing rub resistance. “If a book is going to sell retail, we'll just laminate the covers to make them tough,” says David Blythe, vice president. The printer runs jobs on two Heidelberg halfsize presses, two Hamada duplicators, a Xerox DocuTech and a Canon digital color machine.
One example of a Westerfield-Bonte combination job is a technical manual, which the customer sells or includes with its products. A common cover featuring the customer's name and logo is printed on an offset press, and then laminated in quantities up to 10,000. Then the side pages of the books are printed digitally in short runs of 200 to 500.
Jobs with laser-compatible inks require a fairly substantial drying and curing time after being offset printed — typically from 24 to 72 hours — before they can pass through a digital press. Printers also should use the minimum amount of spray powder possible, because digital presses are highly sensitive to such contaminants.
“We like to see the digital press first in line, then print color around it on a traditional press, which results in a nice, clean web. This is not always feasible,” admits Dave Dunn, product manager of black-and-white products at digital press manufacturer Xeikon (Wood Dale, IL). He notes that some jobs may require overprinting on a color area with black text. In those instances, he says “the offset ink should be dry before it gets into the digital press. Otherwise, there may be offsetting on the rollers.”
To save production time, printers can execute one large run of the job's lithographic shell, seal it, and then run it through the digital printer as needed. This approach is only practical, however, if a customer plans to not make any changes to the shell's design. Otherwise, it can be a waste of time, ink and paper.
In terms of their relationship to moisture in paper, lithography and digital printing are near opposites: The former adds water, while the latter evaporates it. Digital printing is especially susceptible to excess moisture. Because drying and curing are key to a quality offset/digital piece, experts recommend minimizing ink density and the amount of fountain solution used during the offset press run. This will prevent curling or warped pages after the job is digitally printed.
Choose paper type and basis weight based on the specifications of your digital press. Paper manufacturers have introduced a range of stock optimized for digital printing, characterized by low moisture content, smooth finish and an alkaline chemistry. This last characteristic increases the paper's archival capabilities. Vendors include Domtar, Georgia-Pacific, International Paper, Mohawk Paper Mills, Neenah Paper, Potlatch, Stora Enso and Willamette.
According to technical experts at Mohawk Paper Mills (Cohoes, NY), avoid paper that has an uneven surface (such as embossed or heavily textured stock), as toner will adhere quite poorly. Xerox's Kanonik adds that generic offset enamel stocks can also be problematic; specially coated papers in up to 10-pt. cover are available, however. When running coated paper, make sure it is specified as “laser safe.”
Test the runnability of any new paper — especially envelope format — before beginning a run. Paper vendors offer samples, and you may even get help from your digital-press vendor. Océ (Boca Raton, FL), manufacturer of the DemandStream line of digital presses, will run stock through its equipment free of charge to prospects and clients, to determine if the paper is compatible. Xerox (Rochester, NY) offers its own line of papers formulated for its DocuTech line.
Eighty-five percent of the work digitally printed at Digital Marketing (Minneapolis), a provider of personalized e-mail and direct-mail marketing campaigns, is on pre-printed stock. Cofounder and executive vice president Joel Hoefle explains that the goal of each job is to make it appear as if the black, variable text was printed at the same time as the offset background. The company outsources offset printing, and completes marketing pieces on its Xerox DocuTech 6135 and 6180, DocuColor 70 and Indigo e-Print Turbostream presses.
In order to achieve this goal, Digital Marketing carefully selects and tests new stocks, and sticks with paper that has been approved by the digital-press manufacturer. “You can also contact your paper manufacturer to see what stock is being purchased by other digital printers,” Hoefle adds. He also suggests checking out what types of paper your competition is using, as well as what's new and popular in the marketplace.
“When litho-produced sheets are trimmed to go through a digital press, you have to tighten up cutting operations,” notes Kanonik. “There's no room for error.” Cutter maintenance is especially crucial: Sharpen blades frequently to minimize paper dust, which can be hazardous inside a digital print engine. If enough dust accumulates in the digital press' electronics, it can create an insulating effect that may cause the machine to overheat. It can also layer on the imaging belt, producing toner hickeys. Kanonik recommends changing cutter blades every 10 days.
Also, cut in smaller lifts to prevent top sheets of paper from dragging with the blade, and to ensure uniformity. Decrease clamp pressure to prevent the indentation of the paper edges. According to Kanonik and Mohawk's technical experts, imprecisely trimmed paper will jam a digital printer, so accuracy is key. For jobs that have no room for deviation, Kanonik advises letting the litho-printed job sit for about half a day to acclimate, and then three-knife trimming the pieces to clean up the edges.
When storing preprinted offset material, avoid shrink-wrapping it too tightly. According to Mohawk technical experts, this will curl the papers' edges. They also advise storing the material for 10 days before using — if your timeline allows it — to allow the ink enough time to completely cure and the paper to fully acclimate. An environment of 50 percent relative humidity and 70ÞF is ideal.
Combining offset with digital printing sounds like an exact science, and in fact, each application needs to be run like a experiment. Sound scientific procedure calls for controlling all of the variables, and in this print application, it's crucial for consistent success. Because so many factors — from the amount of fountain solution and spray powder to ink density and humidity — can affect a job's outcome, they need to be controlled aggressively.
“Our biggest challenge is having a stock work in a test run or in a controlled production run, and then the second time getting different results,” notes Hoefle. “Locking down the variables is key.”
Say you have a stable of offset presses and would like to offer variable printing to your customers. What type of digital press would best complement your equipment? The answer: that which can keep up with your offset presses, or at least won't slow them down so much that the job begins to lose profitability. This is especially pertinent in applications where a digital press is placed in line with an offset press.
“If you're doing strictly black-and-white variable information, and already have offset equipment available, then combining digital printing with lithography makes a lot of sense — if you can get a digital press with a high enough speed,” notes Dave Dunn, product manager of black-and-white products at Xeikon (Wood Dale, IL). Xeikon's magnetographic digital VaryPress is often placed in line with offset iron to provide speeds from 300 to 400 fpm. Applications typically include business forms and shipping labels.
Dunn adds, however, that if you have to cut the speed of your offset press by two-thirds so as not to outrun the digital press, it will be at the expense of productivity.
“You have to weigh the value of digital printing against productivity,” the exec concludes.
On the offset side, direct-imaging (DI) presses can offer a time-savings advantage over an analog press. “You can get the job done a lot faster on a DI press than on an offset press. It's more cost-effective for short runs,” observes Frank Kanonik, national program manager for Xerox Corp.'s (Rochester, NY) Marketing Partnership Program, and PIA Digital Printing Council technical advisor. He says that the only other difference between the two presses is required ink types — waterless for DI, laser-compatible for offset.
Toner-based printing introduces new considerations when designing a piece that will go through an offset and digital print run:
Keep toner edges 0.125 to 0.25 inches from the edge of the piece; otherwise, the toner might crack when going through the mail.
Avoid overprinting digital print on offset ink, as toner adheres poorly to areas with a high ink density. If a job requests it, use a halftone screen to achieve the specified color.
Allow sufficient space between digitally printed images and preprinted areas to accommodate variations in the printing processes and to avoid unintended overprinting.
Keep the paper grain long when printing digitally.
Source: Frank Kanonik, Xerox; and Mohawk Paper Mills