What about that disappointing curl that can put a dent in your otherwise flawless piece? Bob Hieronymus, Georgia-Pacific senior marketing project manager, communication papers, reports that mills are adjusting papers to better meet the atmospheric demands of digital print engines.
"For instance," Hieronymus explains, "the high temperature fusing process that takes place during toner adhesion can cause paper to curl. By reducing the moisture content for digital presses to 4.5 percent, as opposed to 5.5 percent commonly used for offset paper, the risk of curling is greatly reduced. A smaller moisture percentage also allows for better control of electrostatic conductivity, which improves toner adhesion."
Mavis Motherway, marketing manager at Sappi Fine Paper (formerly S.D. Warren), says that most manufacturers recommend a pressroom temperature of 68 to 77 degrees and a 50 percent (±4) relative humidity to prevent curling.
If brighter is good, smoother is even better. The two work together to allow improved reproduction of color and graphics.
"The surface smoothness of a paper plays a critical role in toner adhesion—the smoother the surface, the better the adhesion," Hieronymus says.
Paper smoothness is measured in Sheffield units, with lower numbers representing smoother surfaces. Sheffield units for high-quality digital printing paper, Hieronymus says, usually range between 50 and 75; by comparison, offset papers generally range between 100 to 200 Sheffield units.
Motherway adds that "dull" finish papers, particularly in premium coated grades, are becoming popular. Though appealing to look at, problems can occur because the surfaces are more sensitive to marking. So a protective coating helps, she advises.
Printing On Demand
The print-on-demand environment has drastically changed in-plants' options, and manufacturers are offering new papers.
• Georgia-Pacific has a Microprint line that Hieronymus contends is a high performance paper engineered with the on-demand market in mind. For printers using roll-fed systems, the paper is offered in imaging rolls.