Digital and offset papers share some of the same characteristics, but are designed and manufactured for specific printing processes.

Some paper attributes that make for successful offset printing apply to digital papers, notes Rob Watson, director of marketing, printing papers, xpedx. “Image production on paper is influenced by base stock formation, print surface uniformity, coating or surface treatment. Press and finishing equipment productivity is affected by moisture content, dimensional stability, basis weight and caliper, grain direction, crack resistance for scoring and folding, among other factors. Paper quality has significant impact on the perceived value of printed media. Smoother papers tend to produce cleaner colors and higher contrast. Substrate brightness, whiteness, color, gloss, matte, silk or dull finishes combine with the printing method, image design and finishing techniques to produce the final printed piece.”

Papers for digital printing are manufactured specifically for the type of ink used in the printing process, notes Ron Pergande, director of digital media, GPA, Specialty Substrate Solutions. Papers for HP Indigo presses, for example, are either optimized in the manufacturing process, or given a surface treatment to ensure proper surface chemistry and compatibility with HP ElectroInks and consumables (such as the blanket) to maximize performance.

GPA offers over 500 Ultra Digital substrates that are Rochester Institute of Technology and HP Indigo certified for consistent performance on HP Indigo presses. They have been engineered for the proper chemistry and moisture content, precisely cut to size and carefully packaged to ensure flawless ink transfer and ink adhesion on press, as well as exceptional performance in finishing

Substrates are put through a battery of tests to see how well they will perform in the digital environment. The sheet’s ability to run smoothly, its ink adhesion properties and the effect it has on the press consumables are a few of these tests that result in how well the substrate rates.

Similar, But Not the Same

To the untrained eye (and even to most “trained” eyes) digital coated papers look very similar to paper that is designed for offset printing, says Pat Semrow, technical department, Nekoosa Coated Products.

“The reason for that is they are quite often a modified version of an existing offset sheet,” says Semrow. “However, because dry toner imaging depends upon electrical charges on the sheets surface, digital sheets are designed to be within a specified range of electrical conductivity or resistivity. The sheet then needs to have a very smooth surface so the toner lies flat. Finally, because toner is fused to the sheet surface by temperatures that can exceed 400°F, the sheet must have lower moisture content than would a comparable offset sheet. As the sheet passes through the fuser, excessive moisture can cause blistering similar to when high moisture sheets pass through the dryers on a web offset press.”

While moisture and smoothness are usually not issues with Digital Synthetics, there are some inherent properties of synthetics must be addressed, says Semrow. As with paper, synthetic substrates need to have a coating that will receive toner or Electroink, and quite often the same coating will not work for both dry and liquid toner. This is the reason that some substrates might be recommended for dry toner machines or for HP Indigo, but not for both. Since most synthetic sheets act as electrical insulators, it is also critical that the toner receptive layer has the correct conductivity.

“Sheet stability at high temperature is another concern with most dry toner digital printers,” says Semrow. Since the fuser area in some printers can reach temperatures in excess of 400°F, there can be some stretching, melting, or deforming in the fuser area. For this reason, Biaxially-oriented polyesters generally make the best base sheet for digital synthetics, because they have higher melt temperatures than many other synthetics and their biaxial fiber orientation makes them much more stretch resistant in all direction. Other synthetic base sheets can be dry toner printed but might only be recommended for low temperature or high speed machines, so the sheet doesn’t absorb too much heat.