The Best Paper for the Job

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.

Synaps OM and Synaps Digital XM are examples of products that have similar base sheets but have surfaces that are engineered for different print application methods. While both products start with a biaxially-oriented polyester base sheet Synaps OM is two side coated with a proprietary ink receptive layer that can be offset printed with standard inks and the ink will dry faster than it would on many coated paper stocks. Synaps OM is also Rochester Institute of Technology certified for use on HP Indigo presses. Synaps Digital XM on the other hand is two-sided coated with a proprietary dry toner accepting coating and is specifically designed for use on dry toner printers.

To ensure its customers understand the differences between digital and offset papers, Mohawk has created www.MohawkMakeReady.com, an online resource for all graphic producers The site, which includes video links as well as articles, explains why each printing process requires its own specific substrate. In fact, one of its hot topics is why cutting down offset folio sheets is not a viable option.

Cutting down offset sheets to use in a digital printer can introduce contaminants into your digital device, says Gavin Gaynor, vice president of research and development, Mohawk. Paper that is guillotine-trimmed will often contain more dust and paper bits leading to issues on the printers. Also, there is the possibility that sheets won’t be cut square or won’t be properly separated from each other for feeding. Also to be considered is the importance of grain direction for digital printing, particularly when finishing is involved.

Mohawk Digital Papers—Everyday Digital, Bravo, Via, Navajo, Options, Superfine, Color Copy, and 50/10—are available in popular digital sheet sizes and small rolls.

Coated papers that are built for sheetfed and heatset web offset will not produce acceptable results on digital inkjet web, explains Howard Kirby, Appleton Coated, product manager. Appleton Coated recently introduced Utopia Inkjet—a coated product that features technology developed jointly with HP—for high-speed inkjet web presses. The product line includes both lightweight matte products for the book publishing market and heavier weights in gloss and dull for commercial applications including direct mail.

Some mills are offering digital papers designed to match available offset grades for cross platform production, allowing printers to ensure consistency for jobs that contain offset and digital printing, Mohawk, for example, makes digital products—even envelopes—that match most offset grades.

As digital printing expands into new realms, specialty digital substrates also expand, and now include wide and grand format, pre-converted forms, synthetic sheets, pressure sensitive and magnetic substrates or coatings, says xpdex’s Watson. “Showcase Digital, available through xpedx, is a line of pressure sensitive paper and vinyl manufactured for both toner and HP Indigo application. Web ink jet rolls are now available in different finishes and weights for book publishing, direct mail and commercial print applications.”

All xpedx exclusive digital brands are properly manufactured for optimal performance on specified presses, offering national brand consistency to the printer.

“Using a substrate certified for the specific press is the best way to ensure compatibility between substrate and press,” says GPA's Pergande. “If an incompatible or inferior material is used and runs through the press successfully, there is still the risk that it will not stand up to finishing processes such as UV coating, lamination, folding and scoring. Incompatible materials may not have the moisture content required to withstand the heat of the press and risk cracking when folded or scored. You risk sinking time, money and resources into material that at first may seem like a bargain, but will ultimately cost you in the end when you have to reprint the job.”

TIPS for Choosing the Right Sheets:

(Courtesy of Mohwak papers)

• All press sheets aren’t created equal. They’re designed for specific printing equipment.
• Digital-only substrates are made to precise caliper and moisture levels, and can have surface treatments for dry toner, liquid toner or inkjet applications.
• Risks posed by cutting down litho sheets to use in your digital press include: (1) Introducing contaminants into your equipment, (2) Extra cost — waste, time and labor, (3) Sheets not cut square or won’t easily separate, and (4) Grain direction might be wrong for finishing
• Materials made only for digital — pre-converted forms, synthetics, pressure sesntive materials, magnetics — let you offer more value-added services for a bigger bottom line.