Offset vs. Digital Sheets: What's the Real Difference?

Offset versus digital papers: does the choice matter?

“While it is possible to run some offset sheets on digital presses and vice versa without problems, it’s taking an unnecessary risk,” said Mike Blanco, GPA’s director of R & D and technical services, adding it’s critical to accommodate each press’ unique requirements.

Offset presses use moderate heat, oxidization or UV lamps to cure ink. Dot gain and drying problems are common issues with substrates, said Blanco.

“Certain sheets have greater dot gain, which need to be compensated for through pre-press or color management methods,” he said. “Drying concerns pose issues like having to run jobs in smaller pile heights to cure the ink.”

HP Indigo presses use plastic particles of ink cured through heat and transferred to the substrate with pressure; heat evaporates the oil in the ink and through pressure, the ink adheres to the substrate, said Blanco.

“To run on an HP Indigo press, substrates require the right surface chemistry to achieve optimal ink adhesion,” he added. “Oftentimes, a special coating is applied to the substrate to achieve this.”

Toner digital presses use fuser oil, pressure and high heat to fuse ink to the substrate’s surface.

“Unlike sheets running on an HP Indigo press, many paper substrates for toner digital devices do not require a special coating,” he added. “However, many specialty substrates require a surface treatment. Also – due to high heat – you have to ensure you’re working with a material that can literally handle the heat. Synthetics not approved for toner digital devices can melt in the machine.”

The substrate’s grain direction plays a big role in how well it feeds throughout different press types, said Blanco. “The HP Indigo 7800 and iGen presses feed the sheet in portrait layout; most offset presses feed in landscape layout. HP recommends cover weight substrates to be grain short and text weight substrates to be grain long, allowing the sheet to flex along the grain direction as it transports through the press.

“Offset press recommendations, as well as the new HP 10000 and 30000 presses, would be the opposite since they feed stock in landscape layout,” said Blanco. “These combined factors allow substrates to transport through presses with more ease. The wrong sheet could result in jamming or transport issues.”

Laura Williams, product marketing specialist for Nekoosa, said a key variable is run length.

“Assuming static print – not variable – you need enough impressions to justify inking up an offset press,” she said. “The bigger the press, the more finished pieces you will need to justify going offset. Make-ready alone on some offset presses can eat up an appreciable amount of paper and sometimes press time.”

On a small sheet-fed offset press, printers usually will opt to print offset if number of impressions exceeds 5,000, Williams said.

“This breakpoint number may be less if operating a direct imaging press where plates are imaged electronically on press,” she adds. “PSPs vary somewhat on where the breakpoint lies for them to go offset versus digital.”

If the job calls for variable data printing for personalization or versioning, “it almost always makes sense to print digitally,” noted Williams. “If a versioned job requires sizeable runs of each version – such as different languages – it may pay to make multiple sets of plates and still run job offset. It may simply depend on the size of the total print job or the size of each component.”
If the printing substrate is not available in a suitable digital or offset printable grade, the printer may not have a choice of which way to go, Williams points out.

“Many synthetic sheets are problematic to print on,” she said. “Some can be treated for various digital print platforms. Many digital presses have limitations on the caliper or basis weight of what will successfully image or transport. It will depend largely on the specific equipment mix in the printers shop: 40-inch press, duplicator press, direct imaging press, xerographic digital printing device, or HP Indigo, among others.”

Gavin Gaynor, vice president, research and development, Mohawk, said it’s important to select a digital sheet optimized for digital printing presses:

• Sheet-feeding and the transport mechanism. “Sheet pick-up systems in digital presses are often less forgiving than in an offset press. A curled or non-flat sheet can lead to misfeeds and jams,” he notes. Co-efficient of friction is another consideration: sheets that don’t slide easily over one another tend to cause double feeds. Cut quality’s important: use sheets without welded edges that can cause double feeds. Squareness and dimensional accuracy gives good registration, particularly side-to-side. No ragged edges means no slitter dust, which can contaminate the inside of the press. Static can cause jams and double feeds.
• Image transfer and the fixing of the image to the sheet. Surface resistivity that’s too high or low means the charged particles composing the images can’t be effectively transferred to the substrate. Like offset printing, sheet uniformity contributes significantly to overall print quality.
• Post-press considerations. Since most digital presses heat the sheet in one way or another, use a sheet that doesn’t respond to heating with excess curl or static.

Daniel Dejan, Sappi’s creative manager, said printers use the term “crossover” to define the point at which an advantage can be gained by switching from one printing process to another.
“For many years, this was a fairly simple question of per-unit cost. Sheetfed, web and digital all had clear points at which they started—and stopped—making financial sense,” he said. “Continued innovation in each of these printing processes has made them competitive over a wider job range, providing a new level of printing flexibility but expanding crossover decision-making well beyond simple per-unit calculations.”

Sheetfed and web offset printing are still considered optimal for the highest level of print quality, with digital making up ground quickly, Dejan noted. Digital printing offers turnaround time advantages due to not needing make-ready or drying time, he said.

All three processes have distinct finishing advantages, Dejan said. “Sheetfed allows for extraordinary special effects. Web presses offer the most powerful in-line techniques that reduce post-production time. Because digital sheets finish dry, they’re usually quicker to get out the door.”

For personalization, digital wins for text and visual flexibility, said Dejan. Digital might not work for larger projects because of digital press size limitations of 13x19 inches, he added.
Sheetfed and web printing’s higher upfront costs make them less common for smaller print runs, noted Dejan, adding continued improvements in digital’s print quality enables its use for higher-volume jobs.

PSPs need to be cognizant in using coated papers due to heat and toner issues during the printing process, said Dennis Essary, Verso Paper’s director for digital papers.

“In coated papers, there has to be something for the toner to adhere to, whereas with uncoated, the toner is going to stick to the fiber,” he added.

Essary hears PSPs talk of using offset papers for cost savings, only for them to encounter machine cleanliness or higher maintenance problems.

“Moisture content changes electrical properties in your sheets,” he said, adding PSPs should choose a sheet with the most consistent electrical properties.

Cut consistency is important for moisture control and registration as well as keeping the machine clear of debris, said Essary.

“True digital papers are formulated to lay flat through extreme heat and are made with a surface sizing to attract and adhere toner,” said Beth A. Povie, Finch’s marketing and communications director. “Other papers are simply cut to a digital press size.”

In any digital printing, “the PSP needs to look at job demands and determine the required quality,” noted Rick Williams, digital printing expert, International Paper Company. “Low-quality single color tends to lean towards standard offset. Multi-color works needs higher-quality digital papers made for these applications.”

Howard Kirby, product manager for Appleton Coated, said that for the most part, especially with coated papers, a PSP can use the same product, but just a different size between the offset and digital sheetfed equipment.

There are exceptions to that, such as Indigo, which needs products that have been designed or treated for it, and newer sheetfed inkjet presses, he added.

In sourcing paper for digital platforms, a PSP can either purchase the digital size from a distributor or mill or buy folio size that also fits offset equipment and cut it down for the digital equipment.

“I’ve seen estimates as high as 45 percent of the coated paper that’s going to digital sheetfed platforms is cut down from folio,” said Kirby. “If that’s what you’re going to do, you have to trim four sides. I know people who don’t do that today and I would say it’s very ill-advised from the standpoint of mostly everybody who does sheeting in North America today does precision sheeting, meaning that the size and the square of the sheet is perfected for the size that it is cut to if you want absolute perfect registration side to side and sheet to sheet.”

Side-to-side registration control is different for every digital platform, noted Kirby.

“The faster the platform is, the more sophistication that goes into that and the people who make the equipment have the expectation that when you bring paper to that platform, you’ve done all of the necessary steps to make sure it’s all cut the right size and that it’s square. If you start with either of those two not correct, you’re never going to hold side to side with the digital platform.”

Buying from a distributor or mill ensures a product sheeted specifically for an application, Kirby pointed out. PSPs who want to sheet on their own “really need to have the equipment and people come in to make sure they can do it with the precision that’s necessary and also find out the number of cuts than can be done.”