The Science of Solvent, UV, and LED Inkjet

He was a bit odd but Fred Hein also was a good teacher. As we walked the halls between classes in high school, students would hear Fred—“Don’t call me Mr. Hein,” the one-time hippie instructed us—slowly drawl the word “physics” outside his lab room door. The tall, slender scientist’s frizzy-long hair framed his smiling-yet-eerie face as the single word oozed enthusiastically from his thin lips: “Physics,” he repeated again and again. This tactic was Fred’s own, strange way of recruiting prospective students for his popular course – and it worked. That, plus getting to go on a year-end field trip to a Six Flags amusement park because physics, like printing, is everywhere; it’s all around us.

“You do not have to be very observant to see the growth occurring in wide format graphics,” said Bruce Carson, CEO of diversified print firm The Dot Printer and Bindery (Irvine, CA), which tripled its large-format digital capacity last August by adding an EFI VUTEk GS2000 flatbed inkjet printer. The eight-color machine prints up to 80-inch graphics imaged direct to rigid substrates with resolutions up to 1,000 dots per inch (dpi). “From special event flags, to vehicle graphics, to signage at baseball parks, to floor graphics, to shopping malls, it is everywhere and expanding rapidly,” Carson observed.

Inkjet printing science is a blend of chemistry and physics; chemical physics, if you will. The challenge for inkjet inks is the conflicting requirements for a coloring agent that will stay on the surface versus rapid disbursement of the carrier fluid. There are academic papers on the topic of strong ultraviolet (UV) radiation exposures used to instantly cure ink, which I’m sure would make my old physics teacher smile, God rest his free-loving soul. But the difference between UV-cured and solvent-based technologies goes further than the ink chemistries. Which type of formulation is used depends on many factors – and these aren’t always obvious at first glance. It all comes down to what you want to produce now and in the future, and what you are producing it on: the media.

“Printing is the simple part,” half jested Larry Salomon, VP of Wide Format for Agfa Graphics in North America. “The complex part is [all] the media and finishing choices,” stressed the 25-year wide-format print veteran, who has had stints with Charette and Pitman. “Agfa has over 25,000 media SKUs [stock-keeping units], from 24 inches to five meters wide.”

With so many variables, when consulting with customers and prospects on which inkjet route to take, Salomon said there is “no Swiss-Army-Knife approach.” Agfa typically conducts an informal survey that includes two key questions:

  1. What is your current production system in place?
  2. What are your most common print applications? (What percentage of work is going outdoors, for example, or is printed on rigid vs. flexible media?)

Most people ballpark the second answer, but some are able to provide detailed, 12-month analyses of the type of work produced in their shops. It’s not always a case of either/or—solvent or UV; flatbed or roll-to-roll. More often than not, Salomon added, if a wide-format print firm has more than five employees and/or at least $500,000 in annual sales, it most likely has multiple pieces of production equipment on the floor. Some print firms do a little of everything (posters, banners, backlit, vehicle graphics), he explained, while some specialize in a niche such as exhibits.

Image duration is no longer an issue, as both solvent and UV inkjet printed pieces can survive up to five years “if done on the ‘right’ media and laminated,” Salomon noted. And where image quality is concerned, “it’s impossible to tell the difference from three feet or further away,” he added.

The deciding factor is end-use application. Solvent and latex inkjet tends to be a superior choice for roll-to-roll printing on flexible media, for such products as banners and car wraps, because the ink is more pliable and better able to go over rivets and seams. (Also see “Latex Printing: Hope or Hype?” Agfa solvent-based inks feature low profiles, lending them even to fabrics and sheer materials. UV-cured inkjet, on the other hand, is ideally suited for rigid media, including plastic, PVC, Styrene, board, and even granite. (Other UV applications include second-surface imaging and reverse printing inside polycarbonate glass.)

Inkjet 101

Each inkjet process has its reputation. Solvent-based inks are known to be more cost-effective, while UV-curable inks cost more per liter but are more sustainable. Industry analyst Henry Freedman compared the two technologies. “Significant differences between solvent and UV inks exist in both sustainability and economics,” said Freedman, who also edits the quarterly Tech Watch newsletter. “A UV system must be very complete in exposing all the ink chemistry so [as] not to release uncompleted drying chemicals into the environment. Solvent impact is more fully defined, being a more mature chemistry.”

Steve Emery, senior director of EFI’s ink business, added, “Solvent—up to 80 percent—is inherent in solvent ink. It is the carrier, in essence carrying the pigment dispersion [the color]. Solvent softens or swells the media so that the ink can penetrate into the top layer,” Emery explained. “The media and the ink almost become one, then the solvent evaporates off.” Agfa’s Salomon noted that so-called eco-solvent ink contains diluted solvent.

There is a chemical reaction within the UV curing process, too, of course. “The ultraviolet light is a catalyst,” Salomon went on to explain. “When UV rays hit the ink, it transforms from a liquid to a solid.”

Emery of EFI said, “At some point UV [inks] will crack. There is a deformation of ink, and it won’t come back together.” At the same time, UV inks are becoming more flexible. “Their elongation is getting better,” Emery noted, “so we’re beginning to see UV displace solvent on some car-wrap applications, but not on banners so much.” He added that EFI, along with media partner 3M, will be launching more flexible, fade-resistant UV ink sets later in 2013.

For UV-cured inkjet technology, monomers and oligomers are used to carry the pigment, Emery continued, which is dried (cured instantly) by a mercury arc lamp light source. According to Wikipedia, “an oligomer is a molecule that consists of a few monomer units, in contrast to a polymer that … consists of a nearly unlimited number of monomers.” A polymer is a chemical compound or mixture of compounds consisting of repeating structural units created through a process of polymerization (a.k.a. plasticization).

In physical chemistry, photoinitiated polymerization is a chain reaction of monomer to polymer initiated by a photogenerated radical or ion (source: McGraw-Hill Dictionary of Scientific & Technical Terms). In an UV inkjet printing context, ultraviolet light triggers photoinitiators that typically react to specific wavelengths within the 200- to 400-nanometer range. In the case of more heat-efficient, light-emitting diode (LED) curing, Emery noted, photons from the LED do the drying. “These photons interact with photoinitiators, which are a component of the ink set,” he pointed out.

Agfa’s Salomon is quick to counter with his own scientific equation, supporting the belief that inkjet is more about the media than the ink: “Ink represents only one or two percent of the mass of a finished graphic,” he summed up.

Economic Trade-Offs and ROI

In terms of upfront costs, eco-solvent inkjet printers range in price from about $20,000 to $25,000, Salomon said, while UV-curable device price points start at around $70,000 and can ramp up to $1 million, depending on the speed and sophistication of the output device. For Agfa’s high-speed :Anapurna flatbed models, prices range from $70,000 to $180,000, he added. Salomon encourages print firm owners to think in terms of productivity, not sticker price. “It’s all about how the work is getting done today, and technology drives consolidation. One M-Press can do the work of 100 devices from 10 years ago,” he explained, commenting on the commoditization of wide-format print. “At $10,000 a month, an entry-level machine could have a three- to four-year payback.”

Process workflow efficiencies are keys to defining an economic comparison, Freedman added. “A more expensive ink price per unit could cost you less in the end.” For example, solvent ink prices are considerably lower, Emery said. However, because UV inks are not absorbed into the media like solvent-based inks are, UV’s coverage is some 25 percent greater.

When comparing inkjet applications, the UV price per square foot is somewhere between five cents and 15 cents for printing on 4x8-foot board (rigid), noted EFI’s Emery. But such statistics can be deceiving when discussing total cost of ownership. For one thing, there is no evaporation of pigments or carriers in the UV curing process, Salomon pointed out, “so what [ink] goes on the board, stays on the board.” With solvent and latex inkjet printing, there is “out-gassing,” and print shop owners “pay for that vaporization,” he emphasized. Plus, there are productivity advantages to UV’s instantaneous cure attribute. “There is no waiting,” Salomon continued. “For big store chains that want to turn around 10 pieces per store, that means there’s no warehouse storage [costs].”

Also consider that UV-cured inkjet systems print directly onto rigid substrates and do not require adhesive, which saves money on consumables (no glue) as well as labor (no trimming and mounting time). Depending on the volume being produced, these savings can be substantial, said Agfa’s Salomon, “ as much as $1 per square foot.”

(For more in UV curing, please see “Enter UV’s Brave New World:”

Inkjet Market Stats

Business research firm Markets and Markets has predicted strong growth for wide-format printing, which it said is expected to reach more than $12 billion globally by 2016. At competitor InfoTrends, wide-format director Tim Greene reported that the solvent squeeze is on, eco or otherwise, especially in North America. His company’s latest report, published in December, echoes 2011’s findings, contending that wide-format solvent inkjet is under attack as a production technology.

The global market for wide-format, UV-curable inkjet printers and supplies is expected to grow to more than $3 billion in 2015, doubling its 2010 figure. In fact, 7,500 new UV inkjet units were sold worldwide in 2012; some 1,900 of those are in North America. Annual global UV shipments are expected to grow at double-digit rates, to more than 11,000 by 2016 (>3,100 in North America), according to InfoTrends. To date, some 16,000 wide-format UV inkjet devices are used in production daily around the world.

Conversely, solvent inkjet compound growth is crawling at less than 2.5 percent, while the installed base is relatively flat (0.8 percent). Of some 52,000 solvent and eco-solvent units sold worldwide last year, less than 10 percent were sold in North America, with only about a 3 percent compounded increase projected through 2016. Why the disparity in the global numbers? In a phrase: environmental regulations.

With less than .06 percent of volatile organic compounds (VOCs), UV inkjet is a far more sustainable solution. “Solvent is still very prominent in Latin America and the APAC [Asia-Pacific] countries, where the environmental standards have not changed,” explained Steve Emery, senior director of EFI’s ink business. InfoTrends’ Greene added to that geographic list Eastern Europe, parts of which do not abide by ecological laws because they are not aggressively enforced by the government. In the United States, however, printers have the EPA and OSHA with which to contend.

“On one side there is the development of the durable aqueous segment, which promises high quality and durable output but without the harmful environmental characteristics of solvent inkjet,” the 2011 study said. “On the other side there is UV-curable inkjet, which offers faster production speeds, instant-drying performance, printing directly onto rigid substrates, as well as improved environmental characteristics.

“Overall, these alternative wide format technologies are driving customers away from solvent inkjet devices,” InfoTrends continued. “The wide format solvent inkjet printer and supplies market is still valued at more than $2 billion in 2015, so there is plenty of opportunity for suppliers to profit. The focus should be on low-end devices and emerging markets.” InfoTrends also advises solvent inkjet manufacturers to diversify product offerings to stave off losses.

Greene added, “This trend does not just impact the sale of new units, many installed solvent inkjet printers are being replaced by durable aqueous and UV-curable wide format printers as well. Even when the printers aren’t totally replaced, solvent printers are often maintained just to produce a certain set of applications for which the technology is best-suited such as billboards or vehicle graphics.”

A 2011 InfoTrends/FESPA survey revealed that more print service providers intended to invest in UV-curable and durable aqueous inkjet in the future than intend to invest in either eco-solvent inkjet or solvent inkjet. 

(InfoTrends’ complete “2012 Wide Format Solvent Inkjet Printing and Supplies Forecast” is available at

High LED Growth

Ultraviolet (UV) inkjet curing’s downfall may be that the process requires too much heat. “Imagine a hair dryer heating a sheet of paper,” said Steve Emery, senior director of EFI’s ink business. “You’ll end up with print-head strikes and there will be buckling and warping” in what he called roll-to-roll’s “mountain-and-valley” effect.

The lure of light-emitting diode (LED) curing is that it can safely accommodate thinner media materials because the light source doesn’t put out such intense heat. “LED even can handle perforations on adhesives—and the color is just as good,” Emery continued. Another benefit is bulb quality. “LED bulbs stay consistent up to 5,000 hours,” he said, while mercury arc lamps progressively deteriorate and last only about one-fifth as long.

LED is something that EFI looked into eight to 10 years ago, according to Emery. “but the output wasn’t high enough [then] to do proper curing.” Presently in the super-wide-format space, EFI is the only manufacturer with an LED solution, Emery added, of which they have sold a “significant number.”

Tim Greene, director of the wide-format service at market research firm InfoTrends, pointed out another consideration in Third World countries. “It is true that LED’s [running] costs are lower because it uses less energy,” said Greene. “But there are less-than-reliable power grids in other parts of the world, including many parts of Asia.” In such emerging markets, the less power that is needed, the better for print providers.

LED will continue to grow, Emery concluded. “It’s growing day by day.”