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May 1, 1999 12:00 AM
Introduced in the early 1970's, UV inks are showing steady growth in offset applications. These inks produce vibrant, high-gloss color on substrates from metal to plastics to paperboard.
At one time reserved for auto brochures, corporate annual reports and high-end cosmetic packaging, UV inks are now used in virtually every printing process and market. RadTech International, the trade group for the radiation-cured industry, estimates that North American offset printers consumed 4,000 metric tons of UV/EB (electron beam) ink during 1997, compared to 3,000 metric tons just four years ago.
What's fueling this growth? UV inks and curing systems have gotten better as they've gotten older. "UV curing has overcome a lot of hurdles," reports consultant Terry Scarlett. "Initially, skin sensitivity to the new chemical composition of the inks was a problem. De-inking also was more difficult and good lithographic qualities were hard to obtain. Today, most of these problems have been solved."
Scarlett adds that the energy crisis, coupled with environmental concerns, were what first attracted web offset printers to UV 25 years ago. "Unable to get gas for their dryers and, simultaneously facing new environmental regulations aimed at plant stack emissions, web printers began to look at UV drying seriously for the first time," recalls the consultant.
Environmental concerns remain a key factor behind the UV boom. Indeed, a recent RadTech survey found that 89 percent of UV users cited environmental compliance as their main reason for using UV/EB technology. The pressure to eliminate emissions of volatile organic compounds (VOCs) favors UV inks, which contain virtually no VOCs. In some areas, use of UV inks is the only way certain printers can be permitted. More U.S. shops are likely to adopt UV technology as federal and local air quality standards become more stringent.
Another UV benefit is the inks' drying characteristics. Since these inks don't set until exposed to UV light, they stay open on press, resulting in fewer wash-ups. Other ink systems dry largely by evaporation, substrate penetration, oxidation or a combination of these. UV inks, by contrast, polymerize to a solid by exposure to UV light (usually at 200 to 400 nm) almost instantaneously, depending on the ink formulation. The active components of the inks are a prepolymer resin (comparable to the hard resin in a conventional ink), a dilutent that also takes part in the curing reaction, a photoinitiator that responds to the UV radiation and starts the reaction (like the drier), additives (such as waxes to aid scuff resistance) and a pigment for color.
John Petruso, pressroom supervisor at GBF Graphics (Skokie, IL), cites durability as another benefit of using UV inks. Many of GBF's direct mail pieces are printed on coated paper that will be folded and laser-printed. UV ink provides hard, dry, highly cross-linked print that resists rubbing and marking.
Although UV inks typically are two to three times more expensive than regular heatset inks, their versatility often justifies the cost. Ray Busse, president of Busse Printing & Packaging (Phoenix), has used UV ink since 1985 to print brochures, presentation folders, childrens' books and other jobs. "UV can print on substrates other inks can't," notes the exec. "With conventional inks, a 60-inch roll of 18 point board with 12-inch core weight would offset, but UV won't."
Busse Printing & Packaging ("debt-free for 27 years" according to its president) has earned a reputation for tackling unusual jobs, with substrates ranging from nylon to credit cards. A recent run of 100,000 direct mail pieces was printed with oil-based inks on one side and flood UV used on the back to seal it.
New users often have to alter operating routines to accommodate UV technology. It may require different blankets, rollers, plates, e.g., switching to bimetal plates from surface plates. Petruso says that while the UV inks require careful attention to roller settings, fountain solution and ink and water balance, with experience "it's like tying your own shoes."
The most common issue associated with UV inks is its effect on blankets and rollers. Most rollers used with conventional inks are made out of synthetic rubber compounds. However, when UV ink meets this rubber, the acetate monomer and photoinitiators they contain react with the rubber, causing it to swell. Swelling can throw off the impression, affecting registration and softening the image. Users may need special rollers and blankets made specifically for UV inks. These are made with polymers that are resistant to the oxygenated solvents and concentrated acids, which are detrimental to rubber compounds.
The same problem can occur with photopolymer surface printing plates. The monomers and photoinitiators can rapidly degrade dot integrity in much the same manner, forcing plate replacement in anywhere from 300 to 100,000 impressions, according to UV ink users. This wide latitude takes many variables into consideration, such as roller pressure, paper, the press used, its condition and age, etc.
An alternative is to use bimetal printing plates. The inert copper surface of these plates is unaffected by UV ink, which tends to dramatically extend run lengths compared to photopolymer plates with no loss in print quality. "We've gotten as many as four million impressions with a single bimetal plate," reports Busse.
Unlike photopolymer surface plates, bimetal plates can be sharpened to overcome dot gain on press. During processing, both surface and bimetal plates are exposed by light and developer is used to strip away unexposed photopolymer. Bimetal plates can take the process further--the photopolymer sits on a thin layer of copper atop aluminum or stainless steel base. Further developing uses the polymer as a stencil for the copper--an etchant removes copper in non-image areas. Etching is varied from 0 to 20 microns to control how much copper is eliminated beneath the polymer dots. As a result, users can control press dot size to that of the original film.
"The image quality with these plates is three to five percent sharper," relates Petruso. "We run a lot of uncoated papers--it's very forgiving on dot spread. There's no tint problems--it runs clean."
Because UV inks can work differently on press than conventional inks, it is important to consult every vendor in the supply chain. They can be invaluable in making the necessary adjustments regarding blankets, rollers, plates, fountain solutions, and, of course, the ink. Ink suppliers, for instance, often provide training and may even formulate products to help meet the specific needs of a printing plant.