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LED & sheetfed UV

Apr 1, 2010 12:00 AM


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While LED lamps are old news on inkjet presses, they are something of a novelty on the sheetfed side.

“LED inks for inkjet [applications] have been available for a couple years now,” says Mike Sajdak, senior R&D chemist for INX (Schaumburg, IL). “They offer less heat generation, a smaller footprint and faster startup times. Offset LED inks are a little newer.”

At Drupa 2008, Ryobi (xpedx) teamed with Toyo Ink to showcase an LED-UV system for a concept demonstration on the 525GX, a 14 × 20-inch press.

LED systems are expected to offer offset users similar energy and lamp savings. Ryobi, for example, claims LEDs reduce power consumption by 70 percent and last 12 times longer than conventional lamps. (Some pundits note that economic comparisons should consider replacement realities: swapping out single lamps on a conventional UV system vs. installing an entire new LED array.)

Other advantages include faster press startups, since LED lamps don't require any warm up time. And since there's no infrared radiation involved, cooling time and expenses are eliminated, although LEDs do require water cooling.

Narrower wavelengths

Ink vendors have inks ready and waiting for printers installing LED UV equipment. “INX has commercially available products that have been tested on a couple of presses available with LED lamps,” Sajdak explains. “It's a matter of printers installing the presses, and although there are some limitations, inks are available for early [LED offset] adopters. Inks will continue to be developed, but the biggest change will probably come from the LED lamp suppliers.”

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 and UV curing source. 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.

LED lamps for offset presses currently operate in a narrower window of wavelengths vs. the arc lamps typically used on UV equipped presses. To paraphrase Spiderman's dilemma, with less power comes great spectral challenges.

“Depending on which LED supplier you are working with, three wavelengths predominate: 365, 385 and 395 nm,” says Sajdak. “An LED UV light source is required to work with the photoinititator, but only certain photoinititators will start a reaction at those wavelengths. With conventional UV printing using a standard medium pressure mercury bulb, the UV ranges are lower, [typically] 250 up to 395 nm.”

When formulating conventional UV inks, vendors can choose from as many as two dozen photointiators. But raw material suppliers have fewer offerings for LED UV inks. “With LEDs, we have maybe a half dozen photointiators,” says Sajdak. “That's been the biggest challenge. There's also the question of the ideal photoinitiator combination.”

On inkjet presses, LED systems can be positioned as little as 3/8 inch from the printing surface. According to one published report, LED systems on sheetfed presses currently are 4 inches from the substrate.

“Getting the lamps closer or being able to focus them better is a challenge,“ says Sajdak.

INX also is working on some products for LED flexo presses. “We're working with some lamp suppliers,” says Sajdak. “We've got some inks, but we might need a little bit more power.” He adds that food packaging is likely to lag behind other market segments in using LED-equipped presses because few current photointiators are suitable for food packaging.

Special effects and heavy coverage

Bill Bonallo likes the idea of LED UV technology for offset printing. “We believe in LED,” stresses Bonallo, vice president of IST America Corp.'s (Bolingbrook, IL) sheetfed group. “We have had a development team dedicated to LED since before Drupa 2008, where we showed a proto type LED unit actually curing opaque white on a conveyer system.”

Bonallo says he champions the technology, “But I believe as we learn more about it, we'll see greater niche applications vs. sheetfed formats of 28 inches and larger,” he says. “There's a lot of marketing hype around technology that isn't [fully] developed. There are some real limitations, particularly for midsize and large-format sheetfed applications.”

Bonallo says the LED UV sheetfed demonstrations he and his colleagues have seen outside the United States featured simple 4-color jobs run on presses with LEDs at the end of the press to support quick throughput rather than inline coating. “Getting an LED light source close enough for efficient curing between printing units is quite challenging. We haven't seen LED used with the special effects and super heavy coverage or coating so common to this market. Installing an array after the last coating unit on the end of the press to speed production [seems to be the current trend].”

According to Bonallo, coatings typically represent a thicker film on a substrate than inks. “So you've got the issue of curing that level of material,” he submits. “Top cure also is a challenge. LED appears to be a bit better for deep cure vs. surface cure.”

Some vendors maintain that a controlled amount of heat provided via the IR portion of the UV spectrum on conventional lamps can effect actual adhesion of the ink and or coating to the substrate. Ink formulators are researching the impact this lack of IR-generated heat on LED applications.

“I'm not sure how [LED] is going to fit into the midsize to very large sheetfed world as we know it in the United States,” says Bonallo. “We use lots of coatings, lots of printing units and inline specialty materials, and that will definitely pose a challenge for LED.”

INX's Sajdak says the scenario is similar to that faced by today's UV offset printers. “Having sufficient energy to penetrate through an ink layer is one of the inherent issues with UV in general, making sure you have enough power to penetrate the ink/coating layer to the bottom. Part of the solution is lamp intensity, part of it ink formulation. It requires some modifications, but it's the same situation you have with conventional UV.”

Bonallo believes that, existing inkjet applications aside, LED will have the greatest impact on flexo and narrow width presses. “We don't believe it's the universal miracle curing technology for sheetfed, but a technology that will supplement current and upcoming conventional UV curing technology. We absolutely believe in it, but we also believe it will have limitations.”

Bonallo warns against pushing a new technology into the market prematurely, citing his experience helping to develop CoCure (hybrid UV). “With early CoCure, it was all about the ink formulation and the supporting on press chemistry, we were using well established UV technology. There is no doubt over zealous marketing brought that product to the trade prematurely. Until the process and the inks were refined to provide a level of consistent predictable results, there were a lot of issues. Remember, we were only dealing with the ink side of the equation. With LED, beyond the limited number of photoinitiators for the inks at restricted limited wave lengths, we are also creating a new UV light source. The lesson should have been learned that all good things take time.”


Katherine O'Brien is the editor of AMERICAN PRINTER. Contact her at KOB@americanprinter.com.

Silver lining

The same wavelengths that pose an ink challenge for LED UV offset applications offer some environmental benefits. “Because the wavelengths are between 365 to 395 nm, you don't generate ozone using LED light sources,” explains INX's Sajdak. Another advantage: Unlike conventional UV lamps, LED systems contain no mercury. “When [conventional] UV lamps are recycled, this generally isn't an issue, but it can be if bulbs are broken,” says Sajdak. “LED eliminates [that possibility].”

Making the case for recycled UV

In 1992, trade association RadTech commissioned Beloit Corp. to perform a study to dispel some misconceptions about recycling material printed via UV and EB. The results were presented at the 1992 RadTech conference and, by popular demand, reprinted in the May/June 2005 RadTech Report. See www.radtech.org.

Power savings

Because it is VOC-free, UV technology is often cited as environmentally friendly. Less attention is generally given to power requirements for UV equipped offset presses, which can be considerable.

“We're pursuing a higher level of efficiency ink cure with a lower level of energy,” says IST's Bill Bonallo. “We're gaining the ability to run with lower power lamps at lower outputs.”

At Drupa 2008, IST unveiled the BLK-5 UV drying system developed specifically for web printing applications where high process speeds are critical. Two of the units at 180 W/cm achieve the same curing results that previously required three BLK-2 UV units at 200 W/cm. Electricity costs reportedly are reduced by up to half.

A German trade association tested the IST UV system and confirms an increase in efficiency of 40 percent. On June 2, 2008, the trade association awarded IST METZ its certificate for energy-minimized UV. IST METZ is the first global manufacturer of UV systems to earn this certificate. This past November, IST also won an ICE Innovation award for the BLK-5 curing system.

E-mail IST America Corp. at info@USA.IST-uv.com.

The UV files

You'll find these and other UV articles online at www.americanprinter.com:

“Time to shine,” January 2010

“Tough enough,” January 2009

“It's all about UV flexibility,” June 2009

“Fast, fancy & flexible,” May 2008

“Hybrid UV then & now,” May 2008

“Razzle dazzle: inline UV,” February 2007

“A bright future,” March 2006

“Bright lights, better productivity,” October 2005