That said, the estimate of narrow web flexo printers who utilize full UV curing of inks and coatings is, at best, 25 percent. Still, considering that the number was about 5 percent a decade ago, it’s a healthy increase in use.
UV curing is familiar to just about every narrow web converter. The majority employ one UV station at the end of the press to cure the varnish that will protect the product, in those cases where an overlaminate film is not used. The science has not changed over the several decades that the curing technology has been in use. Products, however, have continuously evolved to enhance and improve the process.
Suppliers of UV curing systems report that narrow web converting companies range all over the landscape in their knowledge of the products. Some have mastery of the technology, but most possess the basics alone. On the following pages we revisit the fundamentals of UV curing, as well as explore some recent innovations in the field.
UV curable coatings, inks and adhesives are composed of oligomers, monomers and photoinitiators, as well as pigments and other compounds. When they are exposed to light energy of a certain wavelength — in this case ultraviolet light — the photoinitiators decompose into free radicals, which are highly energetic oxygen molecules. The free radicals flit around through the ink in search of the oligomers and monomers to bond with them. “A successful UV curing operation results in a cross-linked matrix of polymer chains,” says Mark Tausch, R&D engineer at UVTechnology, a Mark Andy division based in Milford, OH. “The completion of the cure yields a solid film of polymer.” (About 90 percent of UV products in use today are free radical formulations. The rest use cationic photoinitiators, which present a different chemistry.)
In the electromagnetic spectrum, UV light is just below visible light, which is just below infrared light (IR is a significant player in the UV curing show.). UV energy is measured in nanometers (nm), and that which falls between 250nm and 445nm is important to industrial curing. That range of UV energy is divided into several parts: UVA, UVB, UVC and UVV. Of these, UVC has the lowest nanometer measurement (meaning the shortest wavelength in the spectrum).
Mercury vapor lamps (see page 74) are commonly used to cure UV inks and coatings. “The mercury vapor lamp produces a lot of UVC,” says Dick Stowe, director of technical communications for Fusion UV Systems, Gaithersburg, MD. “Traditionally it has been used to cure press varnishes, and that has been the state of the art for UV curing for many years. Printers have the idea that it’s easy to cure varnishes and difficult to cure inks, because inks behave differently to UV light.
“Photoinitiators have a kind of photographic response to different wavelengths,” Stowe says, “just as color film does. If we have a short wavelength photoinitiator in a varnish, it will work well with a mercury vapor lamp. But longer wavelengths can pass through a material more easily. Different photoinitiators, and lamps that are strong in the longer wavelengths are often what is missing in the transition from varnishes to inks.”
Additive lamps are those which contain metal halides in addition to mercury vapor. “Together they give an expanded color emission from the lamp,” Stowe says. “It’s a simple way of changing the color of the lamp to effect cure over a broader spectrum.” Color, he adds, is a term exclusive to visible light and technically incorrect, “but we’re stuck with our language. In this industry a lot of wrong terms are used,” he jokes. “It’s an industry filled with oligomers, monomers, and misnomers.”
Certain inks — blacks and some whites — cure at higher wavelengths, and benefit from UVA and UVB energy. These wavelengths are produced by the additive lamps. “A lamp with an iron halide additive has a really rich output between 340nm and 400nm, a tremendous output in that range, and that’s what you need to cure black ink,” says Bob Blandford, vice president of Miltec UV, Stevensville, MD. “Black ink absorbs UV light in certain wavelengths below 340nm. A standard mercury vapor lamp has 60 to 70 percent of its output below 340, and the black acts as a sponge. You can’t cure it; you cure the surface, but where the black has to bond with the substrate, it won’t cure. Do a tape test and it comes right off.”
According to Blandford, “When you use mercury vapor lamps on black ink you slow the press way down. That’s a huge problem in the narrow web industry, because you often have plastics running through the presses, and thermal issues really come into play here. If you have six lamps on a press curing flexo inks and they’re running at half speed, you are dumping a lot of heat onto the web.”
Beat the heat
Heat can be a problem. Less than a quarter of the energy produced by a UV lamp is ultraviolet. A good portion of it is infrared light (IR), which is hot. “Bulbs containing metal halides don’t last as long as pure mercury vapor bulbs,” says Mark Hahn, vice president of sales and marketing for AAA Press International, Arlington Heights, IL. “And they produce more heat: There is 35 to 40 percent more IR from lamps that use metal halides. With films, you want to try to use mercury vapor lamps.”
“It is the quartz of the bulb, the envelope, that emits the IR,” says Stowe of Fusion UV, “not the mercury vapor. The first way to reduce IR is by using a smaller diameter bulb, but users have virtually no control over that. Arc lamps have to be large in diameter. For the user, the main point is that the quartz envelope generates that infrared. The larger the surface area, the more IR.”
Most UV systems utilize cooling systems to disperse the heat. Air cooling is effective, and some use water cooling. According to Stowe, different air cooling systems are used today: one is negative cooling, which draws the heated air from the curing chamber, replacing it with cooler air from outside; a second is positive cooling, using pressurized air blown onto the bulbs. Another method is to use lamp air mixed with room temperature are to cool the substrate.
“With most every complaint about heat you can find that there is an inefficient use of the UV,” Stowe says. “It might be compounded by poor air flow design or by not utilizing a step such as the dichroic reflector. In narrow web, speed is used most to cool the web; that actually reduces the heat to the substrate. With a longer wavelength and higher irradiance, you get more efficient cure. And the more efficient, the faster you go.
“Speed is most important. Achieving speed through lamp and ink matching is the second most important factor.”
The dichroic advantage
The UV lamp is encased in a shroud that has a highly polished aluminum surface on the inside. That surface is critical to the performance of the UV lamp because it reflects most of the UV light that does not travel from the lamp directly onto the substrate. Indeed, the reflectors have been precisely engineered to focus the reflected light onto the critical portions of the substrate.
Several years ago, UV engineers perfected a type of reflector that allows more of the IR energy to pass out and through and away from the substrate, while still reflecting the UV light back onto it.
“It’s basically an infrared filter,” says Mark Hahn. “Sometimes it’s coated onto quartz glass reflectors, but that’s not as reflective as on aluminum. It can take out 40 percent of the IR energy. Using a combination of dichroic coatings and variable power control, you can lower the intensity of the energy to match the speed of the press.”
“They are called cool UV,” says Blandford, “but because such a high amount of IR is produced by the lamps they should be called not-as-hot UV. The reflector, which wraps around 70 percent of the lamp, absorbs some of the IR, and if you can get rid of it, you get rid of the heat it brings.
“That IR has to go somewhere. The reflector will get hot, and you can end up with a warped reflector if it’s not designed right. So you have to extract the heat out.”
With the right lamp and dichroics in the reflector, “if you still have thermal issues you can put quartz glass between the reflector assembly and the web,” Blandford adds, noting that it’s referred to as a hot mirror. “There’s a special coating on the back of that glass, a hot mirror coating that reflects the IR back toward the lamp reflector. For a 12" lamp that can cost an average of between $1,500 and $2,000 for a piece of glass. And that’s a reasonable price. I’ve seen higher.”
The use of dichroic materials, he adds, can cause a reduction in IR from 10 to 60 percent. “A lot of that hinges on line speed. At 20 feet a minute — really slow — you might see a 100 percent temperature reduction with dichroic. At 300 feet per minute, your IR reduction might be under 5 percent. The faster you go, the less effect you’ll see from the reflectors. Speed is your biggest ally in heat issues.”
|A basic look at the elements and performance of dichroic for IR reduction, both in the reflector and in the hot mirror between the lamp and substrate. (Miltec illustration)|
Malcolm Rae, of GEW, sees positive changes coming in the UV curing field.
“For the experienced UV user the opportunities are growing daily,” he says. “New digital power supplies give the printer the benefit of accurately controlling the power required for each print station, giving a potential savings of 30 percent in energy costs. Also, by integrating the UV operating parameters with an MIS system, operating parameters, reflector maintenance and bulb changes can be performed on a scheduled basis.
“One area of UV curing that is set to see significant growth is the use of cationic UV cured inks. The food and beverage industry has been hesitant to adopt UV curing for packaging because of the concerns of odor, taint and migration. Cationic inks and coatings do not utilize odor causing photoinitiators, and once exposed to UV energy they are completely cross-linked. The initiation of the curing of cationic inks requires a higher UV intensity compared to traditional free radical UV inks, which the UV equipment suppliers recognize.”
Talented press operators can understand many of the scientific aspects of UV curing, but would rather leave those details to the suppliers.
“We just want cure,” says Gil Dulong, an experienced pressman and partner in G&D Advanced Flexographic Technologies. “We don’t need the fine detail, just functionality. Cure the ink, at these speeds. That’s all we ask.”