Oce white on black (photo by Kelli Ramirez)

Working With a UV-Cure Flatbed

Ken Mergentime is the former executive editor of Sign & Digital Graphics and WRAPS magazines.

The demand for large-format graphics printed on rigid substrates is stronger than ever. Wide-format printer manufacturers have responded with an overwhelming number of UV-curing flatbed printers that can handle the job. The technology behind these machines has improved vastly over the years, with better ink formulations, better media handling options and improved print speeds.

We asked some flatbed printer manufacturers to offer up some tips on working with UV-curing flatbeds, with an eye toward getting good, consistent results and avoiding common pitfalls that those new to the technology might encounter.

UV-Cure Lamps

One of the key components to any UV-curing printer is the UV light source that serves to cure the inks. Many of the lamps in today's machines use wide-spectrum mercury vapor lamps (also called arc lamps, or H-bulbs). However, a growing number of machines today use LED lamps to effect a cure.

"LED lamps have advantages over mercury vapor," says Guy Cipresso, vice president sales and business development at Novus Imaging, Moultonborough, New Hampshire. "LED lamps run cooler and lamp life is much longer—estimated 1,000 hours for mercury vapor vs. 20,000 hours with LED lamps."

"The market has started shifting away from H bulbs," says Dave Cich, a founding partner with Vanguard Digital Printing Systems, Suwanee, Georgia. "LEDs use less electricity, so overall operation cost is less. They also emit less damaging UV light, so they are safer for the operator to run." And, because LED lights require no warm-up time and can be turned on or off instantly, "so there are no lamp shutters opening and closing that can get covered by overspray," he adds. "Plus, LED machines require less maintenance and are more dependable."

Joe Garcia, managing director-Americas, for StratoJet USA, Santa Fe Springs, California, notes that constantly turning mercury UV lamps on and off can wear them out sooner, so he offers one way to prolong lamp life. "Try to get all of your printing files ready each day so they can run consecutively during production. That will minimize the number of times that you turn off the lamps and can help them last longer," he says.

Also, the UV intensity of a mercury vapor lamp will diminish over the life of the lamp, Garcia says, adding that "most UV printers have lamp power settings, so be sure to start with the lowest setting and move up in intensity as needed over time.  You will know when you need to move to a higher intensity when your prints begin to have some tackiness to the touch.  Once you’ve reached the highest intensity and you start to get that tackiness, that is a sure sign that it's time to replace the UV lamp."

Cich recommends that operators clean the lamp's glass at least once a week because overspray can get on the glass which will negatively affect curing. "Also, be sure that your printer's power source is clean and at the recommended voltage. If power fluctuates, or is below the manufacturer's recommendations, it will shorten the life of the bulbs."

"Inspect and clean the lamp(s) reflectors and quartz plates daily to remove dust, powder, smoke and misting," Cipresso says. "Change the air filters as required to ensure the best air cooling for the lamps. Inspect to make sure the lamp shutters are opening completely."

Cich prefers that operators change the lamps sooner rather than later. "When inks are not fully curing, change the lamps," he says. "In the long run, it probably is not worth the extra week or two you gain by increasing the setting." He also recommends that you "change both bulbs at the same time, because if you change only one, the newer bulb will be stronger and that can lead to cure-banding," Cich says.

Cich, Cipresso and Garcia all seem to prefer LED-based UV-curing systems for flatbed printers. "UV LED cure systems do not have the problems of mercury vapor, since lamp intensity remains constant for the life of the LED," Cipresso says. However, there is a catch.

"The down side of LED lamps is that they can cost a lot more than mercury vapor lamps," says Cipresso. "LED lamps emit a narrower light wavelength than mercury vapor, so inks may not cure properly at the highest print speeds on certain substrates. This means you may have to make additional cure passes which reduces productivity. However, LED technology is evolving rapidly, and as the lamps performance become more robust and demand increases surely the price of LED lamps will decrease," he says.

Static Buildup

Print jobs often must be printed onto plastic sheet substrates such as PVC, Sintra, corrugated plastic, styrene, and etc. But plastics are prone to picking up static electricity, and depending on the ambient conditions within the shop, sometimes the static charge in the plastics can cause printing defects. Ink particles are attracted to the charged areas of the substrate like a magnet, deflecting them from their intended path. This results in printing complications such as clogged nozzles, dust attraction, and overspray. However, there are ways to minimize the levels of static in the shop.

"Always wipe down plastic sheets on both sides with isopropyl alcohol," says Cich. "It cleans the sheet and releases built up static. Humidity suppresses static, so the higher you can keep humidity in the printing area, the better. The ideal level for most printers is around 60 percent relative humidity." He believes that the only true solution is to purchase an ionization bar or static wand. "These printers tend to generate static as the printheads pass back and forth, so it's best to eliminate static right at the area where you are printing," he says.

Garcia says that using a humidifier is a good idea, and he suggests keeping the humidity at levels between 40-70 percent non-condensing. He also suggests using an anti-static spray just before printing.

Cipresso says there is another option as well. "One of the newest technologies is something called a static neutralizer. All materials have a positive or negative charge, and as the static neutralizer passes over the substrate it detects the polarity of the charge and neutralizes it. Our newest flatbed machine has this technology built into it," he says.

Preventing Head-Crashes

A head-crash occurs when the printhead strikes the surface of the substrate being printed. For example, a printhead may strike the substrate as it passes over the high area of a warped sheet. This problem can in some cases be very costly to repair since the printhead is the most complex, delicate and expensive part of the printer. Many flatbeds have a crash sensor feature that can lessen the frequency of head crashes, but prevention should be the byword.

"Store your media flat, so that it does not warp," Garcia says. "You should always make sure the printhead head height is at the manufacturer's spec. Always check for uneven surfaces on the substrate before printing, and make sure that the printer's vacuum table is turned on." The vacuum system can help eliminate minor warps in sheets during printing.

"Always make sure the substrate to be printed has had sufficient time to stabilize in the print environment," Cipresso says. "Inspect the substrates to be printed for warp or other artifacts. On a true flatbed printer it is important to concentrate the bed vacuum to the areas under the substrate to be printed." Operators often use tape to mask off the unused vacuum holes around the substrate and then turn off unused vacuum zones in the table.

"Always check the corners of the sheet to be printed," Cich adds. "Corners are the most vulnerable to becoming damaged or bent. Be aware of any masking tape that is used on the bed. Bits of tape that come up in little rolls will not set the crash sensors off, and this is one of the top reasons for head damage."

Printing onto Glass

As UV-cure ink technologies continue to improve, printing onto super-smooth surfaces like glass becomes more viable. However, it's not a simple "plug-and-play" process. There are some tricks that can help ensure a good bond. 

"When printing onto hard-to-print surfaces such as glass, an adhesion promoter is usually necessary," Cipresso says. "Adhesion promoters can also improve durability." He suggests wiping the excess adhesion promoter off as soon as possible after printing since it gets more difficult to remove later on. "Isopropyl alcohol can be used, but be careful not to wipe over the freshly printed image."  

Cich offers another consideration. "Make sure that the white inks and the colored inks are matched to shrink and expand at the same rate over time," he says. "If they are not matched perfectly, it will lead to an ink failure on glass jobs."

"Clean the glass thoroughly," says Garcia. "Run the printer's table vacuum at its highest setting, and use only the highest-quality print mode on your unit." He agrees that using an adhesion promoter is a good idea.  "Though printing directly onto to glass is okay with most UV inks; it is also true that the printed ink on glass is easy to scratch. To solve this, use a good adhesion promoter (we like to use Supply 55 brand)."

Printing Double-Sided with Perfect Registration

Double-sided printing requires great accuracy. If you have a board that is out of true by as little as one-tenth of an inch, experts say that over the length of the board your registration could wind up being off by a whole inch off at the end of the board, and the prints will not line up.

When printing double-sided with rigid materials make sure you start with substrates that are square and straight," says Cipresso. "This will result in improved front-to-back alignment."

Cich adds that most substrates are not perfectly square. To compensate, he offers a simple solution. "You can print onto one side first, and then take an X and Y measurement, and once flipped, move the substrate the above the measurements, and your registration will be perfectly square every time."

Cipresso says that "a number of flatbed printers have registration pins that can be placed in opposite corners. Aligning the rigid material to these pins will ensure the best accuracy possible when you flip the board to the second side. Of course image orientation will have to be addressed in the image file design for the second side image."

Garcia also says that it's best to use the printer's registration pins, "Or you can use the printer's alignment bar if it is equipped with one. Also, set the file to bleed off; print the image edge to edge for best results."

Printing onto an Irregular Surface

Most flatbed printers feature a vertical adjustment for the printhead assembly that allows users to print onto various thicknesses of rigid sheet—some allowing thicknesses of as much as four inches or more. But what if you need to print onto a substrate that has an irregular surface, such as a door? Remember, the farther away the printhead is from the printed surface, the less accurate the dot placement.

Garcia offers some general advice. "Raise the head height to the max tolerance," he says. "And if you are using mercury vapor lamps, set the lamps to max intensity, and print at a very slow speed."

Printing onto an irregular surfaces such as an iPad cover can be tricky, admits Cipresso. "A Novus customer of ours prints iPad covers his Pictora flatbed. To accomplish this they make a mask in order to concentrate the vacuum on the substrate. The head height was adjusted to allow a sufficient head-to-substrate gap which results in a sellable print with minimal risk of a head strike.

And Cich offers this advice: "First, if your printer allows, print in uni-directional mode," he says. "This allows for any overspray that you would get in a recessed area to be more uniform. Second, if allowed by the printer software, raise the carriage a millimeter above what the printer detects.  This will give you clearance if the substrate warps."