26 November 2018 Analogue Printing Technologies

The silver lining to LED UV printing

The BCF staged seminars in Edinburgh and Birmingham to bridge the knowledge gap that is evident in LED UV for litho and label printing. For any hard pressed printer or converter, there are multiple advantages, speaker after speaker explained.

The rise in interest in LED UV curing is unstoppable. There are more than 500 installations of the technology, some retrofitted to sheetfed litho presses worldwide, numerous installed as new since 2008, and in addition there are around 250 narrow web label presses with LED fitted. In addition the technology is beginning to make inroads into newspaper and heatset web offset printing.

But as the first UK Shining the Light on LED UV seminar organised by the British Coatings Federation revealed, there is still a yawning information gap. This can lead to misleading information both in favour and against the technology and certainly about what is what is not possible with light emitting diodes.

Even during presentations some stated that LEDs were available only at the 365-395 nanometer wavelengths that create UV-A energy for a penetrating cure and are used in printing. This is very different from the broad spectrum UV that is created by conventional mercury vapour lamps and which includes UV-C for a strong surface cure.

The limited range of UV energy limits the abilities of the chemists at ink companies to deliver inks and coatings that meet the requirements that printers have. If LEDs with different wavelengths could be combined, a new set of photoinitiators might be used and new applications open up. This are not yet available said speakers from Sun and IST.

But they are responded Phoseon. The US company has supplied more LEDs than any other (100,000 diodes for printing according to Rob Carson) and now has a 210nm version in its catalogue. It might be too costly, not powerful and enough and not yet readily available. But it exists and over time, a short time, these issues will surely be solved.

It is an indication that LED UV is still a relatively immature technology where interactions are not fully understood. Richard Wilson, a consultant who has a formidable expertise in retrofitting LED technology, says that the technology has been applied to presses from narrow web to a Heidelberg Speedmaster XL162.

“All the main press manufacturers support the technology. And it will break into packaging. It has done this at a heatset web offset printer where the conventional dryer has been switched off and the company is running LED UV with no emissions,” he said.

This will be a spur to adoption in commercial web offset. In many countries legislation is restricting emissions that industry can create, particularly in urban areas where many printers in Europe and the US are located. In Japan where such installations are not unusual, the impetus towards LED has been about saving energy as well as reducing emissions.

Wilson also called for better information. “We need best practice guidelines: the tolerance for running a litho press is a lot tighter than with conventional inks. UCR is essential: ink coverage should be no more than 320%. This is because UV inks can feed back and blend into the yellow ink resulting in a dirty image.”

To remedy this he suggests that printers consider changing the lay down sequence of inks and keeping a close check on density measurements which will also differ from those a printer may be used to. Matched fount solutions, roller coverings and blankets are essential.

And while one of the benefits of running with UV is that it becomes much easier to print on uncoated papers and non standard substrates (both highly absorbent and non-absorbent), it is not an automatic win-win. Rougher papers where there can be abrasion of the surface or on silk papers where the coating may flake from the paper. A lick of seal will stop this.

“Don’t just buy a lamp unit, put it on and run because you will come up against problems,” said the consultant who has been adviser on a large number of retrofit projects.

This means that inks, founts, rollers and press settings need to be taken into consideration. Nick Ivory, Sun Chemical, explained that ink makers are hampered by the limited number of photoinitators that operate in the spectrum of LED UV energy.

And when they do there is a tendency to yellow: not noticeable in dark colours, but obvious in clear coatings. “And as 395nm is very close to visible light, UV products are prone to ambient light curing,” he said. “We would suggest duct covers or filtering films, treatment to the lighting in the pressroom to prevent this.”

There is also the risk of oxygen at the surface of the ink inhibiting the curing action by reacting with the free radical compounds before they can link up. This is a greater risk for low viscosity flexo inks than litho paste inks.

Inkmakers could research new photoinitiators, but the process of having these assessed for safety and then registered for use, is lengthy and expensive. Few suppliers want to endure the trial. “We would like to use UV diodes at 250-365nm so expand the range to one very similar to conventional mercury lamps. It is easier to wait for LED providers to deliver their technology – it will be faster than waiting for new photo initiators to be approved.”

The solution to the yellowing problem he suggested is a combination of LED to cure the process colours and a conventional lamp after the coating unit and running a standard UV coating. Lamp design has leaped forward as energy output has increased. The first units needed to be close to the substrate with relatively low ink coverage. Now an IST unit can be positioned 80mm above the sheet and will deliver a peak power of 40W/cm². “But the distance above the substrate and the optics will bring that down to 13W/cm²,” says Chris Schofield, joint managing director of IST UK.

For narrow web and digital press applications, the diodes are close to the substrate so little help is needed. For sheetfed litho, reflectors and optics are needed to deliver enough power across a swathe of each sheet in order for the press to run at 18,000sph.

This is well below the energy consumed by the hot air dryers on a litho press and below the energy needed by a standard UV system. Craig Bretherton, K&B product marketing manager, explained that a B1 press with two inter deck lamps and three in the delivery will use 107kW/hr; a single LED array will run at 14kW/hr.

The press manufacturer has two UK installations and takes a holistic view over the economic viability of LED. “We don’t just look at the ink costs. There’s lots of areas are not considered: non productive time while waiting for sheets to dry, the saving from reprints, reduced costs in the finishing department from faster running and the well documented ability to print on uncoated substrates.” It amounts to LED UV having a cross over point with conventional litho at around 7,000 sheets.

Nevertheless Kevin Creechan, managing director at J Thomson Colour, one of K&B's customers, told the delegates that the calculations alone were not convincing. He presented one of two case studies, explaining that the investment in a six-colour LED UV press alongside a ten colour perfector was “a leap of faith” as the figures were not clear.

“We asked ourselves could we afford not to do it?” When the company looked at the time it spent cleaning down the coating unit on the existing six-colour press which would no longer be needed when printing UV. Suddenly 12 hours of production time were freed up each week.

“It’s the best thing we have ever done,” he says. “I’m still not convinced we would do it on the ten-colour press. It’s for the high quality work, for difficult substrates and we can turn jobs the same day.”

Creechan admits that the printer is cautious but found each of the press suppliers it spoke were advocating some kind of UV printing leading to further investigation of the technology. “But we didn’t want to go with anything but LED, thinking that other forms of UV will be not be viable three years down the line,” he says.

The figures, however, did not add up for the ten-colour press. This is the machine for the longer run work, which by implication have a longer turnaround time. The six-colour is handling more work than anticipated because of the clear production benefits beyond the press.

It is also using a 1% dusting of spray powder, used not for drying but to prevent any risk of blocking. A lick of coating is applied to silk coated papers, not for any problem with the printing process, but to add a protective film to stop the paper’s surface flaking off when finished brochures were stacked against each other in the delivery box.

The biggest impact, he says, has been on one regular job printed on Naturalis. “We would print one side and leave that for two days before printing the reverse side; then it would sit for another two days before thinking about finishing. Now it takes one hour to do the whole job,” Creechan says.

Dave Stones, sales and marketing director at B&B Press, was equally enthusiastic. The LED UV goes hand in hand with setting a new platform for print, changing the perception in the minds of the 25-30-year-olds that are charged with designing and buying print. “We do what we say we are going to deliver to them,” he says.

It has created the Be Brilliant club to widened the discussion around print and inviting buyers into a working place where jobs are printed cleanly without spray, where there is no waiting time and where the consumables are sustainable. “It took us six months to get the chemistry right,” he says.

There was also a learning curve for operators and to understand how LED affects the quality of a job. Creechan had noted this being called in to a client to explain why a reprint looked different compared to the initial run. “I pointed it out that it looked better,” he says. J Thomson Colour has subsequently won all that customer’s business. This would not have been part of the business plan, nor would there have been a figure against the energy savings from using the instant drying system rather than hot air or IR units. Stones says that this amounted to a 75% reduction.

Instant drying has an impact on colour behaviour on press. Where Stones had been buying spot colours to achieve the result customers want, it has shied away from purchasing spot colours sensitive to LED UV. These are expensive, says Stones. “But the colour gamut is much better so we buy very little spot colour. Just because you can does not mean that you should and customers accept the colours that come from CMYK.

“The biggest frustration is trying to teach and engage people about why they should pay a little more for a quality product that we believe is worth paying a little more for.”

This can mean running comparison sheets between the processes to highlight the difference. It has also attracted other work. A greetings card publisher needed bespoke envelopes to match the cards and B&B supplies flat sheets printed on envelope paper, responding a lot faster than the customer had been able to print previously. “It means that they can work on making envelopes immediately they receive the sheets from us,” he says.

As withWilson, Stones believes that education is needed and not just in bringing together the different elements of the technology. Many printers remain in the dark about LED UV, let alone the customers. Suppliers need to work with the printers on projects to educate the clients in order to get the word out.

This is also the case in label printing. Mark Andy has installed countless LED UV presses around the world, far more in North America than in Europe. This is because Phil Baldwin mused, label printers in Europe had already converted to UV flexo, but using standard mercury vapour lamps. In North America it was a bigger leap from water based inks to UV and it made sense to leap the chasm to the newest technology.

Keith Redmond from Flint Narrow Web puts the global base of LED UV label machines at 250, 160 of which are in the US and 40 in Europe. Many are also swapping a conventional lamp for an LED version and running hybrid press set up. This will enable some energy savings, but may compromise the ability to run thinner films or highly heat sensitive products that is a key part of the appeal for label converters and their customers.

A thinner film will mean less environmental impact and many more labels on a reel and a longer run time on press without stopping to change the reel. There are also no moving parts as the lamps are either on or off, cutting the need to shutter lamps or for cooling down and warming up periods.

Companies like IST UV have responded with systems that are hot swappable between conventional and LED arrays. The switch from one to the other takes only minutes. The same switch on a sheetfed litho press will take a little longer, but the same ethos of changing the lamp unit only and keeping control cabinets, applies.

It is a step forwards towards a mature market. Before that the implications for the technology, whether offset or flexo, are about educating the market that here is a technology that brings many of the benefits associated with digital printing with the known advantages of litho and other analogue printing technologies.

Gareth Ward

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Ink manufacturers have a restricted palette of ingredients to select from. The limited range of UV energy also limits the abilities of the chemists at ink companies to deliver inks and coatings that meet the requirements that printers have. If LEDs with different wavelengths could be combined, a new set of photoinitiators might be used and new applications open up.

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LED UV is still a relatively immature technology where interactions are not fully understood, but the technology has been applied to narrow web presses as well as a Heidelberg Speedmaster XL162. Richard Wilson, a consultant who has a formidable expertise in retrofitting LED technology, says that the next breakthrough will be into packaging.

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