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Taking the LED cure – Print 21 magazine article

Tuesday, 07 October 2014
By Nessan Cleary

UV ink curing using low-energy LED arrays instead of metal halide lamps is gaining traction – particularly in the wide-format sector. Some offset presses have also been retro-fitted with LED-UV curing, with Ryobi pioneering the sale of new presses featuring LED-UV. Can LED eventually displace conventional UV and what are its strengths and weaknesses? Print21’s European correspondent, Nessan Cleary, dives into the short end of the spectrum and sheds even more light.

Naturally we prefer our inks to be wet when we’re printing and to be dry right after printing, so that we can get on with finishing and ultimately selling the printed work. This is the main reason why UV inks with their instantly dry capability have proven so popular. Strictly speaking, UV inks don’t dry – they change from a liquid to a solid state through a chemical reaction: photopolymeri-sation, caused by exposure to a UV light source. That light source could be as simple as sunlight though most printers use an easily controllable mercury vapour or metal halide lamp.

Increasingly, manufacturers are turning to LEDs as a viable UV light source. Stephen Emery, director of ink sales and marketing for EFI, says this trend will continue, adding: “I would say the majority of [wide format] printers will be replaced by LED curing models.”

EFI GS3250LX

LEDs typically last for 15,000 to 20,000 hours, while mercury or metal halide lamps have an average life span of just 1,000 hours. LEDs are turned on or off instantly, with no time wasted waiting for the lamps to come up to temperature. This means that the life span equates directly to hours spent printing so it’s unlikely the LEDs would have to be replaced during the life of a printer. There’s no degradation of the lamps over time, so you get their full performance up until the point they fail, unlike a conventional lamp which has to be monitored and replaced as the performance tails off.

LED lamps use considerably less power and only need to be on when actually curing, which adds up to a lower electricity bill. Indeed, Fogra recently tested an EFI 3.2m wide GS3250LX Pro and found energy savings of up to 82 percent over printers using conventional UV lamps.

Reduced power usage is also good for the environment and a further green benefit is that no mercury is used, eliminating the need for careful waste management. Since LEDs don’t produce ozone, or any other harmful gases, there’s no need for extractive ventilation.

LEDs produce far less heat than conventional lamps, so they can be used with a greater range of heat-sensitive materials, such as thinner plastics.

 Growing power

The basic concept of LED curing is fairly simple but the main issue up until the last couple of years has been the relatively low light intensity they produce. UV curable inks rely on photo initiators, which produce free radicals when exposed to UV light. The major components of these inks are monomers and oligomers, which the free radicals cause to cross-link together, turning the ink into a tough solid and simultaneously bonding it to the substrate surface.

However, photo initiators require a certain intensity of light to kickstart this reaction, hence the development of high output LEDs coupled with inks more sensitive to a narrow band of UV. Emery says that inks have to be formulated differently for use with LED curing, explaining: “The main change is the initiator which responds to UV-B wave length to start the cross linking so we need to make sure that we have the proper photo initiator to respond to the LED wavelength.”

One way to get around this is to use pining, where the LED partially cures the ink, prior to a more powerful LED or a mercury vapour lamp. Here the final cure doesn’t need to be as powerful, saving on energy costs, and the pining immediately after printing helps keep the dots sharp for a better image quality.

EFI has been working with LED curing for over five years now, for both its Jetrion label presses, and Vutek wide format printers. Emery says that the inks are a little more expensive but that EFI has leveraged enough economies of scale to negate this cost.

EFI uses LED curing for a number of its GS series of superwide format printers, including the GS5500LXr Pro, a five metre wide hybrid printer with multiple drop sizes starting at 7 picolitre, and eight colours plus an optional white.

For the much faster HS100 Pro, EFI uses LEDs for pining with mercury vapour lamps for the final cure. This is a 3.2m printer capable of producing 100 rigid boards per hour. LED curing is also used on the Jetrion 4950LX label printer, which allows it to handle more heat sensitive materials across a 330mm web width.

Wide and Superwide embrace LEDs

Fujifilm Acuity LED 160

Fujifilm has developed the Acuity LED 1600, a rollfed printer capable of 20 sqm/hr. It will also accept rigid materials up to 13mm thick. It uses Fuji Dimatix Q class heads with one head per colour, configured CMYK + Lc and Lm plus white and clear inks.

Fujifilm’s Specialty Inks Systems has formulated its ink with a monofunctional monomer, which has high elongation and adhesion, so it is flexible enough for roll fed substrates. There are two LED units – one to pin the inks, which allows them to penetrate the media surface, and another to fully cure them, which gives greater adhesion for PVC media.

Mimaki has used LEDs in most of its UV printers, including the recently launched JFX200, an entry level flatbed that can produce up to 25 sqm/hr. It features six colours including white and clear and a choice of hard or more flexible inksets. LED curing is also used for the roll-fed UJV500, which can print at up to 100 sqm/hr, on flexible media, with enough stretch for use in vehicle wrapping applications.

Roland has also used LED curing for a number of its printers including the LEJ640 hybrid printer, as well as the flatbed derivative of this, the 640F. The LEJ 640 prints white and clear inks as well as CMYK using Roland’s new Eco UV inks, which print to both rigid and flexible media.

Mutoh launched a new LED UV hybrid at the last Fespa Digital show earlier this year. The VJ-1626UH is a 1.6m wide printer that can handle a wide variety of rigid and flexible materials, including heat-sensitive substrates. It can run two sets of CMYK inks for higher speed, or CMYK plus white and a clear varnish.

LED’s Zeppelin lifts off in Offset

But LED curing isn’t only confined to inkjet. Ryobi claims to be the first offset press manufacturer to use LED UV curing, which can be fitted to a number of its presses from the two-up 520 series to the eight-up 920. Ryobi claims that this has cut power consumption by around 70 percent, while the LEDs last up to 15 times longer than conventional lamps.

Some printers have been retrofitting LED curing systems to presses. Air Motion Systems, a leading developer of LED curing systems, already claims over 140 installations to various presses worldwide. The LED systems are relatively small so that they’re easier to fit than conventional lamps would be, and allow offset printers to use UV inks, for immediate finishing and no spray powder.

Carsten Barlebo, European director of AMS, says that presses can operate at their rated maximum speeds – up to 20,000 sheets/hour in the case of a KBA Rapida 106. He notes: “A new series of 100 percent bio-renewable LED UV inks are also coming on stream this year – from more than one supplier – featuring low migration to meet Cradle-to-Cradle (Silver) certification, so it really is ‘Game Over’ in the new generation UV debate.”

In addition, low energy or LE systems such as Komori’s H-UV are a variation on LED curing that’s proved popular with offset presses. The UV inks, and particularly their photo initiators, need to be fine tuned to work with the lamps in use, which does limit the choice of inks, but means jobs are turned around much quicker.

The Flint Group has developed a range of UV flexo coatings that are suitable for LED curing using the EkoCure brand. Toyo, INX, Toka and Arets are also now in the offset LED-UV ink space.

In conclusion, we can expect to see a lot more LED-UV curing over the next couple of years given the energy and cost saving advantages. It seems inevitable that most wide format printers will move to LEDs though it may take a while longer for offset printers to fully embrace the benefits of LED-UV curing.

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