Three answers to the UV or not UV question

Technical article

Ryobi was the first press manufacturer to introduce LED UV-curing, seen here on a Ryobi 920

That seems a long time ago. Despite the reservations, the packaging industry in particular took to UV inks and varnishes and technology companies pushed the development of more powerful and more durable lamps. Press manufacturers mitigated the corrosive effect of the inks by using different types of seals and bushes that could withstand the chemicals and that did not melt if left under a switched-on lamp.

In these early days, the heat that the lamps generated along with the ozone that came as an unwanted by product needed to be dealt with. As lamp power increased from 30-40 watts/cm2 to 120-160 watt/cm2 today, air cooling gave way to water cooling, the lamps could be shuttered off and stand-by modes were introduced to reduce the amount of energy required to power the lamps.

Today, says David Pelling, technical consultant to Benford UV, stand-by power has been cut to 30% of full power. This is still more than needed by LED UV systems, which power on instantly and can even be switched off between sheets. But despite this and other advantages to the LED technology, the printing industry continues to need three styles of UV curing.

This has led Baldwin, with its strong background in conventional UV, to acquire AMS for its expertise in LED technology. Baldwin was instrumental in the development of Komori’s H-UV technology, which has used a single-doped lamp to deliver a narrow spectrum of UV energy to cure litho print instantly.

Pat Keogh, who leads the Baldwin UV division, says that the spectral name, associated with UV since the earliest days in print, is being revived for classic UV alongside AMS for LED. “We will continue developing all types of UV and will try to sell the idea to the OEMs that any press with UV should have our technology inside. We want people to recognise that there will be advantages to having our technology inside,” he says.

Chris Schofield, joint managing director of IST UK, points out that LED accounts for only 10% of the business, with installations on new machines rather than retrofit being the norm. “The most important thing with LED is the temperature control of the diode array to ensure the consistency of output. The cost of a retro fit installation for LED could be as much as the cost of the press itself,” he says.

IST-UV can also offer the three technologies as can Benford. While there is a general acceptance that in the long term LED will eventually become the universal solution, that choice is not so simple today. KBA for example has installed presses with LED UV at J Thomson Colour in Glasgow for commercial printing and at the same time has been installing a number of carton presses equipped with classic UV lamps.

The evolution of classic UV came from a search for a way to improve the efficiency of the mercury vapour lamps. Pelling explains, “The issue with classic UV is that only a very small part of the lamp’s output is in the spectrum that is absorbed by the ink chemistry.

“Manufactures have searched over the years for ways to increase the active short wave output. This is almost impossible due to the physics of mercury vapour lamps, any additive in the lamp uses the shorter wave lengths to enhance the longer [ones],” continues Pelling. “This means that while it’s not simple, it is within the bounds of possibility that by adding various metal hydrides to a medium pressure mercury arc lamp it can provide enhanced outputs in the longer wave lengths.”

With a matched photo-initiator, this can trigger the chemical reaction that cross links the ink or varnish trapping the pigments inside and producing a dry sheet in the delivery. As the costs of the chemistry have fallen, it has become possible to put together a system using a single lamp, using less energy and working with suitable sensitive inks.

This is the concept behind Komori’s H-UV, Heidelberg’s LE-UV sourced from IST UV, KBA HR-UV and the Benford Eco-UV. The inks are still more expensive than conventional inks and more expensive than the families of inks suited to classic UV. This favors short-run printing, especially fast turn-around print, which needs to compete with digital printing. It has been a barrier to the adoption by the packaging industry, which tends to be risk averse where customers are concerned.

In commercial printing, numerous installations report that the additional cost of the ink is mitigated by any number of factors: the press remains clean; there is less work in progress waiting for sheets to dry; energy consumption is down; spray powder is eliminated; there is no need to run a seal.

Classic UV creates ozone, classified as a VOC and therefore needing ducting to remove it from the production environment. This helps explain why classic UV has never made much impact on commercial printing.

Other issues remain, namely around energy consumption and the heat that is generated by the lamps. This has to be managed through cooling systems and limits options when printing on heat sensitive substrates.

On the other hand, this is a mature technology that is relatively low cost. Systems can be installed between print units as well as in the delivery of the press to achieve a wide range of effects. There are inks and varnishes for the different applications, including low migration products for food packaging.

The single-lamp UV technology was the first to break through and was developed by Komori to meet the requirements of the Japanese market, and accelerated by the 2011 earthquake after which the government imposed energy-saving measures on all businesses. Other press manufacturers offered their version and Heidelberg in the UK has installed LE-UV at a handful of UK printers.

The lamp has been doped by the addition of iron compounds to achieve around three times the power at the required wavelength. An end of the press, three-lamp system can be replaced by a single lamp unit. Consequently, the approach results in energy saving of around 60% compared to the classic UV set-up while providing the same full cure that three 120 watt/cm2 lamps would achieve at 15,000 sph.

At the same time, less heat is generated and because the emphasis is on long wavelength UV, there is no ozone to be concerned about, making an installation easier for a commercial printer to contemplate. The lack of heat reaching the substrate means that heat sensitive non absorbent materials can be printed.

On the downside, the unit cost of the system is higher and inks are more expensive. The price will fall, but because some components are necessarily expensive, these will never reach the level of conventionally dried inks. The lamps will need to be watched for any deterioration (something that occurs in a classic UV installation) in output from the lamps. The life of the new type of mercury vapour lamp is also likely to be lower, but it is not possible to predict by how much.

The third technology is LED, promoted as the lowest energy, the simplest and the coolest of all the UV systems. It is being helped by continuing development of the diodes as these gain in power and fall in price. Currently LED remains the most expensive approach, though diodes should last longer than the life of the press before needing replacement.

Newer approaches, pioneered by AMS and Baldwin, are delivering energy at multiple wavelengths to increase the range of consumables that ink companies can produce. The key advantage of the diode approach is that a greater percentage of incoming energy is translated into UV power (about 20% efficiency compared to 4% with classic UV).

It is wrong, however, to believe that LEDs are the perfect solution, says Pelling. “The issue with LED UV is the cooling of the back of the LEDs. While LEDs produce little or no heat from the light emitted, the interconnection at the back produces heat which must be dealt with. That means all LED systems need a well-designed cooling system to maintain the interconnections at around 21%. If this temperature is increased, the output of the LEDs reduces considerably.”

If these are not kept within the recommended temperature range, the cost of replacement will be considerable.
“When it comes to which is best,” says Pelling, “it is horses for courses,” adding: “If printing cartons, particularly with a high gloss finish, then a mid range capital cost with well developed normal priced inks would probably be the best: this is the classic UV. If carrying out the same work where gloss coating is not so much of an issue and increased ink costs can be absorbed but reduced electrical costs are important then low energy UV would more likely fit the bill. But if very heat sensitive substrates are being processed and low electrical running costs are the main drivers and can be offset with a high initial capital cost then LED UV could met this need.”

For a commercial printer looking to take on some carton work, either of the approaches will deliver the dry sheet required. But food safe packaging, which for reasons of certification and customer assurance, is best left to those that are fully prepared. It is going to require the low migration inks that are only readily available for classic UV. A printer would also need to run thorough tests to ensure that gloss varnishes are acceptable on the substrates used.

Benford is able to supply whichever is required. It has numerous installations in the US and in the UK has in Selsey Press, a customer that has both the Eco-UV and now its LED solution. The choice belongs to the customer. A triple UV configuration, or one with a cassette unit able to accept classic, doped or LED lamps as these reach the mainstream might be the approach to look for. It is something that both Benford and IST have in the portfolio.

“We think LED will come for cartons but that it is not quite there yet,” says Benford, managing director at Marc Boden. Schofield reckons that inquiries about LED have started to arrive from carton printers, though the ink options are currently too limited.

“And as most carton presses will have UV in multiple locations, the cost of LED will rule it out for now. But the systems we have sold over the last 6-12 months can be converted at some point,” he adds.

Gareth Ward is the editor of Print Business magazine in the UK.