Can electron beam curing compete with UV in inkjet?
This year saw electron beam curing emerge as a potentially viable commercial alternative to UV in inkjets. Simon Eccles finds out more about the additional benefits it can offer to print projects.
Electron beam curing offers most of the advantages of UV - instant curing, no blocked inkjet nozzles, while avoiding the migration and contamination problem through media. EB inks are food safe and garment safe.
So far we’ve only seen digital technology demonstrations on narrow formats suitable for label applications. There’s no technical reason not to use it with wider single-pass configurations in future. The hardware price of the curing units remains the main limitation for now, but these are coming down. Ink costs should be broadly the same as conventional UV-cure inks.
The main players so far in digital EB are ebeam Technologies, a Swiss manufacturer of EB curing systems that has developed a new compact EB “lamp;” and Kao Collins Inks, a US ink maker that recently introduced an inkjet ink to its Quanta range of EB-cured inks. ebeam and Collins showed a working narrow web inkjet demonstrator at a show earlier this year.
Sun Chemical also makes EB inks for conventional processes, but so far hasn’t announced anything for inkjet.
In October Edale, an established UK builder of printing and converting equipment, announced that it had been working with ebeam to integrate EB curing of coatings into the Digicon 3000 digital label finishing system. This is a mid-web width finishing solution that was co-developed with AB Graphics. It runs inline with digital print units for labels and flexible packaging.
So far, three Digicons with ebeam curing lamps have been sold to converters in the USA and EMEA, with two including EB coatings for print protection and immediate finishing.
Electron beam curing isn’t new, but before now its cost and bulkiness have confined it to conventional analogue print processes such as flexo, lithography and gravure. Here it’s mainly used for primary packaging of food and pharmaceuticals where fast curing onto difficult materials is important but UV ink isn’t allowed for health and safety reasons.
The original EB curing units are “pumped” systems, meaning they need to be plumbed into an external vacuum pump. This makes them bulky, but they can cover wide print widths from 914 mm to 2.74 m.
The new compact system from ebeam uses what it calls “lamps” as electron beam sources. These don’t need pumps. “We call it a lamp as it is as easy to replace as a light bulb, normally in 15 minutes,” says Elsa Callini, business development manager of ebeam Technologies.
“It emits electrons instead of photons. There is a vacuum inside and our lamps are sealed in production so the vacuum stays permanently.” The ease of changing and compact size makes it easy to fit the lamp units on smaller printing and finishing lines. Lamp units are currently available with operating widths of 100 to 460 mm.
Kao Collins has brought its inkjet Quanta EB ink to market, although so far there are no commercial users, says Kristin Adams, marketing manager for Kao Collins. She says: “Since EB inkjet is a brand new technology we’re spending a lot of time communicating exactly what it is, what it isn’t and why it’s a ground-breaking solution for some applications.
“We have many projects from a variety of interested parties and we’re working with technology partners to qualify the inks for use across a range of print heads. We expect them to be commercially available in early 2017.“
The Collins’ EB curable inks pass US FDA migration standards and all of Collins’ EB inkjet components are listed on the Swiss Ordinance and the Nestlé Guidance Note on Packaging Inks – version 02-2014.
Hardware cost will be a limiting factor in the broader take-up of EB, she believes: “EB inks are comparable in price to UV/LED inks. The increase in cost that I’m referring to is the curing equipment.
When we began looking at EB a few years ago, the prices were three times what they are now. As the needs continue to shift towards smaller, more versatile pieces for single-pass applications, I believe that we’ll see the prices continue to come down a bit.
“I think it’s important to stress though, that we see EB and UV/LED as complementary, not competing technologies. They have some similarities and fundamental differences. Ultimately, the selection should be based on the best fit for the process and application. Best fit considerations may include: capital cost, operating costs, end-use properties, fitness for food packaging, enabling of end-use, substrate considerations.”
How EB works
EB curing works very similarly to UV curing: direct energy at a liquid ink, where it triggers short-chain monomer molecules to become tangled and linked to form polymers. The entanglement stops their easy movement, so at the macroscopic level they become solid. No solvent is needed and all the ink applied is turned into a cured film, with no evaporation.
The process is very rapid in most cases – certainly the inks are touch-dry as they come off the printer. This allows them to be handled immediately, for re-reeling, finishing, lamination or application.
The (very) big difference between UV and EB is that the first uses photons in short-wavelength electromagnetic radiation to trigger polymerisation. EB uses a stream of electrons.
UV light is related to the invisible sunbeams that give you sunburn and skin damage, so has to be shielded from operators. Electron beams are close enough to X-rays that some shielding is also necessary in the curing units. In fact ebeam Technologies is a spin-off from Comet Group, a major manufacturer of industrial X-ray equipment.
The problem with UV
Cured UV ink is stable and safe. However, getting it to cure all the way through remains a challenge. The process works on “line of sight,” meaning the UV light has to actually hit the molecules for them to polymerise. However photo initiators in the upper layers can absorb UV light before it reaches the base layers, while colour pigments in the ink also absorb and block some light.
The result is that thick ink films may not cure all the way to the bottom. This affects the strength of the bond to the media, but also means that “low molecular weight” uncured monomers and fragments of photo initiators can soak into permeable substrates such as card or paper or textiles, and get into whatever is on the other side.
Uncured UV ink is a health hazard, as exposure to it through the skin, through breathing or in the eyes, can trigger serious allergic reactions. Today’s inks aren’t as risky as a couple of decades ago, but food and drug regulations place significant restrictions on their use, with only certified “low migration” inks being allowed for primary packaging that contacts food. These inks are not used either in textile garments where any uncured ink may migrate (ie soak through) and contact skin.
EB inks have no photo-initiators, meaning low odour and taint dangers. The electron beam also penetrates all the way through the ink film, with no “line of sight” problems. Apart from a guaranteed complete cure, this also gives enhanced adhesion where the inner layer contacts the substrate.
The EB source is cold so can be used with very thin, heat-sensitive media including shrink labels, unlike conventional mercury vapour UV lamps that run very hot. However the newer low energy UV lamps and LED-UVs are also cool, so the advantage isn’t so pronounced.
Will it take off?
Ink manufacturers think that EB will continue to gain ground in conventional processes and perhaps break into single-pass inkjet for narrow-web labels, but aren’t sure about larger formats.
Adams at Collins doesn’t see EB going into wide format inkjet any time soon. “For wide format, the limiting factor is the curing equipment. Current machines are not compatible with scanning heads, only single pass. Not to say that couldn’t happen in the future, but it would need to be developed.”
“The reason not to use EB for inkjet is more or less to do with cost,” says Pete Saunders, business director of SunJet. “The inkjets are mainly single-pass, so they are pricier but slower than flexo. If on top of that you have to invest in EB it may be a step too far. So people are looking at low-migration UV first.”
However, he points out that EB and UV share the advantage of not blocking inkjet print heads. “The reason they are used is not just because they give good films at the end of the process. The main reason is because they are extremely easy to maintain in printheads.”
In other words, the ink remains fluid in the printheads and never dries or clogs the nozzles. This is particularly important in single-pass inkjets and means that less nozzle redundancy is needed, reducing the head costs.
What are the applications?
“Food-related applications are ideal for EB, but most of the interest is coming from large consumer product companies where product health and safety is priority number one,” says Adams.
“There is a balancing act of incorporating digital imaging without compromising what is deemed ‘safe,’ and now with EB curable inkjet, the game has changed a bit because there are far less risks associated with the inks and curing technology.”
“When our customers are coming to us it’s normally because they have an application where UV or other traditional technologies are having an issue,” says Callini. “Food packaging is one. But it is also normally when the physical principle behind the curing cannot be adapted to UV. I’m thinking of aluminium foil metal coating. Because electrons are particles, they are not affected by reflections from the substrate. In inkjet there is a problem when UV curing is used, but with EB we don’t have these problems.”
Glass, ceramics and textiles are also potential markets, she says. “In the contact we had so far the ceramics industry is a bit conservative. The process of curing the ink at the moment is very well established but they are exploring the advantage of EB can bring to them.”
“For glass we have some on-going activity with direct-to-shape. In that case they are interested in instant and more perfect curing of the ink and better adhesion on the substrate. The interest of the textile market for EB has also been really increasing a lot in the past months. We didn’t engage this market before.”
Adams says that Kao Collins has looked at textiles too: “For textiles, we’ve done some preliminary, experimental EB testing with a large apparel company. The ink performance and print quality was great, not stiff.”