By Cassandra Balentine
The evolution from conventional UV to cooler, UV LED curing brings reduced power consumption, lower heat requirements for curing, longer lamp life, and environmental benefits. The latest UV LED inks are developed to optimize the potential of these advancements.
“The introduction of UV LED offered print service providers (PSPs) a number of unique economical, ecological, and business-generating benefits including a broader scope of print applications, faster operations and higher productivity, high durability, significant energy savings, and eco-friendliness,” shares Deborah Hutcheson, director of marketing, North America, Agfa.
In addition to advancements in UV and UV LED, electronic beam (EB) curing, which does not require the use of photoinitiators (PIs), is also emerging.
Cool Cure Advantage
Conventional UV as well as UV LED lamps are both utilized for UV printing. The available substrates for UV LED are vast, as the cooler cure enables more supported options compared to traditional UV. Originally, conventional UV lamps had the production and outdoor durability advantage, but that gap is closing with continued technological advancements.
In addition to lower energy consumption and reduced heat output, LED lamps are maintenance free and feature a long service life. “There are no consumables, toxic waste, or maintenance downtimes as with mercury vapor systems. All this makes UV LED printers profitable and environmentally friendly,” shares Carmen Eicher, chief sales and marketing officer, swissQprint.
Mike Maxwell, senior manager, corporate strategic development, Mimaki USA, Inc., discusses the evolution from traditional UV bulbs to UV LED. He notes that traditional UV printers use bulbs that require a warm up and cool down period and consume a lot of energy to operate. This adds time to print since the warm up and cool down could not be skipped without damaging the bulb, which are expensive to replace and are good for about 500 hours of use. Turning on the bulb also stresses the filament, which reduces the expected life by approximately five hours each time. “This required shops to schedule jobs to keep the printer running to avoid lost time between the warm up and cool down phases,” he explains.
UV LED technology was slow to take off, partly because the chemistry behind the ink required lower wavelengths to cure and manufacturers were invested in other ink technology at the time. “The LED lamps immediately create the wavelength needed to cure the ink and as a result they are only on when needed and off when not. There is no warm up or cool down phase, and the lamps are rated for thousands of hours compared to traditional UV bulbs,” shares Maxwell.
UV LED curing systems also have less obvious advantages that have a direct bearing on the final image quality, including infinitely adjustable energy and better lamp control.
In terms of adjustable energy, Paul Geldenhuys, technical and operations director, NUtec Digital Ink, says inherent in its functioning, conventional UV cannot be adjusted below around 50 percent lamp power, after which it shuts down. “The infinite adjustability of UV LED means you can affect the gloss levels of the final output,” he offers.
Further, UV LED systems are constructed in segments of around one inch. “Each segment can be individually adjusted. The practical advantage of this is that different areas of the printed image can be cured at different levels, which is useful in controlling gloss banding or creating special effects,” adds Geldenhuys.
Hurdles to Overcome
Despite the many advantages UV LED brings to the table, there are still challenges associated with the technology.
For starters, there is a big installation base of machines using conventional curing systems, so replacing or upgrading to LED lamps requires significant cost and financial planning, admits Geldenhuys. “Conventional UV inks are already well entrenched in the industry with known certification and a long period of proof that the ink/application combination works. Converting to an LED curing system not only requires the capital layout on the equipment, but the ink must be replaced as well. This brings uncertainty and risk, making the introduction of this newer technology slow,” he explains.
Maxwell feels that one of biggest challenges LED curable systems face is the ink itself. He points out that the ink chemistry has to support the difference in UV wavelengths that LED produces, all while offering adhesion compatibility to the print surface. This means that inks are more specified to the intended print surface. “If you look at Mimaki, for example, there are six different ink chemistries with different intended application offerings,” he comments.
Hutcheson adds that UV LED curing for inkjet inks means a limited choice in PI systems, and ink formulations must be tuned to deliver a thorough cure of the printed ink from top to bottom.
Another challenge is the use of UV ink technology for food packaging. Due to the high heat intensity needed to print, many applications are not possible with conventional UV, as the heat reacts with the substrate causing it shrink, curl, or damage. “This has been one of the big reasons traditional UV lights were not able to compete that well in food packaging,” asserts Sohil Singh, VP, StratoJet USA.
“Neither UV or UV LED provide a real solution for food packaging applications. None of the technologies can warrant a 100 percent conversion on the reactions that take place when curing and as inks are formulated with monomers with a low molecular weight, the risk of migration persists. Although improvements are made in both UV and UV LED technologies, solutions still have to be provided for specific applications,” says Pedro J. Martínez, CEO, AFFORD.
Terry Amerine, VP, sales and marketing, Polymeric Group, points out that food packaging and UV technology has always faced the issue of chemical migration. This is due to the fact that even with very efficient curing, there is still a small amount of ink that is not fully polymerized and can migrate to the food. “Typically, UV or LED cured ink require a functional barrier to be used with food. That usually is the packaging material itself.”
Ken Parsley, product manager UV LED, Mutoh America, Inc., adds that many of these inks still contain solvents that can migrate even after curing. This makes the inks unsafe for use in food and beverage packaging applications.
The PIs required for curing UV inks using LEDs have a more stringent chemical classification. “The PI options are limited because of the narrow UV wavelength band that these inks require to cure, making it more difficult to select PIs suitable for use with food packaging,” continues Geldenhuys.
Phil Jackman, global product manager, digital, Sun Chemical, says in terms of food packaging, when using low-migration capable inks, a mercury lamp can facilitate a more complete cure of the ink, so there is less possibility of migration. Overall, it provides a more robust solution for food packaging. Inkjet label presses tend to mostly use conventional UV lamps, although UV LED is growing in popularity.
Singh argues that UV ink has undergone tremendous upgrades allowing it to adhere to more applications than ever. Substrates and inks are the key elements of any solution. A newer edition to UV technology is low-migration inks designed for the food industry to limit the potential migration of ink compounds into the food. “Many manufacturers are working on highly reactive and low-viscosity inks that help in adhering to multiple substrates on higher speed modes,” he shares.
To deliver food-safe printed packaging, Hutcheson says it takes much more than the ink—it is a system approach. The combination of substrate, print system, and curing unit, together with other key parameters such as print file, ink volume, type of food, and process conditions all together can result in food safe printed packaging.
Ink manufacturers continue to evolve to meet new demands and support more applications with UV LED systems and ink.
AFFORD recently expanded the versatility of its general use graphic inks, making them suitable for more substrates. The company also continues to develop specific ink solutions for vertical markets such as credit cards, coding, or thermoforming processes.
The UV LED inks developed and produced by Agfa form a key element of system performance. Hutcheson says pigment dispersion technology and chemical design of dispersions and ink are part of the Agfa DNA. The company’s patented Thin Ink Layer Technology relies on the optional dispersion of pigments combined with its Asanti workflow algorithms to produce the lowest ink consumption in the industry, she comments. In addition, the batch-to-batch consistency of the inks and their long shelf life are also key Agfa ink differentiators.
Additionally, Agfa’s UV LED inkjet ink sets for sign and display applications recently obtained the highest level of GREENGUARD Gold certification in recognition for their environmental friendliness.
For EFI’s wide format series printers, the company launched EFI ProGraphics UV XA high-adhesion inks. These new UV LED inks are well suited for standard acrylic and coroplast and other fluted polypropylene media with no edge chip when cutting. According to Mark Goodearl, senior ink product manager, EFI, the inks are flexible enough for 180 degree heat forming applications.
The line includes EFI 3M ProGraphics UV ink for flexible applications, EFI ProGraphics UV POP ink for rigid board and flexible media printing of indoor and outdoor applications, EFI ProGraphics UV Rigid ink for surface mark resistance and printing on specialty rigid media, and EFI ProGraphics UV Ultra ink for a diverse range of flexible and rigid media applications.
Goodearl says a variation of the high-adhesion ink is also available in the EFI VUTEk HS F4 Series and introduced into other printer products over time.
Flexibility is a key characteristic of Mimaki inks. “We have inks that can be used for anything from banners to car wraps and thermoformed plastics. The addition of specialized inks such as white, clear, and metallic silver have bridged a lot of analog processes that couldn’t easily be achieved prior to Mimaki offering them,” says Maxwell.
Mutoh’s latest advancements involve the lamps that cure the ink. Managing the LED lamps means controlling the curing process. “We have lamps that can be controlled more efficiently to produce a range of finishes from matte to glossy in a single pass,” shares Parsley.
Geldenhuys notes that NUtec inks are designed to be free from N-vinyl caprolactam, support adhesion to a broader range of substrates without the need for primers, feature improved stretchability, produce low odor, and comply to regulatory standards.
Polymeric focuses on developing inks for both conventional and LED cure systems. “We are working on digital inks with high chemical resistance as well as adhesion to a broad range of substrates to provide customers with inks that support as many print applications as possible,” adheres Amerine.
Many of Sun Chemical’s recent advancements in energy curable UV-type inks are in optimizing LED cure. Jackman says LED is often specified in new machines, while many older machines are retrofitted with LED lamps due to the advancements and benefits they offer. The company’s inkjet research and development team is actively trying to overcome some of the deficiencies of UV LED cure so that LED optimized ink formulations feature similar performance and properties to conventionally cured counterparts.
LED lamps have longer wavelengths than conventional UV lamps, and are therefore good at penetrating through the ink, but they are not necessarily as good at surface cure. Therefore, Jackman explains that the company has optimized some PI and synergist combinations to optimize surface cure, while looking at improving single-pass applications where LED is starting to be used as well.
“The lesser UV dose available from single-pass applications as opposed to multi-pass, poses some additional challenges; however, improvements to the chemistry in addition to recent LED lamp technology has resulted in inkjet presses running at impressive speeds for labels and narrow web,” continues Jackman.
Lastly, the PIs used are always under scrutiny for environmental reasons and in terms of health and safety regulations. In several cases, they have been reclassified and deemed as potentially more hazardous than they once were and should be removed from formulations. “Thus, the availability of some of the existing PIs are limited going forward, so Sun Chemical has reformulated many inks with less hazardous materials while maintaining the same cure properties and the same level of end use performance. This was challenging as the safer, alternative PIs are generally not as efficient nor as cost effective,” explains Jackman.
All of swissQprint’s current standard ink sets are GREENGUARD Gold certified. This widens the scope for users, with printed products that are safe for use in critical environments such as schools and hospitals. “We have also launched the spot color orange, enabling the user to reproduce corporate colors even more accurately and expand the overall color gamut. Generally speaking, swissQprint LED printers can cure inks very specifically—as individual LED segments of the lamps can be controlled. Again, this widens the scope of applications since prints on critical media can be cured optimally,” says Eicher.
In addition to UV LED cured inks, EB curing is gaining ground. One of the biggest advantages to EB over UV and UV LED is that inks and coatings do not require PIs.
“No PIs translate to lower cost to produce since fewer chemicals are being used in the ink. It also allows for easier handling as they are not susceptible to low-level curing from ambient light. This means fewer precautions are necessary with ink exposure,” explains Parsley.
Maxwell points out that EB technology has been around for other processes including paint curing, however it is somewhat new to digital printing. “The technology is attractive and could offer an interesting addition to the printing industry if developed correctly. Any reduction of chemicals in the ink formulation could open up more space for other benefits such as lower viscosity and increased pigment loads. Right now UV print speeds are throttled by the chemistry and how quickly it can be jetted and activated to cure. By reducing these chemical processes and changing the viscosity of inks, print speeds could benefit, but more research is needed first.”
Amerine says EB technology does provide benefits in terms of the completeness of the cure of the ink and overall cost as PIs are one of the more expensive components of an ink. The drawback has traditionally been the cost of the EB systems. “Those costs have come down significantly but they are still much higher than conventional UV or LED systems. So, the volume of work becomes a crucial decision factor when considering EB technology,” he adds.
The energy provided by an EB system is enough to activate the monomers and oligomers used in the ink, making the use of PIs unnecessary. Martínez shares that this creates a system that is 100 percent reacted and free of unreacted species, generating a continuous film of high molecular weight and therefore, not liable for migration. However, he cautions that systems are not very popular due to the high cost—although in the last few years the price has dropped substantially.
AFFORD have two sets of inks successfully tested on EB equipment. The 899 series is a flexible ink with excellent adhesion, and the 901 series is a similar ink, but better suited for boards and papers.
While theoretically, EB inks do not need PIs, Hutcheson stresses the importance of understanding the specific nature of UV inkjet inks, especially by its low viscosity needed to be jettable through the fine nozzles of the printhead. “This requires specific ink formulations wherefore general rules do not always apply. Therefore, EB curable inkjet inks need to be tuned to this curing means.”
Agfa is developing radiation curable inkjet inks—this term combines UV (bulb and LED) as well as EB—especially in the field of printed interior decoration. Some of the inks designed originally for UV curing will also work with EB, others can be tuned. “These inks are created in close partnership with inkjet print system OEMs and industry partners that produce the panels,” offers Hutcheson.
Geldenhuys points out that PI classifications include press stability and printhead risk, lower maintenance, site preparation, and lighting. “What is not mentioned as one of the additional major advantages of EB is the cure depth. Both conventional and UV LED suffers curing depth issues on high-density areas, leading to reduced adhesion as well as residual uncured ink, which results in smell and contact irritation. EB curing penetrates all the way through the ink layer and is not affected by the color or density, ensuring full curing,” he adds.
As previously mentioned, there is a diminishing availability of some of the current PIs, meaning that formulating without them could be beneficial moving forward. “When using energy curable UV inks in food packaging, the PI is often deemed to be one of the components that poses a risk, especially as during the UV curing process the PIs break down into materials that are likely to migrate. Not having high levels of PI in the ink helps to remove some risk from these food packaging applications. The energy of the electrons emitted from an EB can directly cure EB optimized inks and coatings very effectively. Since EB can penetrate inks without being absorbed or reflected by pigments, they can cure thick layers and/or dark colors more effectively. Thus, EB can offer low migration and potentially very low odor prints, making for many positives for its use in both packaging and industrial applications,” says Jackman.
Sun Chemical offers its EtiJet ELM inks, designed to provide optimum image quality and reliability in high-speed, single-pass printing on a wide range of narrow web and label stocks. Additionally, its Amphora ELM inks offer low-migration performance for packaging and deliver low-odor prints, flexibility, extended color gamut, and enhanced resistance properties.
Let the Light In
UV technology provides many benefits to the wide format print market, but that doesn’t mean it’s not without challenges. As UV LED curing adoption continues, inks evolve to enable new substrates, applications, and added benefits.
Jan2021, Industrial Print Magazine