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Not All UV Inks Are Created Equal

inkjet_printingThe utilization of inks using Ultra-Violet wavelength light to cure them is increasing all over the world, in all printing processes. Instead of relying on oil/solvent-based inks to cure by evaporation, thereby releasing unwanted VOCs (volatile organic compounds) into the atmosphere, UV energy instantly polymerizes a combination of monomers and oligomers present in inks and coatings, onto a wide range of substrates. If that sounds a little fancy, in practical terms it means UV dries and fixes the ink instantly, so the printed item is ready-to-use straight off the printer.

The uptake of UV is strongest in the industrial inkjet printing market. These are the printers that make billboards, POP displays, posters, window signs, exhibition booth graphics, backlit panels, wall and floor décor, product decoration, printed textiles and, increasingly, labels, cartons and boxes for packaging use. Not so long ago these products would have been printed by the ‘silk screen’ process where thick pigmented inks are squeegeed through a fine mesh directly onto the substrate. The screen process is still around, but inkjet UV has taken vast chunks of its market due to its speed, cost and convenience.

Creating inks suitable for good UV curing is a lot more complex than mixing up pigments, resins, oils and solvents for non-UV application. This is because the inks must be responsive to the precise wavelength of UV light used to excite the photo-initiators and polymerize the monomers and oligomers in the ink. (It’s actually a chain reaction, but don’t be alarmed!) It’s clever chemistry and, without getting too technical, deals mostly with invisible UV light beyond the violet end of the visible spectrum – the same kind of light that gives us a suntan and causes us to wear UV-resistant sunglasses to protect our eyes.

The visible spectrum—colour we can see—occupies wavelengths between 400nm and 780nm (nanometers), of the electromagnetic spectrum. UV starts at 400nm and goes down to around 100nm in wavelength and is divided into UV-A, B and C; these can be high-energy wavelengths from our Sun and UV-C is harmful but, thankfully, is absorbed by our atmosphere. UV-C is, however created by the mercury vapour lamps used in curing UV inks, which is why they are shielded.

Recently, UV using low-energy LED lamps has gained pace. This is good because LEDs use less energy than mercury vapour lamps, they last a lot longer and are lower cost to replace. Inks for LED curing need to be precisely matched to the LED wavelength, which is typically right on the cusp of visible/invisible light at 395nm. The Fujifilm Acuity LED1600 printer has a perfect blend of ink + LED curing + machine proving popular in the 1600mm wide format sector.

Some printers use a combination of LED and mercury vapour curing. The LED ‘pins’ the ink down and prevents it spreading (dot gain), while the UV-C rich mercury vapour lamp gives it the final surface cure. This again requires the right ink formulation, with photo-initiators tuned to both wavelengths. In short, there is no ‘one size fits all’ with UV curing ink—they need to be application-specific.

Designed, Not Mixed

In the world of UV curable inks for digital inkjetting, they are not simply ‘mixed’ as in offset or screen applications; they are designed. The demands placed on inks by modern high-speed printers capable of jetting billions of tiny ink droplets per second, leave no margin for error. Inks passing through precision printheads costing thousands of dollars to replace, need to be designed in such a way that pigments do not clump together, colour is always consistent with high saturation, curing is instant and prints emerge dry, ready for die cutting or other finishing.

Fujifilm Speciality Ink Systems (FSIS) has been at the heart of designing inks for UV applications since well before inkjet technologies emerged. When the company was known as Sericol, it developed the world’s first Screen process UV ink way back in 1978. Sericol was also the first with inkjet UV ink when, in 1999 it partnered with Inca Digital of the UK to produce the world’s first flatbed UV printer. Fujifilm acquired Sericol in 2005.

Today, FSIS UV inks come under the Uvijet brand and are designed for various applications:

  • Uvijet KV Inks for Thermoforming: Thermoforming of decorated plastics, such as polystyrene, PETG, polycarbonate, acrylic, PVC and ABS, used in the signage industry presents particular challenges for the inks used; regular inks are far too brittle when dry. Such inks must be able to ‘stretch’ or elongate while retaining excellent colour fidelity. They do not crack or flake, even when bent at a 90˚ angle. Equally, they must be able to tolerate extremes in temperatures used in the thermoforming process, followed by exterior display that could involve sub-zero temperatures.
  • Uvijet KA inks: Ink adhesion is a major concern for all printers who lay images down on a wide variety of substrates. KA inks are formulated for improved performance with corrugated substrates specifically with superior adhesion, higher scratch resistance and improved tolerance to print artifacts caused by operator fingerprints while costing less per litre than previous generation UV inks. When coupled with Fujifilm Acuity Advance flatbed UV printers, the combination is unparalleled for reliability and versatility.
  • Uvijet KO & KI inks: These are the popular wide-gamut inks which have helped cement the Acuity range as the world’s leading flatbed UV printers with more than 4,000 installed. With high gloss levels and the broad-spectrum of colour rendition, KO and KI inks are even used for fine art and photographic reproduction.
  • Uvijet LL inks: These are the inks for the lower energy curing characteristics of LED-UV. LED offers great advantages in energy use, lamp life and emissions. Uvistar LL ink was specially developed for the Acuity 1600LED hybrid roll/flatbed device and continues to amaze customers with its versatility.
  • Uvijet E & O series inks: These are the inks designed for the Inca Digital printers, where flatbed UV began. Inca continues to lead the world at the high-end of industrial flatbed UV printing, with inks designed for specific printers and purposes. The O series is for the ‘Onset’ high-productivity models, including the new OW ink that provides superior resistance to post print finishing such as cutting, creasing and routing.
  • Uvijet Q series inks: Fujifilm’s Uvistar series of roll/flatbed hybrid printers have been a stellar success all over the world. No other printers in the 3.5 and 5 metre wide widths offer the versatility, productivity and output quality of Uvistar II and Pro 8 models. Q series inks are designed for these machines.

FSIS also makes inks on an OEM basis for other printer manufacturers, plus a universal flatbed fusion primer that aids ink adhesion onto difficult substrates, such as Acrylic, PVC, Polycarbonate, Fluted Polypropylene and PETG.

As UV inkjet continues to grow, FSIS and its research facilities in the UK, USA and Japan will continue to be at the forefront of developing formulations that prove not all UV inks are created equal!

Further information and technical data sheets can be found here.

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Topics: inkjet printing, UV inkjet

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