Consistency is key

Ink viscosity is commonly measured in centipoise (cP); water has a viscosity of 1cP at 20º C. The temperature at which viscosity is measured is important, especially in the case of inkjet inks, as will become clear later. This is because many inks are what is known as non-Newtonian fluids, with viscosities that change with temperature, becoming runnier (having a lower viscosity) as they get warmer.

The inks in desktop and wide-format aqueous inkjet printers from the likes of Canon, Epson and HP typically have a ­viscosity of 3-5cP at room temperature, which is somewhere between water and light oil, according to Xaar ink product manager Jill Woods. She adds that inks used in her firm’s printheads are in the range of 8-12cP, gravure inks have a ­viscosity of 50cP, flexo inks are in the range of 35-100cP and litho inks are a paste with “a viscosity up in the thousands of centipoise”.

When an ink hits the substrate, the viscosity is important in determining what happens. Gravure inks, for example, need to be runny so that they flow into the cells on the cylinder and can from there be absorbed by the paper. Even so they are still 10 times the viscosity of aqueous inkjet inks, so it is clear why they need special coatings to stop them from bleeding and feathering.

Wide-ranging use
Xerox’s answer to its inkjet paradox is what it terms ‘gel ink’. This is based on the 20-year-old solid ink technology used in its office products, which centres on a waxy ink melted down to a liquid to be jetted. It then solidifies when it hits the substrate. Gel ink is slightly different. It is “a peanut butter consistency” at room temperature more like offset ink, whereas solid inks are more like wax crayons. When gel ink is heated to a temperature “below 100ºC” its viscosity falls to 10cP, which the Piezo printhead can jet. “In my experience, if you get above 10cP you get problems with jetting,” says Xerox Research Center Webster inkjet program manager Jim Larson. “I know of no inkjets that work much above that.”

On cooling, the ink returns to a gel consistency, but to really bond it to the substrate it is UV-cured.

“The range of applications is what we’re really excited about. It can print onto plain and coated paper, films and foil, and we’ve tested it on labels and packaging, as well as ­documents,” says Larson. “Part of our exploration is to find the sweet spot; this will be commercialised, but the question is when?”

While Xerox has its project in the labs, Océ unveiled its own hybrid toner and inkjet technology, called CrystalPoint, just before Drupa. CrystalPoint is being used in the company’s ColorWave 600 wide-format printer, which is designed primarily for the CAD market.

“The reason to make it a gel was important, because we want good quality on cheap paper,” says Océ international product manager Neil Westhof. “Ink is largely water and tends to run; when these drops land, they don’t run.”

Océ’s technology is different to Xerox’s in detail, but the principle is the same in that what start as solid spheres or toner pearls, which are about 1cm across, are melted at 130ºC for jetting and then cool and solidify once they hit the substrate. Océ uses a crystalisation agent, rather than UV-curing, to control the way the ink solidifies on the substrate.

“The crystalisation agent makes sure the ink solidifies,” says Westhof, adding that it also controls the time the ink takes to cure, which enables the control of dot gain and minimises banding by allowing a couple of seconds to fully solidify.

Xerox and Océ aren’t unique in having phase-change inks. According to Fujifilm Dimatix Technology Integration director of business development Rich Baker, the use of paste-like inks is an established concept, and one that the company demonstrated at Drupa onto a variety of porous substrates including wood and sponge. SunJet, Markem and Hexion all make this type of ink, but currently they are limited to industrial applications.

“We have more than 100 single-pass industrial systems, which include printing onto highly expensive devices,” says Baker. “Using paste-like inks gives you a chance to inspect and remove the image if it’s wrong. You can’t screw up the print because it’s the last step in the production process.”

These inks use additives that cause micro-crystalisation, which causes them to become a Vaseline-like gel at room temperature. But at the jetting temperature of 70-80ºC they have a viscosity of 12cP, which is ideal for Dimatix printheads.

Just like Xerox’s, says Baker, paste-like inks are substrate-independent, as they don’t spread and bleed on porous substrates or de-wet – that is, bead up like water drops on a waxed car – on non-absorbent surfaces. “It’s all about controlling dot gain”, he says.

The way forward
And while they may not be used in any commercial print applications today Baker is sure that every inkjet company is evaluating their use. In fact, he believes that as inkjet techno­logy moves from the multiple-pass print modes used in today’s wide-format printers to build up an image, to single-pass operation needed for the next-generation applications, paste-like inks will become more important.

Nowadays, to stop wet-on-wet inks mixing together and producing muddy colours, they are ‘pinned’, which is where a low dose of UV is applied to harden the ink enough to stop it mixing, before a final blast of UV after all the colours have been applied sets the ink.

“In single-pass you put all your ink down at once, with the risk that you flood the substrate” says Baker. “So you need the ink to stop moving very quickly.”

While phase-change inks may be the future, exploiting the change in viscosity with temperature is something that is commonplace today in UV-cured, flatbed, wide-format machines. Printheads from Dimatix and Xaar can handle fluids with a viscosity of up to 12cP, while most UV-cured inks are about 25cP at room temperature. But hit 9-12cP when heated above 40ºC and the heads can comfortably operate at these higher temperatures.

Baker believes that with paste inks coming to commercial print there isn’t a problem with the inkjet paradox, and there’s nothing to stop its onward march to become the dominant printing technology. “For most inks and most substrates, I’m not sure we need much more.”

While some companies believe the future of inkjet relies on exploiting inks that change properties with temperature, others are taking a different tack. At Drupa, Epson revealed its plans to address commercial and industrial print markets with new ink and printhead technologies. Epson Europe’s senior business manager commercial and industrial printing, Marc Tinkler, made the point that inkjet inks for commercial and industrial print don’t have to have the same physical properties of offset, flexo or gravure inks, but they have to deliver the same high-quality results without feathering, mottling and coagulation on the same broad range of coated and uncoated stocks. He won’t go into detail about what Epson is incorporating into its ink, which is water-based and not UV-cured, but he does believe the firm has solved the problems that beset today’s technologies.

“No stock we have tried so far has required a special coating,” he says. “We use the same stocks for short-run digital as are used for long runs.”

The inks are much thicker than those in Epson’s current machines – 3-5cP –and to jet those inks it has developed a new Type-D printhead that can handle inks of 15-20cP and, if needed, be heated too.

Epson is not alone in developing heads for thicker inks. According to Xaar ink product manager Jill Woods, its next-generation 1001 heads can handle fluids at least twice as thick as its current products’ 10-12cP capability and it is testing inks in this range.

“Everything is moving in the right direction to improve the range of substrates you can print on and the print quality, and reduce bleeding,” she says.

Other novel solutions may also emerge: Ricoh has a gel ink technology called GelSprinter, which is currently used in office printing kits and offers speeds of up to 8ppm. The technology is designed to deliver low-cost, high-quality colour on plain, coated and glossy paper stocks, and the company says it is working to develop products that offer faster speeds and new applications for the technology.

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