Water-based ink has a long history, going back at least 2500 years. However when printing was first invented in Europe in the 15th century, the existing inks failed to adhere to Gutenberg’s revolutionary movable type, and oil-based inks were used for all the early printed works. When we fast-forward to the late 20th century, we find water-based technology has made great advances, and water-based inks are widely used for narrow web flexo and letterpress, but less widely for digital inkjet.
By John Penhallow
Pigments and dyes
For today’s digital inkjet presses, two classes of inks are used: aqueous and UV, and before examining the advantages and drawbacks of aqueous inkjet we need to look briefly at these inks and how they work. Aqueous inks get their colour either from dyes or from pigments, and the difference is important. A dye dissolves completely in water, whereas pigments are tiny colour particles that are held in suspension. Dye-based inks penetrate into the substrate, and generally give more vivid colours, but are less moisture, light and scratch resistant. By way of contrast pigments remain on the surface of the substrate, and are the preferred choice for any labels that need to be long lasting. Print heads for both types of aqueous inks are similar but not interchangeable.
Another type of aqueous inkjet ink has been developed by Ricoh: this ink is a heat-fusing type, which allows printing on a wide variety of uncoated substrates even though it is water-based. According to the manufacturer, handling is similar to water pigment ink, but it can be fused to the substrate like a solvent ink, making it waterproof and robust against scratches. Ricoh furthermore points out that aqueous resin inks, unlike solvent/UV ones, contains no hazardous material, and therefore requires no special ventilation. These resin inks are described as “safe, and with a wide colour gamut, broad base-material adaptability, and high jetting-reliability, allowing printing on materials, such as PVC and PET films often used for labels”. So far, however aqueous resin inks are used mostly for signage and other wide-format applications.
The arguments for and against
For many labelling purposes, aqueous inkjet inks are quite sufficient, giving good print quality. They are more environmentally friendly, and are also generally cheaper than UV inks. Why then have they not been more widely adopted for narrow web inkjet?
Part of the answer is given by Domino’s Christophe Dousset, interviewed for this article: “Our technology is high-speed drop-on-demand inkjet and our first digital label press with Kyocera heads used water-based inks. When we showed it at Labelexpo some years back, the US visitors said it was OK for quality, but the Europeans found the colours ‘unexciting, lacking in brilliance’. Also, we found that aqueous inks did not adhere well on filmic substrates, be they PE, PP or PET. We still use Kyocera heads but they are slightly modified ones designed for UV inks. With the UV option we have the brilliance, we have the white inks (which you can’t have with the aqueous option), and the fast drying that UV provides.”
This statement by one of the leading manufacturers of digital inkjet presses highlights the most powerful drawback to aqueous inkjet. UV inks dry much faster, offering higher press speeds. Strictly speaking, with UV it is not drying at all but “curing”, a chemical reaction to being bombarded with UV radiation. Much technological progress has gone into making the curing process almost instantaneous, and the advent of LED UV inks promises to make the process more energy-efficient. Water-based inkjet must be air-dried, and this takes time. The air cannot be too hot, nor can the drying unit be too long. And since most labels are printed roll-to-roll, the ink must be 100% dry before the printed labels are rewound. To counterbalance this problem is the argument that aqueous inks are less expensive, and the same is true of the press itself, and in particular the drying unit.
Aqueous ink technology
Most digital inkjet label press manufacturers have decided that UV is the future. Some have not, and in particular Epson and Memjet have developed competitive presses using aqueous inks.
Epson is a Japanese company and is one of Europe’s three leading producers of narrow-web digital inkjet printers. When it comes to choosing between aqueous and UV inks, according to Epson’s François Le Bas, a number of factors come into play. “The first is price,” he says, “Water-based inkjet presses cost, on average, half as much as UV-based models”. This being so, why would any label converter opt for UV? The answer, according to Le Bas, depends on what the press will be used for. “With speeds around 16 fpm (5 m/min), aqueous inkjet is three times slower than UV, because our water-based inks penetrate into the substrate, and must be dried using a relatively slow hot air unit”. For converters needing lower print volumes, Epson’s water-based units therefore are a cost-effective solution, and moreover one which gives off no harmful ozone.
Epson offers both possibilities, because the UV inkjet option, as well as offering speeds of 49 fpm (15 m/min) or more, gives a wider range of print options. The use of a digital varnish means that the converter can print on a very wide range of substrates, both paper and filmic, including those with structured surfaces (of the kind much used for wine labels). UV inkjet can even replicate the 3D effect of screen printing. Water-based inks are more limited in their special effects, and in the substrates they can print on. In the case of Epson’s presses, the LED UV inks are dried individually, and this can be followed by a digital UV varnish.
What about print quality? Le Bas says the Epson presses, be they for aqueous or UV, both give excellent quality with just three picolitre droplets. And is there a difference in environmental aspects? “Not a significant one”, says Le Bas. “Our customers know that we collect and recycle all ink cartridges free of charge”. Epson has also recently announced the development of an aqueous white, which could overcome one of the weaknesses of water-based inkjet.
Desktop and bolt-on label print units
Epson’s aqueous inkjet label presses cover much the same range of sizes as their UV presses. Most of the interest in aqueous inkjet is however directed at smaller presses, those selling at under EUR 10,000. Many of these are desktop models, like Epson’s Colorworks which comprises a range of industrial colour label printers starting at EUR 1350 for an entry-level model, and upwards of EUR 8000 for the top-of-the-range with all accessories. All these models use aqueous pigment inks in four-colours only.
In a slightly lower price range, Memjet’s Sirius print engines are widely sold, often under distributors’ names, and claim very fast production speeds. Memjet uses dye based aqueous inks and requires pre-coated substrates. It is often dismissed as a low cost technology, but Miyakoshi and Colordyne are successfully selling label presses with Memjet print engines. These digital print units can also be found in some hybrid label presses (Colordyne for example is in partnership with Mark Andy).
In January 2016, Colordyne upgraded the performance of its 3600 Series with a new Aspen Memjet engine. The upgrade is said to give running speeds up to a scarcely believable 276 fpm (84 m/min) at a resolution of 1600 x 1375 dpi.
Primera favours aqueous
Another major label printer manufacturer using aqueous inks is Primera. This US-based manufacturer has, for many years, manufactured digital label presses using laser technology, but has recently been hitting the “under EUR 10,000” sector with a range of smaller aqueous inkjet machines.
Andreas Hoffman, Primera’s European CEO, said that, in his opinion, the label market in general was increasing constantly, but especially the market for end-user labelling. “This is one of the main target markets for our desktop label printers” he said, “and it is increasing by approximately 10% annually”.
These end-users can be found in many business sectors, but are particularly small and medium companies making beverages, foods and many other retail goods. They account for between 90 and 95% of Primera’s sales of small label printers, the rest being label converters and printing houses.
Asked why label converters are not satisfying this sector of the market, Hoffmann replied “End-users are getting tired of long waiting times, minimum order quantities or fixed label sizes and designs the main part of the label converters and print shops still have or request. Only a small number of them realize the chance they have to increase their customer base by answering the needs of small and medium-sized clients, which they can do by using our CX/FX1200e system and our LX series of desktop colour label printers”.
Questioned on the most important selling arguments for Primera’s water-based ink desktop label printers, Hoffmann answered “Outstanding print quality up to 4800 dpi, flexibility, ease of use, and the very good workmanship”
Sometimes the best option
Of the 30 or more manufacturers offering digital inkjet label presses, nearly all now favour UV/solvent inks. Epson, Memjet and Primera are among the few OEMs offering the aqueous ink alternative. This does not make water-based inkjet technology inferior. In like-for-like comparisons it is generally less expensive, both for the initial investment and for the ink itself. These inks do not require the use of hazardous chemicals, and can be seen as more eco-friendly. Their Achilles heel is the drying process, which slows their speed. However with print runs getting shorter and shorter, speed may come to matter less. This seems to be favouring the aqueous inkjet option, particularly for end-users wanting to print their own labels.