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Finishing for commercial digital print
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HP Indigo Digital Art
07 Nov 2017
Paper mills now offer a wide choice of inkjet-compatible papers, though usually at a premium
Paper is one of the biggest on-going issues in the fast-growing sector of high speed commercial inkjet printing, finds Simon Eccles.
While you could say that the paper's been an issue for any print-on-paper process going back to Gutenberg, inkjet does pose a particular set of needs that have proved harder to solve than for the other main digital processes that use dry and liquid toners.
Mills are responding to demand for inkjet-capable papers that don't cost a fortune due to expensive coatings, and there's a now a healthy choice of several hundred grades. Prices are reducing compared to offset, though all paper prices are rising for various reasons including Brexit effects. Meanwhile press and printhead suppliers are working with ink developers to produce fluids that increasingly work with more-or-less standard uncoated or lightly coated papers, so that special coatings or primers are only needed for the highest quality applications.
The relationship between ink and paper is always the big issue. The ink is almost always aqueous, meaning the carrier fluid is primarily water with a few additives, plus the colourant, which may be a pigment or a dye. Aqueous ink has a lot of advantages - it's non-toxic and dries with heat without emitting many VOCs. However, paper loves water, so if the ink absorbency isn't controlled the image can spread and the fibres will swell to give misregister and finishing problems.
One answer is the original one used by photo-quality desktop and wide-format inkjets: apply an ink-receptive microporous coating to the paper. Desktop and wide format aqueous photo printers can yield spectacular quality, but the microporous coating costs a fortune and wouldn't be economical for general commercial use.
Mitsubishi Paper Mills was an early pioneer of high speed inkjet papers. It chose a high quality coating route for its JetScript papers, which are recognised even by competitors as giving excellent results. These grades are suited to presses from HP (T-series), Kodak (Prosper), Canon/Océ (JetStream, ColorStream, ProStream etc), Ricoh (InfoPrint), Screen (TruePress Jets) and Xerox-Impika, plus the Fujifilm JetPress 540W, no longer sold. However, the coating makes these papers comparatively expensive, so they tend to be confined to prestige work.
Other major mills have also produced papers with tweaked sizing or coatings to make them more inkjet-friendly, without going to the extent of really expensive coatings. The main names here are Mondi and Stora Enso, which are large international operations, but a range of smaller mills are also offering inkjet grades too. For instance Arctic Papers (Sweden) and Crown Van Gelder (Netherlands), which are small/medium sized operations with the flexibility to experiment and innovate.
Mondi's current grades include BioTop (chlorine-free off-white); DNS (smooth high white); DNS high speed CF ('coated feel') or NF ('natural feel'); Neujet (high density smooth for wide gamuts in matte, matte premium, silk and silk premium; and ProCycle (a recycled grade in "classic" low-white and super-white).
Stora Enso has two ranges of papers for high speed inkjet, intended for applications such as books, direct mail, transactional and transpromo, manuals and catalogues. BergaJet is a high bulk wood-free suited to multi-colour work with pigment or dye inks. It can also be used for offset or laser print. SuperiorJet is a bright white paper intended for pigment inkjets, again good for multi-colour work. Both types can be supplied as A4 sheets or fanfold as well as reels.
Crown Van Gelder has a wide range of inkjet papers in its Letsgo brand series. These include Bright Silk, Silk, Matte, Universal, High Performance, Eco and grades, in dye or pigment ink versions (or both). The spec sheet lists all the main inkjet web presses as compatible with the higher quality grades, while the rest will basically run with anything. There's a good choice of reel formats.
Arctic Paper's GPrint Highway is a high bulk, stiffness and opacity wood-free paper, with a thin coating suited to "almost all" high speed inkjets, with the main applications being direct mail and publishing. Amber Highway is intended for transactional printing with inkjets.
Primers and special inks
Increasingly, some makers of aqueous ink printers say they don't need special inkjet coatings. One of these is Fujifilm's sheet fed B2 JetPress 720S, as the company's product manager for digital press systems Mark Stephenson explains: 'As we coat standard offset stocks on the JetPress 720S with a Rapid Coagulation Primer (RCP) we don't really need any special preparation. The one exception to this is synthetic materials, where a standard inkjet coating is useful to give us a surface to key to. Otherwise our RCP is all that's needed for paper, board and canvas and the same rules as litho apply: smooth silk and gloss coated offset stocks give a better result than uncoated and recycled. Having said that I would say we get a better result on uncoated stocks than a standard offset press, more like an LED/H-UV result but with a wider gamut.'
Tim Taylor, head of continuous feed market, production printing, at Ricoh also says that for many types of work no special coating or ink is needed. 'Normal uncoated litho book or photocopier-type uncoated paper works fine with inkjets. For book wove, and bonds, this is fine. It is only when you start to go up the quality ladder that you need either a special coating or a special ink.'
Screen can supply a special SC ink with a higher polymer content for its TruePress Jet 520HD inkjet web press, that will work with offset gloss coated papers. Ricoh, which sells the same press hardware as the InfoPrint VC6000, has chosen not to offer this ink but to stick with conventional aqueous ink. 'It's a matter of balancing what you expect to print on against the costs,' says Mr Taylor. 'The special ink costs more, so you always have to pay that premium even if you are running uncoated papers that don't need it. We think it is better to run normal ink and then run a primer or undercoater on the press only when needed. If you want gloss there's no doubt that it is cheaper to print with a primer or special ink than to buy an expensive inkjet gloss paper. 'Running the special ink on coated, or a primer for that matter, slows down the press, because the ink isn't absorbed by the paper so much and it takes longer to dry.'
All wound up - output from a Kodak Prosper press
Kodak's message is similar, that in many cases no special paper is needed for its Prosper colour and mono inkjet presses and heads, but when top quality is needed then pre-coating can work well. 'We have a salt-based fluid for pre-coating that works well with our inks and keeps them high up on the surface, for less bleed and more colour vibrancy,' says Will Mansfield, worldwide director of marketing and sales for enterprise inkjet systems. In many cases no pre-treatment is needed for Prosper inks, he says, though high density colour on gloss coated papers does need either special coatings or pre-treatment.
HP has always built-in a facility for in-line primer on its PageWide T-series web presses, that it calls Bonding Agent: this is an extra set of printheads that put the fluid only where image drops are going to go. Thus standard non-inkjet papers can be printed when necessary. Canon uses a similar system called ColorGrip on its VarioPrint i300 B3 sheet fed inkjet. ColorGrip fluid is also available on ProStream high speed web press, but applied as an overall undercoat.
According to Jo Oliphant, Xerox's subject matter expert on high speed inkjet, 'Xerox does work with various paper mills to best tune the response of the inks in combination with some papers, and that includes inkjet treated paper too. Our focus really is about trying to take advantage of the vast number of papers in existence already and that's enabled through what I'd term "more forgiving" inks like our HD and HF ink protecting the gamut and at the same time excluding the need for any type of pre-coat.'
Impika (now owned by Xerox), developed its HD (High Density) ink from the outset to print with a decent gamut on many standard papers without pre-coating. Now it has HF (High Fusion) ink, which Mr Oliphant says 'will enable customers to print on a range of existing offset coated papers, gloss, matt or silk, again with no requirement for any form of pre-treatment on the paper or any kind of protective coat.'
Impika Extreme, French technology bought by Xerox in 2014, now using HD inks so no pre-treatment is needed
UV-cured inks avoid the water problem altogether so will work with practically any paper or plastic. So far there is only one UV-cured commercial inkjet press, the Konica Minolta Accuriojet KM-1 (also sold by Komori as the Impremia IS29). According to Mark Hinder, UK business development manager, the big advantage is that UV-cured inks 'can be printed directly onto standard offset substrates without any pre-treatment or damaging effects caused by excessive heat drying. This provides real cost efficiencies for printers, especially if they standardise their stocks to maximise their purchasing powers.'
Landa's new Nanography process, a form of offset inkjet, prints aqueous ink onto a heated moving belt. The heat drives off the water before transferring the remaining sticky resin pigments to the substrate. With no water to worry about, any grade of paper (or plastic) from low to high can be printed. Landa has come in for a lot of stick for its delays, but if it all works as promised then there will be no need for stories like this about paper choice!