21 November 2018 Digital Printing Technologies

Inkjet is knocking on the door

Inkjet is on a long journey for customer acceptance and printers have to understand the challenges for the technology before widespread adoption is possible.

Inkjet is hard. Very hard. Logically the billions of dollars of research and the time since the Inkjet Drupa was declared in 2008 ought to have resulted in a litho replacement press by now. Such high quality machines as exist are few and far between. Printers are still buying litho presses, still buying established electrophotographic technologies. The only conclusion is that building the perfect inkjet press is harder than it first appeared and harder than most anticipated.

Inkjet has transformed the display print market with firstly solvent ink presses, more recently latex or UV machines. No one in this part of the market will be buying screen presses again. Nor will book printers be investing in new litho machines when inkjet presses can deliver the runs, the quality and the overall unit cost that publishers require. Label printing is rapidly succumbing to the appeal of inkjet. More digital presses were sold in Europe last year than conventional flexo presses according to labelling association Finat.

Inkjet is beginning to deliver a digital transformation to textile printers and there are some extraordinary applications for inkjet in electronics, embroidery, wall­papers, wood laminates and ceramic tiles that are absolutely transformational.

But commercial printers are slow to be convinced that inkjet can be the solution to their problems. In part this is because inkjet has to overcome not just the analogue litho process but also established digital processes. Printers cannot afford any let up in quality simply because they are using inkjet technologies. Printers do not want to relinquish the advantages that conventional technologies provide in terms of versatility of format or substrate, or productivity.

But there are incursions. Fujifilm has a handful of Jetpress 720S users in the UK; the Lettershop Group is using a Kodak Prosper to replace conventional printing on direct mail; Screen has a couple of TruepressJet 520HD users to print what has been printed using litho or electrophotographically until now. But these are exceptions.

HP has a number of PageWide presses in direct mail and book printers, but as yet none replacing Indigos; Canon has both inkjet web presses for book and transactional print and a limited number of i300 users; Ricoh likewise has customers for theVC60000, but not as yet the VC70000; Xerox has samples of its continuous feed and cut sheet inkjet technology in use. Heidelberg has yet to ship a B1 Primefire inkjet press to a UK customer, despite its undoubted quality. There is no queue of buyers. Perhaps the exception to this is Landa, where the nano ink is applied using an indirect inkjet process.

This is in all a poor payback for the hype and excitement generated at successive Drupas from 2008. By now inkjet ought to be if not the dominant technology, at least one that every printer has to consider. And the key reason is that inkjet is tremendously hard.

On paper it is a simple process. There is no contact with the substrate, no ink and water to balance, no plates to make and load into position and on a web press, no restrictions in terms of page format. It should be the ideal printing process. The inside of an inkjet press looks simple, compared say to the inners of an Indigo. But in reality there are numerous considerations that printers and developers are still coming to terms with.

The first is the nature of the ink and paper mix. Inkjet ink has to be fluid enough to pass through microscopically small nozzles able to fire a 2.5pl droplet thousands of times a second. This means using water as the solvent to carry the colour to the paper. And paper does not enjoy being soaked by what can be 5 litres of water a minute. This has to be dried and the vapour has to go somewhere to leave the image on the paper.

Standard offset papers are designed to absorb some water. Consequently results with inkjet have not been good. Book printing can get away with it because ink coverage on the page is low, minimising the water problem. The scramble to tackle this has been the key to delivering an acceptable technology and printed quality.

Papermakers like Mitsubishi or CVG have come at the problem with special coatings to absorb the shock of drowning without the water penetrating into the fibres. Production is relatively limited and the paper is more expensive than an offset paper. And the printers customers do not want to compromise when migrating work from offset to inkjet.

The second approach is with a pre coating on a standard offset paper. This can be applied as a universal fluid using an anilox system or where the ink will be applied. This is the bonding agent that HP has used on its PageWide machines. The bonding fluid is only applied where the image will be so does not affect the look and feel of the paper as an overall coating might.

The pre and sometimes post coat approach is one that many developers have taken, though this can affect the cost of the overall machine and introduces a further process. But it brings down the cost of the paper, because standard papers are used, and enables easier acceptance of a new technology.

The third approach is with the ink. If the ink can be made to dry more quickly on the paper, or perhaps be engineered so that it creates its own protective layer, the problem may be solved. This opens up the various inkjet technologies. Kodak's continuous inkjet approach has advantages here because the continual flow of ink through the nozzles, whether needed on the paper or not, keeps them open.

A piezo approach where the nozzle fires “on demand” may need agents in the ink that prevent the nozzle being blocked or partially blocked by dried ink. Glycol is a favourite addition for this purpose, but creates other drying issues on the paper. HP's thermal inkjet approach is also a drop on demand technology that is dependent on vaporising water to expel a droplet of ink through the nozzle.

The aim is for the ink to sit on the surface of the paper, without running around to combine with other droplets. The most successful approach to date is to include an agent to promote rapid coagulation. Fujifilm uses this on the Jetpress 720S and Landa also creates a film for the pigments to sit in as part of its nano ink.

Fujifilm uses a combination of the rapid coagulating ink and a coating design to enhance this characteristic. Konica Minolta, also with a B2 sheetfed machine, is using a UV ink instead of water. UV inks are more viscous than water based inks so are harder to push through the piezo nozzle. KM gets around this by heating the ink to make it more fluid through the print head with the extra advantage that as the droplet flies the 1mm or so to the paper, it cools and holds its shape better when it strikes the surface. A fraction of a second later the UV energy delivers the final cure.

Development on web presses is towards an ink, not unlike the latex ink that is used in large format printing, that is an ink which contains its own coating to prevent penetration of pigment into the paper. To date, Screen’s SC ink for the TruepressJet 520HD is the best known example. Ricoh has taken a similar approach for the ink on its equivalent machine the VC70000. This is only 50% water and delivers the lightfastness that is needed in display applications and the density needed in commercial print applications without the need to apply more ink than is needed, which in turn creates drying problems.

The drying issue is a big one for water based inkjet. In labels and packaging applications it has encouraged the development of energy cured inks and presses, almost always UV of some kind, but also now including electron beam. The web is passed through a modified X-ray scanner to cure at speed with no risk of the migration of photo initiator particles that cause major problems for food brands in particular.

As most labels are applied to the outside of an already impermeable glass or plastic bottle, this is not a great problem, but for flexible packaging with films rather than glass, UV is unwelcome. Inkjet print for flexible packaging, in Europe at least, must either be water based and electron beam cured.

This is not a solution for commercial printing however. And drying technology has become a fascination for the different suppliers. Even HP with the three different widths of press in its PageWide series has three different approaches to drying the web, partly explained by the different centres that developed the machines.

Canon has also come up with different approaches for each generation of machine and as the demands on drying have increased with greater ink coverage and demand to use standard papers. It has used heated drums, hot air knives and combinations of these. With the ProStream 1000, its latest offset replacement machine, it has adopted an offset approach. The web is dried in what is to all intents and purposes an oven from a heatset web offset press. The web runs unsupported through a long drying section beneath the print heads.

The concept is that this dries the paper more gently than a high heat approach and avoids contact with rollers that may cause unwanted marking. Ricoh's VC70000 takes almost the opposite approach. Its web path winds around under IR dryers and then around a heated drum. Again the purpose is to dry the ink not cook the paper by using the longest path possible and to keep the paper smooth by eliminating any tendency to cockling.

The third way, adopted by Kodak initially and now by Screen is to use a near infra-red technology. Adphos developed the concept to use tune energy to evaporate only the water molecules present in the ink. There is almost no heating of the paper which retains its shape without the stretching and shrinking that is a risk when dowsing the web with water.

In a Kodak Prosper there are NIR drying units between the print colours, a configuration that creates a design that is closer to commercial heatset web press than to a design originating in the laser printing world. The design means that the press is printing wet on dry rather than wet on wet. An additional dryer is used for the highest quality version of the press. As the Prosper is currently the fastest press available, it needs the most effective drying that can be found.

Now Screen is adopting Adphos for its TPJ 520HD rather than rely on hot air and heated drums. The drum approach dries the ink through the paper so can have a disproportionate impact on its stability. It will allow the press to deliver dried paper at 75m/min rather than 50m/min currently and will go to 120m/min, though this will mean printing at 600x600dpi rather than 1200dpi or 1200x600dpi.

The resolution question is one that frequently confuses. While finer droplets will mean higher quality, larger droplets at 1200dpi for example confer little advantage. Binary systems like the thermal inkjet or continuous inkjet can fire only one size of droplet, though HP addresses this by having two print heads in one on its HDNA heads, one firing a normal sized droplet and a second array of nozzles to fire a half-sized droplet to add smoothness or fine detail to an image.

The piezo systems can be set to deliver different sizes, perhaps three in a four-level greyscale head according to the level of detail required with larger droplets used for greater efficiency. The resolution therefore is not a de facto indicator of quality, but refers to the addressable points on the substrate, not the amount of ink at that point.

This has raised a further complication to the development process. Higher ink coverage means more water, meaning more drying is needed. It makes sense therefore to reduce ink without losing quality. In the offset world this is achieved with under colour removal, grey component removal or similar technologies.

In inkjet, without screening mechanisms needed for an analogue process, algorithms are needed to optimise quality and avoid artefacts that can occur. This is where Global Graphics is pitching ScreenPro, an ultrahigh speed screening engine designed to overcome some of the unwanted characteristics inherent to inkjet printing at speed.

There are two versions, Pearl to tackle the absorbency of the paper and Mirror which tackles the orange peel effect that can afflict UV inkjet printing on non absorbent materials. It calls on a long background in Rips and screening algorithms covering both AM and as one of the early developers of FM screening for digital imaging.

With version 2 of the software announced earlier this year, Global Graphics is taking on variations that may be noticeable between printheads, within a head due to wear, by addressing individual nozzles regardless of the head and the electronic systems used to integrate the printhead into a printing press. This will mean an end to banding issues. The baseline densities that each head has can be compensated for in the software at high speed and by crunching millions of calculations a second.

Simply put, that is not easy. It was perhaps not accounted for when inkjet first arrived, hence why it has become an issue now, ten years on from the first machines. For all the manufacturers the decade has perhaps been an extended beta test for the different approaches to delivering a reliable, productive inkjet press that can replace offset litho in future. Much needs to be done to refine the process: lower costs would also help, though those are a factor of business models rather than trying to match litho in ink costs for example.

This is a long and expensive game. The potential remains vast even if some of the early forecasts have proved way off the mark. The slow development is putting a strain on some of the smaller players. The integrators will always find projects, solving specific problems with a combination of smart thinking, experience and the right printheads. For a producer of print heads, demand is dependent on the success and speed of take up of the larger end customers.

Kodak's next generation Ultrastream technology for example ought to have shipped a year or so ago. 18 months ago the company fully expected to be able to announce first partners for the heads at the end of 2017. Those discussions continue behind closed doors though Kodak remains confident that something positive will emerge from the kits that are now being shipped. Commercial products will come in 2020 it said in its most recent results.

Likewise Xaar is holding its breath for its thin film technology print heads and uptake in new high volume applications in commercial print, packaging and textile. CEO Doug Edwards has said that the company might need to find a larger partner to achieve this.

The predictions for the long term remain extremely positive, and perhaps on the high side to judge by recent experience. The opportunity continues to attract the dollars. At Ricoh, head of production print Peter Williams has said that the company is constantly being approached about new opportunities for its piezo printheads: “We might be approached about 300 projects. We filter those so that 200 may receive a test kit and then 100 will get our detailed support.”

And those projects may be in textiles, in printing the threads used in embroidery, in printing large format, in printing direct to shape. All may be less demanding that the graphic arts applications that will replace offset printing. Inkjet is hard.

Gareth Ward

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