30 January 2017 Digital Printing Technologies

Turning drops of ink into drops of gold

Inkjet is not simple. It is a new technology that is in the relatively early stages of development and application for commercial printing. But developers are working feverishly to design a press that delivers the speed, quality and cost that the market can bear. In order to do that, there are barriers to overcome.

Inkjet is not easy. This may be the year in which inkjet makes the breakthrough into commercial printing on a significant scale. If it fails it will be due to a failure in imagination or the numbers rather than because the technology is not applicable.

After a decade of continuous development, swallowing millions of dollars across half a dozen companies, continuous feed inkjet can meet quality expectations in many markets. But inkjet is not easy.

To date, continuous feed inkjet has taken the low-hanging fruit. It has replaced laser overprinting in transactional printing, though it failed to establish a new sector called transpromo. Many book printers have invested in high speed inkjet, certainly for mono book printing where the cost of inkjet and the quality are not in question, and increasingly for some colour applications.

Now the next peak that inkjet has to scale is commercial printing proper. Conversations are taking place with a number of UK printers where inkjet could replace sheetfed litho printing and more than one printer has commented that this will be the last sheetfed litho press that he buys. With a five-year replacement cycle there is a good chance that inkjet will be well established by then and the decision will be straightforward: inkjet will complement litho as much as replace it.

In the meantime inkjet press suppliers are scrambling to ensure that their machines will meet the requirements of this market. Digital printing has reached around 10% of all pages printed worldwide. The challenge is how to increase the share that digital has and inkjet is the way to achieve this.

The first generation of inkjet presses had been adapted from continuous feed laser machines, using the same reel handling equipment, similar paper paths and replacing the laser and fusing units with inkjet heads and dryers. When ink coverage is in low percentages, this has proved adequate, but as coverage increases and as speeds and performance expectations grow to meet the requirements of commercial printers, new approaches are necessary. The newest generation of inkjet machines has been designed from the ground up.

The decisions start with the reel handling equipment, should a printer continue with the static systems that have been traditional in transactional printing or opt for some kind of automation; should this try to mimic web offset’s flying reel changes or meet a different set of expectations?

There is also an early decision about the paper width, is 520mm sufficient, or should suppliers offer a choice of reel widths as HP has done?

Once inside the press the paper has to be brought under control in terms of tension; it may need to be primed to create a consistent surface for ink to adhere to. The paper needs to be under full control as it passes beneath the print head array: developers are trying to aim ultra small droplets of ink at high speed on a moving target, even it this is just 1mm away. Even a slight deviation will have a big impact on overall result.

The developer’s choice of inkjet technology may be less open as different technologies are associated with particular suppliers: HP is wedded to thermal inkjet heads, Kodak to continuous inkjet. Those opting for piezo have a choice of different head suppliers, each with a set of characteristics which will determine the inks that can be used.

With piezo there are key choices about the droplet sizes that are fired. In theory, each greyscale head can produce a number of sizes from the same head. In practice, the developer will limit this. A smaller droplet will be useful for fine detail, a larger droplet in covering areas of colour. However, the smaller droplet will be more susceptible to air turbulence as the paper runs beneath the heads and the larger droplet can result in coalescence where droplets merge with each other to create unwanted artefacts.

Inks are another choice. While dye inks are not going to come into the reckoning and UV will be too expensive for almost all applications, manufacturers must decide on straight aqueous inks, or else an ink that includes its own protective layer so that it can print on a range of papers without the need for a primer.

The challenge is to lay down an ink that can comprise up to 90% or more of water and prevent that ink becoming absorbed into the paper so creating show through or swelling of the paper fibres themselves. On a coated paper, the ink has to adhere strongly to a surface that is far from porous and avoid rubbing problems.

Then comes drying. The faster the press runs, more drying is needed and water has to be expelled. When heat is added to the web, this must be removed before the second side is printed and without affecting the dimensional stability of the web.

The amount of energy required for drying should not be underestimated. A press can be drawing vast amounts of power to maintain the temperature of a saddle drying system even if the press is not running.

This has been a key consideration in developing the latest generation of presses. HP has tried different positions for dryers on each of the T300, T200 and T400 series machines. The developers can use a combination of heated drums or saddles, familiar hot air dryers or near infra red systems which seek to evaporate the water in the ink without heating the paper to the same extent.

Then should the drying occur after each set of four colours has been applied or after two colours as Kodak has chosen to do with Prosper? Its Prosper 6000C for commercial printing has more drying power than the Prosper 6000P even though it runs at 200m/min rather than 300m/min. Ink coverage dictates the difference.

Drying can cause problems in other ways. If all the water is not removed quickly from the press, it can condense on other surfaces, including the inkjet heads and then drip back on to the paper with unintended consequences. There is a lot of attention paid to control this misting, IMI’s Mike Willis, points out.

Fortunately he adds that inkjet presses tend not to need the controlled environments that some electrophotographic digital presses demand. “Most are relatively friendly in terms of temperature and humidity,” he says. “Conditions will affect drying efficiency.”

After printing the press developer and printer must decide whether re-reeling the web is the best option or whether to move to some kind of inline finishing. Both GI Solutions and First Move are finishing in line with their latest inkjet presses, as does LSI with its Canon for book printing, while Ashford Colour Press and TJ International re-reel and feed the book lines with paper that has had a short time to dry more thoroughly. KP Services also finishes off line, each Kodak Prosper feeds two Hunkeler newspaper lines because press speed is twice that of finishing.

On the way, there will need to be decisions about maintenance especially how to keep print heads in good condition. If unused the ink may dry on the head, leading to blocked nozzles. Many will cap the inkjet head with a nozzle friendly material when not in use or during a paper change. Others will fire individual nozzles at seemingly random points to ensure that they stay open.

Head replacement itself can be an issue. On the HP PageWide T presses, the head is considered a consumable element and is user replaceable. On others a replacement head may need a service engineer to replace. For all developers there is the question of how much to trust to the customer and how much to lock away critical elements from a customer who under time pressure finds the machine has gone down.

The alignment of printheads is absolutely critical to the success of the process, so too is the approach of the web. John Corrall, managing director of IIJ, which is commissioned to develop inkjet systems for integration into various applications, the latest being a carton printing operation in Asia where the heads are mounted 3m above the floor. The Cambridge company has built up huge knowledge in driving Konica Minolta printheads and how to implement these for different applications.

While it would be possible to monitor the web and to reject any cartons that fall short of the precise requirements, the aberrant blanks are marked and human operators are expected to remove these. A camera system is deployed to make sure the people are fulfilling the task. It is a demanding application with a failure rate set at 6 cartons per 3 million printed. Maintenance is crucial and the line alerts operators when a stop to clean heads and other elements is needed. Inkjet is not easy.

A key part of what IIJ does is to test paper transport systems. These cannot vary by more than 0.1º from a true straight path. “The angle of movement is critical,” he says. “If it varies register can be 20 microns out, stitching between heads will fail. We have completed more than 200 installations and know pretty much what specifications we need.”

This is now being applied to a mono press that IIJ has designed and which will probably become available through Konica Minolta. More details can be expected at the Hunkeler Innovation Days.

Not surprisingly IIJ has also developed strong relationships with ink companies, developing tests to marry the printheads to the applications. It is a seven step testing routine that culminates in a two day running test. If the unit then passes, the company knows that the press will be usable and will not end up in the carpark.

Among the newest types of ink being tested are the water based UV inks where water is the carrying fluid which delivers speed, but the drying method is UV. The challenge is getting rid of water in fractions of a second before the substrate and ink reaches the UV heads. This is an application for corrugated rather than commercial printing, at least in the first instance.

Printheads themselves need to be controlled to match the temperature required to ensure the correct viscosity of the ink. It can mean heating on the one hand and chilling on the other, where Technotrans has some expertise having acquired a company experienced in cooling industrial lasers in the last year or so.

Technotrans is also working on the chilling aspect for the paper and entire press to bring the web into the same condition for reverse side printing as it was when the first side is printed.

Corrall says that inkjet developers face further challenges. On many presses the machine will only start printing when the press is up to speed and all the systems are designed around this. Speed variations, he says, “can be really critical.

“When the printing head is positioned accurately, any time delay from the front sensor, 2mm in front of the print head, or in the data stream may move the image by 1 pixel, which can result in a complaint from the client. Likewise static electricity can be really scary.”

Domino Printing Sciences is also beginning to build its own presses as well as selling the technology to be integrated into mailing or coding lines. Its label press has been a success and the mono press shown at Drupa is now being used in anger at Ashford Colour Press. Like the proposed IIJ/KM machine, this heralds a new style of compact, high focused print engine.

Domino has experience in all styles of inkjet, from continuous inkjet and thermal to aqueous and UV piezo. “UV is really interesting,” says product manager of digital print solutions, Brian Palfreyman. “It allows the ink to stick to any substrate which suits some of the difficult substrates you encounter in mailing, but there is no cost justification for using UV on paper substrates.

“We see a big drive towards 1200dpi as the standard for high quality printing. It requires slightly more expensive printheads, but these are coming. This will be the limit because of the difficulty of maintaining drop placement accuracy on a web moving at 8m/sec. At 2400dpi it is very difficult to maintain any kind of accuracy at speed.

“We will all continue to push the levels of quality. Once you have designed a racing car you continually want to squeeze more performance from the engine.”

Domino’s focus is on aqueous piezo even though it has continuous inkjet in the stable. To date, only Kodak has pursued this line of development with Prosper and now Ultrastream. The first uses actuators to create larger or small droplets (even the large ones are just 6-7pl in volume) and then air to push the lighter drops away from the paper path.

The rate of firing and speed of the ink is higher than with other technologies giving Kodak a speed advantage which others would need banks of heads to match. This has made Prosper attractive to newspapers where additional heads can be added to conventional presses without a need to slow production down, or else to be able to print newspapers at acceptable speeds.

However, Kodak has acknowledged that for the most demanding commercial print applications, Prosper may not be the answer. It is also a complex technology making it more expensive than third party integrators want to pay. The result is Ultrastream which reverts to using charged deflection plates to divert unwanted droplets of ink. The print head is simpler and cheaper, Kodak aiming at $5,000 each long life head, and while it is not as fast, can deliver higher quality.

Quality and speed are also functions of the software surrounding the print head and the profiling of the papers which are being used. Global Graphics has recently taken a strong interest in the problems in inkjet printing, culminating in its acquisition of integrator Meteor at the end of last year.

This company creates the drivers and circuitry to allow third party developers to implement the inkjet heads available. Global Graphics can deliver the software, specifically the colour screening software required to optimise and stabilise print output.

CTO Martin Bailey says: “Developers can print something, but that is not good enough to be saleable, they need colour management and do not understand colour management because they have previously built litho or flexo presses and colour management is built into the imaged plate.

“So the need is first to fix colour management within the digital front end. And this is not the same as analog printing. Some inkjet printed colours will not look good using existing colour management. This can result in striking unwanted effects, especially at speed.

“We have done a lot of work on building half toning engines to reduce the chances of these effects and to achieve acceptable colour quality. It has meant working with partners on the electronics side to make sure that we understand the waveforms for the media used and the speed. That is vital.”

This is a problem that many developers have bumped up against. When Bailey spoke about this work at a developer’s conference at the end of last year, he was approached by a series of people seeking help.

“The same errors occur time after time and across all technologies. The effect can be small, but it is very visible,” he says. The approach that Global Graphics has taken uses a dispersed screen algorithm, optimised for each press and each media profile. Others, he explains, have used an error diffusion approach which works where the file is constant, on packaging or a label perhaps, but is not suited to changing content. “That approach is simply too slow. It delivers the necessary quality, but it is not fast enough.”

The choices facing developers of aqueous inkjet presses on drying have been combinations of hot air and IR, a technology that is known from sheetfed offset printing. The problem is that this suits lower speed printing and is relatively crude, relying on brute force to dry the ink before it can be absorbed.

It will also heat the paper, though not as much as the drum or saddle systems. These heat the paper on contact on the side away from that being printed and rely on the heat being conducted through the paper.

The ink is dried before it can be absorbed to too great an extent, but the paper is heated and will need cooling before printing on the reverse side. Some combination of heat from above and from below can also be used.

The third option has been promoted by Adphos which has a technology it calls Near Infra Red. The energy works on the water in the ink to break chemical bonds and separate pigment and carrier chemicals instantaneously, before the ink can sink into the paper, before droplets can run into each other and to dry the ink on coated papers with greater resilience to rubbing than other methods.

There is no need to add moisture back into the paper or to chill it before printing on the reverse side, says Adphos. “Drying can have a strong impact on colour colour and inkjet applicability,” says CEO Kai Bär.

And speed is required. Along with versatility over media and cost. This golden triangle is gradually coming about. The presses that are coming can print to the quality and at speeds to make them viable. As volume builds, the costs of ink and paper will come down, but never to the levels of litho printing.

The real opportunity that inkjet printing like this presents is after all not about replicating the established models associated with litho printing, but in using the technology to do what litho either finds difficult or impossible to do.

In short, inkjet is not simple.

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Inkjet Droplets

Inkjet Droplets

The first generation of inkjet presses had been adapted from continuous feed laser machines, using the same reel handling equipment, similar paper paths and replacing the laser and fusing units with inkjet heads and dryers.

When ink coverage is in low percentages, this has proved adequate, but as coverage increases and as speeds and performance expectations grow to meet the requirements of commercial printers, new approaches are necessary.

The newest generation of inkjet machines has been designed from the ground up.

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