05 June 2018 Digital Printing Technologies

Digital printing breaks free from short run chains

The latest generation of digital presses tackles the problems of colour consistency that have been a feature of the technology – until now.

Digital printing has always been suited to short runs, not only because of the cost and the limited format and speed compared to litho printing, but because too of quality and consistency – or lack of it.

One book printer always argued it was better to print covers on a litho press because doing the work on a digital press would mean that batch to batch colour variations on the shelf would be glaringly obvious.

It is a problem that the technology providers have been working to solve, and one that they believe they now have under control.

The answer in the old days was constant calibration, every day, at the start of a job and perhaps again in the afternoon: colour particles had a habit of dropping to the bottom of the toner during the day resulting in a difference job to job and between jobs printed one day and the next.

Temperature and humidity have their effect on consistency too. With care it would be possible to deliver consistent quality, but care means time to set the machine, constantly checking sheets and to reset it when colour quality slips. In every day production such aspects simply get in the way of getting sheets on the floor.

As digital printing has improved, the finish has become more and more litho-like on a greater range of papers. This is perfect for job of one products like photobooks, but problematic if a job runs to several thousand or a few hundred copies. If the job is a repeat, or one where production is spread across different locations, additional care has been needed. Consistency has been the glass ceiling of digital print.

A calibration routine at the start of each day may not be too onerous, but having to perform the same task several times a day will mean that the machine is left to its own devices. This generally means an inline spectrophotometer and quality assessment cameras or sensors to spot and correct streaks, registration issues or other unwanted artefacts.

Such technology has been restricted to the flagship machines, the Indigo from HP, the iGen4 and iGen5 from Xerox or the Kodak Nexpress and not worth the bother on less costly presses.

Now the same kind of technology is coming to the next generation of machines from Konica Minolta, Xerox, Ricoh and Canon. Where once digital printing was all about producing short runs because the technology was not capable of longer runs, this rule no longer applies.

To date the best examples of this have been inkjet presses. Inkjet, lacking the fusing belt and imaging unit of electrophotographic presses, ought to be a more consistent print process. And according to Mark Stephenson, Fujifilm inkjet evangelist, it is. The Fujifilm Jetpress 720S, he says, “just works”.

“It is very repeatable, which is one reason why Push Print has invested in one. We ran a packaging trial for a well known brand that asked us to print the same job over a number of months and at different rimes of day. When we measured the results there was a delta E difference of 1-2.

“Their verdict? ‘This beats anything else digitally printed we have ever seen’,” he says. Likewise Push had deferred an investment in digital until it could be sure that output would remain consistent.

It was not always the case for the Fuji press. In the first version, the inspection unit and inline spectrophotometer was positioned in the delivery. By the time a flawed sheet reached the array of sensors and applied corrections, a further handful of inaccurate sheets were already on their way.

In the Jetpress 720S, the colour device, measuring a control strip, and the quality array to check for streaks or other flaws, are positioned immediately after the inkjet heads. Any corrections can be implemented immediately.

While nozzle compensation is in place, allowing an adjacent nozzle to replace one that is blocked, the operators need to run a regular cleaning routine to ensure that nozzles are kept clear of lint or partial blockages that will affect print quality. This may be needed every 1,000 sheets, the equivalent of a blanket wash on a litho press.

The colour gamut with just four colours is wider than the standard offset litho CMYK colour space. Hence Fujifilm prefers to accept an RGB file to the DFE where it can be ripped to match the ISO 12647-2 colourspace, or run to make use of the additional gamut that is possible. It means that the press will achieve a wide range of PMS colours. The predictability and consistency of colour is essential to manage this.

Fujifilm holds the profiles of 150 commonly used papers within its profile library. With more unusual papers specified for a project, the user is likely to run a test or three to meet the client’s expectations. This profile can be locked into the system.

For Stephenson, inkjet is the future. “As run lengths decrease people are looking at what the alternatives are to offset. Hence our main market is not the digital printer looking to improve what they already have, but offset printers,” he says. The inkjet technology means there is no need to run up to colour, nor to achieve an ink water balance. There are no waste sheets. With shorter runs and designers specifying expensive sheets with the feel and colour that their project needs, the absence of waste can become a crucial factor.

This was vital for a company like Push, Stephenson explains. “When we did the sums, the ROI was obvious,” he says.

The HP Indigo has been the acme of digital print quality for many years, unchallenged as the closest match to the look, feel and quality of offset printing. While the gap to rivals is diminishing fast, HP can still deliver a consistency that allows it to be used in distributed print networks. Gemalto, for example, is distributed print management business that sends the same artwork to be printed on a network of HP Indigos close to the point of consumption, for a network of car dealerships, for example.

Members of the network are assessed for quality before signing up and are expected to run a calibration check at the start of each day. The accuracy of this will determine the types of job that are directed to the print service provider. Colour management is built around media fingerprinting. A colour patch is run on each media type and the results measured, analysed and adjusted to produce an ICC profile. Once achieved this becomes the media fingerprint for that paper and the DFE creates a match to the chosen colour target, usually ISO, Fogra or similar.

The press has an onboard spectrophotometer and with the latest Indigo 12000 HD a new imaging head able to address 1600 points per inch. This leads to the equivalent of 300lpi in litho terms says HP.

The extra resolution will be noticeable in subtle graduated tints, flesh tones and skies where the step between one pixel and the next is smoothed out and so less noticeable to the viewer.

To date this is available only on the flagship press, though HP Indigo will generally introduce any new technology on its highest profile machine before rolling it out to other platforms.

HP’s interest in labels and packaging where precise colour control is essential has unquestionably rubbed off on the commercial presses, helping improve consistency of the output across the times that a job is reprinted.

The quality control systems on the latest Kodak Nexpress, the Nexfinity, are equally impressive. At heart is a new UK designed imaging head operating at 1200x1200dpi. This is also capable of delivering 256 exposure levels, through micro adjustments to the writing head, in turn changing the amount of toner transferred to the imaging belt.

The Nexfinity was announced two years ago at Drupa having been in development for two years before that says Brad Kruchten, as head of the print services division before the recent announcement of his retirement. “We don’t expect it to replace the existing Nexpress. This is for companies that have a larger number of jobs and a need for a lower cost per page,” he says.

And in addition, the Nexfinity has the ability to handle a greater range of substrates, can work with a choice of ten extra toners in the fifth print station and can switch the sequence of these colours so that CMYK can print over white. Crucial, however, to the aim of taking on more of the pages that have been printed on litho presses, is the ability to print at higher quality and with improved consistency.

A series of control patches printed on the belt between sheets is measured as a first level of colour control. An inline spectrophotometer feeds back to the imaging head to control the data flow to each of the diodes. Kodak has also introduced a new screening system, allowing differential screens in the same page, which creates a hard dot screen and very consistent tints at an equivalent of 175lpi litho printing. It will also print on almost all standard litho papers, matching the reflectance value of the paper and print through profiling the paper to be used.

This comes under the Substrate Assistant feature which will match fusing temperature and pressure to control the adhesion of toner to paper on both sides of the sheet, which can be a standard offset paper.

The Intelligent Calibration System used maintains colour consistency over time during an extended run, while Technotrans technology maintains conditions inside the press at 33% relative humidity and at a constant 75ºF. Positive pressure inside the press pushes any dust to the outside. This allows any Nexpress to be used outside a controlled environment, Nexfinity included.

The press will handle a sheet to 1,260mm and will duplex a sheet that is 780mm long. “In offset printing , the B2 format overcomes some of the limitations of the process. In digital we do not need to print wider to be more productive,” says Kruchten. The length can give six- or eight-page folds, as well as banners.

The comparison with the offset world is key because this is the target for the press. It is about removing the need for printing plates (Kodaks included) and providing extra flexibility to a company with a single litho press.

“We can address more total pages in the market,” he says. The 152ppm output speed is matched by a lower cost per page thanks to a 30% gain in the productive life of the Operator Replaceable Components.

The colour pigments remain the 6 micron size used previously with ten options to cover additional colours, gold and now silver metallic effects, security, pearlescent, tactile and varnish effects. The Nexfinity has been designed to enable a swift switch between these, using a cart to hold the toner unit being removed while the next unit slides into place.

Ricoh also offers a switchable fifth colour unit and the option of printing white ahead of CMYK or to place white in the fifth position, but not on its most productive press. This comes with the new Pro C7200, a successor to the Pro C7100 that made a reputation for quality and has been used to substitute more expensive top end digital print options.

It has now make the same change with the higher speed machine, the Pro C9200 replacing the Pro C9100, addressing issues that perhaps limited the take up and application of the previous generation press.

This has just been released, a few weeks only after the Pro C7200, the press with the option of a fifth print unit for value added neon yellow, neon pink, white, clear toner and now an invisible toner that responds to infra red light.

Both share the use of a new toner, increased media versatility (though perhaps not as much as some might like in the less productive machine), and features developed to improve colour quality and consistency on longer runs or for repeat jobs.

The VCSEL imaging head is the same, offering 2400x4800dpi imaging, while the real improvement is with the quality control unit. For registration, each press uses six sensors aimed at printed points on the edge of the sheet.

By measuring the reverse relative to the front positions the unit can correct for alignment, back to front registration and skew. This takes 30 sheets, ten to align the front, ten to align the second side and ten to deliver a precise back to front match.

A final sheet is a reference sheet with crops and text to show what deviations have been reported. This is stored as the media profile for that paper and is recalled for automatic use when the same substrate is used on a subsequent occasion.

The integrity of the content printed is monitored against the raster file held in the controller. This will spot and flag up artefacts introduced during the printing process, automatically rejecting sheets that come short of perfect.

Colour is measured at the start of each day to calibrate the press using a test sheet read by an ES2000 handheld spectrophotometer into the EFI Fiery Rip. This creates the reference for that day’s production. Settings for each job are then adjusted according to the media profile for that substrate, and consistency is delivered through an inline spectrophotometer feeding back adjustments to the imaging unit on the fly in the Dynamic Shading Correction feature.

This also allows a job to be split across multiple machines in the same location according to Ricoh, perhaps to machines in different locations though Ricoh is not yet talking about this.

The idea is to maximise productivity through minimising operator intervention. For the Pro C9200 this is about swallowing longer run jobs that might otherwise go to litho. It is rated to run at 135ppm on paper to 450gsm.

There has ben a change to sheet alignment, from measuring along the front edge as previously to adopting a side lay style unit, which is more effective on the longer sheets the press can handle, and will control both sides to a maximum of 700mm.

“People want stable colour, greater media versatility and reduced cost of ownership and to run longer runs,” says xx yy for Ricoh. The Pro C9200 is the press it hopes will deliver this.

The same sorts of automation of colour are available in the Konica Minolta printers fitted with its IQ 501 quality device. This is built around a completely new approach, using Konica Minolta’s expertise in colour measurement and sensors as well as its knowledge of toners and printing.

It is built around the Lab colour space sitting between the RGB world of monitors and spectrophotometric devices and the printing world of CMYK and density. A spectrophotometer will provide a highly accurate Lab colour value, but is relatively slow, while CCD array is faster buy only measures RGB values. Translation of RGB into Lab is problematic because the relationship between different colours measured this way is not constant, so beyond a simple algorithm.

The conventional way around this is measurement of colour patches to build profiles to create look up tables to understand how scanned information needs to be treated: the more patches that are measured, the greater the accuracy, but the more patches that are measured, the slower the process.

Paper and the way that light reflects from its surface into the sensor heads will affect the information read. The Konica Minolta IQ501 is an approach which combines a scanner with spectrophotometer and with a new test chart with colour patches arranged int a different way. It uses technology from the Japanese company’s sensors and measurement division as well as its printing operations.

It has been released in the C3080 and will appear in all Konica Minolta machines in future. First the company worked with one of Japan’s largest printers to validate that this approach would work. After a few tweaks to improve usability, it has been released to the market.

Konica Minolta introduced it with a demonstration where 20,000 sheets of the same job are piled in numbered sequence and customers asked to assess any sheet against any other. The delta E difference is not noticeable by human eye. In Japan the company ran 10,000 sheets with a delta E of 0.4. As well as colour, the IQ-501 will also control register.

This colour accuracy is within the consistency of the KM-1 inkjet press, which like the Fujifilm Jetpress 720S can claim to be offering both colour consistency and a wider colour gamut.

The company has acquired insights from the Konica Minolta Marketing Services operation, built around the former Charterhouse print management business.

This has provided feedback about what its customers, the marketing departments of world-wide brands require. Consistent and predictable colour is key, whether for marketing material or for packaging.

It is a requirement that lies at the heart of Landa’s thinking since the outset. By using an intermediate image carrying belt, it overcomes the problems of maintaining consistency across different papers.

With as many sensors to check colour as there are inkjet heads, the closed loop system will bring automated quality control to the press, and take a step towards guaranteed output quality and lights out operation.

At that point, digital printing will be the most controllable, highest quality and automated print technology available.

Six colours good, long life better

Xerox’s launch of the six-colour Iridesse digital press seems to head in a different direction, stressing the added value possibilities of digital embellishment via metallics, white and varnish rather than colour accuracy and quality.

However, the headline grabbing features perhaps overshadow capability towards longer runs and colour control. The imaging drum has a new coating designed to extend its useful life by 50% while the imaging belt will give twice the expected life of previous belt technology.

The CMYK stations use a new Ultra HD fine 4.7 micron particle sized toner. This improves the controllability of four-colour printing, the combination of particle size, screening and imaging resulting in finer lines, smoother gradients and flesh tones for quality and the inclusion of inline sensors and automated adjustment for control consistency.

Imaging is at a resolution of 1200x1200dpi using a 2400x2400 dpi VCSEL laser. A 10 bit colour depth offers 1024 grey levels and demands vast consumption of data and a suitable DFE to generate this.

The DFE is the EX-P 6 Fiery from EFI, developed for Xerox’s particular requirements. Media management, and other critical set up tasks are swift and consistent. The press includes automatic registration control and front to back alignment, density control, tone curve adjustment, image transfer monitoring and colour calibration in one full width array and an optional sheet curl correction through a second array.

Front to back registration uses a reference chart which is printed and then scanned to compare with the base data held in the press. A correction value is applied to the profile of the paper so that compensation for stretch or shrinkage of the paper is applied to the job file.

Likewise measurement and analysis of test charts feeds into automatic adjustment of density and colour curves to become part of the media profile of the machine.

Gareth Ward

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