Martin highlights the award-winning PrintFlat™ technology, which gives smooth, uniform tints and accurate tone reproduction via a simple ‘fingerprint’ calibration of the screening process, and the value of creating optmized PDF files so they don’t slow down your digital press and disrupt the production process.
As digital presses become wider and higher resolution, supporting additional inks beyond CMYK, the total number of nozzles required per press increases, which in turn raises the likelihood of at least one of those nozzles failing to jet or becoming deviated (and needing to be disabled). Each nozzle that is out will result in a lack of ink where it should have been in a straight line along the direction of print.
One obvious response would be to fix the problematic nozzles, when they have been detected, but this kind of physical intervention interrupts production and can be impractical. Replacing the affected printhead altogether is another possibility, but again this will have an unwelcome financial impact.
Our sister company, Meteor Inkjet, offers NozzleFix™, an embedded software/hardware solution to compensate for missing nozzles. But for digital printing equipment which does not incorporate Meteor printhead electronics, software compensation may also be beneficial. In this post, we will look at a software-based approach where good working nozzles can compensate for any misbehaving neighbors as required.
Nozzle-Out compensation in software
As the word “compensation” suggests, this solution works by telling certain nozzles to jet more ink than they otherwise would have to make up for a deficit caused by nozzles that were not jetting or had to be turned off because they had deviated.
For one-drop screening, this could mean that whenever a drop should have been output by a nozzle that’s out, a nozzle on either side could be asked to jet that drop instead. An alternative to this, which can accommodate any drop size, is to apply compensation before screening, working with intensities rather than drops.
Here’s how it works: when a nozzle is off, the intensity for each pixel associated with that nozzle is used to boost the intensity of neighboring pixels, which are then screened as normal.
This is best understood by way of an example. Consider an 8-bit continuous tone input TIFF™ which has the value 160 for every pixel. If we are using one neighbor on either side of a pixel to compensate, we will add 80 to their intensity before screening. If we are using two neighbors on each side, we would add 40.
The greatly zoomed-in images above illustrate the results after 3-drop screening for this example: the left image has one nozzle out and no compensation; the middle image shows compensation with one neighbor either side; the right image shows the use of two neighbors on each side.
Of course, the impact is best judged from the actual prints, where the combination of high resolution and ink results in an impressive optical illusion!
The scans above were printed at 1200dpi. The image on the left simulates a number of missing nozzles, which can be seen by the presence of thin horizontal white lines (you may need to enlarge it for the full effect). The image on the right shows the impact of applying compensation to each nozzle neighboring a missing nozzle.
Rapid response and adjustment
A key benefit of this software-based approach is that a rapid response and adjustment can happen as soon as a problematic nozzle is detected. There is no need to wait for engineer and parts availability. Additionally, there is no trade-off to make between taking costly action each time a problematic nozzle presents itself or having a tolerance or threshold where nothing is done until a certain number of quality issues have accumulated.
Learn more about the technologies available to improve the quality of your inkjet output by visiting our website.
About the author
Ian has over 15 years’ experience in industry as a software engineer focusing on high performance. With a passion for problem-solving, Ian’s role as product manager for the Direct range gives him the opportunity to work with printer OEMs and break down any new technology barriers that may be preventing them from reaching their digital printer’s full potential.
With fewer design limitations, a faster turnaround, no minimum run length and higher margins (not to mention reduced use of power and water, and of pollution), it’s not surprising that the digitally printed textile market is growing.1 Inkjet has certainly made textile design and printing much more flexible than screen printing – and that goes for everybody involved, from the designer through the printing company, to the buyer.
But printing textiles on inkjet doesn’t come without its challenges: as a software provider focusing on print quality issues, we often hear from print service providers who can only digitally print two thirds of the jobs they receive because they would not be paid for the quality they could achieve on the others.
Shade or color variation is a common problem. It’s not new in digital printing (it’s always been an issue for screen-printed and dyed textiles as well) and is usually managed by providing a shade band, which printer operators refer to, to check allowable color variations between pieces.
But, unlike screen-printing or dyeing, the color variation on an inkjet press can be visible over a small distance, just a few centimeters, and this results in visible bands across the output. Banding describes features that tend to be 1 – 10 cm across and they’re often caused by variation of inkjet pressure or voltage differences within the head, which typically results in a frown or smile shape. We also see a certain amount of manufacturing variation between heads so that one may print lighter or darker than the head next to it in a print bar. Some types of heads can also wear in use, which can result in less regular banding that can change over time. This means that large areas which should be flat color may not be.
When such a variation occurs it can greatly complicate a lot of post-print steps, especially if you need to put more than one piece of textile together, either in sewing or use (such as a pair of curtains). If that’s the case, then a significant difference may be unacceptable and your printing rejected by your buyer. Ultimately this leads to print service providers rejecting jobs, because they know their digital press can’t handle printing those tricky flat tints or smooth tones.
What can you do about it? The first thing many companies do to try to overcome this banding is to adjust the voltage to the inkjet head, but this is often time-consuming and expensive because it requires an expert technician. A better alternative is to make the correction in software, which is a more cost-effective and faster solution. It means it can be automated and can act at a much finer granularity, so printing is more accurate. There’s no need to mess with controls that could damage the press, and printing companies themselves can make corrections without the vendor sending a technician on-site.
Our solution at Global Graphics Software for improving banding is PrintFlat™. It corrects tonality to hide banding based on measurements from the press. It adjusts every nozzle separately and doesn’t need a specialist engineer to make press adjustments. PrintFlat can be integrated into different digital front ends, using a variety of RIPs, including Caldera and Colorgate and, not to mention, our own Harlequin RIP®.
Over the years of working with many press manufacturers we’ve discovered that many technical issues and solutions are common across different sectors, including transactional, wide-format, commercial, labels and packaging, and industrial, including ceramics, wall coverings, flooring and of course textiles. That means that we already have years of experience in correcting for banding. Using PrintFlat in your press means print service providers can now take on those jobs they would normally reject.
To learn more about how to eliminate shade and color variation when printing on an inkjet press, listen to Global Graphics Software’s CTO Martin Bailey’s talk for FESPA 2020:
Banding, or non-uniformity, is a common problem in inkjet printing that can often result in print production downtime and loss of revenue. In this post, I’ll discuss the challenges printer OEMs and print service providers face when trying to reduce banding and provide an insight into the work we’ve been doing at Global Graphics Software to remove banding and streaking artifacts from the print output, enhancing print quality and raising productivity.
What causes banding in inkjet?
Inkjet printheads produce variable density output both across an individual printhead (known as the inkjet ‘smile’) and when comparing output from one printhead with another. The output from a printhead can also change with time, as the printhead wears or ages. Additionally, the overlapping stitch area between printheads in a single-pass printer, or between overlapping passes in a multi-pass printer, can also cause density variations. Such variable density becomes visible in the printed output as ‘banding’ and ‘stripes’, and means that it is not possible for print providers to digitally print jobs with certain image features (such as flat areas or gradients), or that they must sell the lower quality output produced at a significant discount.
Why is uniformity in inkjet a challenge?
Fixing banding or streaking in inkjet is not without its challenges:
In the printer design phase, the use of interlacing in the printing process can be effective at reducing banding and improving uniformity, but significantly impacts the speed and/or cost of the printer. This approach is especially undesirable in single-pass systems, where the only option to interlace is by doubling the quantity, and hence cost, of printheads in the printer.
Currently most OEMs attempt to correct uniformity issues with hardware solutions such as drive voltage tuning, but these give only limited improvement and are slow, complex and costly to implement. Most printheads have only limited voltage adjustment for banks of many nozzles together, or even the entire printhead as a whole, and do not allow adjustment of drive voltage for individual nozzles – such adjustment does not have the granularity necessary to really eliminate banding. Additionally, adjusting drive voltage to balance output density (drop volume), is undesirable as this is likely to negatively impact drop velocity, printing reliability (jetting stability) and even printhead lifetime. As the printer performance changes over time, and when printheads are replaced, service and support engineers must spend a significant amount of time onsite re-making these complex adjustments to achieve quality that is, at best, a compromise.
A solution in software
Global Graphics Software has been working with printer OEMs and print service providers to significantly enhance the quality of their inkjet output, one such company being Ellerhold AG, a leading poster printing house and press manufacturer in Germany.
Ellerhold wanted to enhance the printing quality of it’s large-format posters. Specifically, the printheads on its digital printing machine showed variation in printed density both between the heads and across each head, which produced clearly visible bands within some types of printed output.
Together with Ellerhold we were able to enhance the quality of the printed output using our ScreenPro™ screening engine with PrintFlat™ technology. ScreenPro is a very fast and efficient multi-level screening engine that mitigates artifacts such as banding or streaking and mottling from the inkjet print process and can be used in any print workflow, including Adobe®, Caldera, Esko, EFI and Sofha, with any combination of inks, substrates, printheads and electronics. In ScreenPro every nozzle can be addressed separately on any head/electronics to achieve very fine granularity. The PrintFlat technology adjusts the density within ScreenPro to produce uniform density across a print bar, thereby optimizing print quality.
The project brought many technical challenges: As it was a multi-pass process we needed to efficiently capture repeating density variations across the entire print area in an unbiased way. We carried out tests, analyzed the scanned prints and created a PrintFlat calibration workflow for the press designed to compensate for the non-uniformity in output across the print bar. The team also used a variant of Global Graphics Software’s Advanced Inkjet Screens™, available with ScreenPro and the Harlequin RIP®, which they adapted specially for scanning-head systems. These proved very effective.
You can watch the short case study film here:
PrintFlat technology provided the ideal solution, giving smooth, uniform tints and accurate tone reproduction via a simple ‘fingerprint’ calibration of the screening process, where the density compensation is then built into the screen halftone definition. This means that the PrintFlat calibration is applied during the screening process at runtime and enhances the quality of your output without any compromise on speed. The PrintFlat approach addresses every individual nozzle, has no negative effect on other printing parameters, and allows drive voltage to be used to maximize printing stability and reliability instead.
A valuable additional benefit is in increasing overall productivity. Achieving higher quality with fewer print passes allows for greater use of faster print modes. Jobs that require 4-pass quality can be printed in 2-pass mode with PrintFlat.
The process can be automated for closed-loop correction and, unlike correction by adjustment of voltages, there is no effect on jetting stability or head lifetime, nor ink pressure and timing/drop speed variation.
PrintFlat can increase the added value of your service engineers’ visits, producing a much higher quality result in less time. Alternatively, the print service provider can operate the PrintFlat calibration process to maximize their output quality themselves.
Before and after images illustrating how effective PrintFlat technology is at improving print uniformity.
If you print on an inkjet press you’ll know that the problem of non-uniformity or banding is a particularly difficult one to resolve. It’s especially acute on areas of flat tints with the result that printed output is unacceptable to you and to your customers. This means you either don’t run certain jobs on your inkjet press or, in some sectors of the market, are forced to sell your output at a discount.
The good news is that with PrintFlat you have a solution that is quick to deploy and cost-effective, and it can be applied to any workflow with or without a RIP. With more press vendors adopting this technology, watch our new explainer video to see how you might benefit.