Has inkjet really achieved offset quality?
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From about the middle of last year, the consensus seemed to be that inkjet had finally attained the same quality as offset lithography or flexography. How has that consensus been reached when the areas that inkjet still finds difficult are things like large regions of mid-tone tints? You simply don’t see press vendors, or buyers, making such comparisons. Inkjet problem areas are mostly caused by non-uniformity at both the microscopic and macroscopic levels. Martin Bailey explores how these spatial variations can be remedied and discusses an emerging ISO Technical Specification (ISO TS 18621-21) that is designed to objectively characterise some of these types of variation.
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Has inkjet really achieved offset quality?
- 1. May 2019Martin Bailey, CTO Has inkjet really achieved offset quality? 190424
- 2. Global Graphics PLC: the broadest offering for inkjet Copyright © Global Graphics Software Limited 2019
- 3. Some of our OEM customers Copyright © Global Graphics Software Limited 2019
- 4. Quality targets for inkjet presses
- 5. Many new inkjet presses All trying to match “offset quality” But what does that mean?
- 6. What do you see in most samples? What do people who print on offset, flexo and gravure look for? • Lots of detail! What do you see in the samples handed out at trade shows? • Lots of detail! And on samples from inkjet press vendors? • Lots of detail! Is a lot of detail the hardest thing for inkjet to get right? I’ll be impressed when I see inkjet vendors giving out flat 50% tints or long grads!
- 7. What should you be looking for in inkjet samples? Small-scale effects (micro non-uniformity) • Mottling • Streaking • Missing/blocked/misaligned nozzles Large-scale effects (macro non-uniformity) • Banding
- 8. Mottling is most common on less absorbent substrates, often caused during drying/curing Micro effects Streaking is most common on more absorbent substrates, caused by ink drops coalescing on the substrate surface Blocked and deviated nozzles can occur on any substrate
- 9. Macro effects Variation within a head • Commonly a ‘smile’ shape • Caused by pressure or voltage changes Variation between heads • Especially as heads become field replaceable Head wear Density ONE HEAD
- 11. The cost for signage, graphic arts, labels and packaging Reduced visual quality of prints • Increases potential for buyers to reject them • May reduce margin achievable Companies using inkjet presses have all independently come up with the same metric: “What proportion of jobs submitted by my buyers can I print on this press and be confident I will get paid?”
- 12. Th cost for décor, textiles, industrial print etc If multiple prints will be used together they must be uniform Wall-coverings Ceramic & vinyl tiles Laminate flooring Curtains Soft furnishing Fashion textiles Labels & cartons etc The same metric applies, but the left and right sides must also match
- 13. Mitigating issues takes time and cost • More expensive to build a press • More frequent and longer maintenance procedures • Printhead purging • Voltage trimming, etc Non-uniform output breaks color profiling/calibration • Patch color can vary by print location • Means poor profiling … or even profiling failures The cost for all sectors
- 14. Enough of problems; we focus on fixing them! The story so far …
- 15. Solving the problems Vendors should make reasonable efforts to tune inks, wave forms, encoders etc. After that software is usually the fastest and most effective solution Micro effects • Advanced Inkjet Screens Macro effects • PrintFlat applied during ScreenPro processing Blocked/deviated nozzles • Meteor Inkjet’s blocked nozzle compensation technology All for both single-pass and scanning
- 16. Specialized screening patterns to improve inkjet performance Pearl – optimized for most inks / substrates Mirror – optimized for difficult and poorly wetting substrates Global Graphics Software – Advanced Inkjet Screens™
- 17. Global Graphics Software – ScreenPro™ Advanced Inkjet Screens are applied using Global Graphics’ ScreenPro stand- alone screening engine High-speed screening usable with virtually any RIP and for virtually any press
- 18. Global Graphics Software – PrintFlat™ Automatically correct for banding based on measurement of uncorrected prints Adjusts every nozzle separately without time- consuming voltage adjustments PrintFlat is an option in Global Graphics’ ScreenPro
- 19. So it looks good Now, how do we prove it? ?
- 20. Visual assessment PrintFlat gives us • Smooth-looking sky • Consistent color patches • Flat tints
- 21. What really matters … … is the buyer being happy to pay for the prints and to come back for more … … is the printer/converter saying that they have been able to substantially raise the proportion of jobs that they can confidently accept to print on inkjet … • … or even that they no longer have to inspect each job before accepting it … … is the vendor telling us that they are so pleased that they would like some help with another press model …
- 22. But those are a bit long-term How can we prove in the lab that we’re getting good quality? Density measurements across and along the output Color differences (∆E) when nominally the same color is printed in multiple locations Vendor staff or focus group visual assessments The porthole test
- 23. The Traditional ‘Porthole’ Test Used with décor printing long before digital • If you can tell the direction of printing then it’s a fail
- 24. Making results directly comparable There are two ISO Technical Specifications in development • Under “Graphic Technology — Image quality evaluation methods for printed matter” ISO TS 18621-21: Measurement of 1D distortions of macroscopic uniformity utilizing scanning spectrophotometers ISO TS 18621-31: Evaluation of the perceived resolution of printing systems with the contrast - resolution chart
- 25. ISO TS 18621-21 – Macro uniformity Print a flat tint and measure various points across it! Very simple and effective But not yet easy to use to independently compare • across sites, presses, vendors etc
- 26. ISO TS 18621-31 – Micro uniformity Nominally a test of perceived resolution, based on the size and contrast of elements of a test chart • Sensitive to addressability, cured/dried drop size, etc But as the graphical elements become finer, and the contrast lower, the microscopic uniformity of the output becomes an important determinant of perceived resolution
- 27. If this is important to you … … get involved! Development in a joint working group • Joint between ISO Graphic Arts, Photography and Office Printing committees • Meetings usually held under the auspices of ISO TC130 (Graphic Arts) US participants can join the US Technology Advisory Group (TAG) to TC130 • For more details contact Debbie Orf at APTech • Debbie Orf dorf@aptech.org From other countries, contact your accredited national standards body
- 28. So, has inkjet matched offset quality? I’ll tell you when we have some decent measurement and reporting methods that allow reliable comparison and independent validation But it’s looking good, at least for some ink technologies and substrates
- 29. For more information: martin.bailey@globalgraphics.com info@globalgraphics.com https://www.globalgraphics.com/screenpro Links to white papers and case studies as well
Editor's Notes
- Hello, and welcome to the next installment of a journey that we’ve been sharing with you over the last few years as we’ve worked with many inkjet press vendors to increase their output quality. Today I’m hearing a lot of people telling me that inkjet has equaled offset print quality, which is a hugely broad and ambitious statement. I’d love to be able to just say that it’s true, and that we’ve been an important part of that achievement. But I’m here with my engineering hat on, so like to explore some aspects on inkjet quality and reach a more considered verdict.
- I firmly believe that you cannot fully understand what somebody is saying unless you know who they are, and why they’re saying it. My name is Martin Bailey, and I’m CTO at Global Graphics Software. We are the company behind the Mako SDK for creating, editing, transforming and visualizing page description languages; the Harlequin RIP The ScreenPro screening engine, which I’ll be covering later in this presentation, and Fundamentals, a foundation for building DFEs for digital presses. And we have a couple of sister companies in Global Graphics PLC: URW designs and develops digital fonts for companies like Siemens and Mercedes. And I’m sure you all know Meteor, developers of printhead electronics and associated drivers. Collectively these technologies represent the broadest technology offering to inkjet OEMs available from a single vendor
- And our technologies are used very widely; I’m sure you’ll recognise a few names in this list of some of our OEM customers. Our best estimate at the moment is that something like the equivalent of 10 billion letter-sized pages are printed per month using Global Graphics Software technologies.
- What does quality mean for an inkjet press?
- Everyone is claiming to match offset quality. And at every trade show they’re giving away sample prints of jewelry, big cats, fast cars and Bavarian castles. I happened to give a different presentation last week at Fespa in Munich where I suggested that I’d seen enough Bavarian castles before I remembered that I was in the capital of Bavaria, but obviously I survived the response. And I guess they are very attractive. But what are all these prints showing us?
- The message from many of them is at least partly “we expect offset and flexo press operators and production managers to be examining these prints, we we’d better give them something familiar”. And that should be taken as a positive sign, because it confirms that many printers/converters etc are expanding or switching to inkjet. So those samples show a lot of detail because that’s what offset and flexo vendors try to show. But … does showing a lot of detail really highlight the strengths of a particular press? Don’t get me wrong, that’s an important aspect of quality, but it’s not the toughest challenge for inkjet. What will really impress me will be seeing very large flat mid-tones and long grads, especially in four-color greys.
- So what should you be looking for on those samples? The answer falls into two groups – micro and macro non-uniformity. Micro includes effects from single nozzles, or from very small numbers of nozzles interacting; things like mottling, streaking and missing or blocked nozzles. I’ve heard many other names for such effects, including graininess etc. Macro are the larger effects that are usually described as banding.
- Over the last several years we’ve developed a consistent set of terminology within Global Graphics so that we can talk sensibly with our customers and be sure that we all mean the same thing. In this terminology ‘mottling’ is the orange-peel like effect that you tend to see on less absorbent substrates which arises during drying or curing. It’s perhaps more common with UV inks, but we have seen it with aqueous as well. Streaking is caused by coalescence of ink drops on the substrate surface, either along or across the print, and usually on more absorbent substrates. And blocked, clogged or deviated nozzles can occur on any substrate and with pretty much any ink technology.
- Macro effects, on the other hand, are features that tend to be in the range of between one and ten or so cm across. They’re often cause by variation of ink pressure or voltage changes within a head, which typically results in a smile or frown shape. And 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 wear in use, resulting in less regular banding which can change over time. All of those three causes apply to single-pass inkjet presses, but they also cause issues on scanning or multi-pass presses. And in that case the overlapping swathes of the print head add extra potential for problems. The typical method of correcting these effects is by tweaking head voltages, but that’s a very manual, time-consuming task which can’t always achieve the desired results.
- These are some real-world examples. The left-hand two are the best that one of our customers could achieve before they invited us in to help; you can see the banding very clearly. The one image on the right is from a single-pass press at a mught higher magnification; I hope you can see the regular pattern of light and dark.
- So, there are things that are hard to get right on inkjet, and you can see the problems at normal viewing distances. What does that mean in practice? For signage, graphic arts, labels and packaging the bottom line is that a buyer is more likely to reject a print, to demand some form of compensation, or simply to take their next job to a different supplier. All of those really mess up your bottom line. In practice we’ve found that pretty much every company printing with high-volume inkjet has developed a simple metric: “what proportion of jobs submitted by my buyers can I print on this press and be confident that I will get paid?” And there are a lot of companies struggling along with a value in the range between 40% and 80%, even as low as 30% in some more specialized markets. We’ve also talked with companies who have unused presses gathering dust in the corner, because they concluded that they could not make a profit running them, often because their payment confidence level was too low. I’m sure none of the vendors in this room today want their equipment to be in that state; I sure as heck know we don’t!
- And then if you look at some of the print sectors touted as “the next big thing”, in décor, textiles, and some parts of labels and cartons … there’s an added complication. This drop of wall-covering will be hung right next to that one; the color and tone had better match. This tile will be laid next to that one. This curtain will hang next to that one. This piece of fabric will be sewn onto that one for fashion or furnishing. This boxed product will be next to that one on the shelf. Sometimes the printed design is such that variation is acceptable, such as when emulating natural wood on laminate flooring. But much of the time it’s not. So these opportunity markets just raise the challenge a bit further.
- And for all inkjet markets, building and maintaining a press without issues takes more time and money than most of us would like. In extreme cases it’s difficult even to make a color profile because color varies so much across the test patches. In other words, you can end up with a real lemon. Except that it’s not even a good lemon!
- OK, enough of my belly-aching. If inkjet was easy then everyone would be doing it! Well, OK, there are a lot of people building presses, and that’s where Global Graphics comes in, to help make them print well. Over the last few years we’ve shared our developing story with you, and I’d like to bring you up to date.
- How do you solve a problem like inkjet? First: do what you can in a reasonable time and at a reasonable cost to fix stuff mechanically, in the electronics, in the ink, etc. At the very least, achieve stability. I’m sure we all know that systematically correcting an unstable system is much harder than a stable one. Once you’re at that point, we’re finding that software is a much faster and cost-effective way to address the remaining non-uniformities than doing more in hardware. From the Global Graphics stable we have Advanced Inkjet Screens for mottling and streaking, and PrintFlat for banding. If you’re struggling with b locked or deviated nozzles I’m sure many of you saw my colleague, Jonathan Wilson, from Meteor presenting their solution yesterday. And all of these work for both single-pass and scanning printing.
- Let’s look at each of those in a bit more detail. Advanced Inkjet Screens comprises two base screen designs. The Mirror screens are designed to mitigate mottling effects on poorly wetting substrates, while the Pearl screens are designed to mitigate streaking. Both of these are being used in production.
- And the primary mechanism to apply Advanced Inkjet Screens is using our stand-alone screening engine, ScreenPro. It’s highly versatile because it just needs un-screened raster as an input, and it will deliver screened raster on the output, configurable to apply any number of drop sizes from 1 up to 15 (although I’m not sure we’ve worked with anyone yet using more than 5 drop sizes in real production). Because of that versatility it can be used with virtually any RIP. It’s in use taking rasters from Esko, ColorGate and Caldera RIPs. And, or course, in sectors where the job is originated in a raster format in device colors you don’t need a RIP at all. And it’s also very scalable, and we can work with you to integrate full shared-memory pipelines direct to electronics for very high-speed presses. As you can see, ScreenPro won a PIA Intertech award in 2018.
- And finally, the most recent addition to our inkjet quality technology is PrintFlat. This is a feature within ScreenPro that corrects tonality to hide banding, based on measurements from the press. Because it’s done in software it can adjust every nozzle separately, and doesn’t need a specialist engineer to make press adjustments. In other words, it greatly reduces the time and cost of building a press and of maintaining it.
- Right, I’ve got all the right buzz-words in and given you all the motherhood and apple pie. But none of that matters unless I can show you it’s real. If you’d like to come to our booth outside after this I can show you a number of sample prints showing before and after prints with ScreenPro and PrintFlat on a variety of different print heads.
- But in my remaining few minutes, I’d like to share some more thoughts. Firstly: the top one of these images is a photograph of a large-format outdoor signage from the same customer who produced the prints with the bands that I showed earlier. This time with PrintFlat. Skies are amazingly hard to print well, almost as difficult as flesh tones. We have another PrintFlat engagement where our customer simply could not make color profiles because the output was too inconsistent. Now they can, and they’re generating very nice color.
- That customer is happy, which brings me to what really matters. It matters when somebody commissioning print comes back to the same printer time after time because they’re producing good output. It matters when the printer or converter significantly raises their payment confidence metric … or even stops needing to examine incoming jobs before accepting them. And it matters when a vendor that we’ve worked with on one press asks us to help them on another because they know we’ve got what they need. And all of those are happening.
- But some of those things take a long time to get to. What do people evaluating our technology do to reach a conclusion faster? Basically, think about the problems that they’re seeing and find simple methods to evaluate those. If you’re seeing banding, then do multiple density methods across the output and look at, for instance the standard deviation. If you’re seeing color variation, measure that. If you want to bring in your customers then have a focus group come up with visual assessments. And a useful support tool for visual assessments is the porthole test.
- This has been used in the décor space for a very long time. Viewers look at a piece of print through some form of window; the porthole. If they can tell in which direction the substrate or print heads moved when printing it, then it’s a fail.
- But none of these tests really help to make results directly comparable. So I’m very glad to report that there are two Technical Specifications being developed in ISO.
- The first of them is explicitly concerned with measuring macro uniformity, essentially by standardizing a method that I mentioned a couple of slides ago: print a big flat tint and measure it in multiple places. So this technical specification is a really good start at standardizing a method to measure and report banding. But it’s not yet there with respect to making those results directly comparable between operators, substrates, presses etc.
- The second of them is really aimed at another very interesting and occasionally controversial topic: perceived resolution. Every time I walk round a trade show I’m amazed at the number of ways that inkjet vendors try to present the quality of their output: as perceived resolution, visual resolution, optical resolution, equivalence to offset line screens etc. But if you have any significant micro non-uniformity, such as mottling or streaking, then that reduces the apparent resolution of the print, just like having fewer nozzle per inch does. Whether this will be a useful way of measuring and reporting micro non-uniformity is not yet clear, but it definitely has potential.
- If this kind of measurement is important to you then I’d encourage you to get involved. The development of these technical specifications is being done in a joint working group between graphic arts, photography and office print, but the meetings are usually held with other graphic arts groups. In fact the next meeting will be next week, in Hong Kong. I’m not suggesting that you drop everything and hop on a plane, of course, but you can get the opportunity to review drafts and to provide input without the need for long-haul travel. If you’re in the US I’ve put the relevant contact details on this slide. From other countries you’d have to contact your national standards body. I may well be able to give you some pointers if you’d like to find me after this presentation.
- All of which brings us back to our original question: has inkjet matched offset quality yet? Well, I haven’t got the independently verifiable, reliably comparable evidence to answer that one yet. But it’s looking good!
- There’s more information on the Global Graphics technologies I’ve talked about today at globalgraphics.com/screenpro. My email is up there as well, but even better, come and talk with us on our booth outside while we’re all here together. And with that, are there any questions?