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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.

Published in: Technology
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Has inkjet really achieved offset quality?

  1. 1. May 2019Martin Bailey, CTO Has inkjet really achieved offset quality? 190424
  2. 2. Global Graphics PLC: the broadest offering for inkjet Copyright © Global Graphics Software Limited 2019
  3. 3. Some of our OEM customers Copyright © Global Graphics Software Limited 2019
  4. 4. Quality targets for inkjet presses
  5. 5. Many new inkjet presses  All trying to match “offset quality”  But what does that mean?
  6. 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. 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. 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. 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
  10. 10. Banding examples Single- pass Multi-pass (scanning)
  11. 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. 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. 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. 14. Enough of problems; we focus on fixing them! The story so far …
  15. 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. 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. 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. 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. 19. So it looks good Now, how do we prove it? ?
  20. 20. Visual assessment PrintFlat gives us • Smooth-looking sky • Consistent color patches • Flat tints
  21. 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. 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. 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. 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. 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. 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. 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  From other countries, contact your accredited national standards body
  28. 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. 29. For more information: Links to white papers and case studies as well