Avoiding the orange peel - The IJC 2017
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Global Graphics CTO Martin Bailey presents an update on mitigating texture artifacts on inkjet presses using halftone screens: streaking, coalescence, tone steps and the dreaded orange peel mottling. But improved screening reveals other imperfections, a bit like HD TV; so what are the next steps?
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Avoiding the orange peel - The IJC 2017
- 1. Oct 2017Martin Bailey, CTO, Global Graphics Avoiding the orange peel TheIJC, Oct 2017, Neuss, Germany 171013
- 2. What is halftoning? Black separation only
- 3. Digital halftones the story so far Screens for offset, flexo etc Binary (1-bit, on or off) Clustered Sometimes called AM screens Stochastic screens Used for offset, EP, some inkjet Binary Dispersed Sometimes called FM screens E.g. Harlequin Dispersed Screening (HDS) Multi-level screens Used for some EP, most inkjet 2-7 ink levels (drop sizes etc) Dispersed
- 4. So whats the problem? Irregular drop positioning leads to drops randomly coalescing on the media: These microscopic effects cause visible artefacts, often described as mottling or streaking: Synthetic image with exaggerated artefacts in order to be visible on a projector, or reproduced on another print process
- 5. In response We developed advanced screening technology for inkjet presses And launched a service to develop custom screens for each press in early 2016
- 6. Several projects later Weve now worked on screening for at least a dozen single-pass inkjet presses Mainly UV, with others Various heads & electronics Lots of different media With and without chillers, corona pre-treatment etc And There is more commonality than we thought What matters most is the media There are two distinct clusters of behavior Reasonably absorbent and/or wettable Non-absorbent, poorly wettable Paper, inkjet coated etc Flexible plastics, metal etc
- 7. Fairly absorbent and/or wettable substrates Drops coalesce on the substrate surface Direction of coalescence is not random Tends to be along the substrate Coalescence causes visible streaking Especially in mid- and three-quarter tones Some influence from: Head geometry, e.g. saw-tooth nozzle patterns Head stitching These issues are quite amenable to correction in a halftone With compensating halftone Uncorrected
- 8. Non-absorbent, poorly wettable substrates Prints are characterized by a mottle effect that looks a bit like orange peel Especially in areas with reasonably high total area coverage (TAC) Appears to be triggered by ink shrinkage during cure
- 9. I can fix that with Inter-station pinning, corona pre-treatment etc to enhance ink adhesion Yes, reduces the tonal mottle BUT increases color mottle by reducing fluid ink mixing on the substrate Ink reduction with UCR/GCR etc Yes, reduces tonal mottle BUT increases apparent noisiness by reducing area covered by ink and increasing local contrast between ink and light colored substrate Can be corrected with a halftone with specially designed characteristics Without increasing color mottle or noisiness Thats in test on real presses, but were not quite ready to show it
- 10. Where does all this lead? We plan to launch two advanced screens for inkjet very soon: One for reasonably absorbent and/or wettable substrates To counteract streaking One for non-absorbent, poorly wettable substrates To counteract the orange peel effect They will greatly improve quality on the majority of inkjet presses Some may benefit from a little more tuning with a Global Graphics Breakthrough project And theyre usable in both the Harlequin RIP and in ScreenPro
- 11. So were we wrong ? To offer a press-by-press halftone tuning service? No.There will always be some presses with unique properties Global Graphics has continued to develop our Chameleon technologies for creation of unique halftones for these cases
- 12. So whats next? Improved screening takes away noise that masks other effects So now we have to fix those! Stepping in graduations? No, fixed in passing in our new halftones! Non-uniformity across the web? Yes; thats the next project Both uncorrected Original Blurred to make the density variation more obvious
- 13. What causes non-uniformity? 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
- 14. I can fix that by tweaking voltages! Not all heads have sufficient adjustment points to correct the smile Often time-consuming work for an expert Increases time and cost of installation or head replacement Not readily automated for closed-loop correction Reduces jetting stability and (anecdotally) head lifetime Causes ink pressure and timing/drop speed variation Which increase ink coalescence on the substrate Which causes texture artifacts That sounds familiar! Thats where I started today!
- 15. Much better to do this in software Very fine granularity Can address every nozzle separately On any head/electronics Almost instant once output is measured Can be automated for closed-loop correction Doesnt affect Jetting stability or head lifetime Ink pressure and timing/drop speed variation Ink coalescence on the substrate Halftoning Corrected in software Original Blurred Original Blurred Uncorrected
- 16. Next steps Were already working with inkjet OEMs using everything Ive discussed today Were always looking for new partners to help expand our knowledge We love to develop new technologies to help bring better products to market faster Martin.Bailey@GlobalGraphics.com www.GlobalGraphics.com