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?

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 what’s 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
 We’ve 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
That’s in test on real
presses, but we’re 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 they’re 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 what’s 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; that’s 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!
That’s 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
 Doesn’t 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
 We’re already working with inkjet OEMs
using everything I’ve discussed today
 We’re 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