Oct 2017Martin Bailey, CTO, Global Graphics

Avoiding the orange peelTheIJC, Oct 2017, Neuss, Germany

171013

What is halftoning?

Black separation only

Digital halftones … the story so far

Screens for offset, flexo etcBinary (1-bit, on or off)ClusteredSometimes called AM screens

Stochastic screensUsed for offset, EP, some inkjetBinaryDispersedSometimes called FM screensE.g. Harlequin Dispersed Screening (HDS)

Multi-level screensUsed for some EP, most inkjet2-7 ink levels (drop sizes etc)Dispersed

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

In response …

We developed advanced screening technology for inkjet presses

And launched a service to develop custom screens for each press in early 2016

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

Reasonablyabsorbent and/or

wettable

Non-absorbent, poorly wettable

Paper, inkjet coated etc

Flexible plastics, metal etc

Fairly absorbent and/or wettable substrates

Drops coalesce on the substrate surfaceDirection 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

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

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

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

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

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

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

I can fix that by tweaking voltages!

Not all heads have sufficient adjustment points to correct the smileOften 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 lifetimeCauses 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!

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

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