Xaar Guide to Single Pass Printing White Paper

Xaar Guide to Single Pass Inkjet Printing

Technical White Paper

The Xaar Guide to Single Pass Printing

Author: Mark Alexander

Date: February 2008

Copyright ©2008 Xaar plc. All Rights Reserved.

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Xaar is not responsible for the performance or support of third party products and does not make any representations or warranties whatsoever regarding

quality, reliability, functionality, or compatibility of these devices or products.

Xaar Guide to Single Pass Inkjet Printing

Contents

Introduction 1

Single Pass Printing: Desirable Characteristics 2

Printhead and Drop Formation Options 5

Quality: Resolution and Shades / Tones 8

Single Pass Printing: Technology Requirements 12

How Xaar Variable Drop Technology Works 13

Variable Drop: Quality & Production Benefits 15

Commercial Applications 17

Summary 20

Contact Us 20

Xaar Guide to Single Pass Inkjet Printing 1

Section 1

Introduction

What is the ideal solution for the fast growing short-run print market?

Until now digital printing has not been able to achieve the speed, quality and reliability

delivered by commercial printing techniques. However the new generation of variable drop

piezoelectric inkjet printheads are capable of delivering both the quality and the speed at high

performance levels and innovative printing systems based on these printheads are quickly

coming to market.

In this guide we explain how ‘single pass’ inkjet printing is now a commercial and ‘industrial’

reality by making the most of tightly controlled variable drop printhead technology. Inkjet

printing’s flexibility is changing the world we live in and variable drop, high quality printing is a

significant development that has moved inkjet printing into new markets, bringing with it new

creative possibilities.

Here we will look at the features and benefits of inkjet printing technology available today and

how it can enable industrial strength ‘single pass’ printing.


Section 2

Single Pass Printing: Desirable Characteristics

High productivity at an optimal cost

High print quality from adjustable and variable sized (greyscale) drops

Reliable and consistent results in a real production environment.

The primary benefit of single pass printing is of course speed and therefore productivity.

Simply, printing directly onto a moving web or sheet minimises any time lost between copies

or pages so you can print as fast as the technology allows.

This is in contrast to other established inkjet applications, namely wide and grand format

graphics printing. These large format scanning devices act just like bigger versions of your

desktop printer. As the printed dots are small, these devices perform many passes (hence

called multi-pass printing) to cover the paper fully and to provide the detail required. The

downside to multi-pass printing is that it is a relatively slow procedure as time is lost at each

end of the scan, plus there is a need to put down lots more drops to cover the paper in the

printed areas.

Inkjet has become well established in the low production large format sector. However, with

the right inkjet technology, there is a considerable impact that can be achieved in high

production, single pass applications.


Key Inkjet Elements

There are four key elements that are important for an industrial strength single pass inkjet

system:

1. High print quality: Achieving high quality print in a single pass requires very small

changes in ink lay down. This can be achieved either with a very large number of

nozzles (with associated high cost) and small drops or variable-sized droplets with small

increments in drop size (greyscale) from the same nozzles.

2. High native resolution: This is necessary to provide the density of nozzles in each

printhead for high quality print, simpler integration and minimum cost. This demands a

high nozzle count (i.e. nozzles per inch) which is enabled by shared wall technology1.

3. Long life: High dots per inch (dpi) printing requires a high number of actuations or drops

fired, whether small drop binary or greyscale. This in turn means you need industrial

strength lifetime of the printhead (over 1012 actuations). Shear mode actuation2 is the

ideal choice for longevity as it has been shown to exceed 1013 actuations with no failure

mode.

4. Reliability: This means sustained, consistent print performance over typical print run

lengths and working shifts as well as self-recovery. This can be achieved with the latest

through-flow techniques which require an acoustic firing mode.

1 Shared Wall Technology

Each wall is shared between 2 channels optimising channel/nozzle density.

2 Shear Mode Actuation

A low stress deformation mode of PZT, the piezoelectric material Xaar uses to create the

inkjet channels that form the printhead actuator, where an electric field is applied

perpendicular to the poling direction.


Key Printhead Characteristics

In order to select the right inkjet printhead you need to consider the following characteristics:

1. The maximum flexibility to choose the right drop size range to achieve the combination of

quality and speed you require.

2. The ability to select the right drop size for the specific job e.g.: label, billboard or point of

sale material.

3. The ability to adjust drop size, and therefore drop volume according to the media being

used (i.e. coated or uncoated paper, card, vinyl, plastics, etc).

4. Flexibility to handle a wide range of inks and functional fluids to suit a variety of

applications.

5. Minimal and efficient ink usage for optimal cost-per-copy.

6. Future-proofing through in-built flexibility to adapt to new media, applications and

customer demands. Choosing a variable drop based system may enable you to address

new markets in future.

7. Industrial strength, high-performance architecture for long life and reliable operation.


Section 3

Printhead and Drop Formation Options

Range of printheads available to suit range of applications

Variable drop printheads critical to ensure commercial quality single pass printing

Xaar multi-pulse greyscale printheads produce 1,000 dpi apparent resolution

There are a variety of printheads available in the marketplace that offer effectively 3 different

drop configurations or 3 different modes of operation. Xaar provides a selection of printheads

that offers companies the flexibility to choose the right printhead for the application.

XaarDOTTM (Xaar Drop Optimisation Technology) encompasses a range of drop formation

options, each with specific features.

XaarDOTTM is incredibly flexible in giving customers the choice of what drop size or sizes and

consequently the DPI to use for a job, both in terms of image quality and substrate flexibility.

By tuning the options, printer manufacturers can produce anything from high quality

photographic results for close-up viewing or larger droplets for high speed coverage on

billboards or other signage. The choice really is yours – and the range of drop sizes means

you can manage dot gain to enable printing on a wide variety of media from glossy paper to

absorbent materials such as textiles.

Drop Formation Options

XaarDOT Name Drop formation method Mode of Operation

Fixed Drop Binary

Fixed drop size for all jobs.

Selectable Drop Multi-pulse Binary Selectable drop size on a per job basis.

All drops are the same size throughout

the job.

Variable Drop Multi-pulse Greyscale Drops are dynamically varied within a job.

the maximum number of Greylevels

selected on a per job basis


1. Fixed Drop (Binary)

Typically Binary printheads are used for larger drop applications. Drop size ranges from 35-

80 picolitre(pl) and produce a lower quality image which is directly related to drop size.

These printheads are less sensitive to mechanical artefacts and more forgiving of the print

environment. When printing with small drops (small drop binary) it is necessary to print

multiple passes to achieve full coverage as it is not practical to get the nozzle density

required to get full coverage in a single pass.. This multi-passing degrades small features

like small text and fine lines even on printing machines with tight mechanical tolerances.

Applications: Outdoor signage/graphics; Building wraps; Vehicle livery; Advertising

banners; Coding & Marking and Postal.

2. Selectable Drop (Multi-pulse binary)

Selectable drop printheads form drops by multi-pulsing, which allow drop size (which directly

effects actual resolution) to be selected per job. You also have the ability to achieve very

precise addressability for industrial applications. This formation allows you to fine tune

results through a combination of speed (passes) and quality (level of detail). The ability to

optimise speed and quality is dependent on the application. You can also use this approach

to tune the drop size to best match the dot gain characteristics of the media.

Applications: Indoor to outdoor graphics; Industrial print applications.

3. Variable Drop (Multi-pulse greyscale)

With piezoelectric multi-pulse greyscale printing there are basically two approaches to

producing different size drops from the inkjet nozzle: either extend the drop growth time to

make a bigger drop (6-24pl) or alternatively apply high frequency multi-pulses to grow a drop.

Xaar takes the multi-pulse approach, which provides, for example, a 6-42pl drop size range

and that is our focus here.

Xaar’s greyscale printheads can generate up to 15 different drop sizes by combining multiple

small sub-droplets (e.g. 6pl), fired in quick succession, into the same printed dot. This allows

the 360 dpi actual resolution to give results equivalent to 1,000 dpi (sometimes called

‘apparent resolution’), but using far fewer nozzles, and this gives results that more closely

match the perception of the human eye. The 1000 dpi ‘apparent resolution’ is directly related

to the 6 pl sub-drop size and not the native pitch of the nozzles.


Variable drop allows you to print with fewer passes and still get full coverage whilst retaining

the integrity of fine text and lines. Clearly the lower number of passes you do the better the

text and lines. This approach also enables you to reduce the number of passes - down to a

single pass if needed. Variable drop formation technology is crucial to successful commercial

level ‘single-pass’ inkjet printing.

Image: Comparison of 8 and 16 level variable drop or greyscale

Xaar 318 CA2: Xaar 318 CA2:

8 LEVEL GREYSCALE 6-42 pL 16 LEVEL GREYSCALE 6-42 pL

Image reproduced courtesy of TTEC


Section 4

Quality: Resolution and Shades / Tones

Resolution can mean slightly different things to different people. When talking about

resolution here we are referring to the ability to see, or “resolve”, the individual dots or pixels

that make up an image. Pick up your Sunday colour supplement, hold it at arms length and

you see a nice colour photograph. Steadily bring it closer to your eyes and you will start to

see small patterns in the image. This is the limit of resolution. The point at which you can

start to see how the image is made up.

The first part of understanding image quality then is to understand viewing distance and the

point of resolution for the application. For commercial print and packaging this would typically

be in the order of 30cm, a slightly close but comfortable viewing distance. At this distance

the viewer shouldn’t be able to see how the pixels or dots are made up. In terms of a quality

piezoelectric printhead this means that the nozzles need to be closer than 1/300 of an inch or

about 70um apart. We refer to this as nozzle pitch and it is the closest that any two dots can

be printed.

Having established resolution it is necessary to look at tonal range, the number of shades or

tones that go into making up a print. In simple terms tonal range is the amount of information

in a print.

The image right has 2 grey levels, white … the paper … and black the ink. It is far from

pleasing to the eye as you read this article but if you put some distance between your eye

and the page it looks better, the further away the better it looks, about 4 foot seems correct.

This isn’t because of too low a resolution; you can’t see the individual dots that make up the

image, but lack of information.

360 dpi 2 levels 720 dpi 2 levels


360 dpi 6 levels

As each dot is a packet of information, adding more dots in other words by increasing the

resolution, goes some way to solving this problem. It is the reason why we all look for high-

resolution printers to use when printing photographs at home.

It’s also the reason why this isn’t a solution for commercial print application, these printers all

have to multi-pass the printhead to make up resolution and are therefore not productive

enough for in line commercial application.

Xaar Greyscale solves the lack of information problem by varying the size of each ink droplet,

which in turn varies the size of dot on the page and therefore the amount of information in

each pixel.

Even with only 6 grey levels, Xaar 1001 can print up to 16, you can see the difference in print

quality. The lack of grain in the background, the subtlety of tone in the wine glass and

texture of the tennis balls. Resolution now becomes about addressability, how often in a

given area you can place a drop of ink. The important image factor now also becomes about

the smallest drop of ink or smallest printed dot on the page. Just how small a dot can you

see, at what viewing distance and just how small would you like to print something.


The chart below maps performance of the human eye against tonal range - expressed as

grey bits - and addressability. There is little point in printing above the limits of perception, as

the eye simply can not see it. It is simply a limit.

Current screen printing uses 64 tones expressed as 6 bits, effectively 64 different drop sizes

at approximately 175 lines per inch to try to match the performance of the eye. Binary 600

dpi performance is plotted in green and falls short of the eye and traditional printing

techniques at normal viewing distance.

8 level greyscale at 360 DPI almost exactly matches the performance of the human eye to

deliver pleasing high quality print results with fine detail and smooth tonal gradations.


Utilising its smallest drop size and the ability to use multiple drop sizes anywhere within the

print dynamically variable greyscale can further enhance fine text detail by filling in the pixels

a binary print would leave behind to create a smooth even edge to each character.

Image reproduced courtesy of TTEC

3 drop sizes combined to

prevent character pixelation and

create a more refined edge to

the text


Section 5

Single Pass Printing: Technology Requirements

Wide range of sub-drops required to vary dot size pixel by pixel to achieve a true greyscale effect

Acoustic wave firing technique to achieve high frequency multipulse sub-drops

Six to eight greyscale levels needed for quality single pass application.

To successfully reproduce a true greyscale image it is necessary to vary the dot size pixel-by-

pixel. The more sub-drops used to make up the final dot the greater the possible tonal range.

Small sub-drops are also necessary as the image quality is driven by the smallest drop size

not by the dots per inch. Dpi is simply the spacing or pitch of the dots. The ability to vary the

drop size within each cell of the matrix is a way of increasing the information content so that

less cells are required to simulate a full continuous tone image.

The range of possible drop sizes and quality of drop formation are linked to the modulation

frequency of the nozzle, the higher the frequency the greater the amount of control. Xaar

uses an acoustic wave method to achieve high frequency multi-pulse drop formation. The

ability to modify waveforms and so modify the way in which drops are formed allows the

printhead to adapt to various fluid behaviours and types. As a result inkjet printing can be

finely tuned to suit the particular application.

Six to eight levels of greyscale

are needed to achieve the high

quality, smooth tones, needed for

single pass printing applications

at 360 DPI. The goal is to print

any multiple of sub-drops in any

position without restriction

allowing maximum flexibility in

creating continuous tones.


Section 6

How Xaar Variable Drop Technology Works

Innovative Hybrid Side-Shooter (HSSTM) head technology ensures fast and reliable variable drop formation

Each inkjet drop fired is dynamically variable producing a wide range of tones

Xaar printhead design ensures all nozzles operate at maximum efficiency.

‘End Shooter’ printheads fire the ink drop out of their nozzle at the end of a long narrow

channel. The potential ‘Achilles heel’ with all end shooter designs is that nozzles can fail,

either by particulate blocking or because air bubbles form. A failure then requires a

wipe/purge maintenance routine to re-prime the nozzles.

However, Xaar’s new Hybrid Side-Shooter printhead technology achieves variable drop

formation – fast and reliably. With HSS the ink flows right past the back of the nozzle. Two

acoustic waves moving through the channel meet in the middle and cause pressure changes

which fire a drop out of the side of the channel i.e. downwards through the inkjet nozzle. Due

to the TF TechnologyTM air or particles are moved away from the nozzles, keeping the

channel primed and operational. Any nozzles that are lost due to a knock or impact will

quickly self-recover.

The HSSTM action produces multiple 6 pl sub-drops which combine to give a range of drop

sizes that go on to form the image on the substrate. Each inkjet drop is dynamically varied by

changing the number of sub-drops combined to make up the final drop volume to give a print

result that typically contains up to 7 different drop sizes producing a wide range of tones, and

hence high print quality.


One of the important benefits of this multi-pulse variable drop formation is that it minimises

satellites, giving a very clean and precise print result.

Xaar’s TF TechnologyTM provides ink through-flow and when combined with the side shooter

architecture is a significant step forward towards running inkjet systems reliably for a full shift

with minimal maintenance. Printheads with all channels (nozzles) working to their full

potential, gets rid of the need for multi-pass printing typically used to compensate for missing

jets. The first single pass, narrow web, inkjet systems are coming onto the market for label

and transaction applications. Within a short time we will see reliable fixed arrays built to the

full width of the printing area, potentially even for wide format printers, which could run at

thousands of square metres per hour.

Long printhead lifetime and high native resolution have been available for a long time within

Xaar’s piezoelectric drop-on-demand technology. The combination of shear mode and

shared wall patented technologies has allowed Xaar and our licensees to produce printheads

that deliver industrial-strength life. The key active part within a piezo printhead is the actuator

and we’ve operated this beyond 100 million million (1013) actuations with no failure. Duty

cycles within typical print applications are currently around 20%, so this typically means over

four years of life.


Section 7

Variable Drop: Quality & Production Benefits

Variable drop produces high quality, high productivity results

High ‘value per nozzle’ as each nozzle behaves as multiple nozzles, prints reliably and has a long lifetime

Cost effective cost per copy through efficient ink use.

The benefits of variable drop printing are:

1. High quality combined with high productivity: image quality is improved even at low

native nozzle pitches. For example 360 dpi appears like 1000+ dpi from a binary

printhead (‘apparent resolution’) when using 8 greyscale levels at normal viewing

distances.

2. Image resolution and quality required by industrial print markets

- Smooth gradation and tonal range: Smoother screening with linear drop volume

increments

- Sharp text and lines produced by the range of drop sizes

- Larger (40+pl) drops to ensure excellent image saturation and coverage

- Allows fine tuning to accommodate media dot gain characteristics.

3. Reduced cost per copy & reduced over-wetting of the substrate by minimising ink

usage.

4. Fast drying: the minimum amount of fluid is used when using multiple drop sizes. This

minimises excess solvent or carrier which in turn means minimal drying time and cuts

solvent use.

5. 6-level greyscale (0-5 dpd) is equivalent to 5 densities of CMYK.

6. Eliminates the need for Light Cyan, Light Magenta and Light Black (Grey): a full size

drop of light ink can effectively be replaced with a smaller full-strength drop. The

resolution (determined by the smallest drop size) is enhanced as the smallest drop is

now effectively 30-40% of the original drop size.


7. Increased ‘value’ per nozzle: So the reality is that you get a much higher value per

nozzle – each nozzle effectively behaves as multiple nozzles e.g. 7 if 7dpd – again,

same cost per nozzle but effectively 7x the number of nozzles per inch to get the same

coverage. This could be up to 15 times.

All these benefits combine to produce better image quality without the overhead and cost of

multiple nozzles. Variable drop capability combined with reliability and printhead lifetime are all

important to reach the levels of productivity required for industrial markets such as commercial

print, transactional print and packaging.


Section 8

Commercial Applications

Some examples of single pass inkjet systems:

Nilpeter ‘Caslon’: Narrow web label print module

Sun Chemical ‘SolarJet’: Short run, narrow-web label UV printer

EFI ‘Jetrion 4000’: Narrow web UV label printer.

Single pass inkjet printing is already becoming a reality. Here are some examples of single-

pass printing machines, some using Xaar TF Technology and all using variable drop printing

in a fixed head mode. The key is combining quality and reliability performance for the end-

use application.

Nilpeter ‘Caslon’ print module

Nilpeter, of Denmark, one of the world’s leading narrow web

label press manufacturers, and FFEI Ltd. (formerly FujiFilm

Electronic Imaging) of the UK have jointly developed Caslon, a

modular digital print solution for labels and narrow web

packaging using 4-colour process UV inkjet technology. The

first commercially available product is in 330 mm (13in) and

420 mm (16in) web widths, with 508 mm (20in) and 559 mm

(22in) available later.

Caslon uses the latest Xaar 1001 printheads integrated into a

digital print engine by FFEI, one of the launch partners for the

Xaar 1001 product. Modular in design, Caslon can be

integrated into Nilpeter’s conventional flexo press lines, or can

function as a stand alone roll-to-roll system. In the former

configuration, Caslon’s speed matches that of conventional print units.

Consumer trends towards limited-run special offers, short-term marketing initiatives and

multiple language variants are driving label and packaging manufacturers to look for

alternative production methods. Caslon enables the commercially viable output of such

shorter runs and fits into a printing company's total workflow solution.


Sun Chemical ‘SolarJet

SolarJet™, Sun Chemical's latest generation UV

inkjet printer designed specifically to benefit the

growing short-run, narrow-web labels market, was

launched at Labelexpo (Sept 2007). Built in

partnership with Imaging Technology International

(iTi) of Colorado, USA, one of Xaar’s integrator

partners, SolarJet allows printers to produce

shorter print runs quickly, cost effectively and

without sacrificing quality. This printer is targeted

at the pharmaceutical, health and beauty,

industrial, electronics and FMCG markets where

printed products are now being updated more and more frequently and commercially viable

run lengths need to be shorter to avoid costly wastage. The SolarJet is equipped with Xaar

760 printheads and the press can print at a speed of up to 25 metres (80 linear feet) per

minute.


EFI ‘Jetrion 4000’

EFI, the global printing group based in the US,

unveiled the Jetrion 4000 last year and it was shown

in full production at LabelExpo Europe 2007. The

Jetrion 4000 Series of UV inkjet printers’ offers narrow

web converters an affordable, full-colour alternative to

toner-based printers. Printers can now harness lower

running costs, superior production capabilities, and

have the flexibility of inkjet technology for applications

such as labels, tags, forms and tickets. The Jetrion

4000 uses Xaar printhead technology with greyscale

capability, which allows it to print photographic quality.

In action – commercial printing

In the commercial print space there are machines producing office print, CDs, packaging and

many more will be launched in the near future. The Riso Office HC 5000 series printers uses

Xaar technology in a single pass mode to produce 115 A4 pages per minute using a special

oil based ink; Copytrax produce CD printers using UV inks through the Xaar 318 greyscale

printheads.

Applications for page-wide HSS arrays could include imprinters that can keep up with high

speed web offset, flexo or gravure print lines, with higher quality than today’s systems, or

complete digital colour presses like the Nilpeter Caslon.


Section 9

Summary

Historically there has been a trade off between speed and quality. Variable drop improves

this by combining high image quality with high productivity for the first time. High productivity

in multi pass applications is largely achieved by reducing the number of passes necessary for

a given quality level.

In this guide we have described the requirements and wide-ranging benefits of variable drop.

The key with single pass printing is the ability to achieve full coverage in just one pass. The

beauty of variable drop is that you can achieve the high image quality with fine detail and

good text you need combined with the productivity benefits of sheet or web fed production.

Once these are in place, the true potential of inkjet printing can be exploited: high quality full

colour variable data printing for personalised marketing, transactional and version

applications and high-speed, short run production.

Contact Us

Mark Alexander Xaar Plc Science Park Cambridge CB4 OXR Tel: +44 (0)1223 423663 Fax: +44 (0)1223 423590 Email: [email protected] Web: www.xaar.com

mailto:[email protected]

http://www.xaar.com/

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