Inks, Heads & Materials for Wide Format Digital

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Printhead TechnologiesPrintheads are a key component of any

wide format printer, so it’s important to

understand how they work.

There are many bits of technology that

go into making a large format printer,

but arguably the printhead is the beating

heart of the machine. It is the thing that

pumps the ink directly to the media and

it’s the printhead that determines the

defining characteristics of the printer. It

dictates the type of ink the printer uses

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Inks, Heads & Materials for Wide Format Digital Wild Format

the ink is drawn back to the nozzle causing

an individual drop to form, rather than

having a continuous stream of ink flowing

out through the nozzle.

Thermal printheads, as the name implies,

heat the ink within the ink chamber until

it vaporises and creates a bubble and

forces a drop of ink through the nozzle.

The main drawback of thermal is that it

is mostly limited to water-based inks. It

has been widely used by Canon for its

ImagePrograf range and by HP for its

DesignJet wide format printers. HP has also

developed water-based resin inks, better

known as latex inks, for use with its thermal

and the resolution, so the heads play a

pretty big part in the overall image quality

and the printer’s productivity. This is why

vendors will often release new versions of

their printers as new printheads become

available.

Most inkjet printers use drop on demand

(DOD) printheads, where each drop of ink

is generated only when it’s needed. There

are two main techniques used in DOD

wide format printing engines: thermal and

piezo-electric. In both systems the trick is

to create enough pressure to force some

ink through the nozzle. The pressure must

subside almost immediately, so that some of

Ricoh's Inkjet Test Centre lab where inks are tested to see how they react with Ricoh's printheads. Photo © Nessan Cleary

Inks, Heads & Materials for Wide Format Digital

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Agfa updated its Anapurna M2050 with a new M2050i model to take advantage of a new generation of Konica Minolta printheads. Photo © Nessan Cleary

printheads. Another issue with thermal

heads is that they have a very short life span

and are usually replaced alongside the ink

cartridges and treated as a consumable

item.

However, the most commonly used DOD

technology is piezo-electric as is used in

the Mimaki UJF-6042. A voltage is applied

to a piezo crystal to change its shape, so

that it expands and pushes against the

ink chamber within the printhead. Some

printheads do still use a piezo crystal

actuator but there are other variations now,

such as using a sound wave. The basic

principle remains the same: when a voltage

is pulsed through the actuator it changes

shape, forcing a drop of ink from the ink

chamber and out through the nozzle.

This approach works with a wide variety

of different inks including solvent and UV-

curable inks. The printheads can have a

reasonably long life – up to two years in

some cases – but they are more expensive

than thermal heads.

Binary or Greyscale?

The actual size of the ink droplet can vary

considerably, with some having a tiny drop

size of just three picolitres and capable of

reproducing very fine details, particularly

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Inks, Heads & Materials for Wide Format Digital Wild Format

can also be more expensive than their

binary counterparts. However many wide

format printing devices use some form

of greyscaling, with anything from three

to seven different drop sizes being quite

common now.

Single pass

Traditionally inkjet printheads cover only

a very small part of the printable area

and must scan back and forth to print a

complete line. This ensures that any gaps

are filled in, thereby generating quite

high resolution images from relatively low

resolution printheads. But this also takes

time so clearly the easiest way to speed up

a printer is to reduce the number of passes,

with the holy grail being just a single pass.

In recent years single pass inkjet printing

has become quite common for both label

and document printers and many vendors

privately believe that it’s only a matter of

on small text and intricate patterns. Others

such as the Fujifilm Acuity Advance HD2545

use a much larger drop size, typically up

to 42 picolitres, which lets them put a

lot of ink down quickly, useful for a high

production printer.

The simplest approach is to use a single

fixed droplet size, which is usually referred

to as binary. But it’s easier to optimise the

printer for different applications byusing

multiple drop sizes, a practice known as

greyscale printing. There are a number of

advantages to using greyscale printing.

Firstly, mixing bigger and smaller dots

makes it easier to deal with gradients and

subtle tonal shifts, such as in a skyline. It

can also lead to reduced ink consumption,

partly because some of the dots are quite

small, but also because it’s easier to get

smoother gradients with four colours

without needing additional colours.

There are several different greyscale

approaches. Some printheads eject different

volumes of ink to make different sized

droplets. Others eject the same amount

of ink, but vary the frequency at which

they fire the ink so that the different drops

merge in flight, or land at the same spot on

the media to form larger drops.

Greyscale printheads can be slower,

and as they are more complex they

Greyscale printheads can be slower, and as they are more complex they can also be more expensive than their binary counterparts.

Inks, Heads & Materials for Wide Format Digital

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time before we see single pass wide format

printers. The main issue is the cost of the

printheads, with only the very expensive

machines such as Fujifilm’s Onset having

enough printheads to cover the full width of

the bed. Even these still use several passes

to achieve reasonable print quality.

Memjet has developed relatively low cost

single pass printheads for wide format use,

having stitched five of its heads together

for a 42ins wide print engine. The actual

heads are 220mm wide and capable of

1600 dpi resolution, with the latest version,

codenamed Aspen, capable of running at

68mpm.

These printers are incredibly fast but for

now this technology is limited to water-

based inks, ruling it out for outdoor

applications. Instead, the Memjet-based

printers are mainly targeted at the CAD

graphics market though Memjet is

developing a version of its printhead that

will take solvent inks as well as a newer

design that would be suitable for other ink

types such as UV-curable.

Optimisation

It is tempting to assume that printers that

use the same printhead will have identical

performance. But the heads can be tuned

to satisfy different needs so that although

the physical characteristics will remain the

same, there will be plenty of differences.

Thus each vendor could use their own

electronics to drive the printheads, which

being analogue need a digital signal in

order to fire the ink.

Also, individual inks will have to be

optimised for each head to ensure, for

example, that the inks don’t corrode or

otherwise damage the heads. The inks must

also have the right viscosity so that they can

form droplets that eject accurately from the

heads.

Finally, it’s worth noting that most

printheads only really fail when they

become clogged, usually with dried ink.

A rigorous approach to cleaning them

can save a fair bit of money both in head

replacement costs and keeping the machine

in service.

– Nessan Cleary