Lexmark International inkjetprinting User Manual

inkjet printing

TECHNOLOGY, INKS AND PAPER FOR

M-real Digital imaging

2

Inkjet Technology: an overview 3

Heads, Modes and Nozzles 5

Putting the INK into INKjet 8

Designs on Inkjet Paper 11

The question is?... 12

Inkjet’s different images 14

Cut to Size 14

The Larger Chunk 16

Colour in Profile 17

Jetting Trends 18

INKJET PRINTING IS A NON-IMPACT

DOT MATRIX PRINTING

TECHNOLOGY IN WHICH SMALL

DROPLETS OR PARTICLES OF INK

ARE JETTED FROM A SMALL

APERTURE

(IN RAPID SUCCESSION

AND UNDER COMPUTER CONTROL

)

DIRECTLY TO A SPECIFIED POSITION

ON THE SURFACE OF A SUBSTRATE

,

IN ORDER TO CREATE AN IMAGE.

Definition

Contents

3

Inkjet Technology Overview

In the early days of inkjet print technology development,
poor colour image quality due to ink spreading and
inter-colour bleeding (that is where the penetration of
ink into the paper is too slow to absorb multiple ink
drops on the same spot in short intervals) were widely
recognized as the critical issues.

The initial solution which has continued to be built upon
was the use of special coated media, the design of which
takes into account drop volume, evaporation rate,
penetration rate, porosity, etc (see Designs on Inkjet
Paper, page 11).

But as inkjet print technology was developed and
alternative solutions to the use of special coated media
were sought, the use of solid (hot-melt) ink was
introduced.

The idea being that on contact with the media, the ink
solidifies almost immediately, without over-absorption or
too much spreading, enabling brilliant colour and image
reproduction. These inks have the advantage of being
able to print on a wide variety of substrates such as
glass and ceramics. However, they do not allow for a
very high print resolution, hence at present the
technology is not a market leader.

With this potential alternative solution, the door was now
open to deeper exploration of the true opportunities of
inkjet printing.

Continuous inkjet technology (The process in which a
continuous stream of ink droplets are given an
electrostatic charge which allows precise placement and
frequency of droplets) and drop-on-demand inkjet
technology (The procedure in which droplets of ink are
forced through a nozzle in a controlled fashion, rather
than in a continuous stream), soon became the two
primary categories (see Chart 1), spawning their own
technology sub-divisions. However, drop-on-demand is
at present the most widely used technology with the vast
majority of applications being printed using this method.

WHEN INKJET PRINTING FIRST CAME ON THE SCENE PRINT QUALITY WAS

GENERALLY UNSATISFACTORY

. HOWEVER, DUE TO DEVELOPMENTS IN HARDWARE

(PRINTERS), SOFTWARE, INKS AND SUBSTRATES, VERY GOOD PRINTED RESULTS

CANNOW BE ACHIEVED

. (IMPROVED PRINT QUALITY IS NOT ONLY DOWN TO BETTER

PAPER

, BUT A VARIETY OF CHANGES IN THE WHOLE ARENA)

4

Continuous inkjet

Continuous inkjet – perhaps most widely used in the
industrial coding, marking and labelling markets - can
be designed using a binary or multiple deflection
system.

Put simply, with a binary deflection system some of the
ink-drops are charged and some are uncharged. It is the
charged drops that fly directly onto the media, while the
uncharged drops are ‘deflected’ into a gutter for
recirculation. Whereas with a multiple deflection system
(also known as raster imaging) the design is essentially
reversed, so while the uncharged ink-drops fly directly
into the gutter for recirculation, the charged drops are
‘deflected’ onto the media at different levels.

Meanwhile, another continuous inkjet concept - the Hertz
concept (named after Professor Hertz of the Lund
Institute of Technology in Sweden) - can be given
separate classification because of its unique way of
obtaining gray scale through a burst of small drops. By
varying the number of drops laid down, the amount of ink
volume in each pixel was controlled by Hertz, therefore
the density in each colour could be adjusted to create the
gray tone desired, making this ideal technology for high
quality colour images at a stage where drop-on-demand
quality could not really compete.

Drop-on-demand

But today, drop-on-demand technology can readily
compete, and the majority of activity in inkjet printing
currently available utilises one of two drop-on-demand
methods: thermal and piezo (or piezo-electric). That is to
say that the printing devices supplied by most of today’s
manufacturers are equipped with either thermal or piezo
print heads. This is mainly due to the cost effectiveness
of this technology over continuous inkjet.

Thermal inkjet print heads receive signals from the
control unit, which causes an internal heating device to
heat up rapidly and boil the ink present to form a bubble.
The heat increases until the bubble bursts and forces the
droplet out through the nozzle onto the substrate at high
speed. Droplet size may vary from half to full size by
heating one or two elements respectively.

Piezo processing on the other hand, works through the
piezoelectric effect. Here, currents pass through
piezoelectric crystals or ceramic chambers. This causes
the chambers to change shape, which squeezes ink from
the nozzles. To produce larger droplets the voltage must
be increased which displaces more ink, resulting in a
larger droplet. Manufacturers have also experimented
more recently with acoustic (also referred to as airbrush)
and electrostatic inkjet, but these technologies are still
very much in the developmental stage and few
commercial products employing them are yet available.

Chart 1

INK JET TECHNOLOGY

Continuous

Drop-on-Demand

Binary

Deflection

Elmjet

Scitex

Image

Hewlett-Packard

Olivetti

Lexmark

Canon

Xerox

Siemens

Gould

Tektronix

Sharp
Epson

On Target Tech.

Dataproducts

Epson

Trident

Spectra

Xaar

Nu-Kole

Brother

Microfab Tech.

Philips

Topaz Tech.

Videojet

Diconix

Domino Amjet

Linx

Iris Graphics Hitachi

Multiple

Deflection

Hertz

Microdot

Thermal

Roof-shooter Side-shooter

Squeeze Tube

Bend Mode Push Mode Shear Mode

Piezoelectric Electrostatic Accoustic

5

Heads, Modes and Nozzles

NOT SO WITH PIEZO INKJET TECHNOLOGY, WHERE

THE ELECTRICALLY CHARGED PIEZOELECTRIC
CRYSTAL PRESSURISES THE FIRING CHAMBER IN
THE PRINT HEAD AND PUSHES THE INK OUT

. EVEN

THOUGH AN ELECTRIC PULSE IS UTILISED

, IT

ESSENTIALLY TRIGGERS A MECHANICAL

‘JETTING’

PROCESS, SO VAPOUR BUBBLES DO NOT HAVE TO BE
GENERATED AND THE INK IS NOT SUBJECT TO
THERMAL SHOCK

.

This gives more flexibility in the inks that can be used,
such as water based and solvent based ink types, and
the ink viscosity can be higher, so the risks of kogation
are reduced. It has also become easier now to develop
special inks for substrates that have special
requirements (see Putting the Ink into Inkjet, page 6).
Besides, the piezo print head is more resistant to
aggressive chemicals. Finally, a major advantage of
piezo is the possibility of making smaller units, resulting
in more nozzles per print head, which allows a higher
resolution of print to be achieved.

A THERMAL INKJET PRINT HEAD IS EQUIPPED WITH A RESISTOR, WHICH HEATS

THE INK USING ELECTRICITY

. THE VAPOUR INSIDE THE PRINT HEAD’S FIRING

CHAMBER EXPANDS AND PUSHES THE INK OUT OF THE NOZZLE

, WHILE THE

REMAINING VAPOUR BUBBLE COLLAPSES AFTER COOLING AND SUCKS NEW INK
INTO THE FIRING CHAMBER

. THROUGH CHANGING THE HEAT ENERGY, THE INK-

DROP SIZE CAN BE TUNED ACCORDING TO THE APPLICATION REQUIRED.

The main drawback with thermal inkjet print heads is
that they have a shorter lifetime. Problems can be
caused either through the collapse of the vapour
bubble - creating a rapid thermal shock, which can
cause damage to the print head - or through early drying
of the heated ink in the nozzle. The ink formulation,
therefore, usually has to be adapted to help the print
head withstand any thermal shock and to minimize any
such kogation.

6

Print head configurations

Inkjet technology is used in many different architectures
and with different operating principles, depending on the
configuration of its print heads. With thermal inkjet
technology, for example, the print head can be a roof
shooter, with an orifice located on top of the heater, or a
side-shooter, where the orifice is located on the side
nearby the heater.

For piezo, there are four main types of print head
configuration - squeeze, bend, push, and shear –
dependent on what is called, the piezoceramic
deformation mode.

A squeeze-mode design usually has either a thin tube of
piezoceramic surrounding a glass nozzle,
or a piezoceramic tube cast in plastic
that encloses the ink channel.

In a typical bend-mode design, the piezoceramic plates
are bonded to the diaphragm forming an array of
bilaminar electromechanical transducers used to eject
the ink droplets.

For a push-mode design, as the piezoceramic rods
expand, they push against ink to eject the droplets.

And in a shear-mode print head, the shear action
deforms the piezoplates against ink to eject the
droplets. Interaction between ink and piezomaterial is
one of the key parameters of this design, as currently
pioneered by Xaar.