5
Heads, Modes and Nozzles
N
OT SO WITH PIEZO INKJET TECHNOLOGY
,
WHERE
THE ELECTRICALLY CHARGED PIEZOELECTRIC
CRYSTAL PRESSURISES THE FIRING CHAMBER IN
THE PRINT HEAD AND PUSHES THE INK OUT
. E
VEN
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
. T
HE 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
. T
HROUGH 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.
