196 PART 4 HOW DIGITAL PRINT-MAKING WORKS
How Photo Inkjet Printers Work
Inkjet printers vary the sizes of their droplets using one of two technologies to shoot ink out
of their print heads. Hewlett-Packard, Lexmark, and Canon use thermal nozzles,
whereas Epson uses proprietary piezoelectric nozzles. Both suck ink from the
cartridges into their firing chambers and shoot the ink through scores of
openings on the bottom of the print head that are too small to see.
When the printer sends an electrical current through a resistor built in
to one of the walls of each of the nozzles, the resistor generates
enough heat to boil the ink and create an air bubble.1
As the bubble expands into the firing chamber, it creates pressure in
the ink until the pressure is strong enough to overcome the surface ten-
sion holding the ink inside the nozzle. A droplet with a size between
6 and 10 picoliters bursts out of the nozzle to create a colored dot on
the paper traveling beneath the print head.2
supply of ink to rush in from the
cartridge to arm the nozzle for another
firing. Heating the ink to higher or
lower temperatures causes larger or
smaller droplets to fire.The walls of a piezoelectric nozzle are made of piezo,
a crystalline substance that bends when electricity
passes through it. When current passes through one
way, the walls expand, drawing in ink from the car-
tridge’s reservoir.1
When the printer suddenly reverses the current’s direc-
tion, the walls contract, squeezing out a droplet of ink.2
By varying the intensity of the current, the printer changes
how much the piezo wall flexes. That, in turn, changes
the size of the droplet, which typically ranges from three
to four picoliters. That is equal to about one-fourth the
diameter of a hair and is at the boundary of resolution
for the human eye. Generally, the droplets from piezo-
electric inkjet printers are smaller than those from thermal
inkjet printers.3
Thermal Nozzle
At the same time, current to the resistor turns off, quickly cooling the
resistor. A partial vacuum forms in the firing chamber, causing a new3
Piezoelectric Nozzle