About Inkjet printers
An inkjet printer is a type of computer printer that creates a digital image by
propelling variable-sized droplets of ink on to paper. Inkjet printers are the
most commonly used type of printer and range from small inexpensive consumer
models to very large professional machines.
The concept of inkjet printing dates back to the 19th century, and the
technology was first developed in the early 1950s. Starting in the late 1970s
inkjet printers that could reproduce digital images generated by computers were
developed, mainly by Epson, Hewlett-Packard and Canon. In the worldwide consumer
market, four manufacturers account for the majority of inkjet printer sales:
Canon, Hewlett-Packard, Epson, and Lexmark.
The emerging ink jet material deposition market also uses inkjet technologies,
typically piezoelectric crystals, to deposit materials directly on substrates.
There are two main technologies in use in contemporary inkjet printers:
continuous (CIJ) and Drop-on-Demand (DOD). Drop-on-demand is further divided
into thermal DOD and piezoelectric DOD
The continuous inkjet method is used commercially for marking and coding of
products and packages. The idea was first patented in 1867, by Lord Kelvin and
the first commercial devices (medical strip chart recorders) were introduced in
1951 by Siemens.
In continuous inkjet technology, a high-pressure pump directs liquid ink from a
reservoir through a gunbody and a microscopic nozzle, creating a continuous
stream of ink droplets via the Plateau-Rayleigh instability. A piezoelectric
crystal creates an acoustic wave as it vibrates within the gunbody and causes
the stream of liquid to break into droplets at regular intervals – 64,000 to
165,000 droplets per second may be achieved. The ink droplets are subjected to
an electrostatic field created by a charging electrode as they form; the field
varies according to the degree of drop deflection desired. This results in a
controlled, variable electrostatic charge on each droplet. Charged droplets are
separated by one or more uncharged “guard droplets” to minimize electrostatic
repulsion between neighbouring droplets.
The charged droplets pass through an electrostatic field and are directed
(deflected) by electrostatic deflection plates to print on the receptor material
(substrate), or allowed to continue on undeflected to a collection gutter for
re-use. The more highly charged droplets are deflected to a greater degree. Only
a small fraction of the droplets is used to print, the majority being recycled.
Continuous ink jet is one of the oldest ink jet technologies in use and is
fairly mature. The major advantages are the very high velocity (~50 m/s) of the
ink droplets, which allows for a relatively long distance between print head and
substrate, and the very high drop ejection frequency, allowing for very high
speed printing. Another advantage is freedom from nozzle clogging as the jet is
always in use, therefore allowing volatile solvents such as ketones and alcohols
to be employed, giving the ink the ability to "bite" into the substrate and dry
The ink system requires active solvent regulation to counter solvent evaporation
during the time of flight (time between nozzle ejection and gutter recycling)
and from the venting process whereby air that is drawn into the gutter along
with the unused drops is vented from the reservoir. Viscosity is monitored and a
solvent (or solvent blend) is added in order to counteract the solvent loss.
Thermal/thermal DOD inkjet
Most consumer inkjet printers, from companies including Canon, Hewlett-Packard,
and Lexmark (but not Epson), use print cartridges with a series of tiny chambers
each containing a heater, all of which are constructed by photolithography. To
eject a droplet from each chamber, a pulse of current is passed through the
heating element causing a rapid vaporisation of the ink in the chamber to form a
bubble, which causes a large pressure increase, propelling a droplet of ink onto
the paper (hence Canon's tradename of Bubble Jet for its technology). The ink's
surface tension, as well as the condensation and thus contraction of the vapor
bubble, pulls a further charge of ink into the chamber through a narrow channel
attached to an ink reservoir.
The inks used are usually water-based (aqueous) and use either pigments or dyes
as the colourant. The inks used must have a volatile component to form the
vapour bubble, otherwise droplet ejection cannot occur. As no special materials
are required, the print head is generally cheaper to produce than in other
inkjet technologies. The thermal inkjet principle was discovered by Canon
engineer Ichiro Endo in August 1977.
Thermal inkjet printers are not the same as thermal printers, which produce
images by heating thermal paper, as seen on older fax machines, cash registers,
ATM receipt printers, and lottery ticket printers.
Piezoelectric/piezoelectric DOD inkjet
Most commercial and industrial inkjet printers and some consumer printers (those
produced by Epson) use a piezoelectric material in an ink-filled chamber behind
each nozzle instead of a heating element. When a voltage is applied, the
piezoelectric material changes shape, which generates a pressure pulse in the
fluid forcing a droplet of ink from the nozzle. Piezoelectric (also called Piezo)
inkjet allows a wider variety of inks than thermal inkjet as there is no
requirement for a volatile component, and no issue with kogation, but the print
heads are more expensive to manufacture due to the use of the specialist
piezoelectric material (usually PZT, lead zirconium titanate). Piezo inkjet
technology is often used on production lines to mark products - for instance the
use-before date is often applied to products with this technique; in this
application the head is stationary and the product moves past. Requirements of
this application are a long service life, a relatively large gap between the
print head and the substrate, and low operating costs. There is a drop-on-demand
process, with software that directs the heads to apply between zero to eight
droplets of ink per dot and only where needed. As of June 2009, the fastest
cut-sheet inkjet printer on the market is the RISO ComColor 9050, which prints
146 USLetter and 150 A4 full-color pages per minute in both one-sided and
two-sided printing modes.Recent developments of the inkjet extend the
operation from printing into manufacturing processes. The newest of these
technologies is to deposit layers of plastic material as digital embossing ove
the top of printed works
The basic problem with inkjet inks are the conflicting requirements for a
coloring agent that will stay on the surface and rapid dispersement of the
Desktop inkjet printers, as used in offices or at home, tend to use aqueous inks
based on a mixture of water, glycol and dyes or pigments. These inks are
inexpensive to manufacture, but are difficult to control on the surface of
media, often requiring specially coated media. HP inks contain sulfonated
polyazo black dye (commonly used for dying leather), nitrates and other
compounds. Aqueous inks are mainly used in printers with thermal inkjet heads,
as these heads require water in order to perform.
While aqueous inks often provide the broadest color gamut and most vivid color,
most are not waterproof without specialized coating or lamination after
printing. Most Dye-based inks, while usually the least expensive, are subject to
rapid fading when exposed to light. Pigment-based aqueous inks are typically
more costly but provide much better long-term durability and ultraviolet
resistance. Inks marketed as “Archival Quality” are usually pigment-based.
Some professional wide format printers use aqueous inks, but the majority in
professional use today employ a much wider range of inks, most of which require
piezo inkjet heads and extensive maintenance:
Solvent inks: the main ingredient of these inks are volatile organic compounds (VOCs),
organic chemical compounds that have high vapor pressures. Color is achieved
using pigments rather than dyes for excellent fade-resistance. The chief
advantage of solvent inks is that they are comparatively inexpensive and enable
printing on flexible, uncoated vinyl substrates, which are used to produce
vehicle graphics, billboards, banners and adhesive decals. Disadvantages include
the vapour produced by the solvent and the need to dispose of used solvent.
Unlike most aqueous inks, prints made using solvent-based inks are generally
waterproof and ultraviolet-resistant (for outdoor use) without special
over-coatings. The high print speed of many solvent printers demands special
drying equipment, usually a combination of heaters and blowers. The substrate is
usually heated immediately before and after the print heads apply ink. Solvent
inks are divided into two sub-categories:
Hard solvent ink offers the greatest durability without specialized
over-coatings but requires specialized ventilation of the printing area to avoid
exposure to hazardous fumes.
Mild or "Eco" solvent inks, while still not as safe as aqueous inks, are
intended for use in enclosed spaces without specialized ventilation of the
printing area. Mild solvent inks have rapidly gained popularity in recent years
as their color quality and durability have increased while ink cost has dropped
UV-curable inks: these inks consist mainly of acrylic monomers with an initiator
package. After printing, the ink is cured by exposure to strong UV-light. The
advantage of UV-curable inks is that they "dry" as soon as they are cured, they
can be applied to a wide range of uncoated substrates, and they produce a very
robust image. Disadvantages are that they are expensive, require expensive
curing modules in the printer, and the cured ink has a significant volume and so
gives a slight relief on the surface. Though improvements are being made in the
technology, UV-curable inks, because of their volume, are somewhat susceptible
to cracking if applied to a flexible substrate. As such, they are often used in
large "flatbed" printers, which print directly to rigid substrates such as
plastic, wood or aluminum where flexibility is not a concern.
UV Curable Ink Properties and Functions:
• Photoinitiators: Absorb the UV energy from the light source on the print head.
Chemical reaction occurs that converts the liquid ink into a solid film.
• Monomers: Used as solvents because of their ability to reduce viscosity
(thickness) and combine with other ink components. 100% percent solids and do
not release VOCs (volatile organic compounds). Monomers also add improved film
hardness and resistance properties.
• Oligomers: Determine the final properties of the cured ink film, including its
elasticity, outdoor performance characteristics and chemical resistance.
• Colorants: Can be dye-based or pigment-based. Usually, pigment-based because
of the greater light fastness and durability of pigments compared with dyes.
Pigments used in outdoor advertising and display applications have similar
requirements to those used in automotive paints. Consequently, there is some
crossover of use. While a pigment is selected on the basis of the required
application, size control and reduction along with dispersion technique are
major components of ink formulation.
UV Ink Printing Process:
Ink is exposed to UV radiation where a chemical reaction takes place where the
photo-initiators cause the ink components to cross-link into a solid.
Typically a shuttered mercury-vapor lamp is on either side of the print head,
and produces a great amount of heat to complete the curing process (this lamp is
used for free radical UV ink, which is what the majority of flatbed inkjet
UV inks do not evaporate, but rather cure or set as a result from this chemical
No material is evaporated or removed, which means about 100% of the delivered
volume is used to provide coloration.
This reaction happens very quickly, which leads to instant drying that results
in a completely cured graphic in a matter of seconds. This also allows for a
very fast print process.
As a result of this instant chemical reaction no solvents penetrate the
substrate once it comes off the printer, which allows for high quality prints.
Dye sublimation inks: these inks contain special sublimation dyes and are used
to print directly or indirectly on to fabrics which consist of a high percentage
of polyester fibres. A heating step causes the dyes to sublimate into the fibers
and create an image with strong color and good durability.
 Inkjet head design
Disposable head (left) and
Fixed head (right) with ink cartridge (middle)There are two main design
philosophies in inkjet head design: fixed-head and disposable head. Each has its
own strengths and weaknesses. Most inkjets are used for photo printing.
The fixed-head philosophy provides an inbuilt print head (often referred to as a
Gaither Head) that is designed to last for the life of the printer. The idea is
that because the head need not be replaced every time the ink runs out,
consumable costs can be made lower and the head itself can be more precise than
a cheap disposable one, typically requiring no calibration. On the other hand,
if a fixed head is damaged, obtaining a replacement head can become expensive if
removing and replacing the head is even possible. If the printer's head cannot
be removed, the printer itself will then need to be replaced.
Fixed head designs are available in consumer products but are more likely to be
found on industrial high-end printers and large format plotters. In the consumer
space, fixed-head printers are manufactured primarily by Epson and Canon.
Hewlett-Packard also offers a few fixed-head models, such as the HP Photosmart
3310. Industrial fixed-head print heads are manufactured by these companies:
Kodak Versamark, Trident, Xaar, Spectra (Dimatix), Hitachi / Ricoh, HP Scitex,
Brother, Konica Minolta, Seiko Epson, and ToshibaTec (a licensee of
The disposable head philosophy uses a print head which is supplied as a part of
a replaceable ink cartridge. Every time a cartridge is exhausted, the entire
cartridge and print head are replaced with a new one. This adds to the cost of
consumables and makes it more difficult to manufacture a high-precision head at
a reasonable cost, but also means that a damaged print head is only a minor
problem: the user can simply buy a new cartridge. Hewlett-Packard has
traditionally favoured the disposable print head, as did Canon in its early
models. This type of construction can also be seen as an effort by printer
manufacturers to stem third party ink cartridge assembly replacements, as these
would-be suppliers don't have the ability to manufacture specialized print
An intermediate method does exist: a disposable ink tank connected to a
disposable head, which is replaced infrequently (perhaps every tenth ink tank or
so). Most high-volume Hewlett-Packard inkjet printers use this setup, with the
disposable print heads used on lower volume models.
Canon now uses (in most models) replaceable print heads which are designed to
last the life of the printer, but can be replaced by the user if they should
become clogged. For models with "Think Tank" technology, the ink tanks are
separate for each ink color.
The primary cause of inkjet printing problems is due to ink drying on the
printhead's nozzles, causing the pigments and dyes to dry out and form a solid
block of hardened mass that plugs the microscopic ink passageways. Most printers
attempt to prevent this drying from occurring by covering the printhead nozzles
with a rubber cap when the printer is not in use. Abrupt power losses, or
unplugging the printer before it has capped the printhead, can cause the
printhead to be left in an uncapped state. Further even when capped this seal is
not perfect, and over a period of several weeks the moisture can still seep out,
causing the ink to dry and harden. Once ink begins to collect and harden drop
volume can be affected, drop trajectory can change, or the nozzle can fail to
jet ink completely.
To combat this drying, nearly all inkjet printers include a mechanism to reapply
moisture to the printhead. Typically there is no separate supply of pure
ink-free solvent available to do this job, and so instead the ink itself is used
to remoisten the printhead. The printer attempts to fire all nozzles at once,
and as the ink sprays out, some of it wicks across the printhead to the dry
channels and partially softens the hardened ink. After spraying, a rubber wiper
blade is swept across the printhead to spread the moisture evenly across the
printhead, and the jets are again all fired to dislodge any ink clumps blocking
Some use a supplemental air-suction pump, utilizing the rubber capping station
to suck ink through a severely clogged cartridge. The suction pump mechanism is
frequently driven by the page feed stepper motor – it is connected to the end of
the shaft. The pump only engages when the shaft turns backwards, hence the
rollers reversing while head cleaning. Due to the built-in head design, the
suction pump is also needed to prime the ink channels inside a new printer, and
to reprime the channels between ink tank changes.
Professional solvent- and UV-curable ink wide-format inkjet printers generally
include a "manual clean" mode that allows the operator to manually clean the
print heads and capping mechanism and to replace the wiper blades and other
parts used in the automated cleaning processes. The volume of ink used in these
printers often leads to "overspray" and therefore buildup of dried ink in many
places that automated processes are not capable of cleaning.
The ink consumed in the cleaning process needs to be collected somewhere to
prevent ink from leaking all over the surface under the printer. The collection
area is known as the spittoon, and in Hewlett Packard printers this is an open
plastic tray underneath the cartridge storage and cleaning/wiping station. In
Epson printers, there is typically a large fibrous absorption pad in a pan
underneath the paper feed platen. For printers several years old, it is common
for the dried ink in the spittoon to form a pile that can stack up and touch the
printheads, jamming the printer with sticky slime. Some larger professional
printers using solvent inks may employ a replaceable plastic receptacle to
contain waste ink and solvent which needs to be emptied and/or replaced when
The type of ink used in the printer can also affect how quickly the printhead
nozzles become clogged. While the official brand of ink is highly engineered to
match the printer mechanism, generic inks cannot exactly match the composition
of the official brand since the actual ink composition is a trade secret.
Generic ink brands may alternately be too volatile to keep the printhead moist
during storage, or may be too thick and jellied leading to frequent printhead
Labyrinth air vent tubes on the top of an Epson Stylus Photo 5-color ink tank.
The long air channels are molded into the top of the tank and the blue label
seals the channels into long tubes. The yellow label is removed prior to
installation, and opens the tube ends to the atmosphere so that ink can be
sprayed onto the paper. Removing the blue label would destroy the tubes and
cause the moisture to quickly evaporate.There is a second type of ink drying
that most printers are unable to prevent. In order for ink to spray out of the
cartridge, air needs to enter somewhere to displace the removed ink. The air
enters via an extremely long, thin labyrinth tube, up to 10 cm long, wrapping
back and forth across the ink tank. The channel is long and narrow to slow down
moisture from evaporating out through the vent tube, but some evaporation still
occurs and eventually the ink cartridge dries up from the inside out. To combat
this problem, which is especially acute with professional fast-drying solvent
inks, many wide-format printer cartridge designs contain the ink in a special
airtight, collapsible bag that does not require a vent as the ink level drops.
The bag merely shrinks until the cartridge is empty.
The frequent cleaning conducted by printers can consume quite a bit of ink and
has a great impact on cost per page determinations.
Clogged nozzles can be detected by printing a pattern on the page. Methods are
known for re-routing printing information from a clogged nozzle to a working
A method to quantify the spatial resolution of printed inkjet images is
described in Tunable Laser Applications. This method consists in printing
transmission gratings with the inkjet printer to be assessed and measuring the
modulation of laser light in an N-Slit interferometer. For gratings with spatial
frequencies in the 0.25-5.0 lines/mm range, modulations in the 0.71-0.87 range
are reported for various printers. The closer the measured modulation is to
unity (~ 1.0) the higher the spatial resolution of the print.
Compared to earlier consumer-oriented color printers, inkjets have a number of
advantages. They are quieter in operation than impact dot matrix or daisywheel
printers. They can print finer, smoother details through higher printhead
resolution, and many consumer inkjets with photographic-quality printing are
In comparison to more expensive technologies like thermal wax, dye sublimations,
and laser printers, inkjets have the advantage of practically no warm up time
and lower cost per page (except when compared to laser printers).
For some inkjet printers, monochrome ink sets are available either from the
printer manufacturer or third-party suppliers. These allow the inkjet printer to
compete with the silver-based photographic papers traditionally used in
black-and-white photography, and provide the same range of tones – neutral,
"warm" or "cold". When switching between full-color and monochrome ink sets, it
is necessary to flush out the old ink from the print head with a cleaning
Inkjet printers may have a number of disadvantages:
The ink is often very expensive. (For a typical OEM cartridge priced at $15,
containing 5 mL of ink, the ink effectively costs $3000 per liter—or $8000 per
gallon.) According to the BBC (2003), "The cost of ink has been the subject of
an Office of Fair Trading investigation. Which? magazine has accused
manufacturers of a lack of transparency about the price of ink and called for an
industry standard for measuring ink cartridge performance".
Many "intelligent" ink cartridges contain a microchip that communicates the
estimated ink level to the printer; this may cause the printer to display an
error message, or incorrectly inform the user that the ink cartridge is empty.
In some cases, these messages can be ignored, but some inkjet printers will
refuse to print with a cartridge that declares itself empty, in order to prevent
consumers from refilling cartridges. Thus, Epson embeds a chip which prevents
from printing when the chip claims the cartridge is empty, although a researcher
who over-rode the system found that in one case he could print up to 38% more
good quality pages, even though the chip stated that the cartridge was
The lifetime of inkjet prints produced by inkjets using aqueous inks is limited;
they will eventually fade and the color balance may change. On the other hand,
prints produced from solvent-based inkjets may last several years before fading,
even in direct sunlight, and so-called "archival inks" have been produced for
use in aqueous-based machines which offer extended life.
Because the ink used in most consumer inkjets is water-soluble, care must be
taken with inkjet-printed documents to avoid even the smallest drop of water,
which can cause severe "blurring" or "running." Similarly, water-based
highlighter markers can blur inkjet-printed documents.
The very narrow inkjet nozzles are prone to clogging with dried ink. The ink
consumed cleaning them - either during cleaning invoked by the user, or in many
cases, performed automatically by the printer on a routine schedule - can
account for a significant proportion of the total ink installed in the machine.
These disadvantages have been addressed in a variety of ways: