Twenty or thirty years ago, many people
thought computers would make
paper obsolete. The Xerox company, which pioneered photocopiers in the
1960s, became so worried that paper was going to disappear (and wipe
out its lucrative business) that it set up a famous laboratory called
PARC to develop computers instead. Ironically, even though Xerox PARC
helped to invent the computers we all rely on today, paper stayed as
popular as ever: people loved it and it was just good too an invention
to replace. Now, thanks to the popularity of home computers and digital
cameras, more people have printing machines in their homes than
ever before—and most of them are inkjet printers. But how exactly do they
work?
Photo: A typical "all-in-one" inkjet printer. This one also works as
a standalone scanner and photocopier without any need to plug it into a computer.
To understand inkjet printers, it helps if you understand how computer printers worked before
inkjet technology came along. That means understanding metal type and the computer printers
that evolved from it.
Printing with metal type
Let's rewind a few hundred years, back to the 15th century. There was
some small-scale printing with wooden blocks before this time, but
printing only really took off when a German printer named
Johannes
Gutenberg (c.1400–1468) invented something called movable
metal type
around 1450. If you've ever seen a typewriter
(a personal
letter-writing machine popular until computers came along in the
1980s), you'll know all about metal type. A typewriter has a
keyboard
like the one on a computer, but instead of making letters appear one at
a time on the screen, it prints them directly onto a piece of paper.
Inside the typewriter, there are metal letters called pieces of type.
As you press the keys, the pieces of type hammer against a
ribbon (a spool of fabric) covered in ink and make an impression on the paper. Gutenberg
was the pioneer of metal type. He made thousands of little metal
letters (printed in relief and in reverse) and moved them around inside
wooden blocks so he could print any page he liked—hence the name "movable
metal type".
Photo: The metal type in a relatively modern typewriter. The letters are backward so
they print the right way round when they press an inked ribbon against the paper.
Typewriters were based on Gutenberg's invention and took off in the
1860s after American journalist Christopher Latham Sholes (1819–1890) and his partners made the first really practical typing machine (dozens
of other people had tried before). Although
typewriters were a brilliant invention, they could make only one copy
of a piece of information at a time. Because they printed directly onto
the paper, typing could be slow and messy and mistakes were difficult
to correct. When business computers started to become popular in the
1960s, many people became interested in using them as word processors:
highly automated typewriters that allowed text to be typed onto a
screen, edited and corrected until it was perfect, and only then printed out
onto paper.
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Impact printing
Early computer printers borrowed heavily from typewriter technology, but
it soon became obvious that better methods were needed for quicker and
more efficient printing. Instead of using rows of metal levers to
hammer letters against the page, as in a typewriter, computer printers
(and electric typewriters, which were
similar) started to use three
other technologies. One of them was called a golf
ball. The
golf ball typewriter or printer has all the letters, numbers, and other
characters it needs to print arranged on the surface of a metal ball.
To print a word, the ball rotates at high speed until the right piece
of type is facing the paper. Then it flips up and bashes the type
against a ribbon, pressing the letter onto the page. Having done
that, it spins round to the next letter... and so on. The second
printing technology was called a daisy wheel,
in which the type
letters are arranged like petals around a central wheel. A bit like a
golf ball, the daisy wheel rotates at high speed, stopping to press
letters against the ribbon when they are in the correct position.
A third printing technology, known as dot-matrix,
was popular from the 1970s until about the early 1990s. In a dot-matrix printer,
there is no metal type at all. Instead, letters are printed by a matrix
(a square or rectangular grid) of several dozen metal needles that press against a
ribbon in different patterns to make whichever letter, number, or other
character is required. Dot-matrix printers produce a characteristic
"dotty" print finish that you still sometimes see on bills, invoices,
and railroad train or movie-theater tickets. They were fast and relatively
inexpensive (to buy and to run), but extremely noisy.
Picture: Using a pattern of dots to create letter shapes is the basic idea of dot-matrix printing. Old-style dot-matrix printers typically did this with a five by seven or seven by seven square (matrix) of metal needles. With smaller needles and more of them, you can make better-looking characters—but the printout still tends to look a bit dotty on close inspection.
What's different about inkjet printing?
Inkjet printers were really an evolution of dot-matrix printers.
Instead of metal needles, they use hundreds of tiny guns to fire
dots of ink at the paper instead. The characters they print are still
made up of dots, just like in a dot-matrix printer, but the dots are so
very tiny that you cannot see them. Different types of inkjet printer
fire the ink in various ways. In Canon printers, the ink is fired by
heating it so it explodes toward the paper in bubbles. This is why
Canon sells its printers under the brand name "Bubble Jet." Epson
printers work a slightly different way. They use an effect called piezoelectricity.
Tiny electric currents controlled by electronic circuits inside the
printer make miniature crystals jiggle back and forth, firing ink in
jets as they do so. You can think of inkjet printers very simply as a
firing squad of nozzles rattling off millions of dots of ink at the
paper every single second!
How inkjet nozzles work
How does the ink get onto the page? It's a slightly different process in a bubble jet and an inkjet...
Bubble jets
In Canon Bubble Jet printers, it goes a bit like this:
Under instructions from your computer, an electronic circuit in the printer figures out which nozzles have to be fired to print
a particular character at a certain point on the page. Hundreds of nozzles are involved in making a single character and
each one is only about a tenth as thick as a human hair!
The circuit activates each of the nozzles by passing an electric current through a small resistor inside it.
When electricity flows through the resistor, it heats up.
Heat from the resistor boils the ink inside the nozzle immediately next to it.
As the ink boils, it forms into a bubble of ink vapor. The bubble expands enormously and bursts.
When the bubble pops, it squirts the ink it contained onto the page in a precisely formed dot.
The collapsing bubble creates a partial vacuum in the nozzle that draws in more ink from the ink tank, ready for
printing the next dot.
Meanwhile the entire print head (light orange) is moving to the side ready to print the next character.
Ink jets
In a piezoelectric inkjet, it's slightly different:
An ink tank (black) supplies the ink dispenser (green) through a narrow tube by capillary action.
A droplet of ink from the tank sits waiting at the very end of the tube.
When the printer circuit (not shown) wants to fire an ink droplet, it energizes two electrical contacts (red) attached
to the piezoelectric crystal.
The energized piezoelectric crystal (dark red) flexes outward (toward the right in this picture).
It squashes against a membrane (dark blue), pushing that toward the right as well.
The membrane pushes against a hole in the ink dispenser (green), increasing the pressure there.
The pressure forces the waiting ink droplet from the tube toward the paper.
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Inside an inkjet
Photo: Here's the print head (sometimes called a print cartridge) removed from
my inkjet and turned upside down, showing the slits where the inkjet nozzles are located. The
single long slit on the right is where black ink comes out.
The three smaller slits on the left are for the three colored inks
that make color prints. Note the pattern of copper connectors on the front that connect the cartridge
to the printer's electronics.
Like dot-matrix printers, inkjets make their print with a pattern of dots. The difference is that where
a dot-matrix uses maybe 64 metal needles to make 64 dots per character, inkjets fire thousands of dots to make much higher print quality. Even an average inkjet can print 600 dots per inch (dpi), which is about ten times better than the crudest dot-matrix. A really good photo-quality inkjet can print at nearly 5000 dpi. Where a dot-matrix printer makes a horrible screaming sound as its print needles tear
across the page, the only noise an inkjet makes comes from the sheets of paper feeding in and out and advancing through the mechanism.
The inkjet nozzles build up a whole page of text or graphics from
millions of separate dots. Controlled by your computer, the ink cartridge scans from left to
right across the page and back again, depositing ink as it goes.
Each time it reaches the end of a line, the paper advances forward slightly so the next line can be printed.
What are the main parts of an inkjet printer?
With the front of my printer open, you can clearly see all the important bits:
Plasticgears driven by an electric stepper motor turn rollers that advance the paper through the printer.
A flexible ribbon cable carries printing instructions from the electronic circuit inside the printer to the moving cartridge. (Inkjet printers contain circuits that translate the instructions from your computer into precise movements of the printhead. The electronic bits and bobs aren't actually visible on this photo.
There's usually a single, large circuit board somewhere in a printer with all the components mounted on it,
including the control switches, LED display lights, and various connections to the printer mechanism and power supply.)
Plastic and rubber rollers pinch the paper tightly so it can be moved through the printer with absolute precision.
A sturdy metal rail guides the printer head as it moves back and forth.
Spiked wheels at the front of the printer help to grip the paper securely and move it precisely.
The print cartridge prints from left to right then reverses the print information and prints backwards from right to left. This is known as bidirectional printing and allows pages to be printed much faster.
Ink is the hidden cost of inkjet printing
Inkjet printers are extremely cheap to buy; even all-in-one printers, with scanning and basic "photocopying" (which involves scanning and then printing) built into the same machine are remarkably inexpensive. Before you marvel at what a great deal you're getting, take a moment to ponder how printer makers can afford to do this: they earn their money not from the printers but from the inkjet cartridges you'll keep feeding into them forever more (the so-called
razor-and-blades business model). Considering that inkjet cartridges are little more than tiny plastic boxes with ink-filled sponges inside, they're very expensive. The printers themselves seem designed to use as much ink as they possibly can with endless nozzle-cleaning maintenance routines (each one of which uses a bit more ink).
Artwork: Printing a modest 20–25 pages a week, you can expect to spend twice as much on ink each year as you spent originally on your computer. So, over 10 years, you'll spend 20 times more on ink than you spent on your printer. Based on real figures for a typical popular printer.
What can you do to make your ink (and money) go further? When you're shopping for a printer, be sure to check detailed consumer reviews. Look out for a model with a refillable ink tank or other ink-saving features; make sure it will accept cheap compatible cartridges to save you buying the manufacturer's expensive originals and (if you don't mind getting your hands dirty from time to time) see if you'll be able
to refill the cartridges yourself using an ink bottle and syringe (YouTube will show you how). Print in draft mode whenever you can (it's faster and uses less ink) and try experimenting with ink-saving fonts. A number of independent testers have concluded that Century Gothic, Calibri, and Times New Roman save a considerable amount of ink compared to Arial, most people's default Microsoft font. You can find more tips for saving money on ink at Consumer Reports.
Artwork: Switching from Arial to Times New Roman uses about a quarter less ink, according to Consumer Reports. But don't get too carried away: saving money on ink won't save money in your company if it takes people longer to read things.
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Don't want to read our articles? Try listening instead
Fundamentals of Inkjet Printing: The Science of Inkjet and Droplets by Stephen D. Hoath (ed). John Wiley & Sons, 2016. Covers the mechanics of how drops form, how printheads work, how drops impact and form characters on the paper, and future directions in inkjet technology).
Inkjet!: History, Technology, Markets, and Applications by Frank J. Romano. Ingram, 2012. A comprehensive survey of inkjets, covering everything from how they work and who invented them to who buys them and their applications beyond simple office printing.
Inkjet Applications by Matt Gilliland and Frank Cloutier. Woodglen Press, 2005. A hobbyist's guide to employing inkjets in robots and other electronic projects.
The Chemistry of Inkjet Inks by Shlomo Magdassi. World Scientific, 2010. Explains how inkjet inks are chemically engineered to improve their permanence, color performance, fluidity (as they shoot through the inkjet nozzles), and surface adhesion to paper and other printing surfaces.
Articles
How can I recycle my inkjet printer? by Rebecca Smithers. The Guardian, April 29, 2018. Despite a few promising tips in this article, there's no easy answer.
When the Sous-Chef Is an Inkjet by David Bernstein. The New York Times, February 3, 2005. How a creative chef prints edible ink with his printer.
Epic take-apart: HP Color LaserJet 2600n: Evil mad Scientist Laboratories, June 2008. Although this article is about dismantling a laserjet, some of the mechanism (the paper feed, for example) is quite similar to what's in an inkjet.
Patents
Although various inventors experimented with inkjets through the second half of the 20th century, inkjet printing technology was only commercialized by Canon in the early 1980s. For the real technical nitty-gritty, patents filed by pioneering inventors are always worth a look. Here's a small selection of the many covering inkjets and bubble-jets:
US Patent 4,045,801: Ink ejection head for printer by Kyuhachiro Iwasaki, Ricoh Company, Ltd., published August 30, 1977. An inkjet that squirts ink using an electrostrictive method (similar to the piezoelectric method).
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