Inkjet printers
Last updated: December 14, 2006.
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 these are inkjet printers. But how exactly do they
work?
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 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".
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 then printed out
onto paper.
Smarter printing
Early computer printers borrow 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 relatively recently. In a dot-matrix printer,
there is no metal type at all. Instead, letters are printed by an array
(square pattern) of 35 or more 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 and
invoices.
Picture (right): Using dots to make larger shapes
is the basic idea
of dot-matrix printing. This design is known as the Hacker Emblem.
How inkjets work
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!
Printing dot-to-dot
Inkjet printers can make any letter, number, or other character from a
pattern of tiny dots. Old-style dot-matrix printers did this too with a
five by seven
square of metal needles. With 64 dots (eight by eight), you can make
virtually
any character—but things tend to look a bit dotty:
Inkjet printers fire thousands of dots to make much better 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.
In my printer, the ink comes out of nozzles in a slit on the
underside
of the print cartridge.
With the printer's front cover open, you can see the print cartridge
sitting inside:
Now here's the cartridge removed from the printer and turned upside
down, showing the slits where the inkjet nozzles are located. The
single long slit on the left is where black ink comes out.
The three smaller slits on the right are for the three colored inks
that make
color prints.
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.
With the front of my printer open, you can clearly
see all the important bits:
- Gear wheels driven by electric motor turn rollers that advance
paper through the printer.
- Flexible cable carries printing instructions from electronic
circuit inside printer to moving cartridge.
- Plastic and rubber rollers trap the paper tightly so it can be
moved
through the printer with absolute precision.
- Sturdy metal rail guides printer cartridge as it moves back and
forth.
- Spiked wheels at front of printer help to grip paper securely and
move it precisely.
- Print cartridge moves from side to side.
