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Inkjet printers

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by Chris Woodford. Last updated: July 29, 2017.

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 portable Canon inkjet printer from the late 1990s. This one also works as a scanner if you replace the print head with a scanner cartridge.

Printing with metal type

Type hammers in a typewriter

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.

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".

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.

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.

Example of dot-matrix computer printout on an old railroad ticket.

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.

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:

Artwork showing how the ink nozzles in a thermal Bubble Jet printer squirt ink onto the page.

  1. 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!
  2. The circuit activates each of the nozzles by passing an electric current through a small resistor inside it.
  3. When electricity flows through the resistor, it heats up.
  4. Heat from the resistor boils the ink inside the nozzle immediately next to it.
  5. As the ink boils, it forms into a bubble of ink vapor. The bubble expands enormously and bursts.
  6. When the bubble pops, it squirts the ink it contained onto the page in a precisely formed dot.
  7. 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.
  8. 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:

Artwork showing how the ink nozzles in a piezoelectric inkjet printer squirt ink onto the page.

  1. An ink tank (black) supplies the ink dispenser (green) through a narrow tube by capillary action.
  2. A droplet of ink from the tank sits waiting at the very end of the tube.
  3. When the printer circuit (not shown) wants to fire an ink droplet, it energizes two electrical contacts (red) attached to the piezoelectric crystal.
  4. The energized piezoelectric crystal (dark red) flexes outward (toward the right in this picture).
  5. It squashes against a membrane (dark blue), pushing that toward the right as well.
  6. The membrane pushes against a hole in the ink dispenser (green), increasing the pressure there.
  7. The pressure forces the waiting ink droplet from the tube toward the paper.

Inkjets in action

Inkjet printer nozzles

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.

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:

Inside an inkjet printer

  1. Gears driven by an electric stepper motor turn rollers that advance the paper through the printer.
  2. 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.)
  3. Plastic and rubber rollers pinch the paper tightly so it can be moved through the printer with absolute precision.
  4. A sturdy metal rail guides the printer head as it moves back and forth.
  5. Spiked wheels at the front of the printer help to grip the paper securely and move it precisely.
  6. 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.
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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:

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Text copyright © Chris Woodford 2006, 2016. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2006/2016) Inkjet printers. Retrieved from http://www.explainthatstuff.com/inkjetprinters.html. [Accessed (Insert date here)]

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