How do printers work?

Back in the dark ages if one wanted to have personally printed material, one would have used a typewriter. Typewriters are those archaic machines you see in museums. All kidding aside, printers have supplanted the use of typewriters, but even printers have undergone an amazing evolution. Not long after the development of the personal computer, dot-matrix printers became the rage. Daisy wheel printers were also around, but they were more expensive.

Basically, dot-matrix printers fell under the 'impact printer' category. This means that tiny metal rods (activated by equally tiny solenoids) smacked against an ink ribbon held against a sheet of paper in order to make a character. Dot-matrix printers were slow, noisy and chewed up ribbons, but they were relatively inexpensive. They were low resolution and not very good for images, but their operational cost was low, only requiring occasional replacement of the ribbon.

A daisy wheel printer was similar except it used a wheel with raised character dies that spun around at high speed. The mechanism smacked the wheel at the precise moment to hit a ribbon held against the paper. It was faster, but more expensive. They suffered from the same things that the dot-matrix printer did.

Today, we have the inkjet printer and the laser printer, and their operations are completely different.

The inkjet printer is the technology that most home based people use because the printer is inexpensive to buy but operational costs are high, especially with heavy use. An inkjet printer deposits tiny droplets of specially formulated inks on paper using a marvelous technology. The most common types of inkjet printers use either a disposable head or a permanent or fixed head. The disposable heads use ink cartridges with the business end of the printer on each cartridge. The head has tiny--and I mean very tiny--microscopic holes, and a thermal method that employs a thin-film resister to heat the ink fast enough to make it shoot out of the hole. All the electronics has to do is send a current pulse to the correct resister to cause a vapor bubble to form that breaks to send a tiny drop of ink to the paper. Some printers use a piezoelectric (a crystal that reacts to voltage) method to shoot the ink out. When a voltage is applied to the piezoelectric unit, it deforms and acts like pump.

The technology that goes into the inks is where this printing method becomes complicated. The inks are the name of the game and their formulations are trade secrets. They consist of dyes or pigments and polymeric materials in a mixture of glycol and water. They also have ingredients that prevent the drop of ink from spreading and make it dry fast. Another consideration is the pigments that are used. They must have light fastness (won't fade) and resistance to water. Another important consideration is how fast the ink in the cartridge ages. This is not easy to do, and I should know because I formulated inkjet printer inks, mostly solvent based.

The other half of this ink jet technology is the special paper that goes with it. The coating that goes on these papers is also a trade secret, especially when it's used as photo paper. The secret here is to formulate the coating that goes on it to achieve quick drying, little or no spreading of the ink drop, and light fastness. I also worked on this part of the process.

The other main printing technology is laser based. This idea arose from the Xerox Corporation's copier technology by substituting the UV image exposure with laser beam raster scanning. In order to make a copy using a Xerox machine, one simply puts a master on a glass plate, pushes a button and a copy spits out. What happens in the machine is rather interesting. A selenium coated cylindrical drum is charged electrostatically using a corona wire. Light exposure causes the charge to be dissipated, so what you end up with is an electrostatic image. Throw toner (carbon black mixed with plastic powder) on the drum and it sticks where there is a charge. Now you have a toner image on the drum. Wrap a paper around the drum and you get the toner transferred to the paper. All you need to do is heat the paper to fuse (melt the plastic) the toner to the paper.

Now, to use this idea with laser printing all you have to do is substitute the light exposure with a laser beam exposure. In this case the laser scans across the drum as it rotates. The resolution is controlled by how small the laser beam is (how well it's focused with lenses) and how much data is fed to the laser as it scans the drum. Raster laser mechanisms use a rotating mirror with many facets to speed up the process.

Color laser printers use three separate drums and toners that contain three different (cyan, magenta and yellow) color pigments. Most color lasers use cartridges with the separate drums and toners whereas a monochrome laser printer uses only one cartridge.

I might add that the interface of printers to computers has changed over the years. Originally, printers were connected by means of a parallel centronics cable. Then, they switched to USB cables. These days many printers connect by WiFi.

Actually, it's amazing how these printers have become very affordable unless you print a lot and have to buy cartridges. That's where the expense is. It's the old razor-blades-are-more-expensive than-the-razor trick.

Thanks for reading.