Foundations of printer technology lie in electrostatics

Vijay Jayaram Sep 11, 2011

Invented by German goldsmith Johannes Gutenberg in 1440, the Gutenberg printing press utilized pressure and movable type to reproduce text. The iron press was invented three and a half centuries later, allowing printing to be done with steam power rather than manually. Only later in the 20th century, with the advent of phototypesetting (projecting text onto photographic paper), was the process no longer entirely mechanical.

Now, in the 21st century, students can print at five cents a page with the press of a button. Given the ubiquitous nature of modern printers and the astounding science behind them, there’s an awful lot that we may take for granted; it’s important to understand what’s really going on.

To begin, there are two common printing methods: ink-based and laser-based. As the older of the two, the inkjet method is a trademark of many printing giants, such as Canon, HP, and Lexmark. These types of printers use what is called the thermal bubble process, in which tiny chambers inside the ink cartridge are heated and electrified, causing the ink to vaporize and greatly increase pressure. According to Discovery.com, this effectively shoots a 70-micron (1 micron = 0.001 mm) droplet out of the chamber and sucks in ink from a reservoir to replace it.

Michael Richmond, a physics professor at the Rochester Institute of Technology, often uses the next step in the process to teach students about electrostatics. According to Richmond’s lecture website, the droplet of ink shot out by the cartridge is charged by what he calls an “electron gun,” or a fluctuating flow of electrons. Before the ink hits the paper, it travels at a speed of about 800 inches per second through a capacitor, consisting of two parallel plates of equal and opposite charge that generate a uniform electric field. Depending on the charge of the droplet, this electric field will deflect its path a certain amount. The greater its charge, the more it is deflected.

In some cases, when the ink needs to go straight onto the paper, it will be given no charge by the electron gun. Discovery.com explains this process, noting that a good inkjet printer shoots out over 1 million droplets per square inch, so even if there are slight deviations in deflection, the resolution of the page will be remarkably clear.

Laser printing, however, is far more precise than its less expensive counterpart. The crux of this technology is photoconductivity, or the ability of a material to be charged (or discharged) by exposure to light. In the center of the printing setup is a photoconductive roller, which is initially given a positive electric charge. When the user presses the print button, the printer’s laser initializes and begins tracing out the letters onto the roller, causing only those letter-shaped areas of the drum to discharge and effectively creating an initially invisible electromagnetic image.

Next, positively charged toner powder, a mixture of pigment and plastic, is sprinkled onto the roller. Since like charges repel, the toner avoids the positive areas of the roller and falls precisely onto the neutral spaces traced out by the laser, forming the letters. Paper entering the printer is given a negative electric charge so that when it slides under the roller, it attracts the positively charged toner onto itself. Finally, the paper passes through the fuser, a set of heating rollers that fuse the toner with the paper fibers. This fusion between the toner and paper fibers is why laser-printing jobs come out hot and are impossible to smudge.

While the printer may not be the zenith of modern science, it is astounding to see how commonplace such a technology has become. Without an understanding of their inner workings, we may take these devices for granted. So next time you press the print button, take a moment to think about the incredible chain of events you just set off.