Ink Jets: High Tech in Low Places

Even the least expensive ink jet today is a precision tool. Think about it: Every second, the guts in that flimsy plastic case turn four or more color inks into millions of tiny drops, hurling them at a piece of paper—and that's the least of it.

Individual drops are spit out with such accuracy and masterly coordination that they all land in exactly the spots needed to merge into readable text, compelling graphics, or crisp photos. That sort of precision took Seurat months, not minutes— none of this is particularly easy. The best way to appreciate the complexity is to understand a little about how the technology works.

All ink jet printers use a printhead with nozzles to create ink drops. But there are two fundamentally different kinds of printhead technology: thermal, which most manufacturers use, and piezo, which is unique to Epson products.

The thermal method works on the same principle as a coffee percolator—in fact, HP says the inspiration came from a coffeemaker sitting on an overly caffeinated scientist's desk. In the device that brews your morning elixir, a heater boils water, which forms bubbles of steam. Those pockets of vapor push the water up in spurts though a funnel that's wide at the bottom and narrows to a tube at the top. The water shoots out, filters down, and makes the potion that lets you walk rather than stumble out the door. Get rid of the coffee (after pouring your last cup, of course), lose the top of the pot, and you have the basic design for a drop generator in a printhead—a heater on the bottom, a chamber for ink, and a funnel-shaped tube. Technically, the nozzle is just the hole at the top of the drop generator, but for our purposes, we'll refer to the entire structure in a thermal printhead as a nozzle.

Piezo printheads use a totally different approach, taking advantage of the piezoelectric effect—a characteristic of certain substances, generally crystals such as quartz. Apply pressure to the right material, and it will generate electricity. Conversely, create a voltage difference across your tiny chunk of matter, and the shape changes. Put a thin layer of such material on one side of an ink chamber to form a diaphragm, put a nozzle on the other side, and you've got the basic design for a piezo drop generator. When you apply voltage to the diaphragm, it distorts suddenly. And just as would happen if you were to briefly and slightly squeeze the bulb of a liquid-filled eye dropper, a drop of liquid—ink in this case—fires out the nozzle.

Even the earliest ink jets, built on these basic principles, were fairly impressive accomplishments. The printhead in HP's original ThinkJet (back in the dim realm known only as 1985) had just 12 nozzles and printed at 96 dots per inch (dpi), but it spit out a total of 14,400 drops per second.

By 1999, the company had printheads with 512 nozzles that managed 600 dpi and dispensed 6,144,000 drops per second. These aren't your ordinary little globs, though. Each generator in a thermal printhead heats for 2 millionths of a second (with a heating rate of 100 million degrees centigrade per second), forming a steam bubble that fires a single shot of ink 1 ten-thousandth of a millimeter thick at 50 kilometers per hour. Numbers for other specifications go higher still.

To take an extreme example, the printhead in the Canon iP8500 contains 6,144 nozzles, each of which is able to produce 24,000 drops per second. This means the printer has a theoretical maximum of 147,456,000 drops per second (possibly McDonald's burgers-per-minute rate, too). When you have potentially millions of drops whizzing toward the page at high speed in any given second, controlling where they land is obviously a major achievement.

The feat requires an almost unimaginable degree of exactness in the placement of nozzles, the timing of their firing, the physics of creating a well-formed drop and aiming it accurately, and more. All of this may be a bit hard to believe, but not only do ink jets operate within unimaginable tolerances, they do it with more precision than ever before. Over the same time span, the number of nozzles and dots per second has risen so dramatically that drop size has shrunk from 180 picoliters in the HP ThinkJet to as little as 1 picoliter in today's Canon Pixma iP5000. That's one millionth of a millionth of a liter—the same relationship to a liter as one second has to roughly 31,688 years (taking leap years into account, of course).

Smaller drops result in smaller dots on the page, of course—and that's responsible for the largely realized goal of fine control over how dots join to form an image. Consider, for example, that almost any ink jet today, using just four, six, or eight colors, can create what looks like continuous-tone photos. That trick is accomplished, for the most part, by putting the right mix of colors in just the right spots on the page. And that takes highly sophisticated technology—the sort you'll find in just about any old ink jet.

Copyright © 2005 Ziff Davis Media Inc. All Rights Reserved. Originally appearing in ExtremeTech.