Rethinking the Inkjet Printer

By applying Micro Electro-Mechanical Systems (MEMS) technology to the design of inkjet printers, recent graduate Fan-Gang "Kevin" Tseng has redesigned the injection nozzle to achieve exceptional resolution.

Tseng first used high-speed video to capture the split-second nuances of inkjet printing. This revealed the existence of what has come to be known as "satellite droplets." These are unwanted drops of ink that spin off from the main droplets causing a haze or blur.

After discovering the presence of these satellite droplets, Tseng's first challenge was to determine what was causing them. Armed with that information, he then set out to devise a method of preventing satellite droplets from forming.

The thermal, bubble jet printer head hasn't changed much since its invention 20 years ago. The principal is simple: ink fills a chamber in the printer head. Then the ink is heated until a bubble forms. The bubble forces the ink from the chamber - and presumably onto the paper - then collapses. Close up and in slow motion, Tseng describes the phenomenon as looking "like a bomb going off!"

Because the device is so small, it can be refilled every 10 to 100 microseconds. Satellite droplets are formed when the "tails" of elongated droplets break off and strike the paper at unintended locations.

Tseng knew that if he could shorten the length of the drops, the surface tension of the main drop would be strong enough to pull back the short tail, preventing it from breaking off. However it is very difficult to make a mechanical part move fast enough to cut the tail off a droplet. Instead of using a mechanical device, Tseng used the bubble itself to cut the tail off the droplet. Through careful chamber and heater design, the bubbles encircling the micro nozzle merge into one capable of efficiently severing the long droplet tail within 30 microseconds. This new design successfully solves the long-lasting problem of satellite droplets in the inkjet industry.

To decrease the time it took to refill the reservoir, Tseng shortened the length of the neck. But doing so caused two other problems: "overfilling" and "crosstalk." Because the chamber was being refilled so rapidly, extra droplets were often ejected. Tseng also discovered that the longer neck acted as a buffer between adjoining chambers. When the neck was shortened, the action of one chamber often affected neighboring chambers.

What Tseng had discovered was that the presence of the neck - which slowed the frequency of the device by hampering the filling process - was an absolute necessity during the firing sequence to prevent crosstalk. If only there were a way to detach the neck during filling then re-attach it just before the inkjet fired. What was needed was a "removable chamber neck," Tseng said.

The novel method Tseng came up with for solving this problem was to use a second bubble to create a "virtual chamber neck." The purpose of the second bubble is to first partition off a section of the print head during the filling sequence. This allowed him to control the inflow of ink and prevent crosstalk. Once the firing sequence begins, the second bubble is also used to shut off the outflow. After the droplet is ejected, the two bubbles collapse, which allows the liquid to refill quickly. As a result, the cross-talk and overfill phenomena are much reduced without sacrificing speed.

Both the first and second bubbles are created by a heating element which encircles the nozzle. The element is narrower on one side, causing that side to heat up slower. This, in turn, produces a second bubble, later in the sequence.

All of these innovations are possible because Tseng's printer head is fabricated from a single block of silicon, using MEMS technology. Conventional inkjet nozzles are manufactured from several parts and bonded together, Tseng said. But by etching it out of silicon, he can produce nozzle resolutions as high as 1200 dots per inch (DPI). (Although "print" resolutions in excess of 1200 DPI are obtainable by using micro-movement of printer heads, Tseng said, the nozzle resolutions do not exceed 300 DPI.)

This method of fabrication also prevents misalignment problems observed in previous designs. "If you have a little misalignment," Tseng said, "the droplet will not have a vertical direction."

Over the past 10 years, the printer industry has hired more than 200 engineers to work on the thermal bubble jet system, according to Tseng. "But none of them could solve this problem. We are the first," said Tseng, who holds a patent on the device he designed while obtaining his Ph.D.

Tseng, who has already discussed his work with Xerox, Canon, Samsung and Acer, said he strayed into the inkjet technology area after first working on fuel injector problems.