Progress and Trends in Ink-jet Printing Technology --The Society for Imaging Science and Technology

Journal of Imaging Science and Technology · Volume 42, Number 1, January/February 1998

Ink-jet is a non-impact dot-matrix printing technology in which droplets of ink are jetted from a small aperture directly to a specified position on a media to create an image. The mechanism by which a liquid stream breaks up into droplets was described¹ by Lord Rayleigh in 1878. In 1951, Elmqvist of Seimens patented the first practical Rayleigh break-up ink-jet device.² This invention led to the introduction of the Mingograph, one of the first commercial ink-jet chart recorders for analog voltage signals. In the early 1960s, Dr. Sweet of Stanford University demonstrated that by applying a pressure wave pattern to an orifice, the ink stream could be broken into droplets of uniform size and spacing.³ When the drop break-off mechanism was controlled, an electric charge could be impressed on the drops selectively and reliably as they formed out of the continuous ink stream. The charged drops when passing through the electric field were deflected into a gutter for recirculation, and those uncharged drops could fly directly onto the media to form an image.⁴ This printing process is known as a continuous ink-jet. By the late 1960s, Sweet's inventions led to the introductions of A. B. Dick VideoJet and the Mead DIJIT products. In the 1970s, IBM licensed the technology and launched a massive development program to adapt continuous ink-jet technology for their computer printers. The IBM 4640 ink-jet printer was introduced in 1976 as a word processing hardcopy-output peripheral application.⁵

At approximately the same time, Professor Hertz of the Lund Institute of Technology in Sweden and his associates independently developed several continuous ink-jet techniques that had the ability to modulate the ink-flow characteristics for gray-scale ink-jet printing. One of Professor Hertz's methods of obtaining gray-scale printing was to control the number of drops deposited in each pixel.⁶ By varying the number of drops laid down, the amount of ink volume in each pixel was controlled, therefore the density in each color was adjusted to create the gray tone desired. This method was licensed to companies such as Iris Graphics and Stork to produce commercial high-quality color images for the computer prepress color hardcopy market.⁷

While continuous ink-jet development was intense, the development of a drop-on-demand ink-jet method was also popularized. A drop-on-demand device ejects ink droplets only when they are used in imaging on the media. This approach eliminates the complexity of drop charging and deflection hardware as well as the inherent unreliability of the ink recirculation systems required for the continuous ink-jet technology.

Zoltan⁸ and Kyser and Sears⁹ are among the pioneer inventors of the drop-on-demand ink-jet systems. Their inventions were used in the Seimens PT-80 serial character printer (1977) and by Silonics (1978). In these printers, on the application of voltage pulses, ink drops are ejected by a pressure wave created by the mechanical motion of the piezoelectric ceramic.

Many of the drop-on-demand ink-jet ideas and systems were invented, developed, and produced commercially in the 1970s and 1980s. The simplicity of the drop-on-demand ink-jet system was supposed to make ink-jet technology more reliable. However, during this period, the reliability of ink-jet technology remained poor. Problems such as nozzle clogging and inconsistency in image quality plagued the technology.

In 1979, Endo and Hara of Canon invented a drop-on-demand ink-jet method where ink drops were ejected from the nozzle by the growth and collapse of a water vapor bubble on the top surface of a small heater located near the nozzle.¹⁰ Canon called the technology the bubble jet. The simple design of a bubble jet printhead along with its semiconductor compatible fabrication process allowed printheads to be built at low cost and with high nozzle packing density. Apparently, during the same time period or shortly thereafter, Hewlett-Packard independently developed a similar ink-jet technology.¹¹

In 1984, Hewlett-Packard commercialized the ThinkJet printer. It was the first successful low-cost ink-jet printer based on the bubble jet principle. Hewlett-Packard named the technology thermal ink-jet. The cost of a ThinkJet printhead consisting of 12 nozzles was low enough that the printhead could be replaced every time the ink cartridge was empty. Hewlett-Packard's concept of a disposable ink-jet printhead was brilliant and original. They solved the reliability problem of ink-jet technology by throwing away the printhead at the end of its useful life. Since then, Hewlett-Packard and Canon have continuously improved on the technology. Their efforts were rewarded with a series of successful product introductions. Ink-jet printer models with higher printing resolution and color capability were made available with very affordable prices. Since the late 1980s, because of their low cost, small size, quietness, and particularly their color capability, the thermal ink-jet or bubble jet printers became the viable alternative to impact dot-matrix printers for home users and small businesses. Currently, thermal ink-jet printers dominate the low-end color printer market.

Throughout the course of ink-jet development, ink chemists and media engineers realized that when a liquid ink droplet contacts the surface of paper, it tends to spread along paper fiber lines as well as penetrate into paper sizing and voids. The spreading of ink droplets is often too excessive and too irregular to maintain the resolution required. The penetration of ink into the paper is often too slow to absorb multiple ink drops on the same spot within very short time intervals. The poor color image quality due to ink spreading and intercolor bleeding is recognized as the critical issue in the development of ink-jet technology.

To obtain a high-quality color ink-jet image, the surface of the media requires a special coating. The special ink-jet-coated media must balance between many design parameters such as drop volume, evaporation rate, penetration rate, coating thickness, porosity, etc. Development activi ties in ink-jet media were started in the early 1980s, predominantly in Japan with paper companies such as Jujo Paper and Mitsubishi Paper Mills leading the industry. Today, because of the popularity of color ink-jet printers, the market demand for better media such as ink-jet glossy and photomedia is more significant. This has attracted a number of companies to ink-jet-media development. Canon, Xerox, Asahi Glass, Arkwright, Folex, 3M and Imation are among the many companies currently active in this field.

Another approach to obtaining better image quality without relying on special media is the use of solid ink (or hot melt or phase-change ink). In operation, the ink is jetting as molten liquid drops. On contact with the media, the ink material solidifies, very little spreading and absorption occurs so that brilliant color and high resolution can be realized almost independent of the substrate properties. The early development of solid ink was initiated at Teletype for electrostatic ink-jet devices.¹² The later application to drop-on-demand devices occurred at Exxon¹³ and Howtek.¹⁴ Today, Tektronix, Dataproducts, Spectra, and Brother are among active companies pursuing solid ink-jet technology.

For more details of the ink-jet printing development paths, there are at least four excellent reviews of ink-jet printing in the past literature.^15–18

Progress and Trends in Ink-jet Printing Technology

Part 1

Hue P. Le*

Le Technologies, Inc., Beaverton, Oregon

Ink-jet Printing Development Path