Design and development of a color thermal inkjet print cartridge - includes related articles on capillary forces in a foam matrix and print quality and pen development

Hewlett-Packard Journal, August, 1988 by Jeffrey P. Baker, David A. Johnson, Vyomesh Joshi, Stephen J. Nigro

Design and Development of a Color Thermal Inkjet Print Cartridge

THE USE OF COLOR MONITORS in business and engineering applications of personal computers and engineering workstations is growing rapidly. Software application packages are available to create color text, graphics, and images on computer displays, often for the ultimate purpose of producing color hard-copy versions of the computer screens for various office and technical applications.

The selection of a technology to generate a color hard copy hinges on the user's needs. The major customer requirements are print quality, speed, available colors, low cost, reliable hardware, multiple media, and excellent software support. Thermal inkjet printing has several advantages compared to other technologies to meet these needs in a cost-effective way. This technology uses heat energy to vaporize a small amount of ink to expel a drop through an orifice. The first product implementation of this concept was the monochrome HP ThinkJet printer introduced in 1984.

The HP Paintjet Color Graphics Printer (Fig. 1) uses thermal inkjet technology to produce vivid color graphics for overhead transparencies and reports. Intended for personal computer applications, it produced text and graphics with 180-dots-per-inch resolution and near-letter-quality (NLQ) text at a speed of 167 characters per second. It can produce a typical page of text in about 35 seconds, merge text and graphics (see Fig. 2), and print a full page of color graphics in about four minutes (Fig. 3).

The PaintJet printer uses two disposable print cartridges, which contain all of the elements necessary to produce a dot on the media. The black printhead has thirty nozzles and the color printhead has cyan, magenta, and yellow inks with ten nozzles per color. The printheads have built-in liquid ink supplies and drop propulsion systems. This technology works on the principle of drop on demand. Each nozzle can supply a drop of ink on demand from the printer as the printhead scans across the media. The drops are ejected by electrical heating of a thin-film resistor to vaporizes a small amount of ink.

The PaintJet print cartridge has the following specifications:

* Resolution: 180 dots per inch

* Drop volume: 100 picoliters

* Drop velocity: 12 meters per second

* Maximum drop rate: 3 kHz

* Energy per drop: 17 microjoules

* Typical drive voltage: 11.5 volts

* Typical pulse width: 5 microseconds.

Thermal Inkjet Advantages

This technology has several key advantages in meeting users' needs for a color printer.

Print Quality. Because this technology uses conventional photolithography and thin-film processing to fabricate the printhead, higher resolutions can be achieved. This concept also helps tremendously in designing the color printhead with its multiple chambers. Remarkable color-to-color alignment is achieved, resulting in high-quality color graphics and images.

Reliability. Before disposable thermal inkjet technology, inkjet printing had always been plagued by nozzle clogs and other reliability issues. The disposable concept made it possible to develop an extremely reliable system and move away from pumps and hoses. However, to maintain an acceptable cost per color hard-copy page, the disposable printhead must be inexpensive. It must also be producible in high volume, and this requires a mature and stable manufacturing process to keep yields up and fabrication costs down. Consequently, the process engineers have had to understand fully all of the details of production and perfect the technology. The result is a cost-effective and reliable printing system solution.

Speed. The printhead architecture and fluid geometrics are designed to the printer can operate at high speeds (167 characters per second) while producing exceptional text and color graphics.

Multiple Media. Customers want to produce the hard copies on both paper and overhead transparencies. Thermal inkjet technology can print on both types of media with the same printhead.

The remainder of this article deals with three aspects of the development of the print cartridge: first, the design of the drop generator portion of the cartridge, second, the design of the ink reservoir, and third, the testing of the combined system to ensure reliable operation.

Drop Generator Design

The architecture of the nozzle/heater region is shown in Fig. 4. This design is the result of trade-offs in manufacturablity and performance issues.

A design goal for the PaintJet printer was to bridge the gap between plotters and printers. To accomplish this, the product needed to have eight primary colors with greater than a thousand dithered colors, resolution greater than 150 dots per inch, near-letter-quality text, and text speed greater than 150 characters per second. Early in the printhead development, the trade-offs among resolution, speed, reliability, and dot quality were made to meet these goals.

Drop Volume Selection

A key decision was the selection of drop volume, since this influences the media and printhead designs. Printheads with drop volumes above 140 pl did not refill quickly enough to meet the goal of 150 characters per second. A printhead with drop volume below 60 pl was deemed difficult to manufacture.