Low-cost plain-paper color inkjet printing - design of HP's new DeskWriter C and DeskJet 500C printers - includes related article on how thermal ink-jet technology works - Technical

Hewlett-Packard Journal,  August, 1992  by Daniel A. Kearl,  Michael S. Ard

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The HP DeskWriter C and DeskJet 500C printers are based on advanced thermal inkjet technology in the form of a 300-dpi three-color inkjet print cartridge. The printers and software drivers that use this cartridge were developed on an aggressive one-year schedule.

The use of color in written communications has revolutionized the printed-output industry. The ability of vivid colors to draw attention and stimulate retention is immense. High-density color monitors and high-quality color applications for personal computers have reset customer expectations. However, an affordable, plain-paper color printing solution has not been available.

The introduction of the HP DeskWriter C printer in the summer of 1991, followed by the HP DeskJet 500C printer, set new standards for low-cost color printing[.sup.1,2] At the heart of these printing solutions is HP's three-color 300-dpi thermal inkjet print cartridge. This print cartridge provides the printer with the ability to deliver high-quality 300-dpi color images and graphics on a wide variety of "plain" office papers. This 300-dpi plain-paper color capability represents a major increase in price/performance for personal printers. It is a good example of a technology-enabled performance increase. Fig. 1 shows the HP DeskWriter C and DeskJet 500C printers and Fig. 2 shows examples of their output.

Color Print Cartridge

The color print cartridge for the HP DeskJet 500C and DeskWriter C printers is another extension of HP's thermal inkjet printing technology (see "Thermal Inkjet Review...," page 67 and references 3, 4, and 5). This technology was first made available in 1984 with the introduction of the HP ThinkJet line of printers. These printers initially used a black print cartridge that had 96-dpi resolution and required special paper. Later enhancements brought plain-paper and single-color printing to the personal printer user. In 1987 HP introduced the HP PaintJet series of printers. These printers provided a fully integrated color printing capability on special paper at 180-dpi resolution. The price and performance of the HP PaintJet printers represented a significant advance at the time of their introduction. The HP DeskJet line of printers was introduced in 1988. These printers offered high-quality 300-dpi black printing on a wide variety of office paper types. As noted above, the color-capable versions of the DeskJet family were introduced in the latter part of 1991, the HP DeskWriter C printer for Macintosh computers and the HP DeskJet 500C for the PC. Print cartridge technology development has played a critical role in the successful introduction of each of these generations of thermal inkjet printers.

Reduced to the simplest terms, the HP DeskJet 500C/DeskWriter C color print cartridge is a compact, low-cost, high-resolution color dot generator. The printer dictates to the print cartridge when and where to deliver the color dots. This particular cartridge delivers three different colors of dots: cyan, yellow and magneta. These colors are known as subtractive primary colors. The size, shape, and optical properties of the dots produced should ideally be independent of the media and the printing environment. These dots should be delivered to the media at a very high rate of speed with a positional accuracy commensurate with the high resolution of the device. All of the dot generator properties should remain constant over the storage and printing life of the print cartridge. These relatively simple color dot generator performance goals represented a major development challenge for the HP Inkjet Components Division.

In outward appearance, the color print cartridge for the HP DeskJet 500C and DeskWriter C printers is very similar to the original HP DeskJet black print cartridge (Fig. 3). The plastic body of the print cartridge has been enlarged some-what to provide room for the three individual ink reservoirs. Electrical interconnection to the printer is accomplished using the same flex circuit technology as the black print cartridge. The location of the pressure interconnection pads is identical to those on the black print cartridge. Provisions have been made in the printer drive electronics to sense which cartridge has been installed and respond with the appropriate drive signals.

In general, the thermal inkjet drop generator portion of the color print cartridge for the HP DeskJet 500C and Desk-Writer C represents a natural extension of the existing HP DeskJet black print cartridge technology. Careful inspection of the gold-plated nickel nozzle assembly shows that the nozzles have been arranged into three groups or primitives, one for each of the subtractive primary colors (Fig. 4). The three primitives all share a common silicon thin-film substrate and circuitry. This thin-film substrate is fabricated using processes very similar to those used for the HP DeskJet black and HP PaintJet print cartridges[.sup.3,4] The primitives are staggered with respect to one another to provide separation for the ink delivery channels on the black side of the substrate. Each of the primitives consists of two columns of eight nozzles each. The nozzles within a given column are spaced on 1/150-inch centers. Each column in a given primitive is offset with respect to its neighbor by 1/300 inch, so that the combination of the two nozzle columns results in an array of 16 nozzles with a vertical resolution of 1/300 inch.