Ink and print cartridge development for the HP DeskJet 500C/DeskWriter C printer family - includes related articles on color science in cartridge development, meeting safety and regulatory requirements - Technical

Hewlett-Packard Journal, August, 1992 by Craig Maze, Loren E. Johnson, Daniel A. Kearl, James P. Shields

A new trichamber print cartridge allows the low-cost HP DeskJet printer platform to print in color. The ink vehicle, dyes, dye concentrations, and interactions had to be carefully traded off to optimize performance with respect to color bleed, color saturation, composite black production, edge acuity, drying time, and resistance to crusting.

Development of the print cartridge for the HP DeskJet 500C and DeskWriter C printers required a combination of ink chemists, print cartridge architects, and design engineers, along with inputs from the fields of color science and product marketing. This article deals with the design and development of the inks and the print cartridge.

The major objective of the printer development project was to provide a low-cost desktop printer that produces laser-quality black print and also offers color capability to the user. To enable the low-cost HP DeskJet printer platform to print in color, a trichamber color print cartridge was designed that can be exchanged for the black print cartridge when color printing is desired.

Incorporating a trichamber color print cartridge into the existing HP DeskJet print platform posed several challenges for ink chemistry and print cartridge design. Like the ink for the black-only HP DeskJet printer family, the color inks had to work on a wide variety of "plain" papers. Plain paper printing using thermal inkjet technology has been reviewed.1 One important consideration is ink drying time. In addition, when inks of different colors are laid down next to one another on the paper, they must not bleed or diffuse into adjacent regions of different color, a fault commonly referred to as color bleed. A significant level of color bleed is undesirable and causes the border between the two colors to appear ragged and undefined.

Another challenge involved the production of composite black. This term refers to the production of black using the three primary colors (cyan, yellow, and magenta) from the trichamber print cartridge. Composite black results from printing dots of cyan, yellow, and magenta directly on top of each other. This requirement exists because only the black print cartridge or the color print cartridge can reside in the printer stall at any given time. Thus, when the color cartridge is installed, any black print must be produced using composite black. As discussed later, this required some trade-offs between the composite black color characteristics, the color quality of the primary and secondary colors, and ink robustness.

Finally, the greatest challenge was perhaps in the development of an ink that performed well on a wide variety of plain papers. These paper types range from high-quality cotton bond papers to the papers used in high-speed copiers. Finding an ink that met all of the challenges involved delicate trade-offs among objectives that were often in conflict.

Evolution of the Ink Vehicle

Water-based color inks have evolved from formulations designed to print on special paper to those capable of printing on plain paper. Many hundreds of different mixtures were tried before obtaining satisfactory performance over a large variety of papers. In general, inks changed from those containing large amounts of diethylene glycol, about 60%, to those containing different organic solvents at greatly lowered concentrations.

Some of the requirements on the ink are in conflict. For example, drying must be slow in the print cartridge to prevent plugging of orifices and the associated loss of print quality, but rapid on paper to facilitate paper handling and maintain printing speed. Color bleed, if too great, will ruin a print sample (see Fig. 1). It is also one of the most difficult parameters to control by ink formulation alone. Composite black text print quality must be high, which means a dark composite black and high edge acuity. Unfortunately, ink formulas that give improved text print quality tend to have increased amounts of color bleed.

It is difficult to meet all of these requirements, and some compromise is unavoidable. Market surveys were used to separate performance factors that are important to our customers from those that matter least. Samples were printed with known defects on different papers over a range of temperature and humidity conditions and with different inks. These were submitted to potential customers for their evaluation and the results were used to guide ink development. For example, high color saturation was a strong customer requirement, and ink component concentrations were adjusted accordingly.

The first major challenge was to provide a formulation that would not bleed and would deliver saturated colors. Bleed was suppressed by adding surface-active reagents to speed penetration of the ink into the paper. This reduces the time available for dyes to diffuse on the paper and for colors to mix. Unfortunately, penetration into the paper reduces color saturation, so allowances were made to keep color saturation high.