Ink and media development for the HP PaintJet Printer - systemic development necessary due to complex interaction between elements

Hewlett-Packard Journal, August, 1988 by Donald J. Palmer, John Stoffel, Ronald J. Selensky, Peter C. Morris, M. Beth Heffernan, Mark S. Hickman

Ink and Media Development for the HP PaintJet Printer

THE DESIGN OF INK, paper and overhead transparency film for the PaintJet Color Graphics Printer required substantial interaction between the ink, media, print cartridge, and product teams. Throughout the program, design issues were approached from a system perspective so that the best overall performance and reliability could be achieved.

Ink Design

The PaintJet printer combines black, magenta, yellow, and cyan ink drops in a 2 X 2-pixel cell to generate a palette of 330 different colors. The inks are composed of a solvent carrier (vehicle) and a colorant (dye). The vehicle functions to provide the essential thermodynamic, kinetic, and fluid properties required to generate a superheated vapor bubble and eject a drop. Additionally, the vehicle acts as a carrier for the dye, bonding it onto the surface media (paper or film) with the necessary spot diameter and permanence to meet user needs for print quality.

The vehicle is a combination of water and hydroxylated alkyl ethers and the dyes are organic compounds that have been solubilized in the vehicle using sulfonates and monovalent species such as Li.sup. and Na.sup. , or cationic organic amines.

The inks have many chemical and physical requirements. They must withstand changes in pH, suppress bacterial and fungal growth, resist decomposition, and have chemical compatibility with the material set used in the print cartridge. Additionally, the ink must exhibit little change in its physical properties as a result of evaporation of the vehicle solvent, which can affect viscocity, wettability, and surface tension. This is important to maintain drops that are ejected with controllable volumes, velocities, and shapes.

During the firing of a drop, the layer of ink covering the surface of the heating element can reach a temperature of about 340[deg.]C. At this temperature, the decomposition of ink can deposit residue on the surface of the heating element, a process known as kogation. Kogation affects the volume, shape, and velocity of the ejected drop, causing the quality of the printed output to vary. Consequently, it is essential to design an ink that resists such decomposition over the useful life of the print cartridge.

The residue from kogation was found to be largely carbonaceous with varying amounts of dye and inorganic salts included in the carbon matrix. In the PaintJet printer, kogation was eliminately by careful selection of vehicle solvents, dyes, and electrical drive characteristics.

Impurities introduced into the ink from the vehicle components, dyes, or other additives can also cause kogation. Therefore, all of the constituents used in the PaintJet ink are purified.

Prevention of Clogging

Traditionally, clogging of the pen has been the biggest complaint of the inkjet printer user. With closer inspection, the types of clogging can be divided into two categories. First, a soft plug can occur, causing the initial droplets to be missing. Second, a hard plug can form, requiring the customers to clear an obstructed nozzle.

The root of the clogging problem lies in the exposure of the ink to air. In a roller-ball or felt-tip pen, the ink is protected against exposure to air by a tight cap. A tight cap could not be used in the PaintJet printer because of the position of the interconnect and the potential problem of forcing air into the nozzle. Therefore, the ink is exposed to air and the water component of the vehicle does evaporate. Consequently, the concentration of dye and cosolvent (nonvolatile component of the vehicle) can increase dramatically at the ink/air interface.

This rise in dye and cosolvent concentration causes the customer to experience clogging by two different mechanisms. First, the viscosity increase in the orifice, caused by increased dye and cosolvent concentration, can prevent the ink droplet from being ejected. This soft plug can usually be cleared by repeated firing of the pen, but the initial characters printed would be misformed or missing (Fig. 1). This condition is especially prevalent under cold and low-humidity conditions. Evaporation of water from the orifice can also cause a hard plug to develop at the ink/air interface and prevent the ink droplet from being ejected. This hard plug (crusting) forms as the dye crystallizes out of solution because of the increased dye concentration and the change in the vehicle composition (less water and more cosolvent) at the ink/air interface (see Figs. 2 and 3).

In the development of the PaintJet inks, a hard plugging problem was experienced with the black ink. Lowering the dye concentration would solve this problem, but he printed image would suffer a loss in optical density. Increasing the initial cosolvent concentration would reduce the increase in dye concentration near the orifice, but would increase the viscosity of the ink, which could actually increase the amount of viscous plugging, force a design change in the pen, or create problems in media development. Still another solution to the crusting problem would be to increase the solubility black dye. Increased solubility would keep the dye in solution as its concentration increased as a result of water evaporation.