Mechanical design of a color graphics printer

Hewlett-Packard Journal, August, 1988 by Chuong Cam Ta, Lawrence W. Chan, P. Jeffrey Wield, Ruben Nevarez

An important reason for visual feedback to tell the user when priming is finished is that the print cartridge will withstand only a limited number of primes. The more the pen is primed and the higher the priming pressure, the larger is the likelihood that the cartridge will develop air paths through the foam that holds the ink. The pumped air pushes the ink in the path of least resistance. In time, a path can be created in which there is no ink, giving the air a path to the nozzle plate. However, if the cartridge is not primed for very long and the ink is allowed to reach a state of equilibrium, the chances of creating an air path are diminished. Once an air path is established, the pen cannot be primed any longer. Visual feedback helps the user see when priming is completed, reducing the tendency to prime longer than necessary.

The higher the priming pressure, the less time is required to prime the cartridge and the easier it is to dislodge air bubbles around the nozzles. The lower the pressure, the more times the cartridge can be primed without creating an air path. The pressure had to be carefully chosen to give high reliability and long life with the user in control of the priming. The cartridge design team also made improvements to the foam insertion procedure, which lessened the likelihood of air paths forming and caused a marked improvement in the robustness of the cartridge.

Another area of concern in the primer was the absorber pad which wicks the ink away from the nozzle plate. The concern was that the absorber pad material would leave some contaminants on the substrate pads or nozzles. The absorber pad needs to have a very fast absorption rate, so that when the color pen is primed, the three different inks are absorbed into the pad and do not mix with each other and contaminate the primary colors. The pad's capacity has to be sufficient to hold ink for approximately four consecutive primes. The logical choices are fibrous materials. Unfortunately, most fibrous materials tend to lose fibers, especially as they become wet. After a large number of different materials were tried, a treated polyester was found that left an acceptably small number of fibers on the cartridge.

Acknowledgments

Robert Beauchamp was responsible for a number of design contributions in the primer, and Barry Mauerman contributed to its simplification. Henry Flournoy's use of digital signal analysis proved to be a valuable diagnostic tool for the media drive. David Ellement's analysis and implementation of the step motor's acceleration profiles resulted in a significant product improvement. Curt Torgerson and Frank Nasworthy's relentless pursuit of high-quality plastic gears has made a definite contribution to plastics technology with patents pending. Special thanks to Todd Russell, who devoted his time to developing a free-body-diagram computer model of the print cartridge body. The model contributed to the success of the cartridge latching design.