Mechanical design of a color graphics printer

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

When the user pushes the primer lid down, the cartridge is pushed against a rubber bladder, which causes a seal to form. As the lid is pushed farther down, the bladder is compressed, forcing air into the cartridge and pushing the ink out towards the nozzles. Air in front of the ink in the nozzles is pushed out. Once the ink reaches the nozzles, the capillary force holds the ink in the nozzles.

In designing the primer, the objectives were to minimize the cost and to make the primer reliable and user-friendly.

To meet the objectives, a number of cost reduction approaches were taken. For example, most of the parts are made of plastic, which allows integration of many features into a single part, thereby reducing the number of parts. Plastic also makes it possible to snap fit the parts together, simplifying the assembly process. The primer lid is a good example. It holds the cartridge for priming, holds the absorber pad that captures the ink that is ejected, holds the chamois that draws ink away from the absorber pad, covers the primer unit when not in use, and snaps into the primer unit.

Having the user do the priming eliminates the need for hardware and software to run the primer. As a result, the challenge in the design was to make the primer reliable and friendly while maintaining user control of the priming.

To make the primer user-friendly, the first step was to develop a single procedure for the user to use whenever there is a printing problem and when the cartridge is first used. The procedure needed to be simple so the user would have a short learning curve and remember it easily.

User Tests

Beginning with the first prototype, and continuing with each successive iteration, a user test was conducted to determine the friendliness of the primer. The test results were analyzed and used to design the next iteration. The tests were conducted by non-R&D personnel and videotaped. In each test, nontechnical people unfamiliar with the PaintJet printer were asked to unpack a print cartridge and follow a set of directions, which included priming and wiping.

The user tests were invaluable. Our first iteration, for example, did not have windows in the lid. The user was simply instructed to push the lid down for three seconds and then allow the lid to spring up. The videotapes showed users pushing down slowly and only part way down, holding the primer lid down too long, and pushing down too many times. It became obvious that some feedback was needed to let the user know how long and how far to push down. A number of ideas were considered. The ideas ranged from controlling the effective priming time to visual, tactile, and audio feedback methods. Visual feedback--ink filling the windows in the lid--was chosen. The advantages of this feedback method are that the solution required simple tools, and most important, that it clearly shows the user what is going on during priming. This is significant since the hold-down time varies somewhat with how fast the primer lid is pushed down and how much ink is in the cartridge. Another advantage is that the amount of ink wasted is diminished, since the cartridge is pumped only until it is primed and no more.