Design and development of a color thermal inkjet print cartridge - includes related articles on capillary forces in a foam matrix and print quality and pen development

Hewlett-Packard Journal, August, 1988 by Jeffrey P. Baker, David A. Johnson, Vyomesh Joshi, Stephen J. Nigro

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Print Quality Testing

Print quality is a very subjective but extremely important specification for printers. Print quality is influenced by inks, media, printing mechanism, and customers. A process was developed to set the design specifications and tolerances for dot size, dot location, color range, and other attributes that determine print quality so that inks, media, and a printing mechanism could be developed to meet user needs for their applications.

There were two major efforts for print quality evaluation during the design phase, one in engineering and one in marketing. The marketing group identified the key market segments, applications, and competition for the PaintJet printer. The engineering group developed a list of approximately thirty measurable print quality attributes and a set of diagnostic print samples to simulate errors (see box, page 14). The two groups also collected application specific print samples that represent actual customer needs. The printing technology was characterized to understand and determine the bounds for the critical attributes and a print quality survey was designed to derive design centers.

Samples were collected for three major application segments: word processing, merged text and graphics, and presentations. The word processing sample was generated in black only to evaluate text quality, while merged text and graphics and presentation samples were printed in full color. For each sample some specific print quality attribute was tested to determine the design center and associated tolerances on the basis of the acceptability of the print quality for that application. The survey also included samples generated on other similar products on the market.

Reliability Testing

The disposal printhead concept makes inkjet printing capable of high reliability. The PaintJet printer reliability testing program has been a major effort to ensure that its performance satisfies both business and engineering customers. The marketing group worked with the engineering team to set realistic and concrete reliability goals based on user needs and the use model. The printer will be used in an office environment (15 to 32.5[deg.]C and 20 to 80% relative humidity) and will have to perform with minimal customer attendance. The print cartridge can be shipped anywhere in the world without any shipping or storage restrictions and should have at least two years shelf life. The print cartridge should print a minimum of a million characters.

These requirements drove the reliability goals for three major testing categories: shipping and storage, shelf life, and system operational testing.

Shipping and Storage. The goal here was to be able to ship the printhead anywhere in the world without any restrictions. When customers receive these printheads, they are assured of 99% reliability with 90% confidence that the printer will operate after the activation process (priming and wiping). At each development phase the reliability was tested against the goal by simulating shipping and storage conditions for temperature (high and low), shock, vibration, and altitude.