High-quality inkjet color graphics performance on plain paper - design of HP DeskJet 1200C and HP DeskJet 1200C/PS printers - Technical

Hewlett-Packard Journal, Feb, 1994 by Catherine B. Hunt, Ronald A. Askeland, Leonard Slevin, Keshava A. Prasad

Realizing the color graphics performance of the HP Deskjet 1200C printer required simultaneous optimization of many interacting parameters of the ink and the architecture to deliver significant improvements in print quality, color gamut, throughput, and cost per copy.

The HP Deskjet 1200C printer is a 300-dpi, plain paper printer that provides vivid, bright, true colors with consistent print quality on all media (plain and special papers, transparency and glossy films).

The color print cartridges (cyan, magenta, yellow) for the Deskjet 1200C represent a significant advance in HP thermal inkjet printing technology. The major contributors to the improved performance, print quality attributes, and throughput are the ink and the architecture. Both the ink and the architecture were optimized to deliver significant improvements in print quality, color gamut, throughput, and cost per copy. One major customer benefit of the Deskjet 1200C cartridge is the small drop volume, which provides industry-leading color cost per copy at any use rate.

During the color development phase of the Deskjet 1200C printer, the HP Deskjet 500C(1) and HP Paintjet XL300 printers were used as benchmarks for color graphics. The team learned a great deal from these products and leveraged some of the acquired knowledge in developing a significantly better product. However, leveraging from these products did not make the development easy; a major effort was still required since new issues emerged as design modifications were made. Test qualification became very important, and since time was a limiting factor, the design-build-test-fix cycle became a challenge. Throughout the project, optimization was done within aggressive schedule objectives.

Addressing Customer Needs

Early in the Deskjet 1200C printer and pen cartridge development, the QFD (Quality Function Deployment) method of addressing customer needs was used.(2) This established a foundation of customer focus throughout the project for both the printer and the print cartridge.

A QFD team was formed to gather customer needs, link one or more engineering metrics to the customer needs, and determine what technical factors engineers could control and measure to satisfy the customer needs. This team was composed of representatives from different areas of the project.

Based on the QFD customer research studies, several fundamental characteristics that define color graphics quality were identified:

* Color quality

* Area fill quality

* Line and edge quality

* Cockle

* Curl

* Whiteness

* Archivability.

Engineering Metrics

Color quality of a printed output is determined using the Munsell color measurement system.(3) Chroma is a measure of the vividness or brightness of the color. Hue refers to the shade or tone of the color. Value is a measure of the darkness or lightness of the color. The color gamut describes this three-dimensional color space:

Color Gamut = Value x Chroma[sup.2] x Hue.

Customer survey restfits indicate that smooth area fills are very desirable. The parameters that can affect area fill quality are:

* Drop placement errors caused by swath advance (see Fig. 1) and theta-Z (printhead rotational misalignment) errors

* Banding caused by pen defects such as misdirection (see Fig. 2) and variations in drop volume or dot size

* Wait time banding (see Fig. 3)

* Mottling (see Fig. 4)

* Bleed caused by different penetration rates of the ink into the media (see Fig. 5)

* Coalescence caused by drops pulling together before the printing surface is wetted

* Print rendering methods.

Mottling, coalescence, and wait time banding are evaluated visually. Several algorithms using a machine vision system were developed to measure swath advance and theta-Z errors, directionality, dot size, and bleed.

Artifacts such as spray (see Fig. 6), leathering, and jaggy horizontal and vertical lines (see Fig. 7) caused by dot placement errors or paper shrinkage can deteriorate the line and edge quality of the printed image. Spray is inherent in thermal inkjet technology and it is made worse by the presence of the vapor removal system in the printer. The vapor removal system consists of the fan and the plenum. Its primary function is to remove local moisture from the writing system area.

Flat paper output is another customer want. Paper cockle is a distortion in which bumps or ridges are randomly produced on the printed paper. Paper flatness is lost and the output has a wavy appearance. This is a side effect of high-density inkjet printing with aqueous inks. Visual measurement is done on paper cockle. Curl is a phenomenon in which the edges of the paper migrate towards the center of the paper (see Fig. 8). In worst-case environmental and printing conditions, the output may take the form of a tube. Curl measurements involve measuring the height of all four corners of a sample resting on a flat surface. This problem can be a major customer dissatisfier since paper curl affects media stacking and curled sheets more difficult to display or store. Cockle and curl get worse as the throughput increases. These print quality defects are inversely related. The more cockle a solid area fill output has, the less curl it has. The cockle appears to reinforce the paper so that it does not curl.