Inkjet printer print quality enhancement techniques - design of HP DeskJet 1200C and HP DeskJet 1200C/PS printers - Technical

Hewlett-Packard Journal, Feb, 1994 by Corinna A.E. Hall, Aneesa R. Scandalis, Damon W. Broder, Shelley I. Moore, Reza Movaghar, W. Wistar Roads, William H. Schweibert

Five print modes, each optimized for quality and throughput, HP Resolution Enhancement technology, heaters to dry the ink and the paper, and accurate print cartridge alignment and paper advance schemes contribute to the high print quality of the HP Deskjet 1200C printer. When the concept for the HP Deskjet 1200C printer was formulated, the design direction chosen was a text printer that also had excellent graphics performance. The initial decisions were fairly easy to make. For example, the printer was to be thermal inkjet with a resolution of 300 dots per inch (dpi). It was to be media independent (i.e., not just "special" media) and it was to have four print cartridges (black, cyan, magenta, and yellow) and most probably fast and high-quality printing options. To accomplish these objectives, the chosen printing system was a scanning carriage to hold the four print cartridges and a heated paper path to dry the ink quickly.

Having chosen a design path, each part of the writing system was investigated to determine how its contributions to print quality could be optimized. A dual approach was taken. Each print mode was designed to give the customer the best output in the least amount of time.

Improvements were made in various mechanical designs that optimize print quality in all print modes. While the Deskjet 1200C has five print mode options, those for transparency film and glossy media are not covered in this article. Print modes for plain paper (e.g., copier paper) are described briefly with an emphasis on optimizations that improve quality or throughput.

The mechanics and firmware that are common to all print modes were optimized with good overall print quality as the goal, and were also optimized specifically to improve known defects within a single print mode. The designs highlighted in this article are the main heater, the media preheater, the carriage encoder and print cartridge alignment scheme, and the paper advance accuracy.

Print Modes and Optimizations

Three plain paper print modes are available to the Deskjet 1200C user: fast, normal, and high-quality. Each print mode is capable of printing both text and graphics. While there are many subtle differences between the modes, the main differences are in the number of passes and the optical density of the black. The rate at which the ink can be dried is the main limit to throughput. Ink applied too fast will cause color bleed because of dye mixing at color boundaries, buckled paper from expansion of paper fibers, and mottled colors. Multipass printing can eliminate these defects by allowing the ink to dry between passes. For example, a two-pass mode prints only half of the ink on the first pass and an additional scanning pass of the print cartridge is used to apply the rest of the ink. Multipass printing takes longer than single-pass printing, but quality is improved. Black optical density, especially in text, follows a similar pattern, trading off speed for quality. Applying more black ink to the same area and increasing the optical density requires more time to dry while improving quality.

Fast mode is just that--text speed is six pages per minute and graphics speed is under one minute per page. This speed is achieved by single-pass bidirectional printing, and consequently there are some of the defects listed above when printing high-density graphics in fast mode. Although text quality is still quite good, all 104 nozzles of the print cartridge are fired without regard to character splitting (i.e., a line of characters may be printed in two separate sweeps, top half first and bottom half second). While this mode is not recommended for high-density graphics, it will give the customer feedback about the colors chosen and the location and size of the graphics. Since this mode is heavily skewed towards speed and not quality, one way in which this mode was optimized was in the amount of time the heater is given to warm up the writing zone. Print quality is best when the writing zone is allowed to stabilize at the optimum temperature, but warm-up time requires the customer to wait. To avoid this wait, the printer will only warm up for the amount of time needed to receive the file. If the file is small the printer will begin printing almost immediately. The compromise works well for text and low-density graphics files.

The speed and print quality in fast mode are different from high-quality mode. Both text and graphics quality are improved in high-quality mode. Throughput is slightly reduced to four pages per minute for text and two minutes per page for graphics. Text is darker, and characters are not split and have smoother edges than text in fast mode. Through the use of a three-pass print mode, color-to-color boundaries are crisp and pass-to-pass paper advance errors are blended and distributed. Trade-offs made in this mode favored quality over throughput, although there are three notable exceptions. As mentioned above, three passes are used. Many numbers of passes were investigated, but when the improvement in quality (with more passes) was measured against print time, the winner was three passes in terms of acceptable quality and speed. A similar decision was made with respect to slowing down a plot for very dense graphics. The longer the inked paper is over the heater, the more the ink dries between passes, reducing bleed and cockle. The printer is able to recognize dense graphics in a plot and slow the printing to allow for more drying. However when quality improvement and slowdown are compared, a point is reached when more slowdown gives only a small amount of quality improvement. For this reason, the slowdown equation has a hard-clip limit on the maximal time to print a plot.