Conventional printers are capable of printing in several different modes. The modes represent tradeoffs between print speed and print quality. For example, a printer may have a "best" mode which prints a high quality image at a relatively slow speed, a "normal" mode which prints a medium quality image at a relatively medium speed, and an "economy" mode which prints a low quality image, typically text, at a relatively fast speed.
One way of increasing print speed in color printers is to minimize the distance the carriage in the printer must travel. The carriage of a color printer typically holds several printheads (e.g., cyan, magenta, yellow, and black) and scans across a medium while depositing drops of ink from the printheads along the way. After the carriage completes one or more scans, the medium is shifted, and the carriage then begins another scan. This process is repeated until a desired image is formed on the medium. Since the carriage must scan this distance numerous times before the desired image is printed on the medium, a significant time savings could be achieved if the distance the carriage had to travel to complete a scan could be shortened.
Another way of increasing effective print speed in color printers is to underprint and overprint the black ink. As used herein, underprinting refers to depositing one or more drops of color ink (or a fixer fluid) on dot locations that will subsequently have one or more drops of black ink deposited thereon, and overprinting refers to depositing one or more drops of color ink (or a fixer fluid) on dot locations that have previously received one or more drops of black ink.
Underprinting and/or overprinting increases print speed because, with certain types of ink, black ink dries more quickly when it comes in contact with the color ink. This is due to the different chemistries of black ink and color inks. Black inks are often pigment based where color inks are often dye based. Pigment based inks dry slower than dye based inks. Since black ink dries relatively slowly, and since the ink must be dry before it is touched or has another sheet of paper dropped on top of it, the black ink drying time frequently becomes the bottleneck for print speed. Underprinting with a dye-based ink effectively wets the medium so that the black pigment vehicle is more quickly absorbed by the medium and dries quickly. Overprinting offsets any color shift by the underprinting.
Additionally, a chemical reaction may take place between the black ink and certain types of underprinted or overprinted ink (or other fluid) to speed up the drying time of the black ink. This also prevents the black ink from bleeding into adjacent color image portions. Accordingly, underprinting and/or overprinting increases print speed by decreasing the time it takes black ink to dry.
Another advantage of underprinting and overprinting is that the optical density (i.e., blackness) of the black printed image is increased if underprinting and/or overprinting is used.
A disadvantage of underprinting and overprinting is that the carriage which holds the cyan (C), black (K), magenta (M), and yellow (Y) printheads must travel a longer distance in order for the color inks (typically cyan and magenta) to underprint and overprint all of the black dot locations. This is especially disadvantageous when print speed is at a premium, for example when printing in the previously described "economy" mode.
Accordingly, what is needed is a technique that allows for underprinting and overprinting while minimizing the distance the carriage must travel when scanning across a medium.