PRINTHEAD ARRANGEMENT TO
ELIMINATE BI-DIRECTIONAL HUE
TECHNICAL FIELD OF THE INVENTION
This invention relates to ink-jet swath printing devices, and more particularly to techniques for eliminating hue shifting due to bi-directional swath printing.
BACKGROUND OF THE INVENTION
An ink jet printer forms a printed image by printing a pattern of individual dots at particular locations of an array denned for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes "dot locations", "dot positions", or "pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet printers print dots by ejecting very small drops of ink onto the print medium, and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
Color ink jet printers commonly employ a plurality of printheads, for example four, mounted in the print carriage to produce different colors. Each printhead contains ink of a different color, with the commonly used colors being cyan, magenta, yellow, and black. These base colors are produced by depositing a drop of the required color onto a dot location. Secondary or shaded colors are formed by depositing drops of different colors on adjacent dot locations; the human eye interprets the color mixing as the secondary or shading, through well known optical principles.
Print quality is one of the most important considerations of competition in the color ink jet printer field. Since the image output of a color ink jet printer is formed of millions of individual ink drops, the quality of the image is ultimately dependent upon the quality of each ink drop and the arrangement of the ink drops on the print medium.
Print head arrangements employed in the past have typically used linear arrays of print elements, wherein the pens of different color are one next to the other. There are several negative consequences of such an arrangement. One is that, when the carriage is going from left to right, the colors are laid down in one order, say YMC for example. When the carriage goes in the other direction, from left to right, the colors are laid down in the opposite order, CMY in this example. The problem with this is that the blue made by first printing cyan and then magenta is slightly different from the blue made in the reverse order. This is because the final dot will inevitably cover a bit of the first dot. This problem is illustrated in FIGS. 1A and IB. FIG. 1A shows the case of movement left to right, depositing magenta before cyan, so that some cyan overlaps an adjacent magenta dot. FIG. IB shows the case of movement in the right to left direction, depositing cyan before magenta, so now some magenta overlaps the adjacent cyan dot. Of course, the blue color shifting is only one example; other colors will also suffer from hue shifting in a similar fashion.
A fixer pen can be employed to deposit droplets of a fixer liquid on the print medium at dot locations to subsequently
receive droplets of a colored ink. Fixer liquids can be used to cause the subsequently applied colored ink droplets to precipitate very quickly. The fixer liquids can be a cobalt salt, or any other substance by which the colored inks are
5 enhanced. Another problem is that if one wishes to add a fixer ink, which must be printed first, the printer must have two fixer ink pens, one on either side of the colored ink pens, to achieve bi-directional printing. This adds to the expense of the printer. Moreover, the required width of the carriage
10 is increased, due to the need to accommodate two fixer pens. For some printer embodiments, a top coat could also be applied by a top coat pen after applying the colored ink to the print medium, in a post-printing top coat step. An exemplary top coat material is a transparent polymer. Two
15 such top coat pens would be needed for bi-directional printing, further adding to the expense and width of the printer.
Print throughput is degraded with wide carriages due to the increased overtravel. This is illustrated in the diagram
20 matic illustration of FIG. 2, wherein a printer carriage 10 is mounted for swath movement along a scan axis 12 over a print zone 14 of width A—A. The carriage supports six ink jet pens 16A-16F in a lateral arrangement. FIG. 2 shows the carriage 10 at the leftmost position needed to support
25 printing on the width of the print media, i.e. over the print zone width A—A. The carriage overtravel past the left edge of the print zone is relatively large, due to the pen to pen width and the required acceleration distance needed to accelerate the carriage to achieve the carriage printing speed.
30 Six pens are used here, including two pens 16A, 16F with fixer ink at each end of the carriage. Disposed intermediate the fixer pens are the color pens, e.g. 16B (cyan), 16C (magenta), 16D (yellow) and 16E (black). Two fixer pens are needed in this case to support bi-directional printing. Only
35 the leftmost position of the carriage is shown in FIG. 2; the same carriage overtravel distance must be accommodated on the right side of the print zone.
SUMMARY OF THE INVENTION
40 In accordance with an aspect of the invention, a technique is described for color inkjet printing to eliminate bi-directional hue shifting. The technique includes:
mounting a printer carriage for bidirectional movement along a print swath axis;
45 providing a plurality of ink jet printheads of different colors, each printhead having a nozzle array for emitting droplets of ink of non-black color;
supporting the plurality of printheads on the printer car
5Q riage such that the respective nozzle arrays do not overlap in a direction along the print swath axis;
moving the printer carriage in a first direction along the swath axis from one side of a print area to a second opposite side of the print area while driving one or more of the
55 printheads to emit droplets under computer control onto a print medium;
providing relative motion between the print medium and the carriage in a direction transverse to the swath axis;advancing the print media; 60 moving the printer carriage in a second direction along the swath axis from the second side of the print area to the first side while driving one or more of the printheads to emit droplets under computer control onto the print medium;
providing relative motion between the print medium and 65 the carriage in a direction transverse to the swath axis,
wherein the order of laying down droplets of different colors is the same during the movement in the second