Patent Publication Number: US-6663236-B2

Title: Ink jet printing with color-balanced ink drops mixed using colorless ink

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned, U.S. patent application Ser. No. 09/466,977 entitled CONTINUOUS COLOR INK JET PRINT HEAD APPARATUS AND METHOD, filed in the name of John A. Lebens on Dec. 17, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to ink jet printing and, more particularly, to methods and apparatus for generating color balanced ink drops in a drop on demand ink jet printer. 
     BACKGROUND OF THE INVENTION 
     Ink jet printing is a prominent contender in the digitally controlled electronic printing arena in part because of its non-impact and low-noise characteristics, its use of plain paper, and its avoidance of toner transfers and fixing. Ink jet printing mechanisms can be categorized as either continuous ink jet or drop-on-demand ink jet. 
     Drop-on-demand ink jet printers selectively eject droplets of ink toward a printing media to create an image. Such printers typically include a print head having an array of nozzles, each of which is supplied with ink. Each of the nozzles communicates with a chamber, which can be pressurized in response to an electrical impulse to induce the generation of an ink droplet from the outlet of the nozzle. Many such printers use piezoelectric transducers to create the momentary pressure necessary to generate an ink droplet. 
     Drop-on-demand printers utilizing thermally-actuated paddles have also been suggested. Each paddle would include two dissimilar metals and a heating element connected thereto. When an electrical pulse is conducted to the heating element, the difference in the coefficient of expansion between the two dissimilar metals causes them to momentarily curl in much the same action as a bimetallic thermometer, only much quicker. A paddle is attached to the dissimilar metals to convert momentary curling action of these metals into a compressive wave that effectively ejects a droplet of ink out of the nozzle outlet. 
     Printing images in a plurality of colors is highly desirable. This has been effected by means of a plurality of streams of ink droplets emitted from a plurality of nozzles. However, the images produced in this way are in general binary in the sense that the number of colors available for each drop is limited to that of the number of associated ink reservoirs and nozzle sets. 
     Commonly assigned U.S. Pat. No. 5,606,351, which issued to Gilbert A. Hawkins on Feb. 25, 1997, discloses a system having the ability to control the intensity of color droplets by mixing two or more fluid ink components (dyes, pigments, etc.) drawn into a chamber from refill channels. As such, each ink ejector squirts an ink of a particular color of varying intensity and is not capable of altering the color. That is, only the tone of the color is altered. 
     Commonly assigned U.S. Pat. No. 6,097,406, which issued to Anthony A. Lubinsky et al. on Aug. 1, 2000, discloses an apparatus for mixing and ejecting mixed colorant drops. A mixing chamber receives the appropriate amounts of primary colors and a drop is ejected. However, a residual amount of dye is left in the chamber and needs to be removed by flushing with a clear cleaning fluid before the next color is prepared. A separate diluent chamber is used to control color density. 
     Commonly assigned, co-pending U.S. patent application Ser. No. 09/466,977 entitled CONTINUOUS COLOR INK JET PRINT HEAD APPARATUS AND METHOD, filed in the name of John A. Lebens on Dec. 17, 1999, discloses a scheme for color mixing in a continuous ink jet print head. By selectively restricting flow of two or more different color inks to a nozzle, a range of colored inks can be ejected from the nozzle. 
     U.S. Pat. No. 4,614,953, which issued to James M. Lapeyre on Sep. 30, 1986, discloses a color inkjet printing mechanism in which real time color mixing is achieved in a single channel. The method is said to be applicable to either drop-on-demand or continuous stream inkjet printer heads. According to the Lapeyre patent, the relative sizes of a mixing chamber line and its subsequent drive chamber mixed ink drive interior are such that a continuous flow of in is maintained without significant mixing or blurring of different colors sequentially provided within the ink flow. 
     U.S. Pat. No. 4,382,262, which issued to Joseph Savit on May 3, 1983, discloses a method for ink jet printing in which a first dye component is printed on a receiver. One of several complementary dye components is selectively provided by dedicated nozzles, thereby producing a selected color. 
     Commonly assigned U.S. Pat. No. 6,055,004, which issued to Werner Fassler et al. on Apr. 25, 2000, discloses a microfluidic printing array print head. Micropumps are used to deliver various colors into a nozzle area to create a drop of desired color. The colored drop is then transferred to a receiver by contact. A shutter plate is used to control ink flow. 
     DISCLOSURE OF THE INVENTION 
     According to a feature of the present invention, a drop-on-demand ink jet printing system includes a print head having at least one mixing chamber having a nozzle opening. A plurality of sources of color liquid ink and a source of colorless liquid ink communicate with the mixing chamber. A flow controller is adapted to selectably meter ink from the sources to the mixing chamber, whereby ink droplets of selectable color are prepared in the mixing chamber for delivery from the nozzle. The flow controller is further adapted to meter colorless ink into the mixing chamber after a droplet is delivered from the nozzle opening to thereby dilute color ink remaining in the mixing chamber sufficiently such that a next desired color can be attained by adding ink of appropriate color to the mixing chamber. 
     According to one preferred embodiment of the present invention, the mixing chamber is flushed with colorless ink after a droplet is delivered from the nozzle opening and before adding ink of appropriate color to the mixing chamber to attain a next desired color. 
     According to another preferred embodiment of the present invention, the flushed ink is bleached before being returning to the source of colorless ink. 
     According to yet another preferred embodiment of the present invention, flushed ink is converted to black by addition of appropriate amounts of color ink before being returning to a source of black ink. 
     Advantages associated with the present invention include the ability to produce continuous tone images without the associated need to print with smaller drops to avoid image pixels being filled by only one drop. For example, the image pixel of a 300 dpi printer is approximately 84 micron square, requiring a 60 micron diameter drop for a spread factor of two when the drop impacts paper. The nozzle diameter may therefore be close to 60 microns. Such large nozzles are less likely to clog and therefore are more robust. Furthermore, large nozzles are easily cleaned. Large nozzles may also employ more viscous inks putting less demand on ink formulation. 
     The method of controlling color by adding dye and bleach provides a unique means of obtaining color balance on demand. This method allows single drop per image pixel printing with any color of choice color with many levels of intensity. 
     The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiments presented below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
     FIG. 1 is a schematic view of an ink jet printer according to a preferred embodiment of the present invention wherein flushed liquid is treated by bleaching to a colorless ink; 
     FIG. 2 is a perspective view of a print head suitable for use in the printer of FIG. 1; 
     FIG. 3 is a schematic view of an ink jet printer according to a second preferred embodiment of the present invention wherein flushed liquid is treated by adding CMY and colorless ink and returned as black ink; and 
     FIG. 4 is a perspective view of a print head suitable for use in the printer of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, an ink jet printer  10  uses a drop-on-demand print head  12  having a plurality of nozzle openings  14  in FIG. 2 for delivering ink drops of varying color to a receiver  16  moved relative to print head  12  by a computer-controlled transducer  17 . Each nozzle opening  14  is in communication with an ink mixing chamber  18  which receives selected quantities of cyan (C), magenta (M), yellow (Y), and colorless dye solutions from respective reservoirs  20 - 23  via passages  20 ′- 23 ′, respectively, to create an appropriate color for an ink drop to be delivered at an image pixel being addressed. Optionally, black (K) dye solution may also be received in mixing chamber  18  from a reservoir  24  via passage  24 ′. The reservoirs may be pressurized so that flow occurs once a valve, not shown, is opened. Alternatively, a pump may be used to deliver liquid from the reservoirs to the mixing chamber. 
     A color mixture corresponding to the color to be deposited on an image pixel is prepared by metering in the appropriate amount of dyes, colorless ink and bleach. A mixer  26  is optionally provided in mixing chamber  18 . Any device that causes a disturbance in the liquid present in the mixing chamber would function as a mixer. For example, mixer  26  may be a heater, piezoelectric transducer, micropump, thermally actuated flipper, piezoelectrically-driven flipper, or electrostatically driven vibrating plate. Once the color inks are mixed, an ink drop is ejected by an ink drop ejector  28 . The ink ejector provided in mixing chamber  18  may be a resistor layer, such as TaAl, deposited of the floor of the mixing chamber. The resistive layer may be coated with an electrical passivation layer (e.g., SiNi and/or SiC) and also with a nonwetting passivation layer. When current is passed through the resistive layer, mixed ink is rapidly heated, causing an expanding gas bubble to eject a drop of mixed ink. Another type of ink ejector may be thermally-driven such as a bimetallic flipper paddle that bends toward nozzle opening  14  when energized with electricity. Heat released by the energized resistive strip causes differential expansion of one of the metallic layers in the bimetallic strip, causing the bimetallic paddle to flip rapidly and eject a drop of the mixed ink. 
     If the next image pixel to be addressed requires a different color, requiring a lower intensity of one of the constituent colors, an appropriate amount of colorless ink is added to sufficiently dilute that one constituent color. Dyes of the other constituent colors are added as needed to attain desired levels. If the image pixel to be addressed requires a color in which more than one of the constituent colors are of lower intensity, then sufficient colorless ink is added from reservoir  23  to obtain the lowest intensity color, followed by make-up dye solution to adjust the intensity of the other constituent colors. 
     If the just-mentioned solution for color correction is not possible due to volume constraints as determined such as by a computer  30  that controls the printing operation, then the computer commands that mixing chamber  18  be flushed with colorless ink before CMY color dyes are added according to image requirement. The ink flushed is collected via a passage  32 ′ in a bleach station  32  where it is to be treated and converted to colorless ink by adding appropriate amounts of chemical bleach and colorless ink. Other bleaching methods such as thermal bleaching and photobleaching may be appropriate in particular circumstances. The bleach station is equipped with a bleach source  34 , a mixing pump  36 , a detector (dye and viscosity sensor)  38  and appropriate valves  40  to generate colorless ink, which is then cycled to colorless ink reservoir  23  for future use. A purifier  42  such as a bead pack may be used to remove breakdown products created by the bleaching process. The bleached and purified liquid may also be filtered at  44  before being returned to the colorless ink reservoir. 
     If the next image pixel to be addressed requires only more intense color or colors, appropriate dye is simply added to mixing chamber  18 . Colorless ink may also be added to restore optimum volume and to maintain ink viscosity in the mixing chamber. 
     Black color may be prepared by combining CMY colors. Alternatively, black (K) ink may be provided. When black ink is prepared in mixing chamber or supplied to mixing chamber, color bleaching may not be necessary because the black dye would overwhelm any residual ink left in mixing chamber. 
     Fluid flow control throughout the system may be effected by microvalves and micropumps. Any of many microvalves disclosed in the literature may be used in systems according to the present invention. For example, a bimetallically driven diaphram is disclosed in Understanding Microvalve Technology, 26 Sensors, September 1994. Other types of microvalves are disclosed in U.S. Pat. Nos. 5,178,190; 5,238,223; 5,259,757; 5,367,878; 5,400,824; and 5,880,752. Any of many micropumps disclosed in the literature may be used with the present invention, as for example, electroosmotic pumps, acoustic pumps, or piezoelectrically driven membrane pumps. 
     In the embodiment illustrated in FIG. 1, the flushed fluid is converted to colorless ink. Referring to FIGS. 3 and 4, another embodiment of present invention is illustrated wherein the flushed fluid is converted to black ink. An ink jet printer  50  uses a drop-on-demand print head  52  having a plurality of nozzle openings  54  in FIG. 5 for delivering ink drops of varying color to a receiver  56  moved relative to print head  52  by a computer-controlled transducer  57 . Each nozzle opening  54  is in communication with an ink mixing chamber  58  which receives selected quantities of cyan, magenta, yellow, colorless, and black dye solutions from respective reservoirs  60 - 64  via passages  60 ′- 64 ′, respectively, to create an appropriate color for an ink drop to be delivered at an image pixel being addressed. As in the first-illustrated embodiment, the reservoirs may be pressurized so that flow occurs once a valve is opened or, a pump may be used to deliver liquid from the reservoirs to the mixing chamber. 
     A color mixture corresponding to the color to be deposited on an image pixel is prepared by metering in the appropriate amount of dyes and colorless ink. A mixer  66  is optionally provided in mixing chamber  58 . Once the color inks are mixed, an ink drop is ejected by an ejector  68 . 
     If the next image pixel to be addressed requires a different color, requiring a lower intensity of one of the constituent colors, an appropriate amount of colorless ink is added to sufficiently dilute that one constituent color. Dyes of the other constituent colors are added as needed to attain desired levels. If the image pixel to be addressed requires a color in which more than one of the constituent colors are of lower intensity, then sufficient colorless ink is added from reservoir  63  to obtain the lowest intensity color, followed by make-up dye solution to adjust the intensity of the other constituent colors. 
     If the next image pixel to be addressed requires only more intense color or colors, appropriate dye is simply added to mixing chamber  58 . Colorless ink may also be added to restore optimum volume and to maintain ink viscosity in the mixing chamber. 
     If the just-mentioned solution for color correction is not possible due to volume, then a computer  69  commands that mixing chamber  58  be flushed with colorless ink before CMYK color dyes are added according to image requirement. The ink flushed via a passage  70 ′ is collected in a dye addition station  70  where it is to be treated and converted to black ink by adding appropriate amounts of CMY dye and colorless ink from a source  72 . The dye addition station is equipped with a mixing pump  74 , a detector (dye and viscosity sensor)  76  and appropriate valves to generate black ink, which is then cycled to black ink reservoir  64  for future use. A filter  78  may be used to clean up the recycled ink. Colorless ink is added at the dye mixing station to maintain ink viscosity. 
     Examples of colorants which may be mixed to form ink may be one of many found in the literature. For example, a colored ink may be formed by mixing acid blue 6 (cyan), basic red 29 (magenta) and Zeneca yellow 132 (yellow). A bleach that may be used to reduce or eliminate color is a 5% solution of sodium hypochlorite. Other bleaches that may be used include acids, bases, ozone, hydrogen peroxide, and nucleophiles. 
     The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.