Patent Publication Number: US-7213898-B2

Title: Color ink-jet printer

Description:
The present application is based on Japanese Patent Application No. 2002-043800 filed Oct. 30, 2002, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a color ink-jet printer capable of ejecting ink droplets of different colors. 
   2. Discussion of Related Art 
   U.S. Pat. No. 6,416,149 B2 (in particular, FIGS. 4–6, and col. 1, lines 46–53 and col. 5, lines 43–50) corresponding to JP-2001-301206A discloses an ink-jet printer operable such that at least one droplet of an ink each having a predetermined volume is ejected from each nozzle, so as to form a dot of ink on a recording medium in the form of a paper sheet, such that where a plurality of ink droplets are ejected from the nozzle, the ink droplets overlap each other so as to form one ink dot. Thus, a desired gray-scale value can be established at each picture element of an image corresponding to each ink dot to be formed on the paper sheet according to image data (printing data), by suitably selecting one of three different total volume values (large, medium and small values) of the above-indicated at least one ink droplet, for each of the ink dots, so that each ink dot has the corresponding one of three different sizes or diameters which are determined by the respective three different total volume values. 
   Publication JP-4-364961A of Japanese Patent Application (in particular, paragraphs [0020] and [0021]) discloses a color ink-jet printer capable of ejecting droplets of inks of different colors which have different compositions so that the different-color inks are given different rates of fixing (different drying speeds) on a recording medium, to prevent undesirable color mixing due to bleeding of the different-color ink droplets at their boundaries. Namely, the color mixing at the boundaries can be prevented since one of the adjacent two dots of the inks of the different compositions is dried at a higher rate than the other dot. 
   If the technique disclosed in the above-identified U.S. Patent is applied to a color ink-jet printer having a plurality of rows of nozzles that are arranged to eject droplets of inks of respective different colors (e.g., yellow [Y], magenta [M], cyan [C] and black [B]), the same number of the ink droplets corresponding to one ink dot are ejected for each of the different colors, that is, the total volume of the ink droplets corresponding to one ink dot is the same for all of the different colors, when the gray-scale value at a picture element in question is the same for all of the four colors. Where the different-color inks have the respective different compositions as disclosed in the above-identified Japanese Publication, it is possible to reduce a risk of deterioration of quality of a printed image due to the color mixing at the adjacent ink dots of different colors. However, the different-color inks of different compositions have respective different drying speeds, and the viscosity of the ink having a relatively high drying speed may be excessively increased at the meniscus surface of the ink remaining in a given nozzle, due to evaporation of an aqueous component of the ink at the meniscus surface, which takes place if the ejection of the ink droplets from that nozzle is absent for a relatively long time. In this case, the nozzle may suffer from so-called “plugging” due to increased viscosity of the ink at the meniscus surface, leading to a failure to subsequently eject the ink droplets from that nozzle. In this respect, a color ink-jet printer is required to operate without not only the color mixing at the adjacent ink dots of different colors, but also the plugging of the nozzles due to the different compositions of the different-color inks. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a color ink-jet printer which is arranged to minimize deterioration of quality of a printed image due to color mixing at adjacent ink dots of different colors while avoiding plugging of nozzles with dried ink. 
   After an extensive study in an effort to solve the drawbacks experienced in the prior art, the present inventor had a finding that deterioration of quality of a printed image due to color mixing, as recognized by an observer of the image, is primarily due to ink bleeding at the boundary between the black ink dots and the adjacent ink dots of the other colors. In other words, the inventor had a finding that where the color mixing at the boundary between the black ink dots and the adjacent ink dots of the other colors is substantially prevented, the color mixing if any at the boundary between the adjacent ink dots of those other colors would not significantly deteriorate the quality of the printed image. The present inventor conceived the present invention, based on this finding along with a recognition that the black ink is used at a considerably higher frequency than inks of the other colors, to effect solid printing so as to emphasize letters, for example, in a comparatively large area on the recording medium. 
   The object indicated above may be achieved according to a first aspect of this invention, which provides a color ink-jet printer comprising: a first ink ejecting portion operable to eject droplets of a black ink; a second ink ejecting portion operable to eject droplets of an ink of a color other than black, a first control portion operable to control the first ink ejecting portion such that a total volume of at least one droplet of the black ink ejected by the first ink ejecting portion to form each dot of the black ink on a recording medium is equal to a selected one of a plurality of different total volume values; and a second control portion operable to control the second ink ejecting portion such that a total volume of at least one droplet of the ink of the color other than black, which is ejected by the second ink ejecting portion to form each ink dot of the color other than black on the recording medium, is equal to another of the plurality of different total volume values which is smaller than the above-indicated selected one of the different total volume values. 
   In the color ink-jet printer constructed according to the above-indicated first aspect of the present invention, the total volume of the at least one ink droplet forming each ink dot of at least one color other than black is made smaller than that of the at least one ink droplet forming each black ink dot. Accordingly, the relatively small ink dots of the at least one color other than black are dried on or absorbed by the recording medium at a higher rate, than the relatively large black ink dots. Therefore, substantially no bleeding of the inks takes place at the boundary between the dots of the black ink and the ink dots of the other color. Further, the present color ink-jet printer is not required to use the black ink and the ink of the other color to have different compositions. Namely, the present printer may use the black ink and the other color ink which have substantially the same composition which is formulated to avoid undesirable plugging of nozzles of the first and second ink ejecting portions. 
   In the present invention, the total volume of the at least one droplet forming each black ink dot is made larger than that of the at least one droplet forming each ink dot of the other color, in view of a fact that the black ink is generally used at a considerably higher frequency than the ink of the other color, to effect solid printing so as to emphasize letters of words, for example, in a comparatively large area on the recording medium. In this respect, the printing speed of an image can be increased even where the printing of the image involves such solid printing with the black ink only. 
   In a first preferred form of the color ink-jet printer of the above-indicated first aspect of the present invention, the first and second control portions are operable to control the first and second ink ejecting portions such that the total volume of the above-indicated at least one droplet forming each dot of the black ink, and the total volume of the above-indicated at least one droplet forming each ink dot of the color other than black are respectively kept at the above-indicated one and another of the plurality of different total volume values, throughout entire operations of the first and second ink ejecting portions to print an image on the recording medium, irrespective of gray-scale values at respective picture elements of the image. 
   In one arrangement of the first preferred form of the invention described above, the first and second control portions are operable to select the above-indicated one and another of the plurality of different total volume values, on the basis of a presently selected one of a plurality of different print modes which correspond to respective different values of resolution of the image. In this case, the sizes of the black ink dots and the ink dots of the other color are determined so as to meet a desired value of resolution of the image. For instance, one of the different print modes is selected by the operator of the printer. 
   In a second preferred form of the color ink-jet printer of the first aspect of this invention, the first and second control portions are operable to select the above-indicated one and another of the plurality of different total volume values, on the basis of respective gray-scale values at respective picture elements of the image at which respective ink dots are to be formed by the first and second ink ejecting portions according to print data. 
   In a third preferred form of the color ink-jet printer of the first aspect of the invention, the first and second control portions are operable to select the total volume of the above-indicated at least one droplet forming each dot of the black ink, and the total volume of the above-indicated at least one droplet forming each ink dot of the color other than black, differently depending upon local areas of the image, as long as the total volume value of the above-indicated at least one droplet forming each black ink dot is made larger than the total volume value of the above-indicated at least one droplet forming each ink dot of the color other than black, in each local area of the image in which the black ink dots are adjacent to the ink dots of the color other than black. 
   In a fourth preferred form of first aspect of the invention, the color ink-jet printer further comprises a pulse-waveform-data memory for storing pulse-waveform data indicative of a plurality of different waveforms of drive pulse signals to be applied to the first and second ink ejecting portions to eject the droplets of the black ink and the ink of the color other than black, the plurality of different waveforms corresponding to the plurality of different total volume values, respectively, and the first and second control portions are operable to select respectively one and another of the plurality of different waveforms which respectively correspond to the above-indicated one and another of the plurality of different total volume values. 
   In the fourth preferred form of the color ink-jet printer of the first aspect of the invention described above, the total volume of at least one droplet forming each black ink dot and the total volume of at least one droplet forming each ink dot of the color other than black can be controlled with high degrees of efficiency and accuracy, by reading the appropriate waveforms of the drive pulse signal from the pulse-waveform-data memory. 
   In a fifth preferred form of the first aspect of the invention, the color ink-jet printer further comprises first and second pulse generators operable to generate drive pulse signals to be applied to the first and second ink ejecting portions such that the total volume of the above-indicated at least one droplet forming each dot of the black ink and the total volume of the above-indicated at least one droplet forming each ink dot of the color other than black are variable as the number of the above-indicated at least one droplet to be ejected from each of the first and second ink ejecting portions is changed while the volume of each of the above-indicated at least one droplet is kept constant. 
   In a sixth preferred form of the first aspect of the invention, the color ink-jet printer further comprises first and second pulse generators operable to generate drive pulse signals to be applied to the first and second ink ejecting portions such that the total volume of the above-indicated at least one droplet forming each dot of the black ink and the total volume of the above-indicated at least one droplet forming each ink dot of the color other than black are changed by changing the volume of at least one of the above-indicated at least one ink droplet to be ejected from each of the first and second ink ejecting portions. For instance, each ink dot is provided by only one ink droplet, and the volume of this ink droplet is changed to change the size of the ink dot. 
   In another preferred form of the color ink-jet printer of the first aspect of the invention, the second control portion is operable to control the second ink ejecting portion such that the total volume of the above-indicated at least one droplet ejected by the above-indicated second ink ejecting portion to form each ink dot of the color other than black is next smaller than the above-indicated selected one total volume value. 
   In a further preferred form of the first aspect of the invention, the color ink-jet printer further comprises a third ink ejecting portion operable to eject droplets of an ink of another color other than black, and a third control portion operable to control the third ink ejecting portion such that a total volume of at least one droplet of the ink of the above-indicated another color other than black, which is ejected by the third ink ejecting portion to form each ink dot of the above-indicated another color other than black on the recording medium, is equal to the above-indicated another of the plurality of different total volume values. 
   The object indicated above may be achieved according to a second aspect of the present invention, which provides a color ink-jet printer comprising: a first ink ejecting portion operable to eject droplets of a first ink of a first color; a second ink ejecting portion operable to eject droplets of a second ink of a second color other than the first color; a first control portion operable to control the first ink ejecting portion such that a total volume of at least one droplet of the first ink ejected by the first ink ejecting portion to form each dot of the first ink on a recording medium is equal to a first value, when an image is formed on the recording medium with a predetermined resolution, with a predetermined gray-scale value at a picture element corresponding to the above-indicated each dot of the first ink; and a second control portion operable to control the second ink ejecting portion such that a total volume of at least one droplet of the second ink ejected by the second ink ejecting portion to form each dot of the second ink on the recording medium, is equal to a second value smaller than the first value, when said image is formed on the recording medium with the above-indicated predetermined resolution, with the above-indicated predetermined gray-scale value at a picture element corresponding to each dot of the second ink. 
   In the color ink-jet printer constructed according to the above-indicated second aspect of the invention, the total volume of the at least ink droplet forming each dot of the second color is made smaller than that of the at least one ink droplet forming each dot of the first ink. Accordingly, the relatively small dots of the second ink are dried on or absorbed by the recording medium at a higher rate, than the relatively large dots of the first ink. Therefore, substantially no bleeding of the inks takes place at the boundary between the ink dots of the first and second colors. Further, the present color ink-jet printer is not required to use the inks of the first and second colors to have different compositions. Namely, the present printer may use the first and second inks of the respective different first and second colors which have substantially the same composition which is formulated to avoid undesirable plugging of nozzles of the first and second ink ejecting portions. Where dots of a third ink of a third color different from the first and second colors are formed, the total volume value of at least one droplet of this third ink is selected to be a third value, which is equal to be the first or second value indicated above. The present printer is advantageous particularly when the first color is black. 
   The object indicated above may be achieved according to a third aspect of the invention, which provides a color ink-jet printer comprising: a first ink ejecting portion operable to eject droplets of a black ink; a second ink ejecting portion operable to eject droplets of an ink of a color other than black; a pulse-waveform-data memory for storing pulse-waveform data indicative of a plurality of different waveforms corresponding to respective different total volume values of at least one droplet of each of the black ink and the ink of the color other than black; a first control portion operable to select, for the at least one droplet for forming each dot of the black ink on a recording medium, one of the plurality of different waveforms stored in the pulse-waveform-data memory, and control the first ink ejecting portion to eject the at least one droplet, on the basis of the selected one of the plurality of different waveforms; and a second control portion operable to select, for the at least one droplet for forming each dot of the color other than black on the recording medium, another of the plurality of different waveforms stored in the pulse-waveform-data memory, and control the second ink ejecting portion to eject the at least one droplet, on the basis of the selected another of the plurality of different waveforms, wherein a total volume of the at least one droplet of the black ink ejected by the first ejecting portion is larger than a total volume of the at least one droplet of the ink of the color other than black ejected by the second ink ejecting portion. 
   In a first preferred form of the above-indicated third aspect of the present invention, the color ink-jet printer further comprises a first pulse generator operable to generate a drive pulse signal to be applied to the first ink ejecting portion, on the basis of the above-indicated one of the plurality of different waveforms selected by the first control portion, and a second pulse generator operable to generate a drive pulse signal to be applied to the second ink ejecting portion, on the basis of the above-indicated another of the plurality of different waveforms selected by the second control portion. 
   In a second preferred form of the color-ink-jet printer of the above-indicated third aspect of the present invention, the first and second control portions are operable to select the above-indicated one and another of the plurality of different waveforms, on the basis of a presently selected one of a plurality of different print modes which correspond to respective different values of resolution of an image to be printed on the recording medium, irrespective of gray-scale values at respective picture elements of the image. 
   In a third preferred form of the color ink-jet printer of the above-indicated third aspect of the present invention, the first and second control portions are operable to select the above-indicated one and another of the plurality of different waveforms, on the basis of respective gray-scale values at respective picture elements of an image at which respective ink dots are to be formed by the first and second ink ejecting portions according to print data. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features, advantages and technical and industrial significance of the present invention will be better understood by reading the following detailed description of a preferred embodiment of the invention, when considered in connection with the accompanying drawings, in which: 
       FIG. 1  is a perspective view schematically showing an internal arrangement of a color ink-jet printer constructed according to one embodiment of this invention; 
       FIG. 2  is an exploded perspective view showing a head unit included in the color ink-jet printer of  FIG. 1 , when the head unit is vertically inverted; 
       FIG. 3  is a fragmentary elevational view in cross section showing one of ink-jet heads of the head unit of  FIG. 2 ; 
       FIG. 4  is a block diagram illustrating a control portion of the color ink-jet printer of  FIG. 1 ; 
       FIGS. 5A–5D  are views indicating patterns of drive pulse signals to be applied to the ink-jet head of  FIG. 3 ; 
       FIG. 6  is a time chart illustrating an example of a combination of a total volume of droplets of one black ink dot on a paper sheet, and a total volume of droplets of one yellow ink dot, one magenta ink dot and one cyan ink dot on the paper sheet, when the color ink-jet printer of  FIG. 1  is placed in a fine print mode; and 
       FIG. 7  is a view illustrating an example of a pattern of ink dots of the different-color inks formed on the paper sheet in the color ink-jet printer of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the drawings, one preferred embodiment of the present invention will be described. 
   Reference is first made to the schematic perspective view of  FIG. 1  showing the internal arrangement of a color ink-jet printer constructed according to one embodiment of this invention. As shown in  FIG. 1 , the color ink-jet printer indicated at  1  incorporates a head unit  63  having a frame  68  on which there are fixed four piezoelectric ink-jet heads  6   a ,  6   b ,  6   c  and  6   d  arranged to eject droplets of inks of respective four colors, namely, yellow (Y), magenta (M), cyan (C) and black (K). On the frame  68 , there are also removably fixed four ink cartridges  61  which are filled with the inks of respective four colors and which are arranged to supply the inks to the respective ink-jet heads  6   a – 6   d . The frame  68  is attached to a carriage  64  which is reciprocated along a straight line by a drive mechanism  65 . The present color ink-jet printer  1  is further provided with a platen roller  66 , which is disposed such that its axis of rotation is parallel to a direction of reciprocation of the carriage  64 . The platen roller  66  is rotated by a drive device (not shown) to feed a recording medium in the form of a sheet of paper  62  in a feeding direction perpendicular to the direction of reciprocation of the carriage  64 . The four ink-jet heads  6   a – 6   b  are arranged in the direction of reciprocation of the carriage  64 , along a straight line which is parallel to the axis of rotation of the platen roller  66  and located adjacent to a circumferential surface of the platen roller  66 . 
   The carriage  64  is supported by a guide shaft  71  and a guide plate  72  which are disposed so as to extend in the axial direction of the platen roller  66 . The carriage  64  is slidable on these guide shaft and plates  71 ,  72  by the above-indicated drive mechanism  65 , which includes a pair of pulleys  73 ,  74  rotatably disposed near the respective opposite ends of the guide shaft  71 , and an endless belt  75  which connects the two pulleys  73 ,  74  and to which the carriage  64  is fixed. 
   The drive mechanism  65  further includes a drive motor  76  which is connected to the driving pulley  73  to rotate this pulley  73  in a selected one of opposite directions, for reciprocating the carriage  64  along the guide shaft and plate  71 ,  72 , to thereby reciprocate the head unit  63  in the direction of reciprocation of the carriage  64 . 
   The printer  1  is further provided with a sheet feeding mechanism (not shown) arranged to feed the paper sheet  62  from a sheet feeder cassette (not shown). The paper sheet  62  fed from the sheet feeder cassette is passed through a clearance or gap between an array of the ink-jet heads  6   a – 6   d  and the circumferential surface of the platen roller  66 , when an image is printed on the paper sheet  62 , with the droplets of inks ejected from the ink-jet heads  6   a – 6   d . The paper sheet  62  with the printed image is ejected onto a paper tray (not shown) by a sheet ejecting mechanism (not shown). 
   The color ink-jet printer  1  is further provided with a purge mechanism  67  arranged to remove, by suction, poor-quality inks which remain in the ink-jet heads  6   a – 6   d  and which contain air bubbles and foreign matter. The purge mechanism  67  is located near one of the opposite axial ends of the platen roller  66  such that the purge mechanism  67  is spaced apart from the above-indicated one end of the platen roller  66  in the axial direction away from the other end, so that the four ink-jet heads  6   a – 6   b  are sequentially aligned with the purge mechanism  67 , one after another, when the head unit  63  is returned to a predetermined home position by the drive mechanism  65 . The purge mechanism  67  has a purge cap  81  which is arranged to cover a multiplicity of nozzles  109  which are open in the lower surface of each ink-jet head  6   a ,  6   b ,  6   c ,  6   d , as shown in  FIGS. 2 and 3 . That is, the purge cap  81  is arranged to contact with an area of the lower surface of each head in which the nozzles  109  are open. 
   As shown in  FIG. 1 , the purge mechanism  67  includes a pump  82  which is driven by a cam  83 , when the head unit  63  is located at a position near its home position and the nozzles  109  of a selected one of the ink-jet heads  6   a – 6   d  are covered by the purge cap  81 . The poor-quality inks removed from the ink-jet heads  6   a – 6   d  by the purge mechanism  67  are discarded into a waste ink reservoir  84 . Thus, the purge mechanism  67  functions to sequentially restore the four ink-jet heads  6   a – 6   d  to their normal state, by removing the air bubbles from the inks within the ink-jet heads upon initial filling of the inks, for thereby preventing failures of the ink-jet heads to eject the ink droplets due to growth of the air bubbles during a printing operation of the head unit  63 . As shown in  FIG. 1 , four caps  85  are disposed near the purge cap  81 , so that the nozzles  109  of the four ink-jet heads  6   a – 6   d  are covered by the respective four caps  85  when the head unit  63  is located at its home position after a printing operation. 
   Referring next to the exploded perspective view of  FIG. 2  showing the head unit  63  in its vertically inverted posture, the frame  68  has a generally rectangular box structure which is open upwards as seen in  FIG. 1  (downwards as seen in  FIG. 2 ), so that the four ink cartridges  61  can be removably accommodated in a cartridge installation space within the box structure. 
   The frame  68  has a bottom wall  5  having an upper surface which partially defines the above-indicated installation space for accommodating the ink cartridges  61 , and a lower surface (upper surface as seen in  FIG. 2 ) to which the ink-jet heads  6   a – 6   d  are attached. This bottom wall  5  has four ink supply holes  51  formed therethrough between the above-indicated upper and lower surfaces such that the ink supply holes  51  are held in communication with ink outlet portions of the ink cartridges  61  accommodated in place in the cartridge installation space of the box structure of the frame  68 . To the lower surface of the bottom wall  5 , there are attached four rubber joints  47  which are arranged to connect the respective ink supply holes  51  to ink inlet portions of the respective ink-jet heads  6   a – 6   d.    
   As shown in  FIG. 2 , the bottom wall  5  has four head support portions  8  in the form of rectangular stepped recesses formed in its lower surface, so that the four ink-jet heads  6   a – 6   d  are partially received in the respective recesses, and fixed to the bottom wall  5  with a UV-curable adhesive agent which fills respective slots  9   a ,  9   b  formed through the thickness of the bottom wall  5 , near the recesses. The ink-jet heads  6   a – 6   d  thus supported by the head support portions  8  are covered by a covering member  44 , which has four elongate oval openings as shown in  FIG. 2 , so that the rows of the nozzles  109  of the ink-jet heads  6   a – 6   d  are exposed through the respective openings. As also shown in  FIG. 2 , the four ink-jet heads  6   a – 6   d  are provided with respective flexible printed-circuit (FPC) boards  40  fixed thereto for applying drive pulse signals to their actuator units  106  ( FIG. 3 ). As described below by reference to  FIGS. 5A–5D , each drive pulse signal selectively has a ground potential and a positive potential. 
   Referring further to the fragmentary cross sectional view of  FIG. 3 , there is shown the ink-jet head  6   a , by way of example. The other three ink-jet heads  6   b ,  6   c  and  6   d  have the same construction as the ink-jet head  6   a , which will be described in detail. It is noted, however, that the four ink-jet heads  6   a ,  6   b ,  6   c  and  6   d  are assigned to eject droplets of a yellow (Y) ink, a magenta (M) ink, a cyan (C) ink and a black (K) ink, respectively. In the present embodiment, the ink-jet head  6   d  functions as a first ink ejecting portion, while the other ink-jet heads  6   a ,  6   b  and  6   c  function as a second ink ejecting portion. Alternatively, the ink-jet heads  6   a ,  6   b ,  6   c  and  6   d  are considered to function as respective four ejecting portions. For instance, the head  6   d  functions as the first ink ejecting portion, and the head  6   a  functions as the second ink ejecting portion, while the head  6   b  or  6   c  functions as a third ink ejecting portion. 
   As shown in  FIG. 3 , the ink-jet head  6   a  has an actuator unit  106  and a flow-passage unit  107  superposed on each other. The actuator unit  106  is driven or operated according to a drive pulse signal generated from a control portion  11  (shown in  FIG. 4 ) of the printer  1 , and the flow-passage unit  107  has a multiplicity of ink passages communicating with the respective nozzles  109 . The actuator unit  106  and the flow-passage unit  107  are bonded together with a thermosetting adhesive agent such as an epoxy resin. Although the FPC boards  40  are bonded to the upper surface of the actuator unit  106 , these FPC boards  40  are not shown in  FIG. 3 , in the interest of brevity. 
   The flow-passage unit  107  is a laminar structure consisting of three thin metal plates (a cavity plate  107   a , a spacer plate  107   b  and a manifold plate  107   c ) formed of a metallic material, and a nozzle plate  107   d  formed of a synthetic resin material such as polyimide. The uppermost cavity plate  107   a  is bonded at its upper surface to the actuator unit  106 . 
   The ink-jet head  6   a  has two parallel rows of pressure chambers  110  formed through the cavity plate  107   a  such that the pressure chambers  110  in each row are arranged and spaced apart from each other by partition walls  110   a , in the longitudinal direction of the ink-jet head  6   a . The pressure chambers  110  are filled with the yellow ink, so that droplets of the ink are ejected from the selected ones of the nozzles  109  upon selective operation of the corresponding local active portions of the actuator unit  106 . The spacer plate  107   b  has a communication hole  111  for communication of each pressure chamber  110  at one of its opposite ends with the corresponding nozzle  109 , and another communication hole (not shown) for communication of each pressure chamber  110  at the other end with a manifold passage (not shown) formed in the manifold plate  107   c.    
   The manifold plate  107   c  has a communication hole  113  for communication between the communication hole  111  and the corresponding nozzle  109 . Since the pressure chambers  110  are arranged in the two rows, the manifold plates  107   c  has two manifold passages corresponding to these two rows of the pressure chambers  110 . Each of the manifold passages is elongated so as to extend in a direction of arrangement of the pressure chambers  110  of the corresponding row, and is located below that row. Each manifold passage is connected at one of its longitudinally opposite ends with the corresponding one of the four ink cartridges  61  through the corresponding one of the four ink supply holes  51  (shown in  FIG. 2 ). Thus, the flow-passage unit  107  has a multiplicity of ink passages each of which extends from the manifold passage to the corresponding nozzle  109  through the above-indicated another communication hole, the pressure chamber  110  and the communication holes  111 ,  113 . 
   The actuator unit  106  is a laminar structure consisting of six piezoelectric ceramic plates  106   a – 106   f  formed of lead zirconate titanate (PZT). Two common electrodes  121  are formed between the piezoelectric ceramic plates  106   b  and  106   c , while two common electrodes  123  are formed between the piezoelectric ceramic plates  106   d  and  106   e , such that the two common electrodes  121  are aligned with respective two areas of the flow-passage unit  107  in which the respective two rows of pressure chambers  110  are formed, and the two common electrodes  123  are aligned with those two areas, respectively. Two rows of multiple individual electrodes  122  are formed between the piezoelectric ceramic plates  106   c  and  106   d  such that the individual electrodes  122  are aligned with the respective pressure chambers  110  of the two rows, while two rows of multiple individual electrodes  124  are formed between the piezoelectric ceramic plates  106   e  and  106   f  such that the individual electrodes  124  are aligned with the respective pressure chambers  110  of the two rows. 
   The common electrodes  121 ,  123  are kept at the ground potential, and the individual electrodes  122 ,  124  are selectively energized according to the drive pulse signals. The portions of the piezoelectric ceramic plates  106   c – 106   e  which are located between the common electrodes  121 ,  123  and the individual electrodes  122 ,  124  function as active portions  125  which have been polarized in the direction of lamination of the plates  106   a – 106   f , with an electric field applied thereto through the electrodes  121 – 124 . When each individual electrode  122 ,  124  is given a predetermined positive potential, the corresponding active portion  125  is subjected to an electric field and is expanded in the direction of lamination while the corresponding local portion of the piezoelectric ceramic plates  106   a ,  106   b  maintain the original state, so that the active portion  125  is expanded so as to partially protrude into the corresponding pressure chamber  110 , whereby the volume of the pressure chamber  110  is reduced, with a result of application of a pressure to the ink in the pressure chamber  110 , causing the ink to be ejected from the nozzle  109 . 
     FIG. 3  shows the two adjacent pressure chambers  110  placed in different states, for explaining the operation of the actuator unit  106 . The individual electrodes  122 ,  124  corresponding to the left one of the pressure chamber  110  are given the predetermined positive potential, and the corresponding active portion  125  of the actuator unit  106  is expanded so as to be convex toward the left pressure chamber  110 , so that the volume of the left pressure chamber  110  is reduced, whereby the ink is ejected from the nozzle  109  communicating with the left pressure chamber  110 . On the other hand, the drive pulse signal to be applied to the individual electrodes  122 ,  124  corresponding to the right pressure chamber  110  is such that the individual electrodes  122 ,  124  are held at the ground potential, like the common electrodes  121 ,  123 , so that the ink is not ejected from the nozzle  109  communicating with the right pressure chamber  110 . 
   In the present embodiment, the ink-jet heads  6   a – 6   d  are operated to perform so-called “fill-before-fire” actions for ejecting droplets of ink. Where the fill-before-fire action is performed by the ink-jet head  6   a , for example, all of the pressure chambers  110  are normally placed in a reduced-volume state, like the left pressure chamber  110  shown in  FIG. 3 . Namely, all of the individual electrodes  122 ,  124  are normally kept at the predetermined positive potential, so that the active portions  125  are all expanded to be convex toward the respective pressure chambers  110 . The individual electrodes  122 ,  124  corresponding to each nozzle  109  from which the ink is required to be ejected are given the ground potential at appropriate timings, so that the volume of the corresponding pressure chamber  110  is increased, like the right pressure chamber  110  shown in  FIG. 3 . As a result, a negative pressure wave is generated in the pressure chamber  110 , and the generated pressure wave propagates through the pressure chamber  110  in its longitudinal direction. When the negative pressure wave is changed into a positive pressure wave, the individual electrodes  122 ,  124  are again given the predetermined positive potential, so that the corresponding active portion  125  is expanded so as to be convex toward the pressure chamber  110 , with a result of pressurizing the ink within the pressure chamber  110 . This fill-before-fire action permits a high rate of ejection of the ink droplets with a comparatively low drive voltage. 
   Referring further to the block diagram of  FIG. 4 , there will be described a control portion  11  of the color ink-jet printer  1 , which is arranged to control the operations of the ink-jet heads  6   a – 6   d . The control portion  11  includes a print-data memory portion  12  for storing print data received from an external device such as a personal computer. The print data to be stored in the print-data memory portion  12  includes bit map data representative of a gray-scale value (eight-bit data indicative of one of 256 gray-scale values) at each picture element of an image, for each of the four colors (YMCK). 
   The control portion  11  further includes a print-mode memory portion  22  and a pulse-waveform-data memory portion  24 . The print-mode memory portion  22  is provided to store print-mode data indicative of one of a plurality of print modes (four modes consisting of a SUPER FINE mode, a FINE mode, a NORMAL mode and a DRAFT mode, in this specific embodiment), which has been selected by an operator of the printer  1 , depending upon a desired quality of an image to be printed. 
   The pulse-waveform-data memory portion  24  is provided to store pulse-waveform data indicative of four different waveforms of the drive pulse signal to be applied to the individual electrodes  122 ,  124  of the actuator unit  106 , to eject at least one droplet of ink from the corresponding nozzle  109 . The four different waveforms correspond to respective four ink dots of different sizes, namely, a large dot, a medium dot, a small dot and a very small dot, which correspond to respective four different total volume values of 36 pl, 24 pl, 12 pl and 5 pl of at least one ink droplet, as described below in detail. 
     FIGS. 5A–5D  show the four different waveforms of the drive pulse signal corresponding to the respective large, medium, small and very small dots. In these figures, high level (H) and low level (L) respectively correspond to a low voltage and a high voltage applied to the individual electrodes  122 ,  124 . The waveform of the drive pulse signal of  FIG. 5A  for the large ink dot (36 pl) has four high-level periods H 11 , H 12 , H 13  and H 14  (for placing the pressure chamber  110  in an increased-volume state, like the right pressure chamber  110  shown in  FIG. 3 ), and low-level periods (for placing the pressure chamber  110  in the reduced-volume state, like the left pressure chamber  110  shown in  FIG. 3 ) adjacent to the high-level periods. Upon termination of the first three high-level periods H 11 , H 12  and H 13  (each of which is about 4–6 μs), respective three ink droplets (each having a volume of 12 pl) are ejected from the corresponding nozzle  109  by the above-indicated fill-before-fire actions during a feeding movement of the carriage  64 , so that one large ink dot (36 pl) is formed by the three ink droplets which overlap each other on the paper sheet  62 . The fourth high-level period H 14  (which is about 3 μs) is provided not for ejecting an ink droplet, but for offsetting a variation in the ink pressure remaining in the pressure chamber  110  in question, in order to avoid an adverse influence of the present ink ejection on the next ink ejection associated with the same pressure chamber  110 . 
   The waveform of the drive pulse signal of  FIG. 5B  for the medium ink dot (24 pl) has three high-level periods H 21 ,  22  and H 23 , and the adjacent low-level periods. Upon termination of the first two high-level periods H 21  and H 22  (each of which is about 4–6 μs), respective two ink droplets (each having a volume of 12 pl) are ejected from the corresponding nozzle  109  by the above-indicated fill-before-fire actions, so that one medium ink dot (24 pl) is formed by the two ink droplets which partially overlap each other on the paper sheet  62 . The third high-level period H 23  (which is about 3 μs) is provided not for ejecting an ink droplet, but for offsetting a variation in the ink pressure remaining in the pressure chamber  110  in question. 
   The waveform of the drive pulse signal of  FIG. 5C  for the small ink dot (12 pl) has two high-level periods H 31  and H 32 , and the adjacent low-level periods. Upon termination of the first one high-level period H 31  (which is about 4–6 μs), one ink droplet (having a volume of 12 pl) is ejected from the corresponding nozzle  109  by the above-indicated fill-before-fire action, so that one small ink dot (12 pl) is formed by the one ink droplet on the paper sheet  62 . The second high-level period H 32  (which is about 3 μs) is provided not for ejecting an ink droplet, but for offsetting a variation in the ink pressure remaining in the pressure chamber  110  in question. 
   The waveform of the drive pulse signal of  FIG. 5D  for the very small ink dot (5 pl) has three high-level periods H 41 , H 42  and H 43 , and the adjacent low-level periods. The first one high-level period H 41  (which is about 4–6 μs) for ejecting one ink droplet (having a volume of 12 pl) is followed by the relatively short first low-level period which precedes the second high-level period H 42  (which is about 2 μs), so that a trailing end portion of an ink droplet being ejected from the corresponding nozzle  109  is fed back into the nozzle  109  by the drawing action caused by the second short high-level period H 42 , whereby the ink droplet actually ejected from the nozzle  109  has a volume of about 5 pl. As a result, one very small ink dot (5 pl) is formed on the paper sheet  62 . The third high-level period H 43  (which is about 3 μs) is provided not for ejecting an ink droplet, but for offsetting a variation in the ink pressure remaining in the pressure chamber  110  in question. 
   The control portion  11  further includes four ink-volume determining portions  13 ,  14 ,  15  and  16  corresponding to the respective four colors Y, M, C and K, that is, a Y-ink-volume determining portion  13 , an M-ink-volume determining portion  14 , a C-ink-volume determining portion  15  and a K-ink-volume determining portion  16 . The ink-volume determining portions  13 – 16  are arranged to determine the total ink volume value to be ejected from each nozzle  109  of the corresponding ink-jet heads  6   a – 6   d , irrespective of the gray-scale value at the picture element in question, but on the basis of the presently selected print mode indicated by the print-mode data stored in the print-mode memory  22 . The Y-ink-volume, M-ink-volume and C-ink-volume determining portions  13 – 15  determine the total volume of each of the yellow, magenta and cyan inks such that this total volume is smaller than the total volume of the black ink to be determined by the K-ink-volume determining portion  16 , whatever the presently selected print mode is. Thus, the total volume of the at least one droplet forming each black ink dot ejected onto the paper sheet  62  is kept at a value which is determined depending upon the presently selected print mode, for all of the black ink dots throughout the operations of the ink-jet heads  6   a – 6   d  to print an image on the paper sheet  62 , irrespective of the gray-scale value at each picture element of the image. Similarly, the total volume of the at least one droplet forming each dot of the other three colors ejected onto the paper sheet is kept at another value which is determined depending upon the presently selected print mode, for all of the ink dots of those other colors throughout the operations of the ink-jet heads  6   a – 6   d , irrespective of the gray-scale value at each picture element of the image, such that the above-indicated another value for the dots of the other colors is smaller than the value for the dots of the black color. Thus, the total volume values of each black ink dot and each ink dot of the other colors, that is, the sizes of the black ink dots and the ink dots of the other colors are determined by the print mode selected by the operator of the printer  1 , so that the image can be printed on the paper sheet  62 , with a quality desired by the operator, namely, so as to meet an operator&#39;s desired value of resolution of the image. In this embodiment, the gray-scale image may be obtained by a suitable gray-scale reproduction technique such as a dither method and a density pattern method. 
   Described in detail by reference to Table 1 given below, the large total volume value (36 pl) is selected to form a large ink dot of the black color, while the medium total volume value (24 pl) is selected to form medium ink dots of the other colors, when the DRAFT mode is selected as the print mode. When the NORMAL mode is selected, the medium total volume value (24 pl) is selected to form a medium ink dot of the black color, while the small total volume value (12 pl) is selected to form small ink dots of the other colors. When the FINE mode is selected, the small total volume value (12 pl) is selected to form a small ink dot of the black color, while the very small total volume value (5 pl) is selected to form very small ink dots of the other colors. When the SUPER FINE mode is selected, the very small total volume value (5 pl) is selected to form a very small ink dot of the black color and very small ink dots of the other colors. Accordingly, the resolution of a printed image (density of the ink dots formed per unit area on the paper sheet  62 ) is increased as the print mode is changed in the direction from the DRAFT mode toward the SUPER FINE mode through the NORMAL and FINE modes. In the SUPER FINE mode wherein the very small total volume value (5 pl) is selected for not only the ink dot of the black color but also the ink dots of the other colors, substantially no bleeding of the black ink from the black ink dot to the ink dots of the other colors takes place, so that the printed image does not suffer from a problem of color mixing at the boundary of the ink dots of the different colors. 
   
     
       
         
             
           
             
               TABLE 1 
             
           
          
             
                 
             
             
               Total Volumes of Ink Dots of Different Colors 
             
          
         
         
             
             
             
          
             
                 
               Yellow (Y), Magenta (M), 
               Selected Print 
             
             
               Black (K) Dot 
               and Cyan (C) Dots 
               Mode 
             
             
                 
             
             
               LARGE 
               MEDIUM 
               DRAFT 
             
             
               MEDIUM 
               SMALL 
               NORMAL 
             
             
               SMALL 
               VERY SMALL 
               FINE 
             
             
               VERY SMALL 
               VERY SMALL 
               SUPER FINE 
             
             
                 
             
          
         
       
     
   
   The control portion  11  further includes four pulse generators  17 ,  18 ,  19 ,  20  for the respective colors Y, M, C and K, namely, a Y-pulse generator  17 , an M-pulse generator  18 , a C-pulse generator  19  and a K-pulse generator  20 . The pulse generators  17 – 20  are arranged to generate drive pulse signals of appropriate waveforms to be applied to the respective ink-jet heads  6   a – 6   d , on the basis of the total volume values of at least one droplet of ink determined by the ink-volume determining portions  13 – 16 , and according to the waveform patterns stored in the pulse-waveform-data memory portion  24 , so that the ink dots of the black color and the ink dots of the other colors which are ejected from the nozzles  109  of the ink-jet heads  6   a – 6   d  have the sizes corresponding to the determined total volume values. The drive pulse signals generated by the pulse generators  17 – 20  are applied to the respective ink-jet heads  6   a – 6   d.    
   The control portion  11  including the various portions  12 – 20 ,  22 ,  24  is constituted by a central processing unit (CPU), a random-access memory (RAM), a read-only memory (ROM), etc. The ROM serves as the pulse-waveform-data memory portion  24  storing the pulse-waveform data, and stores other software such as control programs and data for various operations to be performed by the control portion  11 . 
   In the present embodiment, the print-data memory portion  12 , print-mode memory portion  22  and K-ink-volume determining portion  16  cooperate with each other to constitute a first control portion, while the print-data memory portion  12  and the print-mode memory portion  22  cooperate with each of the Y-ink-volume, M-ink-volume and C-ink-volume determining portions  13 ,  14  and  15  to constitute a second control portion. The first control portion is operable to control the actuator unit  106  of the ink-jet head  6   d  such that a total volume of at least one droplet of a black ink ejected by the ink-jet head  6   d  to form each black ink dot on the paper sheet  62  is equal to a selected one of a plurality of different total volume values (36 pl, 24 pl, 12 pl and 5 pl), while the second control portion is operable to control the actuator unit  106  of the ink-jet heads  6   a ,  6   b  and  6   c  such that a total volume of at least one droplet of each of yellow, magenta and cyan inks, which is ejected by the ink-jet heads  6   a ,  6   b ,  6   c  to form each ink dot of these colors on the paper sheet  62  is equal to another of the different total volume values which is smaller than that of the ink-jet head  6   d . Alternatively, the determining portions  13 – 16  are considered to function as parts of respective four control portions. For instance, the determining portion  16  functions as part of the first control portion, and the determining portion  13  functions as part of the second control portion, while the determining portion  14  or  15  functions as part of a third control portion. 
   Referring further to  FIGS. 6 and 7 , there will be described an example of a printing operation of the present color ink-jet printer  1  to form an image on the paper sheet  62 .  FIG. 6  is a time chart illustrating an example of a combination of the total volume of one black ink dot to be formed on the paper sheet  62 , and the total volume of each of one yellow ink dot, one magenta ink dot and one cyan ink dot to be formed on the paper sheet  62 , and  FIG. 7  is a view illustrating an example of a pattern of ink dots of the different-color inks formed on the paper sheet  62  by the color ink-jet printer  1 . 
   In the specific example of  FIG. 6 , the FINE print mode is selected, so that the small total volume value is selected to form the small black ink dots on the paper sheet  62 , as indicated in  FIG. 7 , while the very small total value is selected to form the very small yellow, magenta and cyan ink dots on the paper sheet  62 , as also indicated in  FIG. 7 , throughout the printing operation in the FINE print mode. Accordingly, the yellow, magenta and cyan ink dots are dried at a higher rate or absorbed by the paper sheet  62  at a higher rate, than the black ink dots. Therefore, substantially no bleeding of the inks takes place at the boundary between the dots of the black ink and the dots of the yellow, magenta and cyan inks. Even where some bleeding of the inks takes place at the boundary between the yellow, magenta and cyan ink dots, this bleeding does not have a significant influence on the quality of the printed image. Further, the black, yellow, magenta and cyan inks used by the present color ink-jet printer  1  have substantially the same composition which is formulated to avoid undesirable plugging of the nozzles  109  of the ink-jet heads  6   a – 6   d  due to drying of the inks. 
   In the present embodiment wherein the total volume of the at least droplet forming each blank ink dot is made larger than that of the at least one droplet forming each ink dot of the other colors, except in the SUPER FINE print mode, the printing speed of an image can be increased even where the printing of the image involves solid printing with the black ink only. In this respect, it is noted that the black ink is generally used at a considerably higher frequency than the inks of the other colors, to effect the solid printing so as to emphasize letters of words, for example, in a comparatively large area on the recording medium. 
   As shown in TABLE 1, the total volume of at least one droplet forming each black ink dot and the total volume of at least one droplet forming each ink dot of the other colors are selected to be the predetermined different volume values (e.g., 12 pl and 5 pl in the FINE print mode) which are adjacent to each other, so that the difference between these two total volume values is relatively small. Accordingly, even where the black ink dots and the ink dots of the other colors are formed alternately in the direction of feeding of the carriage  64  (ink-jet heads  6   a – 6   b ), it is not necessary to change the feeding speed of the carriage  64  depending upon the size of the ink dot being formed, so that the printing operation can be performed with high efficiency, at a relatively high feeding speed of the carriage  64 . If the above-indicated difference is relatively large, the feeding speed of the carriage  64  must be determined to be relatively low depending upon the ink dots of the smaller size (the smaller total volume value). 
   It will be understood from the foregoing description that the present color ink-jet printer  1  is capable of performing a printing operation with high efficiency at a relatively high feeding speed of the carriage  64 , while minimizing undesirable deterioration of quality of a printed image due to color mixing at the boundary of the ink dots and plugging of the nozzles with the dried ink. 
   The present embodiment is further arranged such that the pulse-waveform-data memory  24  stores pulse-waveform data indicative of four pulse waveforms corresponding to the large, medium, small and very small ink dots, and such that the ink-volume determining portions  13 – 16  read out the appropriate pulse waveforms from the pulse-waveform-data memory  24 , for the respective ink-jet heads  6   a – 6   d  corresponding to the four different colors, on the basis of the presently selected print mode indicated by the print-mode data stored in the print-mode memory portion  22 . This arrangement permits efficient and accurate control of the ink-jet heads  6   a – 6   d  so as to suitably determine the ink dot sizes for the different colors. 
   While the embodiment described above is arranged such that the size of the black ink dots and the size of the ink dots of the other colors are determined or selected on the basis of the presently selected print mode, the sizes of the black ink dots and the ink dots of the other colors may be selected on the basis of the gray-scale value at picture elements of the image at which respective ink dots are to be formed by the ink-jet heads  6   a – 6   d  according to the print data. In the NORMAL print mode of TABLE 1, for instance, the total volume of one black ink dot may be determined such that the total volume is variable in steps from among the medium total volume value, the small total volume value, and the very small total volume value, according to the gray-scale values at corresponding picture elements, while the total volume of each ink dot of the other colors may be determined such that the total volume is variable in steps from among the small total volume value and the very small total volume value, according to the gray-scale values at corresponding picture elements. In this second embodiment, too, the size of each blank ink dot (total volume value of at least one droplet forming each black ink dot) is made larger than that of the ink dots of the yellow, magenta and cyan inks, except when the very small black ink dot (very small total volume value of 5 pl) is selected, as in the first embodiment. In this embodiment, the size of each black ink dot is determined such that the size increases with an increase in the gray-scale value at the corresponding picture element. 
   In the first and second embodiments described above, the pulse generators  17 – 20  are arranged to generate the drive pulse signals to be applied to the ink-jet heads  6   a – 6   d  such that the number of at least one ink droplet (12 pl) to be ejected from the nozzle  109  to form each dot on the paper sheet  62  is changed depending upon the presently selected print mode or the gray-scale values at the corresponding picture elements, to select one of three sizes of each ink dot, namely, to select one of the large dot (36 pl), medium dot (24 pl) and small dot (12 pl), while the volume of each ink droplet is kept constant. In a third embodiment of this invention, however, the size of each ink dot is changed by changing the volume of each of at least one ink droplet to be ejected from the nozzle  109  to form each dot, by controlling a drive voltage to be applied to the individual electrodes  122 ,  124 , or the waveform of a drive pulse signal to apply the drive voltage. 
   In the illustrated embodiments described above, the total volume values of the dots of the yellow, magenta and cyan inks are next smaller than the total volume value of the dots of the black ink, in the DRAFT, NORMAL and FINE print modes, as indicated in TABLE 1. However, the combinations of the total volume values are not limited to those indicated in TABLE 1. For instance, when the large black dots are selected, the small or very small dots of the yellow, magenta and cyan inks may be selected. Although the four ink-jet heads  6   a – 6   d  corresponding to the four different colors (Y, M, C and K) are provided in the illustrated embodiments, a color ink-jet printer may include two, three, five or more ink-jet heads which correspond to respective different colors, provided these colors include black. 
   While the illustrated embodiments are arranged such that a desired one of a plurality of different print modes is selected by the operator of the printer  1 , the principle of the present invention is equally applicable to a color ink-jet printer in which only one print mode is available. 
   In the first and third embodiments described above, the size of the black ink dots and the size of the ink dots of the other colors are determined or selected to be respective constant values in the entire area of the image, irrespective of the gray-scale values at the corresponding picture elements at which respective ink dots are to be formed by the ink-jet heads  6   a – 6   d  according to the print data which are stored in the print-data memory portion  12  to print a desired image. However, the determination of the sizes of the black ink dots and the ink dots of the other colors may be made differently depending upon local areas of the image, as long as the total volume value of at least one droplet forming each black ink dot is made larger than the total volume value of at least one droplet forming each ink dot of the other colors, in each local area of the image in which the black ink dots are adjacent to the ink dots of the other color or colors. Described more specifically, in the NORMAL print mode of TABLE 1 of the illustrated first embodiment, for instance, the total volume of one black ink dot is kept constant at the medium total volume value while the total volume of each ink dot of the other color or colors is kept constant at the small total volume value in the entire area of the image. However, in one local area of the image, the medium total volume may be selected to form the medium ink dot of the black color and the small total volume may be selected to form the small ink dot of the other color or colors, while, in another local area of the image, the small total volume may be selected to form the small ink dot of the black color and the very small total volume may be selected to form the very small ink dot of the other color or colors. Further, the total volume value of at least one droplet forming each black ink dot need not be made larger than the total volume value of at least one droplet forming each ink dot of the other color or colors in a local area of the image in which the black ink dots are not adjacent to the ink dots of the other color or colors.