Abstract:
A complete image is obtained by performing printing scan for M times (M is an integral number: M ≧1) using N printing elements (N is an integral number: N≧2) on the same printing area of in a front side of the printing medium in the direction of transporting the printing medium. On the other hand, a complete image is also obtained by performing printing scan for K times (K is an integral number: K &gt;M) using L printing elements (L is an integral number: L≦N) on the same printing area in rear side of the printing medium in the direction of transporting the printing medium.

Description:
[0001]    This application is based on Patent Application No. 2000-335179 filed Nov. 1, 2000 in Japan, the content of which is incorporated hereinto by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Feild of the Invention  
           [0003]    The present invention relates to an ink jet printing method and apparatus on a multi-path printing system in which a desired image is formed completely on a printing medium by a plurality of scanning movements of a printing head on the same printing area of the printing medium.  
           [0004]    2. Description of the Related Art  
           [0005]    [0005]FIGS. 1A, 1B, and  1 C are schematic diagrams that illustrate a transport stage for transporting a printing medium in a typical serial type printing apparatus.  
           [0006]    In those figures, the reference numeral  509  denotes a printing medium (e.g., a sheet of paper or a plastic sheet). The printing medium  509  can be fed in a sub-scan direction from the right side to the left side as indicated by an arrow in each figure. In other words, the printing medium  508  receives the sheet-feed force when it is sandwiched between the transport roller  501  and the pinch roller  503 . The transport roller  501  can be driven by a sheet-feed motor (not shown). The pinch roller  502  is pressed against that transport roller  501  by the spring pressure, so that it can be rotated together with the transport roller  501 . The sheet-ejection rollers  503 ,  504  are also arranged as a two-stage sheet-ejection roller so that they work together with the transport roller  501 . In addition, sheet-ejection spurs  505 ,  506  are pressed against the rollers  503 ,  504  by the spring pressures, respectively.  
           [0007]    The feed amount of the printing medium  508  due to the rotational movements of the sheet-ejection rollers  503 ,  504  is adjusted to about 100.3% of the feed amount of the printing medium  508  due to the rotational movement of the transport roller  501  because the printing medium swells by the application of ink under printing in an ink jet printing system. If the feed amount of the printing medium  508  defined by the sheet-ejection rollers  503 ,  504  were set to the same level as one defined by the transport roller  501 , the swell amount of the printing medium  508  would be accumulated, causing a failed sheet-feed between these rollers.  
           [0008]    In the sheet-feed stage shown in FIG. 1A, the printing medium  508  becomes caught between the rollers using the upper portions of the sheet-ejection rollers  503 ,  504 , and the transport roller  501 , respectively. If the printing medium  508  were not swelled under the condition shown in FIG. 1A, the printing medium  508  would be slipped over the contacted surfaces of the respective sheet-ejection rollers  503 ,  504 . In this case, therefore, the feed amount of the printing medium  508  could be depended on the transport thereof by the transport roller  501 .  
           [0009]    Furthermore, the reference numeral  507  denotes a printing head that performs a printing scan in the direction perpendicular to the plane of each of FIGS. 1A, 1B, and  1 C (i.e., main-scan direction). The reference numeral  509  denotes a printing area of the printing medium  508  to be printed by the printing head  507 .  
           [0010]    [0010]FIG. 1A illustrates the state in which two distinctive sheet-feed forces are being applied on the printing medium  508  by the transport roller  501  and sheet-ejection rollers  503 ,  504 , respectively. FIG. 1B illustrates a moment when the rear end of the printing medium  508  left the space between the pinch roller  502  and the transport roller  501 . FIG. 1C illustrates the state in which the sheet-feed force is being applied on the printing medium  508  only by the sheet-ejection rollers  503 ,  504 .  
           [0011]    Referring now to FIG. 2, a two-path printing system will be described as an example of the conventional multi-path printing system, in which an image formation is completed by moving the printing head  507  for two scans per pixel.  
           [0012]    The printing head  507  in FIG. 2 is represented as one that relatively moves toward a lower place of the figure with respect to the printing medium  508  intermittently transported in the sub-scan direction. As the present example is the two-path printing system, as those of the steps S 201  to S 207 , the position of the printing head  507  in the sub-scan direction relatively deviates downward by one-half of a printing element width in the sub-scan direction. An image can be formed on each of the printing areas P 0  to P 14  on the printing medium  508  by two printing scans of the printing head  507 . In the figure, “A 2 ” denotes a mask that allows 50% of image data remaining. “B 2 ” denotes a mask that interpolates the image data that mask A 2  does not remain.  
           [0013]    The images are formed on the printing medium  508  one after another by alternately repeating: the printing scan of the printing head  507  on the basis of image data thinned out by alternately using the masks A 2 , B 3 ; and the transport of the printing medium  508  in the sub-scan direction by shifting it by one-half of the printing element width of the printing head  507 . More specifically, in the step S 201 , the printing scan is performed on the basis of the image data thinned out using the mask A 2 , and thereafter, the printing medium  508  is fed in the sub-scan direction by one half of the printing element width of the printing head  507 . Subsequently, in the step S 202 , the printing scan is performed on the basis of the image data thinned out using the mask B 2 , and thereafter, the printing medium  508  is fed in the sub-scan direction by one-half of the printing element width of the printing head  507 . Thereafter, the same procedure is successively repeating to form images on the printing medium  508  in succession.  
           [0014]    In the multi-path printing system of FIG. 2, for example, the following description will be intended to represent that a period until step S 204  correspond to the state shown in FIG. 1A and a period from step S 05  correspond to the state shown in FIG. 1B. In this case, during the transport of the printing medium  508  from S 204  to S 205 , the rear end of the printing medium  508  paths through a point PA between the steps S 204  and S 205 , where it comes out the pinch roller  502  (i.e., in the state shown in FIG. 1B).  
           [0015]    The printing head  507  may be an ink jet printing head having a plurality of printing elements possible to eject ink from their nozzles. In this example, the number of nozzles is 256 with 1200-dot/inch resolution. In this case, it is possible to form ink dots uniformly on the printing medium  508  as shown in FIG. 3 under the condition until S 204  that correspond to the state of FIG. 1A. In the state of FIG. 1A, the printing medium  508  may be intermittently fed in the sub-scan direction to distances of about 2700 μm that correspond to 128 nozzles. In the period from S 205  corresponding to the state of FIG. 1C, the feed amount of the printing medium  508  may be about 2708 μm corresponding to about 100.3% of about 2700 μm. As a result, as shown in FIG. 4, each of positions on which ink dots have been formed deviates by about 8 μm from its ideal positions in the sub-scan direction, where such a deviation corresponds to the difference between the feed amounts of the printing medium  508  obtained at the states of FIG. 1A and FIG. 1C. Furthermore, if the printing medium  508  is shifted from the stage of FIG. 1A to the state of FIG. 1C through the state of FIG. 1B, the feed amount of the printing medium  508  may be additionally increased by about 8 μm in addition to that difference feed amount 8 μm. Because, it is conceivable that the difference between the feed amounts of the sheet-ejection rollers  503 ,  504  and the transport roller  501  cannot be substantially removed by the slippage of the sheet-ejection rollers  503 ,  504  and the stresses stored in the printing medium  508  and the sheet-ejection rollers  503 ,  504  are then released at the instant when the printing medium  508  comes off the pinch roller  502 . In other words, when the printing medium  508  is shifted to the state of FIG. 1C through that of FIG. 1B, distances of about 8 μm corresponding to the difference between the feed amounts of the sheet-ejection rollers  503 ,  504  and the transport roller  501  is added to distances of about 8 μm depending on the detachment of the printing medium  508  from the pinch roller  502 . As a result, as shown in FIG. 5, the ink-dot-forming position is displaced from the ideal ink-spotting position up to that sum ((i.e., about 16 μm in total) as the maximum displacement thereof.  
           [0016]    The printing density, i.e., the number of ink dots which can be formed per unit area of the printing medium  508 , in each of FIGS. 3, 4, and  5  shows a correlation with the coverage of ink on the printing medium  508 , which is given by the following expression.  
           [Printing density in FIG.  3 ]&gt;[Printing density in FIG.  4 ]&gt;[Printing density in FIG.  5 ] 
           [0017]    In the case of FIG. 2, the printing medium  508  comes off the pinch roller  502  during the transition from the step S 204  to the step S 205 . Thus, the printing areas up to P 0  in FIG. 1 become those of having the printing density shown in FIG. 3, the printing areas from P 5  become those of having the printing density shown in FIG. 4, and the printing areas from P 1  to P 4  become those of the printing density shown in FIG. 5. As a result, uneven density can be observed as different density bands generated in the resulting image as shown in FIG. 6.  
         SUMMARY OF THE INVENTION  
         [0018]    An object of the present invention is to provide a method and apparatus of printing an image on a printing medium, in which the printing speed is reduced as much as possible, furthermore the deterioration of the image quality on the latter part of the printing medium in the transport direction can be substantially reduced.  
           [0019]    In a first aspect of the present invention, there is provided a printing apparatus for printing an image on a printing medium by repeating a printing scan of a printing head having a plurality of printing elements in a main-scan direction and a transport of the printing medium in a sub-scan direction perpendicular to the main-scan direction, comprising:  
           [0020]    first printing control means for providing a complete image by performing printing scan for M times (M is an integral number: M≧1) using N printing elements (N is an integral number: N≧2) on the same printing area of the printing medium;  
           [0021]    second printing control means for providing a complete image by performing printing scan for K times (K is an integral number: K≧M) using L printing elements (L is an integral number: L≧N) on the same printing area of the printing medium; and  
           [0022]    Printing-control switching means for allowing an image printing using the first printing control means for each printing area in a front side of the printing medium in the direction of transporting the printing medium and an image printing using the second printing control means for each printing area in a rear side of the printing medium in the direction of transporting the printing medium.  
           [0023]    In a second aspect of the present invention, there is provided a printing method for printing an image on a printing medium by repeating a printing scan of a printing head having a plurality of printing elements in a main-scan direction and a transport of the printing medium in a sub-scan direction perpendicular to the main-scan direction, comprising the steps of:  
           [0024]    providing a complete image by performing printing scan for M times (M is an integral number: M≧1) using N printing elements (N is an integral number: N ≧2) on the same printing area in a front side of the printing medium in the direction of transporting the printing medium; and  
           [0025]    providing a complete image by performing printing scan for K times (K is an integral number: K≧M) using L printing elements (L is an integral number: L ≧N) on the same printing area in a rear side of the printing medium in the direction of transporting the printing medium.  
           [0026]    In a third aspect of the present invention, there is provided a printing apparatus for printing an image on a printing medium by repeating a printing scan of a printing head having a plurality of printing elements in a main-scan direction and a transport of the printing medium in a sub-scan direction perpendicular to the main-scan direction, comprising:  
           [0027]    first printing control means for providing a complete image by performing printing scan for M times (M is an integral number: M≧1) on the same printing area of the printing medium;  
           [0028]    second printing control means for providing a complete image by performing printing scan for K times (K is an integral number: K&gt;M) on the same printing area of the printing medium; and  
           [0029]    printing-control switching means for initiating an image printing by the first printing control means in a printing operation on the printing medium and switches from the image printing using the first printing control means to an image printing using the second printing control means depending on a transporting position of the printing medium.  
           [0030]    In a fourth aspect of the present invention, there is provided a printing method for printing an image on a printing medium by repeating a printing scan of a printing head having a plurality of printing elements in a main-scan direction and a transport of the printing medium in a sub-scan direction perpendicular to the main-scan direction, comprising the steps of:  
           [0031]    initiating an image printing on the printing medium by a printing operation in which a complete image is obtained by performing printing scan for predetermined times on the same printing area of the printing medium; and  
           [0032]    switching from the printing operation to another printing operation in which a complete image is obtained by performing printing scan for more times than the predetermined times on the same printing area of the printing medium.  
           [0033]    According to the above aspects of the present invention, the printing movement of the printing head can be altered so that the number of the printing scan of the printing head for the same printing area on the printing medium can be increased. Consequently, the printing speed is reduced as much as possible, furthermore the deterioration of the image quality on the latter part of the printing medium in the transport direction can be substantially reduced.  
           [0034]    The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    [0035]FIGS. 1A, 1B, and  1 C are schematic diagrams for explaining the different feed stages of the printing medium in the common serial type printing apparatus;  
         [0036]    [0036]FIG. 2 is a schematic diagram for explaining an example of the conventional multi-path printing system;  
         [0037]    [0037]FIG. 3 is a schematic diagram for explaining the positions on which ink dots are formed in the state shown in FIG. 1A;  
         [0038]    [0038]FIG. 4 is a schematic diagram for explaining the positions on which ink dots are formed in the state shown in FIG. 1C;  
         [0039]    [0039]FIG. 5 is a schematic diagram for explaining the positions on which ink dots are formed in the state shown in FIG. 1C after passing through the state shown in FIG. 1B;  
         [0040]    [0040]FIG. 6 is a schematic diagram explaining the results of the printing based on the printing system shown in FIG. 2;  
         [0041]    [0041]FIG. 7 is a schematic diagram for explaining the printing system as one of the embodiments of the present invention;  
         [0042]    [0042]FIG. 8 is a flow chart for explaining the procedure of controlling the printing movement in one of the embodiments of the present invention;  
         [0043]    [0043]FIG. 9 is a schematic diagram for explaining the results of the printing based on the printing system shown in FIG. 7;  
         [0044]    [0044]FIG. 10 is a schematic diagram for explaining the printing system as another embodiment of the present invention;  
         [0045]    [0045]FIG. 11 is a schematic diagram for explaining the printing system as still another embodiment of the present invention;  
         [0046]    [0046]FIG. 12 is a schematic diagram for explaining the printing system as further still another embodiment of the present invention;  
         [0047]    [0047]FIG. 13 is a schematic perspective diagram that illustrates the main constructive part of the ink jet printing apparatus applicable to the present invention;  
         [0048]    [0048]FIG. 14 is a schematic perspective diagram that illustrates the head cartridge of the printing apparatus shown in FIG. 13, where the main structural part of the ink-discharge part is partially indicated by the broken lines; and  
         [0049]    [0049]FIG. 15 is a block diagram of the control circuit of the ink jet printing apparatus shown in FIG. 13. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0050]    Hereinafter, the preferred embodiments of the present invention are described with reference to the attached figures. Unless indicated otherwise, the same reference numbers are provided on the similar parts as those of the conventional example described above for omitting the explanations thereon.  
         [0051]    [An Example of the Configuration of the Printing Apparatus]  
         [0052]    [0052]FIG. 13 illustrates a main structural component of an ink jet printing apparatus that is applicable to the present invention. In the figure, a plurality of head cartridges (three head cartridges in this example)  1 A,  1 B,  1 C are provided as printing means and exchangeably mounted on a carriage  2 . In addition, the cartridge  1 A- 1 C has a connector for receiving a signal for driving its printing head portion. By the way, each cartridge  1 A- 1 C may be in the form an ink cartridge that comprises a printing head portion and an ink tank portion for supplying ink to the printing head part. Alternatively, it may be in the form of a printing head that receives ink from a distinctive ink tank.  
         [0053]    In the following description, all of the printing means  1 A- 1 C or any one of them may be simply referred to as a printing means, a printing head, or a head cartridge. The head cartridge  1  ejects ink droplets of different colors from their respective printing head portions to print an image on a printing medium. The printing head portions receive color inks from their respective ink tank portions that store cyan, magenta, and yellow color inks, respectively.  
         [0054]    The head cartridge  1  is exchangeably mounted on the carriage  2 . The head cartridge  1  and the carriage  2  are electrically connected to each other. That is, the carriage  2  has a connector holder (electrical contact portion) to be electrically contacted with the above connector for transmitting a driving signal or the like from the carriage  2  to the head cartridge  1 . A guide shaft  3  is mounted on the body of the apparatus to guide the carriage  2  without restraint in the main scan direction. As the carriage  2  is connected to a timing belt  7  that runs between a motor pulley  5  to be driven by a main-scan motor  4  and a driven pulley  6 . Therefore, the carriage  2  can be transported in the main scan direction by the driving force of the main-scan motor  4 .  
         [0055]    A printing medium (a printing material)  8  such as a sheet of paper or a thin plastic plate is transported by the rotations of a pair of transport rollers  9 ,  10  and another pair of transport rollers  11 ,  12 . The printing medium  8  is transported in the sub-scan direction, passing through a position (a printing position) facing to an ink-ejection orifice surface of the printing head portion of the head cartridge  1 . The back surface of the printing medium  8  is supported on a platen (not shown) so that the front surface (printing surface) thereof becomes flattened. The ink-ejection orifice surface of the head cartridge  1  mounted on the carriage  2  is arranged so that it protrudes downwardly from the surface of the carriage  2 . The ink-ejection orifice surface is faces to the flat portion of the printing medium  8  within the space between the pair of transport rollers  9 ,  10  and the pair of transport rollers  11 ,  12 .  
         [0056]    In the present embodiment, the pair of transport rollers  9 ,  10  corresponds to the rollers  501 ,  502  in FIG. 1 and the pair of transport rollers  11 ,  12  corresponds to rollers  503 ,  505  or the rollers  504 ,  506  in FIG. 1.  
         [0057]    The printing head portion of the head cartridge  1  in the present embodiment may be provided as one of components that make up an ink jet printing means for ejecting ink droplets from its orifices using thermal energies. The printing head portion comprises a plurality of ejecting heaters (electro-thermal transducers) to generate the thermal energies. As described later, each thermal electro-thermal transducer provides thermal energy to ink for causing a film-boiling phenomenon. The film-boiling phenomenon produces pressure-changes by growth or shrinkage of a bubble in the ink so that an ink droplet can be ejected from the ink-ejection orifice.  
         [0058]    In FIG. 13, furthermore, the reference numeral  14  denotes a recovery means for keeping an excellent ink-ejection state of the printing head portion. The recovery means  14  comprises caps  15  for capping the front surfaces (ink-ejection surfaces) of the printing head portions of the head cartridges  1 A- 1 C, respectively. The inside of each cap  15  is communicated with a pump  16  through a tube  27 , so that the pump  16  forms a negative pressure in the cap  15  by suction to draw ink from the ink-ejection orifices of the printing head portion to the inside of the cap  15  (a suction-recovery operation). Alternatively, idle ink (does not contribute for printing) may be ejected from the ink-ejection orifices of the printing head portion to the cap (an ejection-recovery operation). Therefore, such recovery operations allow the persistence of excellent ink-ejection state. Furthermore, the reference numeral  18  denotes a blade that is held in a holder  17  so that the blade  18  can be positioned on a path along which the ink-ejection surface of the printing head portion moves. Thus, the blade  18  wipes the ink-ejection surface of the printing head portion as the printing head moves on the blade  18 .  
         [0059]    [0059]FIG. 14 is a schematic perspective diagram that partially illustrates principal structural components of an ink-ejection portion  13  of the printing head portion. In the figure, an ejection surface  21  of the ink-ejection portion  13  is positioned at a predetermined distance (about 0.5 to 2 mm) from the surface of the printing medium  8 , facing each other. In addition, a plurality of ejection orifices  22  formed on such an ejection surface  21  with predetermined pitches. In this embodiment, there are 256 orifices at intervals of 360 dpi. Furthermore, as described above, an ejecting heater (an electro-thermal transducer such as a heating resistance element)  25  for generating a thermal energy to be used for ejecting of the ink. The ejecting heaters are arranged along a wall surface of each channel  24  communicating with a common liquid chamber  23  and an ejection orifice  22 . The head cartridge  1  of the present invention is mounted on the carriage  2  so that the ejection orifices  22  of the printing head portion can be lined up in the direction perpendicular to the main-scan direction of the carriage  2 . Then, a film boiling phenomenon is caused in the ink in the channel  24  by actuating (applying power to) the ejecting heater  25  on the basis of an image signal or a ejecting signal. And then, pressure-changes is produced by growth or shrinkage of a bubble in the ink to eject an ink droplet from the ink-ejection orifice  22 .  
         [0060]    Referring now to FIG. 15, there is shown a block diagram that illustrates the principal configuration of the control circuit equipped in the ink jet printing apparatus of FIG. 13. In FIG. 15, a controller  100  is provided as a main control portion that comprises, for example, a central processing unit (CPU)  101  in the form of a micro computer, a read only memory (ROM)  103  that stores programs, desired tables, and other fixed date, and a random access memory (RAM)  105  in which unfolding image data and work areas are formed. A host device  110  is image-data supplying source (e.g., a computer for forming image data or the like to be printed or recorded, processing such data, and so on, or alternatively any device such as a reader for reading the image data). Transmission and reception of any data of images, commands, status signals and so on can be performed between the host computer  110  and the controller  100  through an interface (I/F)  112 .  
         [0061]    An operation portion  120  comprises a group of switches that receive any instruction from the operator. These switches include an electric power switch  122 , a switch  124  for instructing the initiation of a printing operation, a recovery switch for instructing the initiation of an absorbing-recovery operation, and so on. In FIG. 15, a head driver  140  actuates the ejecting heaters  25  in each ink jet cartridge  1 A- 1 C in response to the print data or the like. The head driver  140  comprises a shift resistor that brings the print data into line so as to correspond to the respective ejecting heaters  25 , a latch circuit for latching the print data in appropriate timing, a logic circuit element for actuating the ejecting heaters in synchronization with the drive-timing signals, and a timing-adjusting portion for appropriately adjusting the drive-timing (ejecting-timing) for adjusting the position on which an ink dot is formed.  
         [0062]    In the printing head  1 , there is also provided a sub-heater  142  that performs a temperature control for stabilizing the ink-ejection characteristics. The sub-heater  142  may be simultaneously formed with the ejecting heater  25  on the substrate of the printing head  1 , or equipped on the body of the printing head  1  or the head cartridge. A motor driver  150  is provided for driving a main-scan motor  152 , while a motor drive  160  is provided for driving a sub-scan motor  162  that transports the printing medium  8  in the sub-scan direction.  
         [0063]    [Fist Embodiment]  
         [0064]    In this embodiment, an ink jet printing apparatus performs a printing operation based on a two-path printing system (i.e., a first printing operation) on a front half area of a printing medium such as a sheet of printing paper. The front half area is positioned on the front side of the printing medium in transporting direction of it. On the other hand, it performs another printing operation based on a six-path printing system (i.e., a second printing operation) on the rear half area of the printing medium. The rear half area is positioned on the rear side of the printing medium in transporting direction of it. During the first printing operation, a printing scan using 256 nozzles of the printing head portion and a transport of the printing medium by distances of corresponding to 128 nozzles are repeated to perform the two-path printing. During the second printing operation, on the other hand, a printing scan using 192 nozzles and a transport of the printing medium by distances of corresponding to 32 nozzles are repeated to perform the six-path printing.  
         [0065]    [0065]FIG. 8 is a flowchart that illustrates the control procedure of the printing operation of the ink jet printing apparatus in accordance with the present embodiment.  
         [0066]    In the procedure of controlling the printing operation, as described below, the printing of an image on a printing medium is initiated by a predetermined printing operation, followed by switching the printing operation to another one on the basis of the position on which the printing medium is transported. The predetermined initial printing operation at the time of initiating the printing corresponds to the first printing operation described above. On the other hand, the printing operation after the switching in the middle of the image printing on the printing medium corresponds to the second printing operation described above. Therefore, in the middle of printing the image on the printing medium, the printing operation is changed so that the number of scanning the printing head  1  over the predetermined printing area of the printing medium for completing the image formation can be increased by the following steps.  
         [0067]    First, the printing is started at the step S 9001 , followed by feeding the printing medium in the sub-scan direction at the step S 9002 .  
         [0068]    In the step S 9003 , random masks (A 2 , B 2 ) for two-path printing system are expanded in the RAM  105  mounted on the body of the printing apparatus (see FIG. 15).  
         [0069]    Then, the first printing operation including a printing scan using 256 nozzles and an operation of transporting the printing medium by distances of corresponding to 128 nozzles is performed using the masks A 2 , B 2  expanded in the RAM  105  at the step S 9004 .  
         [0070]    In the step S 9005 , it is determined whether the image is formed completely on the whole printing area. If the image formation is finished, then the printing medium is ejected from the printing apparatus at the step S 9010 , followed by completing the printing at the step S 9011 . If the image formation is not finished (step S 9005 ), then the process proceeds to the step S 9006  to determine whether the position of the finished image on the printing medium reaches the switching position of the printing control (the position where the first and second printing operations are switched). If the image formation before reaching at the switching position is not completed, then the steps S 9004 , S 9005 , and S 9006  are repeated. In the step S 9006 , if the image formation before reaching at the switching position is completed, then the process proceeds to the step S 9007 .  33   
         [0071]    In the step S 9007 , random masks A 6 , B 6 , C 6 , D 6 , E 6 , and F 6  for 6-path printing system are expanded in the RAM  105  in the body of the printing apparatus.  
         [0072]    Then, in the step S 9008 , the second printing operation including a printing scan using 192 nozzles and an operation of transporting the printing medium by distances of corresponding to 32 nozzles is performed using the masks expanded in the RAM  105  at the step S 9007 .  
         [0073]    In the step S 9009 , furthermore, if it is determined whether the image formation on the whole printing area is completed. If it is completed, then the process proceeds to the step S 9010  to discharge the printing medium from the printing apparatus, followed by completing the printing at the step S 9011 . In the step S 9009 , if the image formation is not completed, then the process proceeds to the step S 9008  to repeat the second printing operation until the image formation is completed. Therefore, the second printing operation is not performed until the printed image reaches the switching position.  
         [0074]    [0074]FIG. 7 is a schematic diagram that illustrates the first and second printing operations before and after passing through the switching position (PC), respectively. In this figure, just as in the case with FIG. 2, an area of using the nozzles of the printing head  1 , masks to be used, and the relative position between the printing head  1  and the printing medium. The switching position PC for switching between the first and second printing operations corresponds to the boundary between the areas P 0  and P 1 .  
         [0075]    Firstly, in the steps from S 101  to S 103 , the first printing operation is performed as a two-path printing system by alternately using the random masks A 2  and B 2 . In the first printing operation, a printing scan operation using all of 256 nozzles and an operation of transporting the printing medium by distances of corresponding to half of the nozzles (i.e., 128 nozzles) are repeated.  
         [0076]    In the step S 104 , the printing medium is not transported, while the nozzles positioned on the upstream side of the feeding direction of the printing medium (the lower side of FIG. 7) are used. Therefore, the image formation, which should be completed before passing through the switching position PC, is completed.  
         [0077]    In the next step S 105  and the subsequent steps, the second printing operation of 6-path printing system is performed. In the second printing operation, a transport of the printing medium by distances of corresponding to 32 nozzles and a printing scan operation using random masks A 6 , B 6 , C 6 , D 6 , E 6 , and F 6  in that order is repeated.  
         [0078]    Furthermore, in the printing scan operations at the step S 104  and the subsequent steps, blank image data is provided about the area where the image is already completed while the printing data is abandoned. In the step S 104 , for example, the printing data corresponding to the area on which the image formation is completed at the preceding steps S 102 , S 103 , i.e., the printing data for 128 nozzles in the downstream side of the feeding direction of the printing medium (the upper side of FIG. 7), receives blank data instead of the printing data being abandoned. Therefore, substantially, the printing operation using those 128 nozzles is not performed. The steps S 105  and the subsequent steps are performed in an analogous fashion, so that the nozzles corresponding to the area on which the image formation is completed at the preceding steps are not used. In FIG. 7, the nozzle marked by “x” means that blank data is provided with respect to an printing scan area corresponding to such an x-marked nozzle.  
         [0079]    In FIG. 7, the printing operation until the step S 104  corresponds to the first printing operation described above, while the printing operations from the steps S 105  corresponds to the second printing operation described above. The boundary time PB for switching these printing operations is adjusted so that the printing medium comes off the pinch roller  502  (10) at the time point PA in a specified period after the boundary time PB. The specified period corresponds to the period of transporting the printing medium after the printing scan on an area (area P 0  in the present embodiment) immediately preceding an area on which the image formation is completed at first by the second printing operation (area P 1  in the present embodiment). In other words, the specified period is the transporting period of the printing medium after the step S 103 . As shown in FIG. 1B, if the distance from the pinch roller  502  (10) to the printing area  509  of the printing head  507  (1) is defined as “L”, the distance from the rear end of the printing medium  508  (8) to the switching position PC is defined as “D”, the feeding amount of the printing medium at a time in the second printing operation is defined as “F” (corresponding to 32 nozzles in the present embodiment), the number of paths in the second printing operation is defined as N (6 paths in the present embodiment), and the accuracy of detecting the rear end of the printing medium is defined as ±ΔA the most efficient printing rate can be attained when these factors are represented by the following equation.  
           D=L+ΔA+F× ( N− 1) 
         [0080]    The means for detecting the position of the rear end of the printing medium may be one using a sensor located on the position in the upstream of the sheet-feed (the position on the right side of FIG. 1) with respect to the pinch roller  502  (1). Alternatively, as the detection means, another means, such as a means for determining the position of the rear end of the printing medium may be used. That is, the means determines the position of the rear end of the printing medium on the basis of the data of the whole length of the printing medium and the transport distance from the point at the time of detecting the front end of the printing medium. An appropriate switching position PC is defined from the above equation by determining the position of the rear end of the printing medium. In FIG. 7, the printing medium comes out of the pinch roller  502  (10) at the time of transporting the printing medium when the process proceeds from the step S 110  to the step S 111  (such a time point is referred to as a coming-off time point PA).  
         [0081]    [0081]FIG. 9 is a schematic diagram that illustrates the image, which is uniformly printed all over the printing area of the printing medium using the printing procedure described above. In FIG. 9, the image formation can be performed by ideally spotting ink droplets on the printing area up to the area P 1 . In one of the printing scan (printing scan of step S 111 ) in the sixth printing scan, the image formed on the area P 2  is under the influences of the printing medium that comes out of the pinch roller  502  (10), and the difference between the feed amount of the transport roller  501  (9) and the feed amount of the sheet-ejection rollers  503  to  506  (11, 12). In the area P 2 , therefore, the position on which the ink is spotted can be deviated from the predetermined in proportional to the sum of those influences. Each of the random masks A 6 , B 6 , C 6 , D 6 , E 6 , and F 6 , macroscopically, has its own uniform rate of thinning out macroscopically, so that the printing by a single printing scan using one of the above random masks becomes the printing in which ⅙ of the printing image data is thinned out.  
         [0082]    In the conventional 2-path printing system as shown in FIG. 2, for example, the declined amount of the image density due to the fact that the printing medium comes out of the pinch roller is defined as ΔDA, and the declined amount of the image density at the time of transporting the printing medium over distances of corresponding to 128 nozzles in response to the difference between the feed amount of the transport roller and the feed amount of the sheet-ejection roller is defined as ADB. It may be simply considered that there is a proportionality relation between the declined image density and the amount of deviation from the expected ink-spotted point. In that case, the decried amount of the image density on the area printed by the printing method of the present invention can be represented by the following equations  
         [0083]    In addition, in this embodiment, the 6-path printing system has been already performed when the printing medium comes out of the pinch roller. Therefore, in the following equations, the decried amount of the image density is calculated with the assumption that the declined amounts of the image densities in the present embodiment may be ⅓ (=2 paths / 6 paths) of the declined amounts of the image densities ΔDA, ΔDB in the conventional one shown in FIG. 2. In addition, in the interests of simplicity, a proportionality relation between the declined image density and the amount of deviation from the expected ink-spotted point is considered. Thus, the calculations are performed in consideration of the ratio between the feed amount of the printing medium corresponding to a distance from the coming-off point PA to each printing area and the feed amount of the printing medium corresponding to 128 nozzles.  
         [0084]    (Decline in the image density at P 2 )=ΔDA/3+ΔDB× (32/128)/3=0.33ΔDA+0.08ΔDB  
         [0085]    (Decline in the image density at P 3 )=ΔDA/3+ΔDB× { (32+64) / 128} / 3=0.33ΔDA+0.25ΔDB  
         [0086]    (Decline in the image density at P 4 )=ΔDA/3+ΔDB× { (32+64+96) / 128} / 3=0.33ΔDA+0.50ΔDB  
         [0087]    (Decline in the image density at P 5 )=ΔDA/3+ΔDB× { (32+64+96+128) / 128} / 3=0.33ΔDA+0.83 ΔDB  
         [0088]    (Decline in the image density at P 6 )=ΔDA/3+ΔDB×{ (32+64+96+128+160) / 128} / 3=0.33ΔDA+1.25ΔDB  
         [0089]    (Decline in the image density at P 7 )=ΔDA/ 3+ΔDB×{ (32+64+96+128+160+192)/128}/3=0.33 ΔDA+1.75ΔDB  
         [0090]    (Decline in the image density at P 8 )=ΔDB×{ (32+64+96+128+160+192+224)/128}/3=2.33 ΔDB  
         [0091]    Furthermore, if it is defined as ΔDA=ΔDB×2, the difference between the image densities of the adjacent areas may be represented by the following equations.  
         [0092]    (The density difference between P 1  and P 2 )=−0.33ΔDA −0.08ΔDB=−0.75ΔDB  
         [0093]    (The density difference between P 2  and P 3 )=−0.17ΔDB  
         [0094]    (The density difference between P 3  and P 4 )=−0.25ΔDB  
         [0095]    (The density difference between P 4  and P 5 )=−0.33ΔDB  
         [0096]    (The density difference between P 5  and P 6 )=−0.42ΔDB  
         [0097]    (The density difference between P 6  and P 7 )=−0.50ΔDB  
         [0098]    (The density difference between P 7  and P 8 )=−0.33ΔDA −0.58ΔDB=0.08ΔDB  
         [0099]    On the other hand, the density difference between the adjacent areas in the conventional example shown in FIG. 6 may be represented by the following equations.  
         [0100]    (The density variation between P 0  and P 1 )=−ΔDA−ΔDB =−3.00ΔDB  
         [0101]    (The density variation between P 4  and P 5 )=ΔDA−ΔDB =ΔDB  
         [0102]    From the results of the above comparisons, the density difference between the adjacent printing areas in the present embodiment is substantially smaller than that of the conventional one. Therefore, by performing the printing control of the present embodiment, the amount of change in the density between adjacent areas can be reduced compared with that of the conventional one, allowing the decrease in the image deficiencies which can be visually recognized.  
         [0103]    In the present embodiment, the nozzle on the sheet-feed side (the lower side of FIG. 7) is used in the control of the second printing operation. However, the nozzle on the sheet-ejection side (the upper side of FIG. 7) may be used in the steps from S 305  for the second printing operation as shown in FIG. 10. In the case of FIG. 7, as described above, the printing medium comes off the pinch roller when the printing medium is transported at the boundary between the step S 110  and the step S 111 . In the case of FIG. 10, on the other hand, the position of the nozzle group used in the control of the second printing operation is shifted toward the sheet-ejection side, so that the printing medium is able to come off the pinch roller when the printing medium is transferred at the boundary between the step S 308  and the step S 309 . Therefore, the width of the printing area for the first printing operation using the nozzles on the sheet-ejection side as shown in FIG. 10 becomes narrow compared with one using the nozzles on the sheet-feed side as shown in FIG. 7.  
         [0104]    (Other Embodiments)  
         [0105]    In the above embodiment, the first printing operation of two paths, in which the printing scan using 256 nozzles and the transport of the printing medium over distances corresponding to  126  nozzles are repeated, is performed on each of the predetermined printing areas located on the front side portion of the printing medium such as a sheet of paper in the transport direction. In addition, the second printing operation of six paths, in which the printing scan using 192 nozzles and the transport of the printing medium over distances corresponding to 32 nozzles are repeated, is performed on each of the predetermined printing areas located on the rear side of the printing medium in the transport direction. By the way, it is not appropriate to increase the number of paths in the multi-path printing system in the case that the printing medium comes off the pinch roller with little influence on the feed amount of the printing medium because of the following reasons. That is, if the number of paths is increased, the feed amount of the printing medium at the time of printing the image on the same printing area during the first and last paths are increased, respectively, to increase the degree of reducing the image density depending on the difference between the feed amount of the transport roller and the feed amount of the sheet-ejection roller.  
         [0106]    Therefore, in the case that the printing medium comes off the pinch roller with little influence on the feed amount of the printing medium, it is preferable to reduce the number of nozzles to be used by the application of 3-path printing system as a second printing operation as shown in FIG. 11. In the 3-path printing system, a printing scan using the 96 nozzles on the sheet-feed side and an transporting the printing medium over distances of corresponding to 32 nozzles are repeated.  
         [0107]    In another embodiment, furthermore, a first printing operation and a second printing operation may be performed as illustrated in FIG. 12. That is, the first printing operation of 4 paths repeatedly performs a printing scan using 256 nozzles and a transport of the printing medium over distances of corresponding to 64 nozzles on each printing area in the front side of the printing medium in the transport direction. On the other hand, the second printing operation of 8 paths repeatedly performs a printing scan using 256 nozzles and a transport of the printing medium over distances of corresponding to 32 nozzles on each printing area in the remaining rear side of the printing medium in the transport direction.  
         [0108]    Furthermore, the control procedure shown in FIG. 12 and the control procedure shown in FIG. 7 may be alternately performed depending to the type of the printing medium.  
         [0109]    The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspect, and it is the intention, therefore, in the apparent claims to cover all such changes and modifications as fall within the true spirit of the invention.