Patent Publication Number: US-8985722-B2

Title: Printing apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Paten Application No. 2009-045112, filed on Feb. 27, 2009, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing apparatus which carries out printing on a printing medium by discharging a liquid from a nozzle. 
     2. Description of the Related Art 
     In an ink-jet printer described in Japanese Patent Application Laid-Open No. 2009-06550 as a printing apparatus which carries out printing by discharging a liquid from a liquid discharge head, an ink-jet head includes: a flow passage unit in which a plurality of nozzles and an ink flow passage including a plurality of pressure chambers communicating with the nozzles are formed; and a piezoelectric actuator arranged on the upper surface of the flow passage unit to apply a pressure to an ink inside the pressure chambers. The piezoelectric actuator has three piezoelectric layers which are stacked on the upper surface of the flow passage unit, and common electrodes and individual electrodes which are formed respectively on the lower surface and upper surface of the uppermost piezoelectric layer so as to sandwich the uppermost piezoelectric layer on the portions facing the plurality of pressure chambers. Further, the ink-jet head is a so-called line head which extends across a full length of a recording paper in its width direction and is fixed to the ink-jet printer. 
     Then, when a drive pulse is applied to an individual electrode to generate a potential difference between the individual electrode and a common electrode which is maintained at the ground potential, in the same direction as the direction of polarization of the piezoelectric layer, a downward electric field is generated in the portion of the piezoelectric layer sandwiched by those electrodes. This electric field causes the uppermost piezoelectric layer to contract in a horizontal direction, thereby deforming the three piezoelectric layers at the portions (a piezoelectric element) facing a pressure chamber, as a whole, to project toward the pressure chamber side. Thus, a pressure is applied to the ink inside the pressure chamber. 
     Here, in the ink-jet head described in Japanese Patent Application Laid-Open No. 2009-06550, as a total number of times of driving a piezoelectric element increases, the polarization of the piezoelectric layer weakens, etc., thereby degrading the drive performance of the piezoelectric element. Therefore, when a drive pulse of a certain height (drive potential) is applied to drive a piezoelectric element, the maximum drivable number of times is predetermined for the piezoelectric element. Thus, if a particular piezoelectric element has a high usage frequency, and the total number of times of driving the piezoelectric element with the high usage frequency will reach the maximum drivable number of times described above, the life of the piezoelectric element ends at an early stage. In this case, even if the other piezoelectric elements are still drivable, the ink-jet head will have to come to the end of its life. 
     At this time, if the ink-jet head is a so-called serial head which discharges ink while moving in a scanning direction and therefore differs from that described in Japanese Patent Application Laid-Open No. 2009-06550, it is possible to prolong the ink-jet head life by, for example, discharging the ink from another nozzle to the position to which the nozzle corresponding to the expired piezoelectric element should have discharged the ink on the recording paper. However, it is difficult for a line head as described in Japanese Patent Application Laid-Open No. 2009-06550 to carry out such kind of process. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a printing apparatus which has a long-life liquid discharge head. 
     According to a first aspect of the present invention, there is provided a printing apparatus which performs printing by discharging a liquid onto a printing medium, the printing apparatus including: a liquid discharge head which includes a nozzle which discharges the liquid to the printing medium, and a discharge energy applying section which applies a discharge energy to the liquid to be discharged from the nozzle; a drive section which drives the discharge energy applying section; and a controller which controls the drive section based on a parameter with respect to a usage amount of the discharge energy applying section; and when one dot is formed on the printing medium by discharging the liquid from the nozzle, if a predetermined threshold value is not exceeded by the parameter, the controller controls the drive section to drive the discharge energy applying section so that a first discharge energy is applied to the liquid, and if the predetermined threshold value is exceeded by the parameter, the controller controls the drive section to drive the discharge energy applying section so that a second discharge energy which is lower than the first discharge energy is applied to the liquid. 
     The discharge energy applying section gets more degraded as its usage amount increases and the total amount of a discharge energy applied to the liquid becomes greater. According to the first aspect of the present invention, when one dot is formed on the recording medium by discharging the liquid from the nozzle which corresponds to the discharge energy applying portion, if a predetermined threshold value is exceeded by a parameter with respect to the usage amount of the discharge energy applying section, the discharge energy which is applied to the liquid from the discharge energy applying section is defined as a second discharge energy which is lower than a first discharge energy, which is applied when the predetermined threshold value is not exceeded by the parameter. Accordingly, it is possible to prolong the life of the discharge energy applying section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic construction diagram of a printer according to an embodiment of the present invention; 
         FIG. 2  is a plan view showing a part of an ink-jet head of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along the line of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view taken along the line IV-IV of  FIG. 2 ; 
         FIG. 5  is a block diagram of a controller of  FIG. 1 ; 
         FIG. 6  is a flow chart showing a control flow of the controller when a printer  1  carries out printing; 
         FIG. 7A  is a diagram showing a drive pulse(s) applied to an individual electrode from a driver IC as usual (under normal conditions); 
         FIG. 7B  is a diagram showing a drive pulse applied to an individual electrode from a driver IC when a predetermined value is exceeded by an integrated value of the number of times of applying the drive pulse; 
         FIGS. 8A and 8B  show a flow chart of Modification 1, corresponding to  FIG. 6 ; 
         FIG. 9  is a flow chart of Modification 2, corresponding to  FIG. 6 ; 
         FIG. 10A  is a diagram of Modification 3, corresponding to  FIG. 7A ; 
         FIG. 10B  is a diagram of Modification 3, corresponding to  FIG. 7B ; 
         FIG. 11A  is a diagram of Modification 4, corresponding to  FIG. 7A ; and 
         FIG. 11B  is a diagram of Modification 4, corresponding to  FIG. 7B . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, a preferred embodiment of the present teaching will be described. 
     As shown in  FIG. 1 , a printer  1  includes four ink-jet heads  2 , transport rollers  3 , etc. Further, the printer  1  operates under the control of a controller  50 . 
     The four ink-jet heads  2  as liquid discharge heads are so-called line heads which extend across a full length or width of a recording paper P as a printing medium in a left-right direction (scanning direction) of  FIG. 1 , and which are fixed to the printer  1 , respectively. These four ink jet heads  2  discharge inks of black, yellow, cyan, and magenta, respectively, from a plurality of nozzles  15  formed on the under surfaces of the ink-jet heads  2 , in the order of arrangement of the ink-jet heads  2  from the uppermost down to the lowermost in  FIG. 1 . 
     The transport rollers  3  are arranged above and below the four ink-jet heads  2  in  FIG. 1  such that the four ink-jet heads  2  are located therebetween, to transport the recording paper P toward a lower portion of  FIG. 1  (in a paper feeding direction). 
     Then, the printer  1  carries out printing on the recording paper P by discharging the inks from the nozzles  15  of the four ink-jet heads  2  onto the recording paper P transported by the transport rollers  3  in the paper feeding direction. 
     Next, an ink-jet heads  2  will be explained. As shown in  FIGS. 2 to 4 , the ink-jet head  2  has a flow passage unit  31  and a piezoelectric actuator  32 . 
     The flow passage unit  31  is formed by stacking a cavity plate  21 , a base plate  22 , a manifold plate  23 , and a nozzle plate  24  each other. Among these four plates  21  to  24 , except for the nozzle plate  24 , the plates  21  to  23  are all formed of a metallic material such as stainless steels, etc. while the nozzle plate  24  is formed of a synthetic resin material such as polyimide, etc. Alternatively, the nozzle plate  24  may also be formed of a metallic material like the other plates  21  to  23 . 
     A plurality of pressure chambers  10  are formed in the cavity plate  21 . The plurality of pressure chambers  10 , each of which has a planar shape of an approximate ellipse with its longitudinal direction in the paper feeding direction (the up-down direction of  FIG. 2 ), are aligned in two rows in the scanning direction (the left-right direction of  FIG. 2 ). A plurality of through holes  12  and  13  are formed in the base plate  22 . Each of the through holes  12  and  13  has a shape of an approximate circle in a plane view, and is located in a portion facing either one end portion or the other end portion of a pressure chamber  10  in their longitudinal direction in a plane view. 
     A manifold flow passage  11  is formed in the manifold plate  23 . The manifold flow passage  11  extends in the scanning direction so as to face both the approximately upper half of the plurality of pressure chambers  10  which are aligned on the upper part of  FIG. 2  and the approximately lower half of the plurality of pressure chambers  10  which are aligned on the lower part of  FIG. 2 . Further, in the manifold plate  23 , a plurality of though holes  14 , each of which has a planar shape of a approximate circle, are formed at the positions facing the plurality of though holes  13  in a plane view. 
     A plurality of nozzles  15  are formed in the nozzle plate  24  at positions facing the plurality of through holes  14  in a plane view. Further, in the flow passage unit  31 , the manifold flow passage  11  communicates with the pressure chambers  10  via the through holes  12 , and the pressure chambers  10  communicate with the nozzles  15  via the through holes  13  and  14 . In such a manner, in the flow passage unit  31 , a plurality of individual ink flow passages are formed from the exits of the manifold flow passage  11  through the pressure chambers  10  to the nozzles  15 . 
     The piezoelectric actuator  32  includes a vibration plate  41 , a piezoelectric layer  42  and a plurality of individual electrodes  43 . The vibration plate  41  is formed of a metallic material such as stainless steels, etc., and joined to the upper surface of the flow passage unit  31  so as to cover up the openings of the plurality of chambers  10  on the upper surface of the flow passage unit  31 . Further, the conductive vibration plate  41  also acts as a common electrode which is constantly maintained at the ground potential for generating a potential difference between the common electrode and individual electrodes  43 . 
     The piezoelectric layer  42  is made of a piezoelectric material which is composed mainly of lead zirconium titanate which is a mixed crystal of lead titanate and lead zirconate, and arranged in a continuous manner on the upper surface of the vibration plate  41  so as to cover the plurality of chambers  10 . 
     The plurality of individual electrodes  43 , each of which has a planar shape of an approximately ellipse sufficiently smaller than each of the pressure chambers  10 , are arranged on the upper surface of the piezoelectric layer  42  at the positions facing the approximately central portions of the pressure chambers  10 , respectively. Further, the portions of the piezoelectric layer  42 , which are sandwiched between the individual electrodes  43  and the vibration plate  41  which acts as the common electrode, are polarized downwardly in their thickness direction. 
     Further, the end portions of the individual electrodes  43  on the side opposite to the nozzles  15  with respect to the paper feeding direction, extend to positions which no longer face the pressure chambers  10 ; the leading ends thereof form connecting terminals  43   a  (not shown). The connecting terminals  43   a  are connected to a wiring member (not shown) such as a flexible printed circuit (FPC), etc., and further connected to a driver IC  45  (see  FIG. 5 ) via the FPC. The driver IC  45 , as will be described hereinbelow, applies one to three drive pulse(s) (see  FIG. 7 ) to an individual electrode  43  according to the size of a dot to be formed so as to land an ink droplet on the recording paper P to form one dot. Namely, the driver IC  45  performs as a drive section which drives piezoelectric elements  46  as discharge energy applying sections as described below. 
     Then, in the piezoelectric actuator  32  of such a structure as described above, the portions facing the plurality of pressure chambers  10  (the portions encircled with the dashed-dotted lines in  FIGS. 3 and 4 ) define piezoelectric elements  46  as discharge energy applying sections for applying pressures as discharge energies to the ink inside the pressure chambers  10 , respectively. That is, the plurality of piezoelectric elements  46  are provided for the plurality of pressure chambers  10  (nozzles  15 ), respectively. 
     Here, a drive method for driving the piezoelectric actuator  32  (piezoelectric elements  46 ) will be explained. In the piezoelectric actuator  32 , the individual electrodes  43  are maintained at the ground potential in advance. Then, when the potential of an individual electrode  43  becomes a predetermined drive potential V 1  by applying a drive pulse to the individual electrode  43 , a potential difference is generated between the individual electrode  43  and the vibration plate  41  which acts as the common electrode, thereby producing a downward electrical field in the same direction as the polarization direction in the piezoelectric layer  42  at the portion sandwiched by the electrodes. This electrical field causes the same portion in the piezoelectric layer  42  to contract in a horizontal direction perpendicular to its thickness direction, thereby deforming the portions, in the vibration plate  41  and the piezoelectric layer  42 , which face a pressure chamber  10 , as a whole, to project toward the side of the pressure chamber  10  and thus decrease the volume thereof. Consequently, the pressure on the ink inside the pressure chamber  10  increases, that is, as a discharge energy, a pressure is applied to the ink inside the pressure chamber  10 , and thereby an ink droplet is discharged from a nozzle  15  which communicates with the pressure chamber  10 . After that, as the potential of the individual electrode  43  returns to the ground potential, the piezoelectric actuator  32  also restores itself to the previous or afore-deformation state. 
     At this time, if one drive pulse is applied to the individual electrode  43  from the driver IC  45 , the movement described above takes place only once, and only one ink droplet is discharged from the nozzle  15 . Then, the one ink droplet is landed on the recording paper P to form one dot. 
     Further, if two drive pulses are successively applied to the individual electrode  43  from the driver IC  45 , the movement described above takes place twice; thereby, two ink droplets are successively discharged from the nozzle  15 . Then, the two ink droplets are landed on the recording paper P to form one dot. 
     Furthermore, if three drive pulses are successively applied to the individual electrode  43  from the driver IC  45 , the movement described above takes place three times; thereby, three ink droplets are successively discharged from the nozzle  15 . Then, the three ink droplets are landed on the recording paper P to form one dot. 
     Hereinafter, a dot formed by one ink droplet, a dot formed by two ink droplets and a dot formed by three ink droplets will be referred to as a small dot, a medium dot and a large dot, respectively. Further, in the embodiment, in order to form a medium dot and a large dot on a recording paper, a drive pulse is applied two and three times, respectively, to an individual electrode  43 . This corresponds to an aspect of the present teaching that a drive pulse is applied multiple times to a piezoelectric element from drive section to form one dot on the printing medium. 
     Next, as a control portion, a controller  50  which controls the operation of the printer  1  will be explained. The controller  50  is composed of a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), etc. As shown in  FIG. 5 , these components operate as a mode determination portion  51 , a drive pulse counter  52 , a drive pulse control portion  53 , etc., respectively. 
     The mode determination portion  51  determines an operation mode of the printer  1 . More particularly, the printer  1  is capable of selectively operating in either of the following modes based on the information inputted from a user: a normal print mode in which it is possible to print an image having a high proportion of the large dots such as texts, graphs and the like at a comparatively high speed though the image quality is more or less degraded; and a high-resolution print mode in which it is possible to print an image having a high proportion of the small dots such as photographs and the like with a high image quality though the printing time is more or less long. Thus, the mode determination portion  51  determines the printer  1  to operate in either of the normal print mode and the high-resolution print mode, according to a mode indication signal which is inputted by the user to select the mode. 
     The drive pulse counter  52  individually counts the number of times of applying a drive pulse each time to the individual electrode  43  of each piezoelectric element  46  from the driver IC  45 , and stores an integrated value Cs, as a parameter with respect to the usage amount, of the number of times of applying the drive pulse to each individual electrode  43  up to the present. In other words, the drive pulse counter  52  functions as a storage section which updates and stores the parameter. As will be described hereinbelow, the drive pulse control portion  53  controls the number of drive pulses to be applied to the individual electrode  43  from the driver IC  45 , according to a print signal which is inputted from outside and indicates the size of a ink droplet to be landed on the recording paper P, the print mode determined by the mode determination portion  51 , and the integrated value Cs stored in the drive pulse counter  52 . 
     Next, a printing process carried out by the printer  1  will be explained. Here, the controller  50  actuates the ink-jet heads  2  (the piezoelectric elements  46 ), transport rollers  3 , etc., to carry out the following operation. 
     Normally, the printer  1  stands by until a print signal is inputted from outside (S 101 : NO). When a print signal is inputted and printing is started and carried out in a high-resolution print mode (S 102 : YES), a certain number of drive pulse(s) according to the print signal are/is applied to an individual electrode  43  from the driver IC  45 . That is, when the print signal indicates a large dot, a medium dot, or a small dot to be landed on the recording paper P, three, two, or one of the drive pulse(s) are/is applied accordingly, as shown in  FIG. 7A , to the individual electrode  43  from the driver IC  45  (S 103 ). 
     On the other hand, when printing is started and carried out in the normal print mode (S 102 : NO), it is determined whether a predetermined value Cp, as a predetermined threshold value, is exceeded by an integrated value Cs which is stored in the drive pulse counter  52  and which corresponds to the piezoelectric element  46  to be driven (S 104 ). 
     Then, if the predetermined value Cp is exceeded by the integrated value Cs which corresponds to the piezoelectric element  46  to be driven (S 104 : YES), after that, the number of drive pulses applied to the individual electrode  43  of the corresponding piezoelectric element  46  from the driver IC  45 , is reduced (lessened) to be less than usual (previous). 
     In particular, as shown in  FIGS. 7A and 7B , when the print signal indicates a large dot to be landed on the recording paper P, the number of drive pulses applied to the individual electrode  43  from the driver IC  45  is reduced from three to one; when the print signal indicates a medium dot to be landed on the recording paper P, the number of drive pulses applied to the individual electrode  43  from the driver IC  45  is reduced from two to one. Thereby, the discharge energy applied to the ink inside the pressure chamber  10  from the piezoelectric element  46  becomes lower than previous. In addition, when the print signal indicates a small dot to be landed on the recording paper P, the number of drive pulse applied to the individual electrode  43  from the driver IC  45  remains at one. 
     Here, in a piezoelectric element  46  of the piezoelectric actuator  32 , as the total number of times of driving, that is, the usage amount, becomes greater, namely, as the total discharge energy applied to the ink inside the pressure chamber  10  from the piezoelectric element  46  becomes higher, the polarization of the piezoelectric layer  42  weakens, etc., and thereby the drive performance of the piezoelectric element  46  degrades. Therefore, if the drive pulse height (drive potential) stays constant, the maximum drivable number of times Cm is predetermined such that the piezoelectric element  46  can maintain a predetermined drive performance. Hence, if the total number of times of driving the piezoelectric element  46 , that is, the integrated value Cs, reaches the maximum drivable number of times Cm, then the piezoelectric element  46  can no longer maintain the predetermined drive performance. Hence, if the usage amount of a particular piezoelectric element  46  is great, then the piezoelectric element  46  having the great usage amount may no longer maintain the predetermined drive performance at an early stage. Consequently, even if the other piezoelectric elements  46  still maintain a predetermined drive performance, the printer  1  will inevitably become unable to print. Further, suppose the number of times of driving be the same, the drive performance is easier to degrade for a piezoelectric element  46  driven on a high drive voltage than a piezoelectric element  46  driven on a low drive voltage. Therefore, the maximum drivable number of times Cm for a piezoelectric element  46  may be determined dependending on its drive voltage. 
     At this time, it would be possible for the printer  1  to continue to print if a so-called serial head were employed. In such an ink-jet head which is different from that in the embodiment and which discharges inks from nozzles while moving in the scanning direction, another nozzle can discharge the ink instead of the nozzle which corresponds to the un-drivable piezoelectric element. However, since the ink-jet heads  2  in the embodiment are line heads, it is difficult to carry out such kind of control. 
     To address this problem, in the embodiment, when a predetermined value Cp is exceeded by the number of times of driving the piezoelectric element  46 , that is, the integrated value Cs of the number of times of applying a drive pulse to the individual electrode  43  from the driver IC  45 , after that, the number of drive pulses applied to the individual electrode  43  of the corresponding piezoelectric element  46 , is reduced to be less than usual. Further, the predetermined value Cp is a value smaller than the maximum drivable number of times Cm for the piezoelectric element  46 , for example, about 80% to about 90% of the maximum drivable number of times Cm. 
     With this, since a small dot is landed on the area where a medium dot or a large dot should have been landed on the recording paper P, there are more or less effects on image quality of the printed image. However, since the number of times of applying the drive pulse to the individual electrode  43  of the corresponding piezoelectric element  46 , becomes less in printing, the number of times of driving the piezoelectric element  46  decreases. Namely, the discharge energy applied to the ink inside the pressure chamber  10  from the piezoelectric element  46  becomes lower. This makes it possible to prolong the life of the corresponding piezoelectric element  46 . 
     The piezoelectric actuator  32  applies a pressure to the ink inside a pressure chamber  10  by being applied a drive pulse to an individual electrode  43 . Therefore, it is possible to easily lower the discharge energy applied to the piezoelectric element  46  by reducing the number of drive pulses applied to the individual electrode  43  from the driver IC  45 . 
     On the other hand, when the predetermined value Cp is not exceeded by the integrated value Cs which corresponds to the piezoelectric element  46  (S 104 : NO), it is a further determined (S 105 ) whether a predetermined rate Rp is reached or exceeded by a rate R of the piezoelectric elements  46  of which the integrated value Cs exceeds the predetermined value Cp in any of the four ink-jet heads  2  (one ink-jet head  2  including the piezoelectric element  46  to be driven, and the other three ink-jet heads  2 ). 
     Then, when the predetermined rate Rp is not reached nor exceeded by the rate R of the piezoelectric elements  46  of which the integrated value Cs exceeds the predetermined value Cp in any of the four ink-jet heads  2  (S 105 : NO), in the same manner as above, a certain number of drive pulse(s) according to the print signal are/is applied to the individual electrode  43  from the driver IC  45  (S 103 ). 
     On the other hand, when the predetermined rate Rp is reached or exceeded by the rate R of the piezoelectric elements  46  of which the integrated value Cs exceeds the predetermined value Cp in any of the four ink-jet heads  2  (S 105 : YES), even if the predetermined value Cp is not exceeded by the integrated value Cs which corresponds to the piezoelectric element  46  to be driven, after that, the number of drive pulses applied to the individual electrode  43  from the driver IC  45 , is reduced to be less than previous (S 106 ), that is, the discharge energy applied to the ink inside the pressure chamber  10  is lowered. 
     Here, even if the predetermined value Cp is not exceeded by the integrated value Cs which corresponds to the piezoelectric element  46  to be driven, in the ink-jet head  2  which includes the corresponding piezoelectric element  46 , a high rate R of the piezoelectric elements  46  of which the integrated value Cs exceeds the predetermined value Cp, may result in the following possible consequence: when drive pulse(s) are/is applied as usual to the individual electrode  43  of the corresponding piezoelectric element  46 , in the printed image, a great difference in image quality may occur between the portions formed by the landed ink droplet(s) discharged from the nozzles  15  which correspond to the piezoelectric elements  46  to which the number of drive pulses applied is reduced, and the portions formed by the landed ink droplet(s) discharged from the nozzles  15  which correspond to the piezoelectric elements  46  to which a usual number of drive pulses is applied. 
     Further, in any of the four ink-jet heads  2 , a high rate R of the piezoelectric elements  46  of which the integrated value Cs exceeds the predetermined value Cp, may cause dot sizes to vary greatly among the different colors, if drive pulses are applied as usual to the individual electrodes  43  of the piezoelectric elements  46  which are included in the other ink-jet heads  2 . This may greatly degrades the quality of the printed image. 
     Nevertheless, in the embodiment, in any of the four ink-jet heads  2 , when the predetermined rate Rp is reached or exceeded by the rate R of the piezoelectric elements  46  of which the integrated value Cs exceeds the predetermined value Cp, even if the predetermined value Cp is not exceeded by the integrated value Cs which corresponds to the piezoelectric element  46  to be driven, the number of drive pulses applied to the individual electrode  43  of the corresponding piezoelectric element  46 , is reduced. Therefore, in the ink-jet head  2  which includes the piezoelectric element  46  to be driven, a high rate R of the piezoelectric elements  46  of which the integrated value Cs exceeds the predetermined value Cp, degrades the entire image quality of the printed image. Therefore, it is possible to prevent a great difference in image quality from occurring between different portions in the printed image. 
     Further, in any of the four ink-jet heads  2 , when the rate R of the piezoelectric elements  46  of which the integrated value Cs exceeds the predetermined value Cp, has become higher, regardless of the color of ink to be discharged from the nozzle  15  which corresponds to the piezoelectric element  46  to be driven, the number of drive pulses applied to the individual electrode  43  of the piezoelectric element  46 , is reduced. This regulates the variation in dot size among the different colors, and is thereby able to prevent great degradation in image quality. 
     Further, when printing is started and carried out in the high-resolution print mode described above (S 102 : YES), if the number of drive pulses applied to the individual electrode  43  from the driver IC  45  is reduced, differently from the embodiment, the quality of the printed image, in particular, degrades conspicuously. 
     However, in the embodiment, when printing is started and carried out in the high-resolution print mode, regardless of the integrated value Cs which corresponds to the piezoelectric element  46  to be driven, the number of drive pulses applied to the individual electrode  43  is not reduced but applied as usual. Accordingly, it is possible to prevent the printed image from degrading in image quality. 
     Further, when printing is carried out in high-resolution, since small dots are used more than medium and large dots, there is a low frequency of applying two or three drive pulses to the individual electrode  43  to form one dot. Accordingly, even if the number of drive pulses applied to the individual electrode  43  is not reduced, the life of the piezoelectric element  46  will not be greatly shortened. 
     On the other hand, when printing is started and carried out in the normal print mode, as described above, even if the number of drive pulses applied to the individual electrode  43  from the driver IC  45  is reduced, degradation in image quality of the printed image is not as conspicuous as degradation when the printing is started and carried out in the high-resolution print mode. 
     Next, a few modifications which apply various changes to the embodiment will be explained. Note that, the constitutive parts or components, which are the same as or equivalent to those of the embodiment described above, are designated by the same reference numerals, any explanation of which will be omitted as appropriate. 
     In one modification (Modification 1), as shown in  FIG. 8 , when a print command is inputted (S 101 : YES) and the printing is performed in the high-resolution print mode (S 102 : YES), print mode  1  is set (S 203 ). In the print mode  1 , the printing is performed by applying drive pulse(s) as usual. When the printing is not performed in the high-resolution print mode (S 102 : NO), with respect to an ink-jet head  2  including the piezoelectric element  46  to be driven, a determination whether a degradation flag, which will be described later, is on or not is made (S 204 ). When the degradation flag is not on (S 204 : NO), print mode  2  is set (S 205 ). In the print mode  2 , the number of pulses which will be applied to the piezoelectric element  46  of which flag “1” is on is reduced. The meaning of flag “1” will be described later. When the degradation flag is on (S 204 : YES), print mode  3  is set (S 206 ). In the print mode  3 , the number of pulses which will be applied to each of the piezoelectric elements  46  included in the ink-jet head  2  is reduced. The printing is performed in the set print mode (S 207 ), and then, the integrated value Cs of each of the piezoelectric elements  46  is updated (S 208 ). After that, for each of the piezoelectric elements  46 , a determination whether the integrated value Csn exceeds the predetermined value Cp or not is made, and when the integrated value Csn exceeds the predetermined value Cp, the flag “1” is set on with respect to the piezoelectric element  46  (S 209 -S 213 ). After the flag is updated for each of the piezoelectric elements  46  (S 213 : YES), it is determined whether a rate R of the piezoelectric element  46  of which integrated value Cs exceeds the predetermined value Cp among the piezoelectric elements  46  in the ink-jet heads  2  is not less than a predetermined rate Rp (S 214 ). When the rate R of the piezoelectric element  46  of which integrated value Cs exceeds the predetermined value Cp among the piezoelectric elements  46  in the ink-jet heads  2  is not less than the predetermined rate Rp, the degradation flag is set on with respect to the ink-jet head  2  (S 215 ). Namely, the degradation flag means that the rate R of the piezoelectric element  46  of which integrated value Cs exceeds the predetermined value Cp among the piezoelectric elements  46  in the ink-jet heads  2  is not less than the predetermined rate Rp. 
     In this case, in the ink-jet head  2  which includes the piezoelectric element  46  to be driven, when the predetermined value Rp is not less than the rate R of the piezoelectric elements  46  of which integrated value Cs exceeds the predetermined value Cp, even if the predetermined value Cp is not exceeded by the integrated value Cs which corresponds to the piezoelectric element  46  to be driven, the number of drive pulses applied to the individual electrode  43  of the corresponding piezoelectric element  46 , is reduced. Accordingly, in the same manner as the embodiment described above, it is possible to prevent a great difference from occurring in image quality among different portions of the printed image. 
     In Modification 1, the number of drive pulses applied to the individual electrode is controlled based on the rate R of the piezoelectric elements  46 , of which the integrated value Cs exceeds the predetermined value Cp, in the ink-jet head  2  which includes the piezoelectric element  46  to be driven. However, the control as described above may also be carried out for each row of nozzles included in one ink-jet head  2 , in other words, for each row of the piezoelectric elements  46 . For example, in a row of the piezoelectric elements  46  including the piezoelectric element  46  to be driven, when the predetermined rate Rp is reached or exceeded by the rate R of the piezoelectric element  46  of which the integrated value Cs exceeds the predetermined value Cp, even if the predetermined value Cp is not exceeded by the integrated value Cs of the corresponding piezoelectric element  46 , the control may also be carried out to reduce the number of drive pulses applied to the individual electrode  43  of the corresponding piezoelectric element  46 . By such a control, it is possible to prevent a great difference from occurring in image quality among different portions of the printed image. 
     Alternatively, in a row of the piezoelectric elements  46  including the piezoelectric element  46  to be driven, when the predetermined value Cp is exceeded by the integrated value Cs of a piezoelectric element  46  adjacent to the piezoelectric element  46  to be driven, even if the predetermined value Cp is not exceeded by the integrated value Cs of the corresponding piezoelectric element  46  to be driven, the control may also be carried out to reduce the number of drive pulses applied to the individual electrode  43  of the corresponding piezoelectric element  46  to be driven. By such a control, it is possible to prevent a great difference from occurring in image quality among different portions of the printed image. 
     Further, in another modification (Modification 2), as shown in  FIG. 9 , when the predetermined value Cp is not exceeded by the integrated value Cs which corresponds to the piezoelectric element  46  to be driven (S 104 : NO), the determination as shown above in S 105  and  5214  (see  FIGS. 6 and 8 ) is not made, and drive pulses are applied as usual (S 103 ). 
     Even in this case, if the predetermined value Cp is exceeded by the integrated value Cs which corresponds to the piezoelectric element  46  to be driven, after that, the number of drive pulses applied to the individual electrode  43  of the corresponding piezoelectric element  46  is reduced to be less than usual; thus a small dot is landed to the area where a medium dot or a large dot should have been landed on the recording paper P. Therefore, although there are more or less effects on image quality of the printed image, it is possible to prolong the life of the corresponding piezoelectric element  46  because the number of drive pulses applied to the individual electrode  43  of the corresponding piezoelectric element  46  becomes less in printing. 
     Further, in the embodiment described above, when the predetermined value Cp is exceeded by the number of times of applying the drive pulse to the individual electrode  43  of the piezoelectric element  46 , after that, even if the print signal indicates either a large dot or a medium dot to be landed, the number of drive pulses applied to the individual electrode  43  from the driver IC  45  is reduced to one. However, it is not limited to this manner to reduce the number of drive pulses. 
     In yet another modification (Modification 3), when the predetermined value Cp is exceeded by the number of times of applying the drive pulse to the individual electrode  43  of the piezoelectric element  46 , after that, as shown in  FIGS. 10A and 10B , if the print signal indicates a large dot to be landed, the number of drive pulses applied to the individual electrode  43  from the driver IC  45  is reduced from three to two; if the print signal indicates a medium dot to be landed, the number of drive pulses applied to the individual electrode  43  from the driver IC  45  is reduced from two to one. Further, in this case, in the same manner as the embodiment described above, if the print signal indicates a small dot to be landed, the number of drive pulse applied to the individual electrode  43  from the driver IC  45  also remains at one. 
     Even in this case, if the predetermined value Cp is exceeded by the integrated value Cs which corresponds to the piezoelectric element  46  to be driven, after that, the number of drive pulses applied to the individual electrode  43  of the corresponding piezoelectric element  46  is reduced to be less than usual; thus a small dot and a medium dot are landed to the areas where a medium and a large dot should have been landed on the recording paper P, respectively. Therefore, although there are more or less effects on image quality of the printed image, since the number of drive pulses applied to the individual electrode  43  of the corresponding piezoelectric element  46  becomes less in printing, the number of times of driving the piezoelectric element  46  decreases; thereby the life of the corresponding piezoelectric element  46  extends. 
     Further, in the embodiment described above, when the predetermined value Cp is exceeded by the number of times of applying the drive pulse to the individual electrode  43  of the piezoelectric element  46 , after that, the number of drive pulses applied to the individual electrode  43  from the driver IC  45  is reduced. However, there is no limitation to this. 
     In yet another modification (Modification 4), in a case such as the integrated value Cs described above exceeds the predetermined value Cp, as shown in  FIGS. 11A and 11B , the drive pulse control portion  53  does not change the number of times of applying the drive pulse to the individual electrode  43  from the driver IC  45 , but lowers the height thereof to be lower than usual (to be a height at which the drive potential becomes a drive potential V 2  which is lower than the usual drive potential V 1 ). 
     Here, when the height of drive pulses stays constant, the maximum drivable number of times Cm is predetermined for the piezoelectric element  46 . However, if the height of drive pulses is lowered to be a lower drive potential, the discharge energy applied to the ink inside the pressure chamber  10  from the piezoelectric element  46  also becomes lower. Thus, when the piezoelectric element  46  is driven, the maximum drivable number of times Cm increases. Therefore, when the predetermined value Cp is exceeded by the number of times of applying the drive pulse to the individual electrode  43  of the piezoelectric element  46 , after that, by lowering the height of drive pulses applied to the individual electrode  43  from the driver IC  45 , it is possible to prolong the life of the piezoelectric element  46 . 
     Further, in this case, because the piezoelectric actuator  32  applies a pressure to the ink inside the pressure chamber  10  by applying drive pulse(s) to the individual electrode  43 , it is possible to easily lower a discharge energy applied to the piezoelectric element  46  by lowering the height of drive pulses. 
     Further, in the embodiment described above, when printing is started and carried out in the high-resolution print mode, regardless of the integrated value Cs which corresponds to the piezoelectric element  46  to be driven, the number of drive pulses applied to the individual electrode  43  is not reduced. However, even if printing is started and carried out in the high-resolution print mode, the number of drive pulses may also be reduced in the same manner as printing is started and carried out in the normal print mode. 
     In the embodiment described above, the predetermined value Cp, of the number of times of applying the drive pulse, is set in one phase. However, it may be set in two or more phases. For example, when the predetermined value, of the number of times of applying the drive pulse, is set in two phases to Cp 1  and Cp 2  (Cp 1 &lt;Cp 2 ), if Cp 2  is not exceeded by the integrated value Cs of the number of times of driving any of the piezoelectric elements  46 , in the same manner as the embodiment described above, printing may be carried out in the high-solution print mode without reducing the number of drive pulses applied to the individual electrode  43 ; if Cp 2  is exceeded by the integrated value Cs of the number of times of driving any of the piezoelectric elements  46 , even in the high-solution print mode, the number of drive pulses may also be reduced in the same manner as in the normal print mode. 
     Further, in the embodiment described above, the printer  1  has four ink-jet heads  2  which discharge inks of different colors from each other; however, the number of the ink-jet head  2 , and the kind and number of the color of ink discharged from the ink-jet head  2  are not limited thereto. 
     Further, in the embodiment described above, the ink-jet head  2  is a so-called line head; however, it is not limited thereto but may also be a so-called serial head which discharges ink from a nozzle while reciprocating in a scanning direction. 
     Further, the above description has given an example of applying the present teaching to a printer which has an ink-jet head discharging an ink droplet from the nozzle  15  by applying a pressure to the ink inside the pressure chamber  10  through the piezoelectric element  46 ; however, different in method therefrom and not limited thereto, it is also possible to apply the present teaching to a printer which has an ink-jet head applying a discharge energy for discharging an ink from a nozzle. 
     As an example, if the ink-jet head is such a one that discharges an ink droplet from a nozzle through raising the pressure on the ink inside a pressure chamber (applying a discharge energy) by heating the pressure chamber with a heater and thereby inflating the air bubbles inside the pressure chamber, as a total length of time of heating with the heater becomes longer, the heater gets more degraded in thermal capability due to the effect of burnt deposits and the like produced on the heater surface. 
     In view of such a case, suppose a timer or the like be provided in the controller instead of the above-described drive pulse counter  52 , for example, to measure the heating time of the heater and store an integrated value thereof at the same time. When the integrated value exceeds a predetermined value, after that, the time length of heating the pressure chamber with the heater is shortened to be shorter than previous to form one dot. Accordingly, the discharge energy applied to the ink inside the pressure chamber from the heater becomes lower, thereby restraining the heater from degrading in thermal capability and making it possible to prolong the life of the ink-jet head. 
     Nevertheless, in this case, because the time of heating the pressure chamber with the heater is shortened, the inflation amount of the air bubbles decreases and thereby the ink droplet discharged from the nozzle become smaller in volume. Therefore, the printed image degrades more or less in image quality. 
     Further, with respect to the piezoelectric element  46  in the embodiment described above, it is understood that as the driving time becomes longer, the integrated value Cs of the number of times of applying the drive pulse to the individual electrode  43  from the driver IC  45  increases. Therefore, in the embodiment described above, a timer as described above may also be provided instead of the drive pulse counter  52 . When a predetermined value is exceeded by an integrated value, stored in the timer, of the time of driving the piezoelectric element  46 , after that, the number of drive pulses applied to the individual electrode  43  from the driver IC  45  may, for example, be reduced to be less than usual. 
     Furthermore, the above description has given an example of applying the present teaching to an ink-jet printer which prints an image by discharging an ink from a nozzle; however, not limited thereto, it is possible to apply the present teaching to a printing apparatus which has a liquid discharge head for discharging a liquid other than ink, such as printing apparatuses for printing a wiring pattern by discharging a liquid droplet of a conductive material from a nozzle to form a wiring.