Patent Publication Number: US-8985726-B2

Title: Inkjet recording apparatus

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2012-190716 filed on Aug. 30, 2012, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to inkjet recording apparatuses. 
     An inkjet recording apparatus has, for example, a plurality of nozzles, pressurizing chambers, and piezoelectric elements. Ink droplets can be ejected from the plurality of nozzles. The pressurizing chambers are provided so as to communicate with the plurality of nozzles, respectively. Ink is charged into the pressurizing chambers. The piezoelectric elements are deformed by a driving voltage being applied, to eject, as ink droplets, the ink having been charged into the pressurizing chambers, from the nozzles. 
     Depending on an image formed on a recording paper, ink droplets may not be ejected from some of the plurality of nozzles for a long time period. The nozzles from which no ink droplets are ejected, are left in a state where a meniscus of ink is at a stop. In this case, for example, solvent contained in the ink near the meniscus evaporates, thereby increasing viscosity of the ink in the nozzles. As a result, ejection of ink droplets from the nozzles may be disturbed, or the nozzles may be clogged and thus ink droplets may not be ejected. 
     In recent years, an ink having a highly volatile solvent blended therein is used in order to enhance a quick-drying property for dots formed on a recording paper. Therefore, disturbance in ejection of ink droplets or non-ejecting of ink droplets as described above is more likely to occur. 
     On the other hand, a technique has been known in which a meniscus of an ink droplet is vibrated by minutely vibrating a piezoelectric element in order to reduce occurrence of non-ejecting of the ink droplets as described above and solve a problem that may arise after occurrence of the non-ejecting. In the technique, for example, ejection signal generation means that generates an ejection signal for ejecting ink, and minute-vibration signal generation means that periodically generates a minute-vibration signal for generating minute vibration to such a degree that ink does not eject, are provided. In the technique, waveform elements contained in the ejection signal and the minute-vibration signal are combined to generate a new driving pulse, the new driving pulse is inputted to a pressure generation element, and ink in a waiting state is minutely vibrated to prevent increase in viscosity of the ink. 
     SUMMARY 
     An inkjet recording apparatus according to one aspect of the present disclosure includes a nozzle, pressuring chambers, a piezoelectric element, an ambient condition obtaining portion, an application portion, and a control portion. The nozzle is implemented as a plurality of nozzles each of which can eject ink droplets toward a recording medium, and configured to have a meniscus of ink formed therein. The pressurizing chambers communicate with the plurality of nozzles, respectively, and ink is charged into the pressuring chambers. The piezoelectric element is deformed by a driving voltage being applied, and operates to eject the ink having been charged into the pressurizing chambers, as ink droplets, from the nozzles. The ambient condition obtaining portion can obtain an ambient condition. The application portion applies the driving voltage to the piezoelectric element. The control portion causes the application portion to output an ejection driving voltage by which the piezoelectric element is deformed so as to eject droplets from the nozzles in a printing period in a printing state, and causes the application portion to output a vibration driving voltage by which the piezoelectric element is vibrated a predetermined number of times so as to vibrate a meniscus in each nozzle in a non-printing period between printing periods in the printing state. The control portion changes the number of times the piezoelectric element is vibrated in the non-printing period, based on the ambient condition obtained by the ambient condition obtaining portion, or a zero shear viscosity of ink which is calculated or obtained according to the obtained ambient condition, and causes the application portion to output a vibration driving voltage by which the piezoelectric element is vibrated the number of times having been changed. 
     An inkjet recording apparatus according to another aspect of the present disclosure includes a nozzle, pressuring chambers, a piezoelectric element, an ambient condition obtaining portion, an application portion, and a control portion. The nozzle is implemented as a plurality of nozzles each of which can eject ink droplets toward a recording medium, and configured to have a meniscus of ink formed therein. The pressurizing chambers communicate with the plurality of nozzles, respectively, and ink is charged into the pressuring chambers. The piezoelectric element is deformed by a driving voltage being applied, and operates to eject the ink having been charged into the pressurizing chambers, as ink droplets, from the nozzles. The ambient condition obtaining portion can obtain an ambient condition. The application portion applies the driving voltage to the piezoelectric element. The control portion causes the application portion to output an ejection driving voltage by which the piezoelectric element is deformed so as to eject droplets from the nozzles in a printing period in a printing state, and causes the application portion to output a vibration driving voltage by which the piezoelectric element is vibrated a predetermined number of times so as to vibrate a meniscus in each nozzle in a non-printing period between printing periods in the printing state. The control portion changes the number of times the piezoelectric element is vibrated in the non-printing period, based on the ambient condition obtained by the ambient condition obtaining portion, and causes the application portion to output a vibration driving voltage by which the piezoelectric element is vibrated the number of times, per unit time, which has been changed. 
     An inkjet recording apparatus according to still another aspect of the present disclosure includes a nozzle, pressurizing chambers, a piezoelectric element, an ambient condition obtaining portion, an application portion, a zero shear viscosity calculating/obtaining portion, and a control portion. The nozzle is implemented as a plurality of nozzles each of which can eject ink droplets toward a recording medium, and configured to have a meniscus of ink formed therein. The pressurizing chambers communicate with the plurality of nozzles, respectively, and ink is charged into the pressurizing chambers. The piezoelectric element is deformed by a driving voltage being applied, and operates to eject the ink having been charged into the pressurizing chambers, as ink droplets, from the nozzles. The ambient condition obtaining portion can obtain an ambient condition. The application portion applies the driving voltage to the piezoelectric element. The zero shear viscosity calculating/obtaining portion calculates or obtains a zero shear viscosity of ink, based on the ambient condition obtained by the ambient condition obtaining portion. The control portion causes the application portion to output an ejection driving voltage by which the piezoelectric element is deformed so as to eject droplets from the nozzles in a printing period in a printing state, and causes the application portion to output a vibration driving voltage by which the piezoelectric element is vibrated a predetermined number of times so as to vibrate a meniscus in each nozzle in a non-printing period between printing periods in the printing state. The control portion changes the number of times the piezoelectric element is vibrated in the non-printing period, based on the zero shear viscosity calculated or obtained by the zero shear viscosity calculating/obtaining portion, and causes the application portion to output a vibration driving voltage by which the piezoelectric element is vibrated the number of times having been changed. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description with reference where appropriate to the accompanying drawings. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical cross-sectional view schematically illustrating an outline of an inkjet recording apparatus, as viewed from the front thereof, according to a first embodiment of the present disclosure. 
         FIG. 2  is a plan view illustrating vicinities of a recording portion and a conveyor unit in a state where a cap unit is mounted to line heads in the inkjet recording apparatus of the first embodiment. 
         FIG. 3  illustrates side faces of the line heads and a conveyor belt shown in  FIG. 1 . 
         FIG. 4  is a plan view illustrating the conveyor belt of the inkjet recording apparatus shown in  FIG. 3 , as viewed from thereabove. 
         FIG. 5  is a cross-sectional view of one of the head lines shown in  FIG. 1 . 
         FIG. 6  illustrates a process of meniscus vibrating in a meniscus vibration operation. 
         FIG. 7  is a block diagram illustrating a configuration of the inkjet recording apparatus according to the first embodiment. 
         FIG. 8  shows a table that represents a relationship between a temperature and the number of times a piezoelectric element is vibrated. 
         FIG. 9  illustrates nozzle arrays of the line head. 
         FIG. 10  is a flow chart showing a meniscus vibration operation performed by the inkjet recording apparatus of the first embodiment. 
         FIG. 11  is a block diagram illustrating a configuration of an inkjet recording apparatus according to a second embodiment. 
         FIG. 12  shows a table that represents a relationship among a temperature, a zero shear viscosity, and the number of times a piezoelectric element is vibrated. 
         FIG. 13  shows a relationship at 15° C. between shear viscosities and values of sin 0. 
         FIG. 14  shows a graph that represents zero shear viscosities plotted against temperatures. 
         FIG. 15  is a flow chart illustrating a meniscus vibration operation performed by the inkjet recording apparatus of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an inkjet recording apparatus  1  according to an embodiment of the present disclosure will be described with reference to the drawings. Firstly, the inkjet recording apparatus  1  of the first embodiment will be described with reference to  FIG. 1  to  FIG. 9 . 
     In the below description, line heads  22 K,  22 C,  22 M, and  22 Y for four colors, four ink tanks  23 K,  23 C,  23 M, and  23 Y, four pump mechanisms  24 K,  24 C,  24 M, and  24 Y, and four cleaning portions  25 K,  25 C,  25 M, and  25 Y described below may be simply referred to as “line head  22 ”, “ink tank  23 ”, “pump mechanism  24 ”, and “cleaning portion  25 ” by omitting the identification characters, “K”, “C”, “M, and “Y”, unless each of them is to be particularly specified. 
     As shown in  FIG. 1  and  FIG. 2 , the inkjet recording apparatus  1  of the first embodiment includes, in a main body  2 , a recording portion  20 , the cleaning portion  25 , a conveyor unit  30 , a lifting and lowering device  40 , a cap unit  50 , a first horizontal moving mechanism (not shown), and a second horizontal moving mechanism (not shown). The lifting and lowering device  40  lifts or lowers the conveyor unit  30 . The first horizontal moving mechanism horizontally moves the cap unit  50 . The second horizontal moving mechanism horizontally moves the cleaning portion  25 . The inkjet recording apparatus  1  of the first embodiment further includes a sheet feed cassette  3 , a sheet feed roller  4 , a sheet conveying path  5 , a pair of registration rollers  6 , a drying device  7 , a pair of sheet discharge rollers  8 , a sheet discharge port  9 , and a sheet discharge tray  10 . 
     As shown in  FIG. 1  and  FIG. 2 , the conveyor unit  30  includes a belt drive roller  32 , a follower roller  33 , a conveyor belt  31 , a tension roller  34 , and an air suction unit (not shown). The conveyor belt  31  is extended on the belt drive roller  32  and the follower roller  33 . A tension of the conveyor belt  31  is adjusted by the tension roller  34 . The air suction unit is disposed below a conveying surface of the conveyor belt  31  (on a side opposite to the recording portion  20  side). On each of the upper surfaces of the conveyor belt  31  and the air suction unit, multiple through holes (not shown) for suction are formed. Also, the belt drive roller  32  and the follower roller  33  may be named a belt roller. 
     The belt drive roller  32  and the follower roller  33  rotate counterclockwise as viewed from the front thereof. Thus, a conveying surface  31 A formed on an upper face portion of the conveyor belt  31  is horizontally moved in a sheet conveying direction P from one side toward the other side on a horizontal plane (X-Y plane). Namely, on the conveying surface  31 A of the conveyor belt  31 , the sheet conveying direction P is almost equal to the horizontal direction X. The air suction unit (not shown) is disposed below the conveying surface  31 A of the conveyor belt  31  (on a side opposite to the recording portion  20  side), and applies, to the conveying surface  31 A, a suction force for suctioning a recording paper T as a recording medium onto the conveying surface  31 A of the conveyor belt  31 . 
     As the conveyor belt  31 , an endless belt obtained by both end portions being overlapped and bonded to each other, a (seamless) belt having no joints, or the like, is used. 
     As shown in  FIG. 2 , the recording paper T as a recording medium is introduced onto the conveying surface  31 A of the conveyor belt  31  from one side in the sheet conveying direction P for a predetermined recording. On the conveying surface  31 A, a suction force to be applied to the conveyor belt  31  through the through holes (not shown) for suction as described above is generated according to an operation of the air suction unit (not shown). The recording paper T introduced onto the conveying surface  31 A of the conveyor belt  31  is suctioned onto the conveying surface  31 A due to the suction force, and conveyed downstream in the sheet conveying direction P. Thus, ink is ejected from the line head  22  of the recording portion  20  described below toward the recording paper T that is suctioned and conveyed on the conveying surface  31 A of the conveyor belt  31 , thereby recording (printing) an image or the like on the recording paper T. 
     As shown in  FIG. 1 , the recording papers T are stored in the sheet feed cassette  3  in a stacked state. The sheet feed cassette  3  is disposed upstream of the conveyor unit  30  in the sheet conveying direction P, and disposed in the lower inside portion of the main body  2 . The sheet feed roller  4  is disposed above the sheet feed cassette  3 . The recording paper T is fed from the sheet feed cassette  3  shown in  FIG. 1 , in the upper right direction, by the sheet feed roller  4 . 
     The sheet conveying path  5 , the pair of registration rollers  6 , the recording portion  20 , and the conveyor unit  30  are disposed downstream of the sheet feed cassette  3  in the sheet conveying direction P. The recording paper T fed from the sheet feed cassette  3  is conveyed through the sheet conveying path  5  and reaches the pair of registration rollers  6 . The pair of registration rollers  6  corrects diagonal feeding of the recording paper T, and feeds the recording paper T again. A leading end of the recording paper T is detected by a sheet leading end detection sensor (not shown) provided in the sheet conveying path  5  between the recording portion  20  and the pair of registration rollers  6 . The recording portion  20  executes ink ejecting operation as described below, based on a time at which the leading end is detected. 
     As shown in  FIG. 1 , the drying device  7  is disposed downstream of the conveyor unit  30  in the sheet conveying direction P in an upper inside portion of the main body  2 . Ink on the recording paper T on which recording has been performed with the ink ejected by the recording portion  20  is dried by the drying device  7 . 
     The pair of sheet discharge rollers  8 , the sheet discharge port  9 , and the sheet discharge tray  10  are disposed in order, respectively, downstream of the drying device  7  in the sheet conveying direction P. The recording paper T on which the ink has been dried by the drying device  7  is fed downstream in the sheet conveying direction P by the pair of sheet discharge rollers  8  including a discharge roller  8   a  and a follower roller  8   b  that is pressed against the discharge roller  8   a  and rotates so as to follow the discharge roller  8   a . Then, the recording paper T is fed through the sheet discharge port  9  to the sheet discharge tray  10  disposed outside the main body  2 , to be discharged to the outside of the main body  2 . 
     As shown in  FIG. 1  and  FIG. 2 , the recording portion  20  has the line heads  22  (head portions) corresponding to four colors. The line heads  22  corresponding to the four colors includes a line head  22 K for black color, a line head  22 C for cyan color, a line head  22 M for magenta color, and a line head  22 Y for yellow color. The line heads  22 K,  22 C,  22 M, and  22 Y for the four colors extend longitudinally along a sheet width direction Y orthogonal to the sheet conveying direction P (the horizontal direction X). The line heads  22 K,  22 C,  22 M, and  22 Y are aligned in order, respectively, from the upstream side toward the downstream side in the sheet conveying direction P along the sheet conveying direction P of the conveyor belt  31 . 
     The line heads  22 K,  22 C,  22 M, and  22 Y corresponding to the four colors have a nozzle surface  221  in which ink ejecting nozzles are formed. The nozzle surface  221  is formed as lower surfaces of the line heads  22 K,  22 C,  22 M, and  22 Y for the four colors. The nozzle surface  221  of each of the line heads  22 K,  22 C,  22 M, and  22 Y is opposed to the conveying surface  31 A of the conveyor belt  31 . An image is recorded, by means of the line heads  22 K,  22 C,  22 M, and  22 Y for the four colors, on the recording paper T with ink ejected from the ink ejecting nozzles formed in the nozzle surface  221 . 
     As shown in  FIG. 1 , an ink supply portion  100  has four ink tanks  23 K,  23 C,  23 M, and  23 Y, and four pump mechanisms  24 K,  24 C,  24 M, and  24 Y. 
     The four ink tanks  23 K,  23 C,  23 M, and  23 Y are disposed below the conveyor unit  30  so as to correspond to the line heads  22 K,  22 C,  22 M, and  22 Y for the four colors, respectively. Inks to be supplied to the line heads  22 K,  22 C,  22 M, and  22 Y for the four colors are stored in the four ink tanks  23 K,  23 C,  23 M, and  23 Y. The inks of the four colors stored in the four ink tanks  23 K,  23 C,  23 M, and  23 Y are supplied to the four pump mechanisms  24 K,  24 C,  24 M, and  24 Y, respectively, described below. The four ink tanks  23 K,  23 C,  23 M, and  23 Y are aligned in order, respectively, from the upstream side toward the downstream side in the sheet conveying direction P along the sheet conveying direction P of the conveyor belt  31 . 
     The four pump mechanisms  24 K,  24 C,  24 M, and  24 Y are disposed above the conveyor unit  30  so as to correspond to the four ink tanks  23 K,  23 C,  23 M, and  23 Y, respectively. The four pump mechanisms  24 K,  24 C,  24 M, and  24 Y are aligned in order, respectively, from the upstream side toward the downstream side in the sheet conveying direction P along the sheet conveying direction P of the conveyor belt  31 . The four pump mechanisms  24 K,  24 C,  24 M, and  24 Y temporarily store the inks of the four colors having been stored in the four ink tanks  23 K,  23 C,  23 M, and  23 Y, respectively. The inks of the four colors stored in the four pump mechanisms  24 K,  24 C,  24 M, and  24 Y are supplied to the line heads  22 K,  22 C,  22 M, and  22 Y for the four colors from the four pump mechanisms  24 K,  24 C,  24 M,  24 Y, respectively. The ink supply portion  100  will be described below in detail. 
     Inks of the four colors are ejected by the line heads  22 , respectively, of the recording portion  20  toward the recording paper T set on the conveying surface  31 A of the conveyor belt  31  according to image data information (for example, characters, figures, patterns) received from an external computer (not shown). As shown in  FIG. 2 , each line head  22  is supported by a line head support member  21  having a rectangular-plate-like shape, and each line head  22  and the line head support member  21  are both secured to the main body  2 . Inks of the four colors are sequentially ejected from the respective line heads  22  at predetermined times, according to the rotation of the conveyor belt  31 , and therefore the inks of the four colors, that is, black, cyan, magenta, and yellow colors, are superimposed on the recording paper T, thereby printing a color ink image. 
     As shown in  FIG. 1 , the lifting and lowering device  40  is disposed below the conveyor unit  30 . The lifting and lowering device  40  lifts or lowers (moves) the conveyor unit  30  relative to the line head  22  in a direction Z (hereinafter, also referred to as “up-down direction Z”) perpendicular to the horizontal plane (the X-Y plane). By the movement of the conveyor unit  30  in the up-down direction Z by the lifting and lowering device  40 , the conveying surface  31 A of the conveyor belt  31  can be moved so as to be relatively closer to or farther from the nozzle surface  221  of the line head  22 . 
     As shown in  FIG. 1 , the lifting and lowering device  40  includes two eccentric cams  41 . The eccentric cams  41  are disposed below the conveyor belt  31  on the upstream side and the downstream side, respectively, in the sheet conveying direction P. The eccentric cams  41  are disposed such that the two eccentric cams  41  are disposed near each of a front face side and a rear face side of the conveyor unit  30 , and the total number of the eccentric cams  41  is four. The eccentric circumferential surface of each eccentric cam  41  approaches the outer bottom surface of the conveyor unit  30  from below the conveyor unit  30 . As shown in  FIG. 1 , each eccentric cam  41  includes: a shaft  42  that extends in the sheet width direction Y; and a cam having a rotation axis line that is eccentrically located. Each eccentric cam  41  rotates about the shaft  42  by means of a motor (not shown). Each eccentric cam  41  includes a plurality of bearings  43  on the circumferential edge portion. A portion of the circumferential surface of each bearing  43  projects outward from the circumferential surface of the eccentric cam  41 . 
     Each bearing  43  is rotatable about an axis line parallel to the rotation axis line of the eccentric cam  41 . The bearings  43  are sequentially disposed from the top end side toward the rotation axis line side in each eccentric cam  41 . In a normal printing state, as shown in  FIG. 1 , the bearing  43  that is farthest from the shaft  42  abuts against the outer bottom surface of the conveyor unit  30  from therebelow. Thus, the conveyor unit  30  is lifted to an uppermost position. 
     In this state, the eccentric cams  41  on the upstream side in the sheet conveying direction P are rotated counterclockwise as viewed from the front thereof, and the eccentric cams  41  on the downstream side in the sheet conveying direction P are rotated clockwise as viewed from the front thereof. Thus, the plurality of bearings  43  sequentially abut against the outer bottom surface of the conveyor unit  30  in the order starting from the bearing  43  farthest from the shaft  42  toward the bearing  43  closest to the shaft  42 . Therefore, the conveyor unit  30  can be lowered. 
     The plurality of bearing  43  are spaced from each other so as to include a period in which two of the bearings  43  adjacent to each other in the circumferential edge direction simultaneously abut against the outer bottom surface of the conveyor unit  30  when the eccentric cams  41  rotate. 
     The conveyor unit  30  is lowered by the eccentric cams  41  of the lifting and lowering device  40  being rotated, whereby the conveying surface  31 A of the conveyor belt  31  in the conveyor unit  30  is moved downward of the line head  22  so as to be spaced from the line head  22 . 
     As shown in  FIG. 1 , the cap unit  50  is disposed below the recording portion  20  and above the conveyor unit  30  (between the recording portion  20  and the conveyor unit  30 ). As shown in  FIG. 2 , the cap unit  50  includes a plurality of cap cases  52  disposed so as to correspond to the line heads  22 , respectively, and cap base members  53  that fix and support the plurality of cap cases  52  so as to satisfy a predetermined positional relationship. 
     The cap unit  50  can be lifted or lowered in conjunction with the conveyor unit  30  being lifted or lowered by the lifting and lowering device  40  in a state where the cap unit  50  is positioned between the recording portion  20  and the conveyor unit  30 . The conveyor unit  30  is lowered by the eccentric cams  41  of the lifting and lowering device  40  being rotated, whereby the cap unit  50  is moved downward of the line head  22  so as to be spaced from the line head  22  in conjunction with the conveying surface  31 A of the conveyor belt  31  being lowered. 
     Thus, the cap unit  50  is detached from the line head  22 . Ink is emitted from ink emitting nozzles described below in the nozzle surface  221  of the line head  22  in a state where the cap unit  50  is detached from the line head  22 , and thus the inkjet recording apparatus  1  can execute an ejection recovery process of emitting ink that has a high viscosity and is left in the nozzles, to remove the clogging with ink, that is, execute purging. 
     On the other hand, when the eccentric cams  41  of the lifting and lowering device  40  are rotated in the direction opposite to the direction described above, to lift the conveyor unit  30 , the conveyor unit  30  is returned to a normal recording position (printing position). 
     In a state where the cap unit  50  is disposed between the recording portion  20  and the conveyor unit  30 , the cap unit  50  can be mounted to the nozzle surface  221  of the line head  22 . Further, in a state where, by the cap unit  50  having been moved by the first horizontal moving mechanism (not shown) described below, the cap unit  50  is not disposed between the recording portion  20  and the conveyor unit  30 , the line head  22  is allowed to eject ink toward the recording paper T on the conveying surface  31 A of the conveyor belt  31 . 
     The cap unit  50  can be moved horizontally in the sheet conveying direction P (see  FIG. 1 ) by the cap base members  53  being horizontally moved by the first horizontal moving mechanism (not shown). 
     The cap unit  50  is moved, by the first horizontal moving mechanism, to a mounting and detaching position at which the cap cases  52  can be mounted to or detached from the line heads  22 , or to a retracting position that is distant form the mounting and detaching position in the horizontal direction. The cap unit  50  is positioned at the retracting position when the recording portion  20  performs a recording operation. 
     The cleaning portion  25  can be disposed below the cap unit  50  and above the conveyor unit  30  (between the cap unit  50  and the conveyor unit  30 ). The cleaning portion  25  can be lifted or lowered in conjunction with the conveyor unit  30  being lifted or lowered by the lifting and lowering device  40  in a state where the cleaning portion  25  is positioned between the cap unit  50  and the conveyor unit  30 , similarly to the cap unit  50 . 
     Further, the cleaning portion  25  can be moved horizontally in the sheet conveying direction P (see  FIG. 1 ) by the second horizontal moving mechanism (not shown). The cleaning portions  25  are moved by the second horizontal moving mechanism to wiping positions, below the line heads  22 , at which the cleaning portions  25  are allowed to clean the line heads  22 , respectively, or moved to retracting positions that are distant from the wiping positions in the horizontal direction. When the recording portion  20  performs a recording operation or the cap unit  50  is mounted to the nozzle surface  221  (see  FIG. 3 ) of the line heads  22 , the cleaning portions  25  are positioned at the retracting positions. 
     Subsequently, with reference to  FIG. 3  to  FIG. 5 , the line heads  22 K to  22 Y will be described in detail. As shown in  FIG. 3 , the sheet feed cassette  3  in which the recording papers T are stored is disposed in the right side portion of the inkjet recording apparatus  1 . The sheet feed roller  4  is disposed at one end portion of the sheet feed cassette. The sheet feed roller  4  operates to convey the stored recording papers T one by one sequentially from the recording paper T at the uppermost position, to the conveyor belt  31  described below. 
     The conveyor belt  31  that conveys the recording paper T is rotatably disposed downstream (the left side in  FIG. 3 ) of the sheet feed roller  4  in the sheet conveying direction. The conveyor belt  31  is extended on the belt drive roller  32  that is disposed on the downstream side in the sheet conveying direction P and driven so as to rotate, and the follower roller  33  that is disposed on the upstream side in the sheet conveying direction P and rotates so as to follow the belt drive roller  32  by means of the conveyor belt  31 . The conveyor belt  31  conveys the recording paper T in the sheet conveying direction by the belt drive roller  32  being driven to rotate counterclockwise. The recording paper T is preferably conveyed at, for example, 50 m/min., or faster. However, the present disclosure is not limited thereto. 
     Since the belt drive roller  32  is disposed on the downstream side in the sheet conveying direction P, the sheet conveying side portion (the upper side portion in  FIG. 1 ) of the conveyor belt  31  is drawn by the belt drive roller  32 . Therefore, the conveyor belt  31  is drawn at a predetermined belt tension, whereby the recording paper T can be stably conveyed. For the conveyor belt  31 , a dielectric resin sheet is used. As the conveyor belt  31 , for example, a (seamless) belt having no joints is particularly advantageously used. 
     Further, the pair of sheet discharge rollers  8  is disposed downstream of the conveyor belt  31  in the sheet conveying direction P. The pair of sheet discharge rollers  8  is driven to rotate counterclockwise in  FIG. 3 , and discharges the recording paper T on which an image is recorded, externally from the main body of the apparatus. The sheet discharge tray  10  on which the recording papers T discharged externally from the main body of the apparatus are stacked, is disposed downstream of the pair of sheet discharge rollers  8 . 
     Further, the line heads  22 K,  22 C,  22 M, and  22 Y are disposed above the conveyor belt  31 . The line heads  22 K,  22 C,  22 M, and  22 Y are supported at such a height that the line heads  22 K,  22 C,  22 M, and  22 Y are each spaced from the upper surface of the conveyor belt  31  by a predetermined distance, and an image is recorded, by the line heads  22 K,  22 C,  22 M, and  22 Y, on the recording paper T conveyed on the conveyor belt  31 . In the inkjet recording apparatus  1 , a color image is formed on the recording paper T by inks corresponding to the colors being ejected from the line heads  22 K to  22 Y, respectively. 
     As shown in  FIG. 3  and  FIG. 4 , each of the line heads  22 K to  22 Y includes nozzle arrays each having a plurality of nozzles aligned in a direction (the up-down direction in  FIG. 4 ) orthogonal to the conveying direction. The line heads  22 K to  22 Y each have a recording region having a dimension greater than or equal to a width of the recording paper T to be conveyed, and are able to record, at one time, one line of an image on the recording paper T conveyed on the conveyor belt  31 . 
     In the present embodiment, the inkjet recording apparatus uses a line head type recording mode in which a line head is structured so as to include a recording region that has a dimension greater than or equal to a width of the recording paper T, by a plurality of nozzles being aligned in a longitudinal direction of a head main body having a longer dimension greater than or equal to a width dimension of the conveyor belt  31 . However, the present disclosure is not limited thereto. The inkjet recording apparatus may use a line head type recording mode in which, for example, a line head is used in which a plurality of head units each having a plurality of nozzles are aligned, so as to have a shorter dimension, in a width direction of the conveyor belt  31 , thereby recording an image over the entirety of a region, in the width direction, of the conveyed recording paper T. 
     Further, in the present embodiment, a mode in which inks are ejected from the line heads  22 K to  22 Y is a piezoelectric element mode in which piezoelectric elements described below are used to eject ink droplets by utilizing pressure generated in pressurizing chambers  76  for the line heads  22 K to  22 Y. In the piezoelectric element mode, ink droplets are ejected by utilizing pressure generated in each pressurizing chamber  76  through voltage control, thereby facilitating control of an ejection amount. Further, in the piezoelectric element mode, by voltage control, pressure generated in each pressurizing chamber  76  is adjusted to vibrate a meniscus of the ink, and further the magnitude and cycle of the vibration can be adjusted. 
     Subsequently, as shown in  FIG. 5 , the line heads  22 K to  22 Y using the piezoelectric element mode each includes: a water-repellent film  73   a  that covers a portion other than an ink outlet  75  of an ejection surface  73 ; the pressurizing chamber  76  provided for each ink outlet  75 ; an ink tank (not shown) in which ink is stored; and a common flow path  77  through which ink is supplied from the ink tank to the plurality of pressurizing chambers  76 . Each pressurizing chamber  76  and the common flow path  77  communicate with each other through a supply opening  78 . Ink is supplied from the common flow path  77  through the supply opening  78  to each pressurizing chamber  76 . 
     A nozzle  74  and the pressurizing chamber  76  connect to each other through a nozzle flow path  76   a . A plurality of the nozzles  74  (see  FIG. 4 ) are formed in the line head  22 . Ink droplets can be ejected from each of the plural nozzles  74  toward the recording paper T (recording medium). A meniscus M can be formed in each of the plural nozzles  74 . The plurality of the pressurizing chambers  76  are provided so as to communicate with the plurality of nozzles  74 , respectively, and an ink W is charged into each pressuring chamber  76 . 
     A wall of each pressurizing chamber  76  on a side opposite to the ejection surface  73  side is formed as a vibration plate  79 . The vibration plate  79  is continuously formed over a plurality of the pressurizing chambers  76 . Over a surface of the vibration plate  79  on a side opposite to the pressurizing chamber  76  side, a common electrode  80  is layered. The common electrode  80  is continuously disposed over regions corresponding to the plurality of the pressurizing chambers  76 . 
     On a surface of the common electrode  80  on a side opposite to the vibration plate  79  side, a plurality of piezoelectric elements  72  are disposed. The plurality of piezoelectric elements  72  are disposed on the surface of the common electrode  80  on the side opposite to the vibration plate  79  side so as to correspond to the pressurizing chambers  76 , respectively. On surfaces of the plurality of piezoelectric elements  72  on a side opposite to the common electrode  80  side, a plurality of individual electrodes  81 , respectively, are disposed. The plural piezoelectric elements  72  are deformed due to a driving voltage applied through the individual electrodes  81 , respectively, described below by a line head control circuit  66 , and enable the ink W having been charged into the plural pressurizing chambers  76  to be ejected as ink droplets from the plural nozzles  74 , respectively. Further, each of the plural piezoelectric elements  72  is deformed due to a driving voltage applied through a corresponding one of the individual electrodes  81  by the line head control circuit  66 , and is vibrated a predetermined number of times per unit time. The meniscus M is vibrated by each of the plural piezoelectric elements  72 . 
     Each of the plural individual electrodes  81  is disposed so as to sandwich a corresponding one of the plural piezoelectric elements  72  between the individual electrode  81  and the common electrode  80 . The plurality of individual electrodes  81  are disposed so as to correspond to the pressurizing chambers  76 , respectively. Each individual electrode  81  outputs (applies), to a corresponding one of the piezoelectric elements  72 , an ejection driving voltage and a vibration driving voltage outputted from the line head control circuit  66 . Each individual electrode  81  outputs, to a corresponding one of the piezoelectric elements  72 , the ejection driving voltage outputted from the line head control circuit  66  in a printing period, and outputs, to the corresponding one of the piezoelectric elements  72 , the vibration driving voltage outputted from the line head control circuit  66  in a non-printing period. 
     As described above, when the vibration driving voltage is applied through the plurality of individual electrodes  81  by the line head control circuit  66 , each of the plural piezoelectric elements  72  vibrates a predetermined number of times per unit time for a predetermined vibration period. In other words, each of the plural piezoelectric elements  72  vibrates a predetermined number of times. Thus, the meniscus M formed in each of the plural nozzles  74  vibrates. 
     Subsequently, with reference to  FIG. 6 , vibration of the meniscus in a meniscus vibration operation will be described. Firstly, in a state where the ink W has been charged into the pressurizing chambers  76 , the meniscus M is formed in each of the plural nozzles  74 . The meniscus M near the ink outlet  75  in a stationary state as shown in (a) of  FIG. 6 , is drawn into the pressurizing chamber  76  as shown in (b) of  FIG. 6 , when a vibration driving voltage is applied to the piezoelectric element  72 . Subsequently, in a state where no vibration driving voltage is applied, the meniscus M expands outward of the pressurizing chamber  76  as shown in (c) of  FIG. 6 , in reaction to the meniscus having been drawn into the pressurizing chamber  76 . The meniscus M repeatedly vibrates (oscillates) plural times with an amplitude being gradually reduced, and converges to a stationary state as shown in (a) of  FIG. 6 . Thus, a meniscus vibration including a series of operations as shown in (a) to (c) of  FIG. 6  occurs in the meniscus M by a driving voltage being applied once to the piezoelectric element  72 . 
     A vibration manner for the meniscus M is determined according to vibration conditions (the number of times the piezoelectric element is vibrated, the number of vibrations per unit time, vibration period) in the piezoelectric element  72 . The number of times the piezoelectric element  72  is vibrated is determined by a control portion  60 . The control portion  60  changes the number of times the piezoelectric element  72  is vibrated. The control portion  60  changes the number of times the piezoelectric element  72  is vibrated, by changing the vibration period, the number of vibrations per unit time, and the like. For example, when the number of vibrations per unit time is constant, the control portion  60  increases the vibration period, thereby increasing the number of times the piezoelectric element  72  is vibrated. 
     Specifically, the control portion  60  determines the number of times the piezoelectric element  72  is vibrated, according to a temperature obtained by a temperature sensor  70 . The control portion  60  outputs, to the line head control circuit  66 , a vibration driving signal that includes information about the number of times the piezoelectric element  72  is to be vibrated. When receiving the vibration driving signal, the line head control circuit  66  applies a vibration driving voltage to the piezoelectric element  72  so as to vibrate the piezoelectric element  72  the number of times indicated in the vibration driving signal. Thus, the meniscus M vibrates according to the vibration of the piezoelectric element  72 . For example, the number of times the piezoelectric element  72  is vibrated is set so as to be less than or equal to 2000, and is preferably set so as to be greater than or equal to 400, and not greater than 1500. 
     The control portion  60  causes the line head control circuit  66  (application portion) to output a vibration driving voltage by which the piezoelectric element  72  is vibrated a predetermined number of times so as to vibrate the meniscus M in a non-printing period in a printing state in the inkjet recording apparatus  1 . The printing state represents a state where a printing job is being executed by the inkjet recording apparatus  1 . The printing state includes a printing period during which characters and/or images are formed (inks are ejected from the nozzles), and a non-printing period between the printing period and the immediately following printing period. 
     The control portion  60  causes the line head control circuit  66  (application portion) to output a vibration driving voltage by which the piezoelectric element  72  is vibrated a predetermined number of times so as to vibrate the meniscus M, in the entirety or a portion of the printing period in the printing state. The control portion  60  may cause the line head control circuit  66  (application portion) to output the vibration driving voltage as described above in a non-printing period in the printing state, such as in a period when the nozzle  74  opposes a region between the papers T being conveyed. The control portion  60  may cause the line head control circuit  66  (application portion) to output the vibration driving voltage so as to vibrate the meniscus M in a so-called paper interval. 
     In this case, for example, in a case where the inkjet recording apparatus  1  performs printing at a rate of 150 pieces/minute, the control portion  60  may cause the line head control portion  66  (application portion) to output the vibration driving voltage so as to vibrate the meniscus M during only a predetermined time period (for example, 0.1 seconds) in a period (for example, 0.15 seconds) from a time when a rear end of the paper T is positioned so as to oppose a predetermined one of the nozzles  74 , to a time when a leading end of the immediately following paper T is positioned so as to oppose the nozzle  74 . 
     The control portion  60  may cause the line head control circuit  66  (application portion) to output the vibration driving voltage so as to vibrate the meniscus M in each and every paper interval. Further, the control portion  60  may cause the line head control circuit  66  (application portion) to output the vibration driving voltage so as to vibrate the meniscus M every predetermined number of paper intervals, instead of in each and every paper interval. 
     Subsequently, with reference to  FIG. 7  and  FIG. 8 , a configuration of the inkjet recording apparatus of the first embodiment will be described. The inkjet recording apparatus  1  includes an interface  59 , the control portion  60 , a ROM  62 , a RAM  63 , an encoder  64 , a motor control circuit  65 , the line head control circuit  66 , a voltage control circuit  67 , a humidity sensor  69 , and the temperature sensor  70 . 
     The interface  59  performs data transmission and reception with, for example, a not-illustrated host device such as a personal computer. 
     The control portion  60  subjects an image signal received through the interface  59  to a scaling process or a gradation process as appropriate, to convert the image signal to image data. The control portion  60  outputs control signals to various control circuits described below. 
     Further, the control portion  60  can output an ejection driving signal and a vibration driving signal to the line head control circuit  66  (application portion). The control portion  60  causes the line head control circuit  66  to output an ejection driving voltage by which the piezoelectric element  72  is deformed so as to eject droplets from the nozzle  74  in the printing period, and causes the line head control circuit  66  to output a vibration driving voltage by which the piezoelectric element  72  is vibrated a predetermined number of times so as to vibrate the meniscus M in the nozzle  74  in the non-printing period. 
     The control portion  60  outputs, to the line head control circuit  66 , a vibration driving signal containing information about the number of times the piezoelectric element  72  is to be vibrated, thereby causing the line head control circuit  66  to output the ejection driving voltage or the vibration driving voltage. 
     Further, the control portion  60  controls, via the line head control circuit  66 , ejection of the ink W from each nozzle  74  and vibration of the meniscus M in the ink W. 
     In the present embodiment, the control portion  60  changes the number of times the piezoelectric element  72  is vibrated by changing a vibration period during which the piezoelectric element  72  is vibrated, and causes the line head control circuit  66  (application portion) to output a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of times having been changed. The control portion  60  outputs, to the line head control circuit  66 , a vibration driving signal containing information about the number of times the piezoelectric element  72  is to be vibrated per unit time, and a vibration period, thereby causing the line head control circuit  66  to output the vibration driving voltage. The control portion  60  outputs, to the line head control circuit  66 , the vibration driving signal containing information about the vibration period having been changed. Thus, the line head control circuit  66  outputs a vibration driving voltage by which the piezoelectric element  72  is vibrated for the vibration period having been changed. 
     The control portion  60  changes the number of times the piezoelectric element  72  is vibrated in the non-printing period in the printing state, based on a temperature (ambient conditions) obtained by the temperature sensor  70  (ambient condition obtaining portion), and causes the line head control circuit  66  to output a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of times having been changed. 
     Namely, the control portion  60  determines (changes) the number of times the piezoelectric element  72  is vibrated, according to a temperature obtained by the temperature sensor  70 . The control portion  60  outputs, to the line head control circuit  66 , a vibration driving signal (for example, pulse signal) containing information about the number of times the piezoelectric element  72  is to be vibrated. The line head control circuit  66  outputs, to the piezoelectric element  72 , a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of times having been determined (changed). 
     In other words, the control portion  60  determines (changes) a vibration period during which the piezoelectric element  72  is vibrated, according to a temperature obtained by the temperature sensor  70 . The control portion  60  outputs, to the line head control circuit  66 , the vibration driving signal containing information about the number of times the piezoelectric element  72  is vibrated per unit time, and the vibration period. The line head control circuit  66  outputs, to the piezoelectric element  72 , a vibration driving voltage by which the piezoelectric element  72  is vibrated the predetermined number of times (which is constant in the present embodiment) per unit time, during the vibration period having been determined (changed). 
     The control portion  60  determines (changes) the number of times the piezoelectric element  72  is vibrated, with reference to a table  500  (see  FIG. 8 ) stored in the ROM  62  described below, based on the temperature obtained by the temperature sensor  70 . 
     In the present embodiment, when the temperature is higher than or equal to T1 and less than T2, the control portion  60  causes the line head control circuit  66  to output a vibration driving voltage by which the piezoelectric element  72  is vibrated S1 (times). Further, when the temperature is higher than or equal to T2 and less than T3, the control portion  60  causes the line head control circuit  66  to output a vibration driving voltage by which the piezoelectric element  72  is vibrated S2 (times). When the temperature is higher than or equal to T3, the control portion  60  causes the line head control circuit  66  to output a vibration driving voltage by which the piezoelectric element  72  is vibrated S3 (times). Each of S1, S2, and S3 represents the number of times the piezoelectric element  72  is vibrated in each non-printing period (for example, each paper interval). 
     Further, the control portion  60  executes a first mode and a second mode. In the first mode, the number of times the piezoelectric element  72  is vibrated in the non-printing period is changed based on a humidity condition (ambient conditions) obtained by the humidity sensor  69  (ambient condition obtaining portion), and the line head control circuit  66  is not caused to output a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of times having been changed. In the second mode, the number of times the piezoelectric element  72  is vibrated in the non-printing period is changed based on a temperature condition (ambient conditions) obtained by the temperature sensor  70  (ambient condition obtaining portion), and the line head control circuit  66  is caused to output a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of times having been changed. The control portion  60  can switch between the first mode and the second mode, based on the humidity condition obtained by the humidity sensor  69  (ambient condition obtaining portion). 
     Namely, when a humidity obtained by the humidity sensor  69  is higher than a predetermined threshold value, the control portion  60  switches to the first mode. In the first mode, the control portion  60  changes the number of times the piezoelectric element  72  is vibrated in the non-printing period, and does not cause the line head control circuit  66  to output a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of times having been changed. In the first mode, the control portion  60  does not change the number of times the piezoelectric element  72  is vibrated in the non-printing period, or does not cause the line head control circuit  66  to output a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of time having been changed. 
     On the other hand, when a humidity obtained by the humidity sensor  69  is less than or equal to the predetermined threshold value, the control portion  60  switches to the second mode. In the second mode, the control portion  60  changes the number of times the piezoelectric element  72  is vibrated in the non-printing period, and causes the line head control circuit  66  to output a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of times having been changed. 
     In the ROM  62 , a control program and the like according to which the line heads  22 K to  22 Y are driven to record an image, are stored. In the RAM  63 , data of images having been subjected to a scaling process or a gradation process by the control portion  60  is stored in a predetermined region. 
     Further, as shown in  FIG. 8 , in the ROM  62 , the table  500  representing a relationship between temperatures and the number of times the piezoelectric element  72  is to be vibrated (the number of times flushing is performed), is stored. In the table  500  stored in the ROM  62 , temperature ranges and the number of times the piezoelectric element  72  is vibrated (the number of times flushing is performed) are stored so as to associate each temperature range with the corresponding number of times the piezoelectric element  72  is vibrated. In the present embodiment, in the table  500 , information indicating that the number of times of the vibrations is S1 (times) in the case of a low temperature (higher than or equal to T1 and less than T2), the number of times of the vibrations is S2 (times) in the case of an intermediate temperature (higher than or equal to T2 and less than T3), and the number of times of the vibrations is S3 (times) in the case of a high temperature (higher than or equal to T3), is stored. 
     The temperature satisfies 0° C.&lt;T1° C.&lt;T2° C.&lt;T3° C. Further, the number of times of vibrations satisfies SX&gt;S1&gt;S2&gt;S3&gt;0. SX represents the number of times by which ejection of the ink W from the nozzle  74  does not occur. 
     Further, the control portion  60  described above may set, for example, S2 as a standard value of the number of times of vibrations in the non-printing period. In this case, when a temperature obtained by the temperature sensor  70  is low or high, the control portion  60  changes the number of times the piezoelectric element  72  is vibrated and causes the line head control circuit  66  to output a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of times having been changed. 
     The encoder  64  is connected to the belt drive roller  32  which is disposed on the sheet discharge side and drives the conveyor belt  31 , and outputs a train of pulses according to rotational displacement of a rotation shaft of the belt drive roller  32 . The control portion  60  counts the pulses transmitted from the encoder  64  to calculate the number of rotations, thereby obtaining a recording paper feed rate (sheet position). The control portion  60  outputs control signals to the motor control circuit  65  and the line head control circuit  66  based on a signal from the encoder  64 . 
     The motor control circuit  65  drives a recording paper conveyance motor  68  according to an output signal from the control portion  60 . The motor control circuit  65  drives the recording paper conveyance motor  68  to rotate the belt drive roller  32 , thereby rotating the conveyor belt  31  counterclockwise in  FIG. 1  and  FIG. 3 . Thus, the recording paper is conveyed in the sheet conveying direction. 
     The line head control circuit  66  (application portion) transfers image data stored in the RAM  63  to the line heads  22 K to  22 Y according to a control signal from the control portion  60 , and controls ejection of the inks from the line heads  22 K to  22 Y based on the transferred image data. Through this control and a control of conveying the recording paper by the conveyor belt  31  driven by the recording paper conveyance motor  68 , a process of recording on the recording paper is performed. 
     Further, the line head control circuit  66  outputs a vibration driving voltage to the piezoelectric elements  72 , based on a vibration driving signal from the control portion  60 . The line head control circuit  66  vibrates the piezoelectric element  72  to control meniscus vibration in the meniscus M of each of the line heads  22 K to  22 Y. 
     The line head control circuit  66  applies a vibration driving voltage to the piezoelectric element  72 , based on a vibration driving signal received from the control portion  60 . The line head control circuit  66  applies, to the piezoelectric element  72 , a vibration driving voltage by which the piezoelectric element  72  is vibrated a predetermined number of times, based on information about the number of times of vibrations which is contained in the vibration driving signal having been received. 
     Further, when receiving, from the control portion  60 , a vibration driving signal representing the number of times of vibrations having been changed, the line head control circuit  66  outputs, to the piezoelectric element  72 , a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of times having been changed. 
     The voltage control circuit  67  generates an alternating electric field by applying a voltage to the follower roller  33  on the sheet feed side, based on an output signal from the control portion  60 , thereby electrostatically attaching the recording paper to the conveyor belt  31 . The electrostatic attaching can be cancelled by the follower roller  33  or the belt drive roller  32  being grounded based on an output signal from the control portion  60 . In the description herein, a voltage is applied to the follower roller  33  on the sheet feed side. However, a voltage may be applied to the belt drive roller  32  on the sheet discharge side. 
     The humidity sensor  69  (ambient condition obtaining portion) obtains a humidity condition (humidity information). The humidity sensor  69  is disposed, for example, in the vicinity of the recording portion  20 . The humidity sensor  69  measures an ambient humidity near the recording portion  20 . The temperature sensor  70  (ambient condition obtaining portion) obtains a temperature condition (temperature information). The temperature sensor  70  is disposed in the vicinity of the recording portion  20 , and measures an ambient temperature near the recording portion  20 . In the present embodiment, the humidity sensor  69  and the temperature sensor  70  are allowed to obtain ambient conditions (humidity, temperature) for the vicinity of the line head  22 . The humidity sensor  69  and the temperature sensor  70  output, to the control portion  60 , the humidity information and the temperature information having been obtained. 
     Subsequently, with reference to  FIG. 9 , a dotting operation performed by the inkjet recording apparatus  1  will be described. Among the line heads  22 K to  22 Y, the line head  22 Y will be described as an example. The description for the other line heads  22 K to  22 M is exactly the same as that for the line head  22 Y. 
     As shown in  FIG. 9 , in the line head  22 Y, nozzle arrays N1 and N2 each including a plurality of nozzles are aligned in the sheet conveying direction P. Namely, the line head  22 Y includes, as a nozzle for forming one dot sequence in the sheet conveying direction P, one nozzle in each of the nozzle arrays N1 and N2 (for example, nozzles  74   a  and  74   a ′ for a dot sequence L1), that is, two nozzles in total. For the convenience of description, among nozzles for forming the nozzle arrays N1 and N2, nozzles  74   a  to  74   p  and  74   a ′ to  74   p ′ corresponding to the dot sequences L1 to L16, that is, 16 nozzles in each nozzle array, are illustrated. However, in practice, the number of nozzles aligned in a direction orthogonal to the sheet conveying direction P is greater than the number described above. 
     In the inkjet recording apparatus  1 , the nozzle arrays N1 and N2 are sequentially used to form an image on a recording medium. For example, while the recording medium is conveyed in the sheet conveying direction P, a dot sequence D1 corresponding to one line, in the width direction (the right-left direction in the drawings), on the recording medium is formed by ink ejection (solid arrow in the drawings) from the nozzle array N1, and a dot sequence D2 corresponding to the immediately following one line is then formed by ink ejection (dashed arrow in the drawings) from the nozzle array N2, and a dot sequence D3 corresponding to one line immediately following the dot sequence D2 is then formed again by ink ejection from the nozzle array N1. Thereafter, the nozzle arrays N1 and N2 are alternately used also for and after a dot sequence D4, to similarly form dot sequences. 
     Subsequently, with reference to  FIG. 10 , a meniscus vibration operation in the ink W performed by the inkjet recording apparatus  1  of the first embodiment will be described. 
     Firstly, in step ST 101 , the inkjet recording apparatus  1  (the control portion  60 ) vibrates the piezoelectric element  72  the number of times which is set for a normal vibration operation. The inkjet recording apparatus  1  (the control portion  60 ) vibrates the piezoelectric element  72  the number of times per unit time, which is set for the normal vibration operation, for a predetermined vibration period which is set for the normal vibration operation. 
     Subsequently, in step ST 102 , the inkjet recording apparatus  1  (the humidity sensor  69 ) measures a humidity. 
     Subsequently, in step ST 103 , the inkjet recording apparatus  1  (the control portion  60 ) determines whether or not a measured humidity A (%) is less than or equal to a predetermined humidity B (%). When the inkjet recording apparatus  1  (the control portion  60 ) determines that the humidity A (%) is less than or equal to the predetermined humidity B (%) (step ST 103 , YES), the process is advanced to step ST 104 . On the other hand, when the inkjet recording apparatus  1  (the control portion  60 ) determines that the humidity A (%) is higher than the predetermined humidity B (%) (step ST 103 , NO), the process is advanced to step ST 105 . 
     Subsequently, in step ST 104 , the inkjet recording apparatus  1  (the control portion  60 ) switches a mode for the control portion  60  to the second mode. In the inkjet recording apparatus  1 , the process is advanced to step ST 106 . 
     On the other hand, in step ST 105 , the inkjet recording apparatus  1  (the control portion  60 ) switches a mode for the control portion  60  to the first mode. In the inkjet recording apparatus  1 , the process is advanced so as to be prior to step ST 110 . 
     Subsequently, in step ST 106 , the inkjet recording apparatus  1  (the temperature sensor  70 ) measures a temperature. 
     Subsequently, in step ST 107 , the inkjet recording apparatus  1  (the control portion  60 ) sets (changes) the number of times the piezoelectric element  72  is vibrated, based on the measured temperature. For example, the inkjet recording apparatus  1  (the control portion  60 ) sets (changes), with reference to the table  500 , the number of times the piezoelectric element  72  is vibrated. 
     Subsequently, in step ST 108 , the inkjet recording apparatus  1  (the control portion  60 ) outputs, to the line head control circuit  66 , a vibration driving signal containing information about the number of times of vibrations having been set (changed). 
     Subsequently, in step ST 109 , the inkjet recording apparatus  1  (the line head control circuit  66 ) outputs, to the piezoelectric element  72 , a vibration driving voltage so as to vibrate the piezoelectric element  72  the number of times that has been set (changed) based on the information contained in the vibration driving signal. 
     Subsequently, in step ST 110 , the inkjet recording apparatus  1  (the control portion  60 ) determines whether or not a printing instruction has been issued. When no printing instruction is issued (step ST 110 , NO), the process is returned so as to be prior to step ST 102  by the inkjet recording apparatus  1  (the control portion  60 ). When a printing instruction is issued (step ST 110 , YES), the inkjet recording apparatus  1  (the control portion  60 ) ends the vibration operation. 
     Thus, the inkjet recording apparatus  1  of the present embodiment can suppress reduction in ejection of ink droplets from nozzles. 
     Further, the inkjet recording apparatus  1  reduces vibration of the meniscus under a high temperature environment, and increases vibration of the meniscus under a low temperature environment. Thus, the inkjet recording apparatus  1  can suppress entry of air into nozzles due to vibration under a high temperature environment, thereby suppressing reduction in ejection from the nozzles  74 . 
     Further, the inkjet recording apparatus  1  changes the number of times the piezoelectric element is vibrated, based on a temperature that greatly affects ejection of ink. Thus, reduction in ejection from the nozzles can be more advantageously suppressed. 
     Further, according to the present embodiment, the control portion  60  causes the line head control circuit  66  to output a vibration driving voltage by which the number of times the piezoelectric element  72  is vibrated in the non-printing period is less than or equal to 2000. Thus, the inkjet recording apparatus  1  is allowed to more advantageously suppress reduction in ejection from the nozzles. 
     Further, according to the present embodiment, the control portion  60  can switch between the first mode and the second mode. 
     Thus, the inkjet recording apparatus  1  can be configured such that, for example, the number of times the piezoelectric element is vibrated is not changed under a high humidity environment, and the number of times the piezoelectric element is vibrated can be changed according to a temperature under a low humidity environment. Thus, in the inkjet recording apparatus  1 , the piezoelectric element is vibrated the standard number of times under a high humidity environment where ejection from the nozzles is less likely to be reduced, whereas the number of times the piezoelectric element is vibrated is changed according to a temperature in order to minutely address a situation under a low humidity environment where ejection from the nozzles is likely to be reduced. Thus, in the inkjet recording apparatus  1 , reduction in ejection from the nozzles can be more advantageously suppressed. 
     Subsequently, with reference to  FIG. 11  to  FIG. 15 , an inkjet recording apparatus according to a second embodiment will be described. 
     Hereinafter, difference from the inkjet recording apparatus of the first embodiment will be mainly described. Description of components common to those of the first embodiment is not given. For components that are not described in the second embodiment, description for the first embodiment is to be referred to. 
     An inkjet recording apparatus  1 A of the second embodiment is different from the inkjet recording apparatus  1  of the first embodiment in that, in the inkjet recording apparatus  1 A, the number of times the piezoelectric element  72  is vibrated is changed based on a zero shear viscosity in ink. 
     In non-Newtonian fluid such as ink, since a relationship between shear rates and viscosities is not a proportional relationship, it is difficult to make a comparison in physical properties of ink based on values of shear viscosities. Therefore, in the present embodiment, in the inkjet recording apparatus  1 A, the number of times the piezoelectric element  72  is vibrated (the number of times flushing is performed) is changed based on a zero shear viscosity. 
     A zero shear viscosity of ink cannot be directly measured. For example, a zero shear viscosity of ink is measured by using a rolling ball type viscometer. Specifically, a tilt angle θ (see, for example,  FIG. 13 ) is changed at predetermined angular intervals to make a measurement, and a viscosity at a predetermined angle is measured. When the horizontal axis represents values of sin θ and the vertical axis represents shear viscosities, a value at sin θ=0 (θ=0) in a sin θ-viscosity curve obtained by plotting the values of sin θ and the shear viscosities against each other is calculated as the zero shear viscosity. 
       FIG. 13  shows a relationship at 15° C. between shear viscosities and values of sin θ. Zero shear viscosities at various temperatures are obtained in the same manner as described above to obtain a graph indicated in  FIG. 14 . A relationship between temperatures and zero shear viscosities as shown in  FIG. 12  is obtained based on the graph indicated in  FIG. 14 . 
     As shown in  FIG. 11 , the inkjet recording apparatus  1 A includes a zero shear viscosity calculating/obtaining portion  71  in addition to the components of the inkjet recording apparatus  1  of the first embodiment. Based on a temperature measured by the temperature sensor  70 , the zero shear viscosity calculating/obtaining portion  71  calculates a zero shear viscosity by using, for example, a predetermined expression, a graph, or a program, or obtains a zero shear viscosity from, for example, a table  500 A stored in the ROM  62 . 
     When calculating the zero shear viscosity, the zero shear viscosity calculating/obtaining portion  71  calculates the zero shear viscosity at a predetermined temperature according to, for example, the graph indicated in  FIG. 14 . Further, as described above, the zero shear viscosity calculating/obtaining portion  71  may calculate the zero shear viscosity at a predetermined temperature according to a predetermined expression. Furthermore, the zero shear viscosity calculating/obtaining portion  71  may calculate the zero shear viscosity at a predetermined temperature by using a predetermined program. 
     Further, when obtaining the zero shear viscosity, the zero shear viscosity calculating/obtaining portion  71  obtains the zero shear viscosity at a predetermined temperature according to, for example, the table  500 A indicated in  FIG. 12 . The ROM  62  is a storage portion for storing zero shear viscosities of the ink W ejected from the nozzles  74 , and storing the table  500 A in which predetermined temperatures and zero shear viscosities are stored so as to associate each temperature with the corresponding zero shear viscosity. 
     Further, in the present embodiment, the control portion  60  changes the number of times the piezoelectric element  72  is vibrated in the non-printing period, based on the zero shear viscosity calculated or obtained by the zero shear viscosity calculating/obtaining portion  71 , and causes the line head control circuit  66  to output a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of times having been changed. The control portion  60  changes the number of times the piezoelectric element  72  is vibrated in the non-printing period, based on not a temperature but the zero shear viscosity calculated or obtained according to a temperature. The control portion  60  causes the line head control circuit  66  to output a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of times having been changed. 
     Further, in the present embodiment, the control portion  60  causes the line head control circuit  66  to output a vibration driving voltage by which the number of times the piezoelectric element  72  is vibrated in the non-printing period is as follows. That is, the number of times the piezoelectric element  72  is vibrated in the non-printing period is less than or equal to 500, and preferably greater than or equal to 400 and not greater than 500, in the case of the zero shear viscosity being less than or equal to 5.0 mPa·s. Furthermore, the number of times the piezoelectric element  72  is vibrated in the non-printing period ranges from 500 to 1000 in the case of the zero shear viscosity being higher than 5.0 mPa·s and less than 9.0 mPa·s. Moreover, the number of times the piezoelectric element  72  is vibrated in the non-printing period is greater than or equal to 1000, and preferably greater than or equal to 1000 and not greater than 1200 in the case of the zero shear viscosity being higher than or equal to 9.0 mPa·s. The control portion  60  vibrates the piezoelectric element  72  under advantageous conditions according to the zero shear viscosity. 
     Further, as in the first embodiment, the control portion  60  executes a first mode and a second mode, and can change (shift) a mode to one of the first mode and the second mode. Specifically, in the first mode, the control portion  60  changes the number of times the piezoelectric element  72  is vibrated in the non-printing period, based on a humidity measured by the humidity sensor  69 , and the control portion  60  does not cause the line head control circuit  66  to output a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of times having been changed. In the second mode, the control portion  60  changes the number of times the piezoelectric element  72  is vibrated in the non-printing period, based on a humidity measured by the humidity sensor  69 , and the control portion  60  causes the line head control circuit  66  to output a vibration driving voltage by which the piezoelectric element  72  is vibrated the number of times having been changed. The control portion  60  can change a mode to one of the first mode and the second mode, based on a humidity measured by the humidity sensor  69 . 
     Subsequently, with reference to  FIG. 15 , a meniscus vibration operation in the ink W performed by the inkjet recording apparatus  1 A of the second embodiment will be described. 
     Firstly, in step ST 201 , the inkjet recording apparatus  1 A (the control portion  60 ) vibrates the piezoelectric element  72  the number of times which is set for a normal vibration operation. The inkjet recording apparatus  1 A (the control portion  60 ) vibrates the piezoelectric element  72  the number of times per unit time, which is set for the normal vibration operation, for a predetermined vibration period which is set for the normal vibration operation. 
     Subsequently, in step ST 202 , the inkjet recording apparatus  1 A (the humidity sensor  69 ) measures a humidity. 
     Subsequently, in step ST 203 , the inkjet recording apparatus  1 A (the control portion  60 ) determines whether or not a measured humidity A (%) is less than or equal to a predetermined humidity B (%). When the inkjet recording apparatus  1 A (the control portion  60 ) determines that the humidity A (%) is less than or equal to the predetermined humidity B (%) (step ST 203 , YES), the process is advanced to step ST 204 . On the other hand, when the inkjet recording apparatus  1 A (the control portion  60 ) determines that the humidity A (%) is higher than the predetermined humidity B (%) (step ST 203 , NO), the process is advanced to step ST 205 . 
     Subsequently, in step ST 204 , the inkjet recording apparatus  1 A (the control portion  60 ) switches a mode for the control portion  60  to the second mode. In the inkjet recording apparatus  1 A, the process is advanced to step ST 206 . 
     On the other hand, in step ST 205 , the inkjet recording apparatus  1 A (the control portion  60 ) switches a mode for the control portion  60  to the first mode. In the inkjet recording apparatus  1 A, the process is advanced so as to be prior to step ST 211 . 
     Subsequently, in step ST 206 , the inkjet recording apparatus  1 A (the temperature sensor  70 ) measures a temperature. 
     Subsequently, in step ST 207 , the inkjet recording apparatus  1 A (the zero shear viscosity calculating/obtaining portion  71 ) calculates or obtains a zero shear viscosity based on the temperature. 
     Subsequently, in step ST 208 , the inkjet recording apparatus  1 A (the control portion  60 ) sets (changes) the number of times the piezoelectric element  72  is vibrated, based on the calculated or obtained zero shear viscosity. For example, the inkjet recording apparatus  1 A (the control portion  60 ) sets (changes), with reference to the table  500 A, the number of times the piezoelectric element  72  is vibrated. 
     Subsequently, in step ST 209 , the inkjet recording apparatus  1 A (the control portion  60 ) outputs, to the line head control circuit  66 , a vibration driving signal containing information about the number of times of vibrations having been set (changed). 
     Subsequently, in step ST 210 , the inkjet recording apparatus  1 A (the line head control circuit  66 ) outputs, to the piezoelectric element  72 , a vibration driving voltage so as to vibrate the piezoelectric element  72  the number of times that has been set (changed) based on the information contained in the vibration driving signal. 
     Subsequently, in step ST 211 , the inkjet recording apparatus  1 A (the control portion  60 ) determines whether or not a printing instruction has been issued. When no printing instruction is issued (step ST 211 , NO), the process is returned so as to be prior to step ST 202  by the inkjet recording apparatus  1 A (the control portion  60 ). 
     When a printing instruction is issued (step ST 211 , YES), the inkjet recording apparatus  1 A (the control portion  60 ) ends the vibration operation. 
     Thus, the inkjet recording apparatus  1 A of the present embodiment can suppress reduction in ejection of ink droplets from nozzles. 
     Further, the inkjet recording apparatus  1 A changes the number of times the piezoelectric element is vibrated, according to a state of ink. Thus, the inkjet recording apparatus  1 A is allowed to advantageously suppress reduction in ejection from the nozzles  74 . 
     Further, the inkjet recording apparatus  1 A reduces vibration of a meniscus in ink having a low zero shear viscosity, and increases vibration of a meniscus in ink having a high zero shear viscosity. Thus, the inkjet recording apparatus  1 A suppresses reduction in ejection from the nozzles  74 . 
     Further, the inkjet recording apparatus  1 A can suppress entry of air into the nozzles  74 , which occurs due to vibration of a meniscus in ink having a low zero shear viscosity. Thus, the inkjet recording apparatus  1 A can suppress reduction in ejection from the nozzles. 
     Further, the inkjet recording apparatus  1 A changes the number of times the piezoelectric element is vibrated, based on a zero shear viscosity that greatly affects ejection of ink. Thus, reduction in ejection from the nozzles can be more advantageously suppressed. 
     Further, according to the present embodiment, the control portion  60  causes the line head control circuit  66  to output a vibration driving voltage by which the number of times the piezoelectric element  72  is vibrated is less than or equal to 500 in the case of a zero shear viscosity being less than or equal to 5.0 mPa·s, the number of times the piezoelectric element  72  is vibrated ranges from 500 to 1000 in the case of a zero shear viscosity being higher than 5.0 mPa·s and less than 9.0 mPa·s, and the number of times the piezoelectric element  72  is vibrated is greater than or equal to 1000 in the case of a zero shear viscosity being higher than or equal to 9.0 mPa·s. The control portion  60  vibrates the piezoelectric element  72  under advantageous conditions according to the zero shear viscosity. Thus, the inkjet recording apparatus  1 A can more advantageously suppress reduction in ejection from the nozzles. 
     Further, according to the present embodiment, the control portion  60  can change a mode to one of the first mode and the second mode. Thus, the inkjet recording apparatus  1 A can be configured such that, for example, the number of times the piezoelectric element is vibrated is not changed under a high humidity environment, and the number of times the piezoelectric element is vibrated can be changed according to a temperature under a low humidity environment. Thus, in the inkjet recording apparatus  1 A, the piezoelectric element is vibrated the standard number of times under a high humidity environment where ejection from the nozzles is less likely to be reduced, whereas the number of times the piezoelectric element is vibrated is changed according to a temperature in order to minutely address a situation under a low humidity environment where ejection from the nozzles is likely to be reduced. Thus, in the inkjet recording apparatus  1 A, reduction in ejection from the nozzles can be more advantageously suppressed. 
     Subsequently, the number of times the piezoelectric element is vibrated will be further described. 
     According to the above description, in the inkjet recording apparatus, it is preferable that, for example, the number of times the piezoelectric element is vibrated is less than or equal to 500 in the case of a zero shear viscosity being less than or equal to 5.0 mPa·s, the number of times the piezoelectric element is vibrated ranges from 500 to 1000 in the case of a zero shear viscosity being higher than 5.0 mPa·s and less than 9.0 mPa·s, and the number of times the piezoelectric element is vibrated is greater than or equal to 1000 in the case of a zero shear viscosity being higher than or equal to 9.0 mPa·s. The grounds for the values that are determined as above will be described below. 
     Ink for use in the inkjet recording apparatus was evaluated for variation in landing, concentration, and satellite droplets under environments where the zero shear viscosities were 4 mPa·s, 5 mPa·s, 9 mPa·s, and 10 mPa·s, and it was confirmed that the values described above were appropriate. A composition of ink and an ink production method, an evaluation method, and evaluation results are indicated below. 
     &lt;Ink&gt; 
     (Synthesizing of Acrylic Resin) 
     An acrylic resin will be described below. As a method for synthesizing an acrylic resin, a macromonomer synthesis method was used. The macromonomer synthesis method is a polymerization method executable with ease and stability. An oligomer (Mn=6,000, product name: AS-6, manufactured by Toagosei Kabushiki Kaisha) having a (meth)acryloyl group bonded to one of molecular ends of a polystyrene, was used, and other monomers were added according to a ratio for the resin. The monomers were polymerized in MEK together with a publicly known initiator such as an azo compound like 2, 2′-azobisisobutyronitrile, or 2,2′-azobis(2,4-dimethylvaleronitrile), to obtain an acrylic resin having a molecular weight of 60000. After the reaction, a solvent was distilled under a reduced pressure. Further, as the molecular weight of the obtained resin, a weight average molecular weight was confirmed by using gel filtration chromatography. The acid value was confirmed by titration. 
     (Dispersion of Pigment) 
     As a pigment, cyan pigment: P. B-15:3 was used. As a pigment dispersion used in an experiment, a particulate pigment dispersion covered with a resin having a molecular weight of tens of thousands is appropriate in the case of an image quality being required. As a material satisfying the aforementioned conditions, a styrene acrylic resin is appropriate, that is, a resin having an acid value ranging from 150 to 300, is appropriate. (When an acid value is low, dispersion of pigments is poor, it is difficult to form fine particles, and color development property and coloring property are poor. On the other hand, when an acid value is high, ink storage stability is poor). In addition, various materials were blended according to the aforementioned composition, and kneading was performed by means of a media type dispersing machine. As a dispersing machine, a wet-type dispersing machine (NANO GRAIN MILL: manufactured by Asada Tekko Kabushiki Kaisha) was used. 
     Conditions for the dispersion were as follows. That is, beads each having a small diameter (0.5 mm·1.0 mm zirconia beads) were set in a vessel, and the pigment dispersion was dispersed such that a particle size was adjusted so as to have an average particle diameter ranging from 70 nm to 130 nm. Further, a bead type was changed to change a degree of dispersion and an amount of free resin. Needless to say, when a bead diameter is reduced, fine particles are easily formed and covering of pigments with a resin is enhanced. 
     A particle size distribution was measured with a solution obtained by 300-fold dilution with ion-exchanged water, by using Zetasizer (registered trademark) nano (manufactured by SYSMEX CORPORATION) as a measurement device. 
     Blending of the pigment dispersion is as indicated in Table 1. Blending of ink is as indicated in Table 2. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 (Blending of pigment dispersion) 
               
            
           
           
               
               
               
            
               
                   
                   
                 wt % 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Water 
                 80 
               
               
                   
                 Resin 
                 5.0 
               
               
                   
                 Pigment 
                 15 
               
               
                   
                 Olefin E1010 
                 0.5 
               
               
                   
                 Total 
                 100.0 
               
               
                   
                   
               
            
           
         
       
     
     In Table 1, a ratio of an amount of the resin to an amount of the pigment may be optionally changed. The resin is soluble in water (alkali-soluble resin), and is neutralized with an equivalent of KOH. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 (Ink production method) 
               
            
           
           
               
               
               
            
               
                   
                 Material 
                 wt % 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Pigment dispersion (Pig. 15%) 
                 42 
               
               
                   
                 Olefin E1010 
                 0.5 
               
               
                   
                 Triethyleneglycolmonomethylether 
                 10.0 
               
               
                   
                 2-pyrrolidone 
                 5.0 
               
               
                   
                 1,2-octanediol 
                 0.8 
               
               
                   
                 Glycerin 
                 25 
               
               
                   
                 Ion-exchanged water 
                 Remainder 
               
               
                   
                 Total 
                 100.0 
               
               
                   
                   
               
            
           
         
       
     
     The ink was produced according to the blending indicated in Table 2. 
     &lt;Evaluation Method&gt; 
     Ink for use in the inkjet recording apparatus was evaluated for variation in landing, concentration, and satellite droplets under environments where the zero shear viscosities were 4 mPa·s, 5 mPa·s, 9 mPa·s, and 10 mPa·s. Evaluation items and evaluation criterions are as follows. 
     &lt;Evaluation Item and Evaluation Criterion&gt; 
     (Variation in Landing) 
     Inks for an image were simultaneously ejected from all of nozzles (2564 pins) of a 600 dpi head, to print the image under conditions that a speed at which a recording medium was conveyed was 846.7 mm/sec., and a droplet ejection rate was 8 m/s. The printed object was measured by the image processing system device Da-6000 (manufactured by Oji Scientific Instruments). An image was captured and the image was binarized, and image correction was performed. Thereafter, a brightness distribution per one pixel was measured by using Da-6000, averaged, and represented as the second order value. 
     (Evaluation Criterions) 
     Excellent: A value (lower limit value) less than 10 μm was obtained as variation in landing. 
     Average: A value that ranges from 10 μm to 15 μm was obtained as variation in landing. 
     Poor: A value greater than or equal to 15 μm was obtained as variation in landing. 
     (Concentration) 
     Concentrations for printing were measured by using the line head  22 C ( FIG. 2 ). For the measurement, an ink ejecting amount was controlled such that an ink impact on a recording medium was 15 g/m 2 , and a conveying speed was 846.7 mm/sec., thereby printing a solid image. As sheets, a sheet IJW (brand name) manufactured by Oji Paper Co., Ltd. was cut into A4 size sheets and used. An image size was 10 cm×10 cm. The printed image was left as it was all night and all day after the image had been printed, and concentrations for printing were then measured with GretagMacbeth at a view angle of 2° by using a D50 light source. 
     (Evaluation Criterions) 
     Excellent: A concentration for printing was higher than or equal to 0.98. 
     Poor: A concentration for printing was less than 0.98. 
     (Satellite Droplet Evaluation) 
     An image was printed on a glossy paper by using a recording head, and a printed state of dots were observed by using a microscope, and evaluated. 
     (Evaluation Criterions) 
     Excellent: No satellite droplets occurred. 
     Poor: Satellite droplets occurred. 
     &lt;Evaluation Results&gt; 
     In Table 3 to Table 6, evaluation results for variation in landing are indicated below. In Table 7 to Table 10, evaluation results for concentrations are indicated. In Table 11 to Table 14, evaluation results for satellite droplet are indicated. 
     
       
         
           
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 (Variation in landing: 4 mPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 The number of  
                 3000 
                 2500 
                 2000 
                 1500 
                 1000 
               
               
                 vibrations 
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Variation in  
                 Poor 
                 Poor 
                 Poor 
                 Poor 
                 Poor 
               
               
                 landing 
                 (26 μm) 
                 (25 μm) 
                  (22 μm) 
                  (19 μm) 
                  (17 μm) 
               
               
                 The number of  
                  800 
                  600 
                  500 
                  400 
                  300 
               
               
                 vibrations 
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Variation in 
                 Poor 
                 
                   
                 
                 Excellent  
                 Excellent 
                 Excellent 
               
               
                 landing 
                   
                 (12 μm) 
                 (9.0 μm)  
                 (7.0 μm) 
                 (6.0 μm) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 (Variation in landing: 5 mPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 The number  
                 3000 
                 2500 
                 2000 
                 1500 
                 1000 
               
               
                 of vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number  
                   
                   
                   
                   
                   
               
               
                 of times) 
                   
                   
                   
                   
                   
               
               
                 Variation in  
                 Poor 
                 Poor 
                 Poor 
                 
                   
                 
                 Ex- 
               
               
                 landing 
                 (19 μm) 
                  (18 μm) 
                  (17 μm) 
                 (13 μm) 
                 cellent 
               
               
                   
                   
                   
                   
                   
                 (9.0 μm) 
               
               
                 The number  
                   
                   
                   
                   
                   
               
               
                 of vibrations 
                  800 
                  600 
                  500 
                  400 
                  300 
               
               
                 (the number  
                   
                   
                   
                   
                   
               
               
                 of times) 
                   
                   
                   
                   
                   
               
               
                 Variation  
                 Ex- 
                 Ex- 
                 Ex-  
                 
                   
                 
                 
                   
                 
               
               
                 in landing 
                 cellent 
                 cellent  
                 cellent 
                 (11 μm) 
                  (14 μm) 
               
               
                   
                   
                 (8.0 μm) 
                 (9.0 μm) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 (Variation in landing: 9.0 mPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 The number of  
                 3000 
                 2500 
                 2000 
                 1500 
                 1000 
               
               
                 vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Variation in  
                 Excellent  
                 Excellent 
                 Excellent 
                 Excellent 
                 Excellent 
               
               
                 landing 
                 (6 μm) 
                 (6 μm) 
                 (6 μm) 
                  (7 μm) 
                  (8 μm) 
               
               
                 The number of  
                  800 
                  600 
                  500 
                  400 
                  300 
               
               
                 vibrations 
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Variation in  
                 Excellent 
                 Excellent 
                 Excellen 
                 
                   
                 
                 
                   
                 
               
               
                 landing 
                 (7 μm) 
                 (7 μm) 
                 (8 μm) 
                 (10 μm) 
                 (13 μm) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 6 
               
               
                   
               
               
                 (Variation in landing: 10 mPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 The number of  
                 3000 
                 2500 
                 2000 
                 1500 
                 1000 
               
               
                 vibrations 
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Variation in  
                 Excellent 
                 Excellent 
                 Excellent  
                 Excellent 
                 Excellent 
               
               
                 landing 
                  (6 μm) 
                  (6 μm) 
                  (6 μm) 
                  (7 μm) 
                  (9 μm) 
               
               
                 The number of  
                  800 
                  600 
                  500 
                  400 
                  300 
               
               
                 vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Variation in  
                 
                   
                 
                 
                   
                 
                 Poor 
                 Poor 
                 Poor 
               
               
                 landing 
                 (12 μm) 
                 (14 μm) 
                 (16 μm) 
                 (18 μm) 
                 (21 μm) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 7 
               
               
                   
               
               
                 (Concentration: 4 mPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 The number of  
                 3000 
                 2500 
                 2000 
                 1500 
                 1000 
               
               
                 vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Concentration 
                 Excellent 
                 Excellent 
                 Excellent  
                 Excellent 
                 Excellent 
               
               
                 The number of  
                  800 
                  600 
                  500 
                  400 
                  300 
               
               
                 vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Concentration 
                 Excellent  
                 Excellent 
                 Excellent  
                 Excellent 
                 Excellent 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 8 
               
               
                   
               
               
                 (Concentration: 5 mPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 The number of  
                 3000 
                 2500 
                 2000 
                 1500 
                 1000 
               
               
                 vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Concentration 
                 Excellent  
                 Excellent 
                 Excellent  
                 Excellent 
                 Excellent 
               
               
                 The number of  
                  800 
                  600 
                  500 
                  400 
                  300 
               
               
                 vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Concentration 
                 Excellent  
                 Excellent 
                 Excellent  
                 Excellent 
                 Excellent 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 9 
               
               
                   
               
               
                 (Concentration: 9.0 mPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 The number of  
                 3000 
                 2500 
                 2000 
                 1500 
                 1000 
               
               
                 vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Concentration 
                 Excellent  
                 Excellent 
                 Excellent  
                 Excellent 
                 Excellent 
               
               
                 The number of  
                  800 
                  600 
                  500 
                  400 
                  300 
               
               
                 vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Concentration 
                 Excellent  
                 Excellent 
                 Excellent  
                 Poor 
                 Poor 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 10 
               
               
                   
               
               
                 (Concentration: 10 mPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 The number of  
                 3000 
                 2500 
                 2000 
                 1500 
                 1000 
               
               
                 vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Concentration 
                 Excellent 
                 Excellent 
                 Excellent 
                 Excellent 
                 Excellent 
               
               
                 The number of  
                  800 
                  600 
                  500 
                  400 
                  300 
               
               
                 vibrations 
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Concentration 
                 Poor 
                 Poor 
                 Poor 
                 Poor 
                 Poor 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 11 
               
               
                   
               
               
                 (Satellite droplet evaluation: 4 mPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 The number of  
                 3000 
                 2500 
                 2000 
                 1500 
                 1000 
               
               
                 vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number of times) 
                   
                   
                   
                   
                   
               
               
                 Satellite droplet 
                 Poor 
                 Poor 
                 Poor 
                 Poor 
                 Poor 
               
               
                 evaluation 
                   
                   
                   
                   
                   
               
               
                 The number of  
                  800 
                  600 
                  500 
                  400 
                  300 
               
               
                 vibrations 
                   
                   
                   
                   
                   
               
               
                 (the number of times) 
                   
                   
                   
                   
                   
               
               
                 Satellite droplet  
                 Poor 
                 Poor 
                 Excellent 
                 Excellent 
                 Excellent 
               
               
                 evaluation 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 12 
               
               
                   
               
               
                 (Satellite droplet evaluation: 5 mPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 The number of  
                 3000 
                 2500 
                 2000 
                 1500 
                 1000 
               
               
                 vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Satellite droplet 
                 Poor 
                 Poor 
                 Poor 
                 Poor 
                 Excellent 
               
               
                 evaluation 
                   
                   
                   
                   
                   
               
               
                 The number of  
                  800 
                  600 
                  500 
                  400 
                  300 
               
               
                 vibrations 
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Satellite droplet 
                 Excellent 
                 Excellent 
                 Excellent  
                 Excellent 
                 Excellent 
               
               
                 evaluation 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 13 
               
               
                   
               
               
                 (Satellite droplet evaluation: 9.0 mPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 The number of  
                 3000 
                 2500 
                 2000 
                 1500 
                 1000 
               
               
                 vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Satellite droplet 
                 Excellent 
                 Excellent 
                 Excellent 
                 Excellent 
                 Excellent 
               
               
                 evaluation 
                   
                   
                   
                   
                   
               
               
                 The number of  
                  800 
                  600 
                  500 
                  400 
                  300 
               
               
                 vibrations 
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Satellite droplet 
                 Excellent 
                 Excellent 
                 Excellent 
                 Excellent 
                 Excellent 
               
               
                 evaluation 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 14 
               
               
                   
               
               
                 (Satellite droplet evaluation: 10 mPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 The number of  
                 3000 
                 2500 
                 2000 
                 1500 
                 1000 
               
               
                 vibrations  
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Satellite droplet 
                 Excellent 
                 Excellent 
                 Excellent 
                 Excellent 
                 Excellent 
               
               
                 evaluation 
                   
                   
                   
                   
                   
               
               
                 The number of  
                  800 
                  600 
                  500 
                  400 
                  300 
               
               
                 vibrations 
                   
                   
                   
                   
                   
               
               
                 (the number of  
                   
                   
                   
                   
                   
               
               
                 times) 
                   
                   
                   
                   
                   
               
               
                 Satellite droplet 
                 Excellent 
                 Excellent 
                 Excellent  
                 Excellent 
                 Excellent 
               
               
                 evaluation 
               
               
                   
               
            
           
         
       
     
     According to the evaluation results indicated above, in the inkjet recording apparatus, it is preferable that the number of times the piezoelectric element is vibrated is less than or equal to 500 in the case of a zero shear viscosity being less than or equal to 5.0 mPa·s, the number of times the piezoelectric element is vibrated ranges from 500 to 1000 in the case of a zero shear viscosity being higher than 5.0 mPa·s and less than 9.0 mPa·s, and the number of times the piezoelectric element is vibrated is greater than or equal to 1000 in the case of a zero shear viscosity being higher than or equal to 9.0 mPa·s. 
     Preferred embodiments of the present disclosure have been described above. However, the present disclosure is not limited to the above embodiments, and various modifications can be made. For example, in the embodiments, the inkjet recording apparatus (printer) for color printing is described as an inkjet recording apparatus. The present disclosure is not limited thereto. The inkjet recording apparatus may be implemented as, for example, black-and-white printers, black-and-white copying machines, color copying machines, facsimile machines, or multifunctional peripherals having the entirety or some of functions of the printers and machines. 
     Further, a sheet-like recording medium is not limited to a paper T, and may be, for example, a film sheet. 
     Further, a recording mode of the inkjet recording apparatus is not limited to any specific recording mode. A serial mode in which a line head performs recording while scanning the recording paper T as a recording medium may be used, or a line head mode in which a line head fixed to an apparatus body performs recording may be used. In the viewpoint of high speed image forming, a line head mode is advantageous. 
     Further, in the embodiments, the control portion changes the number of times of the vibrations by changing a vibration period. However, the present disclosure is not limited thereto. The number of times of vibrations may be changed by changing the number of times of vibrations per unit time. 
     It is to be understood that the embodiments herein are illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.