Abstract:
There is provided a liquid drop expelling head including: a driving element generating a pressure wave at a liquid within a pressure chamber, and expelling a liquid drop from a nozzle which communicates with the pressure chamber; and a control section applying a driving waveform based on image information to the driving element, and controlling a preparatory waveform, which vibrates a meniscus of the nozzle, on the basis of one of a liquid drop expulsion standby time and a liquid drop amount of a first drop at a time of starting expulsion again.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a liquid drop expelling head which expels liquid drops, and to an image forming device provided with the liquid drop expelling head. 
         [0003]    2. Related Art 
         [0004]    Among liquid drop expelling heads of inkjet recording devices (hereinafter called “image forming devices”), there are those which impart vibration to the meniscus of the ink at the nozzle in order to prevent the ink from thickening (Japanese Patent Application Laid-Open (JP-A) No. 9-201960). 
         [0005]    At times of liquid drop expulsion standby when a liquid drop is not being expelled from the nozzle, vibration is imparted intermittently to the meniscus of the nozzle to the extent that a liquid drop is not expelled therefrom. Further, vibration is continuously imparted to the meniscus immediately before printing begins. 
         [0006]    In this way, by imparting vibration intermittently at the time of liquid drop expulsion standby and imparting vibration continuously before printing starts, fatigue and noise of the driving element are reduced, thickening of the ink is prevented, and clogging of the nozzle is prevented. 
         [0007]    However, at this liquid drop expelling head, there is merely the structure of always applying the same vibration before printing starts, regardless of the extent of thickening of the ink or the expelling conditions of the ink drop to be expelled. Therefore, there are cases in which the effects of imparting vibration are insufficient, and cases in which, oppositely, the effects of imparting vibration are excessive. 
         [0008]    In cases in which the effects of imparting vibration are insufficient, thickening of the meniscus surface progresses, and the problem arises that the expulsion speed of the first drop at the time of starting expulsion again is greatly reduced. However, in this case, because the thickened ink is removed due to the expulsion of the first drop, the expulsion speeds of the drops from the second drop on are hardly reduced at all. 
         [0009]    On the other hand, in cases in which the effects of imparting vibration are excessive, the thickened ink is excessively dispersed within the ink flow path. Therefore, although the amount of reduction of the expulsion speed of the of the first drop is kept to a minimum, the dispersed thickened ink cannot be removed only in that first drop, and thus, there is the problem that the expulsion speeds of the ink drops from the second drop on as well are reduced. 
         [0010]    In this way, problems arise both when the effects of meniscus vibration applied at times of expulsion standby are insufficient and when they are excessive. 
       SUMMARY 
       [0011]    According to an aspect of the invention, there is provided a liquid drop expelling head including: a driving element generating a pressure wave at a liquid within a pressure chamber, and expelling a liquid drop from a nozzle which communicates with the pressure chamber; and a control section applying a driving waveform based on image information to the driving element, and controlling a preparatory waveform, which vibrates a meniscus of the nozzle, on the basis of one of a liquid drop expulsion standby time and a liquid drop amount of a first drop at a time of starting expulsion again. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
           [0013]      FIG. 1A  is a drawing showing a preparatory waveform in a case in which the expulsion standby time is long, in an inkjet recording head relating to a first embodiment of the present invention; 
           [0014]      FIG. 1B  is a drawing showing a preparatory waveform in a case in which the expulsion standby time is short, in the inkjet recording head relating to the first embodiment of the present invention; 
           [0015]      FIG. 2A  is a cross-sectional view of the inkjet recording head relating to the first embodiment of the present invention; 
           [0016]      FIG. 2B  is an enlarged sectional view of a nozzle of the inkjet recording head relating to the first embodiment of the present invention; 
           [0017]      FIG. 3  is a plan view showing the recording head relating to the first embodiment of the present invention; 
           [0018]      FIG. 4  is a perspective view showing the recording head relating to the first embodiment of the present invention; 
           [0019]      FIG. 5  is a schematic structural view of an inkjet recording device in which the inkjet recording head relating to the first embodiment is employed of the present invention; 
           [0020]      FIG. 6A  is a drawing showing a preparatory waveform in a case in which the expulsion standby time is long, in an inkjet recording head relating to a second embodiment of the present invention; 
           [0021]      FIG. 6B  is a drawing showing a preparatory waveform in a case in which the expulsion standby time is short, in the inkjet recording head relating to the second embodiment of the present invention; 
           [0022]      FIG. 7A  is a drawing showing a preparatory waveform in a case in which the liquid drop at the time when expulsion starts again is a large drop, in an inkjet recording head relating to a third embodiment of the present invention; 
           [0023]      FIG. 7B  is a drawing showing a preparatory waveform in a case in which the liquid drop at the time when expulsion starts again is a small drop, in the inkjet recording head relating to the third embodiment of the present invention; 
           [0024]      FIG. 8A  is a drawing showing a preparatory waveform in a case in which the expulsion standby time is long, in an inkjet recording head relating to a fourth embodiment of the present invention; and 
           [0025]      FIG. 8B  is a drawing showing a preparatory waveform in a case in which the expulsion standby time is short, in the inkjet recording head relating to the fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    A first embodiment of an image forming device, in which a liquid drop expelling head of the present invention is employed, will be described in accordance with  FIGS. 1 through 5 . 
         [0027]    As shown in  FIG. 5 , a sheet feed tray  116  is provided at the lower portion of the interior of a housing  114  of an inkjet recording device  110  serving as the image forming device. Sheets P which are stacked within the sheet feed tray  116  can be taken-out one-by-one by a pick-up roller  118 . The sheet P which is taken-out is conveyed by plural conveying roller pairs  120  which structure a predetermined conveying path  122 . Hereinafter, “conveying direction” refers to the direction of conveying the sheet P which is the recording medium, and “upstream” and “downstream” mean upstream and downstream in the conveying direction, respectively. 
         [0028]    A conveying belt  128 , which is endless and which is stretched around a driving roller  124  and a driven roller  126 , is disposed above the sheet feed tray  116 . A recording head array  130  is disposed above the conveying belt  128 , and faces a flat portion  128 F of the conveying belt  128 . This facing region is an expulsion region SE where ink drops are expelled from the recording head array  130 . In the state in which the sheet P, which has been conveyed along the conveying path  122 , is held by the conveying belt  128  and reaches the expulsion region SE and faces the recording head array  130 , ink drops corresponding to image information are adhered onto the sheet P from the recording head array  130 . 
         [0029]    Due to the sheet P being circulated in a state of being held by the conveying belt  128 , the sheet P passes through the expulsion region SE plural times, such that so-called multipass image recording can be carried out. Accordingly, the surface of the conveying belt  128  is the path of circulation of the sheet P. 
         [0030]    Four recording heads  10 , which are elongated such that the effective recording regions thereof are at least as long as the width of the sheet P (the length of the sheet P in the direction orthogonal to the conveying direction thereof) and which serve as liquid drop expelling heads and which correspond to the four colors of yellow (Y), magenta (M), cyan (C), and black (K) respectively, are disposed at the recording head array  130  along the conveying direction, such that a full-color image can be recorded. 
         [0031]    The recording head array  130  can be structured so as to be unable to move in the direction orthogonal to the conveying direction. However, if the recording head array  130  is structured so as to move when needed, in multipass image recording, images of higher resolutions can be recorded, and it is possible to make problems with the recording heads  10  not be reflected in the results of recording. 
         [0032]    Four maintenance units  134 , which correspond to the recording heads  10  respectively, are disposed in the vicinity of the recording head arrays  130  (in the present embodiment, at the both sides in the conveying direction). These maintenance units  134  carry out predetermined maintenance operations (vacuuming, dummy jetting, wiping, capping, and the like). 
         [0033]    A charging roller  136  is disposed at the upstream side of the recording head array  130 . The charging roller  136  can move between a pressing position, at which the charging roller  136  is driven while nipping the conveying belt  128  and the sheet P between itself and the driven roller  126  and presses the sheet P against the conveying belt  128 , and a separated position at which the charging roller  136  is apart from the conveying belt  128 . At the pressing position, a predetermined potential difference arises between the charging roller  136  and the driven roller  126  which is grounded, and therefore, charges are applied to the sheet P and the sheet P can be electrostatically attracted to the conveying belt  128 . 
         [0034]    An unillustrated peeling plate is disposed at the downstream side of the recording head array  130 , and peels the sheet P off of the conveying belt  128 . 
         [0035]    The peeled-off sheet P is conveyed by plural discharging roller pairs  142  which structure a discharge path  144 , and is discharged-out onto a catch tray  146  provided at the top portion of the housing  114 . 
         [0036]    An inverting path  152 , which is structured by plural roller pairs  150  for inversion, is provided between the sheet feed tray  116  and the conveying belt  128 . Due to the sheet P, on whose one surface thereof an image is recorded, being inverted and being held at the conveying belt  128 , image recording onto the both surfaces of the sheet P can easily be carried out. 
         [0037]    Ink tanks  154 , which store inks of the four colors respectively, are provided between the conveying belt  128  and the catch tray  146 . The inks in the ink tanks  154  are supplied to the recording head array  130  by ink supplying pipes (not shown). 
         [0038]    Because the inkjet recording device  110  has the four recording heads  10  which house the inks of the four colors, the head widths in the conveying direction of the sheet P can be made to be small, and the recording head array  130  which is compact can be realized. 
         [0039]    The structure of the recording head  10  will be described next. 
         [0040]    As shown in  FIG. 3 , the recording head  10  is formed by an elongated head which is wider than the maximum width of the sheet P. The recording head  10  is structured by plural head units  12  which are rectangular. The head units  12  are disposed in two rows in a staggered manner so as to be offset by a half of a pitch at the upstream side and the downstream side of the sheet P which is being conveyed. 
         [0041]    A rectangular ejector region (ejector group placement portion)  14  is formed at the head unit  12 . A plurality of ejectors  60 , which have a pressure chamber  36 , a nozzle communicating path  38 , a nozzle  16 , and a driving element  58  serving as a driving portion, which are shown in  FIGS. 2A and 2B , are arrayed at the ejector region  14 . 
         [0042]    In the inkjet recording device  110  (see  FIG. 5 ) in which the recording head  10  is installed, the sheet P is conveyed in the direction of the arrow at a predetermined pitch at the portion facing the ejector regions  14  of the head units  12 , and ink drops corresponding to image information are expelled from the nozzles  16  (see  FIGS. 2A and 2B ). Accordingly, regions, which are recorded by ejector regions  14 A which are positioned at the sheet conveying direction upstream side of the recording head  10 , and regions, which are recorded by ejector regions  14 B which are positioned at the sheet conveying direction downstream side of the recording head  10 , are lined-up alternately along the transverse direction of the sheet P on the sheet P at which image recording has been completed. Here, at the head units  12  which are adjacent to one another in the transverse direction of the sheet P which is being conveyed, the end portions of the ejector regions  14 A,  14 B are disposed so as to overlap one another, so that no region which cannot be printed arises within the printing region. 
         [0043]    As shown in  FIG. 4 , a base plate  18  which fixes the head units  12  is disposed at the sides of the head units  12  opposite the sides at which the ejector regions  14 A,  14 B are provided. Two ink flow paths  20 , which supply ink to the two rows of the head units  12  respectively, are formed in the base plate  18 . Further, two heat dissipating plates  22  are mounted to the end portions at the reverse surface side of the base plate  18 . A controller  24 , which controls the driving waveforms applied to driving elements  58 , is disposed at the heat dissipating plate  22 . Electric wires  26 , which connect the respective head units  12  (at the near side in  FIG. 4 ) and the controller  24 , are supported at the side portion of the base plate  18 . Switch ICs  28  are provided at the electric wires  26 . Note that the electric wires  26  which are connected to the respective head units  12 , the switch ICs  28 , and the controller  24  are similarly provided at the far side in  FIG. 4  although not illustrated. 
         [0044]    As shown in  FIG. 3 , flow path main flows  30 A,  30 B are disposed at the outer sides of the both end portions of the ejector region  14 , at the both end portions of the head unit  12 . The flow path main flows  30 A,  30 B are connected to the ink flow path  20  (see  FIG. 4 ), and ink is supplied from the ink flow path  20  through the flow path main flows  30 A,  30 B to the head unit  12 . Plural common flow paths  32 , which supply ink to the respective ejectors  60  arrayed at the ejector region  14 , are connected to the flow path main flows  30 A,  30 B. The plural common flow paths  32  extend along the longitudinal direction from the both end portions of the head unit  12 , and are divided at the central portion of the ejector region  14 . Namely, the final end portions of the common flow paths  32  are positioned in a vicinity of the central portion of the head unit  12 . Note that, for ease of understanding,  FIG. 3  schematically illustrates four common flow paths  32  connected to the flow path main flows  30 A,  30 B. However, actually, the number of nozzles  16  (see  FIGS. 2A ,  2 B) provided at the ejectors  60  is, for example, 600 npi (nozzle per inch), and a large number of common flow paths  32  is connected. 
         [0045]    More specifically, as shown in  FIGS. 2A and 2B , the nozzle communicating path  38  is provided at the side of the pressure chamber  36  at which side the nozzle  16  is provided, and the nozzle  16  and the pressure chamber  36  communicate with one another by the nozzle communicating path  38 . On the other hand, the pressure chamber  36  and the common flow path  32  communicate with one another by a planar direction communicating path  42 . 
         [0046]    These are formed by laminating plural plates. A flow path plate unit  29  is formed by laminating, in order, a nozzle plate  44  in which the nozzles  16  are formed, an ink pool plate  46  in which the nozzle communicating paths  38  and the common flow paths  32  are formed, a pressure chamber plate  48  in which the pressure chambers  36  and the nozzle communicating paths  38  and the common flow paths  32  are formed, and a path plate  50  in which the planar direction communicating paths  42  are formed. 
         [0047]    A vibrating plate  57  is adhered on the top surface of the path plate  50 . The driving elements  58  are adhered on the top surface of the vibrating plate  57  at positions corresponding to the pressure chambers  36 . The driving elements  58  are driving portions which deform due to the working of electrostriction, and apply pressure to the ink within the pressure chambers  36 . A flexible circuit board  62  is joined via solder bumps  52  to upper portion electrodes  54  of the driving elements  58 . 
         [0048]    In accordance with this structure, the controller  24 , which controls the driving waveforms applied to the driving elements  58 , applies driving waveforms to the driving elements  58  via the flexible circuit board  62 . Due to the driving elements  58  being driven thereby, pressure is applied to the ink filled in the pressure chambers  36 , and the ink can be expelled from the nozzles  16 . 
         [0049]    Next, description will be given of the state after the liquid drop expulsion standby time, until expulsion is started again. 
         [0050]    In cases in which there is an ink drop expulsion standby time in which ink is not expelled, the time over which a meniscus  40  (see  FIG. 2B ) of the nozzle  16  contacts the outside air is long, and the moisture in the ink evaporates from the meniscus  40 . The viscosity of the ink in the vicinity of the meniscus  40  thereby increases, and there are cases in which the expulsion speed of the ink at the time expulsion starts again decreases and becomes off-target, and the image quality deteriorates. 
         [0051]    Thus, in the present embodiment, on the basis of the length of the liquid drop expulsion standby time which is judged from the image information, the controller  24  controls the preparatory waveform which vibrates the meniscus  40  at the time of starting expulsion again, and applies this preparatory waveform to the driving element  58 . 
         [0052]    Concretely, as shown in  FIG. 1A , if the expulsion standby time is long, the viscosity of the ink in a vicinity of the meniscus  40  greatly increases, and therefore, the controller  24  applies a continuous preparatory waveform for a rather long time to the driving element  58  (see  FIG. 2A ). By sufficiently vibrating the meniscus  40  in this way, the expulsion speed of the first drop at the time of starting expulsion again is restored to 70% of the expulsion speed at the time of continuous expulsion. If the expulsion speed can be restored to 70%, image quality of a level which is equivalent to the image quality at the time of continuous expulsion can be obtained. Note that, although it would be ideal to return the expulsion speed to 100% (the continuous expulsion speed), the meniscus  40  would have to be vibrated greatly in order to return the expulsion speed of the first drop to the continuous expulsion speed, and as a result, the expulsion speeds of the drops from the second drop on would conversely decrease and the image quality would deteriorate. Therefore, 70% is desirable from an overall standpoint. 
         [0053]    As shown in  FIG. 1B , if the expulsion standby time is short, the viscosity of the ink in the vicinity of the meniscus  40  increases, but does not increase that much as compared with a case in which the standby time is long. Therefore, the controller  24  applies a continuous preparatory waveform to the driving element  58  (see  FIG. 2A ) for a short time. By vibrating the meniscus  40  a proper degree in this way, the expulsion speed of the first drop at the time when expelling is started again is made to be 70% of the expulsion speed at the time of continuous expulsion. 
         [0054]    Due to the controller  24  controlling the length of the time of application of the preparatory waveform on the basis of the liquid drop expulsion standby time in this way, the expulsion speed of the first drop at the time that expelling is started again can be made to be 70% of the expulsion speed at the time of continuous expulsion. The image quality at the time of starting expulsion again can thereby be improved. 
         [0055]    A second embodiment of an inkjet recording device, in which an inkjet recording head of the present invention is employed, will be described next in accordance with  FIGS. 6A and 6B . 
         [0056]    Note that the same members as those of the first embodiment are denoted by the same reference numerals, and description thereof is omitted. 
         [0057]    As shown in  FIGS. 6A and 6B , in the present embodiment, the application time of the preparatory waveform is constant, and instead, by controlling the amplitude of the preparatory waveform, the expulsion speed of the liquid drops at the time of starting expulsion again is made to be 70% of the expulsion speed at the time of continuous expulsion as in the first embodiment. 
         [0058]    Concretely, as shown in  FIG. 6A , if the expulsion standby time is long, the viscosity of the ink in the vicinity of the meniscus  40  increases, and therefore, the controller  24  apples a preparatory waveform of a large amplitude to the driving element  58  (see  FIG. 2A ). By greatly vibrating the meniscus  40  in this way, the expulsion speed of the first drop at the time of starting expulsion again is made to be to 70% of the expulsion speed at the time of continuous expulsion. 
         [0059]    Further, as shown in  FIG. 6B , if the expulsion standby time is short, the viscosity of the ink the vicinity of the meniscus  40  increases, but does not increase that much as compared with a case in which the standby time is long. Therefore, the controller  24  applies a preparatory waveform having a small amplitude to the driving element  58  (see  FIG. 2A ). By vibrating the meniscus  40  a proper degree in this way, the expulsion speed of the first drop at the time when expelling is started again is made to be 70% of the expulsion speed at the time of continuous expulsion. 
         [0060]    Due to the controller  24  controlling the amplitude of the preparatory waveform on the basis of the liquid drop expulsion standby time in this way, the expulsion speed of the first drop at the time that expelling is started again can be made to be 70% of the expulsion speed at the time of continuous expulsion. The image quality at the time of starting expulsion again can thereby be improved. 
         [0061]    A third embodiment of an inkjet recording device, in which an inkjet recording head of the present invention is employed, will be described next in accordance with  FIGS. 7A and 7B . 
         [0062]    Note that the same members as those of the first embodiment are denoted by the same reference numerals, and description thereof is omitted. 
         [0063]    As shown in  FIGS. 7A and 7B , in the present embodiment, the controller  24  does not control the application time of the preparatory waveform on the basis of the expulsion standby time as in the first embodiment. Instead, by controlling the application time of the preparatory waveform on the basis of the liquid drop amount at the time of starting expulsion again, which is judged in accordance with the image information, the expulsion speed of the first drop at the time of starting expulsion again is made to be 70% of the expulsion speed at the time of continuous expulsion. 
         [0064]    Concretely, as shown in  FIG. 7A , if the liquid drop which is the first drop at the time when expulsion starts again is a large drop, the expulsion force is great, and the effect of an increase in ink viscosity in a vicinity of the meniscus  40  on the expulsion speed is small. Therefore, the controller  24  applies a preparatory waveform to the driving element  58  for a short time. By vibrating the meniscus  40  a proper degree in this way, the expulsion speed of the first drop at the time of starting expulsion again is made to be to 70% of the expulsion speed at the time of continuous expulsion. 
         [0065]    Further, as shown in  FIG. 7B , if the liquid drop which is the first drop at the time when expulsion starts again is a small drop, the expulsion force is small, and the effect of an increase in ink viscosity in a vicinity of the meniscus  40  on the expulsion speed is great. Therefore, the controller  24  applies a preparatory waveform to the driving element  58  for a long time. By sufficiently vibrating the meniscus  40  in this way, the expulsion speed of the first drop at the time of starting expulsion again is made to be to 70% of the expulsion speed at the time of continuous expulsion. 
         [0066]    A fourth embodiment of an inkjet recording device, in which an inkjet recording head of the present invention is employed, will be described next in accordance with  FIGS. 8A and 8B . 
         [0067]    Note that the same members as those of the first embodiment are denoted by the same reference numerals, and description thereof is omitted. 
         [0068]    As shown in  FIGS. 8A and 8B , in the present embodiment, the application time of the preparatory waveform is constant, and instead, by controlling the frequency of the preparatory waveform, the expulsion speed of the first drop at the time of starting expulsion again is made to be 70% of the expulsion speed at the time of continuous expulsion as in the first embodiment. 
         [0069]    Concretely, as shown in  FIG. 8A , if the expulsion standby time is long, the viscosity of the ink in the vicinity of the meniscus  40  increases, and therefore, the controller  24  applies a preparatory waveform of a high frequency to the driving element  58  (see  FIG. 2A ). By sufficiently vibrating the meniscus  40  in this way, the expulsion speed of the first drop at the time of starting expulsion again is made to be to 70% of the expulsion speed at the time of continuous expulsion. 
         [0070]    Further, as shown in  FIG. 8B , if the expulsion standby time is short, the viscosity of the ink the vicinity of the meniscus  40  increases, but does not increase that much as compared with a case in which the standby time is long. Therefore, the controller  24  applies a preparatory waveform of a low frequency to the driving element  58  (see  FIG. 2A ). By vibrating the meniscus  40  a proper degree in this way, the expulsion speed of the first drop at the time when expelling is started again is made to be 70% of the expulsion speed at the time of continuous expulsion. 
         [0071]    Note that, although the present invention has been described in detail with reference to specific embodiments, the present invention is not to be limited to these embodiments, and it will be clear to those skilled in the art that various other embodiments are possible within the scope of the present invention. For example, in the above-described embodiment, the expulsion speed of the first drop at the time when expulsion is started again is controlled by controlling the frequency of the preparatory waveform. However, the expulsion speed of the first drop when expulsion is again started may be controlled by controlling, in combination, the application time of the preparatory waveform, the amplitude of the preparatory waveform, and the frequency of the preparatory waveform. 
         [0072]    Further, by optimizing the preparatory waveform shown in the above-described embodiments, excessive application of vibration can be prevented, and therefore, there is also the effect that deterioration of the vibrating element can be prevented.