Patent Publication Number: US-7581811-B2

Title: Inkjet printer

Description:
INCORPORATION BY REFERENCE 
   The present application is based on Japanese Patent Applications Nos. 2004-330488 and 2004-330489, both filed on Nov. 15, 2004, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an inkjet printer. 
   2. Description of Related Art 
   As a kind of recording apparatus for recording information on a recording medium such as a sheet of paper, there is known an inkjet printer that ejects ink droplets onto a recording medium to record information. Such an inkjet printer includes an actuator, a printhead, and a maintenance unit including a purge cap. The printhead has ink passages formed therein, and a nozzle surface where a plurality of nozzles for ejecting ink droplets therefrom are arranged. In the inkjet printer, the printhead is movable between a recording area and a purge area adjacent to the recording area. The printhead performs printing on the recording medium while reciprocated across the recording area. The printhead is located at the purge area when a purging operation is performed with the nozzle surface covered with the purge cap so that the purge cap receives ink discharged or sucked from the nozzles for purging. 
   In the printhead for the inkjet printer, the nozzle surface is usually constituted by an external surface of a nozzle film that is coated with a water repellent material. The nozzle film is formed of polyimide, for instance. In such an arrangement, the nozzle surface is brought into frictional contact with the recording medium, and the water repellent coating and the nozzle film tend to be damaged. Recently, to meet a demand for a higher print rate, the size of the nozzle surface has been increased, thereby further increasing the risk of damage of the nozzle film and the water repellent coating thereon. 
   As a technique for coping with this disadvantage, there is known an arrangement where the nozzle surface is covered with a metallic cover plate, except at and around a row of the nozzles, so as to constitute a front surface of the printhead by the cover plate and a part of the nozzle surface as exposed through a through-hole formed in the cover plate, as disclosed in JP-A-5-201000 (especially paragraphs 0020, 0021 and FIGS. 1 and 2), for instance. 
   Since the cover plate has such a through-hole, ink remaining on the front surface can not be completely removed when the front surface is wiped with a wiper immediately after a purging operation. 
   After a purging operation, a mixture of small bubbles and ink droplets is inside the purge cap. At a moment when the purge cap is separated from the front surface of the printhead, this mixture is drawn into a nozzle because of a negative pressure in the printhead. The bubbles contained in the mixture and drawn into the nozzle thereafter remain in the ink passage extending from that nozzle, thereby attenuating a pressure wave which is generated in the ink passage when the actuator is driven to eject an ink droplet from the nozzle, during a printing operation. Hence, stability can not be assured in ejection performance of the printhead. 
   As a countermeasure to the instable ejection performance, deaerated ink is used in the inkjet printer so that the bubbles introduced into the nozzles are dissolved in the deaerated ink to restore the ejection performance. However, it takes some time to dissolve the bubbles in the ink, and thus it is necessitated to wait until the time elapse before the next printing is started. 
   SUMMARY OF THE INVENTION 
   The present invention has been developed in view of the above-described situations, and therefore it is an object of the invention to provide an inkjet printer capable of preventing a bubble, as generated in a purging operation during which a purge cap covers a front surface of the printhead, from being introduced into a nozzle upon separation of the purge cap from the front surface after the purging operation. 
   To attain the above object, the present invention provides an inkjet printer including:
         a printhead having a front surface in which is formed at least one nozzle row consisting of a plurality of nozzles from each of which a droplet of ink is ejected;   a maintenance unit including a purge cap which receives ink from the nozzles during a purging operation, the purge cap having a contact surface to be brought into contact with the front surface of the printhead, and at least one main channel extending alongside the nozzle row; and   a plurality of dents formed on at least one of the contact surface of the purge cap and the front surface of the printhead so as to form, at least when the contact surface is in contact with the front surface, a plurality of branches a part of each of which is in communication with at least one of the nozzles, and another part of each of which is in communication with the main channel.       

   The purging operation may be performed by at least one of sucking ink from the nozzles, and pressurizing the ink from the inside of the printhead. 
   The main channel may be formed as a covered channel formed inside the purge cap. However, in most cases, forming the main channel as a non-covered channel that is open in the contact surface makes production of the inkjet printer easier than when the main channel is a covered one. Where the main channel is formed as a covered channel, each of the branches should be covered at least at an end thereof where the branch is connected with the covered main channel. 
   According to the arrangement of the invention, when the front surface of the printhead is covered by the purge cap at the purging position in the purging area, each of the dents forms between the printhead and the purge cap a small void or chamber in communication with at least one of the nozzles. Each small chamber is communicated with a discharge hole via the branch and the main channel. Bubbles generated in an initial phase of the purging operation are discharged or sucked along with the ink. The small chamber into which the at least one nozzle opens is completely filled with the ink quickly, and thereafter a bubble does not occur. An air space surrounding each nozzle and corresponding to the small chamber is defined by the dent, thereby reducing a volume of air present around each nozzle. This is advantageous in reducing generation of bubbles and quickly discharging or sucking generated bubbles. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features, advantages and technical and industrial significance of the present invention will be better understood by reading the following detailed description of preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which: 
       FIG. 1  is a schematic perspective view of a principal portion of an inkjet printer according to a first embodiment of the invention; 
       FIG. 2  is a perspective view of a printhead, a purge cap, and a positioning member of the inkjet printer; 
       FIG. 3  shows the purge cap; 
       FIG. 4  illustrates a front surface of the printhead in which nozzles are arranged; 
       FIG. 5  is a cross-sectional view taken along line A-A in  FIG. 4 ; 
       FIG. 6  is a bottom plan view of the printhead; 
       FIG. 7  is a cross-sectional view taken along line B-B in  FIG. 6 ; 
       FIG. 8  is a perspective view of a printhead, a purge cap, and a positioning member of an inkjet printer according to a second embodiment of the invention; 
       FIG. 9  shows the purge cap; 
       FIG. 10  illustrates a front surface of the printhead in which nozzles are arranged; 
       FIG. 11  is a cross-sectional view taken along line C-C in  FIG. 10 ; 
       FIG. 12  is a bottom plan view of the printhead; 
       FIG. 13  is a cross-sectional view taken along line D-D in  FIG. 12 ; 
       FIG. 14  illustrates a front surface of a printhead of an inkjet printer according to a third embodiment of the invention; 
       FIG. 15  illustrates a front surface of a printhead of an inkjet printer according to a fourth embodiment of the invention; and 
       FIG. 16  is a view of a purge cap in an inkjet printer according to a fifth embodiment of the invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Hereinafter, there will be described presently preferred embodiments of the invention, by referring to the accompanying drawings. 
   Referring to  FIGS. 1 to 7 , there will be described an inkjet printer according to a first embodiment of the invention. As shown in  FIGS. 1 and 2 , the inkjet printer includes an inkjet printhead  11 , a maintenance unit  12  that is a purging device, and an ink receiving portion  14 . In the inkjet printer, the printhead  11  can be reciprocated across three areas, namely, a recording area S 1 , a purging area S 2 , and a flashing area S 3 . The purging area S 2  and the flashing area S 3  are on the opposite sides of the recording area S 1 . The printhead  11  has a plurality of nozzles  11   a  each for ejecting an ink droplet therefrom onto a recording medium such as a sheet of paper. Printing on the recording medium is performed while the printhead  11  is reciprocated across the recording area S 1 . A purging operation is performed when the printhead  11  is located at the purging area S 2  where the maintenance unit  12  is disposed. The maintenance unit  12  includes a purge cap  12   a  for receiving ink discharged or sucked from the nozzles  11   a  of the printhead  11  in the purging operation. The ink receiving portion  14  is disposed at the flashing area S 3 , which is a place where a flashing operation to eject ink droplets from the nozzles is periodically performed during a recording operation, in order to prevent the nozzles from drying. 
   In this inkjet printer, the purging operation is implemented when ink is introduced into the printhead  11  for the first time, and when the inkjet printer has not been used for a predetermined period of time. The purging operation may be implemented in any other suitable situations, such as when an ink ejection characteristic of the printhead  11  has changed to a degree that the flashing operation can not compensate for the change. The purging operation is implemented in order to suck a bubble and foreign matter that may be present in ink passages extending to the nozzles. When the purging operation is to be implemented, the printhead  11  is moved into the purging area S 2  adjacent to the recording area S 1 , and then a front surface of the printhead  11  from which ink droplets are to be ejected is covered by the purge cap  12   a . Thereafter the purging operation is implemented, namely, ink droplets are sucked from the nozzles  11   a.    
   Although not shown specifically, the printhead  11  is mounted on a head holder (not shown) along with a buffer tank. The printhead  11 , the buffer tank, and the head holder constitute a recording device  15  for recording information while being reciprocated in a main scanning direction that is perpendicular to a feeding direction in which the recording medium is fed. Inks of respective colors, that is, black, cyan, magenta, and yellow, are supplied from respective ink tanks (not shown) into the printhead  11  via ink supply tubes  16   a - 16   d  and after temporarily stored in the buffer tank on the head holder. The ink tanks are removably disposed on a frame (not shown) of the printer and stores large amounts of the inks to be supplied to the printhead  11 . When the ink in any ink tank is depleted, replacement of the ink tank is implemented along with a purging operation as a maintenance work. 
   The head holder or the recording device  15  is slidably supported by a pair of guide members (not shown) arranged side-by-side in a front-rear direction and extending in a lateral direction of the printer parallel to each other. The head holder is coupled with an endless timing belt  18  entrained around a drive pulley  16  and a driven pulley  17 . When the drive pulley  16  is rotated by a driving motor (not shown), the recording device  15  coupled with the timing belt  18  is reciprocated in the lateral direction along the guide members. Although not shown, the recording medium is fed by a well known feeder mechanism such that the printhead  11  can record information on the recording medium as passing under the printhead  11  in the direction perpendicular to the main scanning direction which is the reciprocating direction of the recording device  15 . 
   As shown in  FIG. 2 , the nozzles  11   a  are arranged in a plurality of rows in the front surface of the printhead. Each nozzle row is for ejecting droplets of ink of one of the four colors, i.e., black, cyan, yellow, and magenta, and extends in a direction perpendicular to the main scanning direction. Each nozzle  11   a  is open or exposed downward so as to be opposed to an upper surface of the recording medium. 
   The printhead  11  includes a cavity unit  21  and an actuator (not shown), and the cavity unit  21  has a nozzle surface where the nozzles  11   a  for ejecting ink droplets therethrough are arranged, a plurality of pressure chambers in communication with the respective nozzles  11   a , and manifolds for storing introduced inks before the inks are supplied to the pressure chambers through openings connecting the manifolds with the pressure chambers. The actuator operates to have ink droplets eject from the nozzles  11   a , so as to record information on the recording medium.  FIG. 4  is a fragmentary enlarged view of the front surface of the printhead  11 , and  FIG. 5  is a cross-sectional view taken along line A-A in  FIG. 4  and showing one of the nozzles  11   a  and its vicinity in enlargement. As seen in  FIG. 4  showing a part of two adjacent nozzle rows, the nozzles  11   a  are arranged in a staggered fashion such that each nozzle row is misaligned relatively to an adjacent nozzle row by half a pitch at which the nozzles  11   a  are arranged in each row. As shown in  FIGS. 4 and 5 , the nozzle surface of the cavity unit  21  where the nozzles  11   a  are formed is covered with a cover plate  23  having through-holes  23   a  at positions corresponding to the nozzles  11   a  so that the nozzle surface is protected by the cover plate  23 . That is, as shown in  FIG. 5 , each of the through-holes  23   a  is formed in the cover plate  23  with one of the nozzles  11   a  located substantially at a center of the through-hole  23   a , and thus a part of the nozzle surface around the nozzle  11   a  is exposed through the through-hole  23   a . The through-hole  23   a  is configured to have a diameter larger at an open end thereof on the side of the nozzle surface than at the other open end on the side of the external surface of the cover plate  23 . The front surface of the printhead is coated with a water repellent material  24 , such that an inside of each through-hole  23   a  including the part of the nozzle surface surrounding each nozzle  11   a  is also coated with the water repellent material  24 . Thus, damage of a nozzle film (described later), a surface of which constitutes the nozzle surface, at the nozzles  11   a  or therearound due to frictional contact with a recording medium or others is prevented. Hence, stable ink ejection performance is ensured. 
   Referring to  FIG. 7 , there will be described a laminar structure of the cavity unit  21 .  FIG. 7  is a cross-sectional view showing a part of the cavity unit  21  in enlargement, in which a contour of the purge cap  12   a  in contact with the front surface of the printhead is indicated by chain line. 
   As shown in  FIG. 7 , the nozzles  11   a  are formed through a nozzle film  21 A partially constituting the cavity unit  21 . The nozzle film  21 A may be formed of a polyimide film, for instance. The cavity unit  21  is constructed such that the nozzle film  21 A is sandwiched between the cover plate  23  and a stack of other plates superposed one on another and bonded with an adhesive, including a pair of manifold plates  21 B,  21 C and a supply plate  21 D. Through the manifold plates  21 B,  21 C, there are formed manifold holes  21 Ba,  21 Ca constituting manifolds in communication with the pressure chambers, and communication holes  21 Bb,  21 Cb each pair of which is in communication with one of the nozzles  11   a  so that ink flown from a corresponding one of the pressure chambers reaches the nozzle  11   a  via the pair of communication holes  21 Bb,  21 Cb. Through the supply plate  21 D, there are formed communication holes  21 Da each of which is in communication with one of the pairs of communication holes  21 Ba,  21 Ca so that the ink flown from the corresponding pressure chamber reaches the nozzle  11   a  via the communication hole  21 Da also. The cover plate  23  has recesses  23   b  each serving as a damper chamber for attenuating vibration in the ink inside the corresponding manifold, or manifold holes  21 Ba,  21 Ca. That is, the nozzle film  21 A serves as a damper plate. The recesses  23   b  are open toward the nozzle film  21 A. 
   There will be now described how a maintenance operation of the inkjet printer is performed. As described above, the maintenance unit  12  disposed in the frame of the printer and at the purging area S 2  located at an end of a reciprocation range of the printhead  11 . The purging device  12  selectively sucks ink of a color so as to restore the ink ejection performance of the printhead at the corresponding nozzle row to an initial state. At the flashing area S 3  at an opposite end of the reciprocation range of the printhead  11 , the ink receiving portion  14  for receiving ink ejected from the nozzles  11   a  in a flashing operation, which is a periodically performed operation to eject ink droplets from the nozzles  11   a  during a recording operation in order to prevent the nozzles from drying. The purging device  12  includes the purge cap  12   a  capable of contacting the front surface of the printhead  11 . The purge cap  12   a  is vertically displaceable in a similar manner as a known purging device is, so that there can be made a capping action to bring the purge cap  12   a  into contact with the front surface of the printhead, and a separating action to move the purge cap  12   a  away from the front surface of the printhead. 
   Reaching of the recording device  15  to a wait position where recording is not performed is detected by a position sensor disposed in a pathway along which the recording device  15  is moved. Then, the purge cap  12   a  at the wait position is elevated to a first vertical position to contact the front surface of the printhead  11 . On the other hand, when the printhead  11  is located at a position other than the wait position, the purge cap  12   a  is lowered or retracted from the first vertical position to a second vertical position away from the front surface of the printhead. The purge cap  12   a  is connected to a suction pump (not shown) in a similar manner as in a known maintenance unit, so that foreign matter and the ink whose viscosity has increased are sucked and removed from the inside of the nozzles  11   a  when the suction pump is driven. 
   There will be now described a structure of a contact surface of the purge cap  12   a , which structure is the characterizing portion of the first embodiment of the invention, with reference to  FIGS. 2 and 3  in which  FIG. 3  is a perspective view showing in enlargement a part of the purge cap  12   a , which part includes two nozzle rows. 
   The contact surface of the purge cap  12   a  has a plurality of dents  12   b  as branches of ink drain channels, and a plurality of grooves  12   c  each as a main channel of an ink drain channel. When the front surface of the printhead is covered with the purge cap  12   a  at a purging position in the purging area S 2 , a first one of opposite end portions of each of the dents  12   b  forms a small void or chamber into which a corresponding one of the nozzles  11   a  opens. Each of the grooves  12   c  extends along an extending direction of each nozzle row to communicate the dents  12   b  with one another at their second end portions. 
   The dents  12   b  are formed on the contact surface of the purge cap  12   a , at respective positions corresponding to the through-holes  23   a  of the cover plate  23  of the printhead  11 . A part of each of the dents  12   b  which part is opposed to the through-hole  23   a  has an area larger than an area of an open end of the through-hole  23   a . In accordance with the staggered arrangement of the nozzles  11   a  or the through-holes  23   a , the dents  12   b  are also arranged in rows in a staggered fashion. The dents  12   b  are formed such that when the front surface of the printhead is covered with the purge cap  12   a , a plurality of individual small voids or chambers are formed between the purge cap  12   a  and the printhead  11 , more specifically, between the first end portions of the dents  12   b  and the through-holes  23   a  opposed thereto. Each dent  12   b  in a first one of the rows extends from its first end portion in a first direction, and each dent  12   b  in a second one of the rows which is adjacent to the first row extends from its first end portion in a second direction opposite to the first direction. In each dent  12   b , the first end portion opposed to the through-hole  23   a  and an extended portion thereof not opposed to the through-hole  23   a  have a substantially same width. Hence, even in a case where the front surface of the printhead and the purge cap  12   a  contact each other with a positional error therebetween in the reciprocating direction of the printhead  11 , the through-holes  23   b  are reliably positioned to be opposed to the dents  12   b , and also a problem that a single dent  12   b  is opposed to a plurality of through-holes  23   a  does not occur. Thus, this embodiment has a high degree of freedom in respect of an error in the relative position between the printhead  11  and the purge cap  12   a  in the reciprocating direction. 
   Each groove  12   c  extending alongside the corresponding nozzle row is formed to communicate the second end portions of the dents  12   b  corresponding to the nozzle row, with one another. Thus, each small chamber is in communication with a discharge hole  12   e  via the second end portion of the dent  12   b  and the groove  12   c . A depth of the groove  12   c  is larger than that of the dent  12   b , and a cross-sectional area of the groove  12   c  is the largest at the side of the discharge hole  12   e  and gradually decreases toward an end thereof opposite to the discharge hole  12   e . In this specific example, the depth of the groove  12   c  gradually decreases to decrease the cross-sectional area of the groove  12   c . However, the cross-sectional area may be gradually decreased by decreasing a width of the groove. Where the nozzle row is long, it is preferable that the discharge hole  12   e  is formed at both of opposite ends of each groove  12   c  in order to further smoothly discharge ink, and in such an arrangement the groove  12   c  is the deepest at a substantially center of the nozzle row. 
   As shown in  FIG. 2 , the maintenance unit  12  includes a positioning member  22  along which the printhead  11  moves into the purging area S 2  and to the purging position, and a pair of positioning pins  12   d  that are disposed on the contact surface of the purge cap  12   a  so as to precisely position the printhead  11  at the purging position. The positioning member  22  has a base portion  22   a  that is flattened U-shaped in cross section, and a pair of guiding portions  22   b  extending from opposite ends of the base portion  22   a  along the reciprocating direction of the recording device  15 . The purge cap  12   a  is connected to the positioning member  22  to be integrally moved. 
   The front surface of the printhead  11  has a pair of positioning holes  11   b  at positions corresponding to the positioning pins  12   d . As the printhead  11  moves into the purging area S 2  by sliding on the positioning member to more or less push the positioning member  22  in the direction perpendicular to the reciprocating direction, the printhead  11  and the purge cap  12   a  connected to the positioning member  22  are properly positioned relatively to each other in the direction perpendicular to the reciprocating direction, and then the purge cap  12   a  is elevated so that the contact surface of the purge cap  12   a  covers the front surface of the printhead in a predetermined relative positional relationship therebetween while the positioning pins  12   d  are disengageably engaged with the respectively corresponding positioning holes  11   b . While there is established the engaged state where the front surface of the printhead is covered with the purge cap  12   a  at the purging position, the first end portion of each dent  12   b  is communicated with one of the nozzles  11   a , and thus a discrete small chamber into which the nozzle  11   a  is open is formed at the first end portion. This engaged state is shown in  FIGS. 6 and 7 .  FIG. 6  is a plan view illustrating the state where the printhead  11  and the purge cap  12   a  are in engagement, and showing in enlargement a part including two nozzle rows. In  FIG. 6 , edges of the purge cap  12   a  are indicated by alternate long and short dash line, an area corresponding to the recess  23   b  formed in the cover plate  23  is indicated by broken line, and an area at which the contact surface contacts the front surface of the printhead is indicated by hatching.  FIG. 7  is a cross-sectional view taken along line B-B in  FIG. 6 . 
   There are formed a plurality of the grooves  12   c  corresponding to the respective color inks, each of which extends along the nozzle row. Each groove  12   c  is communicated with only dents  12   b  that correspond to nozzles  11   a  for a same color ink, in order to prevent color mixing. As described above, the cross-sectional area of the groove  12   c  gradually increases toward the end thereof on the downstream side, from which the ink is discharged off the purge cap  12   a . Thus, the cross-sectional area gradually increases toward the downstream side of the groove  12   c  with respect to flow of the discharged ink, thereby ensuring smoothness of the ink flow. The end of the groove  12   c  on the downstream side is communicated with the discharge hole  12   e  that is connected to the suction pump. 
   In the above-described arrangement, the recording device  15  first moves in sliding contact with the positioning member  22  to the purging position. In this way, the printhead and the purge cap are roughly positioned relatively to each other so as to ensure that the positioning pins  12   d  can engage with the positioning holes  11   b  when the purge cap  12   a  is elevated. When the purge cap  12   a  is actually elevated thereafter, the positioning holes  11   b  in the printhead  11  and the positioning pins  12   d  on the purge cap  12   a  are disengageably engaged with each other to position the purge cap  12   a  and the printhead  11  in a predetermined positional relationship. In this engaged state, the nozzles  11   a  are communicated with the first end portions of the dents  12   b  and the small discrete chambers into which the respective nozzles  11   a  are open are formed between the purge cap  12   a  and the printhead. 
   As shown in  FIGS. 6 and 7 , when the front surface of the printhead is covered with the purge cap  12   a  at the purging position, voids formed over the contact surface of the purge cap  12   a  and defined inside the dents  12   b  to be opposed to the nozzles  11   a  constitute small chambers into which the respective nozzles  11   a  open. The small chambers are in communication with the discharge hole  12   e  via the second end portions of the dents  12   b  and the groove  12   c . Hence, even when bubbles are generated in an initial phase of a purging operation, the bubbles can be easily and quickly discharged along with the ink since an inner volume of each small chamber is small. Then, the small chamber which is formed under the nozzle  11   a  and from which the bubbles have been discharged is quickly filled with the ink, thereby precluding generation of bubbles thereafter. 
   According to the present embodiment, the purging operation is implemented for each nozzle  11   a  by utilizing the discrete small chambers formed under and around the respective nozzles  11   a . This arrangement reduces a volume of air present around each nozzle  11   a , and is thus advantageous in reducing occurrence of bubbles, compared to a conventional arrangement where a single large chamber is formed for all the nozzles  11   a  of a row and the purging operation is implemented for the nozzles  11   a  of the row all together. 
   When the purge cap  12   a  is separated from the front surface of the printhead, ink adhering to the front surface of the printhead at the vicinity of a nozzle  11   a  is usually drawn into the nozzle  11   a . According to this embodiment, even when such drawing of the ink into the nozzle  11   a  occurs, merely fresh ink remaining around the nozzle  11   a  is drawn into the nozzle  11   a . That is, drawing of a bubble into a nozzle  11   a  which adversely affects the ink ejection characteristic does not occur upon termination of a purging operation. Since the grooves  12   c  as main channels of the ink drain channels are provided separately for the respective color inks, color mixing does not occur, thereby making it unnecessary to implement a flashing operation which would be otherwise necessitated after a purging operation. In some situations, even it is enabled to omit a wiper or a wiping mechanism. Hence, the embodiment is advantageous in reducing the cost. 
   Since the nozzles  11   a  are arranged in the staggered fashion such that each dent  12   b  in the first row extends in the first direction, and each dent  12   b  in the second row adjacent to the first row extends in the second direction opposite to the first direction, a density at which the nozzles  11   a  are arranged can be increased without causing mixing of colors. 
   Although in the above-described embodiment the front surface of the printhead is constituted by the exposed parts of the nozzle surface as well as the external surface of the cover plate  23  in which the through-holes  23   a  are formed, the invention is not limited to this arrangement. That is, the effect of preventing drawing of a bubble into a nozzle  11   a  upon separation of the purge cap  12   a  from the front surface of the printhead, can be obtained even where the cover plate  23  is omitted. When the cover plate  23  is omitted while the nozzle film  21 A is used, the positioning holes  11   b  are formed at respective positions that are outside the nozzle film  21 A or alternatively formed through the nozzle film  21 A. In a case where the nozzles  11   a  are formed through a metallic plate and not through the nozzle film  21 A, the positioning holes  11   b  are formed in the metallic plate. 
   The present embodiment enables to quickly discharge, and replace with ink, bubbles that are generated during a purging operation, as described above. This is achieved by appropriately configuring the contact surface of the purge cap  12   a . More specifically, the small discrete chambers are formed around open ends of the respective nozzles  11   a  when the contact surface contacts the front surface of the printhead. By this, a volume of a space into which each nozzle  11   a  is open is considerably reduced as compared to the conventional arrangement. When the inkjet printer is left, for a long time, in the state where the purge cap  12  is held in contact with the front surface of the printhead  11 , some components of the ink evaporate into the air through the nozzles  11   a . However, in this embodiment, the volume of the space into which each nozzle  11   a  is open is small, and thus a viscosity of the ink virtually does not increase. That is, this embodiment is advantageous in a case where the printhead  11  is not used for a long period of time with the ink stored in the printhead  11 , as well as in terms of a purging operation. 
   There will be now described an inkjet printer according to a second embodiment of the invention, by referring to  FIGS. 8-13 . The parts or elements corresponding to those of the first embodiment will be denoted by the same reference numerals and description thereof is dispensed with. 
   A general structure of a principal portion of the inkjet printer according to the second embodiment is the same as that of the first embodiment, and thus description thereof is omitted. In  FIG. 8 , reference numerals  211  and  212  respectively denote an inkjet printhead and a purging device  212  according to the second embodiment, which differ in configuration from the corresponding elements in the first embodiment. Only the different parts will be described. 
   The printhead  211  is different from that  11  of the first embodiment in a structure of a cavity unit  221 , which will be described by referring to  FIG. 13 .  FIG. 13  is an enlarged fragmentary cross-sectional view of the cavity unit  221  having nozzles  11   a , in which drawing a contour of a purge cap  212   a  line as held in contact with a front surface of the printhead  211  is indicated by chain. 
   The cavity unit  221  is constructed similarly to that  21  of the first embodiment, except the configuration of the cover plate. There will be described the configuration and operation of a cover plate  223  according to the second embodiment, by illustrating how a maintenance work or a purging operation is performed in the second embodiment. 
   The purging device  212  has the purge cap  212   a  that is brought into contact with the front surface of the printhead  211  before a purging operation to suck the ink from the nozzles  11   a  of the printhead  211  is implemented. The purge cap  212   a  is vertically displaceable in a manner similar to that in the first embodiment. 
     FIG. 9  is an enlarged perspective view of a part of the purge cap  212   a , in which two nozzles are included. A structure of the purge cap  212   a  is a characterizing portion of the second embodiment of the invention. The purge cap  212   a  has a contact surface on which are formed a plurality of dents  223   c  into which the respective nozzles  11   a  open, and a plurality of grooves  212   c . Each dent  223   c  includes a branch groove or recessed portion as a connecting portion  223   b , and a through-hole  223   a . That is, at the connecting portion  223   b , a thickness of the cover plate  223  is reduced. When the front surface of the printhead  211  is covered by the purge cap  212   a  at a purging area S 2 , each groove  212   c  extends alongside one of the nozzle rows so as to communicate connecting portions  223   b  of the respective dents  223   c  corresponding to that nozzle row. Each of the grooves  212   c  is disposed to extend under a substantially longitudinal center of the connecting portions  223   b  of the dents  223   c . In  FIG. 9 , there are shown two grooves  212   c  formed correspondingly to two adjacent nozzle rows. When the purge cap  212   a  covers the front surface of the printhead  211 , a part of each dent  223   c  of the two nozzle rows is disposed between the two grooves  212   c . One of opposite ends of each of the grooves  212   c  is in communication with a discharge hole  212   e  through which sucked ink is discharged off the purge cap  212   a.    
   Similarly to the purging device  12  of the first embodiment, the purging device  212  has positioning member  22  and a pair of positioning pins  12   d , and the front surface of the printhead  211  has two positioning holes  11   b  corresponding to the positioning pins  12   d . When the printhead  211  and the purge cap  212   a  are brought into contact, these members  211 ,  212   a  are properly positioned relatively to each other by engagement of the positioning pins  12   d  and the positioning holes  11   b . The engaged state is shown in  FIGS. 12 and 13 , in which  FIG. 12  is a plan view of a principal portion of the printhead  211  and the purge cap  212   a  in engagement, which portion includes two nozzle rows. In  FIG. 12 , the purge cap  212   a  is indicated by chain line, and an area at which the contact surface contacts the front surface of the printhead  211  is indicated by hatching.  FIG. 13  is a cross-sectional view taken along line D-D in  FIG. 12 . As shown in  FIG. 12 , when a closed or contact state where the front surface of the printhead  211  is covered by the purge cap  212   a  is established, a part of the contact surface between the two grooves  212   c  is brought into contact with the cover plate  223 , thereby closing open ends of the nozzles  11   a  of the two nozzle rows, and accordingly the corresponding through-holes  223   a  and the dents  223   c  communicated therewith. Between the purge cap  212   a  and the printhead  211 , there are formed discrete small voids or chambers which are defined by first end portions of the dents  223   c , and into which the nozzles  11   a  open. As shown in  FIG. 13 , a small chamber is partially constituted by each through-hole  223   a . Each small chamber is communicated with a discharge hole  212   e  via one of the grooves  212   c  formed in the purge cap  212   a . Even when the purge cap  212   a  and the front surface of the printhead  211  contact each other with a slight relative positional error in a reciprocating direction of the printhead  211 , the positional relationship between the dents  223   c  and the groove  212   c  reliably ensures that the groove  212   c  is opposed to the dents  223   c  or the nozzles  11   a  (or the through-holes  223   a ). Thus, the present embodiment gives a high degree of freedom with respect to a positional error in the reciprocating direction of the printhead  211 . It is noted that a positional error between the dents  223   c  and the groove  212   c  in a direction perpendicular to the reciprocating direction of the printhead  211  can be accommodated by forming the groove  212   c  to have a length larger than a range across which the dents  223   c  are arranged in a row. 
   A plurality of grooves  212   c  are formed parallel to one another correspondingly to the nozzle rows for the respective color inks. Each groove  212   c  is communicated with the dents  223   c  corresponding to the nozzles  11   a  for a same color ink, in order to prevent color mixing. That is, four grooves  212   c  are provided, and only an ink of a particular color flows in each groove  212   c.    
   Similarly to the groove  12   c  in the first embodiment, each groove  212   c  has a cross-sectional area gradually increasing toward a downstream side with respect to the ink flow, and is communicated with a discharge hole  212   e  at an end thereof on the downstream side. 
   In the inkjet printer constructed as described above, the printhead  211  is guided by the positioning member  22  at the purging position, and then positioning holes  11   b  formed in the printhead  211  disengageably engage the positioning pins  12   d  on the purge cap  212   a , and in this engaged state the front surface of the printhead  211  is covered by and held in contact with the purge cap  212   a . In this contact state, the individual small chambers are formed between the purge cap  212   a  and the printhead  211 , at the first end portions of the dents  223   c.    
   That is, as shown in  FIGS. 12 and 13 , when the front surface of the printhead  211  is covered with the purge cap  212   a  at the purging position, a small chamber into which a nozzle  11   a  opens is defined at the first end portion of each dent  223   c , by a wall surface of the through-hole  223   a  and the contact surface of the purge cap  212   a . The small chamber is communicated with a discharge hole  212   e  via the connecting portion  223   b  and the groove  212   c . With an inner volume of the small chamber being small, even when bubbles are generated in the small chamber in an initial phase of the purging operation, the bubbles are easily and quickly discharged along with the ink. The small chamber into which the nozzle  11   a  opens and from which all the bubbles have been discharged, is filled with ink, and a bubble does not occur thereafter. Thus, in the second embodiment also, a discrete small chamber is formed for each nozzle  11   a  when the purging operation is implemented, so as to reduce a volume of air present around the nozzle  11   a , because this arrangement is considerably advantageous in reducing generation of bubbles. 
   The same effects as the first embodiment can be obtained according to the second embodiment. Namely, when the purge cap  212   a  is separated from the printhead  211 , the ink adhering to the front surface of the printhead  211  at the vicinity of a nozzle  11   a  may be drawn into the nozzle  11   a . However, even when this drawing of ink into the nozzle  11   a  occurs in the printer according to this embodiment, only a fresh ink remaining around an open end of the nozzle  11   a  is drawn into the nozzle  11   a , and drawing of a bubble which may adversely affect the ink ejection characteristic of the printhead  211  does not occur upon termination of the purging operation. Since the grooves  212   c  along which the inks sucked from the nozzles are flown to be discharged off the purge cap  212   a  are provided for the respective color inks, the problem of color mixing is dissolved, thereby omitting the flashing operation which would be otherwise necessary to be implemented after the purging operation. Further, in some situations, a wiper and a wiper mechanism can be omitted, contributing to reduction of the cost. 
   In particular, the nozzles  11   a  are arranged in a staggered fashion, and the connecting portions of two dents  223   c  adjacent in an extending direction of each nozzle row extend from the respective through-holes  223   a  in respective directions opposite to each other, thereby enabling to increase the arrangement density of the nozzles  11   a  without causing color mixing. 
   In the above-described embodiment, the connecting portion  223   b  is formed in the form of a recessed portion in communication with the through-hole  223   a  at one end thereof. However, the connecting portion which connects each through-hole  223   a  with the groove  212   c  may be formed through the thickness of the cover plate  223 , and not a recessed portion formed on the cover plate  223 . 
   There will be now described an inkjet printer according to a third embodiment of the invention, by referring to  FIG. 14 . The inkjet printer of the third embodiment is different from that of the second embodiment in the structure of the printhead. Only the different part will be described, and the parts or elements corresponding to those of the second embodiment will be denoted by the same reference numerals and description thereof is not provided. 
   As seen in  FIG. 14  showing a front surface of the printhead, the printhead of the third embodiment does not include a cover plate covering a nozzle film  321 A. A plurality of dents  321 Aa whose depth is about 5-10 μm are formed on the nozzle film  321 A by irradiating the nozzle film  321 A with an excimer laser beam. 
   By this arrangement, the same effects as the first and second embodiments of the invention can be obtained. 
   There will be now described an inkjet printer according to a fourth embodiment of the invention, by referring to  FIG. 15 . The inkjet printer of the fourth embodiment is different from that of the second embodiment in the structure of the printhead. Only the different part will be described, and the parts or elements corresponding to those of the second embodiment will be denoted by the same reference numerals and description thereof is not provided. 
   In the above-described second embodiment, a dent  223   c  including a through-hole  223   a  and a connecting portion  223   b  is formed through the cover plate  223 , for each nozzle  11   a . However, according to the fourth embodiment shown in  FIG. 15 , a dent  423   c  includes an elongate through-hole  423   a  and a connecting portion  423   b , and the elongate through-hole  423   a  is formed commonly for two adjacent nozzles  11   a , namely, two nozzles are open in a single elongate through-hole  423   a . This arrangement of the fourth embodiment may be employed where the nozzles are arranged in each row at a suitable pitch. To quickly discharge bubbles generated in a small chamber defined by the through-hole  423   a , the connecting portion  423   b  is formed to extend from a longitudinal central portion of the through-hole  423   a  and in a direction to intersect the nozzle row. In this fourth embodiment, similarly to the second embodiment, it is preferable that each connecting portion  423   b  is a groove formed on a cover plate  423 . For instance, the connecting portion  423   b  is formed by etching the cover plate  423  halfway in a direction of its thickness direction. 
   Although not shown, the fourth embodiment may be modified as follows. That is, two adjacent through-holes formed similarly to those in the second embodiment are connected by a groove formed by etching the cover plate  423  halfway in the thickness, so that an inner volume of the small chamber formed when the purging operation is implemented is reduced, contributing to quickly discharging the bubbles. In this arrangement, too, the connecting portion  423   b  is formed to extend from a longitudinal central portion of the groove formed by etching the cover plate halfway in the thickness direction. 
   The effect described above and the other effects obtained by the first embodiment are obtained by the fourth embodiment and its modification also. 
   There will be described an inkjet printer according to a fifth embodiment of the invention, by referring to  FIG. 16 . The fifth embodiment is similar to the fist embodiment, and thus only the different part will be described. The parts or elements corresponding to those of the first embodiment will be denoted by the same reference numerals and description thereof is omitted. 
   In the fifth embodiment, a purge cap  512   a  is formed of a combination of an elastic material  540  and two metal sheets  520 ,  530 . The metal sheets  520 ,  530  are disposed on a surface of the elastic material  540  on a side to be opposed to the printhead. By forming the contact surface with the metal sheet  520 , there is ensured, without increasing the thickness of an upper wall covering branches  512   b , which correspond to the dents  12  in the first embodiment, and grooves  512   c , a sufficient rigidity to assure that the branches  512   b  are not deformed to be closed when the purge cap  512   a  is brought into contact with the printhead, and that when the purge cap  512   a  is separated away from the printhead from a state that the purge cap  512   a  and the printhead are held in contact with each other with the grooves  512   c  and/or branches  512   b  deformed, air bubbles or discharged ink containing air bubbles present in the grooves  512   c  and/or branches  512   b  is not flowed or sucked back into nozzles because of elimination of the deformation of the branches  512   b  and/or the grooves  512   c.    
   As a modification of the fifth embodiment, an entirety of the purge cap  512   a  may be formed of elastic material. 
   The fifth embodiment and its modification also give the same effects as the first embodiment. In addition, according to the fifth embodiment and its modification, the front surface of the printhead is less stained with ink because of the purging operation, as compared to each of the above-described embodiments, since the ink drain channels are exposed at an area smaller than that of each of the above-described embodiments when the purge cap is separated from the printhead.