Abstract:
A head chip has a first face formed with nozzle orifices from which ink is ejected, and a second face formed with at least one ink inlet communicated with the nozzle orifices. An ink passage communicates the nozzle orifices with the ink supply source to supply ink to be ejected. A filter is disposed in the ink passage at an upstream side of the ink inlet. The filter has a first face directed to an upstream side of the ink passage. A cover member covers at least a part of the first face of the filter, the cover member being adapted to allow ink in the ink passage to pass through while preventing air bubbles contained in the ink from coming in contact with the first face of the filter.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an ink jet head unit and a printer incorporating the same. 
     Japanese Patent Publication No. 3-224744A discloses an ink jet head unit (hereinafter, simply referred to as a head unit) provided with a damping chamber for absorbing ink pressure which is generated due to acceleration or the like while a carriage moves to carry the head unit. 
     The head unit is further provided with a filter disposed in an ink passage for preventing invaded foreign matter from flowing to the downstream side of the passage. 
     In such a head unit, air bubbles are accumulated in the damping chamber as a result of various causes. Such air bubbles tend to stay in a stagnant point in the ink passage, at which flow rate of the ink is zero or close to zero (e.g., in the vicinity of the filter where the area of the ink passage needs to be wider). In such a position, the air bubbles tend to grow larger. 
     In a case where the air bubbles which have grown under the high-temperature environment, for example, adhere onto the upstream side surface of the filter, the ink passage in the filter is clogged leading to a printing failure. 
     Further, an air bubble remaining in the damping chamber may be broken up into smaller bubbles due to vibrations or the like during the operation of the carriage. Such smaller bubbles may flow back into the ink passage of the upstream side of the head unit. In such a case, there is a possibility that the air bubbles may grow under the high temperature environment and enter the damping chamber again. As a result, the air bubbles may adhere onto the filter leading to the same problem as described above. 
     Japanese Patent Publication No. 9-300654A discloses a priming operation for discharging air bubbles with ink by forcibly sucking or compressing ink in the ink passage, thereby removing air bubbles in the ink passage. In order to enhance the reliability of the priming operation, the air bubbles to be discharged are passed through narrowed passages and broken up to smaller bubbles, thereby reducing discharging resistance.  FIGS. 12 to 15  show such a structure. 
     As shown in  FIG. 12 , an ink supply case  200  is formed with an ink supply pipe  201  and an opening portion (ink supply port)  205  which connects the ink supply pipe  201  and an ink inlet  203  of a head chip  202 . A nozzle section  204  in which a plurality of nozzle orifices are arrayed with a fixed pitch is provided on an end of the head chip  202  which is opposite to the end face  208  ( FIG. 15 ) formed with the ink inlet  203 . 
     As shown in  FIGS. 13 and 15 , a plurality of thin walls  211  are arrayed at the upstream side of the ink supply port  205 . Each of the thin walls  211  extends perpendicularly to the extending direction of the ink supply port  205  (the ink inlet  203 ). The downstream side end of each thin wall  211  is placed so as to maintain a prescribed distance d from the end face of the head chip  202  having the ink inlet  203 . Accordingly, a plurality of narrowed passages  212  each having a width e is formed at the upstream side of the boundary (end face  208 ) between the ink inlet  203  and the ink supply port  205 . 
     As indicated by dashed lines in  FIG. 14 , the ink inlet  203  is an elongated rectangle having a width K and a height H 2  which is considerably smaller than a height H 1  of the ink supply port  205 . Therefore, air bubbles tend to stay at the boundary  208 . 
     In a case where air bubbles exist in the ink supply pipe  201 , the air bubbles are moved toward the head chip  202  by the priming operation and broken up by the narrowed passages  212 . 
     This publication, however, is silent about countermeasures for solving the above described problems (i.e., the case where the broken bubbles adhere onto the filter or flow back to the upstream side of the ink passage). 
     In addition, high machining accuracy and assembling accuracy are required for forming the narrowed passages  212  in the vicinity of the ink supply port  205  of the ink supply case  200  and for assembling the head chip  202  while maintaining the above-described distance d, resulting in a higher cost. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide an ink jet head unit capable of effectively eliminating the undesired influence due to air bubbles remaining in the damping chamber. 
     It is also an object of the invention to provide a printer incorporating such an ink jet head unit. 
     In order to achieve the above objects, according to the invention, there is provided an ink jet head unit connected to an ink supply source, comprising: 
     a head chip, having a first face formed with nozzle orifices from which ink is ejected, and a second face formed with at least one ink inlet communicated with the nozzle orifices; 
     an ink passage communicating the nozzle orifices with the ink supply source to supply ink to be ejected; 
     a filter, disposed in the ink passage at an upstream side of the ink inlet, the filter having a first face directed to an upstream side of the ink passage; and 
     a cover member, which covers at least a part of the first face of the filter, the cover member being constructed to allow ink in the ink passage to pass through while preventing air bubbles contained in the ink from coming in contact with the first face of the filter. 
     With this configuration, it is possible to prevent an air bubble remaining in the ink passage from adhering to the first face of the filter. Accordingly, it is possible to prevent the filter from being clogged, thereby eliminating printing failure. 
     Preferably, a damping chamber forming member is fixed to the second face of the head chip, the damping chamber forming member having at least one damping chamber for dampening pressure fluctuation occurred therein. The filter and the cover member are disposed between the ink inlet and the damping chamber. 
     Here, it is preferable that the cover member is formed with at least one opening communicating the damping chamber and the first face of the filter. 
     It is further preferable that the opening has a size which prevents an air bubble having a diameter of 1.5 mm from entering thereinto. 
     The at least one opening may at least one elongated slit. 
     Preferably, an ink supply pipe supplies ink from the ink supply source to the damping chamber. An end portion of the ink supply pipe is disposed within the damping chamber. A seal member surrounds the end portion of the ink supply pipe, thereby defining a space for storing ink therein. The end portion of the ink supply pipe is sealable with ink filling the space. 
     With this configuration, it is possible to prevent an air bubble from flowing back to the ink supply pipe, thereby preventing the flowed-back and enlarged air bubble from entering the damping chamber again. 
     Here, it is preferable that the cover member and the seal member are monolithically formed. 
     Alternatively, the seal member is monolithically formed with the damping chamber forming member. 
     Preferably, the seal member comprises an annular wall radially spaced from and surrounding the end portion of the ink supply pipe. 
     According to the invention, there is also provided an ink jet printer, comprising: 
     an ink supply source, which stores ink therein; 
     a head chip, having a first face formed with nozzle orifices from which ink is ejected, and a second face formed with at least one ink inlet communicated with the nozzle orifices; 
     an ink passage communicating the nozzle orifices with the ink supply source to supply ink to be ejected; 
     a filter, disposed in the ink passage at an upstream side of the ink inlet, the filter having a first face directed to an upstream side of the ink passage; and 
     a cover member, which covers at least a part of the first face of the filter, the cover member being constructed to allow ink in the ink passage to pass through while preventing air bubbles contained in the ink from coming in contact with the first face of the filter. 
     According to another arrangement of the invention, there is provided an ink jet head unit connected to an ink supply source, comprising: 
     a head chip, having a first face formed with nozzle orifices from which ink is ejected, and a second face formed with at least one ink inlet communicated with the nozzle orifices; 
     an ink passage communicating the nozzle orifices with the ink supply source to supply ink to be ejected; 
     a filter, disposed in the ink passage at an upstream side of the ink inlet; 
     an ink supply pipe which supplies ink from the ink supply source to the damping chamber, an end portion of the ink supply pipe being disposed within the damping chamber; and 
     a seal member surrounding the end portion of the ink supply pipe, defining a space for storing ink therein, 
     wherein the end portion of the ink supply pipe is sealable with ink filling the space. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein: 
         FIG. 1  is a section view of an ink jet head unit according to a first embodiment of the invention; 
         FIG. 2  is a schematic view of an ink jet printer incorporating the head unit of  FIG. 1 ; 
         FIG. 3  is a perspective view of the disassembled state of the head unit viewed from the front side thereof; 
         FIG. 4  is a perspective view of the disassembled state of the head unit viewed from the rear side thereof; 
         FIG. 5  is a perspective view of a filter cover incorporated in the head unit of  FIG. 1 ; 
         FIG. 6  is an enlarged section view showing an essential portion of the head unit of  FIG. 1 ; 
         FIG. 7  is a partial perspective view of a filter cover according to a second embodiment of the invention; 
         FIG. 8  is a partial perspective view of a filter cover according to a third embodiment of the invention; 
         FIG. 9  is a partial perspective view of a filter cover according to a fourth embodiment of the invention; 
         FIG. 10  is an enlarged section view showing an essential portion of a head unit according to a fifth embodiment of the invention; 
         FIG. 11  is an enlarged section view showing an essential portion of a head unit according to a sixth embodiment of the invention; 
         FIG. 12  is a perspective view of a disassembled state of a conventional ink jet head unit; 
         FIG. 13  is a perspective view of narrowed passages provided in the head unit of  FIG. 12 ; 
         FIG. 14  is a diagram showing an ink passage in the head unit, viewed from the direction of an arrow XIV; and 
         FIG. 15  is a diagram showing the ink passage, viewed from the direction of an arrow XV. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, a description will be given of one preferred embodiment of a head unit for an ink jet printer. 
     As shown in  FIGS. 1 and 2 , an ink jet printer  1  in this embodiment is of a serial type, and an ink jet head unit  4  (hereinafter, simply referred as a head unit) is mounted on a carriage  3  capable of reciprocating along a guide shaft  2 . Ink is supplied to this head unit  4  through a flexible ink tube  6  from an ink tank  5  disposed in a predetermined position. In this embodiment, ink of four colors including cyan, magenta, yellow, and black is supplied from ink tanks  5 - 1  to  5 - 4 , in which the ink is respectively stored, to the head unit  4  through four ink tubes  6 - 1  to  6 - 4 . 
     The head unit  4  will be explained in detail with reference to  FIGS. 1 to 4 . The head unit  4  of this embodiment has a unit cover  11  whose rear face side is open and which has the shape of a rectangular parallelepiped, and a head-unit assembly is accommodated in this unit cover  11 . 
     A head chip  12 , a unit base  13 , a damper film  14  made of rubber, a damper holder  15 , and a relay board  16  are superposed one on top of another in the unit cover  11  in that order from its front face side. Four damping chambers  21 ( 1 ) to  21 ( 4 ) are defined by the unit base  13  and the damper film  14 . The unit cover  11 , the unit base  13 , and the damper holder  15  are formed of, for example, resin moldings. 
     Front end portions  22   a ( 1 ) to  22   a ( 4 ) of ink supply pipes  22 ( 1 ) to  22 ( 4 ), which are formed integrally with the damper holder  15 , communicate with the respective damping chambers  21 ( 1 ) to  21 ( 4 ), while the respective ink tubes  6 ( 1 ) to  6 ( 4 ) are connected to rear end portions  22   b ( 1 ) to  22   b ( 4 ) of the ink supply pipes  22 ( 1 ) to  22 ( 4 ). The respective damping chambers  21 ( 1 ) to  21 ( 4 ) are formed in the head chip  12  so as to communicate with four ink inlets  24 ( 1 ) to  24 ( 4 ) via ink supply ports  23 ( 1 ) to  23 ( 4 ). 
     Accordingly, the ink from the ink tanks  5 ( 1 ) to  5 ( 4 ) is supplied to the respective damping chambers  21 ( 1 ) to  21 ( 4 ) via the ink tubes  6 ( 1 ) to  6 ( 4 ) and the ink supply pipes  22 ( 1 ) to  22 ( 4 ), and is further supplied from the damping chambers  21 ( 1 ) to  21 ( 4 ) to nozzle orifice groups of the respective colors via the ink supply ports  23 ( 1 ) to  23 ( 4 ) and the ink inlets  24 ( 1 ) to  24 ( 4 ). 
     The head chip  12  has the shape of a flat rectangular parallelepiped, and its front face  12   a  is the nozzle formation face, where nozzle orifice rows (not shown) for discharging ink of the respective colors are formed. This nozzle formation face  12   a  is exposed from a front face opening  11   a  of the unit cover  11 . In addition, flexible wiring boards  25 ( 1 ) and  25 ( 2 ) for feeding electric power are adjacent side faces of the head chip  12  and are led out to the rear side along inner sides of the side faces of the unit cover  11 , and are connected to the rear face of the relay board  16 . Head-chip driving ICs  26 ( 1 ) and  26 ( 2 ) are attached to those portions of the flexible wiring boards  25 ( 1 ) and  25 ( 2 ) which are opposed to the respective side faces of the unit cover  11 . 
     The unit base  13  disposed on the rear side of the head chip  12  includes a front wail  31  to which the head chip  12  is bonded and fixed, side walls  32  extending orthogonally from the four edges of the front wall  31  in the rearward direction, and a vertical partition wall  33  and a horizontal partition wall  34  which partition the space defined by the rear face of the front wall  31  and the side walls  32  into a crisscross form. 
     As shown in  FIG. 3 , a recessed portion  31   a  to which the head chip  12  is to be fitted is formed in the front face of the front wall  31 , bonding grooves  31   b  for filling an adhesive are formed in bottom face of the recessed portion  31   a , and upper end portions of these bonding grooves  31   b  extend to the upper faces of the side walls  32 . The head chip  12  is fitted to the recessed portion  31   a  and is bonded to the front wall  31  by the adhesive filled in the bonding grooves  31   b.    
     Four recessed portions  35 ( 1 ) to  35 ( 4 ) to be damping chambers are formed in a rear portion of the unit base  13  by the vertical partition wall  33  and the horizontal partition wall  34 . These recessed portions are substantially of identical shape, and the ink supply ports  23 ( 1 ) to  23 ( 4 ) formed in the front wall  31  are open to bottom faces  35   a  of the respective recessed portions  35 ( 1 ) to  35 ( 4 ). 
     In addition, filters  36  are thermally deposited in such a manner as to cover the bottom faces  35   a , so that ink is supplied from the damping chambers  21 ( 1 ) to  21 ( 4 ) to the head chip  12  while passing through the filters  36 . In a bottom face  35   a  of each of the recessed portions  35 ( 1 ) to  35 ( 4 ), a dent  35   b  is formed so as to define a minute passage between the bottom face  35   a  and the filter  36 . 
     An unfixed filter cover  100  is disposed in each of the damping chambers  21 ( 1 ) to  21 ( 4 ). The filter cover  100  comprises a plate-shaped portion  110  which covers at least a part of the upstream side face of the filter  36 , and a cup-shaped portion  120  adapted to surround a front end portion of an associated one of the ink supply pipes  22 ( 1 ) to  22 ( 4 ) inserted into the damping chambers  21 ( 1 ) to  21 ( 4 ). The filter cover  100  is molded with resin so that the plate-shaped portion  110  and the cup-shaped portion  120  are monolithically formed. 
     As shown in  FIGS. 5 and 6 , the plate-shaped portion  110  is formed with a plurality of circular holes  111  respectively communicating between the upstream side face of each of the filters  36  and the damping chambers  21 ( 1 ) to  21 ( 4 ). These holes  111  are formed to a size, which can prevent a large air bubble G, which is generated in the damping chamber  21 ( 1 ) to  21 ( 4 ), from coming into direct contact with the upstream side face of the filter  36 . In this embodiment, the holes  111  are formed 0.5 mm in diameter. However, the diameter may fall within a range from 0.1 mm to 1.0 mm because a smaller air bubble G has a diameter approximately 1.5 mm. The minimum value of the range depends on the machining technique. 
     The cup-shaped portion  120  is constituted by a bottom wall  122  and an annular wall  121  upright from the bottom wall  122  so as to have an inner diameter slightly larger than an outer diameter of each of the front end portions  22   a ( 1 ) to  22   a ( 4 ) of the ink supply pipes  22 ( 1 ) to  22 ( 4 ). Accordingly, a gap to be a part of the ink passage having a width of 0.2 to 1.0 mm is defined therebetween. The height of the annular wall  121  is determined such that each of the front end portions  22   a ( 1 ) to  22   a ( 4 ) of the ink supply pipes  22 ( 1 ) to  22 ( 4 ) stays within the cup-shaped space even when the bottom face of the filter cover  110  comes in contact with the upstream side face of the filter  36 . 
     Next, the damper film  14  is fixed by being bonded to the rear face of the unit base  13 , i.e., rear end faces of the side walls  32 , the vertical partition wall  33 , and the horizontal partition wall  34 . Further, the damper film  14  is pressed against the unit base  13  by the damper holder  15  attached to the rear face side of the damper film  14 . 
     As for the damper film  14 , its portions  14 ( 1 ) to  14 ( 4 ) opposing the respective recessed portions  35 ( 1 ) to  35 ( 4 ) are thin-walled and are deflectable in outward directions of their faces. The recessed portions  35 ( 1 ) to  35 ( 4 ) are sealed by the thin-walled portions  14 ( 1 ) to  14 ( 4 ) of the damper film  14  to form the damping chambers  21 ( 1 ) to  21 ( 4 ). 
     The damper holder  15  has a rear wall  41 , side walls  42  extending orthogonally from its outer edges in the forward direction, and a vertical partition wall and a horizontal partition wall (neither are shown) which partition the space surrounded by a front face of the rear wall  41  and the side walls  42  into a crisscross form. Four recessed portions  45 ( 1 ) to  45 ( 4 ) opposing the damping chambers  21 ( 1 ) to  21 ( 4 ) are thereby formed. The recessed portions  45 ( 1 ) to  45 ( 4 ) communicate with the atmosphere through ventilation holes  46  formed in the rear wall  41 . The thin-walled portions  14 ( 1 ) to  14 ( 4 ) of the damper film  14  are freely deflectable in outward directions of their faces by the recessed portions  45 ( 1 ) to  45 ( 4 ). 
     In the damper holder  15  in this embodiment, the four ink supply pipes  22 ( 1 ) to  22 ( 4 ) are formed integrally with its portion  47  where the vertical and horizontal partition plates cross, and the damper holder  15  is formed of, for example, a resin molding. The rear-end portions  22   b ( 1 ) to  22   b ( 4 ) of the ink supply pipes  22 ( 1 ) to  22 ( 4 ) project to the rear side by passing through holes  51 ( 1 ) to  51 ( 4 ) formed in the relay board  16 , and are connected to the ink tubes  6 ( 1 ) to  6 ( 4 ). In contrast, the tip portions  22   a ( 1 ) to  22   a ( 4 ) of the ink supply pipes  22 ( 1 ) to  22 ( 4 ) are passed through insertion holes  52 ( 1 ) to  52 ( 4 ) formed in the damper film  14 , and project into the respective damping chambers  21 ( 1 ) to  21 ( 4 ). The insertion holes  52 ( 1 ) to  52 ( 4 ) project into the respective damping chambers  21 ( 1 ) to  21 ( 4 ) and have predetermined lengths, and their inside diameters are formed to be slightly smaller than the outside diameters of the front end portions  22   a ( 1 ) to  22   a ( 4 ) of the ink supply pipes  22 ( 1 ) to  22 ( 4 ). 
     Accordingly, the portions of the damper film  14  where the insertion holes  52 ( 1 ) are formed are in close contact with outer peripheral faces of the front end portions  22   a ( 1 ) to  22   a ( 4 ) by the resilient restoring force of the damper film  14  itself, so that the faces of contact between the damper film  14  and the front end portions  22   a ( 1 ) to  22   a ( 4 ) are in a completely sealed state. 
     The outer diameter of each of the front end portions  22   a ( 1 ) to  22   a ( 4 ) of the ink supply pipes  22 ( 1 ) to  22 ( 4 ) are narrowed so that the large gap is secured between the front end portion and the annular wall  121 , thereby reducing ink flow resistance. 
     The thus constructed head unit  4  performs ink ejection while moving in the directions indicated by arrows A in  FIGS. 1 and 2 . The fluctuation of pressure applied to the ink inside the ink tube  6  is absorbed or alleviated by the four damping chambers  21 ( 1 ) to  21 ( 4 ) provided in the head unit  4 . 
     Since ink is supplied from the damping chambers  21 ( 1 ) to  21 ( 4 ) to the head chip  12  through the filter  36 , it is possible to reliably capture foreign matter flown from the upstream side with the filter  36 , preventing clogging in the nozzles. 
     As shown in  FIG. 6 , for some reason, air bubble G sometimes remains in the damping chamber  21 ( 1 ) to  21 ( 4 ) and is enlarged. However, since the plate-shaped portion  110  of the filter cover  100  covers the upstream side face of the filter  36 , it is possible to prevent the remaining air bubble G from adhering to the upstream side face of the filter  36 . 
     Accordingly, it is possible to prevent the large remaining air bubble G from coming into close contact with the upstream side face of the filter  36  blocking the ink passage. Thus, printing failure due to a clogged ink passage in the filter  36  can be prevented. 
     Since the plate-shaped portion  110  is formed with many holes  111 , it does not interfere with the ink flow. Furthermore, during the priming operation, the air bubble can be moved to the filter  36  side through the holes  111 . Thus, the air bubble can be purged easily. 
     According to the cup-shaped portion  120  formed in the filter cover  100 , the front end portions  22   a ( 1 ) to  22   a ( 4 ) of the respective ink supply tubes  22 ( 1 ) to  22 ( 4 ) can be sealed with liquid. Hence, it is possible to prevent the air bubble G from flowing back from the damping chamber  21 ( 1 ) to  21 ( 4 ) to the ink supply pipe  22 ( 1 ) to  22 ( 4 ). As a result, it is possible to prevent the air bubble, which has been flowed-back and enlarged, from entering into the head unit again. 
     The configuration, number and the shape of the holes  111  formed in the plate-shaped portion  110  of the filter cover  100  may be modified. For example, as a plate-shaped portion  110 B shown in  FIG. 7 , rectangular holes  111   b  may be formed. Alternatively, as a plate-shaped portion  110 C shown in  FIG. 8 , elongated slits  111   c  may be formed. Even in this case, it is necessary to set the length and width of the holes  111   b , and  111   c  to a size such that a large air bubble does not come into contact with the filter  36  in order to attain the same effect as that of the holes  111  in  FIG. 5 . In  FIG. 7 , holes  111   b  may be formed into rectangular holes in which each of the sides has a length of 0.1 to 1.0 mm. In  FIG. 8 , each of the elongated slits  111   c  has a width of 0.1 to 1.0 mm. The minimum value of the size depends on the machining technique the same as the case of the above-described holes  111 . 
     The holes do not always have to penetrate the plate-shaped portion  110  in the thickness direction thereof. For example, in the plate-shaped portion  110 D shown in  FIG. 9 , the ink is allowed to flow in from a side end face, and the ink flows out from the bottom face opposing the upstream side face of the filter  36  may be adopted. 
     Here, horizontal holes  111   d - 1  opened at the side end faces are laterally extended within the plate-shaped portion  110 D, and a large number of vertical holes  111   d - 2  are formed in the bottom face of the plate-shaped portion  110 D so as to communicate with the respective horizontal holes  111   d - 1 . Accordingly, there are formed holes  111   d  constituting ink passages continued from the side faces to the bottom face of the plate-shaped portion  110 D. 
     In this case, even when the upper face of the plate-shaped portion  110 D is covered with an air bubble, since the air bubble does not reach the side end faces, blockage of the ink passage can be prevented. As such, holes where the large air bubble cannot pass through but only the ink can pass therethrough may be formed. 
     Since the filter cover  100  disposed in each of the damping chambers  21 ( 1 ) to  21 ( 4 ) is not fixed, a slight gap may be formed between the upstream side face of the filter  36  and the bottom face of the filter cover  100 . Accordingly, a plate-shaped portion  132  of a filter cover  130  as shown in  FIG. 10 , may be provided without holes. 
     In this case, ink flows through the space between the filter  36  and the plate-shaped portion  132 , or an area equivalent to the thickness of the filter  36  of the end face of the filter  36 . Accordingly, the ink flows to the ink supply port  23 ( 1 ) to  23 ( 4 ) through the filter  36 . 
     Further, in this embodiment, the plate-shaped portion  110  and the cup-shaped portion  120  are not monolithically formed. That is, the cup-shaped portion  120  may be embodied by an annular wall  121  directly protruded from the front plate portion  31  of the unit base  13 . 
     Alternatively, as shown in  FIG. 11 , the plate-shaped portion  110  and the cup-shaped portion  120  may be provided separately while defining a clearance having a width c. This width c is the same size as the diameter of the holes  111  formed in the plate-shaped portion  110 , thereby preventing an air bubble from passing through to the filter  36  side. 
     Even when only one of the plate-shaped portion  110  and the cup-shaped portion  120  is formed, it serves to eliminate the problem of the remaining air bubble. For example, when only the plate-shaped portion  110  is formed, the problem of the remaining air bubble adhering to the filter  36  can be solved. When only the cup-shaped portion  120  is formed, the problem of the back-flow of the air bubble into the ink supply pipe  22 ( 1 ) to  22 ( 4 ) can be solved. 
     Although the above-described embodiment concerns the head unit mounted in a serial-type ink jet printer, the invention is similarly applicable to the head unit for another type of ink jet printer. 
     Although the above-described embodiment concerns the head unit mounted in the ink jet printer performing color printing in four colors, the invention is similarly applicable to the head unit for an ink jet printer performing single color printing or printing with a plurality of colors other than the four colors. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.