Patent Publication Number: US-8991980-B2

Title: Ink jet print head

Description:
This application is a divisional of U.S. patent application Ser. No. 12/684,415, filed Jan. 8, 2010. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink jet print head, and in particular, to an ink jet print head in which a print element configured to generate thermal energy required to eject ink and a driving circuit configured to drive the print element s are formed on the same board. 
     The present invention is applicable not only to general print apparatuses but also apparatuses such as copiers, facsimile machines, and word processors as well as industrial print apparatuses combined with various processing apparatuses. 
     2. Description of the Related Art 
     An ink jet print apparatus is configured to print information on a print medium in response to a print signal by allowing a print head to eject ink through a plurality of fine nozzles. The ink jet print apparatus advantageously enables high-speed printing and offers high resolution and high image quality, while allowing a reduction in noise. The ink jet print apparatus is thus commonly used. 
     Some print heads used in ink jet print apparatuses are of an ink jet type configured to utilize thermal energy for printing. Such a print head allows print elements to be energized to heat ink to generate bubbles. Thus, pressure resulting from the generation of the bubbles is utilized to eject the ink through ejection ports for printing. Furthermore, the ink ejected through the ejection ports flies perpendicularly to the principal surface of a print element board. The ink thus impacts a print medium at a desired position. As a result, high-quality and high-grade printing is achieved. 
     However, if the ejection ports are inclined to the principal surface of the print element board or ink channels are shaped asymmetrically with respect to a corresponding pressure chamber, energy applied to the ink by the pressure resulting from the generation of bubbles is also asymmetric with respect to the pressure chamber. The asymmetry may cause the ejection direction of the ink to be inclined to the direction perpendicular to the principal surface of the print element board. Thus, the ink may impact the print medium at a position different from the desired one, thus lowering the print grade. 
     Thus, for the proper print grade, the ejection direction of the ink needs to be perpendicular to the principal surface of the print element board. In this case, the inclination of the ejection ports and the shape of the ink channels are important. Various methods have been proposed which are intended to reduce the inclination of the ejection direction of the ink to the direction perpendicular to the principal surface of the print element board. 
     Japanese Patent Laid-Open No. 2001-341309 describes that a print element in a recess portion is shaped rotationally symmetrically with respect to the center line of each ejection port, thus preventing ejected ink from flying in an inclined direction. 
     Furthermore, Japanese Patent Laid-Open No. 2008-162270 discloses a print head in which two channels are formed symmetrically with respect to each ejection port. 
     However, Japanese Patent Laid-Open Nos. 2001-341309 and 2008-162270 fail to refer to a phenomenon in which a step that may be formed on the bottom surface of the ink channel may disrupt the symmetry, causing the ejected ink to fly in an inclined direction. The present inventors have newly found that not only the symmetry of the channels but also a step of height several μm resulting from wires formed at the channel may affect the ejection direction. 
     A print element is provided in the pressure chamber and requires wires for energization. The wires connected to the print element normally include an individual wire and a common wire. Furthermore, to allow a reduction in wire installation area, the individual and common wires may be provided separately in the same layer, in a stack board, as that of the print element and in an underlying layer. When such a wiring layer is provided under the ink channel, a step structure is created on the inner bottom surface of the ink channel, that is, the surface of the board. If the step structure is present only in one of the ink channels arranged on the respective opposite sides of the pressure chamber, then the bottom surfaces of the ink channels are asymmetric with respect to the pressure chamber. The asymmetric structure of the ink channels may result in a difference in flow resistance between the ink channels. In this case, during ejection, pressure is generated in a biased manner. As a result, the ejected ink is inclined to the direction perpendicular to the principal surface of the print element board. Consequently, the ink may impact the print medium at an incorrect position or images may be unevenly formed. 
     SUMMARY OF THE INVENTION 
     Thus, an object of the present invention is to provide an ink jet print head configured to reduce the inclination of an ink ejection direction to make improper print conditions such as stripes and density unevenness unnoticeable. 
     In a first aspect of the present invention, an ink jet print head comprising a pressure chamber including a print element configured to heat ink to generate bubbles, two ink channels formed symmetrically with respect to the pressure chamber so that the ink is allowed to flow into the pressure chamber, and a plurality of wires arranged under a bottom portion of each of the ink channels, wherein the same flow resistance is set for the two ink channels. 
     In a second aspect of the present invention, a liquid ejection head comprising: 
     a pressure chamber having energy generation device that generates energy used to eject liquid; 
     two channels provided opposite each other across the pressure chamber so that the liquid is allowed to flow into the pressure chamber; 
     a wire provided under the bottom surface of one of the two channels, the wire for connecting electrically the energy generation device; and 
     a dummy wire arranged under the bottom surface of the other channel of the two channels. 
     The ink jet print head according to the present invention allows substantially the same flow resistance to be set for the two ink channels connected to the pressure chamber. Thus, the ink jet print head provided by the present invention enables a reduction in the inclination of the ink ejection direction to make improper print conditions such as stripes and density unevenness unnoticeable. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the appearance of a mechanism portion of an ink jet print apparatus according to a first embodiment; 
         FIG. 2  is a diagram showing the appearance of a head cartridge used in the ink jet print apparatus according to a first embodiment; 
         FIG. 3  is a diagram showing the appearance of an ink jet print head in the head cartridge; 
         FIG. 4  is a partly-sectional schematic perspective view showing a print head applicable to the present invention; 
         FIG. 5  is an enlarged diagram showing a part of the print head according to a first embodiment; 
         FIG. 6  is an enlarged diagram showing ink supply ports and channel walls in the print head according to the first embodiment; 
         FIG. 7  is a diagram showing wires connected to print elements in the print head according to the first embodiment; 
         FIG. 8  is a diagram showing wires connected to print elements in the print head according to the first embodiment; 
         FIG. 9  is a diagram showing individual wires in a lower wiring layer as a comparative example; 
         FIG. 10  is a diagram overlappingly showing a common wire and an individual wire in the comparative example; 
         FIG. 11  is a sectional view of an ink channel portion of the even-number-th pressure chamber from the end of a print element array, the sectional view being taken along line XI-XI in  FIG. 10 ; 
         FIG. 12  is a sectional view of an ink channel portion of the odd-number-th pressure chamber from the end of the print element array, the sectional view being taken along line XII-XII in  FIG. 10 ; 
         FIG. 13  is a diagram overlappingly showing a common wire and an individual wire in the first embodiment; 
         FIG. 14  is a sectional view taken along line XIV-XIV in  FIG. 13 ; 
         FIG. 15  is a diagram showing wires connected to print elements in a print head according to a second embodiment; 
         FIG. 16  is a diagram overlappingly showing the common wire and an individual wire according to the present embodiment; 
         FIG. 17  is a sectional view taken along line XVII-XVII in  FIG. 16 ; and 
         FIG. 18  is a sectional view of a portion of a print head according to a third embodiment which corresponds to a pressure chamber. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     A first embodiment of the present invention will be described below with reference to the drawings. 
       FIG. 1  is a diagram showing the appearance of a mechanism portion of an ink jet print apparatus to which an ink jet print head according to the present embodiment is applicable.  FIG. 2  is a diagram showing the appearance of a head cartridge used in the ink jet print apparatus in  FIG. 1 . Moreover,  FIG. 3  is a diagram showing the appearance of an ink jet print head in the head cartridge. A chassis  10  of the ink jet print apparatus according to the present embodiment comprises a plurality of plate-like metal members with a predetermined rigidity. The chassis  10  forms the framework of the ink jet print apparatus. The chassis  10  includes a medium feeding section  11  configured to feed a sheet-like print medium (not shown in the drawings) to the interior of the ink jet print apparatus. The chassis  10  further includes a medium conveying section  13  configured to guide the print medium fed from the medium feeding section  11  to a desired print position and from the print position to a medium discharge section  12 , a print section configured to perform a predetermined printing operation on the print medium, and a head recovery section  14  configured to execute a recovery process on the print section. 
     The print section comprises a carriage  16  supported so as to be movable along a carriage shaft  15  for scanning, and a head cartridge  18  mounted in the carriage  16  so as to be removable via a head set lever  17 . 
     The carriage  16  in which the head cartridge  18  is mounted includes a carriage cover  20  configured to position an ink jet print head (hereinafter also simply referred to as a print head)  19  at a predetermined installation position on the carriage  16 . The carriage  16  further includes the head set lever  17  configured to engage with a tank holder  21  in the print head  19  to press and place the print head  19  at the predetermined installation position. The head set lever  17 , serving as removal means according to the present invention, is pivotally movable with respect to a head set lever shaft (not shown in the drawings) located at the top of the carriage  16 . Furthermore, a spring-loaded head set plate (not shown in the drawings) is provided in a portion of the carriage  16  configured to engage with the print head  19 . The spring force of the head set plate allows the print head  19  to be pressed and installed in the carriage  16 . 
     One end of a contact flexible print cable (hereinafter also referred to as a contact FPC)  22  is coupled to another portion of the carriage  16  configured to engage with the print head  19 . A contact portion (not shown in the drawings) formed at this end of the contact FPC  22  electrically contacts a contact portion  23  provided in the print head  19  and serving as an external-signal input signal. This allows the transmission of various pieces of information for printing, the supply of power to the print head  19 , and the like. 
     An elastic member such as rubber (not shown in the drawings) is provided between the contact portion of the contact FPC  22  and the carriage  16 . The elastic force of the elastic member and the pressing force of the head set plate enable the contact portion of the contact FPC  22  to reliably contact the contact portion  23  of the print head  19 . The other end of the contact FPC  22  is connected to a carriage board (not shown in the drawings) mounted on the rear surface of the carriage  16 . 
     The head cartridge  18  according to the resent embodiment includes an ink tank  24  in which ink is stored, and the above-described print head  19  configured to eject ink fed from the ink tank  24 , through ejection ports in accordance with print information. The print head  19  according to the present embodiment is of what is called a cartridge type in which the print head  19  is removably mounted in the carriage  16 . 
     Furthermore, the present embodiment allows six independent ink tanks  24  for the respective ink colors, that is, black (Bk), pale cyan (c), pale magenta (m), cyan (C), magenta (M), and yellow (Y) to be used for the apparatus in order to enable photographic high-quality color printing. Each of the ink tanks  24  includes an elastically deformable removal lever  26  that can be locked on the head cartridge  18 . Operation of the removal lever  25  enables the ink tank  24  to be removed from the print head  19  as shown in  FIG. 3 . Thus, the removal lever  26  functions as a part of removal means according to the present invention. The print head  19  comprises a print element board, an electric wiring board  28 , and the above-described tank holder  21 . The print element board is electrically connected to the electric wiring board  28  via contacts at square slots  25  formed in the electric wiring board  28 . 
       FIG. 4  is a partly-sectional schematic perspective view showing a print head applicable to the present invention. In the print head according to the present embodiment, a plurality of ink channels  44  and a plurality of ink supply ports  41  are arranged on a print element board  48  for a pressure chamber  40 . Moreover, a row of pressure chambers  40  are formed on the print element board  48  by channel walls  46  forming ink channels  44 . Print elements  45  are provided in the respective pressure chambers  40  so as to form a row of print elements. The ink supply ports  41  are arranged along the direction in which the print elements  45  are arranged. Each of the print elements  45  generates heat during printing to heat the ink, thus allowing pressure to be exerted. Consequently, ink can be ejected through ejection ports  42 . 
     The print element board  48  in the ink jet print head according to the present embodiment is a stack board comprising an oxide film provided on a silicon substrate, a lower wiring layer, an insulating layer, print elements  45 , an upper wiring layer, and an insulating layer provided on the oxide film in this order. A nozzle material  47  is used to form nozzles on the insulating layer. Ink is fed from the back surface of the silicon substrate through the ink supply ports  41  formed as holes penetrating the silicon substrate. Electric energy is applied to the print elements  45  to heat and bubble the ink. The ink is thus ejected through the ejection ports  42  for printing. 
     In the present embodiment, two ink channels  44 , through which ink supplied through the ink supply port  41  can flow into the pressure chamber  40 , are formed symmetrically with respect to the pressure chamber  40 . That is, the ink channels  44  are provided opposite each other across the pressure chamber  40 . The thus symmetrically formed ink channels  44  prevent the pressure resulting from heat generated by the print head  45  from acting in a biased manner inside the pressure chamber  40 . The ink can thus be ejected perpendicularly to the print head (the principal surface of the print element board). 
       FIG. 5  is an enlarged view of a part of the print head according to the present embodiment. Wires through which electric energy is supplied to the print element  45  are arranged using a beam  51  formed between the ink supply ports  41  in the board as shown in  FIG. 5 .  FIG. 6  is an enlarged diagram showing the ink supply ports  41  and channel walls  46  in the print head according to the present embodiment. Furthermore,  FIGS. 7 and 8  are diagrams showing the wires connected to the print elements  45  in the print head according to the present embodiment.  FIG. 7  shows a common wire provided in the upper wiring layer.  FIG. 8  shows an individual wire provided in the lower wiring layer.  FIG. 9  is a diagram showing individual wires in the lower wiring layer as a comparative example. A common wire  78  electrically connects the power source and the print element  45  together. An individual wire  83  electrically connects the print element  45  and a driving circuit  50  together. 
     The common wire  78  shown in  FIG. 7  is electrically connected to the individual wire  83  shown in  FIGS. 8 and 9 , via a through-hole  80  formed between the adjacent ink supply ports. Energization of the wires enables the print element  45  to be energized to generate heat. Simply allowing the print element to generate heat can be achieved by connecting the through-hole  80  to the driving circuit row  50  as in the case of the individual wire  83  shown in  FIG. 9 . However, in this case, a step structure is created on the surface of the print element board  48  at the ink channel  44 . The step structure will be described below. 
       FIG. 10  is a diagram overlappingly showing the common wire  78  and the individual wire  83  as a comparative example.  FIG. 11  is a sectional view of the ink channel  44  portion of the even-number-th pressure chamber  40  from the end of the print element array; the sectional view being taken along line XI-XI in  FIG. 10 . As is apparent from  FIGS. 10 and 11 , the common wire  78  and the individual wire  83  are provided under one of the ink channels  44  positioned on the respective opposite sides of the print element  45 . Only the common wire  78  is provided under the other ink channel  44 . Thus, the ink channel  44  under which the common wire  78  and the individual wire  83  are provided is higher than the ink channel  44  under which only the common wire  78  is provided, by an amount corresponding to the individual wire  83 . That is, there is a difference in height between the two ink channels  44 . Such a step may cause the pressure resulting from heat generated by the print element  45  to act in a biased manner. As a result, the ink ejection direction may be bent with respect to the direction perpendicular to the print head. 
       FIG. 12  is a sectional view of the ink channel  44  portion in the odd-number-th pressure chamber  40  from the end of the print element array; the sectional view is taken along line XII-XII in  FIG. 10 . In the odd-number-th pressure chamber  40  from the end of the print element array, the individual wire  83  is not provided under either of the common wires  78 . Thus, the ink channels  44  on the respective opposite sides of the print element  45  can be provided symmetrically with respect to the pressure chamber  40 , with no asymmetric step created. 
     As described above, the shape of the surface of the print element board  48  differs between the odd-number-th pressure chamber  40  and the even-number-th pressure chamber  40  from the end of the print element array. In the configuration in the comparative example, during printing, the step may cause a difference in impact position between the odd-number-th pressure chamber  40  and the even-number-th pressure chamber  40  from the end of the print element array. 
     Thus, in the present embodiment, as shown in  FIG. 8 , the individual wire  83  is extended such that the individual wire  83  (dummy wire) is located under both the common wires  78 . The difference between the comparative example and the present embodiment is obvious from a comparison of the individual wire  83  shown in  FIG. 9  as the comparative example with the individual wire  83  according to the present embodiment in  FIG. 8 . 
       FIG. 13  is a diagram overlappingly showing the common wire  78  and individual wire  83  according to the present embodiment.  FIG. 14  is a sectional view taken along line XIV-XIV in  FIG. 13 . As is apparent from the figures, the common wire  78  and the individual wire  83  are provided under all the ink channels  44 . When the wires are thus arranged in the ink channel, the ink channels  44  on the respective opposite sides of the print element  45  can be provided symmetrically with no asymmetric step created, regardless of whether the pressure chamber  40  is odd- or even-numbered. This allows substantially the same flow resistance to be set for the ink channels  44  arranged on the respective opposite sides of the pressure chamber. Here, the term “flow resistance” as used herein refers to the difficulty with which the ink moves in the channel and which affects the shape of bubbles. The flow resistance is determined by the physical properties of the ink and the shape of the channel. 
     The adverse effect of a wiring pattern on a Y deviation will be described below which is observed when 2.8 pl of droplets are ejected at 15 kHz from a print head including 256 nozzles per row and having a nozzle interval of 600 dpi. Here, the Y deviation refers to the amount of deviation between the ideal ink impact position and the actual impact position measured in the form of a value in the nozzle row direction. The distance between the print head and a print medium is 1. 25 mm. The speed of the print head in the scanning direction is 25 inch/sec. 
     In connection with the Y deviation, in the print head shown in the comparative example, the difference in impact position between the odd-number-th print element  45  and the even-number-th print element  45  is about 10 μm. In contrast, the actual ejection condition in the print head according to the present embodiment indicates that the magnitude of the Y deviation is equivalent between the odd-number-th print element  45  and the even-number-th print element  45 . This in turn indicates that the symmetric wiring pattern in the ink channel serves to reduce the inclination of the ink ejection direction to the direction perpendicular to the element board. 
     In the present embodiment, the ink supply ports are not provided symmetrically with respect to the pressure chamber. However, this does not substantially affect the deviation in the ejection direction. Thus, the present invention is not limited to this aspect. The ink supply ports have only to be able to supply ink to the pressure chamber and may be provided symmetrically with respect to the pressure chamber. 
     As described above, the individual wire is extended so as to lie under the common wire. Thus, the wires under the ink channels arranged on the respective opposite sides of the pressure chamber  40  are made symmetric. Consequently, the equivalent step structure is provided at the bottoms of the ink channels arranged on the respective opposite sides of the pressure chamber. The present embodiment thus makes the ink channels symmetric with respect to the pressure chamber. As a result, the inclination of the ink ejection direction can be reduced, thus making improper printing conditions such as stripes and density unevenness unnoticeable. 
     Second Embodiment 
     A second embodiment of the present invention will be described below with reference to the drawings. The basic configuration of the present embodiment is similar to that of the first embodiment. Thus, only the characteristic arrangements of the present embodiment will be described below. 
       FIG. 15  is a diagram showing wires connected to print elements  45  in a print head according to the present embodiment.  FIG. 15  shows an individual wire provided in the lower wiring layer. In the first embodiment, the individual wire is extended so as to lie under the common wire  78 . However, the present embodiment avoids extending the individual wire  83  but uses a wire not connected to any other wire, as a dummy wire  153  provided under the common wire  78 . That is, the dummy wire  153  does not contribute to energization of the print element  45 . As shown in  FIG. 15 , in the even-number-th pressure chamber  40  from the end of the print element array, one dummy element  153  is provided under the ink channel. In the odd-number-th pressure chamber  40  from the end of the print element array, two dummy elements  153  are provided under the ink channel. 
       FIG. 16  is a diagram overlappingly showing the common wire  78  and the individual wire  153  according to the present embodiment. As is apparent from  FIG. 16 , the dummy wire  153  is provided under the common wire  78  in both the even- and odd-number-th pressure chambers from the end of the print element array. 
       FIG. 17  is a sectional view taken along line XVII-XVII in  FIG. 16 . As is the case with  FIG. 14  for the first embodiment, the individual wire  83  and the dummy wire  153  are provided under the common wire  78  on each of the opposite sides of the print element  45 . Thus, the ink channels  44  on the respective opposite sides of the print element  45  can be formed with no asymmetric step created. This enables substantially the same flow resistance to be set for the ink channels  44  on the respective opposite sides of the print element  45 . 
     The actual ejection condition of the print head according to the present embodiment indicates that the Y deviation is reduced compared to that in the comparative example shown in  FIG. 10 , as is the case with the first embodiment. This in turn indicates that the symmetric wires based on the dummy wire  153  serve to reduce the inclination of the ink ejection direction to the direction orthogonal to the element board. 
     As described above, the present embodiment avoids extending the individual wire but provides the dummy wire not connected to any other wire, under the common wire. Thus, the wires under the ink channels arranged on the respective opposite sides of the pressure channels are made symmetric. Consequently, the equivalent step structure is provided at the bottoms of the ink channels arranged on the respective opposite sides of the pressure chamber. The present embodiment thus makes the ink channels symmetric with respect to the pressure chamber  40 . As a result, the inclination of the ink ejection direction can be reduced, thus making improper printing conditions such as stripes and unevenness unnoticeable. In the present embodiment, the dummy wires  153  are formed on the respective opposite sides of the odd-number-th pressure chamber  40 . However, since no individual wire is provided under the two channels connected to the odd-number-th pressure chamber, the dummy wires  153  may be omitted. 
     Third Embodiment 
     A third embodiment of the present invention will be described below with reference to the drawings. The basic configuration of the present embodiment is similar to that of the first embodiment. Thus, only the characteristic arrangements of the present embodiment will be described below. 
       FIG. 18  is a sectional view of a portion of a print head according to the present embodiment which corresponds to a pressure chamber. In the present embodiment, the individual wire  83  is provided only on one side, but a member  180  formed of the same material as that of the nozzle material  47  is additionally stuck to the surface of the print element board  48  at the ink channel. Thus, a step is formed so as to make the flow resistance equal between the ink channels arranged on the respective opposite sides of the pressure chamber. 
     The actual ejection condition of the print head according to the present embodiment indicates that the Y deviation is reduced compared to that in the comparative example shown in  FIG. 10 , as is the case with the first embodiment. This in turn indicates that by sticking the member  180  formed of the same material as that of the nozzle material  47  to the board for symmetry, the inclination of the ink ejection direction to the direction orthogonal to the element board is reduced. 
     As described above, the member  180  formed of the same material as that of the nozzle material  47  is stuck to the surface of the print element board  48 . Thus, the wires under the ink channels arranged on the respective opposite sides of the pressure channels are made symmetric. Consequently, the equivalent step structure is provided at the bottoms of the ink channels arranged on the respective opposite sides of the pressure chamber. The present embodiment thus makes the ink channels symmetric with respect to the pressure chamber. As a result, the inclination of the ink ejection direction can be reduced, thus making improper printing conditions such as stripes and unevenness unnoticeable. 
     Fourth Embodiment 
     A fourth embodiment of the present invention will be described below with reference to the drawings. The basic configuration of the present embodiment is similar to that of the first embodiment. Thus, only the characteristic arrangements of the present embodiment will be described below. 
     In the third embodiment, the member  180  is formed of the nozzle material. However, in the present embodiment, the member  180  is formed of a material (for example, a polyetheramide-containing resin HIMAL manufactured by Hitachi Chemical Co., Ltd.) allowing the nozzles and the print element board  48  to be tightly contacted. 
     The actual ejection condition of the print head according to the present embodiment indicates that the Y deviation is reduced compared to that in the comparative example shown in  FIG. 10 , as is the case with the first embodiment. This in turn indicates that by sticking the member  180  formed of the material allowing the nozzles and the print element board  48  to be tightly contacted, to the board for symmetry, the inclination of the ink ejection direction to the direction orthogonal to the element board is reduced. 
     As described above, the member  180  formed of the material allowing the nozzles and the print element board  48  to be tightly contacted is stuck to the surface of the print element board  48 . Thus, the wires under the ink channels arranged on the respective opposite sides of the pressure channels  40  are made symmetric. Consequently, the equivalent step structure is provided at the bottoms of the ink channels arranged on the respective opposite sides of the pressure chamber. The present embodiment thus makes the ink channels symmetric with respect to the pressure chamber. As a result, the inclination of the ink ejection direction can be reduced, thus making improper printing conditions such as stripes and unevenness unnoticeable. 
     In the above-described embodiments, the height of the wiring layer is substantially equivalent to that of the step formed by the tight contact layer. However, the present invention is not limited to this aspect. That is, if the wiring layer and the tight contact layer have different film thicknesses and the corresponding films form different heights, the effects of the present invention can be exerted by utilizing the widths of the layers to make the flow resistance substantially symmetric. For example, if the height of the wire is smaller than the thickness of the tight contact layer, the effects of the present invention can be exerted by setting the width of the wire smaller than the arrangement width of the tight contact layer to ensure the symmetry of the flow resistance. 
     The embodiments have been individually described. However, any of the embodiments may be combined together. For example, a combination of the nozzle member and the tight contact layer enables substantially equal flow resistance to be adjustably set for the two channels. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2009-026168, filed Feb. 6, 2009, which is hereby incorporated by reference herein in its entirety.