Patent Publication Number: US-6338550-B1

Title: Inkjet printing head with oval flexible cable configured to be received within oval hollow portion

Description:
This application is a continuation of U.S. application Ser. No. 08/388,831, filed Feb. 15, 1995, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improved inkjet printing head applicable to a printer, a facsimile machine, a plotter and so on. 
     2. Description of the Related Art 
     There are known inkjet printing methods in which printing is performed by jetting ink from nozzles onto a recording medium such as paper without bringing a printing head into contact with the recording medium. One typical example of such a method is the drop-on-demand type inkjet printing method, in which ink drops are produced only when needed. With a printing head operating according to this method, a voltage is applied to a piezoelectric element, which varies the volume of a pressure chamber housing ink. Then, ink is caused to fly at the paper via a nozzle in communication with the pressure chamber. Such a printing head mainly comprises a head plate including a plurality of pressure chambers, a diaphragm, and a plurality of piezoelectric elements. 
     The pressure chambers and piezoelectric elements are arranged in various ways on the head plate in accordance with printing head specifications. For instance, a printing head of a line printer includes pressure chambers and piezoelectric elements which are arranged in a line so that nozzles are aligned along a printing line. With a serial printer, a printing head includes pressure chambers and piezoelectric elements which are radially arranged in a space extending through 180° or more. 
     In the inkjet printing head, a diaphragm in the shape of a thin film is attached onto a head plate carrying a plurality of pressure chambers thereon. A plurality of piezoelectric elements are arranged on the diaphragm such that they respectively correspond to the pressure chambers. The piezoelectric elements are respectively actuated by a voltage applied thereto, thereby causing the corresponding parts of the diaphragm to shudder. The shuddering of the diaphragm is transmitted to pressure chambers, thereby flexing them. Then, ink is jetted from nozzles in communication with the pressure chambers. Conversely, when the voltage application is stopped, the diaphragm restores, sucking ink from an ink delivery area, and preparing for a subsequent ink jetting operation. Specifically, the respective piezoelectric elements are actuated in response to printing data supplied from an external source, and vary the volume of necessary pressure chambers. According to the varied volume, the nozzles jet a desired amount of ink onto a recording medium so as to print an image thereon. 
     The foregoing inkjet printing head comprises a head assembly, a cable, an ink reservoir, and an ink pipe. The head assembly includes a head plate, a diaphragm, and a plurality of piezoelectric elements. The head plate carries a plurality of pressure chambers and nozzles disposed thereon. The cable includes a group of electrodes respectively applying a voltage to their associated piezoelectric elements, and 49 control wires (i.e. at least 48 signal wires and one grounding wire when an inkjet printing head has 48 piezoelectric elements). Ink is supplied to the head assembly via the ink pipe from the ink reservoir. Particularly, it is extremely difficult to precisely contact piezoelectric elements with their corresponding electrodes and connect the ink pipe with them so as to prevent ink leakage when assembling a printing head. Therefore, there is a problem that the printing head takes time to be assembled, and that the cable including the control wires is difficult to handle. This means that the printing head cannot be assembled efficiently. 
     SUMMARY OF THE INVENTION 
     The present invention is aimed at overcoming the foregoing problems of the related art, and providing an inkjet printing head which can be assembled efficiently. 
     According to a first aspect of the invention, there is provided an inkjet printing head comprising: a head assembly which includes a plurality of pressure chambers, each pressure chamber having a nozzle at one end and an ink inlet at the other end thereof, a plate defining an ink conduit in communication with the ink inlets, a diaphragm disposed over the pressure chambers, and a plurality of piezoelectric elements attached to one surface of the diaphragm in such a manner as to individually correspond to the pressure chambers, each piezoelectric element making the diaphragm shudder and varying a capacity of each pressure chamber so as to introduce ink into the pressure chamber and to jet ink via the nozzle; a main frame having a hollow portion at a center thereof and supporting the head assembly on one surface thereof; a flexible cable including a group of electrodes for applying a drive voltage to the piezoelectric elements; a flexible member attached to the other surface of the main frame, the flexible member keeping the piezoelectric elements and the flexible cable in mutual contact via the hollow portion of the main frame, and preventing the flexible cable from resonating due to shudders of the piezoelectric elements; and a sub-frame fixing the flexible cable and the flexible member to the main frame with pressure. 
     In this arrangement, the flexible cable and the flexible member are sandwiched between the main frame and sub-frame, so that it is possible to uniformly and reliably press the electrodes toward their associated piezoelectric elements via the flexible member. 
     Therefore, it is possible to prevent the flexible cable from resonating when the piezoelectric elements are actuated. 
     The main frame includes a recess for receiving the head assembly. The hollow portion of the main frame is shaped similarly to the flexible cable so to house the flexible cable therein. Thus, the main frame, head assembly and flexible cable are precisely and easily positioned with respect to one another. 
     The nozzles are inclined with respect to a printing line by a predetermined angle on the head assembly, both the hollow portion of the main frame and the flexible cable are oval in the shape, and the flexible cable is housed in the hollow portion. This enables not only a printing density to be improved without narrowing a pitch between the nozzles but also assures precise, reliable and easy positioning of the main frame, head assembly and flexible cable. 
     The sub-frame has a rimmed window capable of fitting into the hollow portion of the main frame, and the rimmed window supports the flexible member. Thus, the flexible cable and the flexible member are positioned easily and precisely with respect to each other. Further, it is possible to contact the flexible cable to the piezoelectric elements with a uniform pressure. Still further, the inkjet printing head can be automatically assembled by using a part feeder since no strict positioning of the components is necessary. 
     In accordance with a second aspect of the invention, there is provided an inkjet printing head comprising: a head assembly including a plurality of pressure chambers, each pressure chamber having a nozzle at one end and an ink inlet at the other end thereof, a plate defining an ink conduit in communication with the ink inlets, a diaphragm disposed over the pressure chambers, and a plurality of piezoelectric elements attached to one surface of the diaphragm in such a manner as to individually correspond to the pressure chambers, each piezoelectric element making the diaphragm shudder and varying a capacity of each pressure chamber so as to introduce ink into the pressure chambers and to jet ink via the nozzle; a main frame having a hollow portion at a center thereof and supporting the head assembly on one surface thereof; a flexible cable including groups of electrodes for applying a drive voltage to the piezoelectric elements; a flexible member attached to the other surface of the main frame, the flexible member keeping the piezoelectric elements and the flexible cable in mutual contact via the hollow portion of the main frame, and preventing the flexible cable from resonating due to shudders of the piezoelectric elements; and a sub-frame fixing the flexible cable and the flexible member to the main frame with pressure. The diaphragm includes an ink port for supplying ink to the ink conduit, and an ink pipe in communication with an ink reservoir is disposed close to the ink port. 
     In this arrangement, the ink pipe in communication with the ink reservoir is directly connected to the ink port. 
     Since no ink is in direct contact with the main frame, it is possible to protect the main frame against erosion caused by ink. In other words, since the nozzles are not blocked by metal or resin components in the main frame, the original quality of ink can be reliably maintained without color change. 
     Further, the ink pipe can be directly and intimately connected to the ink port, so that it is possible to supply ink without any leakage. 
     According to a third aspect of the invention, there is provided an inkjet printing head comprising: a head assembly, the head assembly including: a plurality of pressure chambers, each pressure chamber having a nozzle at one end and an ink inlet at the other end thereof; a plate defining an ink conduit in communication with the ink inlets; a diaphragm disposed over the pressure chambers; and a plurality of piezoelectric elements attached to one surface of the diaphragm in such a manner as to individually correspond to the pressure chambers, each piezoelectric element making the diaphragm shudder and varying a capacity of each pressure chamber so as to introduce ink into the pressure chamber and jet ink via the nozzle; a main frame having a hollow portion at a center thereof and supporting the head assembly on one surface thereof; a flexible cable including a group of electrodes for applying a drive voltage to the piezoelectric elements; a flexible member attached to the other surface of the main frame, the flexible member keeping the piezoelectric elements and the flexible cable in mutual contact via the hollow portion of the main frame, and preventing the flexible cable from resonating due to shudders of the piezoelectric elements; and a sub-frame fixing the flexible cable and the flexible member to the main frame with pressure. The diaphragm is smaller than the ink conduit plate so as to have a beginning of the ink conduit exposed from the diaphragm and form an ink port, and the ink pipe in communication with the ink port is positioned close to the beginning of the ink conduit. 
     The ink pipe in communication with the ink reservoir is directly connected to the ink port, so that ink does not come into contact with the main frame. 
     Further, the ink port can be easily formed without specifically modifying the diaphragm or ink conduit plate. This will lead to reduction of the manufacturing cost of the inkjet printing head. 
     The beginning of the ink conduit is joined to the ink pipe via flexible packing so as to seal a joined portion. 
     A curing resin is applied to a step portion between the ink conduit plate and the diaphragm so as to form a slope thereon, which reliably and easily connects the ink pipe. 
     Further, the curing resin is applied so as to be banked against a peripheral area of the beginning of the ink port. This enables the ink pipe to be sealed reliably. 
     When the curing resin is applied so as to be banked around the beginning of the ink conduit, the joined portion of the ink pipe can be reliably sealed. 
     Further, a filter is closely attached to the ink port using the curing resin so as to filter impurities in the ink. The filter is integral with the joined area of the ink pipe, which can reduce the number of components used, and assures reliable connection of the ink pipe without ink leakage. 
     In a fourth aspect of the invention, there is provided an inkjet printing head comprising: a head assembly, the head assembly including: a plurality of pressure chambers, each pressure chamber having a nozzle at one end and an ink inlet at the other end thereof; a plate defining an ink conduit in communication with the ink inlets; a diaphragm disposed over the pressure chambers; and a plurality of piezoelectric elements attached to one surface of the diaphragm in such a manner as to individually correspond to the pressure chambers, each piezoelectric element making the diaphragm shudder and varying a capacity of each pressure chamber so as to introduce ink into the pressure chambers and to Jet ink via the nozzle; a main frame having a hollow portion at a center thereof and supporting the head assembly on one surface thereof; a flexible cable including a group of electrodes for applying a drive voltage to the piezoelectric elements; a flexible member attached to the other surface of the main frame, the flexible member keeping the piezoelectric elements and the flexible cable in mutual contact via the hollow portion of the main frame, and preventing the flexible cable from resonating due to shudders of the piezoelectric elements; a sub-frame fixing the flexible cable and the flexible member to the main frame with pressure; and a driver IC attached on the flexible cable so as to perform central control of the piezoelectric elements. 
     In this arrangement, only control wires for controlling the driver IC extend out from the inkjet printing head. 
     Since a width of a bundle of the control wires can be reduced without thinning respective control wires, the control wires are durable. The reduced width of the control wire bundle can decrease a space for attaching the inkjet printing head to a printer body. This is advantageous for making the printer compact. 
     Such an inkjet printing head can be easily handled and efficiently attached to the printer body. 
     The flexible cable carrying the driver IC is sandwiched between the main frame and sub-frame, which allows the inkjet printing head to be efficiently assembled. 
     An external connection terminal is attached to one end of the flexible cable so as to receive a signal actuating the driver IC. This enables the printer body and the inkjet printing head to be fabricated separately, and to be joined at a later stage. The inkjet printing head becomes easy to handle and to connect to the printer body. Further, a faulty inkjet printing head can be easily replaced with a new one. 
     Since the driver IC is positioned on a recess of the rear surface of the main frame, the main frame and the sub-frame can be brought into close contact with each other. Thus, the electrodes on the flexible cable can be uniformly pressed to the piezoelectric elements on the main frame. 
     Alternatively, when the driver IC is positioned on a recess on a surface of the sub-frame where it is pressed to the main frame, the main frame and the sub-frame can be brought into close contact so as to sandwich the flexible cable carrying the driver IC. Therefore, the electrodes on the flexible can be pressed to the piezoelectric elements on the main frame with uniform force. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     Identical parts are assigned identical reference numerals throughout the drawing figures. 
     FIG. 1 is an exploded perspective view showing an inkjet printing head according to a first embodiment of the invention. 
     FIG. 2 is a front view of an ink conduit plate for a head assembly in the first embodiment. 
     FIG. 3 is a view showing how to attach an ink pipe to the head assembly in which the ink conduit plate carries a diaphragm and piezoelectric elements thereon. 
     FIG. 4 is a front view of a main frame for the inkjet printing head shown in FIG.  1 . 
     FIG. 5 shows a manner in which a flexible cable is attached to the main frame of FIG.  4 . 
     FIG. 6 is a perspective view showing an assembled state of an inkjet printing head according to a second embodiment. 
     FIG. 7 is a perspective view of an inkjet printing head according to a third embodiment. 
     FIG. 8 is a perspective view showing how a flexible cable is fixed in the third embodiment. 
     FIG. 9 is a perspective view showing how a flexible cable is fixed in an inkjet printing head in another example according to the third embodiment. 
     FIG. 10 is a view showing an ink port and an ink pipe in an inkjet printing head according to a fourth embodiment. 
     FIG. 11 shows a manner in which an ink conduit plate and a diaphragm are joined in the fourth embodiment. 
     FIG. 12 shows a modified example in which the ink conduit plate and the diaphragm are joined in the fourth embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     The invention will be described with reference to a first embodiment shown in the drawings. It is assumed that the present invention is applied to an inkjet printing head for a serial type printer. 
     Referring to FIG. 1, the inkjet printing head  10  comprises a head assembly  12 , a main frame  14  made from resins or metal, a flexible cable  16 , a flexible member  18 , and an ink reservoir  20  feeding ink to the head assembly  12 . The head assembly  12  includes an ink conduit plate carrying pressure chambers, a diaphragm and a plurality of piezoelectric elements, all of which will be described later in detail. The flexible cable  16  applies a voltage to the piezoelectric elements. The flexible member  18  prevents the flexible cable  16  from resonating when the piezoelectric elements are actuated. 
     A first feature of the first embodiment is that the flexible cable  16  and the flexible member  18  are sandwiched and fixed between the rear surface of the main frame  14  and a sub-frame  22 . Both the main frame  14  and sub-frame  22  are made of hard resins or metal such as aluminum. A second feature is that a hollow portion  14   a  of the main frame  14  is shaped similarly to the flexible cable  16 , e.g. oval, so as to receive the flexible cable  16  therein. A further feature is that the sub-frame  22  has a rimmed window  22   a,  which can be fitted into the hollow portion  14   a  of the main frame  14 . The rimmed window  22   a  receives the flexible member  18  therein. In other words, the flexible member  18  is positioned with respect to the main frame  14  and the flexible cable  16  via the rimmed window  22   a  of the sub-frame  22 . Further, the flexible member  18  protrudes somewhat from the rimmed window  22   a,  thereby pressing the flexible cable  16  closely to the head assembly  12 . 
     As shown in FIG. 2, in the head assembly  12  attached to the main frame  14 , the ink conduit plate  12   a  is made from a material such as glass, and has an ink conduit  28  which is in communication with the pressure chambers  24  and is formed by a process such as etching on one surface (i.e. the rear side of the plane shown in FIG.  1 ). The ink conduit  28  feeds ink to a plurality of pressure chambers  24  (e.g. 48 pressure chambers  24  in FIG. 2) from the ink reservoir  20  via an ink introducing portion  26 . Each of the pressure chambers  24  has a nozzle  30  at one end (toward the center of the ink conduit plate  12   a ), and an ink inlet  34  at the other end (along the ink conduit  28 ). The nozzles  30  are open on the rear side of the plane shown in FIG.  2 . The ink inlets  34  are thinner than supply channels  32  so as to reduce a resistance which is caused when ink is jetted from the pressure chambers  24 , thereby preventing ink from flowing in a reverse direction. 
     The inkjet printing head  10  including the head assembly  12  is actuated by a drive mechanism, not shown, so as to reciprocate in a direction A (i.e. along a printing line A) as shown in FIG. 2, thereby printing an image. In this case, the nozzles  30  are inclined by a predetermined angle, e.g. 30°, with respect to the printing line direction A, so that a printing pitch can be reduced without reducing a pitch for arranging the nozzles  30 . This assures dot printing with very high density. 
     In this example, a plurality of nozzles  30  are provided on the ink conduit plate  12   a.  Alternatively, a nozzle plate  36  having a plurality of openings serving as the nozzles  30  may be attached on a front surface of the head assembly  12  shown in FIG.  1 . The openings of the nozzle plate  36  have a smaller diameter than the diameter of the nozzles  30 . Thus, the nozzle plate  36  is effective for increasing an ink jetting pressure, thereby improving the quality of printed images. 
     Referring to FIG. 2, the pressure chambers  24  are effectively arranged in a staggered manner in two rows so that ink feeders can be effectively positioned in a limited space. 
     The ink introduction portion  26  may have support members  26   a  and  26   b  so as to support a diaphragm  38  (to be described later). The support members  26   a  and  26   b  are effective to prevent non-smooth flow of ink around the ink introducing portion  26  due to the ink conduit  28  being pressed by the diaphragm  38 . 
     The diaphragm  38  is attached to the front surface of the ink conduit plate  12   a  (i.e. opposite to the surface where the nozzles  30  are located) as shown in FIG.  3 . The diaphragm  38  is a plate such as glass, is approximately 50 μm thick, and has a plurality of piezoelectric elements  40  on its upper surface. The piezoelectric elements  40  correspond, on a one-to-one basis, to the pressure chambers  24  (FIG. 2) on the ink conduit plate  12   a.  When a voltage is applied to actuate a particular piezoelectric element  40  in response to printing data from an external source, the piezoelectric element  40  causes the diaphragm  38  to locally shudder at a position associated therewith. Then, the shudder of the diaphragm  38  is transmitted to a corresponding pressure chamber  24 . The volume of the pressure chamber  24  is reduced, thereby jetting ink therefrom onto the recording medium. Conversely, when the voltage application is stopped, the diaphragm  38  restores to its original state, sucks ink from the ink conduit  28 , and prepares for a subsequent ink jetting operation. 
     Referring to FIG. 3 (in which the ink conduit  28  is shown by a broken line), the diaphragm  38  includes an ink port  42  at the ink introducing portion  26  from which the ink conduit  28  extends. The ink port  42  is made using excimer laser or sand blasting. A sealing member  44  and an ink pipe  46  are fitted into the ink port  42  and are sandwiched between the main frame  14  and the sub-frame  22 . The ink pipe  46  is connected, at one end thereof, to an outlet  20   a  of the ink reservoir  20  via a sealant such as an O-ring  48 . Further, the ink pipe  46  has a filter  50  at the other end thereof (i.e. where the pipe  46  is connected to the ink reservoir  20 ) so as to prevent introduction of impurities into the head assembly  12 . The sealing member  44  is made from a material such as silicon rubber. Therefore, the ink pipe  46  can be intimately fitted and fixed in the ink port  42  with ease, thereby preventing ink leakage. Further, ink can be fed to the head assembly  12  from the ink reservoir  20  via the ink pipe  46  directly connected to the ink port  42 . Since no ink comes into contact with the main frame  14 , not only can the main frame  14  be protected against erosion but also ink can maintain its original quality. 
     As shown in FIG. 4, the main frame  14  is made from a material such as resin or aluminum, and has a substantially oval hollow portion  14   a.  The head assembly  12  is attached to the main frame  14  by a UV type adhesive, an anaerobic, or the like. Referring to FIGS. 1 and 4, the main frame  14  has a shallow recess  14   b  to receive the head assembly  12  therein. In other words, the hollow portion  14   a  is positioned substantially at the center of the shallow recess  14   b.  When the head assembly  12  is put into the shallow recess  14   b,  the piezoelectric elements  40  of the head assembly  12  are exposed on the rear surface of the main frame  14  via the hollow portion  14   a.  A space still remains in the hollow portion  14   a  in which the flexible cable  16  (to be described later) is housed. This structure is effective for preventing vibrations which may be caused when the head assembly  12  performs the ink jetting operation. 
     The flexible cable  16  carries a number of electrodes  16   a  and circularly arranged COM electrodes  16   b  as shown in FIG.  1 . The electrodes  16   a  apply a voltage to the piezoelectric elements  40  (FIG. 3) of the head assembly  12 . The electrodes  16   a  and the circular COM electrode  16   b  are printed on the flexible cable  16  in the same pattern as that of the piezoelectric elements  40  (shown in FIG.  1 ). The oval flexible cable  16  is precisely fitted into the oval hollow portion  14   a  from the rear side of the main frame  14 . Thus, the electrodes  16   a  are easily positioned in such a manner as to precisely correspond to the piezoelectric elements  40  on a one-to-one basis. 
     A conductive film made from a material such as indium tin oxide (ITO) is applied on the surface of the diaphragm  38  where the piezoelectric elements  40  are arranged, serving as a COM electrode for the diaphragm  38 . Thus, the voltage to actuate the piezoelectric elements  40  can be easily applied by arranging the COM electrode  16   b  at the center of the flexible cable  16  and arranging the electrodes  16   a  around the COM electrode  16   b.    
     Referring to FIG. 5, a driver IC  52  is attached to the flexible cable  16  so as to perform central control of the piezoelectric elements  40  (i.e. there are 48 piezoelectric elements in this embodiment). The driver IC  52  includes a data input terminal, a clock input terminal, a strobe terminal, an input terminal inputting a piezoelectric element actuating wave, a power supply terminal, a grounding terminal and so on. Data concerning the piezoelectric elements  40  are sequentially applied to a shift register of the driver IC via the data input terminal. The data in the shift register are shifted in response to signals arriving at the clock terminal. In response to signals input in the strobe input terminal, the shift register provides the data to the piezoelectric elements  40 . Further, the driver IC  52  may also include terminals such as a terminal receiving information on an empty state of the ink reservoir  20  (shown in FIG.  1 ), and an input terminal receiving data concerning an intermediate actuation wave to gradually control the operation of the piezoelectric elements  40  and to stabilize an amount of ink to be jetted. 
     It is therefore possible to thin down the flexible cable  16  extending from the inkjet printing head  10  via the driver IC  52 . This is because the cable  16  can have only a few control wires (e.g. the data input terminal, clock input terminal, strobe input terminal, actuation wave input terminal, power supply terminal, and grounding terminal, and also empty ink reservoir information input terminal and intermediate actuation wave terminal if necessary). In other words, the flexible cable  16  can be disposed and fixed in a reduced space of the inkjet printing head  10 . This means that the printer body where the inkjet printing head  10  can be also reduced in size. 
     Further, even when the inkjet printing head is a movable type or when the flexible cable  16  is arranged in a complicated manner, the flexible cable  16  can be thinned down without reducing a pitch of a control wire pattern. Thus, the flexible cable  16  can be easily disposed in the reduced space without adversely affecting the durability of the control wire pattern. 
     The flexible cable  16  carrying the driver IC  52  is protected by a resin cover  54  on the front surface thereof, and is covered on the rear surface by a support  56  (made from a material such as resin) so as to reliably fix the driver IC  52  on the flexible cable  16 . As shown in FIG. 5, a recess  58  is formed on the rear surface of the main frame  14  (i.e. on the side where the main frame  14  is in contact with the sub-frame  22  shown in FIG.  1 ). The driver IC  52  is received in the recess  58 , thereby enhancing the close contact of the main frame  14  with the sub-frame  22  when the flexible cable  16  is sandwiched between them. 
     The sub-frame  22  made from resin or metal such as aluminum is positioned behind the main frame  14 . The sub-frame  22  has an oval rimmed window  22   a  which is insertable into the hollow portion  14   a  of the main frame  14 . The flexible member  18  is fitted into the rimmed window  22   a.  The flexible member  18  is made from a material such as sponge or rubber, and is substantially annular. The flexible member  18  is preferably thick enough to slightly project from the rimmed window  22   a  when fitted therein. 
     Since the hollow portion  14   a  and the rimmed window  22   a  are the same in shape, both the main frame  14  and the sub-frame  22  can be precisely and easily combined with the rimmed window  22   a  received in the hollow portion  14   a.  The flexible member  18  slightly projecting from the rimmed window  22   a  pushes the flexible cable  16  toward the piezoelectric elements  40  with a uniform pressure. Thus, the electrodes  16   a  and  16   b  of the flexible cable  16  can be reliably brought into contact with the piezoelectric elements  40 . Further, the flexible cable  16  can be effectively protected against resonance when the piezoelectric elements  40  are actuated. 
     Besides the flexible cable  16  and the flexible member  18 , the ink pipe  46 , sealing member  44  and filter  50  are also interposed between the main frame  14  and the sub-frame  22 . The ink pipe  46  provides ink to the pressure chambers  24  via the ink port  42  (of the head assembly  12 ) and an opening  14   c  (formed on a part of the main frame  14 ). The sealing member  44  prevents ink leakage and ink flow to the main frame  14 . The filter  50  removes impurities which may flow into the ink conduit  28 . These members are shown in FIG.  3 . The main frame  14  and sub-frame  22  are fixed using small screws or an adhesive, constituting an independent head unit  100 . 
     The sub-frame  22  has an opening  22   b,  through which the ink pipe  46  passes. 
     The ink reservoir  20  containing ink is located behind the sub-frame  22 , and discharges a predetermined amount of ink with a predetermined pressure via an ink outlet  20   a.  The ink reservoir  20  has an opening  20   b  on the top so as to refurnish fresh ink. The opening  20   b  is usually covered by a cap  64  via an O-ring  62 . 
     The ink pipe  46  extends from the rear surface of the sub-frame  22  of the head unit  100 , and is fitted into the ink outlet  20   a  of the ink reservoir  20  via an O-ring  48 . The head unit  100  and the ink reservoir  20  are combined and fixed using small screws  66  or the like. 
     The head unit  100  and the ink reservoir  20  not only serve as an integral unit but are also separable for replacement when either of them becomes defective. 
     All of the hollow opening  14   a,  flexible cable  16 , flexible member  18  and rimmed window  22   a  have the same oval shape, so that the main frame  14 , flexible cable  16 , flexible member  18  and sub-frame  22  can be precisely and easily positioned with respect to one another. Therefore, the inkjet printing head can be automatically assembled in an assembly line using a component feeder or the like. 
     In the foregoing embodiment, the inkjet printing head includes the pressure chambers and piezoelectric elements which are radially arranged in two rows in an oval space of the head assembly. Alternatively, these members may be radially arranged in a single row in a circular space, or in a line. 
     Second Embodiment 
     An inkjet printing head will be described with reference to a second embodiment shown in FIG.  6 . 
     Referring to FIG. 6 showing an inkjet printing head  200 , a sub-frame  68  differs from the sub-frame  22 , which is in the shape of a plate, and includes a base member  68   a  receiving an ink reservoir  20 , a pair of side walls  68   b  holding opposite sides of the ink reservoir  20  (only one side wall is shown in FIG.  6 ), and a wall (not shown) between the side walls  68   b  and not only pressing a flexible member  18  (not shown) to a flexible cable  16  but also fixing the flexible cable  16 . This pressing wall functions similarly to the sub-frame  22  shown in FIG.  1 . 
     The flexible cable  16  extends downwards from the rear surface of the main frame  14  in a similar manner to that shown in FIG.  5 . The downward end of the flexible cable  16  is connected to an external connection terminal  70 . The external connection terminal  70  includes a plurality of terminal sections which are connected to terminals of the driver IC  52  so as to provide control signals thereto from an external source, not shown. 
     The flexible cable  16  extending from between the main frame  14  and the sub-frame  68  is folded at right angles with respect to the main frame  14 . In this state, the external connection terminal  70  is structured such that a connector  70   a  thereof faces downward and is attached to the rear surface of the base member  68   a.  It is also acceptable to attach a nozzle plate  36  on the front surface of the head assembly  12  in a similar manner to that shown in FIG.  1 . 
     Provision of the external connection terminal  70  allows the inkjet printing head and the printer body to be assembled in separate processes, which improves manufacturing efficiency and reduces manufacturing cost. This structure facilitates replacement of a faulty inkjet printing head or a faulty printer body. 
     The inkjet printing head  200  comprising the main frame  14  and the sub-frame  68  is fixed to a carriage of the printer body using small screws  72  or the like. The carriage has a connector at a position where the connector  70   a  of the external connection terminal  70  is connectable. Both of these connectors can be reliably and easily connected by attaching the inkjet printing head  200  to the carriage using small screws  72 . 
     Since the number of control wires connected to the driver IC  52  can be reduced, the inkjet printing head and the printer body can be electrically connected in a reduced space. Thus, the printer can be reduced in size and simplified easily and reliably. Further, the ink reservoir  20  is enclosed by the sub-frame  68 , so that the carriage can be stably moved during the printing operation. 
     Third Embodiment 
     The invention will be described with reference to a third embodiment shown in FIG. 7, in which the flexible cable  76  is fixed in a different manner. In this embodiment, an inkjet printing head  300  differs from the inkjet printing head  10  (FIG. 1) with respect to the shapes of a main frame  74 , a flexible cable  76 , a sub-frame  78 , and an ink reservoir  80 . The remaining parts are similar to those of the first embodiment, are assigned identical reference numerals, and will not be described in detail. 
     The driver IC  52  is housed in the main frame  14  in the first embodiment. However, in this embodiment, the driver IC  52  is positioned on the rear surface of the sub-frame  78  in stead of the main frame  74 . Specifically, control wires connected to the piezoelectric elements  40  (i.e. 48 signal wires and one grounding wire) extend downwards from between the main frame  74  and the sub-frame  78 , are folded upward along the rear surface of the sub-frame  78 , and are fastened there. The inkjet printing head including the main frame  74  and the sub-frame  78  is preferably fastened to the ink reservoir  80 . For this purpose, a recess  78   a  is formed on the rear surface of the sub-frame  78  as shown in FIG. 8 so as to prevent the driver IC  52 , and external connection terminal  82  (functions similarly to the terminal  70  shown in FIG. 6) from sticking out from the sub-frame  78 . Alternatively, a recess may be made on the ink reservoir  80  so as to receive the driver IC  52  and the external connection terminal  82  of a connector type may stick out and prevent them from sticking out. 
     Even when the driver IC  52  is positioned on the sub-frame  78 , the inkjet printing head of this embodiment can be assembled effectively and be reduced in size. This will lead to a reduced volume of the printer where the inkjet printing head is attached. 
     In the third embodiment, the flexible cable  16  extends from between the main frame  74  and the sub-frame  78 , and is fixedly attached to the rear surface of the sub-frame  78 . Alternatively, a recess is formed on the rear surface of the main frame  74 - 1  so as to receive the flexible cable  76 - 1  therein as shown in FIG.  9 . 
     The external connection terminal may be shaped and oriented like a terminal  82 - 1  shown in FIG.  9 . For instance, the external connection terminal may be of a connector type (box type) like the external terminals  82  and  82 - 1 , or may be in the shape of a card as shown in FIG.  6 . The external terminal can project in any direction as shown in FIGS. 8 and 9, depending upon the shape of the printer body. 
     The inkjet printing head of the third embodiment is described assuming that it is applied to the serial type printer, but is also applicable to a line type printer with similar advantageous results. 
     Further, the flexible cable extending from the inkjet printing head is connected to the external connection terminal separately at a later stage. If necessary, it is also possible to obviate the external connection terminal and make the flexible cable extendable. Such a flexible cable requires a reduced space and is durable. 
     Fourth Embodiment 
     An inkjet printing head of a fourth embodiment differs from those of the first to third embodiments in the shape of the ink port through which ink is supplied to the ink conduit from the ink reservoir. 
     The fourth embodiment features that a diaphragm  84  smaller than the ink conduit plate  12   a  is attached to the ink conduit plate  12   a.  Thus, only the beginning of the ink conduit  28 , i.e a portion corresponding to the ink introducing portion  26 , is exposed. In this arrangement, the ink pipe  46  in communication with the ink reservoir  20  can be directly connected to the ink port  86 . 
     Thus, it is possible to prepare the ink port  86  without particularly modifying the diaphragm  84  and the ink conduit plate  12   a.    
     There is a difference of height between the ink conduit plate  12   a  and a diaphragm  84  around the ink introducing portion  86 . This difference is equal to a height of the diaphragm  84 . An ink pipe  46  is connected to an ink port  86  via a sealing member  44  applied to the portion where there is the foregoing difference, thereby preventing ink leakage. In other words, the ink pipe  46  and sealing member  44  are sandwiched between the main frame  14  and sub-frame  22  (both shown in FIG.  1 ), so that the ink pipe  46  is pushed toward the ink port  86  via the sealing member  44 . Alternatively, as shown in FIG. 11, a curing resin  88  (e.g. silicon-based adhesive which is resistant to ink) may be applied to the opposite sides of the ink port  86  in the shape of a slope, thereby contacting the sealing member  44  to the ink port  86  more intimately and preventing ink leakage more reliably. 
     With the foregoing arrangement, ink can be supplied to the ink port  86  via the ink pipe  46  without passing through the main frame  14 . This reliably prevents erosion of the main frame  14  by ink and deterioration of the ink quality. Further, the ink pipe  46  is in pressure-contact with the ink port  86  via the sealing member  44 , which enables ink to be sealed from the main frame  14  and enhances prevention of the ink leakage. 
     The curing resin  88  may be applied around the ink port  86  in the annular shape as shown in FIG. 12 as an alternative measure. In this case, the ink pipe  46  can be connected to the ink port  86  in an optimum manner without using the sealing member  44 . The curing resin  88  preferably has elasticity which is equal to or greater than that of the sealing member  44 . 
     A filter  50  is fused to the ink pipe  46  on a side adjacent to the ink reservoir  20  as shown in FIG.  10 . Alternatively, the filter  50  may be placed on the curing resin  88  and stuck together with the curing resin  88 . In this case, the inkjet printing head  10  can be assembled without the sealing member  44  and filter  50  included therein. Thus, it is possible to provide a tubular projection on the ink reservoir  20 , and connect this projection directly to the head assembly  12 . This is advantageous in that the number of components and the number of assembling processes can be reduced.