Abstract:
The present invention relates to an inkjet recording head, which includes an ink cavity and an ink discharge port, a mounting member covering the ink cavity, an oscillating member, and a substrate to support the oscillating member. Various parts of the inkjet recording head are held together with adhesive. In attaching the piezoelectric element to the mounting member, opposing surfaces of the oscillating member and the mounting member are fixed to one another with adhesive. Then, the two members are pressed together, thereby causing excessive adhesive to emerge from between opposing surfaces of the oscillating member and the mounting member. The emerged adhesive is formed into chamfer to provide the oscillating member and the mounting member with areas of attachment that extend beyond the area of attachment that exists between the opposing surfaces.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an inkjet recording head that applies pressure to eject ink accommodated in an ink cavity by deforming a piezoelectric element in accordance with input signals. 
     As depicted in FIG. 1, one known example of a drop-on-demand (DOD) type inkjet recording head comprises, a channel plate  10  having an ink cavity  12  and ink discharge port  14 , a panel  16  covering the ink cavity  12 , a piezoelectric element  18 , a substrate  20  to support piezoelectric element  18 , and an adhesive  24  for attaching the piezoelectric element  18  to the panel  16 . In this recording head, the ink  22 , which is accommodated in the ink cavity  12 , is subjected to pressure so as to be discharged from the ink discharge port  14  via the deformation of panel  16  in accordance with the deformation of piezoelectric element  18 . As a result, it is necessary that the deformation of piezoelectric element  18  is reliably transmitted to the panel  16 , and subsequently, to the ink  22  accommodated in the ink cavity  12 . Therefore, the piezoelectric element  18  is fixedly attached to panel  16  by an adhesive  24 . 
     The above-described inkjet recording head, however, requires a degree of durability so that the piezoelectric element  18  does not separate from the panel  16  under continuous oscillation, for example, 4 KHz per 80 hrs (about 1×10 9 ) or more. Therefore, such an inkjet recording head has a disadvantage because sufficient durability cannot be obtained simply by an application of an adhesive between the piezoelectric element  18  and the panel  16 . 
     The inkjet recording head&#39;s durability can be increased to a certain degree by increasing the thickness of the adhesive  24 . However, this is not advantageous because the deformation of piezoelectric element  18  is absorbed by the adhesive  24  when the thickness of adhesive  24  is increased. Consequently the discharge force and pressure applied to ink  20  are reduced. 
     FIG. 2 is a table that shows the results of analysis of the relationship between displacement loss and adhesive thickness and amount of displacement loss using a finite element method. In particular, the table shows the amount of displacement for various thicknesses of adhesive at the contact area (point a, FIG. 1) between the adhesive  24  and the panel  16 , and the average amount of displacement at other contact areas (center: point b; edge: point c; midpoint between point b and point c: point d) between the adhesive  24  and the panel  16 . Also, FIG. 2 shows the percentage of displacement loss relative to the amount of displacement loss of the panel, and the average amount of displacement loss of the piezoelectric element at various adhesive thicknesses (5, 25, and 50 μm). As can be clearly understood from Table 1, displacement loss increases in conjunction with the increase in the adhesive thickness, thereby reducing the discharge force of ink  22 . (The aforesaid analysis assumes that the adhesive has a Young&#39;s modulus of 30 kgf/cm 2 .) 
     SUMMARY OF THE INVENTION 
     The inkjet recording head of the present invention has an oscillating member attached to a mounting member by an application of adhesive to the opposing surfaces of these members, wherein the adhesive on the external surfaces of the oscillating member and the mounting member forms a chamfer. 
     Furthermore, the inkjet recording head of the present invention is formed attaching an oscillating member to a mounting member adheres an oscillating member and mounting member by applying adhesive to at least a surface of the oscillating member or the mounting member, and thereafter pressing together the oscillating member and the mounting member so as to cause adhesive to emerge from the adhering surfaces to attach the members together. 
     According to the aforesaid inkjet recording head, opposed surfaces of an oscillating member and a mounting member are fixed to one another with adhesive, and a chamfer is formed in the adhesive that emerges from between the surfaces of the oscillating member and the mounting member at the adjacent external surfaces, so that the oscillating member and the mounting member have areas of attachment that extend beyond the area of attachment that exists between the opposed surfaces. This arrangement provided an advantage in that the adhesion strength and separation resistance between the oscillating member and the mounting member are improved. Furthermore, the absorption of oscillation of the oscillating member by the adhesive is suppressed to a lower limit by reducing the thickness of the adhesive by the aforesaid arrangement. 
     The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description taken in conjunction with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial section view of a conventional inkjet recording head. 
     FIG. 2 is a table showing the results of analysis of the relationship between displacement loss and adhesive thickness and amount of displacement loss using a finite element method. 
     FIG. 3 is a section view of an inkjet recording head of an embodiment of the present invention. 
     FIG. 4 is a partial enlarged section view of FIG.  3 . 
     FIGS.  5 ( a ) and  5 ( b ) illustrate the method of adhering a piezo-actuator to a panel. 
     FIGS.  6 ( a ) and  6 ( b ) illustrate a method of shaping extruded adhesive into a curved surface. 
     FIG. 7 illustrates the amount of applied adhesive. 
     FIGS.  8 ( a )- 8 ( d ) show the relationship between the radius of curvature and the maximum tension when a predetermined force acts upon the panel. 
     FIGS.  9 ( a ) and  9 ( b ) are tables showing the results of continuous durability tests. 
     FIG. 10 is a section view of an inkjet recording head of another embodiment of the present invention including an enlarged sectional view of a portion of the section view. 
     FIG. 11 is a section view of an inkjet recording head of another embodiment of the present invention including an enlarged sectional view of a another portion of the section view. 
     FIG. 12 is a partial section view of an inkjet recording head of still another embodiment of the present invention. 
     FIGS.  13 ( a ) and  13 ( b ) show a modification of the shaped curved surface of the adhesive. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention are described hereinafter with reference to the accompanying drawings. 
     FIG. 3 is an enlarged section view of a DOD-type inkjet recording head  30 , and FIG. 4 is a partial enlarged section view of the recording head  30 . In recording head  30 , a piezoelectric element  34 , which is formed of piezoelectric material, is fixedly attached to a substrate  32 , which is formed of a non-piezoelectric material. Piezoelectric element  34  comprises bilateral substrates or support members  36 , and a plurality of piezo-actuators  38  arranged with an equidistant spacing between said substrates  36 . Each piezo-actuator  38  has electrodes (not illustrated) disposed on the top and bottom surfaces thereof, and is subjected to a polarization process in the opposite direction of said electrodes, so as to be deformed when a voltage is applied between the electrodes. A panel or mounting member  40  is provided above the piezoelectric element  34 . The support members  36  and the piezoelectric element-actuators  38  are fixedly attached to the panel  40  by an adhesive  42 . A channel plate  44 , comprising a non-piezoelectric material, is fixedly attached on the panel  40 . The channel plate  44  is provided with an ink cavity or cutout  46  to accommodate or hold ink  45  in an area facing the piezoactuator  38 . The bottom of the ink cavity  46  is closed by panel  40 . The channel plate  44  is also provided with a nozzle  48  to discharge the ink  45  accommodated in each ink cavity  46 . 
     In the inkjet recording head of the aforesaid construction, a piezo-actuator  38  is deformed when a voltage is applied between its electrodes, such that a panel area of the panel  40  in contact with said piezo-actuator  38  is bent toward ink cavity  46 . As a result, the ink  45  held in said ink cavity  46  is subjected to pressure and caused to be discharged from a nozzle  48 . 
     The bond between piezo-actuator  38  and panel  40  is described below with reference to FIGS.  5 ( a ) and  5 ( b ). The bond between a piezo-actuator  38  and the panel  40  is accomplished by applying a predetermined amount of adhesive  42  to the surface of piezo-actuator  38  facing panel  40  by a method, such as for example, screen printing, pad printing or the like. The amount of adhesive  42  that is applied is described later with reference to FIG.  6 . The panel  40  is pressed against piezo-actuator  38  until any surplus adhesive  42  emerges from between the panel  40  and the piezo-actuator  38 . The thickness of adhesive  42  between the piezo-actuator  38  and the panel  40  can be set below the surface roughness of a piezo-actuator  38  (i.e., normally 3 to 5 μm). 
     The extruded adhesive, which emerges from between panel  40  and piezo-actuator  38 , forms a chamfer by a method described later, so as to form a concave curved surface. When the cross section of the shaped curved surface is assumed to be a circular arc, the radius r of the curved surface is desirably 3 to 200 μm. This radius r is desirably less than one half the distance between adjacent piezoelectric elements, i.e., less than one half the height of the piezoelectric element. 
     The shaping of the extruded adhesive may be accomplished by finishing the concave curved surface on the exterior side of the point angle with a shaping fixture  50  (FIGS.  6 ( a ) and  6 ( b )). Prior to applying a shaping fixture to the adhesive, the shaping fixture is subjected to a processing to achieve excellent separation characteristics or is coated with a fluororesin or the like,which has excellent separation characteristics, at least on the parts of the fixture that will contact the adhesive. 
     As illustrated in FIGS.  6 ( a ) and  6 ( b ), the shaping fixture  50  is pushed toward the extruded adhesive to shape a desired curvature  52 . 
     Alternatively, vibration and/or centrifugal force may be applied to a piezo-actuator  38  that has been attached to a panel  40  to achieve similar shaping, depending on the surface tension and viscosity of the adhesive used. 
     The amount of applied adhesive  42  is described below with reference to FIG.  7 . The width of the piezo-actuator  38  is designated W, the final thickness of the adhesive applied between the piezo-actuator  38  and the panel  40  is designated D, and the radius of the shaped curved surface of the extruded adhesive  42  is designated r. The amount of adhesive per unit length initially applied to piezo-actuator  38  can be expressed as [2(4−π)r 2 +4DW]/4W. 
     The radius r for the shaped curved surface  52  of the adhesive  42  was variously set at 80, 40, 10 and 0 μm. The maximum tension generated in adhesive  42  was determined using the finite element method when a force of 100 gf/mm 2  was applied on panel  40  toward ink cavity  46 . The results are shown in FIGS.  8 ( a )- 8 ( d ). As shown in FIG.  8 ( d ), without the curved surface the maximum tension was 2,295 gf/mm 2 . However, the maximum tension decreased in conjunction with the increase in the radius of the curved surface. Thus, when the radius of curvature r was 80 μm (FIG. 8 ( a )), the maximum tension was 708 gf/mm 2  (about ⅓ the maximum tension without curvature). 
     Continuous operation durability tests were conducted when the adhesive  42  extruded from between the piezo-actuator  38  and the panel  40  was not shaped into a curved surface, and when adhesive was not extruded from between these members. These durability tests were performed by confirming the occurrence or lack thereof of separation of the panel at predetermined intervals until 1,000,000,000 oscillations (10 9 ) were attained, and checking the number of oscillations (number of oscillations of drive durability) when the moment of separation occurred. The adhesive used was epoxy AZ-15 (Ciba-Geigy). 
     FIGS.  9 ( a ) and  9 ( b ) are Tables that show the results of the continuous durability tests. In the tables, [E+n] represents 10+ n , e.g., E+08 is 10+ 8 , and E−03 is 10− 3 . The voltage and frequency applied between the electrodes was 30 V, 4 KHz. 
     As shown in FIG.  9 ( b ), at a thickness of 5 μm without extruded adhesive, the panel separated at 2,400,000 continuous oscillations. It was necessary to have an adhesive thickness of 50 μm or more to prevent separation of the panel prior to the target of 1,000,000,000 oscillations. 
     On the other hand, when the adhesive was extruded but not shaped into a curved surface, the durability could be improved by enlarging the cross section area of the extrusion, as shown in FIG.  9 ( a ). Separation of the panel wall could be prevented for the 1,000,000,000 continuous oscillations, when the adhesive thickness was 5 μm and the extrusion cross section area was set at about 1.0×10 −2  (mm 2 ) or more. However, when the extruded adhesive was shaped as a curved surface, e.g., having an adhesive thickness of 5 μm and radius of curvature of 25 μm or more, separation of the panel could be prevented prior to the target of 1,000,000,000 continuous oscillations. In particular, the durability was remarkably better with a shaped extruded adhesive compared to when the extruded adhesive was not shaped. 
     Although the preceding examples have been described in terms of adhesive applied between a piezo-actuator  38  and a panel  40 , adhesive may be applied between a panel  40  and the head unit or channel plate  44 , as shown in FIG.  10 . Similar effectiveness can be obtained by extruding the adhesive from between these two members, and applying vibration or centrifugal force to shape the extruded adhesive into a curved surface. 
     In the description that follows, like parts are designated by like reference numbers and are not described in further detail. 
     As shown in FIG. 11, the bonding of a piezoactuator  38  and substrate  32  may be accomplished to form an ink recording head by fixedly attaching a piezoactuator  38  and substrate  32  by using an adhesive  42 , or by partially adhering these members with local applications of the adhesive  42 . 
     As shown in FIG. 12, in the case of a laminate type piezo-actuator  54 , it is desirable that the radius of curvature r of the adhesive  42 , extruded between piezo-actuator  54  and panel  40 , is smaller than the thickness of the final exterior layer, i.e., inactive layer  56 , opposite panel  40 , so as to not have the extruded adhesive  42  adhere to an active layer  58 . Consequently, there is no restriction in the deformation of the piezo-actuator  54  by the adhesive attached to an active layer after the adhesive has hardened. It is further desirable that piezo-actuator  54  and substrate  32  be similarly bonded. 
     Although the preceding embodiments have been described using a recording head wherein a panel  40  is deformed in conjunction with the deformation of piezo-actuator  38  to apply pressure on ink held in an ink cavity  6 , alternate arrangements are contemplated. For example, the present invention may be applied to the bonding of a substrate to fixedly attach a piezo-actuator to a piezo-actuator in a recording head, wherein a piezo-actuator is arranged within an ink cavity, which is not provided with a panel. In this arrangement, the ink can be discharged by direct pressure of the piezo-actuator in conjunction with the oscillation of said piezo-actuator. In another variation, the panel  40  can be a thin film. 
     Further, although an extruded adhesive was shaped to form a curved surface in the aforesaid embodiments, the adhesive may be shaped in a plurality of shapes. For example, as shown in FIG.  13 ( a ), a multi-angular shape may be formed. Or as shown in FIG.  13 ( b ), a linear shape may be formed. 
     Furthermore, although an adhesive was applied to a piezo-actuator, which was subsequently pressed against a panel to extrude adhesive from between the bonding surfaces in the above description, as a variation, the adhesive may be applied to the panel beforehand. 
     Of course, it should be understood that a wide range of changes and modifications can be made to the preferred embodiment described above and that the foregoing description be regarded as illustrative rather than limiting. It is therefore intended that it is the following claims, including all equivalents, which are intended to define the scope of this invention.