Inkjet recording head having a driving source attached by a chamfered adhesive layer

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'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's modulus of 30 kgf/cm 2 .)

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.

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.

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.