Patent Publication Number: US-7722165-B2

Title: Liquid-droplet jetting apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The resent application claims priority from Japanese Patent Application No. 2005-353123, filed on Dec. 7, 2005, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid-droplet jetting apparatus constructed to jet (discharge) liquid-droplets of a liquid from a cavity unit by displacement of an active portion in a piezoelectric actuator. 
     2. Description of the Related Art 
     As a liquid-droplet jetting apparatus, there is an ink-jet head and the like. In Japanese Patent Application Laid-open No. 2004-291543 or the like, an embodiment of the ink-jet head is described which is constructed such that a jetting pressure is applied from a piezoelectric actuator to a cavity unit having nozzles so as to jet droplets of an ink (ink-droplets) from the nozzles. For example, in an embodiment disclosed in the Japanese Patent Application Laid-open No. 2004-291543, the cavity unit is formed in a substantially flat shape, and inside the cavity unit, ink supply channels, each of which is formed to range from one of pressure chambers, formed to open on one wide surface of the cavity unit, to reach one of nozzles formed to open on the other wide surface thereof, are provided for the nozzles respectively. 
     On the other hand, the piezoelectric actuator has a plurality of piezoelectric layers, individual electrodes provided for the pressure chambers respectively, and common electrodes each of which is arranged to cover the plurality of pressure chambers. In this piezoelectric actuator, areas of the piezoelectric layers, sandwiched between the individual electrodes and the common electrodes from thereabove and thereunder, are active portions which displace or deforms by a drive voltage applied between the individual electrodes and the common electrodes. Then, the piezoelectric actuator is stacked and fixed on the one wide surface of the cavity unit so that the active portions correspond to the pressure chambers respectively. 
     In the ink-jet head constructed in such a manner, displacement of an active portion changes the volume of a pressure chamber to thereby jet an ink filled in the pressure chamber from a nozzle. Therefore, to jet ink-droplets in a predetermined amount and at a predetermined speed, it is necessary to generate a predetermined amount of volumetric change in the pressure chamber. 
     With respect to the ink-jet head as an liquid-droplet jetting apparatus, there are tendencies to increase the degree of integration (densification) in a plane arrangement of nozzles and to decrease the plane area dimension of pressure chambers, so as to correspond to the miniaturization of the ink-jet head, the highly densified recording, and to the micronization of liquid-droplet in recent years. Accordingly, the reduction of the length of a channel (including a pressure chamber) needed for one nozzle not only makes it possible to realize the adaptation to the miniaturization of the ink-jet head and to the micronization of liquid-droplets, but also shortens an inherent cycle of a pressure fluctuation generated in the ink, thereby increasing a driving frequency of the jetting, which in turn is effective to realize the high-speed recording. However, this inevitably leads to the reduction in the plane area dimension of the active portions provided for the pressure chambers respectively, and thus it is necessary to increase the displacement amount of the active portions so that the volumetric change is applied, to the pressure chambers, in a predetermined amount by the active portions as a whole. Consequently, the drive voltage required for driving the active portions is needed to be set high. Further, the cavity unit is not a perfectly rigid body. Therefore, the displacement of active portion or portions is absorbed by the displacement of the cavity unit, causing a problem such that a predetermined jetting speed cannot be obtained without further setting the drive voltage higher. 
     SUMMARY OF THE INVENTION 
     The present invention is made to solve the above-described problems, and an object thereof is to realize a liquid-droplet jetting apparatus capable of applying a volumetric change sufficient for the jetting to a pressure chamber so as to obtain a predetermined jetting speed, without increasing a drive voltage for a piezoelectric actuator even when the length of a pressure chamber is reduced accompanying with the highly densified or integrated arrangement of the nozzles. In the following description, reference numerals in parentheses added to respective elements or components are just for illustrating these elements or components merely as examples, and are not intended to limit these elements or components. 
     According to a first aspect of the present invention, there is provided a liquid-droplet jetting apparatus ( 100 ) which jets liquid-droplets of a liquid from a plurality of nozzles ( 4 ), the apparatus including: a cavity unit ( 1 ) which has the nozzles ( 4 ) and a plurality of pressure chambers ( 36 ) corresponding to the nozzles ( 4 ) respectively and extending on a predetermined plane ( 17 ); and a piezoelectric actuator ( 2 ) which has a plurality of active portions ( 54 ) extending corresponding to the pressure chambers ( 36 ) respectively, and which is formed on the cavity unit ( 1 ) so as to cover the plane ( 17 ); wherein a length (L 1 ) in a longitudinal direction of each of the active portions ( 54 ) is not more than 1.5 mm; a height (T 1 ) of each of the pressure chambers ( 36 ) is 40 μm to 60 μm; a thickness (T 2 ) of a member ( 16 ) which defines surfaces, of the pressure chambers ( 36 ), on a side facing the piezoelectric actuator ( 2 ) is 100 μm to 150 μm; and volume of the pressure chambers ( 36 ) in which liquid is filled is changed by displacement of the active portions ( 54 ) so as to jet the liquid-droplets from the nozzles ( 4 ). 
     In the liquid-droplet jetting apparatus ( 100 ) of the present invention, the following fact was confirmed by an experiment. Namely, even when the length (L 1 ) of each of the active portions ( 54 ) is reduced to be not more than 1.5 mm, it is possible to stably jet liquid-droplets having a minute volume at a predetermined speed without increasing a drive voltage applied to the active portions ( 54 ), by setting the height (T 1 ) of each of the pressure chambers ( 36 ) to be 40 μm to 60 μm, and the thickness (T 2 ) of the member ( 16 ) which defines the surfaces, of the pressure chambers ( 36 ), on a side facing the piezoelectric actuator ( 2 ) to be 100 μm to 150 μm. 
     In the liquid-droplet jetting apparatus ( 100 ) of the present invention, a length (width) (W 1 ) in a short direction of each of the pressure chambers ( 36 ) may be 240 μm to 280 μm; the piezoelectric actuator ( 2 ) may have a plurality of base piezoelectric layers ( 51 ) which are stacked and a plurality of electrode layers ( 49 ) which sandwich the base piezoelectric layers ( 51 ) respectively therebetween; the electrode layers ( 49 ) may include a plurality of individual electrode layers in each of which a plurality of individual electrodes ( 46 ) extending corresponding to the pressure chambers ( 36 ) respectively are formed, and a plurality of common electrode layers in each of which a common electrode ( 47 ) is formed to cover the pressure chambers ( 36 ); areas, of each of the base piezoelectric layers ( 51 ), between the individual electrodes ( 46 ) and the common electrode ( 47 ) respectively may be formed as the active portions ( 54 ); a thickness (T 51 ) of each of the base piezoelectric layers ( 51 ) may be 15 μm to 40 μm; and a length (width) (W 3 ) in a short direction of each of the individual electrodes ( 46 ) may be 140 μm to 160 μm. When the thicknesses (T 51 , T 52 , T 53 ) of the piezoelectric layers and the width (W 3 ) of each of the individual electrodes ( 46 ) are changed, a displacement amount and an electrostatic capacitance of the active portions ( 54 ) are changed. In this case, by setting the thickness (T 51 , T 52 , T 53 ) of each of the piezoelectric layers ( 51 ,  52 ,  53 ) to 15 μm to 40 μm, and setting the width (W 3 ) of each of the individual electrodes ( 46 ) to 140 μm to 180 μm with respect to the width (W 1 ) that is 240 μm to 280 μm in a direction orthogonal to the longitudinal direction of each of the pressure chambers ( 36 ), then the displacement amount and the electrostatic capacitance of the active portions ( 54 ) can be optimized further provided that the above-described conditions are satisfied regarding the length (L 1 ) in the longitudinal direction of the active portions ( 54 ), the height (T 1 ) of the pressure chambers ( 36 ), and the thickness (T 2 ) of the member ( 16 ) which defines the surfaces, of the pressure chambers ( 36 ), on the side facing the piezoelectric actuator ( 2 ). 
     In the liquid-droplet jetting apparatus ( 100 ) of the present invention, the piezoelectric actuator ( 2 ) may further include: a top layer ( 53 ) arranged on a side opposite to the cavity unit ( 1 ) with respect to the base piezoelectric layers ( 51 ); and a bottom layer ( 52 ) arranged on a side opposite to the top layer ( 53 ) with respect to the base piezoelectric layers ( 51 ); the active portions ( 54 ) may be included only in each of the base piezoelectric layers ( 51 ); and a thickness (T 52 ) of the bottom layer ( 52 ) and a thickness (T 53 ) of the top layer ( 53 ) may be greater than the thickness (T 51 ) of each of the base piezoelectric layers ( 51 ). Specifically, the thickness (T 53 ) of the top layer ( 53 ) and the thickness (T 52 ) of the bottom layer ( 52 ) may be 25 μm to 40 μm; and the thickness (T 51 ) of each of the base piezoelectric layers ( 51 ) may be 15 μm to 30 μm. In this case, by making the thickness (T 53 ) of the top layer ( 53 ) greater than the thickness (T 51 ) of each of the base piezoelectric layers ( 51 ), displacement of the active portions ( 54 ) can be transmitted efficiently to the side of the pressure chambers ( 36 ) without allowing the displacement to escape to side of the top layer ( 53 ). Further, by making the thickness (T 52 ) of the bottom layer ( 52 ) greater than the thickness (T 51 ) of each of the base piezoelectric layers ( 51 ), it is possible to enhance an effect of preventing the ink filled in the pressure chambers ( 36 ) from permeating or infiltrating to the side of the piezoelectric actuator ( 2 ). Further, by making the thickness (T 53 ) of the top layer ( 53 ) and the thickness (T 52 ) of the bottom layer ( 52 ) to be great, it is possible to prevent a warpage which would be otherwise caused due to the unbalance or difference in thickness between the layers near to the top and bottom, respectively, of the piezoelectric actuator ( 2 ) when the piezoelectric actuator ( 2 ) is sintered during the production process thereof. Therefore, it is possible to make the active portions ( 54 ) in the piezoelectric actuator ( 2 ) act on the pressure chambers ( 36 ) respectively, in a substantially uniform manner. Further, by setting the thickness (T 53 ) of the top layer ( 53 ) to be 25 μm to 40 μm and setting the thickness (T 52 ) of the bottom layer ( 52 ) to be 25 μm to 40 μm, and by setting the thickness (T 51 ) of each of the base piezoelectric layers ( 51 ) to be 15 μm to 30 μm, these layers can be formed stably during the production of the piezoelectric actuator ( 2 ). 
     In the liquid-droplet jetting apparatus ( 100 ) of the present invention, the piezoelectric actuator ( 2 ) may further include a top layer ( 53 ) arranged on a side opposite to the cavity unit ( 1 ) with respect to the base piezoelectric layers ( 51 ), and a bottom layer ( 52 ) arranged on a side opposite to the top layer ( 53 ) with respect to the base piezoelectric layers ( 51 ); the active portions ( 54 ) may be included only in the base piezoelectric layers ( 51 ); and a thicknesses (T 51 ) of a base piezoelectric layer ( 51 ), among the plurality of base piezoelectric layers ( 51 ), which is closest to the top layer ( 53 ) and a thickness (T 52 ) of the bottom layer ( 52 ) may be greater than thicknesses (T 51 ) of base piezoelectric layers ( 51 ), among the plurality of base piezoelectric layers, which are different from the piezoelectric layer ( 51 ) closest to the top layer ( 53 ). Specifically, the thickness (T 51 ) of the base piezoelectric layer ( 51 ) closest to the top layer ( 53 ) and the thickness (T 52 ) of the bottom layer ( 52 ) may be 25 μm to 40 μm; and the thicknesses (T 51 ) of the base piezoelectric layers ( 51 ), which are different from the base piezoelectric layer ( 51 ) closest to the top layer ( 53 ), may be 15 μm to 30 μm. In this case, by making the thickness (T 51 ) of the base piezoelectric layer ( 51 ) which is closest to the top layer ( 53 ) and the thickness (T 52 ) of the bottom layer ( 52 ) to be great, it is possible to prevent the warpage which would be otherwise cause due to the difference in thickness between the layers nearer to the top and bottom portion of the piezoelectric actuator ( 2 ) when the piezoelectric actuator ( 2 ) is sintered during the production of the piezoelectric actuator ( 2 ). Accordingly, it is possible to make the active portions ( 54 ) in the piezoelectric actuator ( 2 ) act on the pressure chambers ( 36 ) in a substantially uniform manner. Further, by making the thickness (T 52 ) of the bottom layer ( 52 ) greater than the thickness (T 51 ) of each of the base piezoelectric layers ( 51 ), it is possible to enhance the effect of preventing the ink filled in the pressure chambers ( 36 ) from permeating to the side of the piezoelectric actuator ( 2 ). Further, by setting the thickness (T 51 ) of the base piezoelectric layer ( 51 ) which is closest to the top layer ( 53 ) and the thickness (T 52 ) of the bottom layer ( 52 ) to be 25 μm to 40 μm; and by setting the thickness (T 51 ) of each of the base piezoelectric layers ( 51 ), among the plurality of base piezoelectric layers ( 51 ), which are different from the base piezoelectric layer ( 51 ) closest to the top layer ( 53 ), to be 15 μm to 30 μm, these layers can be formed stably during the production of the piezoelectric actuator ( 2 ). 
     In the liquid-droplet jetting apparatus ( 100 ) of the present invention, in the cavity unit ( 1 ), a member ( 17 ) in which the plurality of pressure chambers ( 36 ) is formed and the member ( 16 ) which defines the surfaces, of the pressure chambers ( 36 ), on the side facing the piezoelectric actuator ( 2 ) may be made of a nickel alloy steel plate. 
     In the liquid-droplet jetting apparatus ( 100 ) of the present invention, the length (L 1 ) in the longitudinal direction of each of the active portions ( 54 ) may be not more than 1.2 mm. The inventor confirmed the following fact by the experiment that, even when the length (L 1 ) in the longitudinal direction of each of the active portions ( 54 ) is reduced to be not more than 1.2 mm, it is possible to stably jet a liquid-droplet having a minute volume at a predetermined speed without increasing the drive voltage applied to the active portions ( 54 ), by setting the height (T 1 ) of each of the pressure chambers ( 36 ) to be 40 μm to 60 μm and by setting the thickness (T 2 ) of the member ( 16 ) which defines the surfaces, of the pressure chambers ( 36 ), on the side facing the piezoelectric actuator ( 2 ) to be 100 μm to 150 μm. 
     In the liquid-droplet jetting apparatus ( 100 ) of the present invention, when the length (L 1 ) in the longitudinal direction of each of the active portions is 0.9 mm to 1.3 mm, a drive voltage for jetting the liquid-droplets at a jetting speed of 9 m/s may be 23.5 volts to 27 volts. 
     The liquid-droplet jetting apparatus ( 100 ) of the present invention may be an ink-jet head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of an ink-jet head as a liquid-droplet jetting apparatus; 
         FIG. 2  is an exploded perspective view of a cavity unit; 
         FIG. 3  is a cross-sectional view taken along a line indicated by arrows III-III in  FIG. 1 ; 
         FIG. 4  is a cross-sectional view taken along a line indicated by arrows IV-IV in  FIG. 3 ; 
         FIG. 5  is an explanatory view showing a positional relationship between pressure chambers and active portions; 
         FIG. 6A  is a table showing conditions of nozzle rows used in an experiment, and  FIG. 6B  is a graph showing a relationship between the thickness of a top piezoelectric layer and a drive voltage; 
         FIG. 7A  is a graph showing a relationship between the thickness of the cavity plate and the drive voltage, and  FIG. 7B  is a graph showing a relationship between the thickness of a base plate and the drive voltage; and 
         FIG. 8A  is a table showing a relationship between the thickness of the cavity plate and a jetting speed of ink (ink-jetting speed), and  FIG. 8B  is a graphic presentation of  FIG. 8A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, a basic embodiment of the present invention will be explained using  FIGS. 1 to 7 . 
       FIG. 1  is an exploded perspective view of an ink-jet head  100  as an embodiment of a liquid-droplet jetting apparatus. The ink-jet head  100  is constructed such that a plate-shaped piezoelectric actuator  2  is joined to a cavity unit  1  provided with a plurality of plates. A flexible flat cable  3  for connection to an external apparatus is stacked on and joined to the upper surface of this plate-shaped piezoelectric actuator  2 . An ink is jetted downward from nozzles  4  (see  FIG. 3 ) which are open on the side of the lower surface of the cavity unit  1 . 
     As shown in  FIG. 2 , the cavity unit  1  is constructed such that eight thin flat plates in total, namely a nozzle plate  11 , a spacer plate  12 , a damper plate  13 , two manifold plates  14   a  and  14   b , a supply plate  15 , a base plate  16 , and a cavity plate  17  are stacked and joined together in a laminated form with an adhesive so that the respective flat plate mutually face at surfaces thereof. In this description, a direction in which these flat plates are stacked is referred to as “stacking direction” as appropriate. 
     In the embodiment, each of the plates  11  to  17  has a thickness of approximately 40 μm to 150 μm, and the nozzle plate  11  is made of synthetic resin such as polyimide, and the plates  12  to  17 , other than plates  11 , are made of a 42% nickel alloy steel (steel to which nickel is added) plate. In the nozzle plate  11 , a large number of nozzles  4  each having a minute diameter (approximately 20 μm) are bored at minute spacing distances. These nozzles  4  are arranged in five rows along a longitudinal direction (X direction) of the nozzle plate  11 . Although a nozzle pitch between adjacent nozzles in a row is set to 75 dpi (dot per inch), the nozzles may be highly integrated by a pitch of not less than 75 dpi. 
     As shown in  FIG. 3 , the nozzles  4  are connected to pressure chambers  36 , of the cavity plate  17 , respectively, via through passages  38  which are bored through the spacer plate  12 , the damper plate  13 , the two manifold plates  14   a ,  14   b , the supply plate  15 , and the base plate  16 . As shown in  FIG. 2 , in the cavity plate  17 , a plurality of pressure chambers  36  are arranged in five rows (pressure-chamber rows) in parallel to a long side (X direction) of the cavity plate  17 . Each of the pressure chambers  36  has a slender (elongated) shape in plan view and is bored penetrating the plate thickness of the cavity plate  17  so that a longitudinal direction of each of the pressure chambers  36  is in parallel to a short direction (Y direction) of the cavity plate  17 . As shown in  FIG. 3 , each of the pressure chambers  36  communicates with a common ink chamber  7 , at one end  36   a  thereof in the longitudinal direction, via a communication hole  37  and a connection channel  40 , as will be described later; and each of the through passages  38  is connected to one of the pressure chambers  36  at the other end  36   b  thereof in the longitudinal direction. Each of the pressure chambers  36  is formed in a shape which is long along a direction in which the ink flows (ink-flow direction). 
     The pressure chambers  36  are bored in (formed to penetrate through) the cavity plate  17  by a pitch corresponding to the aforementioned nozzle pitch of 75 dpi for the nozzles  4 . Accordingly, for assuring the stability or the like in the production of the pressure chambers  36  in the cavity plate  17 , it is desirable that a width W 1  (as shown in  FIGS. 4 and 5 ), of each of the pressure chambers  36  in a direction orthogonal to the ink flow, is 240 μm to 280 μm. In this case, a spacing distance W 2  between adjacent pressure chambers  36  in a row is about 80 μm. Further, it is desirable that a height T 1  of each of the pressure chambers  36  is 40 μm to 60 μm. Note that the term “height” of each of the pressure chambers  36  means a length, in the stacking direction, of the pressure chambers  36 , in other words, a thickness T 1  (see  FIGS. 3 and 4 ) of the cavity plate  17 . The results of an experiment conducted with respect to the height T 1  of each of the pressure chambers  36  will be described later. Note that the length L 2  in the ink-flow direction (length in the longitudinal direction) of each of the pressure chambers  36  is set to be greater, than the length of an active portion  54  (to be described later), approximately by 0.1 mm to 0.3 mm, and there are prepared two types of the pressure chambers having two L 2 , respectively, one being 1.4±0.1 mm to 1.5±0.1 mm (hereinafter referred to as “1.4 mm”), and the other being 1.1±0.1 mm to 1.2±0.1 mm (hereinafter referred to as “1.1 mm”). Note that the above-mentioned width and height are common for these two types. These two types of the pressure chambers are prepared for corresponding to two types of liquids which are mutually different in a volume of liquid-droplets to be jetted. 
     In the base plate  16  adjacent to the lower surface of the cavity plate  17 , communication holes  37  each connecting to the one end  36   a  of one of the pressure chambers  36  are bored. This base plate  16  forms the surfaces, of the pressure chambers  36 , on a side facing the piezoelectric actuator  2 . Since the rigidity of the base plate  16  also have an effect to the transmittance of the jetting pressure, in order to efficiently transmit a jetting pressure, applied from the piezoelectric actuator  2  to the pressure chambers  36 , to the ink, it is conceivable to make the thickness T 2  of the base plate  16  (see  FIGS. 3 and 4 ) as great (thick) as possible. However, this in turn increases the channel length, the channel diameter, and/or the like for the through passages  38  and the communication holes  37 , thereby causing an adverse effect such as an occurrence of disturbance in the frequency of pressure wave generated in the pressure chambers. Therefore, it is desirable that the thickness T 2  of the base plate  16  is 100 μm to 150 μm. Note that the thickness T 2  of the base plate  16  (member which defines the surfaces of the pressure chambers  36  on the side facing the piezoelectric actuator  2 ) means a thickness in the stacking direction of the base plate  16 . The results of an experiment conducted with respect to the thickness T 2  of the base plate  16  will be described later on. 
     In the supply plate  15  adjacent to the lower surface of the base plate  16 , there are provided connection channels  40  which supply the ink, from the common ink chambers  7 , to the pressure chambers  36  respectively. As shown in  FIG. 3 , each of the connection channels  40  is provided with an inlet hole  40   a  to which the ink from one of the common ink chambers  7  enters, an outlet hole  40   b  which opens to face one of the communication holes  37 , and a throttle (narrowed portion)  40   c  located between the inlet hole  40   a  and the outlet hole  40   b  and formed with a small cross-sectional area so as to have the largest channel resistance therein among portions in the connection channel  40 . This throttle  40   c  is provided for preventing the reverse flow of the ink to the side of the common ink chamber  7  and for advancing toward the ink efficiently to the nozzle  4  when the pressure chamber  36  receives a jetting pressure for jetting the ink from the nozzle  4 . 
     In the two manifold plates  14   a ,  14   b , five pieces of the common ink chambers  7  are formed. Each of the common ink chambers  7  is long in a longitudinal direction (X direction) of the manifold plates, extends along one of the rows of nozzles  4  (nozzle rows) and penetrates through the plate thicknesses of the manifold plates  14   a ,  14   b . Namely, as shown in  FIGS. 2 and 3 , the five common ink chambers (manifold chambers)  7  in total are formed by stacking the two manifold plates  14   a ,  14   b , and by covering the upper surface and the lower surface thereof by the supply plate  15  and the damper plate  13 , respectively. Each of the common ink chambers  7  overlaps with portions (parts) of the pressure chambers  36  in one of the pressure-chamber rows and is elongated (extended) in the stacking direction of the plates along a row direction of the pressure chambers  36  (row direction of the nozzles  4 ) in plan view. 
     As shown in  FIGS. 2 and 3 , at a side of the lower surface of the damper plate  13  adjacent to the lower surface of the manifold plate  14   a , damper chambers  41  are formed as dents isolated from the common ink chambers  7 . As shown in  FIG. 2 , the position and shape of each of the damper chambers  41  are matched with one of the common ink chambers  7 . Since this damper plate  13  is made of a metal material which can elastically deform as appropriate, a ceiling portion in a thin plate shape at the upper side of each of the damper chambers  41  can freely vibrate toward both the common ink chamber  7  and the damper chamber  41 . When a pressure fluctuation generated in a certain pressure chamber  36 , among the pressure chambers  36 , upon the ink-jetting is jetted is propagated to one of the common ink chambers  7 , then the ceiling portion elastically deforms and vibrates to generate a damper effect to absorb and damp the pressure fluctuation, thereby preventing a cross-talk which is a phenomenon that the pressure fluctuation in the certain pressure chamber  36  is propagated to another pressure chamber  36 . 
     Further, as shown in  FIG. 2 , four ink supply holes  42  are bored, in the cavity plate  17  at one end thereof in the short direction, as inlets for the ink to the cavity unit  1 . Four connection holes  43  are bored in each of the base plate  16  and the supply plate  15 , corresponding the positions, of the four connection holes  43 , in the up and down direction to those of the four ink supply holes  42 . The ink from an ink supply source is supplied to each of the common ink chambers  7  at one end in a longitudinal direction thereof, via one of the ink supply holes  42  and one of the connection holes  43 . A filter body  20 , having filtering parts  20   a  corresponding to openings of the ink supply holes respectively, is adhered to the four ink supply holes  42  with an adhesive or the like. 
     In this embodiment, five pieces of the common ink chambers  7  are provided while four pieces of the ink supply holes  42  and four pieces of the connection holes  43  are provided; and among the ink supply holes, only an ink supply hole  42  located on the left end in  FIG. 2  is constructed to supply the ink to two pieces of the common ink chambers  7 ,  7 . This ink supply hole  42  is arranged to be supplied with a black ink, taking into consideration that the black ink is used more frequently than other color inks. To the remaining ink supply holes  42 , a yellow ink, a magenta ink and a cyan ink are independently supplied respectively. 
     On the other hand, similarly to a known structure, for example, one disclosed in Japanese Patent Application Laid-open No. 2002-254634 (corresponding to U.S. Pat. No. 6,595,628) or the like, the piezoelectric actuator  2  is provided with a plurality of ceramics layers which have a flat shape and a size to cover all the pressure chambers  36  and which are stacked in a direction orthogonal to a flat direction thereof, and a plurality of electrode layers arranged on a surface in the flat direction of the ceramics layers. Here, the electrode layers are formed with a conductive paste by a printing method or the like on sheet surfaces of an appropriate number of green sheets. The green sheets are obtained from a plurality of green sheets of piezoelectric ceramics materials which are formed to have a flat shape and made of a mixture of ceramics powder, binder, and solvent. Each of the green sheets is made to have a thickness of approximately 15 μm to 40 μm. The green sheets are stacked and burned to form the piezoelectric actuator  2 . 
     As the electrode layers, there are provided layers of drive electrodes including layers each of which has individual electrodes  46  formed therein for the pressure chambers  36  respectively, and layers each of which has a common electrode  47  formed to cover the plurality of the pressure chambers  36 ; and a layer of surface electrodes  48 . In the layers of drive electrodes, the layers of individual electrodes  46  and the layers of common electrodes  47  are arranged alternately in a direction in which the ceramics layers are stacked (stacking direction of the ceramic layers) so as to sandwich these ceramics layers therebetween. The layer of surface electrodes  48  is arranged on the uppermost surface of the piezoelectric actuator  2  (on the side opposite to the cavity unit) to thereby form the surface electrodes  48  separately connected to the individual electrodes  46  and the common electrodes  47 , respectively, via electrical through holes (see  FIG. 1 ). The surface electrodes  48  are each connected electrically to the flexible flat cable  3 . 
     In the piezoelectric actuator  2  in which electrode layers are provided in such a manner, a high voltage is applied between the individual electrodes  46  and the common electrodes  47  in a publicly known manner, so as to polarize portions of the ceramics layer sandwiched between the individual and common electrodes, thereby forming these portions as active portions  54  having a piezoelectric characteristic. In this embodiment, since active portions  54  are formed in a plurality of ceramics layers (hereinafter referred to as base piezoelectric layers  51 ) as will be described later, these active portions  54  are in a state of being overlapped in a direction in which the piezoelectric layers are stacked (stacking direction of the piezoelectric layers). Then, in a plan view in the stacking direction, each of the individual electrodes  46  has an elongated shape corresponding to the shape of one of the pressure chambers  36 , and each of the common electrodes  47  has a wide shape continuously covering the plurality of the pressure chambers  36 . Accordingly, the shape in plan view of the active portions  54  overlapped is the shape of a portion at which the individual electrodes  46  and the common electrodes  47  are overlapped (see  FIG. 5 ). 
     In the ceramics layers, there are provided the base piezoelectric layers  51 , each of which is sandwiched by the individual electrodes  46  and the common electrode  47  thereabove and thereunder, and in each of which the active portions  54  are formed; a bottom layer  52  arranged between the cavity unit  1  and an lowermost base piezoelectric layer  51  among the base piezoelectric layers  51  and including no active portions  54 ; and a top layer  53  arranged on an uppermost base piezoelectric layer  51 , among the base piezoelectric layers  51   a , on a side thereof opposite to the cavity unit  1  and including no active portions  54 . 
     The top layer  53  is provided for efficiently transmitting the displacement of the active portions  54  to the side of the pressure chambers  36  by preventing the displacement of the active portions  54  from escaping to the side opposite to the pressure chambers  36  (to the side of top layer  53 ). The bottom layer  52  is provided for preventing short-circuit between electrodes or the like which would be otherwise caused by the ink in the pressure chambers  36  permeating the piezoelectric actuator  2  covering the openings of the pressure chambers  36 . In this embodiment, the plurality of base piezoelectric layers  51  and a plurality of top layers  53  are provided while one piece of the bottom layer  52  is provided.  FIG. 4  illustrates an embodiment constructed of four base piezoelectric layers  51 , one bottom layer  52 , and two top layers  53 . Note that the term “one layer” used herein means a layer formed of one piece of the green sheet, and in a case, for example, in which two pieces of the green sheet are stacked and burned without sandwiching any electrode layer, and the two green sheets appear to be integrated, it is considered in this case that there are formed two layers. 
     The plate-type piezoelectric actuator  2  constructed in such a manner is stacked on and adhered and fixed to the cavity unit  1  so that the stacking direction of the piezoelectric layers matches with the stacking direction of the piezoelectric actuator  2  and the cavity unit  1 . The individual electrodes  46  of the piezoelectric actuator  2  are arranged so as to correspond to the pressure chambers  36 , respectively. Further, the aforementioned flexible flat cable  3  (see  FIG. 3 ) is joined to the upper surface of this piezoelectric actuator  2  so as to electrically connect various types of patterns (not shown) in this flexible flat cable  3  to the surface electrodes  48 , respectively. 
     In the ink-jet head  100  having the above-described structure, in view of highly integrating (desifying) the pressure chambers  36  corresponding to a highly integrated nozzle arrangement, and in view of improving the image quality by micronizing the liquid-droplet volume, the length L 1  in a longitudinal direction of each of the active portions  54  is set to be not more than 1.5 mm, preferably approximately 1.2 mm to 1.3 mm when the length of each of the pressure chambers  36  is 1.4 mm. When the length of each of the pressure chambers  36  is 1.1 mm, the length L 1  in the longitudinal direction of each of the active portions  54  is set to approximately 0.9 mm. Then, the inventor have conducted various experiments for jetting desired minute liquid-droplets at a predetermined speed even when the active portions  54  with such a short length are used. As a result, it was found out that, with respect to the pressure chambers  36  having the aforementioned width (W 1 ) of 240 μm to 280 μm, it is suitable to set the width W 3 , of the individual electrodes  46 , which is parallel to the width of the pressure chambers  36 , to be 140 μm to 160 μm. The shape of an area at which the individual electrode  46  and the common electrode  47  are overlapped (overlapping area) is reflected to the shape in plan view of each of the active portion  54  as it is. Therefore, the width (length in a short direction) of the shape in plan view of each of the active portions  54  becomes W 3  (=140 μm to 160 μm) (see  FIG. 5 ). 
     Further, as the result of the experiments, it was found out that the thickness of one piece of the layers in the piezoelectric actuator is preferably 15 μm to 40 μm. More specifically, it was found out that the thickness of each of the base piezoelectric layers  51  is preferably 15 μm to 30 μm, whereas the thickness of the top layers  53  and the thickness of the bottom layer  52  are preferably 25 μm to 40 μm, which are greater than the thickness of each of the base piezoelectric layers  51 . Further, it is allowable that the thickness of a base piezoelectric layer  51  closest to the top layer among the base piezoelectric layers  51 , is set to be 25 μm to 40 μm, instead of allowing the top layers  53  to have the thickness of 25 μm to 40 μm. In such a manner, by making the layers nearer to the top and bottom portions, respectively, of the piezoelectric actuators have greater thicknesses substantially in a vertically symmetrical manner, it is possible to prevent the warpage which would be otherwise caused due to the unbalance, in thickness, the layers nearer to the top and bottom portions, respectively, of the piezoelectric actuators when the piezoelectric actuator is subjected to burning during the production of the piezoelectric actuator. This makes it possible to make the active portions in the piezoelectric actuator act on the plurality of the pressure chambers in a substantially uniform manner. 
       FIG. 6B  shows results of the experiment to investigate as to how the drive voltage (voltage V) changes according to the thicknesses of the top layers  53 . As shown in  FIG. 6A , this experiment was performed for five types of nozzle rows A to E which are mutually different in PZT active-portion length (L 1 ), pressure chamber length (L 2 ), and nozzle diameter. In the nozzle row A, L 2 =1.2 mm and L 1 =0.9 mm; in the nozzle row B, L 2 =1.1 mm and L 1 =0.8 mm; in the nozzle row D, L 2 =1.5 mm and L 1 =1.2 mm; in the nozzle row E, L 2 =1.6 mm and L 1 =1.3 mm; and in the nozzle row C, for comparison purpose, L 2 =1.8 mm and L 1 =1.7 mm. Then, drive voltage values (described as “voltage” in the vertical axis) for obtaining a desired jetting speed of 9 m/s were compared among the nozzle rows. Note that the diameter of the nozzles  4  is set to 18.0 μm for the pressure chambers having lengths 1.2 mm and 1.1 mm; and the diameter of the nozzles  4  is set to 20.5 μm for the pressure chambers having lengths of 1.8 mm, 1.5 mm and 1.6 mm. As a result of the experiment, in the four nozzle rows A to D, other than the nozzle row E, the drive voltage are same or lower in a case in which the thicknesses of the top layers  53  are made greater (30 μm) than the thicknesses of the other layers, than the drive voltage in another case in which the thicknesses of the top layers  53  are equal (24 μm) to the thicknesses of the other layers. Therefore, it was confirmed that the drive voltage for obtaining the desired jetting speed can be lowered by making the thicknesses of the top layers  53  thicker than the thicknesses of the layers other than the top layers. 
     Further, it was confirmed that, when the nozzle rows A and B are compared (L 2 =1.1±0.1 mm), the drive voltage can be lowered in the nozzle row A (L 1 =0.9 mm) than the drive voltage in the nozzle row B (L 1 =0.8 mm). Furthermore, it was confirmed that, when the nozzle rows D and E are compared (L 2 =1.4±0.1 mm), the drive voltage is hardly different between the nozzle row D (L 1 =1.2 mm) and the nozzle row E (L 1 =1.3 mm). From these results, it can be appreciated that the PZT active-portion length L 1  affects the drive voltage more largely in a case where the pressure chamber length L 2  is 1.1±0.1 mm than in a case where the pressure chamber length L 2  is 1.4±0.1 mm. 
     Next, a comparative experiment regarding the height of the pressure chambers  36  is shown in  FIG. 7A . As the cavity plate  17  and as the base plate  16 , which defines the surfaces of the pressure chambers  36  on the side facing the piezoelectric actuator  2 , a 42% nickel alloy steel plates was used in the experiment. There were prepared three types of the cavity plate  17  with thicknesses of 40 μm, 50 μm, 80 μm, respectively (described as “cavity thickness” on the horizontal axis), and the four conditions of nozzle rows A, B, D, E shown in  FIG. 6A  are combined with these three types of the cavity plate so as to compare drive voltage values (described as “voltage” on the vertical axis) for obtaining a desired jetting speed of 9 m/s. As a result, as shown in  FIG. 7A , it was found out that the drive voltage becomes lower in a case, in which the thickness T 1  of the cavity plate  17  (height of each of the pressure chambers) is 50 μm, than in a case in which the thickness T 1  is set to thicknesses other than 50 μm (namely, 40 μm, 80 μm). Further, in the nozzle rows A, D, E, the drive voltage is lower than that of the nozzle row B; and that particularly the nozzle rows D, E are hardly different in drive voltage. Furthermore, it is presumable from  FIG. 7A  that the thickness of not more than 60 μm makes it possible to drive not only the nozzle rows D, E but also the nozzle row A sufficiently by a low voltage. Therefore, it was found out that as the height T 1 , including tolerances, of each of the pressure chambers, a value of the aforementioned 40 μm to 60 μm is optimum; and that as the length L 1  of each of the active portions, a length of 1.3 mm to 0.9 mm is optimum. In these cases, the drive voltage value for obtaining the desired jetting speed of 9 m/s can be made to fall in the range of 23.5 V to 27 V. 
     Next, a comparative experiment regarding the thickness of the base plate  16  as the member which defines the surfaces of the pressure chambers  36  on the side facing the piezoelectric actuator  2  is shown in  FIG. 7B . As the cavity plate  17 , and as the base plate  16  which defines the surfaces of in the pressure chambers  36  on the side opposing the piezoelectric actuator  2 , a 42% nickel alloy steel plate was used in this experiment. In the above-described embodiment, there were prepared four types of the base plate  16  having thicknesses of 50 μm, 100 μm, 150 μm, 200 μm respectively; and four conditions of nozzle rows A, B, D, E shown in  FIG. 6A  are combined with these four types of the base plate  16  so as to compare drive voltage values (described as “voltage” on the vertical axis) for obtaining a desired jetting speed of 9 m/s. As a result, as shown in  FIG. 7B , it was found out that the drive voltage becomes lower in a case, in which the thickness of the base plate  16  is 100 μm to 150 μm, than in cases other than this case. Further, in the nozzle rows A, D, E, the drive voltage was lower than that in the nozzle row B; and particularly in the nozzle rows D, E, the drive voltages are hardly different from each other. Therefore, it was found out that as the thickness T 2 , including tolerances, of the base plate  16 , a value of the aforementioned 100 μm to 150 μm is optimum; and that as the length L 1  of each of the active portions  54 , a length of 1.3 mm to 0.9 mm is optimum. 
     It is necessary that the stiffness of the base plate  16  is high for transmitting a jetting pressure from the piezoelectric actuator  2  efficiently to the ink in the pressure chambers  36 . Therefore, it is conceivable to make the thickness T 2  of the base plate  16  as thick as possible, but the drive voltage is high when the thickness T 2 =200 μm. The cause for this can be conceived that, as the thickness of the base plate  16  is increased, the channel length, channel diameter, and the like of the through passages  38  and the communication holes  37  are also increased to cause effects such as the disturbance in the cycle (frequency) of pressure wave generated in the ink in the pressure chambers, or the like. 
     Next, to verify the optimum values for the cavity thickness obtained from the results shown in  FIG. 7A , a simulation was performed. The simulation was conducted to see, in a case that the drive voltage of the piezoelectric actuator is constant, how the jetting speed of the ink is changed when the thickness T 1  of the cavity plate  17  is changed. This simulation is based on the principle of operation of the piezoelectric actuator as follows. When a drive voltage is applied to the electrode layers in the piezoelectric actuator, active portions  54  extend in the thickness direction of the base piezoelectric layer  51 , which decreases the volume of a pressure chamber  36 , corresponding to the active portions  54 , so as to increase the pressure of the ink inside the pressure chamber  36 , thereby jetting the ink from a nozzle corresponding to the pressure chamber. Here, when the thickness T 1  of the cavity plate  17  (namely, the height of the pressure chambers  36 ) is changed, the volume change rate of the pressure chambers  36  becomes different, so that an amount in which the volume of the pressure chamber  36  is decreased (volume decrease amount) changes even when the same drive voltage is applied. Therefore, the pressure applied to the ink inside the pressure chamber  36  is changed also, and consequently the jetting speed of ink is changed, too. The simulation was carried out that in the piezoelectric actuator used in the simulation, the width W 1  of the pressure chamber  36  was 260 μm and the width W 3  of the individual electrode  46  was 150 μm; the drive voltage was 20 V; and two types of nozzles for black ink (black nozzle) and for color ink (color nozzle) were used. For the black nozzle, the nozzle diameter was 20.5 μm, the PZT active-portion length L 1  was 1.25 mm, and the pressure chamber length L 2  was 1.35 mm. For the color nozzle, the nozzle diameter was 18 μm, the PZT active-portion length L 1  was 0.85 mm, and the pressure chamber length L 2  was 0.95 mm.  FIG. 8A  shows the results of calculation performed under these conditions for a jetting speed with the black nozzle and a jetting speed with the color nozzle respectively, in cases where the thickness T 1  of the cavity plate  17  was set to 30 μm, 40 μm, 50 μm, 60 μm, 80 μm, 100 μm, respectively; and these results are graphically presented in  FIG. 8B . As shown in  FIG. 8B , with respect to the black nozzle (BK), the jetting speed of ink increases gradually as the cavity thickness increases from 30 μm to 60 μm, and decreases when the cavity thickness exceeds 60 μm. On the other hand, in the case of the color nozzle (Cl), the jetting speed of ink increases gradually as the cavity thickness increases from 30 μm to 50 μm, and decreases when the cavity thickness exceeds 50 μm. From these results, it can be appreciated that, with respect to both of the black nozzle and the color nozzle, a much faster jetting speed can be obtained when the thickness T 1  of the cavity plate  17  is in a range of 40 μm to 60 μm. The following can be considered as a cause of the above-mentioned phenomena. That is, when the thickness T 1  of the cavity plate  17  is 30 μm, the cross sectional area of the pressure chamber  36  is small, and thus the channel resistance in the pressure chamber  36  is large. Accordingly, with this large channel resistance, the speed is small at which the ink flows in the pressure chamber, thereby making the jetting speed to be low. On the other hand, when the thickness T 1  of the cavity plate  17  exceeds 60 μm, then the volume of the pressure chamber  36  is large, and thus a rate is small at which the pressure chamber  36  is deformed due to the displacement of the active portion  54 . Accordingly, it is not possible to obtain any sufficient jetting speed for the ink. 
     According to the experiments conducted by the inventor, the length L 1  of the active portions  54  is set smaller than the length L 2  of the pressure chambers  36 , by approximately 0.1 mm to 0.3 mm. However, it is found out that a difference in this range does not greatly affect the jetting speed of ink-droplets. Therefore, the length L 1  of approximately 1.5 mm can be usable for the active portions  54  with respect to the length 1.6 mm of the pressure chambers  36  in the nozzle row E. 
     Thus, in the present invention, even when the length L 1  in the longitudinal direction of the active portion  54  is set to be a small length such as not more than 1.5 mm, it is possible to suppress the increase in drive voltage, by optimizing the structure of the pressure chambers  36  and the piezoelectric actuator  2  as described above. Therefore, it is possible to highly integrate the pressure chambers  36  and to improve image quality by jetting small ink-droplets at a predetermined speed. 
     In the above-described embodiment, the present invention is applied to an ink-jet head for jetting ink, but the present invention is applicable also to a device for coating coloring liquid to a medium, a device for forming a thin film on a medium, or the like.