Inkjet head and image forming apparatus

Abstract

Provided are an inkjet head capable of discharging ink droplets of high viscosity, and an image forming device. The inkjet head is provided with: a piezoelectric element having a plurality of drive regions arranged in a first direction corresponding to a plurality of nozzles; a plurality of individual electrodes which are arranged in the piezoelectric element for each drive region and to which a drive voltage for discharging ink from the corresponding nozzle is individually applied; and a plurality of common electrodes disposed in the piezoelectric element alternately with the individual electrodes in the first direction. The piezoelectric element is polarized in a second direction orthogonal to the first direction so that the piezoelectric element is sheared and deformed by an electric field generated in the first direction when the drive voltage is applied to the individual electrodes, causing the ink to be discharged from the nozzle, and at least one electrode of the individual electrodes and the common electrodes is arranged inside the piezoelectric element.

Description

Technical Field

[0001] The present disclosure relates to an inkjet head and an image forming apparatus.

Background Art

[0002] Conventionally, an inkjet image forming apparatus is known that discharges ink from nozzles arranged in a plurality on an inkjet head onto a recording medium such as paper to form an image on the recording medium. In such an inkjet image forming apparatus, the inkjet head has a plurality of channels corresponding to the number of nozzles, and for each channel, includes a pressure chamber for storing ink and a piezoelectric element for deforming the pressure chamber.

[0003] In an inkjet head, when a driving voltage is applied to the piezoelectric element, the piezoelectric element deforms according to the driving voltage, so that the pressure chamber deforms, and the pressure on the ink in the pressure chamber supplied to the nozzle changes. As a result, the ink in the pressure chamber is discharged from the nozzle.

[0004] For example, Patent Document 1 discloses a technique of discharging ink by utilizing the deformation of a piezoelectric element that occurs when an electrode is disposed on the surface of the piezoelectric element and a driving voltage is applied to the electrode. This discharges ink by deforming the pressure chamber by utilizing the shear deformation generated in the piezoelectric element due to the fact that the polarization direction of the piezoelectric element and the electric field direction are orthogonal.

[0005] Also, Patent Document 2 discloses an inkjet head in which notches are formed at predetermined positions on both surfaces of a piezoelectric element, and electrodes are arranged so that the polarization direction and the electric field direction in the piezoelectric element are parallel. In this inkjet head, when a driving voltage is applied to the electrode, the deformation direction of the piezoelectric element is changed from a direction along the surface of the piezoelectric element to a direction orthogonal thereto, so that the piezoelectric element deforms so as to bend and ink is discharged.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] U.S. Patent No. 4,584,590 [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2003-008095 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] By the way, recently, an image forming apparatus capable of ejecting high-viscosity ink has been desired so as to cope with various image formations. When ejecting high-viscosity ink, it is necessary to sufficiently generate the force and displacement by a piezoelectric element. For this purpose, it is conceivable to increase the thickness of the piezoelectric element.

[0008] However, in the inkjet head using the technique described in Patent Document 1, the electrodes are arranged only on the surface of the piezoelectric element. Therefore, the electric field generated in the piezoelectric element when a driving voltage is applied to the electrodes is limited to the vicinity of the surface, and a sufficient electric field cannot be generated inside. Therefore, it is difficult to sufficiently generate the force and displacement by the piezoelectric element.

[0009] Even when the thickness of the piezoelectric element is increased, since the electrodes are arranged only on the surface, a sufficient electric field cannot be generated inside the piezoelectric element, so there is a possibility that the piezoelectric element may not be sufficiently deformed. In addition, by increasing the thickness of the piezoelectric element, the rigidity of the piezoelectric element increases, but the increase in rigidity causes a decrease in the displacement amount, and sufficient pressure for ejection cannot be applied to the ink in the pressure chamber.

[0010] In addition, in the inkjet head described in Patent Document 2, a plate-like piezoelectric body that is fixed at both ends by a partition wall of a liquid chamber and has a notch at a location corresponding to the central portion of the liquid chamber is polarized in a direction along the surface of the plate. When an electric field in the same direction as the polarization is applied to this piezoelectric body, the piezoelectric body tries to expand in the direction of polarization due to the longitudinal piezoelectric effect. However, since both ends are fixed, it bends and deforms toward the side without a notch. This bending deformation is a deformation in a direction different from the deformation direction due to the original longitudinal piezoelectric effect. Therefore, the force generated by the deformation of the piezoelectric element is weak, and it is difficult to eject high-viscosity ink. Furthermore, since the deformation cannot be controlled with high precision, the ejection of ink cannot be accurately controlled.

[0011] On the other hand, when the thickness of the piezoelectric element is increased to increase the force generated by the deformation of the piezoelectric element, the rigidity of the piezoelectric element increases, making it less likely to cause bending displacement. Therefore, the displacement amount of the piezoelectric element decreases.

[0012] An object of the present disclosure is to provide an inkjet head and an image forming apparatus capable of ejecting high-viscosity ink droplets.

Means for Solving the Problems

[0013] The inkjet head according to the present disclosure A piezoelectric element having a plurality of drive regions arranged in a first direction corresponding to a plurality of nozzles, A plurality of individual electrodes arranged for each of the drive regions in the piezoelectric element, to which a drive voltage for ejecting ink from the corresponding nozzle is individually applied, A plurality of common electrodes arranged alternately with the individual electrodes in the piezoelectric element in the first direction, to which a common drive voltage is applied to the plurality of drive regions, Comprising The piezoelectric element is polarized in a second direction orthogonal to the first direction so that when the drive voltage is applied to the individual electrodes, the piezoelectric element undergoes shear deformation due to an electric field generated in the first direction and ejects the ink from the nozzle. At least one of the individual electrodes and the common electrode is disposed inside the piezoelectric element.

[0014] In addition, the image forming apparatus according to the present disclosure includes the above-described inkjet head.

Advantages of the Invention

[0015] According to the present disclosure, highly viscous ink droplets can be ejected.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0017] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments, and various modifications can be made without departing from the gist of the present disclosure. Also, in each figure, those denoted by the same reference numerals are the same or corresponding ones, which is common throughout the entire specification.

[0018] Embodiments of the present disclosure will be described with reference to the drawings. The inkjet image forming apparatus according to the present embodiment forms an image by ejecting ink droplets onto a recording medium such as paper.

[0019] [Configuration of Inkjet Image Forming Apparatus 1] FIG. 1 is a schematic diagram showing an example of the configuration of an inkjet image forming apparatus 1 according to the present embodiment. As shown in FIG. 1, the inkjet image forming apparatus 1 includes a supply unit 10, an image forming unit 20, a discharge unit 30, and the like. Under the control of a control unit (not shown), the inkjet image forming apparatus 1 conveys a recording medium M stored in the supply unit 10 to the image forming unit 20, forms an image on the recording medium M in the image forming unit 20, and conveys (ejects) the recording medium M on which the image is formed to the discharge unit 30. As the recording medium M, various media capable of fixing the ink landing on the surface can be used, such as paper like plain paper or coated paper, as well as cloth or sheet-like resin.

[0020] The supply unit 10 includes a paper feed tray 11 for storing the recording medium M, and a medium supply unit 12 for conveying and supplying the recording medium M from the paper feed tray 11 to the image forming unit 20.

[0021] The paper feed tray 11 is a plate-like member provided so that one or a plurality of recording media M can be placed thereon. The paper feed tray 11 is provided so as to move up and down according to the amount (number of sheets) of the recording medium M placed on the paper feed tray 11, and in the vertical movement direction, the uppermost recording medium M is held at a position where it is conveyed by the medium supply unit 12.

[0022] The medium supply unit 12 includes an annular belt supported inside by two rollers, and by rotating the rollers with the recording medium M placed on this belt, the recording medium M is conveyed from the paper feed tray 11 to the image forming unit 20.

[0023] The image forming unit 20 includes a conveyance drum 21, a delivery unit 22, a medium heating unit 23, a head unit 24, a fixing unit 26, a delivery unit 27, and the like.

[0024] The transport drum 21 rotates about a rotation axis extending in a direction perpendicular to the plane of FIG. 1 (hereinafter referred to as the "orthogonal direction") while holding the recording medium M on the outer peripheral curved surface (transport surface) in the shape of a cylindrical surface, thereby transporting the recording medium M in the transport direction along the transport surface (see the arrow in FIG. 1).

[0025] The transport drum 21 is provided with claw portions and air intake portions (not shown) for holding the recording medium M on its transport surface. The recording medium M is held on the transport surface by having its end pressed by the claw portions and being sucked towards the transport surface by the air intake portions. The transport drum 21 has a transport drum motor (not shown) for rotating the transport drum 21 and rotates by an angle proportional to the rotation amount of the transport drum motor. Note that the transport drum 21 and the transport drum motor serve as a transport unit for transporting the recording medium M while facing the head unit 24 (the nozzle surface of the inkjet head).

[0026] The delivery unit 22 delivers the recording medium M transported by the medium supply unit 12 of the supply unit 10 to the transport drum 21. The delivery unit 22 is provided at a position between the medium supply unit 12 of the supply unit 10 and the transport drum 21, holds and picks up one end of the recording medium M transported from the medium supply unit 12 with the swing arm portion 221, and delivers it to the transport drum 21 via the delivery drum 222.

[0027] The medium heating unit 23 is arranged between the arrangement position of the delivery drum 222 and the arrangement position of the head unit 24. The medium heating unit 23 heats the transport surface of the transport drum 21 and the recording medium M so that the recording medium M transported by the transport drum 21 reaches a temperature within a predetermined range. The medium heating unit 23 has, for example, an infrared heater or the like, and heats the infrared heater by supplying power to the infrared heater based on a control signal supplied from a control unit (not shown).

[0028] The head unit 24 ejects ink onto the recording medium M from nozzle openings provided on the ink ejection surface facing the conveyance surface of the conveyance drum 21 at an appropriate timing according to the rotation of the conveyance drum 21 holding the recording medium M, thereby forming (recording) an image. The head unit 24 is arranged such that a predetermined distance is provided between its ink ejection surface and the conveyance surface of the conveyance drum 21.

[0029] In the inkjet image forming apparatus 1 according to the present embodiment, four head units 24 corresponding to four colors of ink, namely yellow (Y), magenta (M), cyan (C), and black (K), are arranged. These head units 24 are arranged in the order of Y, M, C, and K colors at predetermined intervals from the upstream side in the conveyance direction of the recording medium M.

[0030] Further, when forming an image, the head unit 24 is used with its position fixed with respect to the rotation axis of the conveyance drum 21. That is, the inkjet image forming apparatus 1 is a single-pass type apparatus.

[0031] Each head unit 24 includes an inkjet head (hereinafter, appropriately referred to as "head") 240 and an inkjet head drive unit 241.

[0032] When a drive voltage is applied, the head 240 ejects ink according to the drive voltage. Details of the head 240 will be described later. In fact, a plurality of heads 240 are arranged in the head unit 24, but since the number and arrangement of the heads 240 are the same as those in the prior art, detailed description thereof is omitted.

[0033] Based on the control of a control unit (not shown), the inkjet head drive unit 241 supplies a drive voltage corresponding to image data to the piezoelectric actuator 2404 (see FIG. 2) of the head 240 at an appropriate timing to drive the head 240.

[0034] The fixing unit 26 has a light emitting unit arranged across the width of the conveyance drum 21 in the orthogonal direction. The fixing unit 26 irradiates the recording medium M placed on the conveyance drum 21 with energy rays such as ultraviolet rays from the light emitting unit, and imparts predetermined energy to the ink discharged onto the recording medium M. Thereby, the fixing unit 26 cures and fixes the ink on the recording medium M.

[0035] The delivery unit 27 has a belt loop 272 having an annular belt supported by two rollers on the inside, and a cylindrical delivery drum 271 that delivers the recording medium M from the conveyance drum 21 to the belt loop 272. The delivery unit 27 conveys the recording medium M delivered from the conveyance drum 21 onto the belt loop 272 by the delivery drum 271 by the belt loop 272, and sends it to the discharge unit 30.

[0036] The discharge unit 30 has a plate-shaped paper discharge tray 31 on which the recording medium M sent out from the image forming unit 20 by the delivery unit 27 is placed. The paper discharge tray 31 is, for example, a plate-shaped member provided so that the recording medium M on which an image is formed can be placed. The paper discharge tray 31 is provided so as to move up and down according to the amount (number of sheets) of the recording medium M placed on the paper discharge tray 31.

[0037] (Head 240; First example) Next, the configuration and structure of the head 240 will be described. FIG. 2 is a cross-sectional view schematically showing a first example of the structure of the head 240 according to the present embodiment. In the following, the longitudinal direction (left-right direction of the paper surface) of the head 240 will be described as the X-axis direction, the short-side direction (depth direction of the paper surface) of the head 240 will be described as the Y-axis direction, and the height direction (up-down direction of the paper surface) of the head will be described as the Z-axis direction. The X-axis direction and the Z-axis direction respectively correspond to the "first direction" and the "second direction" of the present disclosure. Further, the X-axis direction in FIG. 2 corresponds to the orthogonal direction in FIG. 1. Furthermore, for each part constituting the head 240, the upper surface of the paper surface in the Z-axis direction may be referred to as the "upper surface", and the lower surface of the paper surface in the Z-axis direction may be referred to as the "lower surface".

[0038] As shown in FIG. 2, the head 240 is formed by laminating a nozzle plate 2401, a nozzle communication plate 2402, a pressure chamber plate 2403, and a piezoelectric actuator 2404 in this order.

[0039] The nozzle plate 2401 is formed, for example, in a plate shape and arranged such that the plate surface is orthogonal to the Z-axis. The nozzle communication plate 2402 is formed, for example, in a plate shape and arranged on the upper surface side of the nozzle plate 2401 in the Z-axis direction such that the plate surface is orthogonal to the Z-axis. The nozzle communication plate 2402 is sandwiched between the pressure chamber plate 2403 and the nozzle plate 2401.

[0040] The pressure chamber plate 2403 is formed, for example, in a plate shape and arranged on the upper surface side of the nozzle communication plate 2402 in the Z-axis direction such that the plate surface is orthogonal to the Z-axis. The pressure chamber plate 2403 is sandwiched between the piezoelectric actuator 2404 and the nozzle communication plate 2402. The piezoelectric actuator 2404 is formed by laminating a piezoelectric element 2441 and a protective layer 2442. The piezoelectric actuator 2404 is arranged on the upper surface side of the pressure chamber plate 2403 in the Z-axis direction such that it is orthogonal to the Z-axis.

[0041] The protective layer 2442 is arranged on the surface of the piezoelectric element 2441 to protect electrodes such as the individual electrode 2443 or the common electrode 2444, which will be described later, from corrosion. As a method for forming the protective layer 2442, for example, a method of adhering a polyimide film with an adhesive can be used. Also, without being limited to this, for example, a method of applying a polyimide solution by spin coating or the like and baking it to form a polyimide film, or a known method such as a method of forming a parylene polymer film by CVD (Chemical Vapor Deposition) can be used to form the protective layer 2442.

[0042] Further, the head 240 includes a nozzle 2411, a communication flow path 2421, a pressure chamber 2431, a piezoelectric element 2441, an individual electrode 2443, a common electrode 2444, etc. The nozzle 2411, the communication flow path 2421, the pressure chamber 2431, the piezoelectric element 2441, the individual electrode 2443, and the common electrode 2444 are formed inside the nozzle plate 2401, the nozzle communication plate 2402, the pressure chamber plate 2403, and the piezoelectric actuator 2404, or by their combination.

[0043] The nozzle 2411, the communication flow path 2421, and the pressure chamber 2431 communicate with each other to form one channel for discharging ink. The discharge of ink is controlled for each channel.

[0044] The nozzles 2411 are formed in the nozzle plate 2401 in a plurality along the X-axis. That is, the X-axis direction, which is the first direction, is the direction in which the nozzles 2411 are arranged. The plurality of nozzles 2411 are holes that penetrate the nozzle plate 2401 in the Z-axis direction. Through the nozzles 2411, ink droplets are discharged outward in the Z-axis direction.

[0045] In this example, a case where three nozzles 2411 are arranged in the head 240 is shown, but the number of nozzles 2411 is not limited to this example. For example, the plurality of nozzles 2411 are arranged in such a number that they can cover the width of the region where an image is recorded in the X-axis direction (the orthogonal direction in FIG. 1) of the recording medium M conveyed by the conveyance drum 21 (see FIG. 1).

[0046] The communication flow paths 2421 are formed in the nozzle communication plate 2402 in a plurality along the X-axis. The plurality of communication flow paths 2421 are holes that penetrate the nozzle communication plate 2402 in the Z-axis direction and are formed corresponding to the plurality of nozzles 2411. The communication flow paths 2421 communicate the nozzles 2411 with the pressure chambers 2431.

[0047] The pressure chamber 2431 is formed by closing the open surface (the upper surface in the Z-axis direction) of the recess formed in the pressure chamber plate 2403 with the protective layer 2442 of the piezoelectric actuator 2404. The pressure chamber 2431 is a space for storing the ink discharged from the nozzle 2411. The pressure chambers 2431 are provided side by side in the X-axis direction corresponding to each of the plurality of nozzles 2411 and communicate with the nozzles 2411 through the communication flow paths 2421.

[0048] The pressure chamber 2431 has, for example, a width (in the X-axis direction) of 350 μm, a height (in the Z-axis direction) of 120 μm, and a depth (in the Y-axis direction) of 5 mm. Also, the interval between adjacent pressure chambers 2431 is, for example, 508 μm (50 dpi (dots per inch)). Note that it is preferable that the height of the pressure chamber 2431 is formed lower than the width. This is because if the height of the pressure chamber 2431 is too high, the volume of the pressure chamber 2431 increases, the pressure generated by the displacement of the piezoelectric actuator 2404 becomes small, and the ink cannot be discharged. However, if the height of the pressure chamber 2431 is too low, the flow path resistance increases and it becomes difficult to supply the ink to the pressure chamber. Therefore, it is preferable that the height of the pressure chamber 2431 is about 1 / 4 to 1 / 2 of the width.

[0049] Also, on the pressure chamber plate 2403, a partition wall 2432 serving as the wall surface of the pressure chamber 2431 is formed. The partition wall 2432 serves to separate adjacent pressure chambers 2431 when the plurality of pressure chambers 2431 are formed side by side in the X-axis direction corresponding to the plurality of nozzles 2411.

[0050] The piezoelectric element 2441 is, for example, a plate-shaped lead zirconate titanate (PZT) and is polarized in a predetermined direction. In the first example of the present embodiment, the piezoelectric element 2441 is polarized in the Z-axis direction (polarization direction P in FIG. 2).

[0051] The piezoelectric element 2441 deforms when a driving voltage is individually applied to each individual electrode 2443. As a result, the piezoelectric element 2441 deforms the pressure chamber 2431. In the first example, the thickness of the piezoelectric element 2441 is, for example, 300 μm. Note that the thickness (plate thickness; Z-axis direction) of the piezoelectric element 2441 is preferably formed to be thicker than 0.5 times the width (X-axis direction) of the pressure chamber 2431. This is because of the following reasons.

[0052] For example, when the thickness of the piezoelectric element 2441 is thin, the rigidity of the piezoelectric element 2441 decreases. Therefore, even if a voltage is applied to deform the piezoelectric element 2441 and pressure is applied to the ink, the pressure cannot be effectively applied. Also, when the width of the pressure chamber 2431 is widened, the rigidity of the piezoelectric element 2441 decreases even if the thickness of the piezoelectric element 2441 is the same as when the width of the pressure chamber 2431 is narrow. Thus, there is a preferable relationship between the width of the pressure chamber 2431 and the thickness of the piezoelectric element 2441. That is, when the width of the pressure chamber 2431 is 350 μm, the plate thickness of the piezoelectric element 2441 is preferably formed to be thicker than 175 μm.

[0053] Further, a slit 2445 having a concave shape from one surface toward the other surface may be formed on a surface of the piezoelectric element 2441 that is orthogonal to the Z-axis of the piezoelectric element 2441. In the following description, the "upper surface" and "lower surface" of the piezoelectric element 2441 correspond to the "first surface" and "second surface" of the present disclosure, respectively.

[0054] In this first example, a slit 2445 having a concave shape from the upper surface toward the lower surface is formed on the upper surface of the piezoelectric element 2441. Note that the slit 2445 is not limited to this, and for example, it may be formed on the lower surface of the piezoelectric element 2441 to have a concave shape toward the upper surface. Also, the slit 2445 may be formed on both the upper surface and the lower surface, for example.

[0055] Further, the slit 2445 is formed over the entire piezoelectric element 2441 in the Y-axis direction. Such a slit 2445 is formed, for example, by dicing using a predetermined dicing blade. In the first example, the slit 2445 is formed, for example, with a width (X-axis direction) of 30 μm and a depth (Z-axis direction) of 150 μm using a dicing blade with a diameter of 56 mm and a thickness of 30 μm.

[0056] The individual electrode 2443 is an electrode to which a driving voltage is applied and is arranged for each channel. The individual electrode 2443 is arranged at a position corresponding in the Z-axis direction near the center between two adjacent partition walls 2432 in the X-axis direction. This is to appropriately deform the pressure chamber 2431 for each channel by deforming the piezoelectric element 2441 when a driving voltage is applied to the individual electrode 2443.

[0057] The common electrode 2444 is an electrode arranged in common for each channel and is either grounded or an electrode to which a common driving voltage is applied. The common electrode 2444 is arranged at a position corresponding in the Z-axis direction between two adjacent individual electrodes 2443 in the X-axis direction. Preferably, the common electrode 2444 is arranged at a position corresponding in the Z-axis direction of the partition wall 2432. That is, the individual electrode 2443 and the common electrode 2444 are arranged alternately in the X-axis direction.

[0058] Here, as a driving method of the piezoelectric element 58, for example, there is a method in which the common electrode 2444 is grounded to set the voltage to 0 V and a driving voltage corresponding to each pixel data is applied to the individual electrode 2443. In this driving method, as the driving voltage applied to the individual electrode 2443, there are cases where either a positive or a negative voltage is used and cases where both positive and negative voltages are used.

[0059] Furthermore, for example, when the inkjet head driving unit 241 (see FIG. 1) applies only a positive driving voltage, first, a positive driving voltage corresponding to each pixel data is applied to the individual electrode 2443. Then, a driving voltage that rises from 0 to positive is applied to the common electrode 2444 in accordance with the timing of the falling edge at which the waveform of the applied driving voltage returns from positive to 0. By applying positive driving voltages with different waveforms to both the individual electrode 2443 and the common electrode 2444 in this way, the displacement of the piezoelectric element 58 can be increased.

[0060] In the present embodiment, at least one of the individual electrode 2443 and the common electrode 2444 is disposed inside the piezoelectric element 2441. When both the individual electrode 2443 and the common electrode 2444 are not disposed inside the piezoelectric element 2441, the remaining electrode that is not disposed inside is disposed, for example, on the surface of the plane orthogonal to the Z-axis among the planes parallel to the X-axis of the piezoelectric element 2441.

[0061] In this way, when the individual electrode 2443 is disposed on the piezoelectric element 2441 and the slit 2445 is formed, a plurality of driving regions 2440 each including the individual electrode 2443 and separated by the slit 2445 are formed in the piezoelectric element 2441. The plurality of driving regions 2440 are formed side by side in the X-axis direction corresponding to the plurality of nozzles 2411.

[0062] In the first example of FIG. 2, the individual electrode 2443 is disposed on the lower surface of the piezoelectric element 2441. Further, the individual electrode 2443 is disposed at a position corresponding in the Z-axis direction near the center between two adjacent partition walls 2432 in the X-axis direction. That is, the individual electrode 2443 is disposed above the vicinity of the center of the pressure chamber 2431 formed so as to be sandwiched between the two partition walls 2432.

[0063] The individual electrode 2443 can be formed using a thin film formation method such as sputtering or vapor deposition. Note that the formation method of the individual electrode 2443 is not limited to this example. For example, the individual electrode 2443 may be formed by printing a conductive paste by screen printing and then firing it.

[0064] The common electrode 2444 is disposed inside the piezoelectric element 2441. Also, the common electrode 2444 is disposed at a corresponding position in the Z-axis direction of the partition wall 2432.

[0065] In the first example, the common electrode 2444 is formed in the slit 2445 and disposed inside the piezoelectric element 2441. The depth position (Z-axis direction) of the slit 2445 with respect to the upper surface of the piezoelectric element 2441 is preferably 1 / 4 or more of the plate thickness (Z-axis direction) of the piezoelectric element 2441. This is because if the depth of the slit 2445 (common electrode 2444) is too shallow, the driving efficiency decreases, so a certain depth is required. Also, considering the driving efficiency, the deeper the slit 2445 (common electrode 2444), the better, but if it is too deep, the mechanical strength of the piezoelectric element 2441 decreases, and the risk of the piezoelectric element 2441 being damaged increases. Therefore, the depth of the slit 2445 is preferably about 1 / 4 to 3 / 4. That is, when the plate thickness of the pressure chamber 2431 is 300 μm, the depth position of the slit 2445 is preferably 75 μm or more and 225 μm or less.

[0066] In this case, the common electrode 2444 can be formed, for example, as a nickel (Ni) electrode pattern on the entire inner surface of the slit 2445 by electroless plating. Note that the formation method of the common electrode 2444 is not limited to this example. For example, the common electrode 2444 may be formed by vapor depositing a metal such as aluminum by vacuum vapor deposition, or may be formed by embedding a conductive paste in the slit 2445 and firing it.

[0067] Such individual electrodes 2443 and common electrode 2444 are formed over the entire piezoelectric element 2441 in the Y-axis direction. Also, the individual electrodes 2443 and the common electrode 2444 are connected to a flexible wiring (FPC; Flexible printed circuits) or the like (not shown) by a known connection process such as ACF (Anisotropic Conductive Film; Anisotropic Conductive Film) connection. Thereby, the individual electrodes 2443 and the common electrode 2444 are connected to an external drive power source (not shown).

[0068] Note that the arrangement of the individual electrodes 2443 and the common electrode 2444 is not limited to this example. For example, the common electrode 2444 may be disposed on the lower surface of the piezoelectric element 2441, and the individual electrodes 2443 may be formed in a slit 2445 formed on the upper surface of the piezoelectric element 2441, and thus may be disposed inside the piezoelectric element 2441.

[0069] Also, for example, the slit 2445 may be formed on the lower surface of the piezoelectric element 2441. In this case, one of the individual electrodes 2443 and the common electrode 2444 is formed in the slit 2445 and thus disposed inside the piezoelectric element 2441, and the other of the individual electrodes 2443 and the common electrode 2444 is formed on the upper surface of the piezoelectric element 2441.

[0070] As described above, when the slit 2445 is formed in the piezoelectric element 2441, at least one of the individual electrodes 2443 or the common electrode 2444 is disposed in the slit 2445, so that the electrodes can be disposed inside the piezoelectric element 2441.

[0071] [Operation of the head 240] The operation of the head 240 in the inkjet image forming apparatus 1 according to the present embodiment having the above configuration will be described. First, when a drive voltage from the inkjet head drive unit 241 is applied to the individual electrodes 2443 of the piezoelectric actuator 2404, the piezoelectric element 2441 is deformed.

[0072] At this time, in the piezoelectric element 2441, an electric field in the X-axis direction is generated from the individual electrode 2443 toward the common electrodes 2444 on both sides. Here, since the polarization direction P of the piezoelectric element 2441 is in the Z-axis direction and the electric field direction E is in the X-axis direction, the polarization direction P and the electric field direction E of the piezoelectric element 2441 are orthogonal to each other. Therefore, the piezoelectric element 2441 undergoes shear deformation and is deformed so as to bend in the Z-axis direction (toward the pressure chamber 2431 side) substantially centered on the position of the individual electrode 2443.

[0073] As a result, since the pressure chamber 2431 is deformed, the pressure on the ink in the pressure chamber 2431 changes, and the ink in the pressure chamber 2431 is ejected from the nozzle 2411 through the communication flow path 2421. That is, the second direction (Z-axis direction), which is the polarization direction of the piezoelectric element 2441, is a direction orthogonal to the first direction such that when a drive voltage is applied to the individual electrode 2443, the piezoelectric element 2441 undergoes shear deformation to deform the pressure chamber 2431 and eject ink from the nozzle 2411 by the electric field in the first direction (X-axis direction). In this way, by ejecting ink droplets from the nozzle 2411, an image is formed on the recording medium M.

[0074] (Ejection of High-Viscosity Ink) Recently, an image forming apparatus capable of ejecting high-viscosity ink has been desired so as to be able to cope with various image formations. In this case, in order to eject high-viscosity ink, it is necessary to sufficiently generate the force and displacement by the piezoelectric element.

[0075] As one method of sufficiently generating the force and displacement by the piezoelectric element, it is conceivable to increase the thickness of the piezoelectric element. Also, in this case, it is necessary to generate a sufficient electric field inside the piezoelectric element. In order to generate an electric field in the piezoelectric element, a drive voltage is applied to the electrodes arranged in the piezoelectric element. At this time, if the electrodes are arranged only on the surface of the piezoelectric element, an electric field is generated only in the vicinity of the surface of the piezoelectric element, and a sufficient electric field cannot be generated inside.

[0076] In contrast, in the head 240 according to the present embodiment, electrodes are disposed inside the piezoelectric element 2441. For example, in the first example of the head 240 described above, the common electrode 2444 is disposed in the slit 2445 formed on the upper surface of the piezoelectric element 2441, so that the common electrode 2444 is disposed inside the piezoelectric element 2441.

[0077] As a result, when a driving voltage is applied to the individual electrode 2443, an electric field is sufficiently generated inside the piezoelectric element 2441. Therefore, in the head 240, a large displacement by the piezoelectric element 2441 can be obtained, and high-viscosity ink can be ejected.

[0078] (Electric field leakage) Further, in an inkjet head, when a driving voltage is applied to the individual electrode of the piezoelectric element in a certain channel, the channel is driven and an electric field is generated, but the generated electric field may leak to an adjacent channel. When the electric field leaks to an adjacent channel, it affects the ejection of ink droplets such as the ejection speed and ejection volume, resulting in electric field crosstalk that causes a decrease in the quality of the formed image.

[0079] The amount of electric field leakage at this time varies depending on the cross-sectional area of the piezoelectric element between the driven channel and the adjacent channel. Therefore, in order to suppress the influence of electric field crosstalk, it is necessary to reduce the cross-sectional area between a certain channel and the adjacent channel.

[0080] As one method of reducing the cross-sectional area between adjacent channels, it is conceivable to reduce the thickness of the piezoelectric element. However, when the piezoelectric element is thinned, the rigidity of the piezoelectric element for deforming the pressure chamber decreases, so that the pressure chamber cannot be sufficiently deformed. Therefore, in order to suppress the influence of electric field crosstalk, it is necessary to reduce the cross-sectional area between adjacent channels without reducing the thickness of the piezoelectric element.

[0081] On the other hand, when a slit 2445 is formed in the piezoelectric element 2441 as in the first example of the head 240 according to the present embodiment, the cross-sectional area between the drive channel and the adjacent channel can be reduced. As a result, in the head 240 in which the slit 2445 is formed in the piezoelectric element 2441, the amount of electric field leakage to the adjacent channel is reduced, so that the influence of electric field crosstalk can be reduced.

[0082] [Other examples of the head 240] Next, a specific example of the head 240 according to the present embodiment will be described. The head 240 can be formed as shown in the following second to fourth examples in addition to the first example described above, depending on the arrangement positions of the individual electrodes 2443 and the common electrode 2444, and the arrangement position of the slit 2445.

[0083] (Second example) FIG. 3 is a cross-sectional view schematically showing a second example of the structure of the head 240 according to the present embodiment. As shown in FIG. 3, in the second example, the piezoelectric element 2441 is composed of a plurality of green sheets 2450. Here, the piezoelectric element 2441 is composed of two green sheets 2450a and 2450b. An individual electrode 2443 is disposed on the lower surface of the piezoelectric element 2441, and a common electrode 2444 is disposed inside the piezoelectric element 2441. The green sheet 2450 is a sheet-like ceramic substrate or the like that functions as a piezoelectric element by firing.

[0084] When the piezoelectric element 2441 is configured using the green sheet 2450, for example, a conductive paste material is printed on the surface of the green sheet 2450b by a screen printing method. Then, the green sheet 2450a is laminated on the green sheet 2450b on which the conductive paste material is printed and fired. Thereby, the piezoelectric element 2441 in which the common electrode 2444 is disposed inside is formed.

[0085] In the piezoelectric element 2441 formed in this way, since the common electrode 2444 is disposed inside, an electric field is sufficiently generated inside the piezoelectric element 2441 as in the first example. Therefore, in the second example of the head 240, a large displacement by the piezoelectric element 2441 can be obtained, and highly viscous ink can be ejected.

[0086] In the second example, although the common electrode 2444 has been described as being disposed inside the piezoelectric element 2441, the present invention is not limited to this. For example, the individual electrode 2443 may be disposed inside the piezoelectric element 2441.

[0087] (Third Example) FIG. 4 is a cross-sectional view schematically showing a third example of the structure of the head 240 according to the present embodiment. As shown in FIG. 4, in the third example, slits 2445 are formed on both the upper and lower surfaces of the piezoelectric element 2441. The individual electrode 2443 is formed in the slit 2445 formed on the lower surface of the piezoelectric element 2441 and is disposed inside the piezoelectric element 2441. Further, the common electrode 2444 is formed in the slit 2445 formed on the upper surface of the piezoelectric element 2441 and is disposed inside the piezoelectric element 2441.

[0088] In such a piezoelectric element 2441, since the individual electrode 2443 and the common electrode 2444 are disposed inside, an electric field is sufficiently generated inside the piezoelectric element 2441 as in the first and second examples. Therefore, in the third example of the head 240, a large displacement by the piezoelectric element 2441 can be obtained, and highly viscous ink can be ejected.

[0089] Further, in the third example, slits 2445 are formed in the piezoelectric element 2441, and the cross-sectional area between the drive channel and the adjacent channel is reduced as compared with the case where the slits 2445 are not formed. Therefore, in the third example of the head 240, the amount of electric field leakage to the adjacent channel is reduced, and thus the influence of electric field crosstalk can be reduced.

[0090] In the third example, it was described that the individual electrode 2443 is disposed on the lower surface of the piezoelectric element 2441 and the common electrode 2444 is disposed on the upper surface of the piezoelectric element 2441. However, the present invention is not limited to this. For example, the individual electrode 2443 may be disposed on the upper surface of the piezoelectric element 2441 and the common electrode 2444 may be disposed on the lower surface of the piezoelectric element 2441.

[0091] (Fourth Example) FIG. 5 is a cross-sectional view schematically showing a fourth example of the structure of the head 240 according to the present embodiment. As shown in FIG. 5, in the fourth example, piezoelectric actuators 2404 are disposed on both the upper surface side and the lower surface side of the pressure chamber plate 2403. Further, slits 2445 are formed on both the upper surface and the lower surface of the piezoelectric element 2441 in each piezoelectric actuator 2404, as in the third example.

[0092] The individual electrode 2443 is formed in the slit 2445 formed on the lower surface of the piezoelectric element 2441 and is disposed inside the piezoelectric element 2441. The common electrode 2444 is formed in the slit 2445 formed on the upper surface of the piezoelectric element 2441 and is disposed inside the piezoelectric element 2441.

[0093] Note that, in this case, the nozzle plate 2401 in which the nozzle 2411 is formed is disposed on a plane parallel to the ZX plane, although not shown. Therefore, the ink droplets from the nozzle 2411 are ejected in the Y-axis direction (the direction toward the front or the back side of the paper surface).

[0094] In such a piezoelectric element 2441, since the individual electrode 2443 and the common electrode 2444 are disposed inside, as in the third example, an electric field is sufficiently generated inside the piezoelectric element 2441. Therefore, in the third example of the head 240, a large displacement by the piezoelectric element 2441 can be obtained, and highly viscous ink can be ejected.

[0095] Also, in the fourth example, since two piezoelectric actuators 2404 are arranged so as to sandwich the pressure chamber plate 2403, the amount of deformation of the pressure chamber 2431 due to the deformation of the piezoelectric element 2441 is twice that in the first to third examples. Therefore, in the fourth example of the head 240, a larger displacement can be obtained by the piezoelectric element 2441, and high-viscosity ink can be ejected.

[0096] Furthermore, in the fourth example, a slit 2445 is formed in the piezoelectric element 2441, and the cross-sectional area between the drive channel and the adjacent channel decreases as compared with the case where the slit 2445 is not formed. Therefore, in the fourth example of the head 240, the amount of electric field leakage to the adjacent channel decreases, so that the influence of electric field crosstalk can be reduced.

[0097] Also, the configuration of the piezoelectric element 2441 is not limited to this example, and for example, it may have the same configuration as in the first or second example.

[0098] As described above, in the head 240 according to the present embodiment, at least one of the individual electrode 2443 and the common electrode 244 is disposed inside the piezoelectric element 2441 polarized in the Z-axis direction. As a result, in the head 240, the piezoelectric element 2441 undergoes shear deformation and the pressure chamber 2431 is deformed.

[0099] At this time, since the electrode is disposed inside the piezoelectric element 2441 and a sufficient electric field is generated inside the piezoelectric element 2441, sufficient force and displacement can be generated in the piezoelectric element 2441. Therefore, the head 240 can appropriately eject high-viscosity ink.

[0100] Also, when a slit 2445 is formed on the X-axis surface of the piezoelectric element 2441 and an electrode is disposed in the slit 2445, the cross-sectional area between the drive channel and the adjacent channel can be decreased by the slit 2445. Thereby, the influence of electric field crosstalk on the adjacent channel can be reduced, and a decrease in the quality of the image formed by the inkjet image forming apparatus 1 can be suppressed.

[0101] The above describes the embodiments. However, the present disclosure is not limited to the above-described embodiments, and various modifications and applications are possible without departing from the gist of the present disclosure. In this embodiment, the ink droplets ejected from the nozzle 2411 are described as being ejected in the Z-axis direction. However, the ejection direction of the ink droplets is not limited to this example. For example, the head 240 may be configured such that the nozzle plate 2401 on which the nozzle 2411 is formed is arranged in the ZX plane, and the ink droplets from the nozzle 2411 may be ejected in the Y-axis direction.

[0102] In this embodiment, the case where the individual electrode 2443 and the common electrode 2444 are arranged on different surfaces of the piezoelectric element 2441 has been described. However, the present disclosure is not limited to this, and for example, the individual electrode 2443 and the common electrode 2444 may be arranged on the same surface of the piezoelectric element 2441.

[0103] In particular, in the third example described with reference to FIG. 4, when the individual electrode 2443 and the common electrode 2444 are arranged on the upper surface, after closing the open surface (the upper surface in the Z-axis direction) of the recess formed in the pressure chamber plate 2403 by the piezoelectric actuator 2404 to form the pressure chamber 2431, the individual electrode 2443 and the common electrode 2444 can be formed. Therefore, the degree of freedom in manufacturing can be expanded.

[0104] The disclosures of the specification, drawings, and abstract included in the Japanese application of Japanese Patent Application No. 2023-108379 filed on June 30, 2023 are all incorporated herein by reference.

Explanation of Reference Numerals

[0105] 1 Inkjet Image Forming Apparatus 20 Image Forming Unit 24 Head Unit 240 Inkjet Head 241 Inkjet Head Driving Unit 2401 Nozzle Plate 2402 Nozzle Communication Plate 2403 Pressure Chamber Plate 2404 Piezoelectric Actuator 2411 Nozzle 2421 Communication Flow Path 2431 Pressure Chamber 2432 Partition Wall 2440 Driving Region 2441 Piezoelectric Element 2442 Protective Layer 2443 Individual Electrode 2444 Common Electrode 2445 Slit 2450, 2450a, 2450b Green Sheet

Claims

1. a piezoelectric element having a plurality of driving regions aligned in a first direction corresponding to a plurality of nozzles; a plurality of individual electrodes disposed in the piezoelectric element for each of the driving regions, to which a driving voltage for ejecting ink from the corresponding nozzle is individually applied; a plurality of common electrodes arranged on the piezoelectric element alternately with the individual electrodes in the first direction, and a common driving voltage is applied to the plurality of driving regions; the piezoelectric element is polarized in a second direction perpendicular to the first direction such that an electric field generated in the first direction when the driving voltage is applied to the individual electrode causes the piezoelectric element to shear and deform, thereby ejecting the ink from the nozzle; an inkjet head, wherein at least one of the individual electrodes and the common electrode is disposed inside the piezoelectric element;

2. The piezoelectric element further includes a slit formed in the piezoelectric element so as to divide the piezoelectric element into the driving regions.

2. The ink-jet head according to claim 1.

3. At least one of the individual electrodes and the common electrode is formed in the slit.

3. The ink-jet head according to claim 2.

4. The piezoelectric element is A plurality of green sheets are laminated to form a laminate. At least one of the individual electrodes and the common electrode is disposed between the plurality of stacked green sheets.

2. The ink-jet head according to claim 1.

5. a plurality of pressure chambers arranged in the first direction corresponding to the plurality of nozzles; The length of the piezoelectric element in the second direction is longer than 0.5 times the length of the pressure chamber in the first direction.

2. The ink-jet head according to claim 1.

6. The depth of the electrode disposed inside the piezoelectric element in the second direction is in the range of ¼ to ¾ of the length of the piezoelectric element in the second direction.

2. The ink-jet head according to claim 1.

7. a plurality of pressure chambers arranged in the first direction corresponding to the plurality of nozzles; a length of the pressure chamber in the second direction is shorter than a length of the pressure chamber in the first direction; 2. The ink-jet head according to claim 1.

8. An image forming apparatus comprising the inkjet head according to claim 1 .

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