Inkjet head and image forming device

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

Inkjet head and image forming apparatus

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

[0002] Inkjet image forming devices have been known in the past that eject ink from a plurality of nozzles arranged in an inkjet head onto a recording medium such as paper to form an image on the recording medium. In such inkjet image forming devices, the inkjet head has a plurality of channels corresponding to the number of nozzles, and each channel is equipped with a pressure chamber that stores ink and a piezoelectric element that deforms the pressure chamber.

[0003] In an inkjet head, when a drive voltage is applied to a piezoelectric element, the piezoelectric element deforms in response to the drive voltage, which in turn deforms the pressure chamber, changing the pressure on the ink in the pressure chamber that is supplied to the nozzle, causing the ink in the pressure chamber to be ejected from the nozzle.

[0004] For example, Patent Document 1 discloses a technology in which electrodes are arranged on the surface of a piezoelectric element, and ink is ejected by utilizing the deformation of the piezoelectric element that occurs when a drive voltage is applied to the electrodes. This technology ejects ink by deforming a pressure chamber using shear deformation that occurs in the piezoelectric element when the polarization direction of the piezoelectric element and the electric field direction are perpendicular to each other.

[0005] Furthermore, Patent Document 2 discloses an inkjet head in which notches are formed at predetermined positions on both sides of a piezoelectric element, and electrodes are arranged so that the polarization direction of the piezoelectric element is parallel to the electric field direction. In this inkjet head, when a drive voltage is applied to the electrodes, the deformation direction of the piezoelectric element is changed from a direction along the surface of the piezoelectric element to a direction perpendicular to the surface, causing the piezoelectric element to bend and deform, thereby ejecting ink.

[0006] U.S. Patent No. 4,584,590 JP-A-2003-008095

[0007] Recently, there has been a demand for image forming apparatuses capable of ejecting high-viscosity ink so that they can be used for various types of image formation. When ejecting high-viscosity ink, it is necessary to generate sufficient force and displacement from the piezoelectric element. To achieve this, it is conceivable to increase the thickness of the piezoelectric element.

[0008] However, in an inkjet head using the technology 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 drive 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 generate sufficient force and displacement from the piezoelectric element.

[0009] Even if the thickness of the piezoelectric element is increased, since the electrodes are arranged only on the surface, it is not possible to generate a sufficient electric field inside the piezoelectric element, and there is a risk that the piezoelectric element will not deform sufficiently. Furthermore, although increasing the thickness of the piezoelectric element increases the rigidity of the piezoelectric element, the increased rigidity reduces the amount of displacement, and it is not possible to apply sufficient pressure to the ink in the pressure chamber to eject it.

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

[0011] On the other hand, if the thickness of the piezoelectric element is increased in order to increase the force generated by the deformation of the piezoelectric element, the rigidity of the piezoelectric element increases, making it less likely to undergo bending displacement, and therefore the amount of displacement of the piezoelectric element decreases.

[0012] An object of the present disclosure is to provide an inkjet head and an image forming apparatus that are capable of ejecting highly viscous ink droplets.

[0013] The inkjet head according to the present disclosure comprises: a piezoelectric element having a plurality of driving regions arranged in a first direction corresponding to a plurality of nozzles; a plurality of individual electrodes arranged on the piezoelectric element for each driving region, to which a driving voltage for ejecting ink from the corresponding nozzle is individually applied; and a plurality of common electrodes arranged on the piezoelectric element alternately with the individual electrodes in the first direction, to which a common driving voltage is applied to the plurality of driving regions; wherein the piezoelectric element is polarized in a second direction perpendicular to the first direction so that when the driving voltage is applied to the individual electrodes, an electric field generated in the first direction causes the piezoelectric element to shear and deform, thereby ejecting the ink from the nozzle; and at least one of the individual electrodes and the common electrode is arranged inside the piezoelectric element.

[0014] An image forming apparatus according to the present disclosure includes the inkjet head described above.

[0015] According to the present disclosure, it is possible to eject ink droplets with high viscosity.

[0016] Fig. 1 is a schematic diagram showing an example of the configuration of an inkjet image forming apparatus according to the present embodiment. Fig. 2 is a cross-sectional view schematically showing a first example of the structure of a head according to the present embodiment. Fig. 3 is a cross-sectional view schematically showing a second example of the structure of a head according to the present embodiment. Fig. 4 is a cross-sectional view schematically showing a third example of the structure of a head according to the present embodiment. Fig. 5 is a cross-sectional view schematically showing a fourth example of the structure of a head according to the present embodiment.

[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 are possible without departing from the spirit of the present disclosure. In addition, in each drawing, the same reference numerals are used to denote the same or equivalent parts, and this is common throughout the entire specification.

[0018] An inkjet image forming apparatus according to an embodiment of the present disclosure 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, and a discharge unit 30. Under the control of a control unit (not shown), the inkjet image forming apparatus 1 transports 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 transports (discharges) the recording medium M with the image formed to the discharge unit 30. The recording medium M can be paper such as plain paper or coated paper, or various media such as fabric or sheet-like resin, on whose surface ink can be fixed.

[0020] The supply unit 10 has a paper feed tray 11 that stores the recording medium M, and a medium supply unit 12 that transports and supplies 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 that can accommodate one or more recording media M. The paper feed tray 11 is configured to move up and down depending on the amount (number of sheets) of recording media M placed on the paper feed tray 11, and in the direction of the up and down movement, the topmost recording medium M is held at a position where it can be transported by the medium supply unit 12.

[0022] The media supply unit 12 has a circular belt supported on the inside by two rollers, and by rotating the rollers while the recording medium M is placed on this belt, the recording medium M is transported from the paper feed tray 11 to the image forming unit 20.

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

[0024] The transport drum 21 holds the recording medium M on its cylindrical outer curved surface (transport surface) and rotates around a rotation axis extending in a direction perpendicular to the paper surface of Figure 1 (hereinafter referred to as the "orthogonal direction"), thereby transporting the recording medium M in a transport direction along the transport surface (see arrow in Figure 1).

[0025] The transport drum 21 has claws and an air intake section (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 edges pressed down by the claws and being drawn to the transport surface by the air intake section. 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 amount of rotation of the transport drum motor. The transport drum 21 and the transport drum motor serve as a transport section that transports the recording medium M facing the head unit 24 (the nozzle surface of the inkjet head).

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

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

[0028] The head unit 24 forms (records) an image by ejecting ink onto the recording medium M from nozzle openings provided on an ink ejection surface facing the transport surface of the transport drum 21 at appropriate timing according to the rotation of the transport drum 21 on which the recording medium M is held. The head unit 24 is disposed so that its ink ejection surface and the transport surface of the transport drum 21 are spaced a predetermined distance apart.

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

[0030] Furthermore, when forming an image, the head unit 24 is used while its position is fixed relative to the rotation axis of the transport drum 21. In other words, the inkjet image forming apparatus 1 is a single-pass type apparatus.

[0031] Each head unit 24 has an inkjet head (hereinafter referred to as “head”) 240 and an inkjet head driving section 241 .

[0032] When a drive voltage is applied, the head 240 ejects ink in accordance with the drive voltage. Details of the head 240 will be described later. Note that, although a plurality of heads 240 are actually arranged within the head unit 24, the number and arrangement of the heads 240 are the same as in the conventional case, and therefore detailed description thereof will be omitted.

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

[0034] The fixing unit 26 has a light-emitting unit arranged across the width of the conveying drum 21 in the perpendicular direction. The fixing unit 26 irradiates the recording medium M placed on the conveying drum 21 with energy rays such as ultraviolet rays from the light-emitting unit, and imparts a predetermined amount of energy to the ink ejected onto the recording medium M. In this way, the fixing unit 26 hardens and fixes the ink on the recording medium M.

[0035] The delivery unit 27 has a belt loop 272 having a ring-shaped belt supported on the inside by two rollers, and a cylindrical transfer drum 271 that transfers the recording medium M from the transport drum 21 to the belt loop 272. The delivery unit 27 transports the recording medium M that has been transferred from the transport drum 21 onto the belt loop 272 by the transfer drum 271, using the belt loop 272, and sends it to the discharge unit 30.

[0036] The discharge unit 30 has a plate-shaped discharge tray 31 on which the recording medium M sent from the image forming unit 20 by the delivery unit 27 is placed. The discharge tray 31 is a plate-shaped member that is provided so that the recording medium M on which an image has been formed can be placed, for example. The discharge tray 31 is provided so as to move up and down according to the amount (number of sheets) of recording media M placed on the 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 illustrating a first example of the structure of the head 240 according to the present embodiment. In the following description, the longitudinal direction of the head 240 (left-right direction in the plane of the paper) is referred to as the X-axis direction, the lateral direction of the head 240 (depth direction in the plane of the paper) is referred to as the Y-axis direction, and the height direction of the head (up-down direction in the plane of the paper) is referred to as the Z-axis direction. The X-axis direction and Z-axis direction correspond to the "first direction" and "second direction" of the present disclosure, respectively. The X-axis direction in FIG. 2 corresponds to the orthogonal direction in FIG. 1. Furthermore, for each part of the head 240, the surface above the plane of the paper in the Z-axis direction may be referred to as the "upper surface," and the surface below the plane of the paper 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 is disposed so that its plate surface is perpendicular to the Z axis. The nozzle communication plate 2402 is formed, for example, in a plate shape and is disposed on the upper surface side of the nozzle plate 2401 in the Z axis direction so that its plate surface is perpendicular 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 is arranged on the upper surface of the nozzle connection plate 2402 in the Z-axis direction, so that the plate surface is perpendicular to the Z-axis. The pressure chamber plate 2403 is sandwiched between a piezoelectric actuator 2404 and the nozzle connection 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 of the pressure chamber plate 2403 in the Z-axis direction, so that the plate surface is perpendicular to the Z-axis.

[0041] The protective layer 2442 is disposed to protect electrodes such as the individual electrodes 2443 or the common electrode 2444, which will be described later, formed on the surface of the piezoelectric element 2441 from corrosion. For example, a method of adhering a polyimide film with an adhesive can be used as a method of forming the protective layer 2442. However, the method is not limited to this. For example, the protective layer 2442 may be formed using a known method such as a method of forming a polyimide film by applying a polyimide liquid by spin coating or the like and baking the applied liquid, or a method of forming a paraxylene polymer film by a CVD (Chemical Vapor Deposition) method.

[0042] The head 240 also includes a nozzle 2411, a communication flow path 2421, a pressure chamber 2431, a piezoelectric element 2441, an individual electrode 2443, and a common electrode 2444. 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 combining these together.

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

[0044] A plurality of nozzles 2411 are drilled in the nozzle plate 2401 along the X-axis. That is, the X-axis direction, which is a 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. Ink droplets are ejected to the outside in the Z-axis direction through the nozzles 2411.

[0045] In this example, three nozzles 2411 are arranged in the head 240, but the number of nozzles 2411 is not limited to this example. For example, the nozzles 2411 are arranged in a number that can cover the width in the X-axis direction (the orthogonal direction in FIG. 1 ) of the area on which an image is to be printed, of the recording medium M transported by the transport drum 21 (see FIG. 1 ).

[0046] A plurality of communication channels 2421 are formed along the X-axis in the nozzle communication plate 2402. The plurality of communication channels 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 channels 2421 communicate between the nozzles 2411 and the pressure chambers 2431.

[0047] The pressure chambers 2431 are formed by closing the open surface (upper surface in the Z-axis direction) of a recess formed in the pressure chamber plate 2403 with a protective layer 2442 of the piezoelectric actuator 2404. The pressure chambers 2431 are spaces that store ink ejected from the nozzles 2411. The pressure chambers 2431 are arranged side by side in the X-axis direction corresponding to each of the multiple nozzles 2411, and communicate with the nozzles 2411 via communication channels 2421.

[0048] Each pressure chamber 2431 has, for example, a width (X-axis direction) of 350 μm, a height (Z-axis direction) of 120 μm, and a depth (Y-axis direction) of 5 mm. The spacing between adjacent pressure chambers 2431 is, for example, 508 μm (50 dpi (dots per inch)). It is preferable that the height of each pressure chamber 2431 be smaller than its width. This is because if the height of the pressure chamber 2431 is too high, the volume of the pressure chamber 2431 increases, reducing the pressure generated by the displacement of the piezoelectric actuator 2404 and making it impossible to eject ink. However, if the height of the pressure chamber 2431 is too low, the flow path resistance increases, making it difficult to supply ink to the pressure chamber. Therefore, it is preferable that the height of the pressure chamber 2431 be approximately 1 / 4 to 1 / 2 of its width.

[0049] Furthermore, partition walls 2432 that become the wall surfaces of the pressure chambers 2431 are formed on the pressure chamber plate 2403. When multiple pressure chambers 2431 are formed side by side in the X-axis direction corresponding to multiple nozzles 2411, the partition walls 2432 serve to separate adjacent pressure chambers 2431 from one another.

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

[0051] The piezoelectric element 2441 is deformed by applying a drive voltage to each individual electrode 2443 individually. This causes the piezoelectric element 2441 to deform the pressure chamber 2431. In the first example, the thickness of the piezoelectric element 2441 is, for example, 300 μm. It is preferable that the thickness (plate thickness; Z-axis direction) of the piezoelectric element 2441 be formed to be thicker than 0.5 times the width (X-axis direction) of the pressure chamber 2431. This is for the following reason.

[0052] For example, if 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 apply pressure to the ink, the pressure cannot be applied effectively. Furthermore, if 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. This is because there is a preferable relationship between the width of the pressure chamber 2431 and the thickness of the piezoelectric element 2441. In other words, if the width of the pressure chamber 2431 is 350 μm, it is preferable that the plate thickness of the piezoelectric element 2441 be formed to be thicker than 175 μm.

[0053] Furthermore, the piezoelectric element 2441 may have a slit 2445 formed in a surface perpendicular to the Z axis of the piezoelectric element 2441, the slit 2445 being concave from one surface to the other. 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 that is concave from the top to the bottom is formed on the top surface of the piezoelectric element 2441. Note that the slit 2445 is not limited to this, and may be formed, for example, on the bottom surface of the piezoelectric element 2441 so as to be concave from the top to the bottom. Furthermore, the slit 2445 may be formed, for example, on both the top and bottom surfaces.

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

[0056] The individual electrodes 2443 are electrodes to which a drive voltage is applied, and are arranged for each channel. The individual electrodes 2443 are arranged at corresponding positions in the Z-axis direction near the center between two partition walls 2432 adjacent in the X-axis direction. This is because when a drive voltage is applied to the individual electrodes 2443, the piezoelectric elements 2441 are deformed, thereby appropriately deforming the pressure chambers 2431 for each channel.

[0057] The common electrode 2444 is an electrode commonly disposed for each channel, and is grounded or has a common driving voltage applied thereto. The common electrode 2444 is disposed between two individual electrodes 2443 adjacent to each other in the X-axis direction, at a corresponding position in the Z-axis direction. Preferably, the common electrode 2444 is disposed at a corresponding position in the Z-axis direction on the partition wall 2432. In other words, the individual electrodes 2443 and the common electrode 2444 are disposed alternately in the X-axis direction.

[0058] Here, one method for driving the piezoelectric elements 58 is to ground the common electrode 2444 to set the voltage at 0 V, and apply a drive voltage according to each pixel data to the individual electrodes 2443. In this drive method, the drive voltage applied to the individual electrodes 2443 may be either a positive or negative voltage, or both a positive and negative voltage.

[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, in synchronization with the falling edge of the waveform of the applied driving voltage, where the waveform returns from positive to zero, a driving voltage rising from zero to positive is applied to the common electrode 2444. By applying positive driving voltages with different waveforms to both the individual electrode 2443 and the common electrode 2444 in this manner, the displacement of the piezoelectric element 58 can be increased.

[0060] In the present embodiment, at least one of the individual electrodes 2443 and the common electrode 2444 is disposed inside the piezoelectric element 2441. When neither the individual electrode 2443 nor the common electrode 2444 is disposed inside the piezoelectric element 2441, the remaining electrode that is not disposed inside is disposed, for example, on the surface of a surface of the piezoelectric element 2441 that is parallel to the X axis and perpendicular to the Z axis.

[0061] In this way, the individual electrodes 2443 are arranged on the piezoelectric element 2441 and the slits 2445 are formed, whereby a plurality of driving regions 2440 are formed in the piezoelectric element 2441, each of which includes an individual electrode 2443 and is separated by the slits 2445. The plurality of driving regions 2440 are formed side by side in the X-axis direction corresponding to the plurality of nozzles 2411.

[0062] 2 , the individual electrode 2443 is disposed on the lower surface of the piezoelectric element 2441. The individual electrode 2443 is also disposed near the center between two adjacent partition walls 2432 in the X-axis direction, at a position corresponding to the center in the Z-axis direction. In other words, the individual electrode 2443 is disposed above and near the center of the pressure chamber 2431 formed between the two partition walls 2432.

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

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

[0065] In the first example, the common electrode 2444 is formed in a slit 2445 and disposed inside the piezoelectric element 2441. The depth position (Z-axis direction) of the slit 2445 relative to the top surface of the piezoelectric element 2441 is preferably at least ¼ of the plate thickness (Z-axis direction) of the piezoelectric element 2441. This is because a certain degree of depth is necessary because if the depth of the slit 2445 (common electrode 2444) is too shallow, the driving efficiency decreases. Furthermore, considering the driving efficiency, the depth of the slit 2445 (common electrode 2444) is preferably deeper, but if it is too deep, the mechanical strength of the piezoelectric element 2441 decreases, increasing the risk of damage to the piezoelectric element 2441. Therefore, the depth of the slit 2445 is preferably approximately ¼ to ¾. In other words, if 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 on the entire inner surface of the slit 2445 as a nickel (Ni) electrode pattern by, for example, electroless plating. Note that the method for forming the common electrode 2444 is not limited to this example. For example, the common electrode 2444 may be formed by depositing a metal such as aluminum by vacuum deposition, or by filling the slit 2445 with a conductive paste and firing it.

[0067] Such individual electrodes 2443 and common electrodes 2444 are formed over the entire Y-axis direction of the piezoelectric element 2441. The individual electrodes 2443 and common electrode 2444 are connected to flexible wiring (FPC: Flexible Printed Circuits) (not shown) or the like by a known connection process such as ACF (Anisotropic Conductive Film) connection, thereby connecting the individual electrodes 2443 and common electrode 2444 to an external drive power supply (not shown).

[0068] 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 arranged on the lower surface of the piezoelectric element 2441, and the individual electrodes 2443 may be formed in slits 2445 formed on the upper surface of the piezoelectric element 2441, thereby being arranged inside the piezoelectric element 2441.

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

[0070] In this way, when a slit 2445 is formed in the piezoelectric element 2441, at least one of the individual electrode 2443 or the common electrode 2444 can be placed in the slit 2445, thereby allowing the electrode to be placed inside the piezoelectric element 2441.

[0071] [Operation of Head 240] The operation of the head 240 in the inkjet image forming apparatus 1 according to this 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 electrode 2443 of the piezoelectric actuator 2404, the piezoelectric element 2441 is deformed.

[0072] At this time, an electric field in the X-axis direction is generated in the piezoelectric element 2441, extending from the individual electrode 2443 toward the common electrodes 2444 on both sides. Here, the polarization direction P of the piezoelectric element 2441 is the Z-axis direction, and the electric field direction E is the X-axis direction, so the polarization direction P and the electric field direction E of the piezoelectric element 2441 are perpendicular 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) with the position of the individual electrode 2443 as the center.

[0073] As a result, the pressure chamber 2431 deforms, changing the pressure on the ink in the pressure chamber 2431, and the ink in the pressure chamber 2431 is ejected from the nozzle 2411 via the communicating flow path 2421. In other words, the second direction (Z-axis direction), which is the polarization direction of the piezoelectric element 2441, is a direction perpendicular to the first direction, such that an electric field in the first direction (X-axis direction) generated when a drive voltage is applied to the individual electrode 2443 causes the piezoelectric element 2441 to shear and deform the pressure chamber 2431, thereby ejecting ink from the nozzle 2411. In this way, an image is formed on the recording medium M by ejecting ink droplets from the nozzle 2411.

[0074] (Discharge of High-Viscosity Ink) Recently, there has been a demand for image forming apparatuses that can discharge high-viscosity ink so that they can be used for various image formation. In this case, in order to discharge high-viscosity ink, it is necessary to generate sufficient force and displacement using a piezoelectric element.

[0075] One way to generate sufficient force and displacement from a piezoelectric element is to increase the thickness of the piezoelectric element. In this case, it is necessary to generate a sufficient electric field inside the piezoelectric element. To generate an electric field in the piezoelectric element, a driving voltage is applied to electrodes arranged on the piezoelectric element. However, if the electrodes are arranged only on the surface of the piezoelectric element, the electric field will only be generated near the surface of the piezoelectric element, and a sufficient electric field will not be generated inside.

[0076] In contrast to this, in the head 240 according to the present embodiment, an electrode is 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 a slit 2445 formed in the upper surface of the piezoelectric element 2441, and thereby the common electrode 2444 is disposed inside the piezoelectric element 2441.

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

[0078] (Leakage of Electric Field) In addition, in an inkjet head, when a drive voltage is applied to an individual electrode of a piezoelectric element in a certain channel, the channel is driven to generate an electric field, but the generated electric field may leak to an adjacent channel. If the electric field leaks to an adjacent channel, it affects the ejection speed, ejection volume, etc. of ink droplets, and generates electric field crosstalk that reduces the quality of the formed image.

[0079] The amount of electric field leakage depends on the cross-sectional area of ​​the piezoelectric element between the channel being driven and the adjacent channel, so to suppress the effects of electric field crosstalk, it is necessary to reduce the cross-sectional area between a certain channel and the adjacent channel.

[0080] One way to reduce the cross-sectional area between adjacent channels is to reduce the thickness of the piezoelectric element, but if the piezoelectric element is made thinner, the rigidity of the piezoelectric element for deforming the pressure chamber will decrease, making it impossible to sufficiently deform the pressure chamber. Therefore, in order to suppress the effects 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] In contrast, when slits 2445 are 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 slits 2445 are formed in the piezoelectric element 2441, the amount of electric field leakage to the adjacent channel is reduced, thereby reducing the effects of electric field crosstalk.

[0082] [Other Examples of Head 240] Next, a description will be given of specific examples of the head 240 according to the present embodiment. In addition to the first example described above, the head 240 can be formed as shown in the following second to fourth examples, depending on the positions of the individual electrodes 2443 and the common electrode 2444 and the position of the slits 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 when fired.

[0084] When forming piezoelectric element 2441 using green sheet 2450, for example, a conductive paste material is printed on the surface of green sheet 2450b by screen printing. Then, green sheet 2450a is laminated on green sheet 2450b on which the conductive paste material has been printed, and the laminate is fired. This forms piezoelectric element 2441 with common electrode 2444 disposed therein.

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

[0086] In the second example, the common electrode 2444 is described as being disposed inside the piezoelectric element 2441 , but this is not limitative. For example, the individual electrode 2443 may be disposed inside the piezoelectric element 2441 .

[0087] (Third Example) Figure 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 Figure 4, in the third example, slits 2445 are formed on both the upper and lower surfaces of the piezoelectric element 2441. The individual electrodes 2443 are formed in the slits 2445 formed on the lower surface of the piezoelectric element 2441 and are disposed inside the piezoelectric element 2441. Furthermore, the common electrode 2444 is formed in the slits 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, the individual electrode 2443 and the common electrode 2444 are disposed inside, and so, similar to the first and second examples, an electric field is sufficiently generated inside the piezoelectric element 2441. Therefore, in the third example of the head 240, a large displacement can be obtained by the piezoelectric element 2441, and highly viscous ink can be ejected.

[0089] Furthermore, in the third example, a slit 2445 is formed in the piezoelectric element 2441, and the cross-sectional area between the drive channel and the adjacent channel is reduced compared to when the slit 2445 is not formed. Therefore, in the third example of the head 240, the amount of electric field leakage to the adjacent channel is reduced, and the effect of electric field crosstalk can be reduced.

[0090] In the third example, it has been described that the individual electrode 2443 is arranged on the lower surface of the piezoelectric element 2441 and the common electrode 2444 is arranged on the upper surface of the piezoelectric element 2441, but this is not limited to this, and for example, the individual electrode 2443 may be arranged on the upper surface of the piezoelectric element 2441 and the common electrode 2444 may be arranged 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 arranged on both the upper and lower surfaces of a pressure chamber plate 2403. Furthermore, as in the third example, slits 2445 are formed on both the upper and lower surfaces of the piezoelectric element 2441 in each piezoelectric actuator 2404.

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

[0093] In this case, the nozzle plate 2401 in which the nozzles 2411 are formed is disposed on a plane parallel to the ZX plane (not shown), so that ink droplets from the nozzles 2411 are ejected in the Y-axis direction (toward or away from the paper surface).

[0094] In this type of piezoelectric element 2441, similarly to the third example, the individual electrodes 2443 and the common electrode 2444 are disposed inside, and therefore, similarly to the first to third examples, an electric field is sufficiently generated inside the piezoelectric element 2441. Therefore, in the third example of the head 240, a large displacement can be obtained by the piezoelectric element 2441, and highly viscous ink can be ejected.

[0095] Furthermore, in the fourth example, two piezoelectric actuators 2404 are arranged to sandwich the pressure chamber plate 2403, so compared to the first to third examples, the amount of deformation of the pressure chamber 2431 due to the deformation of the piezoelectric element 2441 is doubled. 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, slits 2445 are formed in piezoelectric element 2441, and the cross-sectional area between the drive channel and the adjacent channel is reduced compared to when slits 2445 are not formed. Therefore, in the fourth example of head 240, the amount of electric field leakage to the adjacent channel is reduced, and the effect of electric field crosstalk can be reduced.

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

[0098] As described above, in the head 240 according to this 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, causing the pressure chamber 2431 to deform.

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

[0100] Furthermore, when a slit 2445 is formed on the surface of the piezoelectric element 2441 in the X-axis direction and an electrode is disposed in the slit 2445, the cross-sectional area between the drive channel and the adjacent channel can be reduced by the slit 2445. This reduces the effect of electric field crosstalk on the adjacent channel and suppresses deterioration in the quality of images formed by the inkjet image forming apparatus 1.

[0101] Although the embodiments have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications and applications are possible within the scope of the gist of the present disclosure. In the present embodiment, the ink droplets ejected from the nozzles 2411 are described as being ejected in the Z-axis direction, but the ejection direction of the ink droplets is not limited to this example. For example, the head 240 may be configured so that the nozzle plate 2401 in which the nozzles 2411 are formed is disposed on the ZX plane, and the ink droplets from the nozzles 2411 may be ejected in the Y-axis direction.

[0102] In addition, in this embodiment, we have described a case where the individual electrode 2443 and the common electrode 2444 are arranged on different surfaces of the piezoelectric element 2441, but this 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] 4, in particular, when the individual electrodes 2443 and the common electrode 2444 are disposed on the upper surface, the open surface (the upper surface in the Z-axis direction) of the recess formed in the pressure chamber plate 2403 can be closed by the piezoelectric actuator 2404 to form the pressure chamber 2431, and then the individual electrodes 2443 and the common electrode 2444 can be formed. This allows for greater freedom in manufacturing.

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

[0105] REFERENCE SIGNS LIST 1 Inkjet image forming apparatus 20 Image forming section 24 Head unit 240 Inkjet head 241 Inkjet head driving section 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 area 2441 Piezoelectric element 2442 Protective layer 2443 Individual electrode 2444 Common electrode 2445 Slit 2450, 2450a, 2450b Green sheet

Claims

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

2. The inkjet head according to claim 1, further comprising slits formed in said piezoelectric element so as to divide said piezoelectric element into said driving regions.

3. The ink jet head according to claim 2, wherein at least one of the individual electrodes and the common electrode is formed in the slit.

4. The inkjet head as described in claim 1, wherein the piezoelectric element is formed by stacking a plurality of green sheets, and at least one of the individual electrodes and the common electrode is disposed between the plurality of stacked green sheets.

5. An inkjet head as described in claim 1, further comprising a plurality of pressure chambers aligned in the first direction corresponding to a plurality of the nozzles, and the length of the piezoelectric element in the second direction is longer than 0.5 times the length of the pressure chambers in the first direction.

6. The inkjet head according to claim 1, wherein the depth of the electrode disposed inside the piezoelectric element in the second direction is in the range of 1 / 4 to 3 / 4 of the length of the piezoelectric element in the second direction.

7. An inkjet head as described in claim 1, further comprising a plurality of pressure chambers arranged in the first direction corresponding to a plurality of the nozzles, wherein the length of each of the pressure chambers in the second direction is shorter than the length of each of the pressure chambers in the first direction.

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

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