Liquid discharge head

Abstract

To provide a liquid discharge head that can normally discharge liquid at high speed while suppressing meniscus from vibrating for a long period.SOLUTION: A liquid discharge head according to an embodiment comprises a resonant chamber, a restrictor flow path, a nozzle, an actuator, an upstream-side liquid manifold, and a downstream-side liquid manifold. The resonant chamber is formed so as to perform liquid-column resonance. One end of the restrictor flow path communicates with one end of the resonant chamber. The nozzle is provided on the middle of the restrictor flow path. The actuator makes the resonant chamber cause the liquid-column resonance. The upstream-side liquid manifold communicates with the other end of the restrictor flow path. The downstream-side liquid manifold communicates with the other end of the resonant chamber. Cross-sectional areas of the restrictor flow path are smaller than cross-sectional areas of a flow path of the resonant chamber.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] Embodiments of the present invention relate to a liquid ejection head.

Background Art

[0002] A liquid ejection head that supplies a predetermined amount of liquid to a predetermined position is known. The liquid ejection head is mounted on, for example, an inkjet printer, a 3D printer, a dispensing device, or the like. An inkjet printer ejects ink droplets from an inkjet head to form an image or the like on the surface of a recording medium. A 3D printer ejects droplets of a modeling material from a modeling material ejection head and cures them to form a three-dimensional object. A dispensing device ejects droplets of a sample and supplies a predetermined amount to a plurality of containers or the like.

[0003] The liquid ejection head has a plurality of channels for ejecting liquid. Each channel includes a nozzle for ejecting liquid, a pressure chamber communicating with the nozzle, and an actuator for changing the volume of the pressure chamber. The liquid ejection head selects a channel for ejecting liquid from among the plurality of channels, and applies a drive signal to the actuator of the selected channel to eject the liquid. A liquid inlet and a liquid outlet are provided in the pressure chamber. Of the ink supplied from the liquid inlet to the pressure chamber, that which is not ejected from the nozzle is returned from the liquid outlet to the supply side for circulation supply. The liquid ejection head may be unable to eject the liquid normally, for example, when bubbles accumulate near the nozzle or sediment components in the liquid gather. The circulation supply of the ink enables the liquid to be ejected normally by discharging the bubbles accumulated near the nozzle and the sediment components in the liquid from the pressure chamber. In an inkjet head, vibration (meniscus vibration) remains in the meniscus of the ink formed at the nozzle after ink ejection. Among the meniscus vibrations, the component caused by the resonance of the ink in the pressure chamber is the vibration necessary for ink ejection, and since the period is shorter than between the printing dots and the vibration can be canceled after ejection by adjusting the drive waveform, it is an acceptable vibration. However, the vibration components caused by the restoring force of the meniscus and the mass of the ink, etc., have a period longer than between the printing dots and cannot be adjusted by the drive waveform, so it is necessary to deal with them by a structural method.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0005] The problem to be solved by the present invention is to provide a liquid ejection head that can suppress the long-period vibration of the meniscus and normally eject a liquid at high speed.

Means for Solving the Problem

[0006] The liquid ejection head according to an embodiment of the present invention includes a resonance chamber, a restrictor flow path, a nozzle, an actuator, an upstream liquid manifold, and a downstream liquid manifold. The resonance chamber is formed to cause liquid column resonance. One end of the restrictor flow path communicates with one end of the resonance chamber. The nozzle is provided in the middle of the restrictor flow path. The actuator causes the liquid column resonance in the resonance chamber. The upstream liquid manifold communicates with the other end of the restrictor flow path. The downstream liquid manifold communicates with the other end of the resonance chamber. The cross-sectional area of the flow path of the restrictor flow path is smaller than the cross-sectional area of the flow path of the resonance chamber.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

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Figure 4

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[0008] Hereinafter, the liquid ejection head according to the embodiment will be described in detail with reference to the accompanying drawings. In each figure, the same components are denoted by the same reference numerals.

[0009] As an example of an image forming apparatus equipped with the liquid ejection head of the embodiment, an inkjet printer 10 that prints an image on a recording medium will be described. FIG. 1 shows a schematic configuration of the inkjet printer 10. The inkjet printer 10 includes, inside a housing 11, a cassette 12 that stores a sheet S, which is an example of a recording medium, an upstream conveyance path 13 for the sheet S, a conveyance belt 14 that conveys the sheet S taken out from the cassette 12, a plurality of inkjet heads 100 to 103 that eject ink droplets toward the sheet S on the conveyance belt 14, a downstream conveyance path 15 for the sheet S, a discharge tray 16, and a control board 17. An operation unit 18, which is a user interface, is arranged on the upper side of the housing 11.

[0010] Image data to be printed on the sheet S is generated, for example, by a computer 200 which is an external connection device. The image data generated by the computer 200 is sent to the control board 17 of the inkjet printer 10 through a cable 201 and connectors 202 and 203.

[0011] The pickup roller 204 supplies the sheets S one by one from the cassette 12 to the upstream conveyance path 13. The upstream conveyance path 13 is composed of a pair of feed rollers 131, 132 and sheet guide plates 133, 134. The sheet S is sent to the upper surface of the conveyance belt 14 via the upstream conveyance path 13. The arrow 104 in the figure indicates the conveyance path of the sheet S from the cassette 12 to the conveyance belt 14.

[0012] The conveyance belt 14 is a net-like endless belt having a large number of through holes formed on its surface. Three rollers, namely a driving roller 141, driven rollers 142 and 143, rotatably support the conveyance belt 14. The motor 205 rotates the conveyance belt 14 by rotating the driving roller 141. The motor 205 is an example of a driving device. The arrow 105 in the figure indicates the rotation direction of the conveyance belt 14. A negative pressure container 206 is arranged on the back side of the conveyance belt 14. The negative pressure container 206 is connected to a decompression fan 207. The fan 207 creates a negative pressure inside the negative pressure container 206 by the formed air flow, and adsorbs and holds the sheet S on the upper surface of the conveyance belt 14. The arrow 106 in the figure indicates the flow of the air flow.

[0013] The inkjet heads 100 to 103, which are examples of liquid ejection heads, are arranged to face the sheet S adsorbed and held on the conveyance belt 14 with a slight gap of, for example, 1 mm therebetween. The inkjet heads 100 to 103 eject ink droplets toward the sheet S, respectively. The inkjet heads 100 to 103 print an image when the sheet S passes below. Each of the inkjet heads 100 to 103 has the same structure except that the colors of the ejected ink are different. The colors of the ink are, for example, cyan, magenta, yellow, and black.

[0014] The supply of ink to each of the inkjet heads 100 to 103 is circulated and supplied by respective ink circulation devices 341 to 344. The detailed configuration of the ink circulation devices 341 to 344 will be described later (see FIG. 8). In FIG. 1, for the convenience of drawing, the ink circulation devices 341 to 344 are each illustrated by a dashed line frame.

[0015] After image formation, the sheet S is sent from the conveyance belt 14 to the downstream conveyance path 15. The downstream conveyance path 15 is composed of the feed roller pairs 151, 152, 153, 154 and the sheet guide plates 155, 156 that define the conveyance path of the sheet S. The sheet S is sent from the discharge port 157 to the discharge tray 16 via the downstream conveyance path 15. The arrow 107 in the figure indicates the conveyance path of the sheet S.

[0016] Subsequently, the configuration of the inkjet heads 100 to 103 will be described. The following describes the inkjet head 100 with reference to FIGS. 2 to 8, but the inkjet heads 101 to 103 also have the same structure as the inkjet head 100.

[0017] FIG. 2 is an external perspective view of the inkjet head 100. The inkjet head 100 includes an ink supply unit 4 which is an example of a liquid supply unit, a nozzle plate 5 which is an example of a nozzle unit, a flexible substrate 6, and a head drive circuit 61. A plurality of nozzles 51 for discharging ink are arranged on the nozzle plate 5. The ink discharged from each nozzle 51 is supplied from the ink supply unit 4 communicating with the nozzle 51. The ink supply unit 4 is connected to an ink circulation device 341 (see FIG. 8) via an ink supply path 311 and an ink discharge path 331. Note that the arrow A2 indicates the rotation direction of the above-described conveyance belt 14 (see FIG. 1).

[0018] FIG. 3 is a partial enlarged plan view of the nozzle plate 5. The nozzles 51 are two-dimensionally arranged in the column direction (e.g., the X direction of the first direction) and the row direction (e.g., the Y direction of the second direction). However, the nozzles 51 arranged in the row direction (Y direction) are arranged obliquely so that the nozzles 51 do not overlap on the axis of the Y axis. Each nozzle 51 is arranged at intervals of a distance X1 in the X-axis direction and a distance Y1 in the Y-axis direction. As an example, the distance X1 is approximately 42.25 μm, and the distance Y1 is approximately 253.5 μm. That is, the distance X1 is determined so that the recording density in the X-axis direction is 600 DPI. Further, the distance Y1 is determined so that printing is also performed at 600 DPI in the Y-axis direction. The nozzles 51 are arranged in a plurality in the X direction with eight nozzles 51 arranged in the Y direction as a set. Although not shown, for example, 150 sets are arranged in the X direction, and a total of 1200 nozzles 51 are arranged.

[0019] The piezoelectric actuator 8 (hereinafter simply referred to as the "actuator 8"), which is a driving source for the operation of discharging ink, is provided for each nozzle 51. The actuator 8 is formed in an annular shape and arranged so that the nozzle 51 is located at its center. A set of nozzles 51 and the actuator 8 constitute one channel. The size of the actuator 8 is, for example, an inner diameter of 30 μm and an outer diameter of 140 μm. Each actuator 8 is electrically connected to an individual electrode 81 respectively. Further, each actuator 8 is electrically connected to eight actuators 8 arranged in the Y direction by a common electrode 82. Each individual electrode 81 and each common electrode 82 are further electrically connected to a mounting pad 9 respectively. The mounting pad 9 not shown serves as an input port for applying a driving waveform to the actuator 8. Each individual electrode 81 applies a driving waveform to each actuator 8 respectively, and each actuator 8 drives according to the applied driving waveform. Note that in FIG. 3, for convenience of explanation, the actuator 8, the individual electrode 81, the common electrode 82, etc. are shown by solid lines, but these are arranged inside the nozzle plate 5 (see FIG. 5). However, the position of the actuator 8 is not limited to the inside of the nozzle plate 5.

[0020] The mounting pad 9 is electrically connected to a wiring pattern formed on the flexible substrate 6 via, for example, an anisotropic conductive film (ACF: Anisotropic Contact Film). Further, the wiring pattern of the flexible substrate 6 is electrically connected to the head drive circuit 61. The head drive circuit 61 is, for example, an IC (Integrated Circuit). The head drive circuit 61 as the control unit of the inkjet head 100 gives a drive waveform to the actuator 8 according to print data.

[0021] FIG. 4(a) is a longitudinal sectional view of the inkjet head 100. FIG. 4(b) is a cross-sectional view taken along line A-A. As shown in FIG. 4(a), the nozzle 51 penetrates the nozzle plate 5 in the Z direction of the third direction. The size of the nozzle 51 is, for example, a diameter of 20 μm and a length of 8 μm. Inside the ink supply unit 4, a plurality of restrictor channels 41 communicating with the respective nozzles 51 are provided. The restrictor channel 41 has a circular planar shape as viewed from the Z direction as shown in the cross-sectional view taken along line A-A, and is a cylindrical channel that is flat in the Z direction. The restrictor channel 41 is a constriction provided in the ink circulation channel and also serves as a movable region when the annular actuator 8 is distorted inward during driving. The restrictor channel 41 has an ink discharge port 42 at one end side and an ink supply port 43 at the other end side. The ink discharge port 42 and the ink supply port 43 are arranged to face each other, for example, on a diameter line of the restrictor channel 41 formed in a flat cylindrical shape.

[0022] The ink supply port 43 of the restrictor channel 41 communicates with the individual supply channel 44. The individual supply channel 44 is a channel formed in the Z direction and having, for example, a rectangular cross-section. The individual supply channels 44 of each channel are commonly connected to an ink supply manifold 45, which is a common liquid chamber arranged on the upper side in the Z direction. The ink supply manifold 45 is an example of an upstream liquid manifold.

[0023] The ink discharge port 42 of the restrictor channel 41 communicates with the resonance chamber 46. The resonance chamber 46 is a liquid chamber, for example, with a rectangular cross-section formed in the Z direction. The resonance chambers 46 of the respective channels are commonly connected to an ink discharge manifold 47, which is a common liquid chamber arranged on the upper side in the Z direction. The ink discharge manifold 47 is an example of a downstream liquid manifold.

[0024] The resonance chamber 46 is formed wider than the restrictor channel 41 so that the cross-sectional area of the restrictor channel 41 is smaller than that of the resonance chamber 46. Therefore, it is preferable that the restrictor channel 41 is formed flat in the Z direction. As will be described in detail later, the resonance chamber 46 is formed as a liquid column resonance tube that causes a quarter-wavelength liquid column resonance when the actuator 8 is driven. Therefore, the ink discharge manifold 47 is communicated with the other end of the resonance chamber 46 to form the open end of the liquid column resonance tube, and a restrictor channel 41, which is a constricted portion, is provided at one end to form the closed end of the liquid column resonance tube. The restrictor channel 41 serves as the closed end of the liquid column resonance tube at one end of the resonance chamber 46 and enables ink to circulate from the ink supply manifold 45 to the ink discharge manifold 47. The flow resistance from the nozzle 51 to one end of the resonance chamber 46 (i.e., the connection point between the ink discharge port 42 and the resonance chamber 46) is made smaller than the flow resistance from the nozzle 51 to the ink supply manifold 45, which is the upstream manifold, by, for example, providing an individual supply path 44. Also, the flow resistance from the nozzle 51 to one end of the resonance chamber 46 (i.e., the connection point between the ink discharge port 42 and the resonance chamber 46) is made larger than the flow resistance from one end of the resonance chamber 46 to the ink discharge manifold 47 by reducing the cross-sectional area of the restrictor channel 41.

[0025] The restrictor flow path 41 is configured, for example, by forming cylindrical holes with a diameter of, for example, 200 μm in a single-crystalline silicon wafer. The ink supply unit 4 is configured to form spaces corresponding to, for example, individual supply paths 44, an ink supply manifold 45, a resonance chamber 46, and an ink discharge manifold 47 in, for example, alumina (Al2O3), and to seal the upper opening in the Z direction with a cover member 48. The circular surface on the nozzle 51 side of the restrictor flow path 41 is blocked by a diaphragm 53 on the nozzle plate 5 except for the hole of the nozzle 51, and the circular surface on the opposite side is blocked by the surface 90 of the ink supply unit 4. That is, the diaphragm 53 on the nozzle plate 5, the surface 90 of the ink supply unit 4, and the cylindrical side wall of the silicon wafer constitute the partition wall of the restrictor flow path 41.

[0026] FIG. 5 is an enlarged view of the actuator 8 of the nozzle plate 5. The nozzle plate 5 has a structure in which a protective layer 52, an actuator 8, and a diaphragm 53 are laminated in this order from the bottom surface side. The actuator 8 has a structure in which a lower electrode 84, a thin plate-shaped piezoelectric body 85 which is an example of a piezoelectric element, and an upper electrode 86 are laminated. The upper electrode 86 is electrically connected to the individual electrode 81, and the lower electrode 84 is electrically connected to the common electrode 82. An insulating layer 54 for preventing a short circuit between the individual electrode 81 and the common electrode 82 is interposed at the boundary between the protective layer 52 and the diaphragm 53. The insulating layer 54 is formed of, for example, a silicon dioxide film (SiO2) with a thickness of 0.5 μm. The lower electrode 84 and the common electrode 82 are electrically connected by a contact hole 55 formed in the insulating layer 54. The piezoelectric body 85 is formed of, for example, PZT (lead zirconate titanate) with a thickness of 5 μm or less in consideration of piezoelectric characteristics and breakdown voltage. The upper electrode 86 and the lower electrode 84 are formed of, for example, platinum with a thickness of 0.15 μm. The individual electrode 81 and the common electrode 82 are formed of, for example, gold (Au) with a thickness of 0.3 μm.

[0027] The diaphragm 53 is formed of an insulating inorganic material. The insulating inorganic material is, for example, silicon dioxide (SiO2). The thickness of the diaphragm 53 is, for example, 2 to 10 μm, preferably 4 to 6 μm. Although details will be described later, the diaphragm 53 and the protective layer 52 are such that the piezoelectric body 85 to which a voltage is applied has d 31It bends inward or outward as the mode changes (see Fig. 13). When the application of voltage to the piezoelectric body 85 is stopped, it returns to its original state. This deformation is reversible. Since the diaphragm 53 forms a part of the partition wall of the restrictor flow path 41, the deformation of the diaphragm 53 is the deformation of the partition wall of the restrictor flow path 41. The deformation of the partition wall changes the volume of the restrictor flow path 41, which transmits a pressure change to one end of the resonance chamber 46. At this time, the ink is ejected from the nozzle 51 by utilizing the liquid column resonance generated in the resonance chamber 46.

[0028] The protective layer 52 is formed of, for example, polyimide with a thickness of 4 μm. The protective layer 52 covers one surface on the bottom side of the nozzle plate 5 and further covers the inner peripheral surface of the hole of the nozzle 51.

[0029] Fig. 6 is a plan view of the ink supply manifold 45 and the ink discharge manifold 47. As shown by an example in Fig. 6, the ink supply manifold 45 and the ink discharge manifold 47 are formed in a comb shape and arranged such that the comb portions of each other are alternately arranged in the X direction. The comb portion extending in the Y direction of the ink supply manifold 45 communicates with the restrictor flow path 41 of each channel via the individual supply path 44 (not shown) of each channel. The comb portion extending in the Y direction of the ink supply manifold 45 is shared by the channels adjacent to each other in the X direction. The comb portion extending in the Y direction of the ink discharge manifold 47 communicates with the restrictor flow path 41 of each channel via the resonance chamber 46 (not shown) of each channel. The comb portion extending in the Y direction of the ink discharge manifold 47 is shared by the channels adjacent to each other in the X direction.

[0030] As shown in FIG. 7, the ink supply manifold 45 and the ink discharge manifold 47 are respectively connected to the ink supply path 311 and the ink discharge path 331 on the upper side in the Z direction. The ink supply path 311 and the ink discharge path 331 are connected to an ink circulation device 341 (see FIG. 8) described later. The ink discharge path 331 is made thicker than the ink supply path 311 to reduce the flow path resistance of the circulation flow. For the ink supply path 311, for example, a tube with an inner diameter of φ3 mm is used. For the ink discharge path 331, a tube with an inner diameter of φ6 mm is used. The opening of the downstream port (OUT), which is the connection point between the ink discharge manifold 47 and the ink discharge path 331, is also made larger than the opening of the upstream port (IN), which is the connection point between the ink supply manifold 45 and the ink supply path 311, to reduce the fluid resistance of the circulation flow. Further, the volume of the ink discharge manifold 47 is also made larger than the volume of the ink supply manifold 45 to reduce the fluid resistance of the circulation flow. It is preferable that a flexible film is attached to the inner surface of the ink discharge manifold 47 as a damper 49 to form a pressure damper chamber and suppress a sudden change in pressure. The damper 49 is, for example, a polyimide film or the like. All of these are effective in controlling the pressure of the downstream flow path described in detail later to be constant.

[0031] Next, an ink circulation device 341 that circulates and supplies ink to the inkjet head 100 will be described with reference to FIG. 8. The ink circulation device 341 is an example of a liquid circulation device for a liquid ejection head. Note that the ink circulation devices 342 to 344 that circulate and supply ink to the inkjet heads 101 to 103 also have the same configuration as the ink circulation device 341. As shown in FIG. 8, the ink circulation device 341 includes an ink tank 315, an ink pump 321, an ink filter F1, an ink supply unit 4 of the inkjet head 100, and the ink supply path 311 and the ink discharge path 331 that connect these components. Further, air valves V1, V2 and air filters F2, F3 are respectively provided in the ink supply path 311 and the ink discharge path 331.

[0032] When initially filling the ink circulation device 341 in FIG. 8 with ink, first close the air valve V2 and open the air valve V1, and supply ink from the upstream channel using the ink pump 321. The ink flows into the ink supply section 4 through the upstream port (IN), and passes through the restrictor channel 41 and the resonance chamber 46 as described above. The ink discharged from the ink supply section 4 through the downstream port (OUT) returns to the ink tank 315 through the downstream channel. When the downstream channel is filled with ink, close the air valve V1 and open the air valve V2, and the nozzle back pressure is pressure-controlled to -ρgh + ΔP, which is ΔP greater than -ρgh. ΔP is due to the pressure loss from the nozzle 51 to the downstream port (OUT). This pressure loss increases as the nozzle 51 is farther from the closed end of the resonance chamber 46 and as the flow rate of the ink flowing through the ink pump 321 increases. The height difference h is set, for example, so that -ρgh + ΔP = -1 kPa. When ΔP is large, when trying to set the height difference h so as to obtain the desired nozzle back pressure, the tank 315 has to be set low, and as a result, the ink discharge path 331 may become too long. If the ink discharge path 331 becomes too long, even if a thick channel is used, the pressure fluctuations due to the channel resistance cannot be ignored, and also a large amount of ink is consumed to fill the channel with ink. In order to prevent the ink discharge path 331 from becoming too long, instead of venting the air release side of the air filter F3 to the atmosphere, it may be connected to an atmospheric pressure control tank (for example, a decompression tank) with a pressure of -P1 (not shown). In that case, set the height difference h and the pressure -P1 so that -ρgh + ΔP - P1 = -1 kPa. That is, the ink circulation device 341 of the inkjet head 100 has a pressure adjustment section downstream of the nozzle 51. The pressure adjustment section controls the pressure of the ink discharge manifold 47, which is a common liquid chamber, to a predetermined pressure. Further, when the ink discharge manifold 47 also serves as a pressure damper chamber, it suppresses fluctuations in the pressure of the pressure damper chamber. Note that the timing of closing the air valve V1 and opening the air valve V2 may be controlled by time, or may be controlled by installing a liquid level sensor or the like in the ink tank 315.

[0033] Here, when attempting to flow ink through a system with a predetermined flow path resistance from the upstream end to the downstream end, since the flow rate is proportional to the pressure difference between the upstream end and the downstream end, it is possible to choose to increase the pressure at the upstream end or decrease the pressure at the downstream end in order to obtain a desired flow rate. However, reducing the pressure of the ink is more difficult and has more drawbacks compared to increasing the pressure of the ink. One reason is that air more easily passes through smaller gaps compared to ink, so it is more difficult to prevent air leakage on the low-pressure side than to prevent ink leakage on the high-pressure side. Another reason is that there is no upper limit to the pressure on the high-pressure side, while there is a constraint that the low-pressure side cannot be below vacuum.

[0034] On the other hand, the nozzle back pressure is desired to be an appropriate pressure so that an appropriate meniscus M (see Fig. 13) is formed at the nozzle 51. For example, it is -0.5 kPa to -3 kPa, preferably -1 kPa, relative to atmospheric pressure. And in order to maintain the appropriate nozzle back pressure without reducing the pressure on the downstream side as much as possible, it is desirable to set a larger flow path resistance on the upstream side of the nozzle 51 and a smaller flow path resistance on the downstream side of the nozzle 51.

[0035] When discharging ink from the nozzle 51, the flow rate of the ink discharged from the nozzle 51 is the difference between the upstream flow rate and the downstream flow rate of the circulation flow. The flow rate of the ink discharged from the nozzle 51 varies depending on the printing content, but it is desirable that the change in the nozzle back pressure is small. The nozzle back pressure is a value that is larger or smaller by the amount obtained by multiplying the flow rate by the flow path resistance from the location where [pressure + potential energy per unit volume due to the height h of the ink] is defined to the nozzle branch in the circulation path. If the location where [pressure + potential energy per unit volume due to the height h of the ink] is defined exists on both the upstream side and the downstream side, the magnitude of the pressure change in the nozzle back pressure due to the change in the flow rate of the ink discharged from the nozzle 51 is determined by the parallel resistance of the flow path resistance on the upstream side of the circulation flow and the flow path resistance on the downstream side of the circulation flow.

[0036] That is, in order to suppress the pressure change of the nozzle back pressure accompanying the flow rate change of the ink ejected from the nozzle 51, it is sufficient to reduce the parallel resistance of the flow path resistance on the upstream side of the circulation flow and the flow path resistance on the downstream side of the circulation flow. Since it is a parallel resistance, if one resistance is reduced, the influence of the magnitude of the other resistance becomes small. That is, by keeping the flow path resistance on the downstream side small, even if the flow path resistance on the upstream side is large, it is possible to suppress the pressure change of the nozzle back pressure accompanying the flow rate change of the ink ejected from the nozzle 51. For this reason, the ink filter F1 that increases the flow path resistance can be arranged on the upstream side of the circulation flow. Arranging the ink filter F1 on the upstream side of the circulation flow is also important in order not to let foreign matter or bubbles flow into the ink supply unit 4.

[0037] Furthermore, since the flow path resistance on the upstream side can be increased, the flow path on the upstream side can be narrowed. Generally, the flow from a narrow flow path to a wide flow path is less likely to leave air during ink filling than in the reverse case. Also, general inkjet ink is easy to fill in a flow path with a diameter of 4 mm or less regardless of the gravity direction, and conversely, it is difficult to fill the ink without air remaining in a flow path having a cross section larger than 4 mm in diameter and facing various directions in the gravity direction. From this, also in terms of ease of ink filling, it is advantageous that the flow path on the upstream side can be relatively narrowed.

[0038] A slight amount of bubbles in the downstream flow path only causes a pressure change due to the decrease in gravity caused by the bubbles. On the other hand, if even a little bubble remains on the flow path on the upstream side, when the bubble accidentally flows into the ink supply unit 4, ink non-ejection may occur. Therefore, in order to perform highly reliable printing, the bubbles on the upstream path must be eliminated.

[0039] As described above, the ink supply unit 4 allows ink to flow in from the restrictor channel 41 side, discharges it to the ink discharge manifold 47, and circulates it. That is, the restrictor channel 41 side is the upstream side of the ink flow. For the reasons described above, the ink filter F1 is provided in the upstream channel. This is because it is desirable to keep the flow rate constant on the upstream side, and the channel resistance on the upstream side can be large. Therefore, the ink supply path 311 may also be made thinner than the ink discharge path 331. As an example, when the ink supply path 311 is a tube with an inner diameter of 3 mm and the ink discharge path 331 is a tube with a diameter of 6 mm, the opening of the upstream port (IN) is 3 mm in diameter and the opening of the downstream port (OUT) is 6 mm in diameter. If the flow rate is kept constant on the upstream side, a channel with a small cross-sectional area can be adopted in this way, so it is easy to fill the ink supply unit 4 of the inkjet head 100 with ink and air bubbles are less likely to remain.

[0040] It is desirable that the flow rate on the upstream side be at least one time the maximum ink discharge flow rate of the inkjet head 100. The maximum ink discharge flow rate is the total flow rate when ink is discharged from all channels. The flow rate of the ink flowing through the restrictor channel 41 decreases as the flow rate of the discharged ink increases. Therefore, if the flow rate on the upstream side is smaller than the maximum ink discharge flow rate, part of the discharged ink will be sucked from the downstream side. Since the ink on the downstream side is not the ink immediately after passing through the ink filter F1, it is not desirable to let it flow into the ink channel (restrictor channel 41) on the back of the nozzle from the perspective of particles. Therefore, the flow rate on the upstream side is set to be at least one time the maximum ink discharge flow rate so that ink can be supplied from the upstream side even when ink is discharged at the maximum ink discharge flow rate.

[0041] The ink supply manifold 45, which is the common liquid chamber on the upstream side, is controlled to a predetermined flow rate, while the ink discharge manifold 47, which is the common liquid chamber on the downstream side, is controlled to a predetermined pressure. Let the pressure of the ink discharge manifold 47 be Pm (negative pressure), the flow resistance of the restrictor flow path 41 from the ink discharge manifold 45 to the back of the nozzle be R, the flow rate of the ink flowing in from the upstream side of the back of the nozzle be Qu, the flow rate of the ink discharged from the nozzle 51 be Qn, and the flow rate of the ink flowing downstream of the back of the nozzle be Qd. Then, Qd = Qu - Qn. The nozzle back pressure Pn is Pn = Pm + RQd.

[0042] When the ink is not being discharged, that is, when Qn = 0, the nozzle back pressure becomes the negative pressure with the smallest absolute value. At this time, Pn = Pm + RQu. Set the nozzle back pressure Pn when the ink is not being discharged to, for example, a negative pressure of -1 kPa. On the other hand, set the steady value of the pressure in the resonance chamber 46 to, for example, -5 kPa. If this negative pressure were transmitted directly to the back of the nozzle, the meniscus M could not be maintained, and the meniscus M would break, causing air to be drawn into the nozzle 51. Therefore, by utilizing the flow resistance of the restrictor flow path 41 between the nozzle 51 and the resonance chamber 46, the pressure at the back of the nozzle is shifted to the positive pressure side with respect to the pressure in the resonance chamber 46. If the flow resistance R is set such that the value obtained by multiplying the flow rate by the flow resistance between the nozzle 51 and the resonance chamber 46 when the ink is not being discharged, RQu = RQd, becomes, for example, 4 kPa, the pressure at the back of the nozzle becomes an appropriate negative pressure of -5 kPa + 4 kPa = -1 kPa, and a concave meniscus M is formed at the nozzle 51. This state is maintained while the ink is not being discharged.

[0043] When ink is ejected, the flow rate Qd flowing downstream from the back of the nozzle decreases by the flow rate Qn of the ejected ink. Then, the pressure difference between the back of the nozzle and the resonance chamber 46 decreases, and the nozzle back pressure shifts to the negative pressure side by RQn. If the upstream flow rate Qu is set so that the downstream flow rate Qd becomes 0 (zero) at the maximum ink ejection flow rate, the steady pressure at the back of the nozzle when ejecting at the maximum ink ejection flow rate is -5 kPa. That is, the greater the ink ejection volume, the more the nozzle back pressure Pn shifts to the negative pressure side, making it difficult for the meniscus M to rise due to continuous ejection. Since the restrictor flow path 41 is between the ink supply manifold 45 and the nozzle 51, even if the negative pressure in the continuously ejecting channel increases, it has little effect on the channels that are not ejecting.

[0044] The flow rate on the downstream side of the nozzle 51 is the value obtained by subtracting the ejection flow rate from the upstream flow rate, so it is not constant and varies depending on the printing content. That is, it becomes the maximum flow rate when not ejecting, and the minimum flow rate when ejecting at the maximum ink ejection flow rate. Instead of making the flow rate constant on the downstream side like the upstream side, it is configured so that the pressure change with respect to the flow rate change is small. Therefore, by making the downstream flow path, port, and manifold larger than the upstream flow path, port, and manifold, the pressure change accompanying the flow rate change is suppressed. Furthermore, if the ink discharge manifold 47 is made into a pressure damper chamber, a rapid pressure change on the downstream side can be suppressed. Do not provide an ink filter or the like that increases the flow path resistance on the downstream side. Since a larger cross-sectional area flow path is required on the downstream side of the nozzle 51, small bubbles tend to remain, but since the purpose is to control the pressure to be constant, small bubbles do not have an adverse effect. Also, since the ink flow is directed downstream, there is little risk of these small bubbles mixing into the resonance chamber 46.

[0045] If there is a resonance chamber 46 between the restrictor flow path 41 and the nozzle 51, the change in the nozzle back pressure during a flow rate change is delayed, resulting in a delay in the action of preventing the meniscus M from rising. Also, when the flow rate decreases after the ejection ends, the change in negative pressure is not fast enough, and air is drawn into the nozzle 51. On the other hand, since the restrictor flow path 41 of the present embodiment is between the nozzle 51 and the resonance chamber 46, the reaction to a flow rate change is fast, and the nozzle back pressure can be effectively increased or decreased when the ejection flow rate increases or decreases.

[0046] FIG. 9 is a block configuration diagram of the control system of the inkjet printer 10. The control board 17 as the control unit of the printer is equipped with a CPU 90, a ROM 91, a RAM 92, an I / O port 93 which is an input / output port, and an image memory 94. The CPU 90 controls the drive motor 24, the ink circulation devices 341 to 344, the operation unit 18, and various sensors through the I / O port 93. Image data from the computer 2 which is an external connection device is transmitted to the control board 17 through the I / O port 93 and stored in the image memory 94. The CPU 90 develops the image data stored in the image memory 94 into, for example, a dot pattern and transmits it to the head drive circuit 61 in the printing order. The head drive circuit 61 gives a drive waveform to the actuator 8 selected according to the image data.

[0047] Here, the quarter-wavelength liquid column resonance used by the inkjet head 100 of the present embodiment for ink ejection will be described. FIG. 10(a) illustrates the quarter-wavelength liquid column resonance in the resonance chamber 46. The dashed line represents the pressure amplitude, and the dotted-dashed line represents the flow velocity amplitude. As shown in FIG. 4, although the direction of ink flow from the restrictor channel 41 to the resonance chamber 46 bends by 90 degrees, it does not affect the mechanism of liquid column resonance. By bending the ink flow direction by 90 degrees, the ink flow in the resonance chamber with a large cross-section becomes upward, opposite to the gravitational direction. Therefore, there is an advantage that even if there are bubbles in the resonance chamber 46 during ink circulation or filling, they are easily discharged. For comparison, FIG. 10(b) shows a resonance chamber 7 that uses half-wavelength liquid column resonance for ink ejection. The dashed line represents the pressure amplitude, and the dotted-dashed line represents the flow velocity amplitude. As shown in FIG. 10(b), since the resonance chamber 7 that uses half-wavelength liquid column resonance for ink ejection has both ends completely open, it is very suitable for an ink circulation type head that circulates ink through the resonance chamber 7. The ink flowing into the resonance chamber 7 from the upstream common ink chamber passes behind the nozzle 51 without any obstacles and flows directly into the downstream common ink chamber. Since the center of the half-wavelength liquid column resonance tube has the maximum pressure amplitude, the nozzle 51 is arranged at the center of the resonance chamber 7, which is a liquid column resonance tube. In principle, it is not necessary to restrict the ink flow at the inlet and outlet of the resonance chamber 7 to increase the pressure at the center of the pressure chamber.

[0048] In addition, since the circulating flow of ink from the inlet to the outlet of the resonance chamber 7 does not affect the ink ejection operation by liquid column resonance, the advantages of circulating ink can be maximally obtained. Specifically, it prevents the retention of sedimentary components (such as pigments), alleviates the increase in viscosity due to solvent evaporation, discharges bubbles, equalizes the temperature, and facilitates cleaning.

[0049] However, for this type of side shooter inkjet head, the braking effect on the meniscus vibration caused by the restoring force of the ink meniscus M formed in the nozzle 51 is ineffective. Therefore, there is a problem that the high-speed followability for continuously discharging ink is not good. When the ink is continuously discharged, as shown in Fig. 10(b), the meniscus M gradually moves or bulges outward, affecting the subsequent ink discharge. As a countermeasure, as shown in Fig. 10(c), restrictors 72 and 73 are provided at the inlet and outlet of the resonance chamber 7 to appropriately brake the meniscus vibration caused by the resonance between the restoring force of the meniscus M formed in the nozzle 51 and the mass of the ink.

[0050] However, when restrictors 72 and 73 are provided at the inlet and outlet of the resonance chamber 7, both ends of the resonance chamber 7 are no longer the release ends of the liquid column resonance. Therefore, the resonance principle of the resonance chamber 7 shown in Fig. 10(c) is not a liquid column resonance type, but a Helmholtz resonance type in which the ink mass in the restrictors 72 and 73 resonates with the volume of the resonance chamber 7. In addition, the circulating flow of the ink must pass through the restrictors 72 and 73 at two locations, the inlet and outlet of the resonance chamber 7. Since the nozzle back pressure when the ink is not discharged is the pressure determined by the resistance ratio of the restrictors 72 and 73 at the inlet and outlet, the nozzle back pressure cannot be maintained at an appropriate value (for example, -1 kPa) unless the restrictors 72 and 73 are manufactured with high precision. Also, problems such as air bubbles and sedimentary components in the ink staying in front of the downstream restrictor 73 are likely to occur.

[0051] On the other hand, the inkjet head 100 that utilizes the 1 / 4 wavelength liquid column resonance for ink discharge solves the problem of high-speed discharge followability while using a liquid column resonance tube with a structure that can open the resonance chamber 46 and is suitable for ink circulation. That is, as shown in Fig. 10(a), by substantially closing one end side of the resonance chamber 46 and opening the other end side, the resonance chamber 46 is made into a 1 / 4 wavelength liquid column resonance tube. Further, a restrictor flow path 41 is provided at the closed end of the liquid column resonance tube, and the nozzle 51 is arranged in the middle of the restrictor flow path 41.

[0052] The resonance chamber 46 has a larger cross-sectional area than the restrictor flow path 41 so that it does not itself become a resistance to the circulating flow. Since the restrictor flow path 41 is provided to damp resonance (meniscus vibration) with a longer period than the time interval between printing dots, which is caused by the restoring force of the meniscus M formed in the nozzle 51 and the mass of the ink, it is desirable that it has no damping effect on the liquid column resonance occurring in the resonance chamber 46 with a shorter period than the time interval between printing dots. Since the restrictor flow path 41 is provided at the closed end of the liquid column resonance tube, it does not damp the liquid column resonance. Thus, the configuration of FIG. 10(a) is not structurally damped by the liquid column resonance and is damped by a damping pulse of a drive waveform described later.

[0053] The damping of the meniscus vibration by the restoring force of the meniscus M can be adjusted by the position of the nozzle 51 in the restrictor flow path 41. That is, if the nozzle 51 is brought closer to the resonance chamber 46 in the restrictor flow path 41, the meniscus vibration appears, and if it is moved farther away from the resonance chamber 46, the meniscus vibration is damped. The flow path resistance from the nozzle 51 to the closed end of the resonance chamber 46 is sufficiently smaller than the flow path resistance from the nozzle 51 to the upstream ink supply manifold 45. The flow path resistance from the nozzle 51 to the closed end of the resonance chamber 46 is larger than the flow path resistance from the closed end of the resonance chamber 46 to the downstream ink discharge manifold 47. Therefore, the damping action of the meniscus vibration is dominated by the flow path resistance from the nozzle 51 of the restrictor flow path 41 to the closed end of the resonance chamber 46.

[0054] For example, when it is desired to adjust the ink viscosity to be low and increase the damping of the meniscus vibration, if the nozzle position is moved away from the resonance chamber 46, the flow path resistance from the nozzle 51 of the restrictor flow path 41 to the closed end of the resonance chamber 46 increases, so the damping of the meniscus vibration becomes stronger. Conversely, for example, when it is desired to adjust the ink viscosity to be high and decrease the damping of the meniscus vibration, if the nozzle position is moved closer to the resonance chamber 46, the flow path resistance from the nozzle 51 of the restrictor flow path 41 to the closed end of the resonance chamber 46 decreases, so the damping of the meniscus vibration becomes weaker. When the nozzle 51 is at the boundary between the restrictor flow path 41 and the resonance chamber 46, the meniscus vibration is not damped, but if the ink viscosity is high and the driving frequency for continuous ejection is low, it is still sufficient.

[0055] In this way, the strength of the damping of the meniscus vibration can be selected by determining the position of the nozzle 51 in the restrictor flow path 41.

[0056] Subsequently, while referring to FIGS. 11 to 13, the results of simulating the behavior of the ink flow velocity ejected from the nozzle 51 in the above-described head structure will be described. FIG. 11 shows the simulation model. FIG. 12 shows the behavior of the ink flow velocity when a DPR waveform is applied to the actuator 8 as an example of the driving waveform. FIG. 13 shows the operation of the actuator 8 when the DPR waveform is applied. As will be described in detail below, it was confirmed that the ink flow velocity is applied to the nozzle 51 by a DRP waveform that pushes after subtracting half a cycle of the pressure vibration to attenuate the residual vibration. The driving waveform applied to the actuator 8 may be other than the DPR waveform.

[0057] First, in a state where the ink is circulated and supplied to the ink supply unit 4 by the above-described ink circulation device 341, as shown in FIG. 12, prior to the ink ejection operation, a driving voltage of, for example, 12 V is applied to the individual electrode 81 of the actuator 8 at time t0. A common potential of, for example, 0 V is applied to the common electrode 82. When a voltage is applied to the individual electrode 81, an electric field is generated in the thickness direction of the piezoelectric body 85 in the actuator 8, and as shown in FIG. 13(a), d 31The mode deforms and curves inward. This state is set as the steady standby state.

[0058] As shown in FIG. 12, when the voltage applied to the individual electrode 81 of the actuator 8 for discharging ink is set to 0 V at time t1, the deformation of the piezoelectric body 85 returns to its original state as shown in FIG. 13(b), and the meniscus M of the ink in the nozzle 51 is pulled. Subsequently, when a voltage of, for example, 12 V is applied to the individual electrode 81 of the actuator 8 at time t2, the piezoelectric body 85 deforms as shown in FIG. 13(c), and the actuator 8 curves inward again. The time from time t1 to time t2 is set to be half of the inherent pressure oscillation period determined by the ink characteristics and the internal structure of the head. As a result, a quarter-wavelength liquid column resonance occurs in the resonance chamber 46, and as shown in FIG. 12, an ink flow rate can be applied to the nozzle 51, and the ink can be discharged. When the pressure oscillation period is, for example, 4 μs, the time from time t1 to time t2, which is the basic pulse width of the drive waveform, is 2 μs.

[0059] Thereafter, when the voltage applied to the individual electrode 81 of the actuator 8 is increased to 24 V at time t3, the actuator 8 curves further inward as shown in FIG. 13(d). Then, when it is lowered to 12 V at time t4, the deformation of the actuator 8 returns to the state shown in FIG. 13(e). The shrinkage and return of the restrictor flow path 41 attenuate the residual amount oscillation. Since the state of FIG. 13(e) is the same as the standby state of FIG. 13(a), ink can be continuously discharged in the next drive cycle.

[0060] According to the above-described embodiment, it is possible to provide an inkjet head 100 that suppresses the long-period oscillation of the meniscus M of the ink in the nozzle 51 and can normally discharge the ink at high speed.

[0061] Note that, as a preferred example, the above-described embodiment showed a configuration in which the resonance chamber 46 and the restrictor flow path 41 communicating with the closed end thereof are sealed by a common nozzle plate 5, but the present invention is not limited thereto. Similarly, as a preferred example, a nozzle 51 and an actuator 8 are provided on the nozzle plate 5, and a configuration is shown in which a diaphragm 53 constituting a part of the partition wall of the restrictor flow path 41 is deformed by the operation of the actuator 8, but the present invention is not limited thereto. The actuator 8 may be arranged to deform the partition wall 90 on the opposite side of the restrictor flow path 41. The actuator 8 only needs to cause liquid column resonance in the resonance chamber 46, and for example, it may be provided so as to deform the partition wall of the resonance chamber 46.

[0062] Also, in the above-described embodiment, the inkjet head 100 of the inkjet printer 10 was described as an example of a liquid ejection head, but the liquid ejection head may be a modeling material ejection head of a 3D printer or a sample ejection head of a dispensing device.

[0063] The embodiments of the present invention are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalent scope thereof.

Explanation of Reference Numerals

[0064] 10 Inkjet printer 100 - 103 Inkjet head 4 Ink supply unit 41 Restrictor flow path 46 Resonance chamber 45 Ink supply manifold 46 Ink discharge manifold 5 Nozzle plate 51 Nozzle 8 Actuator

Claims

1. A resonance chamber formed to cause liquid column resonance, A restrictor flow path with one end communicating with one end of the resonance chamber, A nozzle provided in the middle of the restrictor flow path, An actuator for causing the liquid column resonance in the resonance chamber, An upstream liquid manifold communicating with the other end of the restrictor flow path, A downstream liquid manifold communicating with the other end of the resonance chamber, and comprising: A liquid discharge head, wherein a flow path cross-sectional area of the restrictor flow path is smaller than that of the resonance chamber.

2. The liquid discharge head according to claim 1, wherein the resonance chamber is a liquid column resonance tube having a closed end at one end communicating with the restrictor flow path and an open end at the other end communicating with the downstream liquid manifold.

3. The liquid discharge head according to claim 1, wherein the actuator deforms a part of a partition wall constituting the restrictor flow path.

4. The liquid discharge head according to any one of claims 1 to 3, wherein a flow path resistance from the nozzle to one end of the resonance chamber is smaller than a flow path resistance from the nozzle to the upstream liquid manifold and larger than a flow path resistance from one end of the resonance chamber to the downstream liquid manifold.

5. The liquid discharge head according to any one of claims 1 to 3, wherein the liquid is filled or circulated from the upstream liquid manifold toward the downstream liquid manifold.

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