JP2011067999A - Method of ejecting liquid and liquid ejection device - Google Patents

Abstract

A liquid ejection method and a liquid ejection apparatus capable of realizing a wider gradation expression while stabilizing ejection characteristics using a high viscosity liquid.
An opening area of a nozzle on which ink of 8 millipascal seconds or more is ejected is 1/9 or less of an opening area on the pressure chamber side of an ink supply path, and an ejection pulse PS operates from a reference potential. A first waveform portion that expands the pressure chamber by changing the potential to a potential with a constant gradient; a second waveform portion that maintains the operating potential for a fixed time; and a potential chamber that changes from the operating potential to a reference potential with a constant gradient. And a third waveform portion that pressurizes the discharge pulse PS2 when the discharge pulse PS2 that is generated next to the discharge pulse PS1 that is previously generated within the unit period T is applied to the actuator. Thus, the interval Δt of these pulses was set based on the natural vibration period Tc generated in the ink in the pressure chamber so that the phase of the residual vibration after the liquid was discharged was uniform.
[Selection] Figure 4

Description

  The present invention relates to a liquid ejection method for a liquid ejection apparatus such as an ink jet printer, and a liquid ejection apparatus.

  The liquid discharge apparatus is an apparatus that includes a liquid discharge head having a nozzle that discharges liquid and discharges various liquids from the liquid discharge head. As a representative example of this liquid ejection apparatus, for example, an ink jet recording head (hereinafter simply referred to as a recording head) as a liquid ejection head is provided, and liquid ink is ejected from the nozzle of this recording head to a recording medium such as recording paper. An image recording apparatus such as an ink jet printer (hereinafter, simply referred to as a printer) that records an image or the like by ejecting and landing on a (landing target) to form dots can be given. In recent years, liquid ejecting apparatuses have been applied not only to the image recording apparatus but also to various manufacturing apparatuses such as a manufacturing apparatus for a color filter such as a liquid crystal display.

  For example, the printer has a nozzle row (nozzle group) formed by arranging a plurality of nozzles, and discharge pulses are supplied to a pressure generating unit (for example, a piezoelectric vibrator, a heating element, or an electrostatic actuator). Some are configured to apply a pressure change to the liquid in the pressure chamber by applying and driving the liquid, and to discharge the liquid from a nozzle communicating with the pressure chamber using the pressure change. In a printer that employs an electrostatic actuator as a pressure generator (actuator), a diaphragm electrode that forms a part of the pressure chamber (ink chamber), and an individual electrode (fixed electrode) that is disposed opposite to the diaphragm electrode When the ejection pulse is applied between them, the diaphragm electrode is attracted to the individual electrode, and when the application of the ejection pulse is released, the diaphragm electrode is released from the individual electrode and elastically returns. The pressure of the ink in the pressure chamber fluctuates due to the displacement of the diaphragm electrode, and ink is ejected from the nozzle communicating with the pressure chamber in the process of the diaphragm electrode being released from the individual electrode and elastically returning. The ejection pulse includes, for example, an expansion element whose potential rises at a constant gradient from the reference potential to the expansion potential, a hold element that maintains the expansion potential for a certain time, and a steeper than the potential gradient of the expansion element from the expansion potential to the reference potential. It is composed of a contracting element whose potential drops with a simple potential gradient and a substantially trapezoidal waveform.

  Incidentally, after ink discharge, residual vibration of ink in the pressure chamber becomes a problem. That is, the residual vibration disturbs the behavior of the meniscus, which may adversely affect the subsequent ink droplet ejection operation. In particular, when a very small (for example, several pl) ink droplets are ejected continuously in a very short time (for example, several μs) as the recording speed increases or the resolution of a recorded image increases, It is desirable to suppress the residual vibration of the as much as possible. For this reason, for example, in a printer that employs a piezoelectric vibrator as a pressure generating unit, a damping element is included after a waveform element (ejection element) for ejecting ink droplets in a drive signal, and the residual is caused by this damping element. Vibration is reduced (for example, see Patent Document 1).

  However, in a printer that employs the above-described electrostatic actuator as a pressure generating unit, even if a damping element is included in the ejection pulse, the diaphragm electrode cannot follow a small change in potential, so it is used in such a printer. The ejection pulse is generally a trapezoidal waveform as described above, and generally has no damping element.

  In the case of a printer employing the above-described electrostatic actuator, in the case of a configuration in which gradation representation of pixels formed on a recording medium is performed according to the number of ink droplets ejected within one printing unit cycle, In the case of shot / pixel printing, ink ejection characteristics at each ejection fluctuate when ink droplets are ejected continuously under the influence of the residual vibration. That is, in the case where the ink droplet is discharged after the first ink droplet discharge in the unit cycle, the pressure vibration generated in the pressure chamber is the pressure vibration generated by driving the diaphragm electrode and the previous ink droplet discharging. Overlay with the generated residual vibration. Therefore, due to residual vibration, the displacement state of the meniscus at the time of starting the driving of the diaphragm electrode is a case where only one ink droplet is ejected by one driving and a plurality of ink droplets are ejected by multiple shots. It may be different from the case of doing.

  Therefore, in the prior art, when printing for one pixel by continuous ink droplet ejection, the drive interval from the time when the second and subsequent ejection pulses are applied to the time when the next ejection pulse is applied is set to the ink interval. A method of determining based on the residual vibration waveform of the diaphragm electrode after droplet discharge has been proposed (see, for example, Patent Document 2). That is, in the configuration of Patent Document 2, each pulse interval of the first to nth ejection pulses applied for ejecting each ink droplet is determined by the residual vibration of the diaphragm electrode after ejecting the ink droplet. By setting individually based on the vibration waveform, desired ejection characteristics can be obtained. More specifically, the driving of the diaphragm electrode by the next pulse is started at the timing when the residual vibration of the diaphragm electrode after ink droplet discharge becomes the neutral position (zero cross point). As a result, the ejection of ink droplets is stabilized.

JP 2002-127418 A JP 2005-305866 A

  Ink ejection characteristics for the second and subsequent times when ink droplets are ejected continuously, that is, fluctuations in the amount (weight / volume) of ink ejected from the nozzles and the flying speed, are especially the Helmholtz period generated in the ink in the pressure chamber. This is largely due to residual vibration based on vibration of Tc (hereinafter also referred to as natural vibration period Tc as appropriate). For example, in the case of an ink having a relatively low viscosity (for example, 1 millipascal second) having a viscosity of less than 8 millipascal seconds (mPa · s), such as a water-based ink generally used in a home printer. Since the meniscus displacement due to the residual vibration is likely to be large, the ink ejection characteristics greatly vary. Such characteristic variation becomes more significant as the amplitude of the residual vibration at the time of starting the second and subsequent ink ejection operations becomes larger, and the print quality may be greatly deteriorated. The natural vibration period Tc generated in the ink in the pressure chamber has a predetermined variation for each recording head due to a manufacturing error of the recording head. Therefore, for example, a reference head in which ejection pulses are designed so that a desired ink weight can be obtained in the multi-shot / pixel printing is applied to another recording head different from the reference head. When the ink is ejected, there is a possibility that a desired ink weight cannot be obtained. Therefore, in the conventional printer for water-based ink, the drive of the ejection operation by the next ejection pulse is started at the timing when the residual vibration after the ink is ejected by the previous ejection pulse becomes at or near the neutral position. Thus, the interval between pulses had to be set. As a result, the degree of freedom in setting the pulse interval is narrowed, and the width of pixel gradation expression by multi-shot / pixel printing is limited.

  The present invention has been made in view of such circumstances, and the object thereof is a liquid ejection method capable of realizing a wider gradation expression while stabilizing ejection characteristics using a high-viscosity liquid, and Another object is to provide a liquid ejection device.

The present invention has been proposed to achieve the above-described object, and includes a pressure chamber to which liquid from a liquid supply source is supplied through a supply unit, a nozzle that communicates with the pressure chamber and discharges liquid, A liquid generating head having a pressure generating unit that performs an operation of changing the pressure of the liquid in the pressure chamber in order to discharge the liquid from the nozzle; and operating the pressure generating unit to discharge the liquid from the nozzle A liquid discharge method in a liquid discharge apparatus including a pulse generator that generates a plurality of discharge pulses within a unit cycle,
The liquid has a viscosity of 8 millipascal seconds or more,
The opening area of the nozzle on the side from which the liquid is discharged is 1/9 or less of the opening area on the pressure chamber side of the supply unit,
The discharge pulse includes a first waveform section that operates the pressure generating section to depressurize the pressure chamber by changing a potential from a reference potential to an operating potential with a constant gradient, and a second waveform that maintains the operating potential for a certain period of time. And a third waveform unit that operates the pressure generating unit to pressurize the pressure chamber by changing the potential with a constant gradient from the operating potential to the reference potential,
The phase of residual vibration after the liquid is discharged using the previous discharge pulse when the discharge pulse generated next to the discharge pulse generated earlier in the unit period is applied to the pressure generating unit. The interval between the previous ejection pulse and the next ejection pulse is set based on the natural vibration period Tc generated in the liquid in the pressure chamber so that the values are uniform.

  According to the present invention, when a discharge pulse generated next to a discharge pulse generated earlier in a unit cycle is applied to the pressure generating portion, the residual after discharging the liquid using the previous discharge pulse Since the interval between the previous discharge pulse and the next discharge pulse is set based on the natural vibration period Tc generated in the liquid in the pressure chamber so that the vibration phase is uniform, multiple discharge pulses are generated within the unit period. By continuously applying to the part, the total amount of liquid when the liquid is discharged a plurality of times can be made uniform regardless of individual differences of the liquid discharge heads. In particular, when a highly viscous liquid of 8 millipascal seconds or more is discharged, the liquid itself exhibits a damper effect that suppresses residual vibration. For example, the liquid discharge operation by the next discharge pulse at the timing of strengthening the residual vibration. Even if the pulse interval is set so as to be performed, the movement of the meniscus is suppressed from being excessive, and the liquid discharge can be stabilized. As a result, the timing at which the liquid ejection operation is performed by the next ejection pulse with respect to the phase of the residual vibration, that is, the interval between the previous ejection pulse and the next ejection pulse can be set in a relatively wide range, and can be freely set The degree spreads. As a result, the total amount of liquid when the liquid is ejected a plurality of times using a plurality of ejection pulses within a unit cycle can be determined wider than in the past. Therefore, it is possible to widen the range of pixel gradation expression by multi-shot / pixel printing. Even when using the above high viscosity liquid, the amount of liquid discharged from the nozzle and the amount of liquid supplied to the pressure chamber are optimal depending on the size of the nozzle opening and the size of the supply opening. Therefore, the shortage of liquid supply to the pressure chamber is improved, and the liquid discharge can be stabilized even when the liquid is discharged at a higher frequency.

In the liquid ejection method, it is desirable that an opening area of the nozzle on the side from which the liquid is ejected is 1/20 or more of an area of the opening of the supply unit.
In the liquid ejection method, it is preferable that the length of the nozzle is in a range of 40 μm or more and 100 μm or less.
According to such a liquid ejection method, it is possible to further stabilize liquid ejection.

In the liquid ejection method, the opening of the supply unit has a rectangular shape, and the length of one side of the opening is in a range of 30 μm or more and 500 μm or less, and the other side of the opening has The length is preferably 20 μm or more and 300 μm or less.
According to such a liquid ejection method, a liquid having a viscosity of 8 millipascal seconds or more can be reliably supplied to the pressure chamber.

In the liquid ejection method, it is preferable that an outer edge of the opening of the supply unit is smaller than an outer edge of a surface that partitions the pressure chamber and communicates with the supply unit.
According to such a liquid discharge method, the pressure vibration applied to the liquid can be attenuated by the supply unit. Thereby, the liquid discharge frequency can be increased.

In the liquid ejection method, it is preferable that an inertance of the nozzle is smaller than an inertance of the supply unit.
According to such a liquid discharge method, the liquid can be efficiently discharged by pressure vibration applied to the liquid.

In the liquid ejection method, it is preferable that the pressure generating unit includes a deforming unit that partitions a part of the pressure chamber and applies a pressure change to the liquid by deformation.
In the liquid ejection method, it is desirable that the pressure generating unit deforms the deforming unit to a degree corresponding to a potential change pattern in the applied ejection pulse.
According to such a liquid discharge method, the pressure of the liquid in the pressure chamber can be accurately controlled.

The present invention also provides a pressure chamber in which a liquid from a liquid supply source is supplied through a supply unit, a nozzle that communicates with the pressure chamber and discharges the liquid, and the pressure for discharging the liquid from the nozzle. A liquid generating head having a pressure generating unit that performs an operation of giving a pressure change to the liquid in the room, and a pulse for generating a plurality of discharge pulses for operating the pressure generating unit to discharge the liquid from the nozzle in a unit cycle A liquid discharge apparatus including a generation unit,
The opening area of the nozzle on the side from which the liquid is discharged is 1/9 or less of the opening area on the pressure chamber side of the supply unit,
The discharge pulse includes a first waveform section that operates the pressure generating section to depressurize the pressure chamber by changing a potential from a reference potential to an operating potential with a constant gradient, and a second waveform that maintains the operating potential for a certain period of time. And a third waveform unit that operates the pressure generating unit to pressurize the pressure chamber by changing the potential with a constant gradient from the operating potential to the reference potential,
The phase of residual vibration after the liquid is discharged using the previous discharge pulse when the discharge pulse generated next to the discharge pulse generated earlier in the unit period is applied to the pressure generating unit. The interval between the previous ejection pulse and the next ejection pulse is set on the basis of the natural vibration period Tc generated in the liquid in the pressure chamber so as to be uniform.

1 is a block diagram illustrating a configuration of a printing system. (A) is sectional drawing of a head, (b) is a figure which illustrates the structure of a liquid flow path typically. It is a block diagram explaining structures, such as a drive signal generation circuit. (A) A waveform diagram for explaining an example of a drive signal, and (b) is a schematic diagram for explaining residual vibration after ink ejection due to the previous ejection pulse. It is a wave form diagram explaining an ejection pulse. (A) is a figure which shows a mode that high viscosity ink is discharged stably, (b) is a figure which shows a mode that high viscosity ink is discharged in the unstable state. FIG. 6 is a diagram for explaining ejection of ink droplets by a head in which the area of the nozzle opening is set to about 1/9 of the opening area on the pressure chamber side in the ink supply path. It is a figure explaining discharge of the ink droplet by the head of a comparative example. FIG. 6 is a diagram for explaining ejection of ink droplets by a head in which an opening area of an ink supply path is 0.34 times an area of a pressure chamber. FIG. 4 is a diagram illustrating ink droplet ejection by a head in which an opening area of an ink supply path is 0.32 times the area of a pressure chamber. It is a figure explaining discharge of the ink droplet by the head in the worst state. It is a figure explaining discharge of an ink drop when ink with a viscosity of 5 millipascal seconds is discharged. It is a figure explaining discharge of an ink drop when ink whose viscosity is 6 millipascal seconds is discharged. It is sectional drawing explaining another head. It is a figure explaining the discharge pulse for other heads. (A) is a diagram for explaining a substantially funnel-shaped nozzle, (b) is a diagram for explaining a model for analysis of a substantially funnel-shaped nozzle, and (c) is a diagram for explaining a modification of an ink supply path and a pressure chamber. It is.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the accompanying drawings. In the embodiments described below, various limitations are made as preferred specific examples of the present invention. However, the scope of the present invention is not limited to the following description unless otherwise specified. However, the present invention is not limited to these embodiments. In the following, an ink jet recording apparatus (hereinafter referred to as a printer) will be described as an example of the liquid ejection apparatus of the present invention.

  The printer 1 illustrated in FIG. 1 ejects ink, which is a kind of liquid, toward a recording medium such as recording paper, cloth, or film. The recording medium is a landing target that is a target to which liquid is discharged and landed. A computer CP as an external device is connected to the printer 1 so as to be communicable. In order to cause the printer 1 to print an image, the computer CP transmits print data corresponding to the image to the printer 1.

  The printer 1 includes a paper transport mechanism 2, a carriage movement mechanism 3, a drive signal generation circuit 4, a head unit 5, a detector group 6, and a printer controller 7. The paper transport mechanism 2 transports the paper in the transport direction. The carriage moving mechanism 3 moves the carriage to which the head unit 5 is attached in a predetermined movement direction (for example, the paper width direction). The drive signal generation circuit 4 generates a drive signal COM. This drive signal COM is applied to the actuator 10 (see FIG. 2A) of the recording head 8 at the time of printing on the recording paper (recording medium). As shown in FIG. A series of signals including a plurality of ejection pulses PS in T. Here, the ejection pulse PS is to cause the actuator 10 to perform a predetermined operation in order to eject ink droplets from the recording head 8. The drive signal COM in the present embodiment includes a total of two ejection pulses PS1 and PS2, but may include three or more ejection pulses. Since the drive signal COM includes a plurality of ejection pulses PS, the drive signal generation circuit 4 corresponds to a pulse generator. The configuration of the drive signal generation circuit 4 and the details of the ejection pulse PS will be described later.

  The head unit 5 includes a recording head 8 and a head control unit 11. The recording head 8 is a kind of liquid ejection head, and ejects ink toward a recording medium to land on the recording medium. The head controller 11 controls the recording head 8 based on the head control signal from the printer controller 7. The recording head 8 will be described later. The detector group 6 includes a plurality of detectors that monitor the status of the printer 1. Detection results by these detectors are output to the printer controller 7. The printer controller 7 performs overall control in the printer 1. The printer controller 7 will also be described later.

  FIG. 2A is a cross-sectional view of the main part in the longitudinal direction of the pressure chamber showing the configuration of the recording head 8 of the present embodiment. The recording head 8 is formed by laminating a silicon nozzle substrate 16 on one surface of a silicon flow path substrate 15 and a glass electrode substrate 17 on the other surface of the flow path substrate 15. And each member is joined by the adhesive agent, and it has a 3 layer structure.

  The nozzle substrate 16 is a plate material in which a plurality of nozzles 14 are opened in a row at a pitch (for example, 180 dpi) corresponding to the dot formation density. The channel substrate 15 is formed with a groove portion serving as an ink channel by performing anisotropic etching from the surface thereof, and an opening portion of the groove portion is blocked by the nozzle substrate 16 so that each nozzle is formed. 14 is a series of pressure chambers 19, a reservoir 20 into which ink common to each pressure chamber is introduced, and an ink supply path 21 that communicates between the reservoir 20 and each pressure chamber 19. An ink flow path is defined. The reservoir 20 is a part for temporarily storing ink supplied from an ink cartridge (a kind of liquid supply source, not shown), and corresponds to a liquid storage chamber common to the nozzles 14 constituting the nozzle row. The ink supply path 21 corresponds to a supply unit in the present invention.

  In the flow path substrate 15, an ink introduction port 22 penetrating in the thickness direction of the substrate is formed on the bottom surface of the groove portion serving as the reservoir 20. In addition, a thin portion 23 that functions as an elastic surface that can be elastically displaced in the head stacking direction (vertical direction in FIG. 2A) is formed on the bottom surface (one surface) of the groove portion that becomes each pressure chamber 19. . A common electrode terminal 18 is formed on the flow path substrate 15, and since the flow path substrate 15 has conductivity, the thin-walled portion 23 is a common electrode (a flexible electrode, which is a modification of the present invention). It also functions as a part.)

  The electrode substrate 17 is made of, for example, borosilicate glass. This borosilicate glass has a thermal expansion coefficient similar to that of silicon. For this reason, separation between the head constituent members due to temperature changes is difficult to occur. On the surface of the electrode substrate 17 that is bonded to the flow path substrate 15, a recess 24 that is shallowly etched in a tray shape is formed corresponding to each pressure chamber 19 at a position facing the thin portion 23 of the pressure chamber 19. Has been. Individual electrodes (fixed electrodes) 25 formed by laminating thin films such as indium tin oxide (ITO) are laid on the bottom surface of the recess 24. Each individual electrode 25 includes a segment electrode 25a extending corresponding to each pressure chamber 19, and an electrode terminal portion 25 (b) exposed to the outside. When the electrode substrate 17 is bonded to the flow path substrate 15, the thin portion 23 of each pressure chamber 19 and the segment electrode 25 a of each individual electrode 25 face each other with a narrow gap formed therebetween.

  The electrode substrate 17 is formed with an ink introduction path 17 ′ penetrating the substrate thickness direction. The ink introduction path 17 ′ communicates with the ink introduction port 22 in a joined state with the flow path substrate 15. It is supposed to be. For example, ink from an ink cartridge provided on the printer main body side is introduced into the reservoir 20 through the ink introduction path 17 ′ and the ink introduction port 22. The ink in the reservoir 20 is distributed and supplied to each pressure chamber 19 through an ink supply path 21 branched from the reservoir 20.

  A drive signal (ejection pulse PS) from the printer controller 7 side is applied between the common electrode terminal 18 of the flow path substrate 15 and the individual electrode 25 of the electrode substrate 17. When the drive voltage changes to the + side from the reference potential (or ground potential), an electrostatic force is generated between the thin electrode portion 23 functioning as a flexible electrode or a deformed portion and the individual electrode 25, and this electrostatic force As a result, the thin-walled portion 23 is elastically deformed and bent toward the individual electrode 25, and is attracted to the surface of the segment electrode 25a. As a result, the volume of the pressure chamber 19 increases and ink flows into the pressure chamber 19 from the reservoir 20 side through the ink supply path 21. When the drive voltage suddenly changes to the negative side (ground potential side) and the electrostatic force sharply decreases, the thin portion 23 moves away from the surface of the segment electrode 25a by the elastic force and is displaced to the pressure chamber 19 side. As a result, the volume of the pressure chamber 19 decreases rapidly. As a result, pressure fluctuations occur in the ink in the pressure chamber 19, and ink is ejected (jetted) from the nozzles 14 due to this pressure fluctuation. That is, the thin part 23, the common electrode terminal 18, the individual electrode 25, and the thin part 23 function as the actuator 10 (pressure generating part in the present invention). The actuator 10 can be said to be a means for deforming the thin portion 23 (deformation portion) to a degree corresponding to the potential change pattern in the applied ejection pulse PS. The degree of pressurization and the degree of decompression of the ink in the pressure chamber 19 can be roughly determined by the potential change amount per unit time in the individual electrode 25.

  As described above, the recording head 8 is provided with a plurality of series of ink flow paths (corresponding to liquid flow paths filled with liquid) from the reservoir 20 to the nozzles 14 according to the number of the nozzles 14. In this ink flow path, the narrow flow path nozzle 14 and the ink supply path 21 communicate with the thick flow path pressure chamber 19. Therefore, when analyzing characteristics such as ink flow, the Helmholtz resonator concept is applied. FIG. 2B is a diagram schematically illustrating the structure of the liquid flow path of the recording head 8 based on this concept. For this reason, the ink flow path is shown in a shape different from the actual one.

  In the general recording head 8, the length L19 of the pressure chamber 19 is determined in the range of 200 μm to 2000 μm. The width W19 of the pressure chamber 19 is determined in the range of 20 μm to 300 μm, and the height H19 of the pressure chamber 19 is determined in the range of 30 μm to 500 μm. The length L21 of the ink supply path 21 is determined within the range of 50 μm or more and 2000 μm or less. The width W21 of the ink supply path 21 is determined in the range of 20 μm to 300 μm, and the height H21 of the ink supply path 21 is determined in the range of 30 μm to 500 μm. Further, the diameter φ14 of the nozzle 14 is determined within a range of 10 μm to 40 μm, and the length L14 of the nozzle 14 is determined within a range of 40 μm to 100 μm.

  In the recording head 8 of the present embodiment, the opening area Snzl of the discharge side end of the nozzle 14 is determined based on the opening area Ssup of the ink supply path 21, and the opening area Snzl of the discharge side of the nozzle 14 is the ink. The supply passage 21 is configured to be 1/9 or less of the opening area Ssup on the pressure chamber 19 side.

  Regarding the ink supply path 21, the width W 21 and the height H 21 are determined to be equal to or less than the width W 19 and the height H 19 of the pressure chamber 19. Further, when one of the width W21 and the height H21 of the ink supply path 21 is aligned with one of the width W19 and the height H19 of the pressure chamber 19, the other of the width W21 and the height H21 of the ink supply path 21 is the pressure. The chamber 19 is set to a size smaller than the other of the width W19 and the height H19.

  Here, FIG. 2B is a diagram schematically illustrating the ink flow path. However, as shown in FIG. 2A, the ink supply path 21 is a narrow flow path etched shallower than the recess corresponding to the pressure chamber 19 and the reservoir 20, and is actually a rectangular parallelepiped shape having a rectangular opening. It is configured as a space. Accordingly, the size of the opening of the ink supply path 21 is set smaller than the outer edge of the surface that partitions the pressure chamber 19 and communicates with the ink supply path 21.

  In such an ink flow path, the ink is ejected from the nozzle 14 by applying a pressure change to the ink in the pressure chamber 19. At this time, the pressure chamber 19, the ink supply path 21, and the nozzle 14 function like a Helmholtz resonator. For this reason, when a pressure is applied to the ink in the pressure chamber 19, the magnitude of the pressure changes in a specific period Tc called a Helmholtz period. That is, pressure vibration occurs in the ink.

Here, the Helmholtz period (natural vibration period of ink) Tc can be generally expressed by the following equation (1).
Tc = 2π√ [(Mn + Ms) / (Mn × Ms × (Cc + Ci))] (1)
In equation (1), Mn is the inertance of the nozzle 14 (the mass of ink per unit cross-sectional area, which will be described later), Ms is the inertance of the ink supply path 21, and Cc is the compliance of the pressure chamber 19 (change in volume per unit pressure). , Ci represents the ink compliance (Ci = volume V / [density ρ × sound speed c ² ]).
The amplitude of this pressure vibration gradually decreases as ink flows through the ink flow path. For example, the pressure vibration is attenuated by a loss in the nozzle 14 and the ink supply path 21 and a loss in a wall portion that divides the pressure chamber 19.

  In the general recording head 8, the Helmholtz period in the pressure chamber 19 is determined within the range of 5 μs to 10 μs. For example, in the ink flow path of FIG. 2B, the width W19 of the pressure chamber 19 is 100 μm, the height H19 is 70 μm, the length L19 is 1000 μm, the width W21 of the ink supply path 21 is 50 μm, and the height H21 is 70 μm. When the length L21 is 500 μm, the diameter φ14 of the nozzle 14 is 30 μm, and the length L14 is 100 μm, the Helmholtz period is about 8 μs. Note that this Helmholtz cycle also varies depending on the thickness of the wall section that separates adjacent pressure chambers 19, the thickness and compliance of the thin-walled section 23, and the material of the flow path substrate 15 and the nozzle substrate 16. Therefore, even with the same type of recording head, the Helmholtz period may differ from head to head due to dimensional errors during manufacturing. Details of this point will be described later.

  The printer controller 7 performs overall control in the printer 1. For example, the control target unit is controlled based on print data received from the computer CP and detection results from each detector, and an image is printed on paper. As shown in FIG. 1, the printer controller 7 includes an interface unit 28, a CPU 29, and a memory 30. The interface unit 28 exchanges data with the computer CP. The CPU 29 performs overall control of the printer 1. The memory 30 secures an area for storing a computer program, a work area, and the like. The CPU 29 controls each control target unit according to the computer program stored in the memory 30. For example, the CPU 29 controls the paper transport mechanism 2 and the carriage movement mechanism 20. Further, the CPU 29 transmits a head control signal for controlling the operation of the recording head 8 to the head controller 11 and transmits a control signal for generating the drive signal COM to the drive signal generation circuit 4.

  Here, the control signal for generating the drive signal COM is also called DAC data, and is, for example, digital data of a plurality of bits. This DAC data defines a change pattern of the potential of the generated drive signal COM. Therefore, the DAC data can be said to be data indicating the potential of the drive signal COM and the ejection pulse PS. This DAC data is stored in a predetermined area of the memory 30, read when the drive signal COM is generated, and output to the drive signal generation circuit 4.

  The drive signal generation circuit 4 functions as a pulse generator, and generates a drive signal COM having an ejection pulse PS based on the DAC data. As illustrated in FIG. 3, the drive signal generation circuit 4 includes a DAC circuit 31, a voltage amplification circuit 32, and a current amplification circuit 33. The DAC circuit 31 converts digital DAC data into an analog signal. The voltage amplification circuit 32 amplifies the voltage of the analog signal converted by the DAC circuit 31 to a level at which the actuator can be driven. In this printer 1, the maximum analog signal output from the DAC circuit 31 is 3.3 V, whereas the amplified analog signal output from the voltage amplifier circuit 32 (also referred to as a waveform signal for convenience) is 42 V at maximum. It is. The current amplifying circuit 33 amplifies the current of the waveform signal from the voltage amplifying circuit 32 and outputs the amplified signal as a drive signal COM. The current amplifier circuit 33 is configured by, for example, a push-pull connected transistor pair.

  The head controller 11 selects a necessary portion of the drive signal COM generated by the drive signal generation circuit 4 based on the head control signal, and applies the selected portion to the actuator 10. Therefore, as shown in FIG. 3, the head controller 11 includes a plurality of switches 34 provided for each actuator 10 in the middle of the supply line of the drive signal COM. Then, the head control unit 11 generates a switch control signal from the head control signal. By controlling each switch 34 by this switch control signal, a necessary portion (for example, ejection pulse PS) of the drive signal COM is applied to the actuator 10. At this time, the ejection of ink from the nozzles 14 can be controlled depending on how the necessary portions are selected.

  Next, the drive signal COM generated by the drive signal generation circuit 4 will be described. As shown in FIG. 4A, the drive signal COM includes a plurality of ejection pulses PS that are repeatedly generated. In the present embodiment, a total of two ejection pulses PS1 and PS2 are included in a unit period T that is a repetition period of the drive signal COM and is a period for forming pixels that are units constituting an image on a recording medium. It is. These ejection pulses PS have the same waveform. That is, the potential change pattern is the same. As described above, the drive signal COM is applied to the individual electrode 25 of the actuator 10. As a result, a potential difference corresponding to the change pattern of the potential of the ejection pulse PS is generated between the thin portion 23 having a fixed potential. As a result, the actuator 10 changes the volume of the pressure chamber 19 by displacing the thin portion 23 by electrostatic force according to the potential change pattern.

FIG. 5 is a waveform diagram illustrating the configuration of the ejection pulse PS1. Since the ejection pulse PS2 has the same waveform as the ejection pulse PS1, its description is omitted. In FIG. 5, the vertical axis represents the potential of the drive signal, and the reference potential VB as the reference potential is set to 0V. The horizontal axis is time.
The ejection pulse PS1 is a substantially trapezoidal waveform composed of a first waveform portion P1, a second waveform portion P2, and a third waveform portion P3. The first waveform portion P1 is a portion that is generated from timing t0a to timing t1a. In the first waveform portion P1, the potential at the timing t0a (corresponding to the start potential) is the reference potential VB, and the potential at the timing t1a (corresponding to the termination potential) is the highest potential (a kind of operating potential) VH. Therefore, when the first waveform portion P1 is applied to the actuator 10, the pressure chamber 19 expands from the reference volume to the maximum volume over the generation period of the first waveform portion P1. The first waveform portion P1 expands the pressure chamber 19 as a preparatory operation for ejecting ink droplets.

  The difference between the reference potential VB in the ejection pulse PS1 and the reference potential VB in the ejection pulse PS1 (hereinafter also referred to as the drive voltage Vh) is several tens of volts. With respect to the setting of the drive voltage Vh, for example, using two evaluation pulses having different drive voltages, the actuator 10 is driven to acquire the amounts of ink ejected from the nozzles 14, respectively. Based on the amount, a drive voltage is set that provides a target ejection amount (ink amount when ink is ejected using a single ejection pulse). Specifically, assuming that the change amount of the ejection amount is proportional to the change amount of the voltage, a drive voltage corresponding to the target ink amount in the printer specifications is acquired, and this value is used as the drive voltage of the ejection pulses PS1 and PS2. Set as Vh.

  The second waveform portion P2 is a portion generated from timing t1a to timing t2a. The second waveform portion P2 is constant at the maximum potential VH. For this reason, if the 2nd waveform part P2 is applied to the actuator 10, the pressure chamber 19 will maintain the maximum volume over the production | generation period of the 2nd waveform part P2. The third waveform portion P3 is a portion that is generated from timing t2a to timing t3a. In the third waveform portion P3, the start potential is the highest potential VH and the termination potential is the reference potential VB. For this reason, when the third waveform portion P3 is applied to the actuator 10, the pressure chamber 19 contracts over the generation period of the third waveform portion P3 from the maximum volume to the reference volume (or more than that). Since ink is ejected as the pressure chamber 19 contracts, the third waveform portion P3 corresponds to a portion for ejecting ink droplets. Since the ejection pulse PS1 does not include a vibration damping portion for suppressing residual vibration after ejecting ink droplets, residual vibration occurs after ink ejection.

  The maximum ink ejection frequency is determined by the interval between the ejection pulses PS generated in the unit period T. For example, in the drive signal COM of FIG. 4, ink is ejected every period Δt by sequentially applying the ejection pulses PS1 and PS2 to the actuator 10 in order. Then, by applying only one ejection pulse PS1 to the actuator 10, the weight of ink ejected only once from the nozzle 14 is about 7 ng, for example. That is, when only the ejection pulse PS1 is selected in the unit period T and the actuator 10 is driven to eject ink droplets, dots having a size corresponding to the middle dots are formed on the recording medium. In addition, when both the ejection pulse PS1 and the ejection pulse PS2 are selected and applied to the actuator 10 in the unit period T, ink droplets are ejected twice from the nozzle 14 in large dots on the recording medium. Corresponding size dots are formed.

By the way, in order to improve the image quality of an image printed on a recording medium, it is desired to obtain a certain discharge characteristic regardless of individual differences of the recording heads 8.
FIG. 4B shows the vibration state of the meniscus (the free surface of the ink exposed at the nozzle 14) in the nozzle 14 when ink is ejected using the ejection pulse PS1 generated earlier in the unit period T. It is a graph which shows typically (a more detailed vibration state is explained using Drawings 7-13). In the graph, the horizontal axis represents time, and the vertical axis represents the position in the meniscus ejection direction (axial direction of the nozzle 14). The position of 0 on the vertical axis corresponds to the nozzle surface (specifically, the normal position of the meniscus corresponding to the reference potential VB). With this, the meniscus is further outward (discharge side) than the nozzle surface as the waveform goes upward. Means that the meniscus is drawn toward the pressure chamber as it moves downward. Also, the time indicated by Da in the graph is the ink droplet ejection timing. Further, the time point indicated by Db is the starting end of the discharge pulse PS2, and is the time point when the drive (discharge operation) of the actuator 10 is started by the discharge pulse PS2.

  As shown in the figure, in the present embodiment, it is understood that the residual vibration having the Helmholtz period (natural vibration period Tc) occurs after the ink droplet is ejected (after the time point Da). Since the natural vibration period Tc is different for each recording head, the period of the residual vibration is also different for each head. For example, the period of the residual vibration indicated by the broken line is longer than the period of the residual vibration indicated by the solid line. Accordingly, when the interval Δt between the ejection pulses is set to a constant value for all the recording heads, the ejection pulse PS2 generated after the ejection pulse PS1 generated earlier within the unit period is applied to the actuator 10. The phase of the residual vibration of Db (that is, the timing at which driving of the actuator 10 is started by the ejection pulse PS2) differs for each recording head.

  That is, in the case of the residual vibration shown by the solid line in FIG. 4B, the driving of the actuator 10 is started at the timing of moving from the discharge side to the pressure chamber side at the time point Db, and the meniscus is drawn into the pressure chamber side. Vibration is strengthened. As a result, the weight and flying speed of the ink ejected by the ejection pulse PS2 increase. On the other hand, in the case of the residual vibration indicated by the broken line, the driving of the actuator 10 is started at the timing Db when moving from the pressure chamber side to the discharge side, and thus the vibration is weakened. As a result, the weight and flying speed of the ink ejected by the ejection pulse PS2 are reduced. As described above, there is a possibility that the total amount of ink ejected by continuously applying the ejection pulses PS1 and PS2 to the actuator 10 is different for each head.

  For this reason, in the printer 1 according to the present invention, the interval Δt between the previous ejection pulse PS1 included in the drive signal COM within the unit period T and the ejection pulse PS2 generated next is the Helmholtz period for each recording head. It is set based on (natural vibration period Tc). As a result, the phase of the residual vibration after the ink is first ejected using the ejection pulse PS1 at the time point Db when the ejection pulse PS2 is applied to the actuator 10 is made constant regardless of the individual differences of the recording heads. It is configured. In the present embodiment, the interval Δt between pulses is the interval between the pulse start ends (that is, the start end of the first waveform portion P1), but is not limited to this, for example, the pulse end (third waveform portion P3). May be the interval between the first ends of the third waveform portion P3. In this way, by defining the interval Δt between pulses, a plurality of ejection pulses are continuously applied to the actuator 10 within the unit period T, thereby ejecting ink droplets a plurality of times to form dots of a predetermined size. In the configuration formed above (multi-shot / pixel printing), the total amount of ejected ink can be made uniform regardless of individual differences in the recording heads.

  The natural vibration period Tc for each recording head is measured in the manufacturing process of the recording head. Various methods for measuring Tc have been proposed. For example, the generation time Pwh of the second waveform portion of the ejection pulse PS (the time from the start t1a to the end t2a of the second waveform portion) is within a predetermined range. While changing, the flying speed Vm and the ink weight of the ink when the ink is ejected with the ejection pulse PS are measured, and the change is observed, and the interval between the adjacent maximum values or the minimum values of the change curve is expressed as Tc. An existing method such as a method of acquiring as can be adopted.

  Here, in a printer that targets relatively low-viscosity ink having a viscosity of less than 8 millipascal seconds (for example, about 1 millipascal second) such as a conventional water-based ink, for example, ink is generated by the previous ejection pulse PS1. In the case where the pulse interval Δt is set so that the ink ejection operation by the next ejection pulse PS2 is performed at the timing of strengthening the residual vibration after the ink is ejected, the amount of ink ejected by the ejection pulse PS2 is It increases more than the amount of ink ejected by the pulse PS1. However, in this case, the behavior of the meniscus tends to be too large with low viscosity (for example, 1 millipascal second) ink. As a result, there is a possibility that the ejection becomes unstable, for example, the flying direction of the ink ejected by the ejection pulse PS2 is bent.

  For this reason, in a printer that handles low-viscosity ink, in order to minimize the influence of residual vibration, the next pulse of the residual vibration of the meniscus after ink discharge by the previous discharge pulse PS1 is as close to the neutral position as possible. The pulse interval is set so that the ink ejection operation by PS2 is started. That is, the degree of freedom in setting the pulse interval is limited. Therefore, in this configuration, assuming that the weight of ink when the ink is ejected using only the ejection pulse PS1 is 7 ng, when the ink is ejected continuously using both the ejection pulse PS1 and the ejection pulse PS2. The total amount of ink is only 14 ng which is twice the weight when ejected alone, or a value close thereto, and the degree of freedom of gradation expression in pixels is limited.

  In contrast, in the printer 1 according to the present invention, an ink having a viscosity sufficiently higher than that of a general water-based ink, specifically, an ink having a viscosity of 8 millipascal seconds or more (also referred to as a high viscosity ink for convenience). ) And changing the interval Δt between pulses, the total amount of ink in multi-shot / pixel printing can be arbitrarily changed. For example, when the pulse interval Δt is set so that the ink ejection operation by the next ejection pulse PS2 is performed at the timing of strengthening the residual vibration after the ink is ejected by the previous ejection pulse PS1, the ejection pulse PS2 The amount of ink ejected is greater than the amount of ink ejected by the ejection pulse PS1. High-viscosity ink exhibits a damper effect that suppresses vibrations, so that it is possible to prevent excessive movement of the meniscus. For this reason, even if the pulse interval Δt is set so that the ink is ejected by the ejection pulse PS2 at the timing of strengthening the residual vibration, the ejection of the ink is suppressed from becoming unstable. As a result, the degree of freedom in setting the pulse interval Δt increases. That is, it is possible to set the pulse interval Δt so that the ink ejection operation by the ejection pulse PS2 is started at a phase where the amplitude of the residual vibration is large (maximum or near minimum). Then, assuming that the ink weight when ejecting ink by selecting only the ejection pulse PS1 within the unit period is 7 ng, the total weight when ejecting ink by selecting both the ejection pulse PS1 and the ejection pulse PS2 is calculated. , Not only 14 ng, which is twice the weight when discharged alone, but also more or less than the double value. As a result, the timing at which the ink ejection operation is performed by the next ejection pulse with respect to the phase of the residual vibration, that is, the interval Δt between the previous ejection pulse and the next ejection pulse can be set in a relatively wide range. The degree spreads. As a result, the total amount of ink when ink is ejected a plurality of times using a plurality of ejection pulses within a unit cycle can be determined wider than in the past. Therefore, it is possible to widen the range of pixel gradation expression by multi-shot / pixel printing.

  By the way, when the above high-viscosity ink is continuously ejected by a conventional head, there is a problem that the ejection of the ink becomes unstable particularly when ink droplets are ejected at a higher frequency. . However, as described below, even when high-viscosity ink is used, the amount of ink ejected from the nozzle 14 and the amount of ink supplied to the pressure chamber 19 are determined based on the size of the opening of the nozzle 14 and the ink supply. Since it can be optimized depending on the size of the opening of the passage 21, the shortage of ink supply to the pressure chamber 19 is improved, and even when ink is ejected at a higher frequency, ink ejection can be stabilized.

  FIG. 6A shows a state in which high viscosity ink is ejected in a relatively stable state. On the other hand, FIG. 6B shows a state where high viscosity ink is ejected in an unstable state. Comparing these figures, it can be seen that, in an unstable state, there are ink droplets with insufficient flight speed and ink droplets with ejection bends.

  There are various factors that make ink ejection unstable. One of the factors is considered to be insufficient supply of ink to the pressure chamber 19. High-viscosity ink has characteristics that it is less likely to pass through the ink supply path 21 than ordinary ink. For this reason, if the ink supply to the pressure chamber 19 cannot catch up and the ink discharge operation is performed in a state where the ink is insufficient, it is considered that the ink discharge becomes unstable.

  In view of such circumstances, in the recording head 8 of the present embodiment, the opening area of the nozzle 14 is determined based on the opening area of the ink supply path 21. That is, as shown in FIG. 2B, the opening area Snzl on the ejection side of the nozzle 14 is configured to be 1/9 or less of the opening area Ssup on the pressure chamber 19 side of the ink supply path 21. Thus, the amount of ink supplied to the pressure chamber 19 is ensured while restricting the amount of ink droplets discharged from the nozzle 14. As a result, insufficient supply of ink to the pressure chamber 19 can be solved, and ink ejection is stabilized. This will be described in detail below.

  FIG. 7 is a diagram for explaining ink droplet ejection by the recording head 8 in which the opening area Snzl of the nozzle 14 is set to 1/9 or less of the opening area Ssup of the ink supply path 21. As shown in FIG. 2B, the opening area Snzl is an area of the opening located on the nozzle 14 on the side where the ink droplets are ejected. The opening area Ssup is the area of the opening that communicates with the pressure chamber 19 among the two openings of the ink supply path 21.

  In FIG. 7, the vertical axis indicates the meniscus state by the amount of ink, and the horizontal axis indicates time, as in FIG. 4B. Regarding the vertical axis, 0 ng indicates the position of the meniscus in the steady state. The larger the value on the positive side, the more the meniscus is pushed out in the ejection direction, and the larger the value on the negative side, the more the meniscus is drawn into the pressure chamber 19 side. Note that FIG. 7 is obtained by simulation. Further, other diagrams for explaining the ejection of ink droplets to be described below are also obtained by simulation. The simulation explains the change of the meniscus at the time of ink ejection by the schematic liquid flow path structure shown in FIG.

In this simulation, the width W19 of the pressure chamber 19 is 100 μm, the height H19 is 70 μm, and the length L19 is 1000 μm. The diameter 14 of the nozzle 14 is 25 μm, and the length of the nozzle 14 is 100 μm. The ink supply path 21 has a width W21 of 100 μm, a height H21 of 55 μm, and a length L21 of 500 μm. For this reason, the opening area Snzl of the nozzle 14 is about 500 μm ² (more accurately, 491 μm ² ), and the opening area Ssup of the ink supply path 21 is 5500 μm ² . Therefore, the opening area of the nozzle 14 is 1/9 or less of the opening area of the ink supply path 21.

  When the ink pulse is ejected from the nozzle 14 by applying the ejection pulse PS1 of FIG. 6 to the actuator 10 in such an ink flow path, the meniscus moves as shown in FIG. First, when the first waveform portion P1 is applied to the actuator 10, the pressure chamber 19 expands from the reference volume to the maximum volume. With this expansion, the ink in the pressure chamber 19 becomes negative pressure, and the ink flows into the pressure chamber 19 through the ink supply path 21. Further, as the ink becomes negative pressure, the meniscus is drawn into the pressure chamber 19 side in the nozzle 14.

  The movement of the meniscus toward the pressure chamber 19 is continued even after the application of the first waveform portion P1 is completed. That is, the meniscus moves to the pressure chamber 19 side even during the application period of the second corrugated portion P2 due to the compliance of the wall portion and the thin portion 23 that partition the pressure chamber 19. Thereafter, the moving direction of the meniscus is reversed in a direction away from the pressure chamber 19 (timing indicated by reference numeral A1 in FIG. 7). At this time, since the contraction of the pressure chamber 19 accompanying the application of the third waveform portion P3 is also added, the moving speed of the meniscus is fast. The meniscus moved along with the application of the third waveform portion P3 becomes a columnar shape. Thereafter, a part of the columnar meniscus on the tip side is cut and discharged in a droplet shape (timing indicated by reference numeral B1 in FIG. 7).

  After the ink droplets are ejected, the meniscus gradually changes to a steady state (ink amount 0 ng) while switching the movement direction between the ejection side and the pressure chamber 19 side (for example, timings indicated by reference numerals C1 and D1 in FIG. 7). Approach the position. The vibration period at this time is different for each recording head as described above. The reason why the meniscus approaches the steady state position is thought to be because the ink in the pressure chamber 19 has increased. For this reason, it can be said that ink is supplied from the ink supply path 21 into the pressure chamber 19 while the meniscus approaches the steady state position. The fact that the meniscus has returned to the steady state position means that a sufficient amount of ink has been supplied into the pressure chamber 19. Therefore, if the ejection pulse PS1 is applied to the actuator 10 after this time, ink ejection failure due to insufficient ink supply can be prevented. In the example of FIG. 7, the meniscus has returned almost to the steady state position after 100 μs has elapsed from the start of application of the first waveform portion P1 to the actuator.

  In the present embodiment, the fact that the meniscus has returned to the steady state position after 100 μs has elapsed since the start of application of the first waveform portion P1 can be performed stably even at a high frequency of 40 kHz or higher. It is based on the judgment criteria. Considering only the time of 100 μs, it seems that the discharge frequency is about 10 kHz at the maximum. However, when the ejection frequency is increased, ink droplets are ejected one after another, and therefore the ink flow path (a series of flow paths from the reservoir 20 to the nozzle 14) has an ink flow from the reservoir 20 side toward the nozzle 14 side. It is thought that a flow will occur. This ink flow becomes faster as the ejection frequency is increased. In addition, since the ink is supplied into the pressure chamber 19 by this flow, the above-described determination criteria are set.

  One of the reasons why the meniscus quickly returns to the steady state position is considered to be the ratio between the opening area Snzl of the nozzle 14 and the opening area Ssup of the ink supply path 21. That is, in the recording head 8, the opening area Snzl of the nozzle 14 is set to about 1/9 of the opening area Ssup of the ink supply path 21. Thereby, the ease of ink flow when the pressure of the ink in the pressure chamber 19 is changed can be made different between the inside of the nozzle 14 and the inside of the ink supply path 21. That is, it can be considered that the ink supply path 21 is more likely to flow ink than the nozzle 14. Further, since the opening area Snzl of the nozzle 14 is made sufficiently smaller than the opening area Ssup of the ink supply path 21, it can be said that the ink droplet ejection ability is suppressed.

  As a result, when the ink in the pressure chamber 19 is depressurized, it becomes easier to supply ink from the ink supply path 21 to the pressure chamber 19, and insufficient supply of ink is improved. This can also be understood from the fact that the meniscus has moved greatly between timing C1 and timing D1 in FIG. That is, it is considered that the ink flows from the ink supply path 21 to the pressure chamber 19 side due to the reaction of the ink greatly depressurized at the timing C1, and the meniscus approaches the steady state position at the timing D1.

  FIG. 8 is a diagram illustrating ink droplet ejection by the recording head 8 of the comparative example. In the recording head 8 of the comparative example, the recording head used in FIG. 7 is that the opening area Snzl of the nozzle 14 is set to about 1 / 6.7 (ratio 0.15) of the opening area Ssup of the ink supply path 21. 8 and different. Comparing FIG. 8 and FIG. 7, it can be seen that the recording head 8 of the comparative example ejects more ink. That is, the ink amount at the timing B2 is 12 ng, whereas the ink amount at the timing B1 is 7 ng. It can also be seen that the recording head 8 of the comparative example is larger in terms of the meniscus pull-in amount. That is, the ink amount at the timing C2 is −15 ng, whereas the ink amount at the timing C1 is −10.5 ng. These are considered to be because the ink of the comparative recording head 8 is more likely to flow inside the nozzles 14 than the recording head 8 used in FIG.

  However, the recording head 8 of the comparative example has a smaller meniscus return amount than the recording head 8 used in FIG. Specifically, the ink amount at timing D2 is −6 ng, whereas the ink amount at timing D1 is −2 ng. As described above, the return amount of the meniscus is related to the amount of ink supplied into the pressure chamber 19. That is, the more ink is supplied to the pressure chamber 19, the closer the meniscus is to the steady state position. Therefore, in the recording head 8 used in FIG. 7, it can be said that a sufficient amount of ink is promptly supplied to the pressure chamber 19 through the ink supply path 21 after the ink droplets are ejected. On the other hand, in the recording head 8 of the comparative example, it can be said that the amount of ink supplied to the pressure chamber 19 is smaller than that of the recording head 8 used in FIG. Accordingly, the time until the meniscus returns to the steady state position becomes longer. This indicates that the recording head 8 of the comparative example is more likely to be short of ink supply than the recording head 8 used in FIG.

  Next, the relationship between the area Scav of the pressure chamber 19 and the opening area Ssup of the ink supply path 21 will be described. As shown in FIG. 2B, the area Scav of the pressure chamber 19 is a cross-sectional area of a surface intersecting with the ink flow direction, that is, the thickness of the pressure chamber 19. In the following description, when the area Scav of the pressure chamber 19 is simply described, it means a cross-sectional area of a surface intersecting with the ink flow direction.

  FIG. 9 is a diagram illustrating ink droplet ejection by the recording head 8 in which the opening area Ssup of the ink supply path 21 is 0.34 times the area Scav of the pressure chamber 19. FIG. 10 is a diagram for explaining ink droplet ejection by the recording head 8 in which the opening area of the ink supply path 21 is 0.32 times the area of the pressure chamber 19. The recording head 8 used in FIG. 9 satisfies the condition of Scav <3 × Ssup and can be said to be a boundary head under this condition. On the other hand, the recording head 8 used in FIG. 10 does not satisfy the condition of Scav <3 × Ssup and can be said to be a head at the boundary of this condition. In these drawings, the viscosity of the ink to be ejected is 20 millipascal seconds.

  Comparing FIG. 9 and FIG. 10, with respect to the movement of the meniscus until ink droplets are ejected and the ink in the pressure chamber 19 is depressurized, the recording head 8 used in FIG. 9 and the recording head used in FIG. It can be said that there is no big difference with 8. For example, the ink amount at timing B3 is slightly less than 11 ng, whereas the ink amount at timing B4 is slightly more than 11 ng. The ink amount at timing C3 is slightly more than −15 ng, whereas the ink amount at timing C4 is slightly less than −15 ng.

  However, in these recording heads 8, there is a difference in how the meniscus returns after ink droplet ejection. For example, the ink amount at timing D3 is −3 ng, whereas the ink amount at timing D4 is −4 ng. Further, the ink amount at the timing E3 is -1 ng, whereas the ink amount at the timing E4 is -3 ng. The time required for the meniscus to approach the steady state position is shorter in the recording head 8 used in FIG. 9 than in the recording head 8 used in FIG. From this characteristic, it can be understood that the recording head 8 used in FIG. 9 has a larger amount of ink supplied after ink droplet ejection than the recording head 8 used in FIG.

  Accordingly, it can be said that by using the recording head 8 that satisfies the condition of Scav <3 × Ssup, insufficient supply of ink to the pressure chamber 19 is less likely to occur, and it is possible to further improve the ejection stability for high viscosity ink.

  From the above results, the opening area Snzl of the nozzle 14 (the area of the opening on the ink droplet ejection side) is 1/9 or less of the opening area Ssup of the ink supply path 21 (the area of the opening on the pressure chamber 19 side). As a result, it was found that the balance between the amount of ink ejected from the nozzle 14 and the amount of ink supplied to the pressure chamber 19 can be optimized, and insufficient supply of ink to the pressure chamber 19 can be improved. As a result, it was found that ink supply shortage can be suppressed even with high-viscosity ink, and ink droplet ejection at a higher frequency can be stabilized.

  Incidentally, as described above, the opening area Snzl and the length L14 of the nozzle 14 and the opening area Ssup and the length L21 of the ink supply path 21 can take various values. By changing these values, it is considered that the balance between the ease of ink flow on the nozzle 14 side and the ease of ink flow on the ink supply path 21 side can be changed.

  Here, in consideration of the effect of suppressing the shortage of ink supply to the pressure chamber 19 and stabilizing the ejection, the ink is most likely to flow on the nozzle 14 side and the ink is less likely to flow on the ink supply path 21 side ( It is understood that the above-described effect can be obtained regardless of other factors such as the length L14 of the nozzle 14 and the length L21 of the ink supply path 21 if ink supply is not insufficient even in the worst state.

  Based on this viewpoint, a simulation was performed using the recording head 8 in which the opening area Snzl of the nozzle 14 is set to 1/9 of the opening area Ssup of the ink supply path 21 in the worst state. FIG. 11 is a diagram for explaining the simulation result, that is, the ejection of ink droplets by the worst recording head 8.

In the recording head 8 used in FIG. 11, the nozzle 14 has a diameter φ14 of 50 μm (opening area Snzl: about 1963 μm ² ), the nozzle 14 has a length L14 of 40 μm, the ink supply path 21 has a width W21 of 200 μm, and a height H21. 100 μm (opening area Ssup: 20000 μm ² ), and the length L21 of the ink supply path 21 is 2000 μm. Regarding the pressure chamber 19, the width W19 is 300 μm, the height H19 is 100 μm, and the length L19 is 800 μm. That is, in this recording head 8, the diameter φ14 of the nozzle 14 is the largest, the length L14 of the nozzle 14 is the shortest, the length L21 of the ink supply path 21 is the longest, and the opening area Snzl of the nozzle 14 is the ink supply. The opening area Ssup of the passage 21 is almost 1/10. The viscosity of the ink to be ejected is 20 millipascal seconds.

  In this recording head 8, the amount of ink discharged is larger than in each of the recording heads 8 described above. That is, the ink amount at timing B5 is 30 ng. This is because the diameter φ14 of the nozzle 14 is set to the maximum value that can be taken by the general recording head 8, and the length L14 of the nozzle 14 is set to the minimum value that can be taken by the general recording head 8. It is thought that.

  At timing D5 and timing E5 after ink droplet ejection, the amount of ink is about -11 ng. Thereafter, the meniscus approaches a steady state position, and almost at the timing after 75 μs has elapsed from the start of application of the first waveform portion P1. It has returned to the steady state position. From this, it can be seen that the ink is rapidly supplied to the pressure chamber 19 after the ejection of the ink droplets. Therefore, by setting the opening area Snzl of the nozzle 14 to at least 1/10 or less, preferably 1/9 or less of the opening area Ssup of the ink supply path 21, even if high viscosity ink is ejected, the ink pressure chamber 19 is discharged. It can be said that supply shortage can be suppressed and ejection of ink droplets can be stabilized.

  The above embodiment is an experimental result (simulation result) for a high-viscosity ink having a viscosity of 20 millipascal seconds, but the viscosity of the high-viscosity ink varies. Therefore, the influence of the difference in ink viscosity is examined. FIG. 12 is a diagram illustrating ink droplet ejection when ink having a viscosity of 5 millipascal seconds is ejected. FIG. 13 is a diagram illustrating ink droplet ejection when ink having a viscosity of 6 millipascal seconds is ejected. The recording head 8 used in these drawings is the same as the recording head 8 used in FIG.

  Referring to FIG. 12, the ink amount is convex on the positive side in the period X <b> 1 after ink droplet ejection. This means that the supply of ink to the pressure chamber 19 becomes excessive, and the meniscus is located on the ejection side with respect to the opening edge of the nozzle 14. Such movement of the meniscus toward the convex side is undesirable because it contributes to unstable ink ejection. On the other hand, referring to FIG. 13, the ink amount is on the positive side in the period X2 after the ejection of the ink droplets, but is almost close to the position in the steady state. This means that the meniscus is oscillating slightly near the steady state position. That is, it can be said that the meniscus is stable at the steady state position.

  Accordingly, if the ink viscosity is in the range of 8 millipascal seconds or more and 20 millimeters or less, the opening area Snzl of the nozzle 14 is set to 1/9 or less of the opening area Ssup of the ink supply path 21. It can be said that the ejection of ink droplets can be stabilized.

  As described above, from the viewpoint of stabilizing the ejection of ink droplets, the opening area Snzl of the nozzle 14 may be set to at least 1/10 or less, preferably 1/9 or less of the opening area Ssup of the ink supply path 21. I can say that. Here, as the opening area Snzl of the nozzle 14 is made smaller than the opening area Ssup of the ink supply path 21, the ink hardly flows inside the nozzle 14. For this reason, a large amount of ink pressurized in the pressure chamber 19 flows toward the ink supply path 21. Furthermore, if the opening area Snzl of the nozzle 14 is excessively reduced, ink droplets are no longer ejected from the nozzle 14 even when the pressure chamber 19 is pressurized with ink.

  In order to prevent such ink droplet ejection failure, the opening area Snzl of the nozzle 14 may be set to 1/20 or more of the opening area Ssup of the ink supply path 21. By defining in this way, when the ink is pressurized in the pressure chamber 19, an ink flow can be generated also on the nozzle 14 side, and ink droplets can be reliably discharged.

  Even if the opening area Snzl of the nozzle 14 is 1/20 or more of the opening area Ssup of the ink supply path 21, the diameter φ14 of the nozzle 14 cannot be made smaller than the minimum value. That is, the diameter φ14 of the nozzle 14 cannot be smaller than 10 μm. This is because a structurally necessary amount of ink cannot be ejected.

  From the above description, it can be said that the opening area Ssup of the ink supply path 21 may be set to a range of 9 times or more and 20 times or less of the opening area Snzl of the nozzle 14. In addition, in relation to the area Scav (thickness) of the pressure chamber 19, the opening area Ssup of the ink supply path 21 is equal to the area Scav of the pressure chamber 19 (the surface defining the pressure chamber 19 and the ink supply path 21. It is more preferable to set it to be larger than 1/3 of the area of the communicating surface. Here, in addition to the function of supplying ink from the reservoir 20 to the pressure chamber 19, the ink supply path 21 also has a function of attenuating ink pressure vibration after ink droplet ejection. When attention is paid to this function, the opening area Ssup of the ink supply path 21 is required to be smaller than the area Scav of the pressure chamber 19. This is because the flow path resistance is increased by reducing the opening area.

Here, the channel resistance is an internal loss of the medium. In the present embodiment, the force received by the ink flowing through the ink flow path is a force opposite to the direction in which the ink flows. This flow path resistance can be expressed by the following equations (2) and (3). That is, the flow path resistance R in a substantially rectangular parallelepiped flow path such as the pressure chamber 19 and the ink supply path 21 can be expressed by the following equation (2). Further, the flow path resistance R circle in the circular cross-section flow path like the nozzle 14 can be approximated by the following expression (3).
Channel resistance R straight = (12 × viscosity μ × length L) / (width W × height H3) (2)
Channel resistance R circle = (8 × viscosity μ × length L) / (π × radius r4) (3)
In these equations (2) and (3), the viscosity μ is the viscosity of the ink, L is the length of the flow path, W is the width of the flow path, H is the height of the flow path, and r is a flow having a circular cross section. Each road radius is shown.

  Then, by making the flow path resistance of the ink supply path 21 larger than the flow path resistance of the pressure chamber 19, the ink pressure vibration in the pressure chamber 19 can be effectively attenuated by the ink supply path 21. As a result, the meniscus can be stabilized early after ejection of the ink droplets. That is, it is suitable for discharging ink droplets at a high frequency.

Both the nozzle 14 and the ink supply path 21 can be considered as a tube through which ink (medium) flows. For this reason, when pressure is applied from the outside of the tube, it can be said that the ink in the tube is more easily moved as the diameter of the tube is larger, and the ink in the tube is more difficult to move as the mass of the ink in the tube is larger. Since it has such characteristics, the ease of movement of ink in the tube is expressed by inertance in the acoustic circuit. Here, assuming that the density of ink is ρ, the cross-sectional area of the surface perpendicular to the ink flow direction of the flow path is S, and the length of the flow path is L, the inertance M can be approximated by the following equation (4). Can do. As shown in FIG. 2B, the length L and the cross-sectional area S of the flow path here indicate the length and cross-sectional area of each part in the modeled ink flow path. The length L is the length in the ink flow direction. The cross-sectional area S is the area of a surface that is substantially orthogonal to the ink flow direction.
Inertance M = (density ρ × length L) / cross-sectional area S (4)
From this equation (4), inertance can be considered as the mass of ink per unit cross-sectional area. It can be seen that the greater the inertance, the more difficult the ink moves according to the ink pressure in the pressure chamber 19, and the smaller the inertance, the easier the ink moves according to the pressure in the pressure chamber 19. When ejecting high viscosity ink, it is preferable to make the inertance of the nozzle 14 smaller than the inertance of the ink supply path 21. This is because the meniscus can be efficiently moved based on the pressure vibration applied to the ink in the pressure chamber 19.

  The above-described embodiments are mainly described with respect to a printing system having a printer as a liquid ejection apparatus, but this includes disclosure of a liquid ejection method, a liquid ejection system, an ejection pulse setting method, and the like. Yes. Further, this embodiment is intended to facilitate understanding of the present invention and is not intended to limit the present invention. The present invention can be changed and improved without departing from the gist thereof, and it is needless to say that the present invention includes equivalents thereof. In particular, the embodiments described below are also included in the present invention.

  In the recording head 8 of the above-described embodiment, the actuator 10 is of an electrostatic actuator type that operates to increase the volume of the pressure chamber 19 by using electrostatic force as the potential applied by the ejection pulse PS1 is higher. Although the actuator is used, another type of actuator 10 may be used. The other recording head 8 ′ shown in FIG. 14 employs a so-called flexural vibration type piezoelectric element (piezo element) 37 as the actuator 10. The piezoelectric element 37 is of a type that operates to reduce the volume of the pressure chamber 40 as the potential applied by the ejection pulse PS ′ (see FIG. 15) is higher.

  In brief, the other recording head 8 ′ has a reservoir 38, an ink supply port 39, a pressure chamber 40, and a nozzle 41. A plurality of ink flow paths corresponding to the nozzles 41 are provided from the reservoir 38 to the nozzles 41 through the pressure chambers 40. In the other recording head 8 ′, the volume of the pressure chamber 40 is changed by the operation of the piezoelectric element 37. That is, a part of the pressure chamber 40 is partitioned by the vibration plate 42, and the piezoelectric element 37 is provided on the surface of the vibration plate 42 opposite to the pressure chamber 40.

  A plurality of piezoelectric elements 37 are provided corresponding to the respective pressure chambers 40. Each piezoelectric element 37 has a configuration in which, for example, a piezoelectric body is sandwiched between an upper electrode and a lower electrode (none is shown), and is deformed by applying a potential difference between these electrodes. In this example, when the potential of the upper electrode is raised, the piezoelectric body is charged, and accordingly, the piezoelectric element 37 is bent so as to protrude toward the pressure chamber 40 side. As a result, the pressure chamber 40 is contracted. In the other recording head 8 ′, the portion of the diaphragm 42 that partitions the pressure chamber 40 corresponds to a deformed portion.

  The ejection pulse PS ′ for the other recording head 8 ′ has a waveform shown in FIG. 15, for example. In brief, the ejection pulse PS ′ has a waveform obtained by inverting the ejection pulse PS (PS1, PS2) described above in the potential direction (the height direction). Accordingly, the ejection pulse PS ′ has a first waveform portion P11, a second waveform portion P12, and a third waveform portion P13.

  The first waveform portion P11 has a start potential of the reference potential VB and a termination potential of the reference potential VB, and is generated from the timing t0a to the timing t1b. The second waveform portion P12 is constant at the reference potential VB and is generated from the timing t1b to the timing t2b. The third waveform portion P13 has a start potential of the reference potential VB and a termination potential of the highest potential VH, and is generated from the timing t2b to the timing t3b. The functions of the portions P11 to P13 included in the ejection pulse PS2 for the other recording head 8 'are the same as the functions of the portions P1 to P3 included in the ejection pulse PS described above.

  In the other recording head 8 ′ having such a configuration, if the ink viscosity is 8 millipascal seconds or more, the opening area on the ejection side of the nozzle 41 is set to 1 / of the opening area on the pressure chamber 40 side of the ink supply port 39. By setting it to 9 or less, the ejection of ink droplets can be stabilized.

The ejection pulses PS (PS1, PS2) and PS ′ described above are merely examples. The waveform of the ejection pulse (potential change pattern) is appropriately determined according to the ink ejection amount and the ink viscosity.
Further, the number of ejection pulses included in the drive signal within the unit period T is not limited to the example. That is, it is possible to employ a configuration in which three or more ejection pulses are included in the drive signal within the unit period T. In this case, what is necessary is just to set the space | interval of the ejection pulses adjacent on a time axis by said method.

  In the printer 1, the actuator 10 and the piezoelectric element 37 are used as elements that perform an operation (discharge operation) for discharging ink. Here, the element that performs the ejection operation is not limited to the actuator 10 and the piezoelectric element 37 described above. For example, a so-called longitudinal vibration type piezoelectric element, a heat generating element, or a magnetostrictive element may be used. When the actuator 10 and the piezoelectric element 37 are used as the elements as in the above-described embodiment, the volumes of the pressure chambers 19 and 40 can be accurately controlled based on the potential of the ejection pulse PS.

  In the above-described embodiment, the nozzle 14 has a circular opening shape and is configured by a hole penetrating the thickness direction of the nozzle substrate 16. In other words, it is constituted by a through hole that partitions a cylindrical space. Further, the ink supply path 21 has a rectangular opening shape and is configured by a hole communicating the pressure chamber 19 and the reservoir 20. In other words, it is constituted by communication holes that define a prismatic space.

  Here, the nozzle 14 and the ink supply path 21 can take various shapes. For example, the nozzle 14 may be constituted by a substantially funnel-shaped through hole as shown in FIG. The illustrated nozzle 14 has a tapered portion 14a and a straight portion 14b. The tapered portion 14 a is a portion that defines a frustoconical space, and the opening area decreases as the distance from the pressure chamber 19 increases. That is, it is provided in a tapered shape. The straight portion 14b is continuously provided at the end portion on the small diameter side of the tapered portion 14a. The straight portion 14b is a portion that defines a cylindrical space, and is a portion that has a substantially constant cross-sectional area on a surface that is orthogonal to the nozzle direction.

  In this nozzle 14, for example, as shown in FIG. 16B, the taper portion 14 a can be analyzed by defining a plurality of disk-shaped spaces whose diameters are gradually reduced. it can. Further, as shown in FIG. 16A, the analysis can also be performed by defining a nozzle 14 equivalent to the funnel-shaped nozzle 14 and having a constant cross-sectional area in a plane orthogonal to the nozzle direction.

  Further, for example, as shown in FIG. 16C, the ink supply path 21 is a flow path having an opening that is elongated in the vertical direction (a shape in which two semicircles having the same radius are connected by a common circumscribing line). It may be configured. In this case, the opening area Ssup of the ink supply path 21 corresponds to the area of an oval portion indicated by hatching. The ink supply path 21 having this oval opening may be analyzed by defining a flow path having a rectangular opening equivalent to this. In this case, the height H21 of the ink supply path 21 is slightly lower than the maximum height of the actual ink supply path 21. The same applies even if the opening of the ink supply path 21 is elliptical.

  The same applies to the pressure chamber 19. For example, as shown in FIG. 16C, when the surface orthogonal to the longitudinal direction of the pressure chamber 19 has a horizontally long hexagonal shape, a flow path having a rectangular cross section equivalent to this is defined. You may analyze. In other words, a flow path having a rectangular cross section having a height H19 and a width W19 slightly smaller than the maximum width of the pressure chamber 19 may be defined and analyzed.

  The present invention is not limited to a printer as long as it is a liquid ejection device that can perform ejection control using a plurality of drive signals, and other than various ink jet recording devices such as plotters, facsimile devices, copiers, and recording devices. The present invention can also be applied to a liquid ejecting apparatus such as a display manufacturing apparatus, an electrode manufacturing apparatus, and a chip manufacturing apparatus. In the display manufacturing apparatus, a solution of each color material of R (Red), G (Green), and B (Blue) is discharged from the color material discharge head. Moreover, in an electrode manufacturing apparatus, a liquid electrode material is discharged from an electrode material discharge head. In the chip manufacturing apparatus, a bioorganic solution is discharged from a bioorganic discharge head.

DESCRIPTION OF SYMBOLS 1 ... Printer, 4 ... Drive signal generation circuit, 10 ... Actuator, 14 ... Nozzle, 19 ... Pressure chamber, 20 ... Reservoir, 21 ... Ink supply path, 23 ... Thin part, 25 ... Individual electrode, PS1 ... Discharge pulse, PS2 ... discharge pulse, P1 ... first waveform portion, P2 ... second waveform portion, P3 ... third waveform portion

Claims (9)

A pressure chamber in which liquid from a liquid supply source is supplied through the supply unit; a nozzle that communicates with the pressure chamber and that discharges the liquid; and a pressure change in the liquid in the pressure chamber in order to discharge the liquid from the nozzle A liquid discharge head having a pressure generation unit that performs an operation of applying a pressure, and a liquid including a pulse generation unit that generates a plurality of discharge pulses within a unit period for operating the pressure generation unit to discharge liquid from the nozzle A liquid discharge method in a discharge apparatus,
The liquid has a viscosity of 8 millipascal seconds or more,
The opening area of the nozzle on the side from which the liquid is discharged is 1/9 or less of the opening area on the pressure chamber side of the supply unit,
The discharge pulse includes a first waveform section that operates the pressure generating section to depressurize the pressure chamber by changing a potential from a reference potential to an operating potential with a constant gradient, and a second waveform that maintains the operating potential for a certain period of time. And a third waveform unit that operates the pressure generating unit to pressurize the pressure chamber by changing the potential with a constant gradient from the operating potential to the reference potential,
The phase of residual vibration after the liquid is discharged using the previous discharge pulse when the discharge pulse generated next to the discharge pulse generated earlier in the unit period is applied to the pressure generating unit. The liquid discharge method is characterized in that the interval between the previous discharge pulse and the next discharge pulse is set based on the natural vibration period Tc generated in the liquid in the pressure chamber so that the values are uniform.   The liquid ejection method according to claim 1, wherein an opening area of the nozzle on a side from which the liquid is ejected is 1/20 or more of an area of the opening of the supply unit.   The liquid ejection method according to claim 1, wherein a length of the nozzle is in a range of 40 μm or more and 100 μm or less. The opening of the supply section is rectangular;
The length of one side of the opening is in the range of 30 μm or more and 500 μm or less,
4. The liquid ejection method according to claim 1, wherein a length of the other side of the opening is in a range of 20 μm or more and 300 μm or less. 5.   The outer edge of the opening of the supply unit is a surface that defines the pressure chamber and is smaller than an outer edge of a surface that communicates with the supply unit. The liquid discharge method as described.   6. The liquid ejection method according to claim 1, wherein an inertance of the nozzle is smaller than an inertance of the supply unit.   The liquid according to any one of claims 1 to 6, wherein the pressure generating section includes a deformation section that partitions a part of the pressure chamber and applies a pressure change to the liquid by deformation. Discharge method.   The liquid ejection method according to claim 7, wherein the pressure generation unit deforms the deformation unit to a degree corresponding to a potential change pattern in the applied ejection pulse. A pressure chamber in which liquid from a liquid supply source is supplied through the supply unit; a nozzle that communicates with the pressure chamber and that discharges the liquid; and a pressure change in the liquid in the pressure chamber in order to discharge the liquid from the nozzle A liquid discharge head having a pressure generation unit that performs an operation of applying a pressure, and a liquid including a pulse generation unit that generates a plurality of discharge pulses within a unit period for operating the pressure generation unit to discharge liquid from the nozzle A discharge device,
The opening area of the nozzle on the side from which the liquid is discharged is 1/9 or less of the opening area on the pressure chamber side of the supply unit,
The discharge pulse includes a first waveform section that operates the pressure generating section to depressurize the pressure chamber by changing a potential from a reference potential to an operating potential with a constant gradient, and a second waveform that maintains the operating potential for a certain period of time. And a third waveform unit that operates the pressure generating unit to pressurize the pressure chamber by changing the potential with a constant gradient from the operating potential to the reference potential,
The phase of residual vibration after the liquid is discharged using the previous discharge pulse when the discharge pulse generated next to the discharge pulse generated earlier in the unit period is applied to the pressure generating unit. The liquid ejection apparatus is characterized in that the interval between the previous ejection pulse and the next ejection pulse is set on the basis of the natural vibration period Tc generated in the liquid in the pressure chamber so as to be uniform.

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