Ink discharge method and inkjet head

Abstract

Provided are an ink discharge method and an inkjet head with which it is possible to stably discharge ink while ensuring the width between a discharge surface and a landing surface. This ink discharge method is performed by an inkjet head comprising a nozzle (N) and an ink flow path communicating with the nozzle (N), the ink flow path including a pressure chamber (12) for applying pressure fluctuation to ink inside the ink flow path. In the inkjet head, when the viscosity of the ink is 5.7 mPa·s, the density is 1080 kg / m3, the surface tension is 41 mN / m, and the acoustic velocity is 1521 m / s, a Q value pertaining to the pressure vibration of the ink in the pressure chamber (12) is 10-18, a resonance period pertaining to the meniscus vibration of the ink in the nozzle (N) is 1-6 times a resonance period pertaining to the pressure vibration, the opening diameter of the nozzle (N) is 20 μm or greater, and the width between the ink discharge surface of the inkjet head and the ink landing surface is 5.0 mm or greater.

Description

Ink ejection method and inkjet head

[0001] The present disclosure relates to an ink ejection method and an inkjet head.

[0002] There are inkjet heads that eject ink and land it on a medium. The ink that has landed on the medium forms an image, a coating, or the like. There is a demand for higher precision in the object being formed and for faster formation. To achieve this higher precision and faster speed, inkjet heads are required to eject ink stably. Patent Document 1 discloses a technique for stabilizing the meniscus. The meniscus is the ink liquid surface inside the nozzle.

[0003] Inkjet heads eject ink droplets that land on a medium. In addition to the main ink droplets, microdroplets may also be generated. These microdroplets are called satellites. When satellites land in a different location from the main droplet, quality is reduced.

[0004] There is a width between the ink ejection surface and the ink landing surface of the medium. If this width is wide, images can be recorded on uneven media. Furthermore, the wider the width, the less likely the medium will come into contact with the ink ejection surface if the medium has uneven thickness, warping, or creases. Therefore, the ink ejection surface is less likely to be scratched.

[0005] JP 2007-69374 A

[0006] However, if the distance between the ink ejection surface and the ink landing surface of the medium is large, the positions of the satellites tend to vary. Furthermore, if an attempt is made to reduce the satellites with the distance widened, it becomes difficult to stabilize the meniscus.

[0007] An object of the present disclosure is to provide an ink ejection method and an inkjet head that can eject ink stably while ensuring the width between the ejection surface and the landing surface.

[0008] In order to achieve the above object, one aspect of the present disclosure is an ink ejection method using an inkjet head comprising a nozzle and an ink flow path communicating with the nozzle, the ink flow path including a pressure chamber that applies pressure fluctuations to the ink therein, wherein the inkjet head ejects ink having a viscosity of 5.7 mPa s and a density of 1080 kg / m ３ , surface tension is 41 mN / m, and sound speed is 1521 m / s, the Q value related to the pressure vibration of the ink in the pressure chamber is 10 or more and 18 or less, the resonance period related to the meniscus vibration of the ink in the nozzle is 1 or more and 6 or less times the resonance period related to the pressure vibration of the ink in the pressure chamber, the nozzle opening diameter or a diameter equivalent to the opening diameter is 20 μm or more, and the width between the ink ejection surface and the ink landing surface of the inkjet head is 5.0 mm or more.

[0009] According to the present disclosure, it is possible to achieve the effect of stably ejecting ink while ensuring the width between the ejection surface and the landing surface of the inkjet head.

[0010] 1 is a cross-sectional view including a nozzle in an inkjet head according to a first embodiment; FIG. 2 is a diagram showing an equivalent circuit of the inkjet head according to the first embodiment; FIG. 3 is a diagram showing an example of an oscillation pattern of ink pressure in a pressure chamber; FIG. 4 is a diagram showing an example of a calculation of the frequency spectrum of ink pressure oscillation obtained by numerical simulation; FIG. 5 is a diagram showing an example of meniscus oscillation; FIG. 6 is a diagram showing an example of a calculation of the frequency spectrum of meniscus oscillation by numerical simulation; FIG. 7 is a diagram showing experimentally determined minimum ink droplet velocities at which satellites are generated for each nozzle shape, relative to Qc; FIG. 8 is a table showing ink ejection stability relative to Qc; FIG. 9 is a diagram explaining frequency stability; FIG. 10 is a diagram explaining frequency stability; FIG. 11 is a cross-sectional schematic diagram of an inkjet head according to a second embodiment; FIG. 12 is an equivalent circuit diagram of an inkjet head according to the second embodiment; FIG. 13 is a cross-sectional schematic diagram of an inkjet head according to a third embodiment; FIG. 14 is an equivalent circuit diagram of an inkjet head according to the third embodiment; FIG. 15 is a cross-sectional schematic diagram of an inkjet head according to a fourth embodiment; FIG. 16 is an equivalent circuit diagram of an inkjet head according to the fourth embodiment; FIG. 17 is a cross-sectional schematic diagram of an inkjet head according to a fifth embodiment; FIG. 18 is an equivalent circuit diagram of an inkjet head according to the fifth embodiment; FIG. 19 is a cross-sectional schematic diagram of an inkjet head according to a sixth embodiment. 10 is an equivalent circuit diagram of an inkjet head according to a sixth embodiment; FIG. 11 is a cross-sectional view parallel to a nozzle opening surface of an inkjet head according to a seventh embodiment; FIG. 12 is an equivalent circuit diagram of an inkjet head according to a seventh embodiment;

[0011] Hereinafter, embodiments will be described with reference to the drawings. [First Embodiment] Fig. 1A is a cross-sectional view including a nozzle N of an inkjet head 1 of a first embodiment. Fig. 1B is a diagram showing an equivalent circuit of the inkjet head 1 of the first embodiment. As shown in the cross-sectional view of Fig. 1A, the inkjet head 1 ejects ink from the nozzle N. An ink flow path that supplies ink to the nozzle N is connected. The ink flow path includes a pressure chamber 12 and an upper communication path 13. Ink is sent from an ink tank to a common supply path (not shown). Ink in the common supply path is supplied to the pressure chamber 12 via the upper communication path 13. The upper communication path 13 may include a restriction where the opening diameter is partially narrowed to restrict the amount of ink flowing in.

[0012] A vibration plate 16 faces the pressure chamber 12. A piezoelectric element 15 is positioned along the vibration plate 16. The piezoelectric element 15 deforms in a flexural mode, causing the vibration plate 16 to vibrate. The pressure chamber 12 changes size in response to the vibration of the vibration plate 16. Pressure fluctuations are applied to the ink in response to this size fluctuation. The piezoelectric element 15 is made of, for example, lead zirconate titanate (PZT).

[0013] As shown in the equivalent circuit of Figure 1B, the nozzle N is represented by a combination of a capacitor Cn, a resistor Rn, and an inductor Ln. The pressure chamber 12 is represented by a combination of resistors R1 and R2 and inductors L1 and L2. The pressure chamber 12 also has capacitors C1 and Ce1 connected in parallel to the ground plane. The combination of circuit components is the same in the following embodiments.

[0014] The resistor elements R1 and R2 represent the flow path resistance of the ink flow path. The resistor elements R1 and R2 may be collectively represented by a single resistor element. The inductors L1 and L2 represent the inertance of the ink flow path. The inductors L1 and L2 may be collectively represented by a single inductor. The capacitor C1 represents the elastic compliance of the ink. The capacitor Ce1 represents the elastic compliance of the wall surface of the pressure chamber 12. The pressure chamber 12 has elasticity related to the deformation of the piezoelectric element 15 and the vibration plate 16. The compliance corresponding to this elasticity is the capacitor Ce1. The pressure chamber 12 has, for example, a rectangular parallelepiped shape, but is not limited to this. For example, the pressure chamber 12 may have a rectangular shape with chamfered or rounded corners when viewed from above in a plan view of the pressure chamber 12, i.e., from top to bottom in the figure.

[0015] The upper communication passage 13 has a length and width (shape) different from those of the pressure chamber 12. Therefore, the upper communication passage 13 is represented by a circuit component with parameters separate from those of the pressure chamber 12. The upper communication passage 13 is represented by a combination of resistors R3 and R4 and inductors L3 and L4. The resistors R3 and R4 may be collectively represented as a single resistor. The inductors L3 and L4 may be collectively represented as a single inductor. The upper communication passage 13 has capacitors C3 and Ce3 between it and the ground surface. The capacitor C3 represents the elastic compliance of the ink. The capacitor Ce3 represents the elastic compliance of the wall surface of the upper communication passage 13. A thin portion (compliance plate) for absorbing vibration pressure may be located on the wall surface of the upper communication passage 13. The compliance plate may be made of polyimide or various thin plates. The upper communication passage 13 has, for example, a cylindrical shape when viewed in cross section perpendicular to the ink flow direction, i.e., the vertical direction in the figure, but is not limited thereto. Furthermore, if the upper communication passage 13 does not have a compliance plate, it may be considered as a rigid body, i.e., the capacitance of the capacitor Ce3 is negligible and can be treated as substantially zero.

[0016] Of the circuit components representing the upper communication passage 13, the pressure chamber 12, and the nozzle N, the resistor element and inductor are connected in series. The capacitor is arranged in parallel between the series connection and the ground plane. The resonance period of this equivalent circuit includes a resonance period Tm corresponding to the volume fluctuation of the ink, and a resonance period Tc corresponding to the pressure fluctuation of each ink flow path portion and the entire nozzle. The ink volume fluctuation corresponds to the fluctuation of the liquid surface (meniscus) position in the nozzle N. In other words, the resonance period Tm is also the resonance period related to meniscus vibration. The liquid surface position referred to here is not only inside the nozzle N, but can also be temporarily located outside the nozzle N as ink is ejected.

[0017] A drive signal including a drive pulse having a length of half the resonance period Tc, for example, is applied to the piezoelectric element 15 in approximately synchronization with the resonance period Tc of the pressure fluctuation in the pressure chamber 12. The pressure vibration of the ink in the pressure chamber 12 and the volume vibration of the ink in the ink flow path cause reverberation vibration even after the drive signal is applied. In the case of a multi-drop system in which a drive pulse is applied multiple times within a drive period for recording one dot, new pressure fluctuations are superimposed on the reverberation vibration. Therefore, the amplitude of the second and subsequent drive pulses is greater than the amplitude of the first.

[0018] Actual volume vibrations and pressure vibrations contain a mixture of vibrations associated with the respective resonances. Furthermore, the volume vibrations and pressure vibrations may also contain vibration components associated with the resonance of the upper communicating passage 13 and the resonance of the entire ink flow path. The period of the ink volume vibration is different from the period of the pressure vibration. Here, the resonance period Tm of the volume vibration is longer than the resonance period Tc of the pressure vibration in the pressure chamber 12. Therefore, the position of the meniscus associated with the volume vibration shifts by the resonance period Tm from the position associated with the vibration at the resonance period Tc. When the resonance period Tm is long, the ink surface remains significantly protruding from the nozzle opening N for a long period of time near the maximum of the vibration component associated with the resonance period Tm. As a result, ink overflows onto the nozzle opening surface (ink ejection surface). This overflow adversely affects ink ejection.

[0019] FIG. 2 shows an example of calculations performed by numerical simulation of the vibration pattern of ink pressure within the pressure chamber 12 and its frequency spectrum. This figure shows an example of ink ejection using a combination of an inkjet head and ink with a resonance period Tc = 5.36 μs, a resonance period Tm = 26.78 μs, a Q value Qc = 12.6, and a Q value Qm = 2.03. Qc and Qm will be described later. A multi-drop method is used to eject ink, in which eight droplets of approximately 3 pL each are ejected consecutively at intervals of the resonance period Tc associated with the pressure vibration, and the droplets land on one pixel position (thin dotted lines). Each drive pulse is a back-and-forth pulse with a width of half the resonance period Tc and an amplitude of approximately 250 kPa.

[0020] The pressure in the pressure chamber 12, i.e., the capacitor Ce, can be calculated as the amount of deformation of the piezoelectric element 15 that accompanies pressure fluctuations, i.e., the output voltage. A plurality of modes are further superimposed on the obtained waveform.

[0021] In Fig. 2A, the pressure vibration at point Pc in Fig. 1B is indicated by a thick dotted line. This vibration is a damped vibration in which the amplitude of a sinusoidal vibration corresponding to the resonance period Tc increases along with the drive pulse and then damps after the drive pulse ends. As is well known, damped vibration is expressed as the product of the initial amplitude, the damping term, and the vibration component, and the damping rate increases as the damping coefficient ζ of the damping term increases toward 1.

[0022] The frequency spectrum shown in Figure 2B clearly shows a peak at the resonance frequency fc = 1 / Tc. The resonance period Tc is determined by the position of this peak. A small peak also appears at the resonance frequency fm = 1 / Tm. In Figure 2B, the frequencies fcl and fch indicate the positions where the amplitude spectrum intensity is 1 / √2 of the peak spectrum intensity. In other words, the distance between the frequencies fch and fcl indicates the half-width of the vibration energy of the pressure vibration.

[0023] By extracting the component at the resonant frequency fc from the result of superimposing the waves of the multiple modes, the vibration characteristics at the resonant frequency fc can be obtained. For example, a band-pass filter (BPF) or the like can be used to extract only the vibrations near the target resonant period Tc from the waveform shown by the bold dotted line in FIG. 2A. Alternatively, components outside the band including the resonant frequency fc, for example, outside the range of the half-width, can be eliminated from the frequency spectrum obtained from the result by Fourier transform. The frequency spectrum from which the vibrations in the target frequency band have been selectively extracted is then subjected to an inverse Fourier transform to obtain the time change in pressure vibration of a specific target. In FIG. 2A, the vibration waveform due to pressure vibration at the period Tc after such processing is shown by a solid line. Because the original vibration waveform is mostly composed of components corresponding to the resonant period Tc, the vibration waveform is almost identical before and after applying the BPF. In this waveform after BPF, the envelope shown by the thin solid line represents the attenuation of the vibration at the resonant period Tc. The damping rate ζ will be described later. In addition to the damping coefficient ζ, the Q value is also known as a parameter that indicates the degree of damping in such damped vibrations. The Q value, which indicates the degree of damping of pressure vibrations in the pressure chamber 12, is defined as Qc. It is expressed as ζ≒(1 / 2Qc). Therefore, the larger the Qc value, the longer the reverberation will last.

[0024] On the other hand, the resonance frequency fm=1 / Tm is the resonance frequency of the vibration of the meniscus, which is the liquid surface of the nozzle N. This resonance period Tm depends on the volume vibration of the liquid inside the nozzle N (the entire inside of the individual flow path).

[0025] FIG. 3A is a diagram showing an example of a meniscus vibration pattern. FIG. 3B is a diagram showing an example of a frequency spectrum calculated by numerical simulation. The inkjet head and ink characteristics related to ink ejection, as well as the ejection method conditions, are the same as those in FIG. 2 above. In FIG. 3A, the meniscus vibration at point Pm in FIG. 1 is shown by a thick dotted line. In this vibration, the center of the sinusoidal vibration corresponding to the resonance period Tc fluctuates due to the vibration corresponding to the resonance period Tm. The superposition of these multiple modes of vibration results in both vibrations increasing as the drive pulse increases and then attenuating after the drive pulse ends.

[0026] The meniscus vibration frequency spectrum shown by the dashed line in Figure 3B has a maximum peak at a frequency corresponding to the resonance period Tm. This frequency spectrum also includes a peak at a frequency corresponding to the resonance period Tc. The frequencies fmh and fml shown in Figure 3B indicate the half-width of the vibration energy, i.e., the position where the amplitude spectrum intensity is 1 / √2 of the peak spectrum intensity. Applying a BPF within this half-width results in the vibration waveform shown by the solid line in Figure 3A. This vibration waveform is the component of the meniscus vibration due to the resonance period Tm. In this waveform after BPF, the envelope shown by the thin solid line represents the damping of the vibration with the resonance period Tm. A damping rate ζ and a Q value are also determined for this damped meniscus vibration. Furthermore, the Q value, which indicates the degree of damping of the meniscus vibration, is designated Qm. A larger Qm value indicates a longer reverberation. When the meniscus vibration is a positive value, the tip of the ink protrudes from the opening of the nozzle N. The timing Fm is the point at which this protrusion is at its maximum. If this protrusion continues for a long time, that is, if the period Tm is long, some of the ink that has not separated into droplets may not return to the nozzle N and may overflow from the nozzle opening surface.

[0027] When the vibration characteristics are obtained from actual measurements, the resonance periods Tc and Qc can be obtained, for example, from the amount of deformation of the piezoelectric element 15 due to pressure fluctuations, i.e., the output voltage. The resonance periods Tm and Qm can be obtained, for example, by irradiating a laser beam from the opening of the nozzle N and sequentially obtaining the movement speed of the meniscus in the nozzle N by Doppler measurement of the reflected wave.

[0028] The damping coefficient ζ in an electric circuit such as an equivalent circuit is expressed by the following formula (1): ζ=Rc / (2Lcωc) (1) ωc is the angular velocity of the resonance period Tc, and ωc=2π / Tc. Therefore, Qc is expressed by the following formula (2) in relation to the damping coefficient ζ: Qc=2πLc / (RcTc) (2) The resonance period Tc is expressed by the following formula (3): Tc=2π(LcCc) １／２ … (3)

[0029] Rc, Lc, and Cc respectively represent the combined resistance, combined inductance, and combined capacitance in the pressure chamber 12. However, these values ​​are difficult to determine analytically. Therefore, the combined resistance Rc, combined inductance Lc, and combined capacitance Cc may be determined numerically by numerical simulation. Well-known circuit simulation software such as LTspice may be used for the numerical simulation.

[0030] On the other hand, based on the resonance period Tm of the liquid surface vibration and various parameters, Qm is expressed by the following formula (4): Qm=2πLm / (RmTm) (4) The resonance period Tm is expressed by the following formula (5): Tm=2π(LmCm) １／２ ... (5) The combined inductance Lm is the sum of the inductances of the upper communication passage 13, the pressure chamber 12, and the nozzle N, which are arranged in series. The combined resistance Rm is the sum of the resistances of the upper communication passage 13, the pressure chamber 12, and the nozzle N, which are arranged in series. The combined capacitance Cm is approximately the capacitance of the capacitance Cn of the nozzle N, and is expressed by the following equation (6): Cm=Cn=πφ ４ / (128σ) ... (6) Here, φ is the diameter (nozzle diameter) when the cross section of the nozzle N is circular, i.e., the opening diameter, and σ is the surface tension of the ink. When these are analytically determined, they can be calculated approximately. Alternatively, some or all of the above values ​​may be calculated by numerical simulation. Note that the cross section of the nozzle N may not be circular. For example, when the cross section is elliptical or rectangular, φe = 2 r1 r2 / (r1 + r2), where the long side is r1 and the short side is r2, can be used as a value equivalent to the nozzle diameter φ, i.e., the hydraulic diameter.

[0031] When each component is a rectangular parallelepiped, the resistance R, inductance L, and capacitance C of the component are expressed by the following equations (7) to (9): R=8η(a+b) ２ h / (a ３ b ３ ) … (7) L=ρh / (ab) … (8) C=abh / (ρv ２) ... (9) In the above, a and b are the lengths of the short and long sides, respectively, of the cross section of the rectangular parallelepiped perpendicular to the flow path direction. h is the length of the rectangular parallelepiped along the flow path. η is the ink viscosity, ρ is the ink density, and v is the sound speed of the ink. η, ρ, v, and the above σ are constants depending on the ink.

[0032] When each component has a truncated cone shape with diameters of φ1 and φ2 at both ends, the resistance R, inductance L, and capacitance C of the component are expressed by the following formulas (10) to (12): R=128η / (3π)(d / (φ2-φ1)((1 / φ1) ３ -(1 / φ2) ３ )) … (10) L=4ρ / π(d / (φ2−φ1)((1 / φ1)−(1 / φ2)))…(11) C=πd / 12(φ1 ２ +φ1φ2+φ2 ２ ) / (ρv ２ ) … (12)

[0033] The wall surface of each of the above-mentioned flow paths is not limited to the vibration plate of the pressure chamber 12, but may be made of an elastic material. For example, each flow path configuration may have a thin portion (compliance plate) for absorbing vibration pressure. In addition to the PZT and vibration plate, the compliance plate may include, for example, polyimide. The electric capacitance of the capacitor Ce1 associated with this side wall is expressed by the following equation (13): Ce1=αb ４ (hb) / (2Ed ３ ) ... (13) where h and b are the length and width of the side wall, respectively. E is the Young's modulus of the side wall. d is the thickness of the side wall. α is a coefficient corresponding to (h / b). When h / b > 5, α = 0.0284. Note that the vertical width h of the side wall does not have to be the same as the length h along the flow path direction of each of the rectangular parallelepiped structures described above. When the wall surface is complex, such as when it is made of laminated plates, it is difficult to analytically determine the capacitance of capacitor Ce1. In this case, it can be numerically determined using analysis software such as the finite element method. Examples of analysis software that can be used include software from ANSYS (registered trademark) and COMSOL Multiphysics from COMSOL (registered trademark).

[0034] Qc, Qm and resonance periods Tc, Tm according to the actual structure of the inkjet head and the characteristics of the ink can also be determined from the vibration waveforms of FIGS. 3A and 3B obtained by actual measurements or numerical simulations.

[0035] For example, a plurality of maximum or minimum points are extracted from the vibration waveform, and the damping ratio can be obtained from the ratio of the displacements of these points. The amplitude term in the vibration of the damping coefficient ζ is A·exp(−ζω ０ Therefore, the ratio of the maximum pressure values ​​P(n) and P(n+1) in the pressure oscillation periods n and n+1 is expressed by the following formula (14) based on formula (1): P(n+1) / P(n)=exp(-RcTc / (2Lc)) (14)

[0036] From this equation (14) and the above equation (2), the following equation (15) can be obtained approximately: Qc = -π / ln(P(n+1) / P(n)) (15) Similarly, the ratio V(n+1) / V(n) of the volume V at the maximum timing t1 of period n in the volume vibration to the maximum timing t1+Tm of the next period n+1 is taken. This value is expressed by equation (16): V(n+1) / V(n) = exp(-RmTm / (2Lm)) (16) From this equation (16) and the above equation (2), the following equation (17) can be obtained: Qm = -π / ln(V(n+1) / V(n)) (17) The phases of the vibrations of the volume V and pressure P can be considered to be the same.

[0037] Note that Qc and Qm calculated for only a single period n and period n+1 are likely to have low accuracy due to the influence of noise, etc. Therefore, Qc and Qm may be calculated for multiple periods n=1, 2, ... and then simply averaged. That is, Qc is calculated as the average of N periods using the following formula (18): Qc=(-π / N)Σ ｎ＝１～Ｎ 1 / ln(P(n+1) / P(n)) (18) where N is the number of periods to be referenced.

[0038] Similarly, Qm is calculated as the average of N periods by the following formula (19): Qm=(−π / N)Σ ｎ＝１～Ｎ 1 / ln(V(n+1) / V(n))...(19)

[0039] When Qc is calculated from the results of experiments or simulations, the number of periods N to be averaged may be determined within a range that allows accurate amplitude determination. In other words, the faster the attenuation, the smaller the number of periods N may be.

[0040] Alternatively, Qc and Qm may be calculated directly from the spectral distribution. That is, Qc and Qm are calculated by the following formulas (20) and (21). Qc = fc / (fch-fcl) (20) Qm = fm / (fmh-fml) (21) The resonance frequencies fc and fm are the reciprocals of the resonance periods Tc and Tm, respectively. The upper limit frequencies fch and fmh are frequencies at which the spectral intensity is 1 / √2 (approximately -3 dB) of the spectral intensity at the resonance frequencies fc and fm, respectively. The lower limit frequencies fcl and fml are frequencies at which the spectral intensity is 1 / √2 (approximately -3 dB) of the spectral intensity at the resonance frequencies fc and fm, respectively. fch>fc>fcl, fmh>fm>fml.

[0041] 2B and 3B, the resonance frequencies fc and fm corresponding to the resonance periods Tc and Tm and their peak spectral intensities are specified, respectively. Also, upper frequencies fch and fmh and lower frequencies fcl and fml whose spectral intensities are 1 / √2 (approximately -3 dB) relative to the peak spectral intensities are specified. From these, Qc and Qm can be obtained from equations (20) and (21).

[0042] The vibrations associated with ink ejection, characterized as described above, affect the accuracy of the actually ejected ink droplets. In this disclosure, the above-described resonance periods Tc, Tm, and Qc are used to define a range of ink combinations and structures that enable stable ink ejection. In particular, this disclosure considers image recording on surfaces other than paper and assumes a wide gap between the nozzle opening surface of the inkjet head 1 and the landing surface of the ejected ink. In this disclosure, a wide gap is defined as ink ejection with a width of 5.0 mm or greater. It is also preferable that stable ejection is possible even with a width of approximately 10.0 mm or greater. Ink droplets are decelerated by air resistance and other factors during flight from the nozzle opening surface to the landing surface. Therefore, when the width is large, the deviation in the landing position due to deceleration cannot be ignored. When flying over a width of 10 mm at an ejection speed of 10 m / s, the landing position of an ink droplet decelerated by air resistance, as modeled, has a variation of ±105 μm. In an image of 360 dpi (approximately 70 μm / pixel), this variation corresponds to ±1.5 pixels.

[0043] Furthermore, when the width is wide, if satellites occur, they are more likely to land in positions different from the main liquid. Such variations in landing positions can lead not only to poor design quality but also to a decline in practicality, such as the accuracy of reading barcodes and two-dimensional symbols. Therefore, when performing image recording operations with a wide width, greater ink ejection stability is required.

[0044] That is, the required image quality can be determined as a level at which barcodes, two-dimensional symbols, and the like can be stably read. Standards such as ISO / IEC 15415 and ISO / IEC 15416 are used to evaluate the print quality of barcodes and two-dimensional symbols. In this disclosure, these standards define print quality as stably readable as an overall C rating, i.e., a rating of C or higher for the lowest rating of each individual evaluation criterion. Experiments by the inventors have shown that a deviation of about three pixels is acceptable in a 360 dpi image. That is, the ink and inkjet head are required to be able to stably eject ink at a speed of 10 m / s or higher over a width of 10 mm while reducing the occurrence of satellites for droplets with an ejection volume of about 20-30 pL per shot.

[0045] Furthermore, inkjet recording apparatuses primarily for industrial use also require high-speed recording operations. In this embodiment, stable ejection is desired at a drive frequency of 10 kHz or higher, particularly up to approximately 11 kHz. A drive frequency of 10 kHz or higher corresponds to a drive period of 100 μsec or less. A drive frequency of 11 kHz corresponds to a drive period of approximately 90 μsec (90.9 μsec).

[0046] For example, if Qc related to the pressure vibration in the pressure chamber 12 becomes large and the reverberation of the pressure vibration in the pressure chamber 12 becomes large, the meniscus cannot be maintained and is likely to break. On the other hand, if Qc related to the pressure vibration in the pressure chamber 12 is too small, the trailing edge of the ejected ink droplet is likely to be slowed down compared to the leading edge. As a result, the ejected ink is less likely to gather into a single droplet, and microdroplets (satellites) are more likely to be generated.

[0047] 4 is a diagram showing experimentally determined minimum ink droplet velocity at which satellites are generated for each nozzle shape relative to Qc. In the experiment, ink was ejected while changing Qc for each of four types of nozzle shapes represented by different symbols. The specific nozzle shapes are not relevant to the contents of this disclosure, so explanation will be omitted. The ink ejected had a viscosity of 5.7 mPa·s at 25°C and a density of 1080 kg / m as standard values. ３ , surface tension is 41 mN / m, and the speed of sound is 1521 m / s.

[0048] As described above, Qc is determined by a combination of the inkjet head shape and ink characteristics. Here, the inkjet head shape and ink type are fixed. As the temperature of the ejected ink rises, the combined resistance Rc of the pressure chamber 12 decreases relatively. Utilizing this property, it is possible to change Qc while roughly maintaining other parameters based on the above formula (2). For the same nozzle shape, the larger Qc is, the higher the minimum speed at which satellites occur. It has been shown that if Qc is 10 or higher, satellites are less likely to occur at normal ink ejection speeds.

[0049] FIG. 5 is a graph showing the ink ejection stability versus Qc. Two types of ink, A and B, were used in the experiment. A multi-drop method was used to eject ink, in which eight droplets were ejected consecutively at intervals of the resonance period Tc associated with pressure vibration, landing on a single pixel. The droplet volume per droplet was 3 pL. Each drive pulse was a successive pulse with a width of half the resonance period Tc. With the multi-drop method, the volume of each droplet, especially the final droplet, is smaller than that of a single droplet in the single-drop method, making satellites less noticeable. Note that the drive pulse typically used for ink ejection includes a waveform designed to reduce reverberation vibrations. Therefore, actual ink ejection is further improved within the parameter range determined using this waveform as follows. As a result, ink was ejected appropriately when Qc was between 10 and 18 (Examples 1 to 5). However, when Qc exceeded 18 (Comparative Examples 1 and 3), significant speed variations occurred depending on the ejection frequency, resulting in unstable ejection. Additionally, it can be seen that problems such as the tendency for satellites to occur occur when Qc is less than 10 (Comparative Example 2). It can be seen that by setting Qc to an appropriate value, ink ejection stability can be achieved even if the ink properties change.

[0050] 6A and 6B are diagrams illustrating the frequency stability in Fig. 5. Fig. 6A shows the experimental results of determining the rate of change of the ejection speed at 10 m / s for the drive period under the conditions in the left three columns of Fig. 5.

[0051] When the drive cycle is short, i.e., when the drive frequency is high, reverberation from the previous drive cycle tends to remain. As Qc increases, this effect spreads to the longer drive cycle, causing the ink ejection speed to tend to deviate from the expected speed. For image quality, the allowable deviation in ejection speed of less than about 3 pixels is, for example, about 10%. Figure 6A shows that for each pattern with Qc of 18 or less, the deviation in the expected speed is within 10% in the drive frequency range of 11 kHz or less. From the above, we have obtained the result that, as condition 1, within the range of 10≦Qc≦18, ink is ejected stably while reducing the occurrence of satellites.

[0052] 6B is a graph showing the results of determining frequency stability while varying the surface tension under the conditions of ink X in FIG. 5 and a temperature of 40° C. By varying the surface tension, the resonance period Tm can be decreased while maintaining Qc and the resonance period Tc, and Qm can be increased accordingly. In other words, the ratio Tm / Tc of the resonance periods Tm and Tc changes.

[0053] As shown in Figure 6B, when Tm / Tc is large, the ejection speed tends to become unstable relative to the drive frequency. When Tm / Tc is 6 or less, the ejection speed stabilizes within the above-mentioned allowable range. Furthermore, as described above, a smaller resonance period Tm reduces ink overflow from the nozzle opening surface.

[0054] On the other hand, it has been known that the occurrence of satellites worsens as Tm / Tc decreases (see, for example, International Publication No. 2009 / 107552). This is because the ink column protruding from the nozzle N with the resonance period Tc becomes more likely to separate due to the shorter resonance period Tm. The above suggests that when the nozzle diameter φ is φ≦10 μm and Tm / Tc<1, the occurrence of satellites worsens. This condition is also considered effective for stable ink ejection in the present disclosure. Therefore, as condition 2, within the range of 1≦Tm / Tc≦6, ink is ejected stably while reducing the occurrence of satellites. In other words, the resonance period Tm of the meniscus vibration may be between 1 and 6 times the resonance period Tc of the pressure vibration in the pressure chamber 12.

[0055] When the distance between the nozzle opening surface and the ink landing surface is wide, if the volume of ink droplets being ejected is small, the ink flight direction is likely to vary due to the influence of surrounding wind, etc. Since the volume of ink droplets depends on the nozzle diameter φ, the nozzle diameter φ must be set large enough to allow stable flight of ink droplets due to inertia. Meanwhile, as shown in the above equations (5) and (6), the resonance period Tm depends on the square of the nozzle diameter φ. In other words, increasing the nozzle diameter φ tends to significantly increase Tm / Tc. Furthermore, as the resonance period Tc increases, the upper limit of the drive speed decreases, making high-speed image recording impossible. Therefore, this type of inkjet printer is not suitable for use in industrial inkjet recording devices.

[0056] Therefore, it is preferable that the inkjet head has a large flow path cross-sectional area other than that of the nozzle N and a short flow path length, which is the length along the flow path. The nozzle diameter φ of the nozzle N is set to be large within the range possible by adjusting the structure in this way. In a typical flow path structure, if the nozzle diameter φ is about 20 μm or more, the above conditions 1 and 2 are no longer satisfied. In contrast, the inkjet head of the present disclosure adjusts the flow path cross-sectional area and flow path length as described above to achieve condition 3, such that the nozzle diameter φ is 20 μm or more, thereby maintaining stable ink flight.

[0057] As long as the above-defined preconditions and conditions 1 to 3 are met, the structure of the inkjet head may be modified in various ways.

[0058] [Second Embodiment] An inkjet head 1a according to a second embodiment will now be described. Figures 7A and 7B are schematic cross-sectional views and diagrams showing an equivalent circuit of the inkjet head 1a. In the schematic cross-sectional views, the ink flow paths are located within the substrate, but the substrate other than the nozzle plate is not depicted. As shown in Figure 7A, the inkjet head 1a includes a lower communication path 14 between the nozzle N and the pressure chamber 12. In addition to the nozzle N, a recovery path 22 is connected to the lower communication path 14.

[0059] The recovery flow path 22 recovers ink that is not discharged from the opening of the nozzle N and remains near the nozzle N, and returns it to an ink tank or the like via the common discharge flow path 21 (common liquid chamber). If ink remains for a long period of time, its components may separate, and air and dust may enter through the opening of the nozzle N and become mixed into the ink. This causes the quality of the ink to deteriorate. As a result, the ink ejection accuracy decreases, and / or the quality of the image recorded by the deposited ink decreases. The recovery flow path 22 returns this ink to the ink tank. The recovered ink is agitated again, and dust is removed from the ink by a filter. Ink of an appropriate quality according to the amount of ink ejected or recovered is supplied from the ink tank to the pressure chamber 12 via the common supply flow path 11 (common liquid chamber) and the upper connecting path 13.

[0060] The common supply flow path 11 and the common discharge flow path 21 are connected to the upper communication path 13, the pressure chamber 12, the lower communication path 14, and the recovery flow path 22, which are multiple individual flow paths connected to the multiple nozzles N, respectively.

[0061] As shown in FIG. 7B , the equivalent circuit of the inkjet head 1a includes electronic components corresponding to the lower communication passage 14 and the recovery passage 22. The lower communication passage 14 is represented by resistor elements R5 and R6 and inductors L5 and L6 connected in series, and capacitors C5 and Ce5 connected in parallel to the ground surface. The recovery passage 22 is represented by resistor elements Rr1 and Rr2 and inductors Lr1 and Lr2 connected in series, and capacitors Cr1 and Cre connected in parallel to the ground surface. The walls of the lower communication passage 14 and the recovery passage 22 may be considered rigid bodies. In other words, the capacitances of capacitors Ce5 and Cre are negligible and can be treated as substantially zero.

[0062] The recovery flow path 22 is designed so that pressure vibrations applied in the pressure chamber 12 are efficiently transmitted to the nozzle N. In other words, the recovery flow path 22 has a high impedance compared to the upper communicating path 13, the pressure chamber 12, the lower communicating path 14, and the nozzle N, so that pressure vibrations are not transmitted to the recovery flow path 22 as much as possible. Therefore, approximately, the recovery flow path 22 may be ignored when calculating / experimenting pressure vibrations and volume vibrations.

[0063] The common supply flow path 11 and the common discharge flow path 21 are sufficiently wide compared to the individual flow paths, and are therefore treated as open ends, that is, as ground surfaces.

[0064] That is, the combined inductance Lm of the ink flow path is the sum of the inductances of the upper communicating path 13, the pressure chamber 12, the lower communicating path 14, and the nozzle N. The combined resistance Rm of the ink flow path is the sum of the resistance values ​​of the upper communicating path 13, the pressure chamber 12, the lower communicating path 14, and the nozzle N. As a result, when calculating the resonance periods Tc, Tm and Qc, Qm of the inkjet head 1a, values ​​that include the influence of the lower communicating path 14 for the inkjet head 1 are obtained.

[0065] [Third and Fourth Embodiments] As described above, in order to increase Qc and reduce the resonance period Tm, it is preferable to make the ink flow path short and wide. For this reason, the upper communication path 13 and the lower communication path 14 may be omitted. This shortens the ink flow path by this amount. Furthermore, the upper communication path 13 and the lower communication path 14 are narrower than the pressure chamber 12. Therefore, the combined resistance Rm is obtained by the sum of the resistance values ​​of the pressure chamber 12 and the nozzle N. The combined inductance Lm is obtained by the sum of the inductances of the pressure chamber 12 and the nozzle N. This reduces the resonance period Tm in the inkjet head. Qc can be increased.

[0066] Fig. 8A is a cross-sectional view of an inkjet head 1b according to a third embodiment. Fig. 8B is an equivalent circuit diagram of the inkjet head 1b. In the cross-sectional view of Fig. 8A, the pressure chamber 12 communicates with the common supply flow path 11 via a short upper communication path 13. The pressure chamber 12 communicates directly with the nozzle N. The piezoelectric element 15b located on the side of the pressure chamber 12 may deform in a shear mode instead of the above-mentioned bending mode. The equivalent circuit diagram shown in Fig. 8B is the same as the equivalent circuit in the inkjet head 1 according to the first embodiment.

[0067] Fig. 9A is a cross-sectional view of an inkjet head 1c according to a fourth embodiment. Fig. 9B is an equivalent circuit diagram of the inkjet head 1c. The cross-sectional view of Fig. 9A further omits the upper communication passage 13 from the cross-sectional structure of the inkjet head 1b according to the third embodiment shown in Fig. 8A. Therefore, the pressure chambers 12 are directly connected to the common supply passage 11.

[0068] Accordingly, in the equivalent circuit diagram shown in Figure 9B, the circuit components corresponding to the upper communicating passage 13 have been omitted. By performing a numerical simulation using this equivalent circuit in the same procedure as above, the resonance periods Tc, Tm and Qc, Qm can be determined. In the inkjet heads 1 to 1c shown in the first to fourth embodiments, ink to which pressure fluctuations have been applied in the pressure chambers 12 can be transmitted directly to the nozzles N in a straight line. This type of structure is also called an end shooter.

[0069] Fifth Embodiment The ink flow path for the nozzle N may be arranged substantially in parallel with the ink flow path having the above-described serial configuration. This also reduces the resonance period Tm. Fig. 10A is a schematic cross-sectional view of an inkjet head 1d according to a fifth embodiment. Fig. 10B is an equivalent circuit diagram of the inkjet head 1d.

[0070] 10A , the first pressure chamber 12a and the second pressure chamber 12b are connected in parallel to the lower communication passage 14, which is connected to the nozzle N. The first pressure chamber 12a is connected to the common supply flow path 11 via the first upper communication passage 13a. The second pressure chamber 12b is connected to the common discharge flow path 21 via the second upper communication passage 13b. That is, as a whole, ink is supplied from the common supply flow path 11 to the lower communication passage 14 and the nozzle N via the first upper communication passage 13a and the first pressure chamber 12a. Ink in the lower communication passage 14 that is not ejected from the nozzle N is sent to the common discharge flow path 21 via the second pressure chamber 12b and the second upper communication passage 13b. That is, the second pressure chamber 12b, the second upper communication passage 13b, and the second lower communication passage 14b to the right of the nozzle N in the drawing of the lower communication passage 14 form a recovery flow path 22. That is, the lower communication passage 14 is divided into a first lower communication passage 14a on the side of the first pressure chamber 12a and a second lower communication passage 14b on the side of the second pressure chamber 12b.

[0071] However, during ink ejection, the ink that has flowed into the second pressure chamber 12b is returned to the lower communication passage 14 and can be ejected from the nozzle N. The first pressure chamber 12a and the first upper communication passage 13a, the second pressure chamber 12b and the second upper communication passage 13b, and the lower communication passage 14 are positioned symmetrically with respect to the central axis of the nozzle N. That is, the first pressure chamber 12a and the second pressure chamber 12b apply pressure fluctuations in parallel to the ink to be ejected from the nozzle N. As a result, when viewed from the nozzle N, the cross-sectional area of ​​the ink supply path during ink ejection is the sum of the cross-sectional areas of the first pressure chamber 12a and the second pressure chamber 12b. The pressure fluctuations applied to the ink in the first pressure chamber 12a and the second pressure chamber 12b are transmitted in the lower communication passage 14 perpendicular to the extension direction of the nozzle N toward the connection position with the nozzle N. Therefore, this structure is also called a side shooter.

[0072] 10B, the lower communication passage 14 is divided into two with the connection position with the nozzle N as the boundary. The first upper communication passage 13a, the first pressure chamber 12a, and the first lower communication passage 14a are arranged in parallel with the second upper communication passage 13b, the second pressure chamber 12b, and the second lower communication passage 14b.

[0073] The first pressure chamber 12a is represented by resistors R11 and R12, inductors L11 and L12, and capacitors C11 and Ce1. The first upper communication passage 13a is represented by resistors R13 and R14, inductors L13 and L14, and capacitors C13 and Ce3. The first lower communication passage 14a is represented by resistors R15 and R16, inductors L15 and L16, and capacitors C15 and Ce5. The second pressure chamber 12b is represented by resistors R21 and R22, inductors L21 and L22, and capacitors C21 and Ce2. The second upper communication passage 13b is represented by resistors R23 and R24, inductors L23 and L24, and capacitors C23 and Ce4. The second lower communication passage 14b is represented by resistor elements R25 and R26, inductors L25 and L26, and capacitors C25 and C6.

[0074] Capacitors C11, C13, C15, C21, C23, and C25 represent the elastic compliance of the ink, respectively. Capacitors Ce1 and Ce2 represent the elastic compliance of the wall surfaces of the first pressure chamber 12a and the second pressure chamber 12b, respectively. Capacitors Ce3 and Ce4 represent the elastic compliance of the wall surfaces of the first upper communication passage 13a and the second upper communication passage 13b, respectively. Capacitors Ce5 and Ce6 represent the elastic compliance of the wall surfaces of the first lower communication passage 14a and the second lower communication passage 14b, respectively.

[0075] The resistor elements R11 to R16 and the inductors L11 to L16 are connected in series. The capacitors C11, C13, C15, Ce1, Ce3, and Ce5 are connected in parallel to the resistor elements and inductors of each section between the ground plane and the respective resistor elements and inductors. The resistor elements R21 to R26 and the inductors L21 to L26 are connected in series. The capacitors C21, C23, C25, Ce2, Ce4, and Ce6 are connected in parallel to the resistor elements and inductors of each section between the ground plane and the respective resistor elements and inductors. Therefore, the combined inductance Lm related to the volume vibration is calculated by adding the inductance of inductor Ln to the reciprocal of the sum of the reciprocals of the sum of the inductances of inductors L11 to L16 and the sum of the inductances of inductors L21 to L26. The combined resistance Rm is obtained by adding the resistance value of the resistance element Rn to the reciprocal of the sum of the reciprocals of the sum of the resistance values ​​of the resistance elements R11 to R16 and the sum of the reciprocals of the sum of the resistance values ​​of the resistance elements R21 to R26.

[0076] The parameter values ​​of the individual circuit components may be determined so that the resonance periods Tc and Qm fall within appropriate ranges, as described above. The parameters of the circuit components on the side connected to the common supply flow path 11 and the side connected to the common discharge flow path 21 may or may not be the same. If they are the same, the total inductance Lm is the sum of half of the total inductance of one side and the inductance of inductor Ln. The total resistance Rm is the sum of half of the total resistance of one side and the resistance value of resistor Rn. The total inductance Lc and total resistance Rc for pressure vibration may be obtained by numerical simulation. The resonance periods Tm, Tc, Qm, and Qc can be obtained analytically according to the above equations 1 to 4, or numerically according to equations 17, 18, etc.

[0077] Even in this type of inkjet head 1d with a side shooter, the resonance period Tm can be reduced by reducing the combined inductance Lm. That is, because the flow path cross-sectional area is large and the flow path length is short, it is easy to increase the nozzle diameter φ and the capacitance of the capacitor Cn. In particular, with a side shooter, the combined resistance and combined inductance of elements other than the nozzle N can be made relatively small compared to the resistance and inductance of the nozzle N. Therefore, it is easier to reduce the resonance frequency Tm and increase Qc than with an end shooter. Therefore, this inkjet head 1d can easily incorporate nozzles N with a diameter φ≧20 μm.

[0078] [Sixth Embodiment] Fig. 11A is a cross-sectional schematic diagram of an inkjet head 1e according to a sixth embodiment. Fig. 11B is an equivalent circuit diagram of the inkjet head 1e. As shown in Fig. 11A, in the inkjet head 1e, a first upper communicating passage 13a and a second upper communicating passage 13b are connected to both ends of a pressure chamber 12 which is connected to a nozzle N. The first upper communicating passage 13a connects the pressure chamber 12 to the common supply flow path 11. The second upper communicating passage 13b connects the pressure chamber 12 to the common discharge flow path 21. A piezoelectric element 15 and a vibration plate 16 which deform in a flexure mode are located along the wall surface of the pressure chamber 12.

[0079] That is, the first upper communication passage 13a and the second upper communication passage 13b are positioned in parallel with respect to the pressure chamber 12. In the equivalent circuit shown in Fig. 11B, the pressure chamber 12 is divided into a first pressure chamber 12a and a second pressure chamber 12b with the nozzle N as the boundary. The first pressure chamber 12a is connected to the first upper communication passage 13a, and the second pressure chamber 12b is connected to the second upper communication passage 13b. The circuit components of the first pressure chamber 12a and the first upper communication passage 13a are positioned in parallel with the circuit components of the second pressure chamber 12b and the second upper communication passage 13b.

[0080] In this inkjet head 1e, ink normally flows from the common supply flow path 11 to the first upper communicating path 13a and pressure chamber 12 (first pressure chamber 12a). Ink in the pressure chamber 12 that is not ejected from the nozzle N flows from the second pressure chamber 12b through the second upper communicating path 13b to the common discharge flow path 21. When ink is ejected, a pressure fluctuation is applied to the ink in the second pressure chamber 12b together with the ink in the first pressure chamber 12a, and a portion of it is ejected from the nozzle N.

[0081] This inkjet head 1e does not have the first lower communicating passage 14a or the second lower communicating passage 14b. Therefore, the inductance of the inductors L15, L16, L25, and L26 and the resistance of the resistor elements R15, R16, R25, and R26 are all zero. The combined resistance Rm and combined inductance Lm are smaller than those of the inkjet head 1d by the amount of the reduction in inductance and resistance.

[0082] [Seventh Embodiment] Fig. 12A is a cross-sectional view parallel to the nozzle opening surface of an inkjet head 1f according to a seventh embodiment. Fig. 12B is an equivalent circuit diagram of the inkjet head 1f. As shown in Fig. 12A, in this inkjet head 1f, a common supply flow path 11 and a common discharge flow path 21 are directly connected to both ends of a pressure chamber 12. Therefore, normally, ink flows from the common supply flow path 11 to the common discharge flow path 21 via the pressure chamber 12.

[0083] 12B, the pressure chamber 12 is divided into a first pressure chamber 12a and a second pressure chamber 12b, with the nozzle N as the boundary. When ink is ejected, pressure fluctuations are applied to ink that has passed through the position of the nozzle N and flowed into the second pressure chamber 12b, and a portion of the ink can be ejected from the nozzle N. The circuit components of the first pressure chamber 12a and the circuit components of the second pressure chamber 12b are located in parallel to the circuit components of the nozzle N.

[0084] Compared to inkjet head 1e, inkjet head 1f does not further include the first upper communicating passage 13a and the second upper communicating passage 13b. Therefore, in the combined resistance Rm, the resistance values ​​of resistor elements R13, R14, R23, and R24 are zero. In the combined inductance Lm, the inductances of inductors L13, L14, L23, and L24 are zero. Therefore, the combined resistance Rm and combined inductance Lm are smaller than those in inkjet head 1e.

[0085] As described above, the ink ejection method of this embodiment is performed by the inkjet head 1, which is provided with a nozzle N and an ink flow path communicating with the nozzle N, and the ink flow path includes a pressure chamber 12 that applies pressure fluctuations to the ink therein. This ink ejection method is performed by ejecting ink having a viscosity of 5.7 mPa s and a density of 1080 kg / m ３ , surface tension of 41 mN / m, and sound velocity of 1521 m / s, the following conditions are satisfied: (1) The inkjet head 1 has a Qc of 10 or more and 18 or less, related to the pressure vibration of the ink in the pressure chamber 12. (2) The resonance period Tm of the ink meniscus vibration in the nozzle N is 1 or more and 6 or less times the resonance period Tc of the ink pressure vibration in the pressure chamber 12. (3) The nozzle diameter φ or a diameter φe equivalent to the nozzle diameter φ is 20 μm or more. (4) The width between the nozzle opening surface of the inkjet head 1 and the ink landing surface is 5.0 mm or more. This ink ejection method can reduce meniscus breakage, variations in landing position, and the occurrence of satellites, regardless of the specific shape of the inkjet head 1. Therefore, this ink ejection method can stably eject ink while ensuring the width between the ink ejection surface and the ink landing surface.

[0086] This ink ejection method may also be a multi-drop method in which multiple droplets are ejected consecutively onto one pixel. This reduces the amount of ink per pixel, particularly the size of the last droplet ejected, making it easier to reduce satellites.

[0087] Furthermore, in this ink ejection method, the distance between the nozzle opening surface of the inkjet head 1 and the ink landing surface may be set to 10.0 mm or more. Under the above conditions, stable ink ejection is possible up to a width of 10 mm. If the distance between the nozzle opening surface and the ink landing surface is set sufficiently large in this way, images can be recorded on a wider variety of surfaces.

[0088] The range may also include a range in which the ejection volume from one nozzle is 20 pL or more and 30 pL or less, and the drive cycle for ink ejection is 90 μsec or more and 100 μsec or less. That is, with this ink ejection method, ink can be stably ejected with an ejection volume of 20-30 pL from one nozzle and a drive cycle of about 10-11 kHz. Therefore, this ink ejection method can quickly obtain desired images with appropriate quality through high-speed recording operations required for image recording operations in industrial applications, etc.

[0089] Furthermore, the inkjet head 1 of this embodiment satisfies the structural conditions relating to the ink ejection method described above.

[0090] Furthermore, the inkjet heads 1d to 1f may have a recovery flow path 22 that recovers ink that is not ejected from the nozzles N. In this case, the structure of the ink flow path may be symmetrical with respect to the central axis of the nozzle N. In this way, the recovery flow path side is also used for ejecting ink, so the cross-sectional area of ​​the flow path related to ink ejection appears to be larger. Therefore, this inkjet head 1 is likely to have a small resonance frequency Tm. Accordingly, the inkjet head 1 is likely to have a nozzle diameter φ of 20 μm or more.

[0091] Note that the contents of the present disclosure are merely examples, and various modifications are possible. For example, the above description is based on the premise that the ejection volume from one nozzle is 20-30 pL and the drive frequency is approximately 10 to 11 kHz, but the ejection volume and drive frequency may be outside these ranges.

[0092] Furthermore, the number of consecutive ejections in the multi-drop method does not have to be eight. Any suitable number of ejections may be used. Furthermore, a suitable waveform that reduces reverberation vibrations may be added to the ejection waveform. Alternatively, ink ejection does not have to be performed using the multi-drop method.

[0093] Furthermore, the ink recovered by the recovery passage does not necessarily have to be returned to the ink tank, but may be discarded as is, or may be temporarily stored separately and then subjected to some other processing.

[0094] Furthermore, in the above-described embodiment of the side shooter, the ink flow path is symmetrical with respect to the central axis of the nozzle N, but this is not limited to this. The orientation, shape, and size of the ink flow path may be different as long as it does not adversely affect ink ejection. Furthermore, the specific configurations, structures, and settings shown in the above-described embodiment may be modified as appropriate without departing from the spirit of this disclosure. The scope of the present invention includes the scope of the invention set forth in the claims and their equivalents.

[0095] The present disclosure can be used in an ink ejection method and an inkjet head.

[0096] 1, 1a to 1f inkjet head 11 common supply flow path 12 pressure chamber 12a first pressure chamber 12b second pressure chamber 13 upper communication path 13a first upper communication path 13b second upper communication path 14 lower communication path 14a first lower communication path 14b second lower communication path 15, 15b piezoelectric element 16 vibration plate 21 common discharge flow path 22 recovery flow path N nozzle

Claims

1. A method for ejecting ink using an inkjet head comprising a nozzle and an ink flow path communicating with the nozzle, the ink flow path including a pressure chamber for applying pressure fluctuations to the ink therein, the inkjet head ejecting ink having a viscosity of 5.7 mPa s and a density of 1080 kg / m ３ an ink ejection method, wherein, when the surface tension is 41 mN / m and the speed of sound is 1521 m / s, a Q value associated with the pressure vibration of the ink in the pressure chamber is 10 or more and 18 or less, a resonance period associated with the meniscus vibration of the ink in the nozzle is 1 to 6 times the resonance period associated with the pressure vibration of the ink in the pressure chamber, the nozzle opening diameter or a diameter equivalent to the opening diameter is 20 μm or more, and the width between the ink ejection surface and the ink landing surface of the inkjet head is 5.0 mm or more.

2. The ink ejection method according to claim 1, wherein the ink is ejected by a multi-drop method in which a plurality of droplets are ejected continuously for one pixel.

3. The ink ejection method according to claim 1, wherein the width between the ink ejection surface of the ink jet head and the ink landing surface is 10.0 mm or more.

4. The ink ejection method according to claim 3, wherein the ejection volume from one nozzle is between 20 pL and 30 pL, and the drive cycle for ink ejection is within the range of between 90 μsec and 100 μsec.

5. An ink-jet head according to any one of claims 1 to 4.

6. The inkjet head according to claim 5, wherein the ink flow path has a recovery flow path that recovers ink not ejected from the nozzle, and the structure of the ink flow path is symmetrical with respect to the central axis of the nozzle.

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