Droplet-discharging head and drive control method

Abstract

Provided is a droplet-discharging head and drive control method whereby image recording operations can be more flexibly implemented while suppressing reductions in image quality. An inkjet head (1), which is a droplet-discharging head, comprises: an individual flow channel (F) through which ink passes and which includes a pressure chamber (P) that stores the ink and applies pressure fluctuations; and a nozzle (N) that communicates with the individual flow channel (F) and discharges droplets of a liquid to which the pressure fluctuations have been applied. The inkjet head (1) has a structure configured such that the refill Q value related to the vibration of the liquid surface at the nozzle (N) is 1.17 or greater in a case in which the viscosity of the liquid is 5.7 mPa·s, the liquid density is 1080 kg / m3, the speed of sound in the liquid is 1521 m / s, and the surface tension is 42 mN / m.

Description

Technical Field

[0001] The present invention relates to a droplet ejection head and a drive control method.Background Art

[0002] A droplet ejection head records an image by ejecting droplets, such as ink droplets, from nozzles and landing the droplets on a target recording medium. According to such a droplet ejection head, minute droplets called satellites may be generated in addition to droplets to be ejected. Herein, an image includes a film and a planar structure. When the satellites adhere to the recording medium or the surroundings, such as the droplet ejection head, the satellites may decrease the quality of a formed image or affect normal droplet ejection of the droplet ejection head.

[0003] Patent literature 1 discloses a technique for suppressing satellites by devising drive waveforms of a multi-drop type inkjet recording apparatus that unites a plurality of discharged droplets and lands the united droplets on the same position (pixel) of a target.Citation ListPatent Literature

[0004] Patent Literature 1: Japanese Unexamined Patent Publication No. 2021-020338Summary of InventionTechnical Problem

[0005] A droplet ejection apparatus ejects and lands droplets on a recording medium in a state where the nozzle surface and the recording medium are not in contact with each other and are close to each other. Therefore, the distance between the nozzle surface and the recording medium needs to be accurately adjusted. When the distance is increased, the satellites give a greater influence on image quality and are likely to decrease image quality.

[0006] An object of the present invention is to provide a droplet ejection head and a drive control method capable of performing an image recording operation more flexibly while suppressing a decrease in image quality.Solution to Problem

[0007] In order to achieve the above object, the present invention is a liquid ejection head including: a liquid channel that passes liquid and includes a pressure chamber, the pressure chamber storing the liquid and applying a pressure fluctuation; and a nozzle that communicates with the liquid channel and ejects a droplet of the liquid to which the pressure fluctuation has been applied, wherein a refill Q factor related to oscillation of a liquid surface in the nozzle is 1.17 or greater when a viscosity of the liquid is 5.7 mPa·s, a liquid density is 1,080 kg / m 3< , a speed of sound in the liquid is 1,521 m / s, and a surface tension is 42 mN / m.Advantageous Effects of Invention

[0008] According to the present invention, it is possible to more flexibly perform an image recording operation while suppressing a decrease in image quality.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] [FIG. 1A] This is a diagram illustrating a cross section of an ink channel of an inkjet head according to the present embodiment. [FIG. 1B] This is a diagram illustrating an equivalent circuit of the ink channel of the inkjet head. [FIG. 2A] This is a diagram illustrating a driving operation for ejecting ink. [FIG. 2B] This is a diagram illustrating an example of a drive waveform. [FIG. 3A] This is a diagram illustrating an example of an image to be formed and used for judging image quality. [FIG. 3B] This is a diagram illustrating an example of an image to be formed and used for judging image quality. [FIG. 3C] This is a table showing part of a standard related to image quality judgement. [FIG. 4] This is a table showing combination examples of values of an equivalent circuit according to refill Q factors. [FIG. 5A] This is a table showing examples of results of image quality inspection. [FIG. 5B] This is a table showing examples of results of image quality inspection. [FIG. 6] is a diagram showing a relation between the refill Q factor and the maximum droplet speed of ink ejection by a multi-drop method. [FIG. 7] This is a diagram showing the influence of the reverberation oscillation of the meniscus caused by the first ink ejection among two ink ejections on the droplet speed of the second ink ejection. [FIG. 8] This is a diagram showing another example of a drive voltage waveform related to ink ejection. [FIG. 9A] This is a diagram showing another example of the ink channel. [FIG. 9B] This is a diagram for explaining another example of the equivalent circuit of the ink channel. [FIG. 10A] This is a diagram showing another example of the ink channel. [FIG. 10B] This is a diagram for explaining another example of the equivalent circuit of the ink channel. [FIG. 11A] This is a diagram for explaining another example of the ink channel. [FIG. 11B] This is a diagram for explaining another example of the equivalent circuit of the ink channel. DESCRIPTION OF EMBODIMENTS

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0011] FIG. 1A and FIG. 1B are diagrams illustrating an ink channel of an inkjet head 1 according to the present embodiment.

[0012] FIG. 1A is a cross-sectional view of the ink channel. In the inkjet head 1 (droplet ejection head), ink (liquid) flows from an ink tank into individual channels F (liquid channels) that communicate with individual nozzles N via a common ink channel or a manifold. The inflow ink is supplied to the nozzle N through the individual channel F.

[0013] The individual channel F includes a pressure chamber P. The pressure chamber P is a part that applies pressure fluctuation to the ink in the pressure chamber P by the deformation of the pressure chamber P. The pressure chamber P is wider than the other parts of the individual channel F to have an appropriate volume. The pressure chamber P temporarily stores the ink. The pressure chamber P has, for example, a circular shape or a rectangular shape with rounded corners in plan view but is not particularly limited thereto. An oscillation plate and a piezoelectric element are positioned along the upper surface of the pressure chamber P having such a plan view shape. When a voltage having a voltage pattern corresponding to a pressure fluctuation pattern to the ink is applied to the piezoelectric element, the piezoelectric element is deformed, so that the pressure chamber P is deformed.

[0014] The nozzle N may have a tapered shape (truncated cone shape) the diameter of which becomes smaller toward the ink ejection port (nozzle opening) at the tip end, although not particularly limited. In plan view (bottom view), the nozzle opening has a circular shape having a diameter of D0. When the distance from the nozzle opening in a direction perpendicular to the nozzle opening surface in the nozzle N is defined as x, the nozzle diameter (diameter) at the x is expressed as D(x). D(x) is greater than or equal to D0 and is a linear function that changes depending on x. D0 is determined according to the resolution of a image to be recorded, the ink droplet amount to be ejected, and so forth. In the image recording operation that takes a large gap, which is targeted by the inkjet head 1 of the present embodiment, if the ink droplet amount is small, the droplets tend to greatly decelerate while flying due to air resistance. Therefore, it is preferable that the droplet amount ejected by one time of ink ejection on a pixel, namely the droplet amount per dot, be 10 pL or greater, for example. In a multi-drop method to be described later, the droplet amount herein is the total of the multi drops in one cycle.

[0015] The other part of the individual channel F basically has a prism shape or a cylinder shape of a uniform thickness. Herein, the individual channel F extends in the laminated substrates. The size of the individual channel F, which is expressed by a cross-sectional area, a cross-sectional shape, and a length, is generally determined for each of the substrates. For example, the nozzle N is at the nozzle substrate 11. The nozzle substrate 11 is made of metal or resin, for example, but is not limited thereto. The pressure chamber P is positioned in the pressure chamber substrate 14.

[0016] The channel substrate 12, the intermediate substrate 13, and so forth are positioned between the nozzle substrate 11 and the pressure chamber substrate 14. These are glass (silicon dioxide) substrates and / or metal substrates of SUS or 42 alloy, for example. A downstream individual channel L extends in the channel substrate 12 and the intermediate substrate 13 and connects the pressure chamber P to the nozzle N.

[0017] The spacer substrate 15 is positioned on the upper side of the pressure chamber substrate 14. The spacer substrate 15 has a hollow portion, and the hollow portion includes the pressure chamber P in a plan view position. The oscillation plate 51 is located at the boundary between the spacer substrate 15 and the pressure chamber P. The piezoelectric element 52 interposed between electrodes is positioned in the cavity portion on the upper surface side of the oscillation plate 51. When the piezoelectric element 52 is deformed by application of a voltage, the deformation of the piezoelectric element 52 deforms the oscillation plate 51 and changes the volume of the pressure chamber P. Thus, pressure fluctuation is given to the inside of the pressure chamber P. The oscillation plate 51 may be a conductive metal member and may serve as one of the electrodes sandwiching the piezoelectric element 52. The piezoelectric element 52 is not particularly limited but is lead zirconate titanate (PZT), for example.

[0018] The wiring substrate 16 is positioned on the upper side of the spacer substrate 15. The electrodes sandwiching the piezoelectric element 52 are connected to the wiring of the wiring substrate 16 via bumps or the like. The upper surface of the wiring substrate 16 is connected to a drive substrate. The drive substrate inputs a drive voltage signal related to a voltage to be applied to the piezoelectric element 52. The drive substrate is not particularly limited but may be a flexible printed circuit (FPC), for example. In the pressure chamber substrate 14, the spacer substrate 15, and the wiring substrate 16, an upstream individual channel U extends and communicates with the pressure chamber P. At the upper surface of the wiring board 16, the upstream individual channel U is connected to a manifold, a common ink channel, and so forth that store and send out ink supplied to the nozzles N. The substrates may be bonded to each other with an adhesive or the like.

[0019] In the individual channel F, pressure fluctuation applied to ink by deformation of the pressure chamber P is transmitted and reflected to become oscillation. By this vibration, the ink projects from the nozzle N, separates, and is ejected as an ink droplet. On the other hand, in the nozzle N, the position of the ink liquid surface changes according to the pressure fluctuation, a decrease in ink due to separation of ink droplets, and supply of ink to cover the decrease, and so forth. By the surface tension, a force is applied in such a direction that maintains an appropriate position and shape of the ink liquid surface (meniscus).

[0020] In the equivalent circuit of the ink channel in FIG. 1B, a parameter of the oscillation is expressed by a combination of electrical elements of resistor elements, capacitor elements (capacitors), and inductive elements (coils). As described above, the upstream individual channel U, the downstream individual channel L, and the nozzle N are connected in series with the pressure chamber P interposed therebetween. The upstream individual channel U and the downstream individual channel L may be further divided into channels of the respective substrates and displayed. As the entire equivalent circuit, a combined resistance of the multiple resistor elements, a combined inertance of the multiple inductive elements, and so forth may be obtained.

[0021] Among the oscillation characteristics represented by the equivalent circuit, the oscillation of the ink liquid surface (meniscus) in the nozzle N is important for properly ejecting ink, in particular for suppressing satellites. A once-applied pressure fluctuation remains while being attenuated for a certain period according to oscillation characteristics. If the damped oscillation remains at an appropriate magnitude after ink ejection, satellites are suppressed. On the other hand, if the damped oscillation remains excessively, the remaining oscillation is added to the oscillation for the next cycle of ink ejection. This may affect continuous ejection.

[0022] oscillation characteristics related to the oscillation of the liquid surface, such as a resonance frequency, are variables corresponding to the structure of the individual channel F and the characteristics (e.g., viscosity) of the ink. As described above, the shape and the position of the liquid surface (meniscus) are maintained by the surface tension of the ink. That is, the surface tension influences the restoring force of the liquid surface after ink ejection. Therefore, it can be said that stable ink ejection is enabled by an inkjet head that has individual channels F configured to have appropriate oscillation characteristics according to characteristics of ink to be ejected.

[0023] As a parameter related to the oscillation (damping thereof), a Q factor is known. The Q factor is a dimensionless quantity parameter that becomes smaller as the energy loss in one cycle of oscillation in energy of the system is greater. When the Q factor is too small, the energy loss is large; the oscillation quickly converges; and further, the pressure fluctuation itself related to the original ink ejection is suppressed. When the Q factor is too large, the energy loss is small, and the oscillation remains for a long time for one drive pulse. In the present disclosure, the Q factor related to the liquid surface oscillation is referred to as a refill Q factor.

[0024] The value Q of the refill Q factor is obtained as follows for the individual channel F. Q = ω ⋅ Ln / Rn

[0025] Here, Ln is the combined inertance of the m inductors Lm included in the above equivalent circuit. That is, in a case where the inductors Lm are connected in series, the following is applied. Ln = ∑ j = 1 to m Lj

[0026] In a case where the inductors Lm are connected in parallel, the following is applied. 1 / Ln = ∑ j = 1 to m 1 / Lj

[0027] Rn is the combined resistance of the k resistances Rk included in the equivalent circuit. That is, when the resistances Rk are connected in series, the following is applied. Rn = ∑ j = 1 to k Rj

[0028] When the resistors Rk are in parallel, the following is applied. 1 / Rn = ∑ j = 1 to k 1 / Rj

[0029] Cn is the compliance of the ink liquid surface.

[0030] The angular frequency ω is a value of damped oscillation generated in the RLC series circuit and is expressed as follows. ω = 2 π / T = 1 / Ln ⋅ Cn − Rn / 2 Ln 2 1 / 2

[0031] T is the oscillation period of the liquid surface (meniscus) in the nozzle N, namely the oscillation period of the damped oscillation in the RLC series circuit. Therefore, the refill Q factor is as follows. Q = 1 / Rn ⋅ Ln / Rn − Rn / 2 2 1 / 2

[0032] As described above, in the taper-shaped nozzle N, the position of the liquid surface changes with the capillary force caused by the surface tension of the droplet as the restoring force. In this case, the compliance Cn related to the ink liquid surface in the nozzle N is obtained by an average in the oscillation period of Cn (x) = π· D (x) 4< / (128·σ). That is, the compliance Cn is expressed by multiplying the value D (0) = the fourth power of D0 when x = 0 by a constant π / (128·σ). Here, σ is the surface tension of the ink and determined according to the ink. X is the distance from the opening end of the nozzle N. Therefore, the refill Q factor is expressed as follows. Q = 1 / Rn ⋅ A ⋅ Ln / D 0 4 − Rn 2 / 4 1 / 2

[0033] Here, A is the reciprocal of the above constant and expressed as A = 128·σ / π.

[0034] When part of the individual channel F has a cylindrical shape, the resistance value R and the inertance L of the part are analytically obtained as follows. R = 4 ⋅ ρ ⋅ l / π ⋅ d 2 L = 128 ⋅ η ⋅ l / π ⋅ d 4

[0035] Here, H is the viscosity of the ink; ρ is the density of the ink; d is the radius of the cylinder; and 1 is the length of the cylinder.

[0036] When part of the individual channel F has a prismatic shape, the resistance value R and the inertance L of the part are analytically obtained as follows. R = 8 ⋅ η ⋅ a + b 2 ⋅ w / a 3 ⋅ b 3 L = ρ ⋅ w / a ⋅ b

[0037] Here, "a" and "b" are the lengths of two sides of the cross section of the prism channel, and "w" is the length of the prism channel.

[0038] Alternatively, when part of the individual channel F has a tapered shape (truncated cone), the resistance value R and the inertance L of the part are analytically obtained as follows. R = 128 ⋅ η / 3 π ⋅ w / φ 2 − φ 1 ⋅ φ 1 − 3 − φ 2 − 3 L = 4 ⋅ ρ / π ⋅ w / φ 2 − φ 1 ⋅ φ 1 − 1 − φ 2 − 1

[0039] Here, Φ 1 and φ2 are the base diameter and the top diameter of the truncated cone, respectively.

[0040] The resistance value R and the inertance L of a portion having a complicated shape, such as the pressure chamber P, can be obtained using numerical simulation. Here, the resistance values R and the inertances L of the respective parts arranged in series are combined to obtain a combined resistance Rn and a combined inertance Ln of the individual channel F. As described above, the refill Q factor of the individual channel F is obtained from the parameters (a, b, w, φ1, φ2, and D0) related to the shape of the individual channel F and the parameters (σ, η, and ρ) related to the characteristics of the ink.

[0041] In addition to the above, a channel resistance or the like may occur at the bent portion of the ink channel and so forth. Since such a channel resistance is smaller than the above in a general individual channel F, it is not considered here. However, such a channel resistance can be taken into consideration to obtain the refill Q factor.

[0042] The type of the ink, namely the characteristics of the ink can be determined independently of the inkjet head. However, many inkjet heads, particularly industrial inkjet heads, generally have their corresponding inks to be used. Therefore, it is sufficient that the properties of the corresponding ink be considered.

[0043] The range of an appropriate refill Q factor is determined by causing the inkjet head to eject ink while changing the refill Q factor of the inkjet head and examining the ejection characteristics.

[0044] FIG. 2A and FIG. 2B are diagrams illustrating a driving operation for ejecting ink.

[0045] FIG. 2A illustrates the flow of a drive signal. In the inkjet recording apparatus having the inkjet head 1, the head drive section 5 performs an operation of deforming the piezoelectric element 52 under the control of the signal controller 41 of the head drive controller 4. The signal controller 41 includes a processor, such as a central processing unit (CPU), and performs control operation related to image recording operation. The signal controller 41 may be a general-purpose CPU of the inkjet recording apparatus or may be a dedicated CPU different from the general-purpose CPU. In this case, the signal controller 41 may be located together with the drive circuit 50 on the drive board.

[0046] The head drive section 5 includes the drive circuit 50 on the drive board and the piezoelectric element 52. When the drive circuit 50 outputs a drive voltage signal having an appropriate waveform (drive waveform) to the piezoelectric element 52, the piezoelectric element 52 deforms such that pressure fluctuation for ejecting ink is generated on the ink.

[0047] The drive circuit 50 includes a signal generation section 53. The signal generation section 53 converts the digital waveform into an appropriate analog waveform, amplifies power (voltage and current), and outputs the analog waveform under the control of the signal controller 41. The output drive voltage signal is selectively output to the piezoelectric element 52 corresponding to the nozzle N to eject ink, based on image data.

[0048] FIG. 2B is a diagram illustrating an example of the drive waveform.

[0049] In the method of driving the piezoelectric element 52 for ink ejection according to the present embodiment, ink ejection is done by a one-drop method or a multi-drop method. In the one-drop method, one droplet is ejected by one drive pulse. The multi-drop method uses such a drive waveform that applies multiple drive pulses to unite multiple droplets ejected from the nozzle N and causes the united droplet to land at the same pixel position. The inkjet head 1 may be configured to use both methods or may be mainly specialized for either of the methods. Here, the drive waveform of the multi-drop method will be described.

[0050] For example, three drive pulses per pixel (dot) are output at an interval of twice the acoustic length (AL). The AL is half the resonance period (acoustic resonance period) of pressure oscillation that occurs on the ink in the pressure chamber P. Three drive pulses are output in synchronization with the resonance period, so that the respective ink surface oscillations are utilized to efficiently eject the ink. The resonance period is determined according to the ink ejection frequency required for the inkjet head. A specific resonant frequency is 10 to 250 µs, for example. The currently common range thereof is 70 µs or less. In practice, the interval between rises of the drive pulses, namely the time between rising start timings of the adjacent drive pulses may be slightly deviated from 2AL. For example, the interval is set to about 1.8AL or greater and 2.3AL or less.

[0051] On the other hand, the duration of each drive pulse, namely the time between the timing at which the voltage starts rising to the timing at which the drive voltage ends is 1.2AL, which is slightly longer than the AL. Accordingly, the oscillation of the liquid surface caused by the preceding drive pulse is suppressed from remaining excessively. However, when the width of the drive pulse greatly deviates from 1.0AL, pressure fluctuation is not appropriately applied to the ink. Therefore, it is preferable that the width of the drive pulse be in the range of about 0.8AL to 1.3AL. The drive pulse is, for example, a rectangular wave, but may be a trapezoidal wave. In the case of the trapezoidal wave, the ratio between the voltage change period and the period of the constant drive voltage may be determined as appropriate.

[0052] Of these three drive pulses, the voltage amplitude V1 of the third pulse is greater than the voltage amplitude V2 of the first and second pulses. Thus, the ink droplet that is ejected last certainly catches up with and unites with the preceding ink droplets. It is preferable that the absolute values of the voltage amplitudes V1 and V2 be great, namely that the droplet speed be high, within a range of not causing abnormal ink ejection.

[0053] By such drive pulses, the satellites are suppressed, and the ink is stably ejected. However, how much influence of the satellites on image quality is suppressed ultimately depends on the structure of the inkjet head 1, the properties of ink, and the like. It is possible to judge how much appropriate ink ejection is performed with little degradation in image quality, based on the image quality of an image formed by the ink ejection.

[0054] FIG. 3A to FIG. 3C are diagrams illustrating image quality determination.

[0055] FIG. 3A and FIG. 3B are examples of images to be formed and used for image quality determination. FIG. 3A shows a one-dimensional barcode (code 1), and FIG. 3B shows a two-dimensional code (code 2). When these codes are not accurately shown, a problem may occur in reading them. Therefore, evaluation criteria are defined. The quality evaluation standards for one-dimensional barcodes are defined by ISO15416 (JIS X 0520). The quality evaluation standards for two-dimensional codes are defined by ISO15415 (JIS X 0521).

[0056] FIG. 3C shows partial contents of ISO15415. The inspection contents include the error ratio in a region where positions of white cells and black cells are fixed, such as the contrast between white cells and black cells, imbalance, finder pattern, quiet zone, alignment pattern, and timing pattern, reading abnormality, distortion amount of the two-dimensional code itself, variations in size of each cell (relative uniformity), and the utilization rate of an error correction code (data restoration code) used in reading of stains and losses. Each of the items is evaluated by a numerical value of 0.0 to 4.0 (revised edition in 2016) or by five stages by alphabets of A to D and F (edition in 2000). Among the evaluations of the respective items, the stage of the lowest evaluation is determined to be the overall judgement.

[0057] FIG. 4 is a table showing the combined inertance Ln, the combined resistance Rn, and the compliance Cn of an equivalent circuit corresponding to each refill Q factor when inspection is performed according to the inspection standards of the above code 1 and code 2. The respective refill Q factors are obtained based on the above (Expression 8) or the like. To increase the refill Q factor, the shape of the individual channel F may be changed so as to decrease the combined resistance Rn or increase the combined inertance Ln as described above, for example.

[0058] FIG. 5A is a table showing the inspection result of the code 1 according to the inspection standards. FIG. 5B is a table showing the inspection result of the code 2 according to the inspection standards. These inspection results were obtained by performing tests using inkjet heads having different structural parameters related to the refill Q factor while changing the distance from the ink ejection face to the ink landing surface (image recording surface). The distance, namely the head / media gap, is hereinafter referred to as a gap. For simplification, the overall evaluation of C or better is regarded as OK. Even if the overall evaluation is D or worse, the image quality may not necessarily be NG. However, to stably obtain an image (image quality) having appropriate quality, it is preferable that the overall evaluation be C or better.

[0059] The ink used for image recording in this inspection has a viscosity of 5.7 mPa·s, a concentration (liquid concentration) of 1080 kg / m 3< , a speed of sound in the ink of 1,521 m / s, and a surface tension of 42 mN / m. The recorded code 1 and code 2 were read by a code reader "SR-1000" of Keyence Corporation (registered trademark). The code reader outputs the overall evaluation.

[0060] When the refill Q factor was 1.05, neither the code 1 nor the code 2 had satisfactory image quality. When the gap was 5 mm, a proper-quality image was obtained with the refill Q factor of 1.17. When the gap was 10 mm or 15 mm, a proper-quality image was obtained with the refill Q factor of 1.85 or greater.

[0061] On the other hand, when the refill Q factor was 2.44 and the gap was 15 mm, an appropriate quality was not obtained. When the refill Q factor was 2.32 or less, an image having satisfactory quality was obtained even when the gap was 15 mm. That is, from the viewpoint of a decrease in image quality related to satellites, it is preferable that the refill Q factor be 1.17 or greater, and it is further preferable that the refill Q factor be 1.85 or greater. In addition, it is further preferable that the refill Q factor be 2.32 or less.

[0062] FIG. 6 is a diagram illustrating the relation between the refill Q factor and the maximum droplet speed at which ink can be stably ejected by the multi-drop method.

[0063] Herein, the maximum droplet speed at which ink can be stably ejected is the speed at the distance of 0. 5 mm from the ink ejection face. It is shown that the maximum droplet speed is positively correlated with the refill Q factor. When the refill Q factor is 1.31 or greater, the maximum droplet speed is 5 m / s or greater. On the other hand, when the refill Q factor is less than 1.31, the maximum droplet speed is 5 m / s or less, which is unsatisfactory for practical use. It is considered that, if the refill Q factor is low in the multi-drop method, ink supply to the nozzle N cannot keep up with the high-speed injection where the interval between drive pulses is short and the AL is small. As a result, the ejection may become unstable. Therefore, in the multi-drop method in particular, it is preferable to add a condition that the refill Q factor is 1.31 or greater in addition to the above range of the refill Q factor. Similarly, in a case where the ink is ejected at a frequency corresponding to the multi-drop method, it is preferable that the refill Q factor be 1.31 or greater. In particular, in the case of high-speed ejection, it is preferable that the droplet speed be 7 m / s or greater at the point of the gap of 5 mm. It is therefore preferable that such an inkjet head 1 have the refill Q factor of 1.38 or greater.

[0064] On the other hand, even in the ink ejection by the one drop method, the interval between the drive pulses becomes short when the ink is ejected at a high frequency. As a result, reverberations of the liquid surface oscillation by the previous ink ejection may remain and affect the next ink ejection. It is therefore preferable that the inkjet head 1 have the above-described refill Q factor that allows an appropriate droplet speed, regardless of the ink ejection method.

[0065] FIG. 7 is a diagram showing the influence of the reverberation oscillation of the meniscus caused by the first ink ejection among two ink ejections on the droplet speed of the second ink ejection. FIG. 7 shows the difference between the ink droplet speed at the first ejection and the ink droplet speed at the second ejection with respect to the drive cycle related to the two ink ejections. The lines correspond to six different types of refill Q factors, respectively. The drive cycle is indicated, based on the AL.

[0066] The greater the refill Q factor is, the greater the difference in ink ejection droplet speed tends to be, and the more influence of the drive cycle tends to remain later. When the refill Q factor is 2.12 or less, the difference is about 1 m / s or less. Since the droplet speed is to be about 5 m / s or greater as described above, the difference of 1 m / s is 20% or less of the droplet speed. The influence of such a difference in the droplet speed on the image quality may be acceptable, although the influence also depends on the gap size, the conveyance speed of the recording medium, and the like. Therefore, in addition to the above-described range of the refill Q factor, it is preferable that the refill Q factor be 2.12 or less.

[0067] FIG. 8 is a diagram showing another example of the drive voltage waveform related to ink ejection.

[0068] Regardless of the structure of the inkjet head 1, it is preferable that such an ink ejection waveform or a drive voltage waveform be used that is less likely to cause satellites and that can stably and continuously eject ink, in particular in the multi-drop method. In another example of the drive voltage waveform, a pulse waveform having a voltage amplitude V3 greater than the voltage amplitude V1 of the last pulse waveform is added after the last pulse waveform of FIG. 2B at an interval of 4.0AL, which is twice the normal interval. In this case, the duration (pulse length) of the pulse waveform added to the last is 1.0AL. Further, the pulse length of the pulse waveform having the voltage amplitude V1, which is no longer the last pulse waveform, is set to 1.3AL, which is the same as the pulse length of the previous pulse waveform. According to such drive voltage waveforms, ink ejection can be more stably continued in the multi-drop method.

[0069] FIG. 9A, FIG. 9B, FIG. 10A, FIG. 10B, FIG. 11A, and FIG. 11B are diagrams illustrating other examples of the ink channel.

[0070] As illustrated in FIG. 9A, the specific structure of the ink channel may be different from the structure illustrated in the above-described embodiment. According to the individual channel F1 of the inkjet head 1a, a common ink channel Sc is positioned in the channel substrate 12a between the nozzle substrate 11a and the pressure chamber substrate 14a.

[0071] The ink flows into the pressure chamber P via the upstream individual channel U that separates from the common ink channel Sc. The ink to which pressure fluctuation is applied is ejected from the nozzle N via the downstream individual channel L. FIG. 9B is a diagram illustrating an equivalent circuit of this ink channel. The corresponding resistance value R, inertance L, and so forth are applied to each of the structural portions of the individual channel U, the pressure chamber P, the downstream individual channel L, and the nozzle N. By combining these, the refill Q factor can be obtained in the same manner as described above even if the structure is different.

[0072] For another example, as illustrated in FIG. 10A, the inkjet head 1b may be configured to apply pressure fluctuation using the piezoelectric element 52 that deforms in a shear mode. The ink having flowed into the individual channel Fb is supplied to the nozzle N through the upstream individual channel U, the pressure chamber P of the pressure chamber substrate 14b, and the downstream individual channel L of the intermediate substrate 13b. The piezoelectric element is positioned on a side surface of the pressure chamber P. The piezoelectric element deforms in a direction along the individual supply channel S in a shear mode and applies pressure fluctuation to ink in the individual channel Fb of the pressure chamber substrate 14b. Thus, for the inkjet head 1a having a different deformation mode of the piezoelectric element 52, the refilling Q factor can be obtained according to the parameters of the respective structural portions illustrated in FIG. 10B.

[0073] FIG. 11A shows another example of the ink channel of an inkjet head 1c. The ink flowing into the upstream individual channel U from the common ink channel Sc is ejected from the nozzle N of the nozzle substrate 11c through the pressure chamber P of the pressure chamber substrate 14c and the downstream individual channel L of the intermediate substrate 13c. In addition, the ink may be separated into the individual discharge channels E1 and E2 in the channel substrate 12c and the intermediate substrate 13c. The ink that has passed through the common discharge channels Ec1 and Ec2 may be returned to the ink tank. The inkjet head 1c having such circulation channels can swiftly discharge entrained air bubbles, dust, and so forth and allows separation of components of stagnated ink in the individual supplying channels S including the upstream individual channel U, the pressure chamber P, and the downstream individual channel L. Therefore, the inkjet head 1c can perform image recording operation more stably.

[0074] The equivalent circuit corresponding to the nozzle N, the individual supply channel S, and the individual discharge channels E1, E2 included in the individual channel Fc can also be taken into consideration for such a inkjet head 1c. In this case, as shown in FIG. 11B, the individual discharge channels E1 and E2 are positioned in parallel with the individual supply channel S in the individual channel Fc. The refill Q factor is obtained using the parameters of the resistance elements, the capacitors, and the inductive elements of the circuit configuration and the characteristics of the ink.

[0075] The application of the individual discharge channels E1, E2 and the common discharge channels Ec1, Ec2 is not limited to the individual channel Fc that deforms in the shear mode and applies pressure fluctuation to the ink. These may be applied to the individual channels F and Fa and so forth that deform in the above-described slack mode to apply pressure fluctuation to ink.

[0076] As described above, the inkjet head 1 of the present embodiment includes: the individual channel F that passes liquid (ink) and includes the pressure chamber P that stores ink and applies pressure fluctuation; and the nozzle N that communicates with the individual channel F and ejects droplets of the ink to which the pressure fluctuation is applied. The inkjet head 1 is configured such that the refill Q factor related to oscillation of the liquid surface in the nozzle N is 1.17 or greater when the viscosity of liquid is 5.7 mPa·s, the liquid density is 1080 kg / m 3< , the speed of sound in liquid is 1521 m / s, and the surface tension is 42 mN / m.

[0077] The inkjet head 1 having such a structure can eject ink while reducing the influence of satellites on image quality, even when the distance between the nozzle opening and the recording medium is increased to about 5 mm, which is greater than a conventional distance of about 1 mm. Thus, the inkjet head 1 can suppress deterioration in image quality of images to be recorded. Thus, the inkjet head 1 can be used for various image recording operations more flexibly than in the related art.

[0078] As described above, the structure of the inkjet head 1 is determined, based on the characteristics of liquid as a reference. Therefore, when the characteristics of liquid differ, the actual refill Q factor also changes. By using the inkjet head 1 of the present disclosure, it is possible to somewhat obtain the effect of reducing the influence of the satellites on image quality while widening the gap between the nozzle opening and the recording medium in the range of the generally used ink.

[0079] More preferably, the refill Q factor is 1.38 or greater. When the refill Q factor is increased to this extent, the ink can more appropriately fly the gap of about 5 mm at a high speed. Thus, the inkjet head 1 can more stably and flexibly eject ink for a wide gap.

[0080] More preferably, the refill Q factor is 1.85 or greater. With such a refill Q factor, the distance between the nozzle opening and the recording medium can be further increased to about 15 mm. Therefore, according to the inkjet head 1, it is possible to stably perform recording operation without trouble while suppressing a decrease in image quality, even on a three-dimensional recording medium having unevenness.

[0081] On the other hand, the inkjet head 1 is configured such that the refill Q factor is 2.32 or less under the same configuration and ink conditions described above. Thus, the inkjet head 1 can stably perform image recording operation with a wide gap while suppressing a decrease in image quality, no matter whether the multi-drop method or the one-drop method is used. Thus, the inkjet head 1 can flexibly record images on a wide variety of recording media.

[0082] More preferably, the refill Q factor is 2.12 or less. In addition to stably suppressing the influence of satellites on image quality in a wide gap, it is possible to suppress the influence on the subsequent droplet speed even when droplets are continuously discharged. Thus, it is possible to appropriately suppress a decrease in image quality even when ink is ejected at a high speed.

[0083] Further, according to the drive control method of the inkjet head 1 of the present embodiment, multiple drive pulses are applied to eject droplets toward the same pixel position by the multi-drop method. The drive waveform of the multi-drop method is set such that the width of each of the drive pulses is 0.8 or greater and 1.3 or less of the AL, and the interval between rises of the drive pulses is 1.8 or greater and 2.3 or less of the AL. With such a drive pulse width, it is possible to certainly unite liquid droplets in the multi-drop method, to suppress the influence of satellites on image quality, and to stably land an appropriate droplet amount at a desired position. By ejecting ink from the inkjet head 1 of the present embodiment by such a drive control method, it is possible to stably eject ink while more certainly suppressing the influence of satellites compared to the related art, even when the gap is increased. Thus, according to the drive control method, it is possible to record images more flexibly while suppressing a decrease in image quality.

[0084] Further, in the drive waveform of the multi-drop method, the last drive pulse among the multiple drive pulses may rise at an interval of 4.0 AL or greater from the rise of the drive pulse that is immediately before the last drive pulse. In the multi-drop method, influences of drive pulses are superposed on a later drive pulse, so that satellites tend to occur between ejected droplets. According to the driving control method, the interval between the last two drive pulses is greater than usual to suppress remaining satellites in the end. Therefore, most of the ejected ink can be united and land on the recording medium. By applying such an ejection control method to the inkjet head 1 of the present embodiment, it is possible to record appropriate images while more stably suppressing satellites even when the gap is widened.

[0085] Further, in this control method, the drive waveform may be determined such that the droplet speed at the position of 0.5 mm from the opening end of the nozzle N is 7 m / s or greater. When the flying speed is low, and particularly when the gap is wide, ink flies for a longer time. As a result, the ink may receive a greater influence of outside air flow and so forth during flight. When the droplet speed is about 7 m / s or greater, the ejected ink can more stably land on an appropriate position, and images having appropriate image quality can be formed.

[0086] Further, in this control method, the drive waveform may be determined such that the droplet amount to be ejected per dot is equal to or greater than 10 pL. When the droplet amount is small, and particularly when the droplet flies a wide gap, the droplet is likely to greatly decelerate by air resistance. By determining the drive waveform such that the droplet amount is equal to or greater than 10 pL, the degree of deceleration can be suppressed even when the droplet receives air resistance. Thus, the drive control method can suppress a decrease in image quality when the inkjet head 1 of the present embodiment lands ink on the recording medium with a wide gap.

[0087] The present invention is not limited to the above embodiment, and various modifications can be made.

[0088] For example, as described above, factors of a decrease in image quality when the gap is widened include the droplet size and the droplet speed, in addition to the occurrence of satellites. Therefore, the lower limit value and / or the upper limit value of the refill Q factor may be determined in consideration of preferential conditions, based on the gap, the droplet size, whether to use the multi-drop method, and so forth that are required for recording images or recording media.

[0089] Furthermore, the drive waveform in the multi-drop method described above is an example. Drive signals having other drive waveforms may be generated and output. Furthermore, although trapezoidal drive pulses are combined in the above embodiment as an example, the present invention is not limited to this. The drive pulses may be rectangular wave pulses. In the above description, although the drive pulses change from a reference voltage to the positive side only, the present invention is not limited thereto. The drive pulses may change only to the negative side, or a drive pulse that changes to both the positive and negative sides may be combined.

[0090] Furthermore, the nozzle N may not have a tapered shape. The nozzle N may have a short cylindrical shape or the like. Furthermore, other shapes of the ink channel may be appropriately determined. The combined resistance and the inertance of each part may be obtained analytically or by numerical simulation, depending on the shape. In addition, the required combined resistance, inertance, and the like may not correspond to only the shape of the ink channel. That is, when a filter or the like is disposed, the combined resistance, the inertance, and so forth may be calculated in consideration of these.

[0091] In the above, pressure fluctuation is applied to ink by deformation of a piezoelectric element as an example. However, the present invention is not limited thereto, as long as the inkjet head 1 ejects ink from the nozzle N by applying pressure fluctuation to the ink in the ink channel. In this case, drive signals may be generated, based on a standard different from the above-described standard.

[0092] The signal controller 41, the signal generation section 53, and so forth may be configured as desired. Any configuration may be adopted as long as appropriate drive signals are generated and an output destination is selected according to image data.

[0093] In addition, the specific configurations, the contents and sequence of the processing operations, and the like described in the above embodiment can be appropriately changed without departing from the spirit and scope of the present invention. The scope of the present invention includes the scope of the invention described in the claims and the scope of equivalents thereof.Industrial Applicability

[0094] The present invention can be used for a droplet ejection head and a drive control method.Reference Signs List

[0095] 1, 1A to 1C Inkjet head 4 Head drive controller 41 Signal controller 5 Head drive section 50 Drive Circuit 51 Oscillation plate 52 Piezoelectric element 53 Signal generation section 11, 11A to 11C Nozzle substrate 12, 12A to 12C Channel substrate 13, 13B to 13C Intermediate substrate 14, 14A to 14C Pressure chamber substrate 15 Spacer substrate 16 Wiring substrate E1, E2 Individual discharge channel Ec1, Ec2 Common discharge channel F, Fa to Fc Individual channel L Downstream individual channel N Nozzle P Pressure chamber S Individual supply channel Sc Common ink channel U Upstream individual channel V1 to V3 Voltage amplitude

Claims

1. A droplet ejection head comprising: a liquid channel that passes liquid and includes a pressure chamber, the pressure chamber storing the liquid and applying a pressure fluctuation; and a nozzle that communicates with the liquid channel and ejects a droplet of the liquid to which the pressure fluctuation has been applied, wherein a refill Q factor related to oscillation of a liquid surface in the nozzle is 1.17 or greater when a viscosity of the liquid is 5.7 mPa·s, a liquid density is 1,080 kg / m3, a speed of sound in the liquid is 1,521 m / s, and a surface tension is 42 mN / m.

2. The droplet ejection head according to claim 1, wherein the refill Q factor is 1.38 or greater.

3. The droplet ejection head according to claim 1, wherein the refill Q factor is 1.85 or greater.

4. The droplet ejection head according to claim 1, wherein the refill Q factor is 2.32 or less.

5. The droplet ejection head according to claim 2 or claim 3, wherein the refill Q factor is 2.32 or less.

6. The droplet ejection head according to claim 4, wherein the refill Q factor is 2.12 or less.

7. A drive control method for the droplet ejection head according to any one of claims 1 to 6, wherein in a multi-drop method in which droplets are ejected by multiple drive pulses toward an identical pixel position, a drive waveform of the multi-drop method is set such that: a width of each of the drive pulses is 0.8 AL (acoustic length) or greater and 1.3 AL or less; and an interval between rises of the drive pulses is 1.8 AL or greater and 2.3 AL or less.

8. The drive control method according to claim 7, wherein in the drive waveform of the multi-drop method, a last drive pulse among the drive pulses rises at an interval of 4.0 AL or greater from a rise of a drive pulse that is immediately before the last drive pulse.

9. The drive control method according to claim 7 or claim 8, wherein the drive waveform is set such that the liquid is ejected at a droplet speed of 7 m / s or greater, the droplet speed being at a position of 0.5 mm from an opening end of the nozzle.

10. The drive control method according to any one of claims 7 to 9, wherein the drive waveform is set such that a droplet amount to be ejected per dot is 10 pL or greater.

Images