Droplet ejection head and drive control method

Abstract

To provide a liquid droplet ejection head and a driving control method capable of more flexibly performing an image recording operation while suppressing a decrease in image quality. An inkjet head (1) as a liquid droplet ejection head includes: an independent flow path (F) that allows ink to flow and has a pressure chamber (P) that stores ink and imparts a pressure variation; and a nozzle (N) that communicates with the independent flow path (F) and ejects a liquid droplet of the liquid imparted with the pressure variation. The inkjet head (1) has a configuration in which a refill Q value related to vibration of a liquid surface in the nozzle (N) becomes 1.17 or more when the viscosity of the liquid is 5.7 mPa·s, the density of the liquid is 1080 kg / m 3 , the speed of sound in the liquid is 1521 m / s, and the surface tension is 42 mN / m.

Description

Technical Field

[0001] This invention relates to a droplet ejector head and a driving control method. Background Technology

[0002] In droplet ejectors that record images by ejecting droplets of ink or the like from a nozzle onto a recording medium, microdroplets, sometimes called satellites, are sometimes generated in addition to the droplets ejected from the object. The image referred to here includes coatings and planar structures. These satellites adhere to the recording medium, the droplet ejector, and the surrounding area, sometimes resulting in a decrease in image quality or adversely affecting the normal ejection of droplets using the droplet ejector.

[0003] Patent Document 1 discloses a technique for suppressing small droplets by designing a drive waveform in an inkjet recording device that uses a multi-droplet method to combine multiple ejected droplets into the same position (pixel) of an object.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2021-020338 Summary of the Invention

[0007] In droplet ejection devices, droplets are ejected and allowed to fall at a very close distance to the recording medium without contact with the nozzle surface. Therefore, precise adjustment of the distance between the nozzle surface and the recording medium is required. However, when this distance is increased, the impact of small droplets on image quality becomes more significant, and image quality is easily degraded.

[0008] The purpose of this invention is to provide a droplet ejector and a driving control method that can perform image recording operations more flexibly while suppressing image quality degradation.

[0009] To achieve the above objectives, the droplet ejector of the present invention comprises:

[0010] A liquid flow path, allowing liquid to flow, having a pressure chamber for storing the liquid and imparting pressure variations; and

[0011] The nozzle, connected to the liquid flow path, ejects droplets of liquid subjected to the pressure variation.

[0012] The refill Q-value associated with the vibration of the liquid surface in the nozzle is given when the liquid viscosity is 5.7 mPa·s and the liquid density is 1080 kg / m³. 3 When the speed of sound in a liquid is 1521 m / s and the surface tension is 42 mN / m, the structure becomes 1.17 or higher.

[0013] According to the present invention, it has the effect of being able to perform image recording actions more flexibly while suppressing image quality degradation. Attached Figure Description

[0014] Figure 1A This is a cross-sectional view showing the ink flow path of the inkjet head according to this embodiment.

[0015] Figure 1B This is a diagram showing the equivalent circuit of the ink flow path of the inkjet head.

[0016] Figure 2A This is a diagram illustrating the driving action used to eject ink.

[0017] Figure 2B This is a diagram showing an example of a driving waveform.

[0018] Figure 3A This is a diagram illustrating an example of an object image used in image quality assessment.

[0019] Figure 3B This is a diagram illustrating an example of an object image used in image quality assessment.

[0020] Figure 3C It is a chart that shows part of the standards related to image quality judgment.

[0021] Figure 4 This is a graph showing examples of combinations of values ​​for the equivalent circuit corresponding to the refilled Q value.

[0022] Figure 5A This is a chart showing an example of the results of an image quality check.

[0023] Figure 5B This is a chart showing an example of the results of an image quality check.

[0024] Figure 6 This is a graph showing the relationship between the refill Q-value and the maximum droplet velocity of ink ejection based on a multi-droplet method.

[0025] Figure 7 This is a graph showing the effect of the reverberation of the meniscus caused by the first ink ejection on the droplet velocity associated with the second ink ejection in two ink ejections.

[0026] Figure 8 This is a diagram showing other examples of drive voltage waveforms associated with ink ejection.

[0027] Figure 9A These are diagrams illustrating other examples of ink flow paths.

[0028] Figure 9B This is a diagram illustrating the equivalent circuit of other examples of ink flow paths.

[0029] Figure 10A These are diagrams illustrating other examples of ink flow paths.

[0030] Figure 10B This is a diagram illustrating the equivalent circuit of other examples of ink flow paths.

[0031] Figure 11A These are diagrams illustrating other examples of ink flow paths.

[0032] Figure 11B This is a diagram illustrating the equivalent circuit of other examples of ink flow paths. Detailed Implementation

[0033] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0034] Figure 1A as well as Figure 1B This is a diagram illustrating the ink flow path of the inkjet head 1 in this embodiment.

[0035] exist Figure 1A The image shows a cross-sectional view of the ink flow path. In the inkjet head 1 (droplet ejector), ink (liquid) flows from the ink tank through a common ink flow path or manifold into an independent flow path F (liquid flow path) that is connected to an individual nozzle N. The incoming ink is supplied to the nozzle N via this independent flow path F.

[0036] The independent flow path F includes a pressure chamber P. Pressure chamber P is a portion that imparts pressure variations to the ink within it through its deformation. It is wider than other parts of the independent flow path F and has a suitable volume to temporarily store ink. Pressure chamber P may be, for example, circular in plan view or rectangular with chamfered corners, but is not particularly limited to these. A vibrating plate and a piezoelectric element are provided along the upper surface of the pressure chamber P, which has such a plan view shape. A voltage is applied to the piezoelectric element in a voltage mode corresponding to the pressure variation mode supplied to the ink, causing it to deform, thereby deforming pressure chamber P.

[0037] Nozzle N is not particularly limited and can also be a conical shape (frustum shape) that tapers towards the ink ejection port (nozzle opening) at the front end. The nozzle opening is a circle with a diameter D0 when viewed from above (bottom view). The nozzle diameter (diameter) at which the distance from the nozzle opening to the direction perpendicular to the nozzle opening surface is x is denoted as D(x). D(x) is greater than or equal to D0, and here it becomes a linear function that varies depending on x. D0 is determined based on the resolution of the recorded image, the amount of ink droplets ejected, etc. In the image recording operation in this embodiment where the gap of the inkjet head 1 is set relatively wide, when the amount of ink droplets is small, it is easy to decelerate significantly due to air resistance during flight. Therefore, the amount of ink ejected once per pixel, i.e., the amount of droplets ejected per point, is preferably, for example, 10 pL or more. In the case of a multi-droplet method described later, the amount of droplets referred to here is the sum of the amounts of the multiple drops over one cycle.

[0038] The independent flow paths F, excluding these, are generally prisms or cylinders of uniform thickness. Here, the independent flow paths F extend within the stacked substrates, and for each portion of each substrate, their dimensions, expressed in terms of cross-sectional area, cross-sectional shape, and length, are approximately determined. For example, nozzle N is located on nozzle substrate 11. Nozzle substrate 11 is, for example, metal, resin, etc., but is not limited thereto. Pressure chamber P is located on pressure chamber substrate 14.

[0039] The flow path substrate 12, intermediate substrate 13, etc., are located between the nozzle substrate 11 and the pressure chamber substrate 14. They are, for example, glass (silicon dioxide) substrates and / or metal substrates such as SUS, 42 alloy, etc. In the flow path substrate 12 and intermediate substrate 13, a downstream independent flow path L connecting the nozzle N and the pressure chamber P extends.

[0040] An isolation substrate 15 is located above the pressure chamber substrate 14. The isolation substrate 15 has a cavity portion that, in a top view, includes the pressure chamber P. A vibrating plate 51 is located at the boundary between the isolation substrate 15 and the pressure chamber P. A piezoelectric element 52, clamped by an electrode, is located within the aforementioned cavity portion on the upper surface of the vibrating plate 51. When a voltage is applied to the piezoelectric element 52, causing it to deform, the vibrating plate 51 deforms accordingly, changing the volume of the pressure chamber P. This imparts a pressure variation to the pressure applied within the pressure chamber P. Furthermore, the vibrating plate 51 may also be a conductive metal component and serve as one of the electrodes clamping the piezoelectric element 52. The piezoelectric element 52 is not particularly limited, but may be, for example, lead zirconate titanate (PZT).

[0041] Wiring substrate 16 is located above isolation substrate 15. Electrodes clamping piezoelectric element 52 are connected to wiring on wiring substrate 16 via bumps or the like. Wiring substrate 16 is connected to drive substrate on its upper surface, from which a drive voltage signal related to the applied voltage to piezoelectric element 52 is input. The drive substrate is not particularly limited and can be an FPC (Flexible Printed Circuit) or the like. An upstream independent flow path U extending from pressure chamber substrate 14, isolation substrate 15, and wiring substrate 16 communicates with pressure chamber P. The upstream independent flow path U is connected on the upper surface of wiring substrate 16 to a manifold that stores and discharges ink supplied to multiple nozzles N, a common ink flow path, or the like. The aforementioned substrates can also be bonded together using adhesives or the like.

[0042] Inside the independent flow path F, the pressure fluctuations applied to the ink are transmitted and reflected as vibrations through the deformation of the pressure chamber P. Correspondingly, ink protrudes and separates from the nozzle N, ejecting ink droplets. On the other hand, within the nozzle N, the ink surface position changes according to the aforementioned pressure fluctuations, and due to the reduction of ink caused by the separation of ink droplets, the supply of replenishing ink, etc. Regarding this ink surface position (squirrel), a force is applied in a direction that maintains a suitable position and shape through surface tension.

[0043] exist Figure 1B In the equivalent circuit of the ink flow path shown, the vibration parameters are represented by a combination of electrical components, including resistive elements, capacitive elements (capacitors), and inductive elements (coils). As described above, the upstream independent flow path U, the downstream independent flow path L, and the nozzle N are connected in series, sandwiching the pressure chamber P. The upstream independent flow path U and the downstream independent flow path L can also be further subdivided and represented for each flow path within each substrate. As a whole, the combined resistance of the multiple resistive elements and the combined inertia of the multiple inductive elements can be calculated as an equivalent circuit.

[0044] In the vibration characteristics represented by this equivalent circuit, the vibration of the ink surface (meniscus) in nozzle N is crucial for proper ink ejection, particularly for suppressing small droplets. A single pressure variation, depending on the vibration characteristics, decays and remains for a certain period. By decaying the vibration and leaving it at an appropriate size after ink ejection, small droplets are suppressed. On the other hand, if the decayed vibration remains excessively, this residual vibration overlaps with the vibration associated with ink ejection in subsequent cycles, potentially negatively impacting continuous ejection.

[0045] The vibrational characteristics, such as the resonant frequency, associated with the surface vibration are variables corresponding to the construction of the independent flow path F and the properties of the ink (viscosity, etc.). As mentioned above, the shape and position of the liquid surface (squirrel surface) are maintained by the surface tension of the ink. That is, surface tension affects the restoring force of the liquid surface after ink ejection. Therefore, it can be said that ink can be stably ejected by an inkjet head with an independent flow path F having vibrational characteristics suitable for the properties of the ink to be ejected.

[0046] The Q value is known as a parameter related to vibration (and its decay). The Q value is a dimensionless parameter of the system's energy, decreasing as the energy lost in one cycle of vibration increases. When the Q value is too small, the energy loss is large, resulting in rapid vibration convergence and suppression of the pressure variations inherent to the ink ejection. When the Q value is too large, the energy loss is small, leaving a longer residual vibration for a single drive pulse. In this disclosure, the Q value related to the aforementioned liquid surface vibration is defined as the refill Q value.

[0047] Regarding the independent flow path F, the value Q of the refilled Q is determined as described below.

[0048] Q=ω·Ln / Rn…(Formula 1)

[0049] Here, Ln is the combined inertia of the m inductors Lm contained in the above equivalent circuit, that is, when the inductors Lm are connected in series.

[0050] Ln=Σ (j＝1～m) Lj…(Formula 2)

[0051] When inductors Lm are connected in parallel

[0052] 1 / Ln=Σ (j＝1～m) (1 / Lj)…(Formula 3)

[0053] Rn is the combined resistance of k resistors Rk included in the equivalent circuit, that is, when the resistors Rk are connected in series.

[0054] Rn=Σ (j＝1～k) Rj…(Formula 4)

[0055] When resistors Rk are connected in parallel

[0056] 1 / Rn=Σ (j＝1～k) (1 / Rj)…(Formula 5)

[0057] Cn stands for compliance of the ink surface.

[0058] The angular vibration frequency ω is the value of the damped vibration generated in an RLC series circuit, as described below.

[0059] ω=2π / T=(1 / (Ln·Cn)-(Rn / (2Ln)) 2 ) 1 / 2 …(Formula 6)

[0060] T is the vibration period of the liquid surface (meniscus) in nozzle N, that is, the vibration period of the damped vibration in the RLC series circuit. Therefore, the Q value is filled as described below.

[0061] Q = 1 / Rn·(Ln / Cn-(Rn / 2)) 2 ) 1 / 2 …(Formula 7)

[0062] As described above, in the conical nozzle N, the position of the liquid surface changes with the restoring force of the capillary force caused by the surface tension of the droplet. This is achieved through Cn(x) = π · D(x) 4 / (128 · The average of the vibration period of σ) is used to calculate the force Cn related to the ink level in nozzle N under this condition. That is, the force Cn is expressed as the value of D(0) = D0 raised to the power of 4 when x = 0, multiplied by π / (128·σ) as a constant. σ is the surface tension of the ink, a value determined by the ink. x is the distance from the opening end of nozzle N. Therefore, the Q value is filled in as follows.

[0063] Q = 1 / Rn·(A·Ln / D0) 4 -Rn 2 / 4) 1 / 2 …(Formula 8) A is the reciprocal of the above constant, A=128·σ / π.

[0064] When a portion of an independent flow path F is cylindrical, the resistance R and inertia L of that portion are analytically determined as shown below.

[0065] R = 4·ρ·l / (π) · d 2 )…(Formula 9)

[0066] L=128·η·l / (π · d 4 )…(Formula 10)

[0067] η is the viscosity of the ink, ρ is the density of the ink, d is the radius of the cylinder, and l is the length of the cylinder.

[0068] In addition, when a portion of the independent flow path F is prismatic, the resistance R and inertia L of that portion are analytically determined as shown below.

[0069] R = 8·η·(a+b) 2 ·w / (a 3·b 3 )…(Formula 11)

[0070] L = ρ · w / (a·b)…(Formula 12) a and b are the lengths of the two sides of the flow path profile of the prism, and w is the flow path length of the prism.

[0071] Alternatively, if a portion of the independent flow path F is conical (frustum), the resistance R and inertia L of that portion can be analytically determined as shown below.

[0072]

[0073] These are the diameters of the base and the top surface of the frustum, respectively.

[0074] Numerical simulations can be used to obtain the resistance R and inertia L of complex-shaped components such as pressure chamber P. The resistance R and inertia L of these components, arranged in series, are then combined to obtain the combined resistance Rn and combined inertia Ln of the independent flow path F. As described above, this is achieved using parameters related to the shape of the independent flow path F. The refill Q value of the independent flow path F is obtained by taking parameters (σ, η, ρ) related to the characteristics of the ink.

[0075] In addition to the above, flow resistance may also occur at bends in the ink flow path, but this resistance is smaller than that in a typical independent flow path F, so it will not be considered here. However, such flow resistance can still be taken into account to obtain the refill Q value.

[0076] The type and characteristics of ink can be determined independently of the inkjet head. However, in most inkjet heads, especially industrial inkjet heads, the corresponding inks used are roughly the same. Therefore, it is sufficient to consider the characteristics of that corresponding ink.

[0077] By changing the refill Q value of the inkjet head while ejecting ink and investigating its ejection characteristics, a suitable range of refill Q values ​​is determined.

[0078] Figure 2A as well as Figure 2B This is a diagram illustrating the driving action used to eject ink.

[0079] exist Figure 2AThe flow of the drive signal is shown in the diagram. In an inkjet recording apparatus having an inkjet head 1, the head drive unit 5 performs an operation to deform the piezoelectric element 52 under the control of the signal control unit 41 of the head drive control unit 4. The signal control unit 41 has a processor such as a CPU (Central Processing Unit) and performs control operations related to the image recording operation. The signal control unit 41 can be a general-purpose CPU for inkjet recording apparatuses or a dedicated CPU different from them. In this case, the signal control unit 41 can also be provided on the drive board in conjunction with the drive circuit 50.

[0080] The head drive unit 5 includes a drive circuit 50 and a piezoelectric element 52 on a drive substrate. The drive circuit 50 outputs a drive voltage signal to the piezoelectric element 52 with a suitable waveform (drive waveform) to deform the piezoelectric element 52 in a way that generates pressure changes on the ink to eject the ink.

[0081] The drive circuit 50 includes a signal generation unit 53. Under the control of the signal control unit 41, the signal generation unit 53 converts a digital waveform into a suitable analog waveform and amplifies and outputs electrical power (voltage and current). The output drive voltage signal is selectively output to the piezoelectric element 52 corresponding to the nozzle N that ejects ink based on image data.

[0082] Figure 2B This is a diagram showing an example of a driving waveform.

[0083] In the driving method of the piezoelectric element 52 related to ink ejection in this embodiment, there are single-droplet and multi-droplet modes for ink ejection. In the single-droplet mode, one droplet is ejected with a single drive pulse. In the multi-droplet mode, a drive waveform is used to combine multiple droplets ejected from each nozzle N by multiple drive pulses and drop them onto the same pixel position. The inkjet head 1 can utilize either mode or be specialized to primarily utilize one mode. Here, the drive waveform for the multi-droplet mode will be described.

[0084] For example, for each pixel (dot), three drive pulses are output at intervals of twice the Acoustic Length (AL). AL is half the resonant period (sound resonant period) of the pressure vibration generated in the ink within the pressure chamber P. By outputting three drive pulses synchronously with the resonant period, ink is efficiently ejected by utilizing the surface vibration of each ink column. The resonant period is determined based on the ink ejection frequency required in the inkjet head. Specific resonant frequencies are, for example, 10-250 μs, but are generally below 70 μs in the current common range. In practice, the interval between the rising edges of the drive pulses, i.e., the time between the start timing of the rising edges of adjacent drive pulses, can deviate slightly from 2AL. For example, this interval is between 1.8AL and 2.3AL.

[0085] On the other hand, the width of each drive pulse, i.e., the time from the start of the voltage rise edge timing to the end of the drive voltage timing, is 1.2AL, slightly longer than AL. This suppresses excessive residual vibration of the liquid surface caused by the preceding drive pulse. However, when the width of the drive pulse deviates significantly from 1.0AL, it is not suitable to impart pressure variations to the ink. Therefore, the width of the drive pulse is preferably in the range of 0.8AL or more and 1.3AL or less. The drive pulse can be, for example, a rectangular wave, but it can also be a trapezoidal wave. In the case of a trapezoidal wave, the ratio of the voltage change period to the constant drive voltage period can be appropriately determined.

[0086] In these three driving pulses, the voltage amplitude V1 of the third pulse is greater than the voltage amplitude V2 of the first and second pulses. Therefore, the last ejected ink droplet reliably catches up with the preceding ink droplet and is merged. Regarding the absolute values ​​of the voltage amplitudes V1 and V2, a larger value is preferred within a range that does not produce abnormalities in ink ejection; that is, a larger droplet velocity is preferred.

[0087] By using such a driving pulse, small droplets are suppressed and ink is ejected stably. However, the degree to which the impact of small droplets on image quality is suppressed ultimately depends on the construction of the inkjet head 1 and the characteristics of the ink. The extent to which the preferred ink that minimizes image quality degradation is ejected can be determined based on the image quality of the image formed by the ejected ink.

[0088] Figures 3A-3C This is a diagram illustrating image quality assessment.

[0089] Figure 3A as well as Figure 3B This is an example of an object image used in image quality assessment. Figure 3A The one-dimensional barcode (Code 1) is shown. Figure 3BThe QR code (code 2) is shown. Reading it can be problematic if it is not displayed accurately, therefore evaluation criteria are determined. Quality evaluation criteria related to one-dimensional barcodes are specified in ISO 15416 (JIS X 0520). Quality evaluation criteria related to QR codes are specified in ISO 15415 (JIS X 0521).

[0090] exist Figure 3C This document shows a portion of ISO 15415. The inspection content includes the ratio of errors in the range where the positions of white and black cells are fixed (contrast between white and black cells, offset, seeker pattern, blank areas, alignment pattern, timing pattern, etc.), reading anomalies, the amount of distortion of the QR code itself, deviations in the size of each cell (ratio uniformity), and the utilization rate of error correction symbols (data repair symbols) used when reading dirty or damaged parts. Each item is evaluated using a value from 0.0 to 4.0 (2016 revision) or five stages based on the letters AD and F (2000 version). The overall judgment is the lowest evaluation stage for each item.

[0091] Figure 4 This is a graph showing the combined inertia Ln, combined resistance Rn, and force Cn of the equivalent circuit corresponding to each refill Q value obtained according to the inspection criteria of Code 1 and Code 2 described above. The refill Q values ​​are calculated according to the above (Formula 8), etc. In order to improve the refill Q value, for example, the shape of the independent flow path F can be changed in a way that reduces the combined resistance Rn or increases the combined inertia Ln, as described above.

[0092] Figure 5A This is a chart showing the results of the inspection according to the inspection standard of Code 1 above. Figure 5B This is a chart showing the results of inspections according to the inspection criteria of Code 2. These inspection results were obtained by checking each inkjet head with different construction parameters related to the refill Q value while varying the distance from the ink ejection surface to the ink droplet surface (image recording surface). Hereinafter, this distance, i.e., the head / media gap, will be referred to as gap. For simplicity, a comprehensive evaluation of C or higher is considered "good". Even if the comprehensive evaluation is D or lower, it cannot be said that the image quality is NG, but in order to consistently obtain an image of appropriate quality, a comprehensive evaluation of C or higher is preferred.

[0093] The ink used to record the images in this test had a viscosity of 5.7 mPa·s and a density (liquid density) of 1080 kg / m³. 3The speed of sound in the ink is 1521 m / s, and the surface tension is 42 mN / m. Using a Keyence (registered trademark) code reader "SR-1000," the recorded codes 1 and 2 were read. The code reader outputs the above comprehensive evaluation.

[0094] At a refill Q value of 1.05, neither code 1 nor code 2 recorded images of appropriate quality. With a gap of 5 mm, an image of appropriate quality was obtained with a refill Q value of 1.17. When the refill Q value became 1.85 or higher, images of appropriate quality were obtained even with gaps of 10 mm and 15 mm.

[0095] On the other hand, with a refill Q value of 2.44, adequate quality cannot be obtained when the gap is 15 mm. However, as long as the refill Q value is 2.32 or lower, an image of adequate quality can still be obtained even with a gap of 15 mm. That is, from the viewpoint of image quality degradation related to small droplets, a refill Q value of 1.17 or higher is preferred, and a refill Q value of 1.85 or higher is more preferred. Furthermore, a refill Q value of 2.32 or lower is even more preferred.

[0096] Figure 6 This is a graph showing the relationship between the refill Q value and the maximum droplet velocity that can be stably ejected by the ink ejection under the above multi-drop method.

[0097] The maximum droplet velocity that can be stably ejected is the velocity at a position 0.5 mm from the ink ejection surface. It is known that the maximum droplet velocity is positively correlated with the refill Q value. When the refill Q value is 1.31 or higher, the maximum droplet velocity is 5 m / s or higher; conversely, at points less than 1.31, the maximum droplet velocity is less than 5 m / s, which is slightly insufficient for practical use. That is, it is considered that in multi-drop mode, when the refill Q value is low, the ink supply to nozzle N may not keep up during high-speed ejection with short intervals between drive pulses and small AL values. As a result, ejection may become unstable. Therefore, especially in the case of multi-drop mode, it is preferable to add a refill Q value of 1.31 or higher in addition to the aforementioned range. Similarly, when ejecting ink at a frequency corresponding to multi-drop mode, the refill Q value is preferably 1.31 or higher. Especially in the case of high-speed ejection, it is preferable to obtain a droplet velocity of 7 m / s or higher at a position with a gap of 5 mm. Therefore, the refill Q value of such an inkjet head 1 is more preferably 1.38 or higher.

[0098] On the other hand, even in single-droplet ink ejection, the interval between drive pulses narrows at high-frequency ink ejection. As a result, the residual echo of the liquid surface vibration from the previous ink ejection sometimes affects subsequent ink ejection. Therefore, an inkjet head 1 with the aforementioned refill Q value that is suitable for obtaining droplet velocity is more preferable for ink ejection mode independent of ink.

[0099] Figure 7 This graph illustrates the effect of the reverberation of the meniscus caused by the first ink ejection in a two-stage ink ejection process on the droplet velocity associated with the second ink ejection. For the drive cycle associated with the two ink ejections, the difference between the droplet velocity of the second ink ejection and that of the first ink ejection is shown. Each line corresponds to one of six different refill Q values. The drive cycle is shown with AL as the reference.

[0100] The following trend can be observed: the larger the refill Q value becomes, the greater the difference in ink ejection droplet velocity tends to become, and the effect of the drive cycle tends to linger. If the refill Q value is below 2.12, the difference converges to less than 1 m / s. As mentioned above, the droplet velocity is 5 m / s or higher, so a difference of 1 m / s is less than 20% of the droplet velocity. It is conceivable that, although it still depends on the size of the gap, the transport speed of the recording medium, etc., if the droplet velocity deviates by this extent, the impact on image quality can be tolerated. Therefore, based on the above range of refill Q values, a refill Q value of 2.12 or less is more preferable.

[0101] Figure 8 This is a diagram showing other examples of drive voltage waveforms associated with ink ejection.

[0102] Regardless of the construction of the inkjet head 1, in ink ejection waveforms, especially in multi-droplet configurations, a drive voltage waveform that is less prone to generating small droplets and can stably and continuously eject ink is preferred. In the drive voltage waveform of this other example, in Figure 2B Following the final pulse waveform shown, a pulse waveform with a voltage amplitude V3, larger than the voltage amplitude V1 of the final pulse waveform, is added at an interval of 4.0AL, twice the normal interval. In this case, the width (pulse length) of the last added pulse waveform becomes 1.0AL. Furthermore, the pulse length of the pulse waveform that is no longer the voltage amplitude V1 of the final pulse waveform becomes 1.3AL, the same as the pulse length of the preceding pulse waveform. Based on this driving voltage waveform, more stable ink ejection can be continuously achieved in multi-drop mode.

[0103] Figure 9A , Figure 9B , Figure 10A , Figure 10B , Figure 11Aas well as Figure 11B These are diagrams illustrating other examples of ink flow paths.

[0104] like Figure 9A As shown, the specific ink flow path can also be constructed in a manner other than that described in the above embodiments. In this inkjet head 1a, regarding the independent flow path Fa, the common ink flow path Sc is located on the flow path substrate 12a between the nozzle substrate 11a and the pressure chamber substrate 14a.

[0105] Ink flows into pressure chamber P via an upstream independent flow path U, which separates from the common ink flow path Sc. The ink, subjected to pressure variations, is ejected from nozzle N via a downstream independent flow path L. Figure 9B This is a diagram showing the equivalent circuit of the ink flow path. For each structural component of the upstream independent flow path U, pressure chamber P, downstream independent flow path L, and nozzle N, corresponding resistance values ​​R, inertia L, etc., are applied. Even if the structures are different, the refill Q value is calculated by combining them in the same manner as described above.

[0106] Or, such as Figure 10A As shown, the inkjet head 1b can also be constructed by applying pressure variations to the piezoelectric element 52, which deforms in a shear mode. Ink flowing into the independent flow path Fb is supplied to the nozzle N via the upstream independent flow path U, the pressure chamber P of the pressure chamber substrate 14b, and the downstream independent flow path L of the intermediate substrate 13b. The piezoelectric element is located on the side of the pressure chamber P. Pressure variations are applied to the ink in the independent flow path Fb within the pressure chamber substrate 14b by deforming the piezoelectric element in the direction along the independent supply flow path S, i.e., deformation in a shear mode. Thus, even inkjet heads 1a with different deformation modes of the piezoelectric element 52 can adapt to different ink flow paths. Figure 10B The parameters of each structural part shown are calculated and then the Q value is filled in.

[0107] exist Figure 11A The image shows the ink flow path of another example of an inkjet head 1c. Ink flowing from the common ink flow path Sc into the upstream independent flow path U is ejected from the nozzle N of the nozzle substrate 11c via the pressure chamber P of the pressure chamber substrate 14c and the downstream independent flow path L of the intermediate substrate 13c. Furthermore, the ink can be separated into independent discharge flow paths E1 and E2 in the flow path substrate 12c and the intermediate substrate 13c. The ink discharged via the common discharge flow paths Ec1 and Ec2 returns to the ink tank. In an inkjet head 1c with such a circulating flow path, it is possible to separate components caused by precipitation in the independent supply flow path S, including the upstream independent flow path U, the pressure chamber P, and the downstream independent flow path L, and to rapidly remove mixed air bubbles, dust, etc. Therefore, the inkjet head 1c can perform more stable image recording operations.

[0108] In such an inkjet head 1c, an equivalent circuit can also be considered, corresponding to the independent supply flow path S, the independent discharge flow paths E1 and E2, and the nozzle N, which are included in the independent flow path Fc of the ink corresponding to each nozzle N. For example... Figure 11B As shown, in this case, independent discharge paths E1 and E2 are arranged side-by-side with respect to the independent supply path S in the independent flow path Fc. Using the parameters of the resistive, capacitor, and inductive elements of this circuit structure, as well as the characteristics of the ink, the refill Q value is determined.

[0109] Furthermore, the independent discharge paths E1 and E2, as well as the shared discharge paths Ec1 and Ec2, are not limited to the independent path Fc, which applies pressure variations to the ink through shear-mode deformation. They can also be applied to the aforementioned independent paths F and Fa, which apply pressure variations to the ink through flexural-mode deformation.

[0110] As described above, the inkjet head 1 of this embodiment includes: an independent flow path F for flowing liquid (ink) and a pressure chamber P for storing ink and applying pressure variations; and a nozzle N connected to the independent flow path F for ejecting droplets of ink with applied pressure variations. The inkjet head 1 has a refill Q value in the nozzle N that is related to the vibration of the liquid surface when the liquid viscosity is 5.7 mPa·s and the liquid density is 1080 kg / m³. 3 When the speed of sound in a liquid is 1521 m / s and the surface tension is 42 mN / m, the structure becomes 1.17 or higher.

[0111] With an inkjet head 1 having such a structure, even if the distance between the nozzle opening and the recording medium is widened to 5 mm (more than the previous 1 mm), ink can be ejected while reducing the image quality impact caused by small droplets. Therefore, the inkjet head 1 can suppress the degradation of the recorded image quality. Thus, this inkjet head 1 can be used more flexibly for various image recording operations than before.

[0112] Furthermore, as described above, the structure of the inkjet head 1 is determined based on the properties of the liquid used as a reference. Therefore, the actual refill Q value varies depending on the properties of the liquid. However, by using the inkjet head 1 disclosed herein, it is possible to achieve, to some extent, the effect of increasing the distance between the nozzle opening and the recording medium while reducing the impact of small droplets on image quality, within the range of commonly used inks.

[0113] Furthermore, a refill Q value of 1.38 or higher is preferred. When the refill Q value is increased to this level, it is more suitable for high-speed flight within a gap of about 5 mm. Therefore, the inkjet head 1 can eject ink more stably and flexibly within a wider gap.

[0114] Furthermore, a refill Q value of 1.85 or higher is preferred. With this refill Q value, the distance between the nozzle opening and the recording medium can be further extended to 15 mm. Therefore, according to this inkjet head 1, image recording can be performed stably and without failure, even on recording media with uneven surfaces, while suppressing image quality degradation.

[0115] On the other hand, the inkjet head 1 has a structure in which the refill Q value is 2.32 or less under the same structure and ink conditions as described above. Therefore, regardless of whether it is a multi-drop or single-drop method, image recording can be performed stably with a wider gap while suppressing image quality degradation. Thus, this inkjet head 1 can flexibly record images for a wide range of recording media.

[0116] Furthermore, a refill Q value of 2.12 or lower is even more preferable. In addition to stably suppressing the impact of small droplets on image quality under wide gaps, as mentioned above, it also suppresses the impact on the velocity of subsequent droplets even when droplets are continuously ejected. Therefore, it can appropriately suppress image quality degradation, especially at high-speed ejection.

[0117] Furthermore, in the drive control method of the inkjet head 1 in this embodiment, in the multi-drop drive waveform where droplets are ejected towards the same pixel position by multiple drive pulses, the widths of the multiple drive pulses are 0.8 or more and 1.3 or less of AL, and the interval between the rising edges of the drive pulses is 1.8 or more and 2.3 or less of AL. By setting the drive pulse widths to such a range, droplets can be reliably unified in the multi-drop mode, and the influence of small droplets on image quality can be suppressed, allowing a suitable amount of ink droplets to be stably dropped at the desired position. By using this drive control method to eject ink from the inkjet head 1 of this embodiment, even with increased gaps, stable ink ejection with less influence from small droplets can be achieved more reliably than before. Thus, in this drive control method, image recording can be performed while suppressing image quality degradation more flexibly.

[0118] Furthermore, in the multi-drop drive waveform, the last drive pulse among the multiple drive pulses can rise at an interval of AL 4.0 or more from the rising edge of the preceding drive pulse. In the multi-drop mode, the effects of each drive pulse overlap with subsequent drive pulses, making it easier to generate small droplets between the ejected droplets. In this drive control method, a larger-than-usual interval is formed between the last two drive pulses, thereby suppressing the eventual residue of small droplets. Therefore, it is possible to consolidate most of the ejected ink and drop it onto the recording medium. By applying such an ejection control method to the inkjet head 1 of this embodiment, even with an enlarged gap, it is possible to record an appropriate image while suppressing small droplets more stably.

[0119] Furthermore, in this drive control method, the drive waveform can also be determined such that the droplet velocity at a position 0.5 mm from the opening end of nozzle N is 7 m / s or more. During low-speed flight, especially with wide gaps, the ink's flight time becomes longer. As a result, the influence of external airflow and other factors during flight becomes more pronounced. Therefore, by ensuring a droplet velocity of 7 m / s or more, the ejected ink can more stably drop to the appropriate position, resulting in a recorded image of suitable quality.

[0120] Furthermore, in this drive control method, the drive waveform can be determined such that the amount of droplet ejected at each point is 10 pL or more. When the droplet amount is small, especially during flight within a wide gap, it is easy to decelerate significantly due to air resistance, etc. By determining the drive waveform so that the droplet amount is 10 pL or more, the degree of deceleration can be suppressed even due to air resistance, etc. Therefore, in this drive control method, even when ink is dripped onto the recording medium with a wide gap using the inkjet head 1 of this embodiment, image quality degradation is suppressed.

[0121] Furthermore, the present invention is not limited to the above-described embodiments and various modifications can be made.

[0122] For example, as mentioned above, in addition to the occurrence of small droplets, factors that reduce image quality when the gap is widened are also related to droplet size, droplet velocity, etc. Therefore, the lower and / or upper limits of the refill Q value can be determined based on priority conditions such as the gap required for the recorded image and recording medium, droplet size, and whether a multi-drop method is used.

[0123] Furthermore, the drive waveform shown above in the multi-drop mode is an example. Drive signals with other drive waveforms can also be generated and output. Additionally, in the above embodiment, an example of a combined trapezoidal drive pulse was described, but it is not limited to this. A rectangular wave pulse can also be used. Furthermore, only a drive pulse varying from the reference voltage to the + side is shown above, but it is not limited to this. A drive pulse varying only to the - side can also be used, and drive pulses varying to both the ± and ± sides can also be combined.

[0124] Furthermore, nozzle N may not have a conical shape. It can also be a short cylindrical shape, etc. Additionally, the shapes of other ink flow paths can be appropriately determined. The combined resistance and inertia of each part can be calculated analytically or through numerical simulation based on the shape. Furthermore, the calculated combined resistance and inertia can be based on more than just the shape of the ink flow path. That is, if filters are installed, they can also be taken into account when calculating the combined resistance and inertia.

[0125] Furthermore, the example described above, which uses the deformation of a piezoelectric element to impart pressure variation to the ink, is not limited to this. An inkjet head 1 can eject ink from nozzle N by imparting pressure variation to the ink in the ink flow path. Alternatively, in this case, a drive signal can be generated based on a different reference than described above.

[0126] Furthermore, the specific structure of the signal control unit 41, signal generation unit 53, etc., is arbitrary. It is sufficient to generate a suitable drive signal and select the output destination based on the image data.

[0127] Furthermore, the specific structures, processing actions, and procedures described in the above embodiments can be suitably modified without departing from the spirit of the invention. The scope of the invention includes the scope of the invention as set forth in the claims and its equivalents.

[0128] Industrial availability

[0129] This invention can be used in droplet ejection heads and drive control methods.

[0130] (Explanation of reference numerals in the attached diagram)

[0131] 1. 1a-1c: Inkjet head; 4: Head drive control unit; 41: Signal control unit; 5: Head drive unit; 50: Drive circuit; 51: Vibrating plate; 52: Piezoelectric element; 53: Signal generation unit; 11. 11a-11c: Nozzle substrate; 12. 12a-12c: Flow path substrate; 13. 13b, 13c: Intermediate substrate; 14. 14a-14c: Pressure chamber substrate; 15: Isolation substrate; 16: Wiring substrate; E1, E2: Independent discharge flow path; Ec1, Ec2: Common discharge flow path; F, Fa-Fc: Independent flow path; L: Downstream independent flow path; N: Nozzle; P: Pressure chamber; S: Independent supply flow path; Sc: Common ink flow path; U: Upstream independent flow path; V1-V3: Voltage amplitude.

Claims

1. A droplet ejector head, comprising: A liquid flow path, allowing liquid to flow, having a pressure chamber for storing the liquid and imparting pressure variations; and The nozzle, connected to the liquid flow path, ejects droplets of liquid subjected to the pressure variation. The droplet ejector has the following structure: a dimensionless parameter, whose energy loss during one cycle of vibration decreases as much as possible, is designated as the Q value; the Q value related to the vibration of the liquid surface in the nozzle is designated as the refill Q value; when the liquid viscosity is 5.7 mPa·s and the liquid density is 1080 kg / m³, the desired effect is achieved. 3 When the speed of sound in the liquid is 1521 m / s and the surface tension is 42 mN / m, the refill Q value is 1.17 or higher.

2. The droplet ejector according to claim 1, wherein, The refill Q value is 1.38 or higher.

3. The droplet ejector according to claim 1, wherein, The refill Q value is 1.85 or higher.

4. The droplet ejector according to claim 1, wherein, The refill Q value is below 2.

32.

5. The droplet ejector according to claim 2 or 3, wherein, The refill Q value is below 2.

32.

6. The droplet ejector according to claim 4, wherein, The refill Q value is below 2.

12.

7. A driving control method for a droplet ejector according to any one of claims 1 to 6, wherein, In a multi-droplet driving waveform where droplets are ejected toward the same pixel position by multiple driving pulses, the widths of the multiple driving pulses are 0.8 to 1.3 times the acoustic length (AL), and the interval between the rising edges of the driving pulses is 1.8 to 2.3 times the acoustic length (AL).

8. The drive control method according to claim 7, wherein, In the multi-drop driving waveform, the last driving pulse of the plurality of driving pulses rises at an interval of AL 4.0 or more from the rising edge of the preceding driving pulse of the last driving pulse.

9. The drive control method according to claim 7 or 8, wherein, The driving waveform ejects liquid at a droplet velocity of 7 m / s or higher at a position 0.5 mm from the opening end of the nozzle.

10. The drive control method according to claim 7 or 8, wherein, The driving waveform is determined in such a way that the amount of droplets ejected at each point is more than 10 pL.

11. The drive control method according to claim 9, wherein, The driving waveform is determined in such a way that the amount of droplets ejected at each point is more than 10 pL.

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