Inkjet head, ink discharge method, and inkjet recording apparatus

Abstract

Provided are an inkjet head, an ink discharge method, and an inkjet recording apparatus, which enable discharging of ink having a high viscosity in a more easy and stable manner. This inkjet head comprises a nozzle (N), a pressure chamber (C) that is in communication with the nozzle (N), and a pressure application part (E) that applies pressure fluctuations to ink within the pressure chamber (C). The inkjet head has a structure that has a break-up Q factor, which is expressed as πfpφ2 / (8η) using a nozzle diameter φ and a resonance frequency f with respect to the ink, of 0.37 or more, when the ink protruding out of the nozzle (N) upon receiving the pressure fluctuations by the pressure application part (E) has a viscosity η of 100 mPa·s and a density ρ of 1080 kg·m-3.

Description

Inkjet head, ink ejection method, and inkjet recording device

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

[0002] Inkjet heads can eject a variety of inks. The density and viscosity of ink vary depending on the ink components. Therefore, inkjet heads can have a structure that allows ink to fly and land appropriately depending on the ink.

[0003] Japanese Patent Application Laid-Open No. 2006-129999 discloses a technique in which a diaphragm that applies pressure fluctuations to ink has an uneven structure with locally increased thickness in order to eject high-viscosity ink at high speed and stably. Japanese Patent Application Laid-Open No. 2006-129999 discloses a technique in which a nozzle plate has a structure in which the relationship between the size of the nozzle and the pressure chamber is limited.

[0004] JP 2021-74946 A Patent No. 5659198

[0005] However, conventional techniques have the drawback of requiring complex structures and manufacturing processes, which increases the time and cost required.

[0006] An object of the present disclosure is to provide an inkjet head, an ink ejection method, and an inkjet recording apparatus that are capable of ejecting high-viscosity ink more easily and stably.

[0007] In order to achieve the above object, the present disclosure provides a method for producing an ink jet head comprising: a nozzle; a pressure chamber communicating with the nozzle; and a pressurizing unit that applies a pressure fluctuation to ink in the pressure chamber, wherein the pressure fluctuation is applied by the pressurizing unit and the ink protruding from the nozzle has a viscosity η of 100 mPa s and a density ρ of 1080 kg m －３ When the nozzle diameter φ and the resonance frequency f are used for the ink, πfρφ ２ The ink jet head has a structure in which the tear Q value expressed as / (8η) is 0.37 or more.

[0008] According to the present disclosure, there is an effect that the inkjet head can eject high viscosity ink more easily and stably.

[0009] 1 is a perspective view illustrating an inkjet head according to an embodiment of the present invention; FIG. 2 is a diagram illustrating the shape of a pressure chamber; FIG. 3 is a diagram illustrating the shape of a pressure chamber; FIG. 4 is a diagram illustrating the state of ink in response to pressure fluctuations in a pressure chamber; FIG. 5 is a diagram illustrating the state of ink in response to pressure fluctuations in a pressure chamber; FIG. 6 is a diagram illustrating the state of ink in response to pressure fluctuations in a pressure chamber; FIG. 7 is a diagram illustrating the state of ink in response to pressure fluctuations in a pressure chamber; FIG. 8 is a diagram illustrating the state of ink in response to pressure fluctuations in a pressure chamber; FIG. 9 is a diagram illustrating the state of ink in response to pressure fluctuations in a pressure chamber; FIG. 10 is a diagram illustrating the change in tear-off Q value with respect to the nozzle diameter and AL of an inkjet head, and the actual ink droplet separation results; FIG. 11 is a block diagram illustrating the functional configuration of an inkjet recording device; FIG. 12 is a diagram illustrating an example of a waveform pattern of a first drive signal applied to a piezoelectric element when ejecting ink; FIG. 13 is a diagram illustrating an example of a waveform pattern of a second drive signal applied to a piezoelectric element when ejecting ink; FIG. 14 is a diagram illustrating an example of a waveform pattern of a third drive signal applied to a piezoelectric element when ejecting ink; FIG. 15 is a diagram illustrating the flight speed of a main droplet and the length of a tail extending from the main droplet in response to the waveform pattern of a first drive signal; FIG. 16 is a diagram illustrating the flight speed of a main droplet and the length of a tail extending from the main droplet in response to the waveform pattern of the first drive signal; 1A and 1B are diagrams showing the flight speed of a main droplet and the length of a tail extending from the main droplet according to the waveform pattern of a second drive signal, a diagram showing the flight speed of a main droplet and the length of a tail extending from the main droplet according to the waveform pattern of a third drive signal, and a diagram showing the flight speed of a main droplet and the length of a tail extending from the main droplet according to the waveform pattern of the third drive signal.

[0010] An embodiment will now be described with reference to the drawings. FIGS. 1A to 1D are diagrams illustrating an inkjet head 1 according to this embodiment. As shown in the perspective view of FIG. 1A, the inkjet head 1 has a substrate made of a piezoelectric member E. The piezoelectric member E has a plurality of recesses extending in parallel. These recesses are pressure chambers C (channels) through which ink flows. The recesses are covered above by a cover plate Pc, only a portion of which is shown. The cover plate Pc has a common flow path F that supplies ink to each of the pressure chambers C. The piezoelectric member E is an electromechanical transducer that deforms when a voltage is applied. Each of the piezoelectric members E is sandwiched between a pair of electrodes (not shown) facing adjacent pressure chambers C. The electrodes may be covered with a coating that is non-reactive to ink. The electrodes are connected to wiring (not shown). The piezoelectric member E and the electrodes form a pressure unit. A nozzle plate Pn having a nozzle N is located at the tip of each pressure chamber C. Only a portion of the nozzle plate Pn, corresponding to the cover plate Pc, is shown. The nozzle N is in communication with the pressure chamber C.

[0011] Fig. 1B is a cross-section of the pressure chamber C in a reference state. The cross-section is perpendicular to the axial direction of the nozzle N. When the pressure chamber C expands in response to deformation, as in Fig. 1C, the ink pressure decreases. When the pressure chamber C compresses in response to deformation, as in Fig. 1D, the ink pressure increases. For example, but not limited to, the electrodes may be positioned so that the piezoelectric member E deforms in a shear mode.

[0012] 2A to 2C, 3A, and 3B are diagrams illustrating the state of ink in response to pressure fluctuations in a pressure chamber C. As shown in FIG. 2A, when the ink pressure increases, ink is pushed out from a nozzle N that is in communication with the pressure chamber C and is located in the extension direction of the pressure chamber C. As shown in FIG. 2B, when the ink pressure decreases, ink is drawn back from the nozzle N to the pressure chamber C. Pressure increases and pressure decreases can be appropriately combined. With an appropriate pressure application pattern, ink that has been subjected to pressure fluctuations will partially fly out of the pressure chamber C. Pressure fluctuations include push shots, in which the ink is first pressurized and then depressurized, and pull shots, in which the ink is first depressurized and then pressurized.

[0013] As shown in Fig. 2C, when the ink liquid mass Dp is initially extruded from the nozzle N, the extruded ink liquid mass Dp is connected to the ink in the pressure chamber C. A force that pulls the ink back into the pressure chamber C is appropriately applied to the ink in accordance with the pressure application pattern described above, and as a result, the leading edge of the extruded ink is appropriately separated from the ink in the pressure chamber C, and the separated ink becomes an independent droplet D, as shown in Fig. 3A.

[0014] If the ink lump Dp is not properly cut, a long tail T may form on the droplet D, resulting in misalignment of the droplet D and the tail T on the medium. Alternatively, as shown in FIG. 3B , the long tail T may be further broken up into smaller pieces. The resulting microdroplets S (satellites) may have varying landing positions relative to the main droplet Dm or may land in unintended locations. In particular, when the distance (gap) between the nozzle opening surface (including the nozzle N opening) and the recording medium surface (landing surface) is large, the microdroplets S are likely to experience relatively large air resistance during flight, slowing them down, or their tails T being carried away by the air flow. This can lead to significant misalignment of the landing positions between the main droplet Dm and the microdroplets S and tails T. In this embodiment, a large distance between the opening surface and the landing surface refers to a distance greater than 1 mm, particularly a distance of 2 mm or greater. When this distance is large, the susceptibility to the air resistance and air flow also depends on the flight speed of the main droplet Dm. When the distance is long, the droplet flight speed is preferably at least 4 m / s or more. The flight speed depends on the amplitude of the drive signal (drive pulse) that ejects the ink. Specific waveform patterns of the drive signal will be described later.

[0015] The higher the viscosity of ink, the more difficult it is to break. Therefore, when ejecting high-viscosity ink, an inkjet head structure and / or pressure application pattern that allows it to be more easily broken than normal ink is required. In this embodiment, "high viscosity" refers to the level of viscosity as a liquid, and refers to a viscosity of 50 mPa·s or higher. In other words, gel-like substances with extremely high viscosity that are difficult to eject from the nozzles N of the inkjet head 1 are not considered. However, the boundary between high viscosity and non-high viscosity is not necessarily limited to this value. High-viscosity inks can also include inks containing particles that do not dissolve in solvents, such as pigment inks. The ink referred to here is not limited to color materials used to record images. Ink can also include liquids that solidify to form coatings, conductors that form wiring, or functional materials such as adhesives.

[0016] First, the structure according to viscosity will be explained. Ink is expressed by viscosity η and density ρ. The inkjet head 1 has a resonance frequency f of the pressure fluctuation related to the ink extruded from the nozzle N. Half of the reciprocal of the resonance frequency f, i.e., the length of a half period of the pressure fluctuation, is called the acoustic length (AL). The nozzle N has an opening diameter, i.e., a nozzle diameter φ. The nozzle N may have a tapered shape. The nozzle opening may also be non-circular, in which case the size of the nozzle N may be expressed by a hydraulic diameter instead of the nozzle diameter. Non-circular shapes may include ellipses, rectangles, other polygons, or polygons with rounded corners. A length L is generated by one pressurization of the ink. １ When ink of a certain size is ejected from the opening of the nozzle N, the mass M of the ejected ink is ２ ρL １ / 4. The force Fi acting on the ink is expressed as Fi = Mωv. The angular frequency ω = 2πf. v is the moving speed of the ink surface inside and outside the nozzle N. Therefore, the force Fi acting on the ink can be calculated using Equation 1: Fi = 2π ２ fφ ２ ρL １ v / 4 (Equation 1) This force Fi acting on the ink corresponds to the inertial force of the ink.

[0017] This ink of mass M tries to move in the ejection direction by inertia according to the movement speed v of the ink liquid surface (meniscus) inside the nozzle N in response to the pressurization of the ink. On the other hand, the ink of mass M is subjected to viscous resistance according to the movement speed v of the liquid surface, and ejection from the nozzle N is suppressed. The viscous resistance is determined according to the shear stress τ of the ink. Using the movement speed v of the ink liquid surface and the viscosity η of the ink, the shear stress τ is expressed as τ = η dv / dy. The movement speed v is the average movement speed, and y is the direction perpendicular to the axis of the nozzle N. Therefore, τ = η v / (φ / 4). If the length of the ink column of the ink portion that is about to separate as a droplet is L, ２ As a result, the surface area S of the ink pillar ２ is S ２ = πφL ２ Therefore, the viscous resistance Fr is Fr = τ S ２ = 4πηL ２ The viscous resistance coefficient R is expressed by the following formula 2: R=4πηL ２ ... (Equation 2)

[0018] The ratio Fi / Fr = Mω / R of the viscous resistance Fr to the force Fi acting on the ink is the Q value for the resonance of the ink pushed out from the nozzle N. This Q value also corresponds to the ease with which the ejected ink separates from the ink in the pressure chamber C. Therefore, hereinafter, this Q value will be referred to as the tearing Q value Qt.

[0019] From the above formula, the tear Q value Qt can be calculated by the following formula 3: Qt=(L １ / L ２ )πfρφ ２ / (8η) ... (Equation 3) Therefore, the tearing Q value Qt is proportional to the reciprocal (1 / η) of the ink density ρ and viscosity η. In other words, the greater the viscosity η, the greater the viscous resistance coefficient R, and the smaller the tearing Q value Qt. Therefore, it becomes more difficult for the ejected ink to separate from the ink in the pressure chamber C. The greater the density ρ, the greater the inertia of the ink, and the larger the tearing Q value. Therefore, it becomes easier for the ejected ink to separate from the ink in the pressure chamber C.

[0020] The tearing Q value Qt increases as the nozzle diameter φ increases. Furthermore, the tearing Q value Qt increases as the resonance frequency f increases and the AL decreases. In other words, for an inkjet head 1 that ejects high-viscosity ink, it is best if the nozzle diameter φ is not too small. Furthermore, as is well known, the AL, i.e., the resonance frequency f, depends on the structure of the ink flow path including the pressure chamber C. Examples of structural parameters include length, cross-sectional shape, cross-sectional area, and bending. Therefore, changing the structure of the ink flow path to reduce the AL and increase the resonance frequency f relative to the initial structure can also increase the tearing Q value Qt and facilitate separation of ink droplets from the ink in the nozzle N. The actual AL may be experimentally determined by the width of the drive pulse, for example, the width at which the droplet flight velocity is maximized when droplets are ejected while changing the width of a single contraction pulse. Alternatively, the AL may be determined analytically or numerically from the designed structure.

[0021] 4 is a graph showing the change in tear-off Q value Qt with respect to the nozzle diameter φ and AL of the inkjet head 1, and the actual ink droplet separation results. The density ρ of the ejected ink is 1080 kg / m ３ The viscosity η is 100 mPa·s. The tear Q value Qt is １ and length L ２ Relative to L １ / L ２ is calculated as 1.

[0022] In the example, the ink droplet separation results are determined by photographing the ink during ink ejection to determine whether the ink is separated from the ink in the nozzle N and the length of the tail at the time of separation. In the comparative example, the length of the tail when the ink droplet separates is determined using numerical simulation.

[0023] In Example 3, when the nozzle diameter φ is 50 μm and AL is 14.5, the tear-off Q value Qt is 0.37, and in this case the ink droplets are barely separated from the ink in the nozzle N (Evaluation C). Even when the nozzle diameter φ is 50 μm or less (Examples 1, 2, and 7), if AL decreases as the nozzle diameter φ decreases, the tear-off Q value Qt increases overall, just as in the case when the nozzle diameter φ is greater than 50 μm (Example 4).

[0024] Even when the nozzle diameter φ is 24 μm (Example 7), the ink droplets can be separated from the ink in the nozzle N by reducing the AL to 8.6 (Evaluation C). Even in this case, it is preferable that the nozzle diameter φ is 40 μm or more. If the tear-off Q value Qt is about Qt≧0.50, the ink droplets are properly separated from the ink in the nozzle N (Evaluation B). If the tear-off Q value Qt is further increased, and particularly if the tear-off Q value Qt≧0.76 (Examples 5 and 6), the ink droplets are separated cleanly without unnecessarily long tails (Evaluation A).

[0025] On the other hand, in Comparative Examples 1 to 5, the nozzle diameter φ was small and the tearing Q value Qt was also small at 0.36 or less. In these cases, even if the nozzle diameter φ and AL were changed while maintaining the ratio, the ink droplets did not separate properly (evaluation D).

[0026] 5 is a block diagram showing the functional configuration of an inkjet recording apparatus 100 equipped with the inkjet head 1. In addition to the inkjet head 1, the inkjet recording apparatus 100 is equipped with a control unit 11, a storage unit 12, a communication unit 13, a discharge drive unit 14, a transport drive unit 15, and the like.

[0027] The control unit 11 has a hardware processor such as a CPU (Central Processor Unit) and performs overall control of the operation of the inkjet recording apparatus 100. The control of the control unit 11 includes the image recording operation using the inkjet head 1.

[0028] The storage unit 12 has a RAM (Random Access Memory) and a non-volatile memory. The RAM provides working memory space for the hardware processor and stores temporary data. The storage unit 12 has a large-capacity RAM and stores image data to be recorded by the operation of the inkjet head 1. The non-volatile memory stores control programs and setting data, etc. The non-volatile memory also stores image data acquired from the outside that has not yet completed image recording. The non-volatile memory is, for example, a flash memory, but is not limited to this. The non-volatile memory may also include an HDD (Hard Disk Drive).

[0029] The communication unit 13 controls communication with external devices. The communication unit 13 controls the transmission and reception of data in accordance with, for example, a communication protocol related to a local area network (LAN). The data to be received includes the image data to be recorded. The communication unit 13 may have a connection terminal such as a universal serial bus (USB). Alternatively, the communication unit 13 may have a connection slot for an SD card or the like.

[0030] The ejection drive unit 14 determines whether or not ink is to be ejected from each nozzle at each timing in accordance with the image data to be recorded. The ejection drive unit 14 outputs a drive signal in accordance with whether or not ink is to be ejected to the electrode of the piezoelectric member E corresponding to each pressure chamber C of the inkjet head 1. A voltage is applied to the piezoelectric member E by supplying a voltage to the electrode.

[0031] The transport drive unit 15 moves the recording medium before image recording to a position facing the inkjet head 1, and ejects the recording medium after image recording from the position facing the inkjet head 1. The transport drive unit 15 moves the recording medium relative to the inkjet head 1, and changes the landing position of ink ejected from the inkjet head 1 over time. Note that when the inkjet head 1 records an image using a scanning method, the inkjet recording apparatus 100 may move the inkjet head 1 relative to the recording medium.

[0032] The method of ejecting ink from each nozzle N of the inkjet head 1 in the inkjet recording apparatus 100 will now be described. As described above, when a voltage is applied to the piezoelectric member E in accordance with the waveform pattern of the drive signal, the piezoelectric member E deforms in accordance with the waveform pattern. The waveform pattern is a combination of a plurality of pulse-like voltage waveforms (voltage pulses). The pressure chamber C expands / contracts in response to the deformation of the piezoelectric member E in response to a sudden change in voltage, thereby applying a pressure change to the ink. The difference between the ink pressure and the atmospheric pressure outside the nozzle N causes the ink to move in a direction along the axis of the nozzle N, and the pressure change is converted into kinetic energy.

[0033] The drive signal preferably has a waveform pattern that efficiently obtains kinetic energy, i.e., ink droplet flight speed, relative to the power consumption corresponding to the drive signal output. A relatively slow flight speed with the same drive signal indicates that the deformation of the piezoelectric element E is not efficiently converted into kinetic energy of the ink. A slight decrease in flight speed can be compensated for by increasing the amplitude of the drive signal. On the other hand, the greater the amplitude, the greater the power consumption. Therefore, a waveform pattern is set that does not significantly reduce the flight speed compared to the maximum flight speed obtained with a drive signal of a certain amplitude.

[0034] Depending on the waveform pattern, even if the main droplet at the front of the droplet has a sufficient flight speed, the tail extending behind the main droplet may not travel at the same speed as the main droplet, and may even travel at a slower speed than the main droplet. Such uneven speed can cause the ejected ink droplet to have a long tail, or the tail to break up into small droplets (satellites). Therefore, a waveform pattern that does not result in a very long tail is preferable.

[0035] 6A to 6C are diagrams showing examples of waveform patterns of drive signals applied to the piezoelectric member E during ink ejection. In the inkjet recording apparatus 100 of this embodiment, a positive voltage is applied to expand the pressure chamber C to reduce the ink pressure, and a negative voltage is applied to contract the pressure chamber C to increase the ink pressure.

[0036] The waveform pattern of the first drive signal shown in Fig. 6A is a simple flicker waveform that expands and then contracts the pressure chamber C. Each voltage pulse that constitutes this waveform pattern is a rectangular pulse that changes substantially discontinuously from a reference voltage to positive and negative voltages, but is not limited to this. It may also be a trapezoidal pulse in which the voltage changes continuously over a certain time width.

[0037] The width of the first pulse W1, which has a positive voltage, i.e., the expansion pulse, and the width of the second pulse W2, which has a negative voltage, i.e., the contraction pulse, are each, for example, 1 AL. This allows the ink to be switched from a depressurized state to a pressurized state in nearly synchronization with the resonance of the ink ejection, i.e., the change in pressure. However, the width of each pulse may be different from the above.

[0038] 7A and 7B show the position V of the main droplet and the length L of the tail extending from the main droplet when a certain time has elapsed since the start of the drive signal, relative to the widths of the first pulse W1 and the second pulse W2, in the waveform pattern of the first drive signal. The position V of the main droplet is indicated by a solid line and a white circle relative to the pulse width. The length L of the tail is indicated by a dashed line and a black circle relative to the pulse width. FIG. 7A shows the measurement results when the width of the first pulse is varied around 1 AL while the width of the second pulse W2 is fixed at 1 AL.

[0039] The position V of the main droplet corresponds to the velocity of the main droplet. In other words, the farther the position of the main droplet, the greater the flight velocity of the droplet. Figure 7A shows that, based on a first pulse W1 width of 1 AL as a reference, the flight velocity decreases as the width deviates from 1.00 AL. A slow flight velocity affects the droplet's landing accuracy due to factors such as wind between the opening surface and the landing surface. Furthermore, deviations in the landing position from the intended position can also lead to degradation of image quality. Therefore, it is undesirable for the width of the first pulse W1 to be greater than 1.30 AL. On the other hand, if the tail length L is large compared to the main droplet position V, a range of landing positions within the droplet will arise, resulting in degradation of image quality. In other words, it is preferable for the ink droplet to have a short tail as long as the flight velocity does not decrease significantly. In other words, it is preferable for the ratio L / V of the tail length L to the main droplet position V to be small. When the width of the first pulse W1 is greater than 1.00 AL, the tail length L becomes shorter. Therefore, the width of the first pulse W1 is preferably in the range of 1.10 AL or more, at which the tail length L becomes small, and 1.30 AL or less, at which the decrease in the position V becomes small.

[0040] FIG. 7B shows the measurement results when the width of the second pulse W2 is changed around 1 AL while the width of the first pulse W1 is fixed at 1.00 AL.

[0041] If the second pulse W2 is 0.72 AL or greater, even if the width of the second pulse W2 increases beyond that, the position V of the main droplet, i.e., the flight speed of the main droplet, changes very little. On the other hand, the tail length L decreases when the second pulse W2 is 0.89 AL or less. That is, when the width of the second pulse W2 is in the range of 0.72 AL to 0.89 AL, a relatively small L / V is obtained. Therefore, the width of the second pulse W2 is preferably in the range of 0.72 AL or greater and 0.89 AL or less, which is a range in which the flight speed does not decrease significantly and the tail length L can be shortened.

[0042] In the waveform pattern of the second drive signal shown in FIG. 6B, the ink pressurization period is divided into two. The width of the first pulse W1, which is a positive voltage expansion pulse, is 1.00 AL, as described above. A pause period H, which is a reference voltage period, is located between the second pulse W2, which is the first negative voltage pulse, and the third pulse W3, which is the second negative voltage pulse. The widths of the second pulse W2, the pause period H, and the third pulse W3 are, for example, each approximately 1 AL. In particular, the width of the second pulse W2 and the pause period H can be made equal. Therefore, the total width of the waveform pattern is approximately 4 AL.

[0043] 8A shows the position of the main droplet and the length of the tail extending from the main droplet after a certain time has elapsed since the start of the drive signal for the waveform pattern of the second drive signal, where the width of the second pulse W2 and the width of the pause period H are each varied around 1 AL while maintaining the same width, while the widths of the first pulse W1 and third pulse W3 are fixed at 1.00 AL.

[0044] It can be seen that if the second pulse W2 and the pause period H are 0.80 AL or greater, the flight speed of the main droplet is sufficient and does not change significantly. Furthermore, within this range, the tail length L does not increase significantly. Note that a tail length L of 1.20 AL is undesirable because it can result in satellite droplets separating from the main droplet. Therefore, when ink is ejected using the waveform pattern of the second drive signal, it is preferable that the widths of the second pulse W2 and the pause period H are equal, and both are 0.80 AL or greater and 1.00 AL or less.

[0045] In the waveform pattern of the third drive signal shown in Figure 6C, a fourth pulse W4, which is a positive voltage pulse, is output instead of the reference voltage pause period H between the second pulse W2 and the third pulse W3, which are two negative voltage pulses in the waveform pattern of the second drive signal. The widths of the first pulse W1, the second pulse W2, and the third pulse W3 are each approximately 1 AL. The width of the fourth pulse W4 is shorter than the pause period H, and in this case, is approximately 0.5 AL. Therefore, the total width of the waveform pattern of the third drive signal is approximately 3.5 AL. The expansion pulse, which is the fourth pulse W4, effectively pulls back the portion of the extruded ink near the nozzle N that does not have a sufficient flight speed compared to the main droplet, thereby creating a "tail-cutting" effect.

[0046] 8B and 8C show the position V of the main droplet and the length L of the tail extending from the main droplet after a certain time has elapsed since the start of the drive signal for the waveform pattern of the third drive signal. Fig. 8B shows the measurement results obtained when the width of the second pulse W2, which is the first contraction pulse, is varied while the widths of the first pulse W1, second pulse W2, and third pulse W3 are fixed at 1.00 AL and the width of the fourth pulse W4 is fixed at 0.50 AL. Fig. 8C shows the measurement results obtained when the widths of the first pulse W1, second pulse W2, and third pulse W3 are fixed at 1.00 AL and the width of the fourth pulse W4, which is the second expansion pulse, is varied.

[0047] As shown in FIG. 8B , when the width of the second pulse W2 is smaller than 0.72 AL, the position V of the main droplet, i.e., its flight velocity, drops significantly. On the other hand, when the width is 0.72 AL or greater, the velocity of the main droplet does not change significantly. The second pulse W2 also has the effect of canceling out the periodic pressure fluctuations caused by the first pulse W1. In this case, too, a width of 1.10 AL for the second pulse W2 undesirably causes satellite droplets to separate from the main droplet. As described above, to enhance the tail-cutting effect of the fourth pulse W4, the width of the second pulse W2 can be kept to 1.00 AL or less, particularly less than 1.00 AL, so as not to overwhelm the canceling effect of the first pulse W1. Therefore, it is preferable that the width of the second pulse W2 be 0.72 AL or greater and 1.00 AL or less.

[0048] As shown in Figure 8C, the width of the fourth pulse W4 does not affect the flight speed of the main droplet. On the other hand, when the width of the fourth pulse W4 is 0.39 AL or more, the tail length L can be shortened as the width increases. However, when the width of the fourth pulse W4 is 0.61 AL, satellite droplets separate from the main droplet, which is undesirable. Therefore, the width of the fourth pulse W4 is preferably 0.39 AL or more and 0.50 AL or less.

[0049] As described above, the inkjet head 1 of this embodiment includes a nozzle N, a pressure chamber C communicating with the nozzle N, and a pressure unit having a piezoelectric member E and an electrode that applies pressure fluctuations to the ink in the pressure chamber C. In the inkjet head 1, the pressure fluctuations applied by the pressure unit cause the ink to protrude from the nozzle N to have a viscosity η of 100 mPa s and a density ρ of 1080 kg m －３ When the nozzle diameter φ and the resonant frequency f are used for this ink, πfρφ ２ The inkjet head 1 has a structure in which the tear-off Q value Qt, expressed as / (8η), is 0.37 or greater. In other words, the inkjet head 1 has a structure in which the tails of ejected ink droplets are properly cut off, even when the ink is highly viscous. The inkjet head 1 having such an appropriate structure can stably eject highly viscous ink without requiring complex control. Furthermore, high viscosity makes it difficult for the ejected ink to separate cleanly from the ink in the nozzle, which can easily result in the formation of tiny droplets such as satellites and adversely affect the flight of the ink. The inkjet head 1 suppresses the generation of such tiny droplets, allowing ink droplets to fly and land more stably.

[0050] The tearing Q value may be 0.5 or more, so that the inkjet head 1 can eject ink droplets in a group at an appropriate flying speed with higher precision.

[0051] The tearing Q value may be 0.76 or more. If the tearing Q value Qt is large to this extent, the inkjet head 1 can stably suppress the generation of minute droplets and can accurately land ink droplets at desired positions.

[0052] The nozzle diameter φ may be 24 μm or more. If the nozzle diameter φ is too small, the cost of manufacturing the inkjet head 1 that can obtain the required AL tends to be high. In addition, ink tends to clog. Therefore, by setting the nozzle diameter φ to 24 μm or more, the inkjet head 1 can easily and stably eject ink.

[0053] The nozzle diameter φ may be 40 μm or more. If the nozzle diameter φ of the nozzle N that ejects high-viscosity ink is this large, the inkjet head 1 can suppress the generation of satellites of high-viscosity ink and eject ink droplets so that they land accurately at the appropriate landing positions.

[0054] The pressure applying section may have a piezoelectric element E that applies pressure fluctuations to the ink by deforming in shear mode in response to the application of voltage. The structure of the inkjet head 1 having the tear Q value Qt of this embodiment can be widely and easily used, including piezoelectric elements that deform in flexural mode. The shear mode tends to make the actuator's driving wall thicker and harder, which tends to increase the resonance frequency. Therefore, when the pressure applying section deforms in shear mode, the AL tends to be shorter. On the other hand, the driving amplitude of the driving wall increases, which tends to reduce the driving voltage.

[0055] In the ink ejection method of this embodiment, the drive pulse that the pressure applying unit generates in the ink is determined so that the flight speed of ink droplets when ejecting ink from the inkjet head 1 is 4 m / s or more. As described above, the flight speed increases according to the amplitude of the drive pulse. Furthermore, by bringing the pulse width closer to the AL corresponding to the resonance frequency f of the drive pulse, the pressure fluctuation is efficiently converted into kinetic energy, thereby obtaining speed. As a result, by achieving a flight speed of 4 m / s or more, the effects of air resistance and wind flow on the ink droplets are reduced, making it easier for them to land at the desired position.

[0056] Furthermore, this ink ejection method may be configured such that the distance between the nozzle opening surface and the landing surface of the ink ejected from the nozzle during ink ejection is 2 mm or more. When ink lands at a distance greater than the usual distance of 1 mm or less, the effects of small droplets and long tails are likely to be significant. On the other hand, high-viscosity inks are often found in functional components. High-viscosity inks include inks that have a three-dimensional structure after being ejected and landing, i.e., have thickness. A large distance is required for the ejection of these inks to prevent contact between the opening surface and the landing surface. In such cases, the above ink ejection method can effectively determine the landing position with high precision. Furthermore, this ink ejection method can prevent small droplets from adhering to unintended locations.

[0057] The viscosity η of the ejected ink may be 50 mPa·s or more. The ink ejection method of this embodiment can eject and land the ink with high precision even when the ink has a high viscosity that differs from the standard viscosity η of 100 mPa·s.

[0058] The ejected ink may contain particles that are not dissolved in the solvent. When the tearing Q value is determined under the above conditions, the nozzle diameter φ is basically increased. This increases the number of particles per unit length in the nozzle N, reducing the risk of uneven particle distribution. Therefore, the inkjet head 1 is capable of stable ink ejection with little unevenness in the ejection speed.

[0059] The first drive signal includes a first pulse W1 that expands the pressure chamber C and a second pulse W2 that contracts the pressure chamber C. The width of the first pulse W1 is 1.10 to 1.30 times the AL, and the width of the second pulse W2 is 0.72 to 0.89 times the AL. By setting the pulse width to approximately the AL, the ink is pressurized in accordance with the vibration of the ink surface in the nozzle N. Therefore, the drive pulse is efficiently converted into kinetic energy. In particular, in the first drive signal, the first pulse W1 is slightly longer than the AL, and the second pulse W2 is slightly shorter than the AL. This ink ejection method using this combination range does not significantly reduce the droplet ejection speed and can maintain a short satellite (tail) length. As a result, this ink ejection method efficiently ejects ink droplets and accurately impacts them.

[0060] The second drive signal may include a first pulse W1 that expands the pressure chamber C, a second pulse W2 that contracts the pressure chamber C, a third pulse W3 that contracts the pressure chamber C again after the second pulse W2, and a pause period H between the second pulse W2 and the third pulse W3, during which neither contraction nor expansion of the pressure chamber C occurs. In this case, the width of the second pulse W2 and the width of the pause period H are both 0.8 times or more and 1.0 times or less than AL. According to this second drive signal in which the third pulse W3 is added with the pause period H sandwiched between them, the trailing end of the ejected ink is appropriately pushed out again after the pause period H that is matched with or slightly shorter than the vibration of the ink surface in the nozzle N. Therefore, this ink ejection method using the second drive signal can suppress the generation of microdroplets from the ejected ink droplet and can also suppress long tails from extending from the main droplet.

[0061] The third drive signal includes a first pulse W1 that expands the pressure chamber C, a second pulse W2 that contracts the pressure chamber C, a third pulse W3 that contracts the pressure chamber C again after the second pulse W2, and a fourth pulse W4 that is positioned between the second pulse W2 and the third pulse W3 and expands the pressure chamber C. In this case, the width of the second pulse W2 is 0.72 to 1.00 times the AL, and the width of the fourth pulse W4 is 0.39 to 0.50 times the AL. The third drive signal includes the fourth pulse W4, which is an expansion pulse, and the fourth pulse W4 pulls back the tail of the ejected ink droplet. This makes it easier for the droplet to separate from the ink in the nozzle N. Therefore, the fourth pulse W4 is set to a length that provides an appropriate pull-back effect without causing satellite droplets to separate from the main droplet. As a result, the ink ejection method using the third drive signal prevents the tails of the ejected ink droplets from growing long, and at the same time, prevents the generation of minute droplets. Therefore, this ink ejection method makes it easier to properly align the ink landing positions.

[0062] The inkjet recording apparatus 100 of this embodiment includes the inkjet head 1 and a control unit 11 that causes a pressure unit to output a drive pulse according to any one of the ink ejection methods and apply it to the ink. The inkjet recording apparatus 100 can appropriately cut off particularly high-viscosity ink, suppress the generation of minute droplets, and eject ink easily and stably.

[0063] The above embodiment is merely an example, and various modifications are possible. For example, the nozzle diameter φ does not have to be limited to the above range. It may be appropriately determined depending on the application and required resolution of the inkjet head 1, the manufacturing technology and cost of the inkjet head 1, the temperature dependency of viscosity, etc.

[0064] Furthermore, the flying speed of the ink droplets does not have to be 4 m / s or more, as long as the ink droplets can fly stably and land on the surface.

[0065] The distance between the nozzle opening surface and the landing surface does not have to be 2 mm or more. The inkjet head 1 may be spaced apart from the medium at a normal distance, and the inkjet recording apparatus 100 may record a two-dimensional image or the like on the recording medium. In this case, it is preferable that the ink droplets be separated from the ink in the nozzles N more quickly according to the distance.

[0066] The piezoelectric element E does not have to be positioned so as to deform in a shear mode. The piezoelectric element E may be positioned so as to deform in a flexural mode, for example. The shape of the pressure chamber C may also be any shape other than the illustrated shape.

[0067] In the above description, the nozzle N is described as having a cylindrical shape, but this is not limiting. The nozzle N may have a tapered shape. In this case, the value of the smallest diameter portion may be used as the nozzle diameter φ.

[0068] In the above embodiment, a voltage is applied to the piezoelectric member E to deform the piezoelectric member E, and a corresponding pressure fluctuation is applied to the ink in the pressure chamber C, but this is not limited to this. A member that deforms in response to a physical quantity other than voltage, such as a magnetostrictive element, may also be used. Furthermore, the multiple nozzles N are not limited to being arranged side by side in a one-dimensional manner. They may also be arranged in a two-dimensional plane. Additionally, the structure of the inkjet head 1 may be different from the structure described above.

[0069] The drive signal is not limited to the waveform patterns exemplified above. Ink may be ejected using a drive signal with a different waveform pattern. Furthermore, even with the waveform patterns exemplified above, the width of each voltage pulse may be different from that described above. A drive signal with an appropriate waveform pattern may be selected depending on the ink characteristics and the surrounding environment, such as temperature and humidity.

[0070] Furthermore, the inkjet head 1 does not have to be distributed integrally with the inkjet recording apparatus 100. A user may purchase or lease the inkjet head 1 independently of the inkjet recording apparatus 100. Purchasing or leasing also includes replacing an existing inkjet head 1 in a certain inkjet recording apparatus 100.

[0071] In addition, the specific configurations, contents and procedures of the processing operations, etc. shown in the above embodiments can be modified as appropriate without departing from the spirit of the present disclosure. The scope of the present invention includes the scope of the invention described in the claims and its equivalents.

[0072] The present disclosure can be used in an inkjet head, an ink ejection method, and an inkjet recording apparatus.

[0073] REFERENCE SIGNS LIST 1 Inkjet head 11 Control unit 12 Memory unit 13 Communication unit 14 Discharge drive unit 15 Conveyance drive unit 100 Inkjet recording device C Pressure chamber Dm Main droplet E Piezoelectric member F Common flow path N Nozzle Pc Cover plate Pn Nozzle plate S Microdroplet T Tail

Claims

1. A nozzle, a pressure chamber communicating with the nozzle, and a pressurizing unit that applies pressure fluctuations to ink in the pressure chamber, wherein the pressure fluctuations applied by the pressurizing unit cause the ink to protrude from the nozzle to have a viscosity η of 100 mPa s and a density ρ of 1080 kg m －３ where πfρφ is the nozzle diameter φ and the resonant frequency f for the ink. ２ The inkjet head has a structure in which the tearing Q value, expressed as / (8η), is 0.37 or more.

2. The ink jet head according to claim 1, wherein said tear Q value is 0.5 or more.

3. The ink jet head according to claim 1, wherein the tear Q value is 0.76 or more.

4. The ink jet head according to claim 1, wherein said nozzle diameter is 24 μm or more.

5. The ink jet head according to claim 1, wherein the nozzle diameter is 40 μm or more.

6. An ink jet head according to claim 1, wherein said pressure applying portion has an electromechanical conversion element which applies said pressure fluctuation by deforming in a shear mode in response to application of a voltage.

7. An ink ejection method according to claim 1, wherein the drive pulses for causing the pressure fluctuations in the ink by the pressurizing section are determined so that the flight speed of ink droplets when ejected is 4 m / s or more.

8. The ink ejection method according to claim 7, wherein, during ink ejection, the distance between the nozzle opening surface and the landing surface of the ink ejected from the nozzle is set to 2 mm or more.

9. The ink ejection method according to claim 7, wherein the viscosity of the ejected ink is 50 mPa·s or more.

10. The ink ejection method according to claim 7, wherein the ejected ink contains particles that are not dissolved in the solvent.

11. The ink ejection method according to claim 7, wherein the drive pulse includes a first pulse for expanding the pressure chamber and a second pulse for contracting the pressure chamber, the width of the first pulse is 1.10 to 1.30 times the AL (Acoustic Length), and the width of the second pulse is 0.72 to 0.89 times the AL.

12. The ink ejection method described in claim 7, wherein the drive pulse includes a first pulse for expanding the pressure chamber, a second pulse for contracting the pressure chamber, a third pulse for contracting the pressure chamber again after the second pulse, and a pause period between the second and third pulses during which neither contraction nor expansion of the pressure chamber occurs, and the width of the second pulse and the width of the pause period are both 0.80 times or more and 1.00 times or less of AL.

13. The ink ejection method described in claim 7, wherein the drive pulse includes a first pulse for expanding the pressure chamber, a second pulse for contracting the pressure chamber, a third pulse for contracting the pressure chamber again after the second pulse, and a fourth pulse positioned between the second and third pulses for expanding the pressure chamber, and the width of the second pulse is not less than 0.72 times and not more than 1.00 times AL, and the width of the fourth pulse is not less than 0.39 times and not more than 0.50 times AL.

14. An inkjet recording device comprising: an inkjet head as claimed in claim 1; and a control unit which causes the pressure unit to output the drive pulse according to the ink ejection method as claimed in any one of claims 7 to 13, thereby applying pressure fluctuations to the ink.

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