JP2007190901A - Ink droplet ejection device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ink droplet ejection device capable of forming an ink droplet to be ejected into a smaller one, without decreasing an ejection speed, and by extension, reducing landing precision, and shortening an entire pulse width to increase a driving frequency and achieving a higher recording speed. <P>SOLUTION: In the ink droplet ejection device, when forming an ink droplet to be ejected into a smaller one, as a driving pulse signal applied to an actuator, a prepulse Pp insufficient for ejecting an ink droplet and a stabilizing pulse Ps are respectively output before and after a main pulse Pm ejecting an ink droplet, and a pulse width Tp of the prepulse Pp, a pulse width Tm of the main pulse Pm, a pulse width Ts of the stabilizing pulse Ps, a space Wp between a terminal end of the prepulse Pp and a leading end of the main pulse Pm, and a space Wm between a terminal end of the main pulse Pm and a leading end of the stabilizing pulse Ps, respectively satisfy the relationships of 0.1AL≤Tp≤0.44AL, 0.5AL≤Tm≤0.8AL, 0.1AL≤Ts≤0.5AL, 0.1AL≤Wp≤0.5AL and 0.4AL≤Wm≤0.8AL for a period of time AL when a pressure wave propagates one way in an ink flow path contained in a pressure chamber. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an ink droplet ejection apparatus using an inkjet method.

  An ink jet printer, which is an ink droplet ejecting apparatus, includes an ink jet head. In an ink jet head having a piezoelectric actuator, an ink is ejected from a nozzle by applying an ejection pressure to the ink due to displacement of the actuator by applying a drive pulse signal. A configuration in which droplets are discharged is known.

  According to the configuration of the ink jet head, the ink droplet ejected from the nozzle has the pulse width of the drive pulse signal applied to the actuator, the one-way propagation time of the pressure wave in the ink channel (the ink in the ink channel) When the generated pressure wave coincides with AL, which is a one-way propagation in the longitudinal direction in the ink flow path, the discharge is performed with the highest energy efficiency and the discharge amount is also maximized.

  By the way, in general, in an inkjet printer, in order to express gradation, a plurality of types of ink dots of different sizes (recording areas) are mixed and recorded on a recording medium, and the discharge amount per dot is set. Need to change. For example, a drive pulse signal is set so that a plurality of pulses are continuously applied to form one dot, and one dot is formed with a plurality of ink droplets, or ink droplets that have started to be ejected Some of the dots are pulled back to reduce the size of the dots. In addition, a stabilization pulse (cancellation pulse) may be applied after the main pulse for ejecting ink in order to suppress the vibration remaining in the ink after ejecting the ink droplet from affecting the next ejection.

  When increasing the ejection amount per dot, for example, as disclosed in Patent Document 1 by the present applicant, the second ink droplet is ejected before the first ink droplet leaves the nozzle. There is a way to merge. Specifically, the pulse width of the main (second) drive pulse is made to coincide with the pressure wave propagation time T (corresponding to the AL), and the first pulse width of 0.35T to 0.65T is set before that. The drive pulse is applied. Further, after the main (second) driving pulse, a pulse having a short pulse width that does not eject ink droplets is applied as the third driving pulse. As a result, the ink droplets generated by the first drive pulse are ejected in a state of poor energy efficiency, and the ink droplets generated by the second drive pulse are ejected energy-efficiently and merged before the ink droplet leaves the nozzle. The third drive pulse suppresses the remaining pressure wave component.

  On the other hand, in the case of reducing the discharge amount per dot, a method of reducing droplets by shifting the pulse width of the main (first) drive pulse from the propagation time AL to deliberately lower the discharge energy efficiency, Alternatively, as disclosed in Patent Document 2, there is a method in which a second drive pulse is applied to reduce the size of the ink droplet at a timing at which a part of the ink droplet that has started to be ejected is pulled back.

Further, in Patent Document 3, in order to reduce the size of an ink droplet, an ejection pulse, a droplet miniaturization pulse, and a stabilization pulse are applied in that order within one driving cycle, and the ink droplet is driven at a driving frequency of 10 kHz. Are listed.
Japanese Patent No. 3551822 (see FIGS. 1 and 6) JP-A-11-170515 (see FIGS. 1 and 6) Japanese Patent Laid-Open No. 11-227203 (see FIG. 2, paragraphs 0027 and 0028)

  However, when the ejection amount per dot is reduced, the ink droplet is ejected with an energy-efficient driving pulse whose pulse width is shorter than AL, compared with the case where the ink droplet is ejected efficiently. Not only is the discharge amount small, but the discharge speed is also slow. Also, the ejection speed is slowed by the method of pulling back a part of the ink droplet with the second drive pulse. When both methods are combined in order to make the ink droplets smaller, the ejection speed is further slowed. As a result, there is a problem that the landing position shifts with respect to the recording medium and the landing accuracy is lowered.

  Further, recent inkjet printers are required to increase the recording speed. For this reason, it is desired to increase the driving frequency, that is, to shorten the driving cycle for forming one dot. In the case where the ejection pulse is applied at the beginning of the driving cycle as described in Patent Document 3, a large ejection energy must be generated by one pulse, and therefore, depending on the pulse width synchronized with the pressure wave generated in the ink, It is necessary to generate a large pressure superimposed on the pressure wave, and it is necessary to apply a stabilization pulse for suppressing the large pressure with a sufficient pulse width and at a sufficient interval from the injection pulse. The pulse width of the entire drive pulse signal composed of a plurality of pulses is increased. As a result, the drive cycle becomes long and the recording speed cannot be increased.

  Therefore, in order to shorten the driving cycle, it is necessary to shorten the time AL for one-way propagation of the pressure wave generated in the ink due to the displacement of the piezoelectric actuator, which is related to the width of each of the plurality of pulses, through the ink flow path. For this purpose, it is conceivable to shorten the ink flow path including the pressure chamber. Then, the length of the pressure chamber that receives the displacement from the actuator is also shortened. As a result, in order to give the same discharge pressure, the drive voltage applied to the piezoelectric actuator must be increased, which is limited.

  The present invention solves the above-mentioned problem, and can reduce the size of the ink droplets to be ejected without lowering the ejection speed and, consequently, the landing accuracy, and shortening the overall pulse width to increase the drive frequency. An object of the present invention is to provide an ink droplet ejection device that can increase the recording speed.

  In order to achieve the above object, an ink droplet ejection apparatus according to claim 1 applies an ink droplet by applying a drive pulse signal to an actuator that changes the volume of a pressure chamber filled with ink. In an ink droplet ejection device that ejects to a recording medium, a drive pulse signal for forming one dot includes a main pulse that ejects ink, a pre-pulse that is output before the main pulse, and a stabilization that is output after the main pulse. A time AL in which the pressure wave generated in the ink in the ink flow path including the pressure chamber propagates in one direction in the longitudinal direction in the ink flow path in accordance with the volume change of the pressure chamber. And output a pre-pulse so as not to cause ink droplets to be ejected. It is characterized in that the output to pull back some of the ink droplets started to discharge by impulse.

  According to a second aspect of the present invention, there is provided the ink droplet ejection device according to the first aspect, wherein the pre-pulse causes the ink in the pressure chamber to vibrate by expanding the volume of the pressure chamber and then reducing the volume. The main pulse is a pulse for ejecting ink in the pressure chamber by expanding the volume of the pressure chamber at an interval from the pre-pulse and then reducing the volume, and the stabilization pulse is a main pulse. The volume of the pressure chamber is enlarged at a timing before the ink started to be ejected by the main pulse is separated from the nozzle, and then the ink chamber is further reduced to draw back a part of the ink to the nozzle side and the pressure. It is a pulse that suppresses residual vibration of ink in the room.

  The ink droplet ejection device according to a third aspect of the present invention is the ink droplet ejection device according to the first or second aspect, wherein the main pulse is one of the two voltage values set to drive the actuator. The pulse having a pulse width sufficient to allow the voltage applied to the actuator to reach from one voltage value to the other voltage value, and the stabilization pulse has the voltage from one voltage value to the other voltage value. The pulse is set to have a short pulse width so that it does not reach.

  The ink droplet ejection device according to a fourth aspect of the invention is the ink droplet ejection device according to the third aspect, wherein a pulse width of the stabilization pulse and a pulse interval between the main pulse and the stabilization pulse are Each one-way propagation time AL is less than 1 AL.

  The ink droplet ejection device according to claim 5 is the ink droplet ejection device according to any one of claims 1 to 4, wherein the pre-pulse, the main pulse, and the pulse width of the stabilization pulse and a pulse interval therebetween. Are each less than 1 AL with respect to the one-way propagation time AL.

An ink droplet ejection apparatus according to a sixth aspect of the invention is the ink droplet ejection device according to the first aspect, wherein the pulse width Tp of the pre-pulse, the pulse width Tm of the main pulse, the pulse width Ts of the stabilization pulse, An interval Wp between the end of the pre-pulse and the start of the main pulse, and an interval Wm between the end of the main pulse and the start of the stabilization pulse are respectively set with respect to the one-way propagation time AL.
0.1AL ≦ Tp <0.44AL, 0.1AL ≦ Wp ≦ 0.5AL,
0.5AL ≦ Tm ≦ 0.8AL, 0.4AL ≦ Wm ≦ 0.8AL,
0.1AL ≦ Ts ≦ 0.5AL,
It is characterized by satisfying the relationship.

According to a seventh aspect of the present invention, there is provided the ink droplet ejection device according to the first aspect, wherein the prepulse pulse width Tp, the main pulse pulse width Tm, the stabilization pulse width Ts, An interval Wp between the end of the pre-pulse and the start of the main pulse, and an interval Wm between the end of the main pulse and the start of the stabilization pulse are respectively set with respect to the one-way propagation time AL.
0.20AL ≦ Tp ≦ 0.33AL, 0.11AL ≦ Wp ≦ 0.44AL,
0.56AL ≦ Tm ≦ 0.78AL, 0.56AL ≦ Wm ≦ 0.78AL,
0.11AL ≦ Ts ≦ 0.44AL,
It is characterized by satisfying the relationship.

An ink droplet ejection apparatus according to an eighth aspect of the present invention is the ink droplet ejection device according to the second aspect, wherein the pulse width Tp of the pre-pulse, the pulse width Tm of the main pulse, the pulse width Ts of the stabilization pulse, An interval Wp between the end of the pre-pulse and the start of the main pulse, and an interval Wm between the end of the main pulse and the start of the stabilization pulse are respectively set with respect to the one-way propagation time AL.
0.25AL <Tp <0.50AL, 0.08AL ≦ Wp <0.25AL,
0.50AL <Tm <0.80AL, 0.50AL <Wm <0.80AL,
0.18AL <Ts <0.50AL,
It is characterized by satisfying the relationship.

The ink droplet ejection device according to claim 9 is the ink droplet ejection device according to claim 1 or 2, wherein the pulse width Tp of the pre-pulse, the pulse width Tm of the main pulse, and the pulse width of the stabilization pulse. Ts, the interval Wp between the end of the pre-pulse and the start end of the main pulse, and the interval Wm between the end of the main pulse and the start end of the stabilization pulse, respectively, with respect to the one-way propagation time AL,
0.375AL ≦ Tp ≦ 0.425AL, Wp = 0.125AL,
Tm = 0.625AL, 0.625AL ≦ Wm ≦ 0.675AL,
0.175AL ≦ Ts ≦ 0.375A,
It is characterized by satisfying the relationship.

  The ink droplet ejection device according to a tenth aspect of the present invention is the ink droplet ejection device according to any one of the first to ninth aspects, wherein the actuator is a piezoelectric element that is displaced with respect to the pressure chamber when a voltage is applied. It is characterized by being.

  According to the first aspect of the present invention, since the ejection width is intentionally deteriorated by shifting the pulse width Tm of the main pulse Pm with respect to the one-way propagation time AL, the ejection amount of the ink droplet ejected by the main pulse is reduced. Less. Furthermore, by pulling back part of the ink droplets that started to be ejected by the main pulse Pm by the stabilization pulse Ps, it is possible to reduce the size of the ink dots formed on the recording medium. In addition, since the pre-pulse Pp that does not cause ink droplets to be ejected is first applied and the main pulse Pm is applied in a state where the ink is vibrated in advance, at the time when the main pulse Pm is applied. The ink droplets are easily ejected. For this reason, even if the energy efficiency of ejection by the main pulse Pm is poor, ink droplets can be ejected quickly, and the ejection speed is less likely to decrease. As a result, even if the ink droplet is reduced in size, it is possible to prevent the ejection speed and hence the landing accuracy from being lowered. Further, by adding the stabilization pulse Ps at the end, not only can the ink droplet be reduced in size as described above, but also the residual pressure wave is suppressed and the influence on the subsequent drive pulse signal is eliminated.

  According to the second aspect of the present invention, each pulse expands the volume of the pressure chamber and then shrinks, and first vibrates the ink in the pressure chamber by the pre-pulse, and then supplies the ink by the main pulse. Since the ink is ejected, the ink can be ejected even with a main pulse that is relatively short with respect to the period of the pressure wave of the ink. Furthermore, even if a stabilization pulse is applied at a short interval thereafter, part of the ink that has started to be ejected can be satisfactorily pulled back, and residual vibration of the ink in the pressure chamber can be suppressed. That is, it is possible to shorten the pulse width as a whole of the drive pulse signal composed of a plurality of pulses, increase the drive frequency, and increase the recording speed. In addition, a small volume of ink droplets can be accurately obtained by satisfactorily pulling back part of the ink that has started to be ejected.

  According to the third aspect of the present invention, the stabilizing pulse has a short pulse width, and is set so that the voltage applied to the actuator does not reach from one voltage value to the other voltage value. That is, the heat generation and fatigue of the actuator can be suppressed, and the entire pulse width composed of a plurality of drive pulse signals can be shortened, the drive frequency can be increased, and the recording speed can be increased.

  According to the fourth aspect of the present invention, the pulse width of the stabilization pulse and the pulse interval between the main pulse and the stabilization pulse are set according to the time AL in which the pressure wave propagates in the longitudinal direction in the ink flow path, that is, the pressure. By setting it to less than ½ of the wave fluctuation period, the effect of the invention of claim 3 can be easily realized.

  According to the fifth aspect of the present invention, the pulse widths of the pre-pulse, the main pulse, and the stabilization pulse and the pulse interval therebetween are set as the time AL, that is, the pressure wave when the pressure wave propagates in one direction in the longitudinal direction in the ink flow path. By making the fluctuation period less than ½, the effects of the inventions of claims 1 to 4 can be easily realized.

  According to the sixth and seventh aspects of the invention, by combining Tp, Tm, Ts, Wp, and Wm so as to satisfy the above relationship, the ink droplets can be reduced in size and the ejection speed and landing accuracy can be reduced. Thus, the effect of the invention of claim 1 can be reliably realized.

  According to the eighth and ninth aspects of the invention, Tp, Tm, Ts, Wp, and Wm are combined so as to satisfy the above relationship, thereby reducing the size of the ink droplet and increasing the recording speed. The effect of the invention of claim 2 can be realized with certainty.

  According to the invention described in claim 10, in the configuration of each of the above claims, the piezoelectric element that is displaced with respect to the pressure chamber by application of voltage is preferably used as the actuator.

  Embodiments of the present invention will be described below with reference to the drawings. 1 is a perspective view of an inkjet head applied to the ink droplet ejection apparatus of the present invention, FIG. 2 is an exploded perspective view of the inkjet head, FIG. 3 is an enlarged exploded perspective view of a cavity unit, and FIG. 4 is IV-IV in FIG. FIG. 5 is an enlarged sectional view taken along line VV in FIG. 1, FIG. 6 is a block diagram of a control device, and FIG. 7 is a schematic diagram showing a relationship between a pulse width and a voltage in a driving pulse. FIG. 8 is a schematic diagram showing a drive pulse signal, FIG. 9 is a diagram showing an experimental result for optimizing the drive pulse signal, and FIG. 10 is a diagram showing another experimental result for optimizing the drive pulse signal.

  An ink droplet ejection apparatus according to an embodiment of the present invention is an ink jet printer, and an ink jet head 100 provided in the ink jet printer has a direction (main main direction) perpendicular to a recording medium conveyance direction (sub-scanning direction, hereinafter referred to as X direction). It is mounted on a carriage (not shown) that reciprocates in the scanning direction (hereinafter referred to as the Y direction). In the inkjet head 100, for example, ink cartridges filled with four color inks of cyan, magenta, yellow, and black are detachably mounted on the carriage, or statically mounted on the main body of the inkjet printer. The ink of each color is supplied through a supply pipe or the like.

  As shown in FIG. 1, a plate-type piezoelectric actuator 2 is joined to a cavity unit 1 composed of a plurality of plates made of a metal plate, and an ink jet head 100 is externally connected to the upper surface (rear surface) of the plate-type piezoelectric actuator 2. A flexible flat cable 3 (see FIG. 4) for connection with a device is lap-joined. Then, it is assumed that ink is ejected downward from the nozzle 4 opened on the lower surface (front surface) side of the cavity unit 1.

  As shown in FIG. 2, the cavity unit 1 includes a total of eight plates including a nozzle plate 11, a spacer plate 12, a damper plate 13, two manifold plates 14a and 14b, a supply plate 15, a base plate 16, and a cavity plate 17. It has a structure in which thin plates are joined together with adhesive.

  In the embodiment, each of the plates 11 to 17 has a thickness of about 50 to 150 μm, the nozzle plate 11 is made of synthetic resin such as polyimide, and the other plates 12 to 17 are made of 42% nickel alloy steel plate. In the nozzle plate 11, a large number of nozzles 4 for ejecting ink having a minute diameter (about 25 μm) are formed at minute intervals. The nozzles 4 are arranged in five rows parallel to the long side direction (X direction) of the nozzle plate 11.

  Further, as shown in FIG. 3, a plurality of pressure chambers 36 are arranged in the cavity plate 17 in five rows parallel to the long side (the X direction) of the cavity plate 17. In the embodiment, each of the pressure chambers 36 is formed in an elongated shape in plan view, and the longitudinal direction thereof is bored along the short side direction (Y direction) of the cavity plate 17, and one end portion 36 a in the longitudinal direction is formed. The other end 36b communicates with a common ink chamber 7 described later.

  One end 36a of each pressure chamber 36 is connected to the supply plate 15, the base plate 16 and the two manifold plates 14a and 14b, the damper plate 13 and the small-diameter communication hole 37 formed in the spacer plate 12. The nozzle plate 11 communicates with the nozzles 4.

  The base plate 16 adjacent to the lower surface of the cavity plate 17 has a through hole 38 connected to the other end 36b of each pressure chamber 36.

  The supply plate 15 adjacent to the lower surface of the base plate 16 is provided with a connection flow path 40 for supplying ink from the common ink chamber 7 described later to each pressure chamber 36. Each connection channel 40 is provided between an inlet hole into which ink enters from the common ink chamber 7, an outlet hole opened on the pressure chamber 36 side (through hole 38), and the inlet and outlet holes. 40 is provided with a throttle portion formed with a reduced cross-sectional area so as to have the largest flow path resistance in 40.

  In the two manifold plates 14a and 14b, five common ink chambers 7 extending along the long side direction (X direction) are formed through the plate thickness so as to extend along each row of the nozzles 4. ing. That is, as shown in FIGS. 2 and 4, two manifold plates 14a and 14b are stacked, and the upper surface thereof is covered with the supply plate 15, and the lower surface is covered with the damper plate 13, so that a total of five common plates are used. An ink chamber (manifold chamber) 7 is formed in a sealed state. Each of the common ink chambers 7 extends in the row direction of the pressure chambers 36 (the row direction of the nozzles 4) so as to overlap a part of the pressure chambers 36 when viewed in plan from the stacking direction of the plates.

  As shown in FIGS. 3 and 4, a damper chamber 45 isolated from the common ink chamber 7 is formed as a recess on the lower surface side of the damper plate 13 adjacent to the lower surface of the manifold plate 14a. The positions and shapes of the damper chambers 45 are matched with the common ink chambers 7, as shown in FIG. Since the damper plate 13 is a metal material that can be elastically deformed as appropriate, the thin plate-like ceiling portion above the damper chamber 45 can freely vibrate both on the common ink chamber 7 side and on the damper chamber 45 side. it can. Even when the pressure fluctuation generated in the pressure chamber 36 propagates to the common ink chamber 7 during ink ejection, the ceiling portion elastically deforms and vibrates, thereby producing a damper effect that absorbs and attenuates the pressure fluctuation. This is intended to reduce so-called crosstalk in which pressure fluctuations propagate to other pressure chambers 36.

  In addition, as shown in FIG. 2, four ink supply ports 47 are formed in the end portions on one short side of the cavity plate 17, the base plate 16, and the supply plate 15 so as to correspond to the upper and lower positions. Has been. Ink from the ink supply source communicates with the one end portion of the common ink chamber 7 from these ink supply ports 47. The four ink supply ports 47 are individually labeled 47a, 47b, 47c, and 47d in order from the left side of FIG.

  In the ink flow path from the ink supply port 47 to the nozzle 4, the ink is supplied from the ink supply port 47 to the common ink chamber 7 serving as an ink supply channel, and then connected to the supply plate 15 as shown in FIG. 3. Distribution is supplied to each pressure chamber 36 via the passage 40 and the through hole 38 of the base plate 16. As will be described later, when the piezoelectric actuator 2 is driven, the ink passes from the pressure chambers 36 through the communication holes 37 to the nozzles 4 corresponding to the pressure chambers 36. When a discharge pressure is applied to the pressure chamber 36 by driving the piezoelectric actuator 2 described later, a pressure wave is transmitted from the pressure chamber 36 through the communication hole 37 to the nozzle 4 to discharge ink.

  In this embodiment, as shown in FIG. 2, four ink supply ports 47 are provided, whereas five common ink chambers 7 are provided, and only two ink supply ports 47a are common. The ink chambers 7 and 7 are connected. The ink supply port 47a is set so that black ink is supplied, and it is considered that black ink is used more frequently than other color inks. The other ink supply ports 47b, 47c, and 47d are supplied with yellow, magenta, and cyan inks, respectively. A filter body 20 having a filtration portion 20a corresponding to each opening is attached to the ink supply ports 47a, 47b, 47c, and 47d with an adhesive or the like (see FIG. 1).

  On the other hand, as shown in FIG. 5, the piezoelectric actuator 2 has a structure in which a plurality of piezoelectric sheets 41 to 43 having a thickness of about 30 μm are laminated, and Japanese Patent Laid-Open Nos. 4-341833 and 2002-254634. Similar to the known ones disclosed in Japanese Laid-Open Patent Publication No. H. et al., The upper surface (wide surface) of the piezoelectric sheet 42 of a predetermined number of even-numbered stages from the bottom is placed in each pressure chamber 36 in the cavity unit 1. Narrow individual electrodes 44 are formed in rows along the long side direction (X direction) for each corresponding location. A common electrode 46 common to the plurality of pressure chambers 36 is formed on the upper surface (wide surface) of a predetermined number of odd-numbered piezoelectric sheets 41 from below, and a surface is formed on the upper surface of the uppermost sheet. As the electrode 48, a surface electrode connected to each of the individual electrodes corresponding to the stacking direction via an electrical through hole or the like, and an electrode 48 connected to the common electrode via an electrical through hole or the like. And a surface electrode.

As is well known, by applying a high voltage between the individual electrode 44 and the common electrode 46, the portion of the piezoelectric sheet located between both electrodes is polarized and formed as an active portion 49.

  Then, an adhesive sheet (not shown) made of a non-ink-permeable synthetic resin as an adhesive is attached in advance to the entire lower surface (the wide surface facing the pressure chamber 36) of the plate-type piezoelectric actuator 2. Then, the piezoelectric actuator 2 is bonded and fixed to the cavity unit 1 with the individual electrodes 44 facing the pressure chambers 36 in the cavity unit 1. Further, when the flexible flat cable 3 is pressed against the upper surface of the piezoelectric actuator 2, various wiring patterns (not shown) in the flexible flat cable 3 are electrically connected to the surface electrodes 48. Are joined together.

  Next, the configuration of a control device for controlling the drive pulse signal applied to each electrode will be described with reference to FIG. The control device includes an LSI chip 50 (see FIG. 1) disposed on the flexible flat cable 3. The surface electrode corresponding to each of the individual electrode 44 and the common electrode 46 is connected to this. The LSI chip 50 is also connected with a clock line 51, a data line 52, a voltage line 53, and an earth line 54. In synchronization with the clock pulse supplied from the clock line 51, data corresponding to each nozzle 4 is serially supplied onto the data line 52, and a plurality of drive pulses supplied from the main circuit (not shown) through the voltage line 53. One of the signal data is selected based on the data, and a drive pulse signal having a voltage value suitable for driving the active portion 49 is generated, and ejected to the ink in the pressure chamber 36 corresponding to the active portion 49. Pressure is applied. Then, an ink droplet is ejected from the nozzle 4 by the forward component of the pressure wave from the pressure chamber 36 to the nozzle 4.

  As shown in FIG. 7A, the drive pulse signal is composed of a pulse that varies between voltage values V1 and V2. In this embodiment, V1 is an arbitrary positive voltage value, and V2 is zero. V is set. Before ink discharge, since the positive voltage V1 is applied to all the individual electrodes 44 and the common electrode 46 is grounded, the active portion 49 between the individual electrode 44 and the common electrode 46 extends, and the entire pressure chamber 36 is expanded. Is in a reduced state. When the voltage application to each individual electrode 44 in the stacking direction corresponding to the pressure chamber 36 that is about to eject ink is stopped (switched to V2), the active portion 49 returns to the contracted state and the volume of the pressure chamber 36 is reduced. Expanding. Then, the ink in the pressure chamber 36 becomes negative pressure and a pressure wave is generated. When a voltage is again applied to each individual electrode 44 at a timing when the pressure wave is inverted and becomes a positive pressure, the pressure due to the extension of the active portion 49 and the pressure reversed to the positive pressure are superimposed, and the ink droplet is formed. It is discharged from the nozzle 4.

  As described above, the pulse changes between the preset voltages V1 and V2, but actually, as shown in FIG. 7B, a delay time occurs at the falling edge and the rising edge of the waveform. This is because the piezoelectric layer sandwiched between the individual electrode 44 and the common electrode 46 acts as a capacitor (C), and there is a resistance (R) in the path from the control device that outputs the drive pulse signal to the individual electrode 44. Therefore, even if the control device outputs a rectangular wave as a drive pulse signal, an integration circuit is formed by the CR, and the rise and fall of the pulse at the individual electrode 44 are rounded (delayed). Therefore, by setting the pulse Pm to a sufficient pulse width Tm including the rounded portion, the voltage applied to the actuator can reach the voltage V2 from the voltage V1. On the other hand, by setting the pulse Ps to a short pulse width Ts, the voltage V1 does not reach the voltage V2, that is, the change in the voltage applied to the actuator can be made a low voltage difference.

  Contrary to the above, like the actuator disclosed in Japanese Patent Laid-Open No. 2001-301161, a voltage is applied to the drive electrode to expand the volume of the pressure chamber and generate a pressure wave. By stopping the application of voltage when the wave is reversed, the volume of the pressure chamber may be reduced to eject ink droplets.

  In this ink droplet ejection apparatus, in order to perform gradation expression that changes the diameter (area) of dots formed on a recording medium, data signals of a plurality of drive pulse signals having different ink volumes per dot are set in advance. is doing. When the dot diameter is controlled, the number of pulses for ejecting ink droplets is increased or decreased as is well known, but FIG. 8 shows an ink droplet having a smaller volume than the volume of ink droplets ejected by one normal pulse. A driving pulse signal for discharging one (small dot, small dot) is shown.

  The drive pulse signal shown in FIG. 8 is composed of three drive pulses Pp, Pm, and Ps. As described with reference to FIG. 7, each pulse drives the actuator to expand the volume of the pressure chamber and then reduce the volume.

  The time until the pressure wave of the ink changes from negative pressure to positive pressure is determined by the time AL during which the pressure wave propagates one way through the ink flow path for each nozzle including the pressure chamber 36, the communication hole 37, and the through hole 38. This one-way propagation time AL is influenced not only by the natural frequency of the ink and the length of the ink flow path but also by the flow resistance and the rigidity of each plate constituting the flow path.

  That is, when the time from the rise to the fall of the drive pulse, that is, the pulse width is made to coincide with the one-way propagation time AL of the pressure wave, the highest pressure is superimposed and the energy efficiency is maximized, and the ink droplet ejection speed, The discharge amount (droplet volume) reaches a peak. Therefore, in this embodiment, since the purpose is to eject ink droplets smaller than the standard, the ink pulse is intentionally ejected with reduced energy efficiency, and the pulse width Tm of the main pulse Pm is set to the AL. Shorter (Tm <AL).

  In addition, since the main pulse Pm has low energy efficiency and discharge speed is low, this alone causes a shift in the landing position. For this reason, a pre-pulse Pp having a pulse width Tp set to such a short width as not to eject ink droplets is applied before the main pulse Pm. The pre-pulse Pp has a pulse width that does not cause the ink to be ejected in a substantially calm state where the vibration of the ink is small, and gives vibration to the ink prior to the next main pulse Pm, that is, to generate a pressure wave. is there. That is, since the ink is vibrated to such an extent that no ink droplets are ejected, when the main pulse Pm is applied, the ink is vibrated in advance and easily ejects, and the main pulse Pm becomes a pressure wave of the prepulse Pp. The ink droplets can be ejected from the nozzles 4 without causing a decrease in speed by superimposing and generating a larger pressure wave.

  A stabilization pulse Ps is applied after the main pulse Pm. The stabilization pulse Ps is applied at a timing before the ink droplet started to be ejected from the nozzle by the main pulse Pm is separated from the nozzle, and is set to a short width so as not to eject the ink droplet. By expanding the volume of the pressure chamber 36, the tail (terminal) of the ink droplet ejected by the main pulse Pm is pulled back to the nozzle 4 side, and the ink droplet flying to the recording medium is reduced in size. Further, the stabilization pulse Ps is applied at a phase that substantially cancels the pressure wave of the ink in the pressure chamber 36, and the residual vibration of the ink can be suppressed.

  Next, a study was made to optimize each pulse width value of the drive pulse signal. For the drive pulses Pp, Pm, and Ps, the interval between the end of the pre-pulse Pp and the start of the main pulse Pm is Wp, and the interval between the end of the main pulse Pm and the start of the stabilization pulse Ps is Wm. As combinations of Tp, Tm, Ts, Wp, and Wm, an experiment was performed on 16 types (No. 1 to No. 16) shown in FIG. In FIG. 9, the numerical values described in the “pulse width and interval” column are values to be multiplied by the one-way propagation time AL, and the one-way propagation time AL is determined for each inkjet head 100. In this experiment, an inkjet head 100 having an AL of about 5 μsec was used.

  The experiment shown in FIG. 9 is an experiment conducted for the purpose of preventing the ejection speed from being lowered when ejecting small ink droplets (small balls), and therefore, the evaluation items are set to “stability”, “speed”, and “capacity”. Set. “Stability” is determined by determining whether or not there is a defective portion such as splash, twist, or penetration in the recording state of the recording medium. “Speed” is a judgment as to whether or not it is faster than a reference speed at which landing accuracy is obtained. The “capacity” is determined by determining whether the capacity is smaller than the reference capacity, that is, the capacity suitable for the small ball. In both cases, the case where the result is good is indicated by ◯, and the case where the result is defective is indicated by ×.

As can be seen from the experimental results shown in FIG. 1-No. There were only 5 combinations of 5.
No. 1
Tp = 0.33AL
Wp = 0.11AL
Tm = 0.56AL
Wm = 0.56AL
Ts = 0.33AL
N0.2
Tp = 0.22AL
Wp = 0.11AL
Tm = 0.56AL
Wm = 0.67AL
Ts = 0.44AL
N0.3
Tp = 0.22AL
Wp = 0.11AL
Tm = 0.56AL
Wm = 0.78AL
Ts = 0.33AL
N0.4
Tp = 0.20AL
Wp = 0.30AL
Tm = 0.60AL
Wm = 0.60AL
Ts = 0.20AL
N0.5
Tp = 0.22AL
Wp = 0.44AL
Tm = 0.78AL
Wm = 0.67AL
Ts = 0.11AL
From this experimental result, the optimum drive pulse signal for Kodama is
0.20AL ≦ Tp ≦ 0.33AL,
0.11AL ≦ Wp ≦ 0.44AL,
0.56AL ≦ Tm ≦ 0.78AL,
0.56AL ≦ Wm ≦ 0.78AL,
0.11AL ≦ Ts ≦ 0.44AL,
Was in the range.

Furthermore, no. In No. 16, although Wp, Tm, Wm, and Ts were within the above range, a favorable result was not obtained at Tp = 0.44AL. On the other hand, in other experimental results where Tp = 0.11 AL, other numerical values deviated greatly from the above range, and thus preferable results could not be obtained. In consideration of the tolerance, margin, etc. that the inventor often experiences in this experiment, the same result can be obtained in the range of 0.1AL ≦ Tp <0.44AL. Similarly for other numbers,
0.1AL ≦ Tp <0.44AL,
0.1AL ≦ Wp ≦ 0.5AL,
0.5AL ≦ Tm ≦ 0.8AL,
0.4AL ≦ Wm ≦ 0.8AL,
0.1AL ≦ Ts ≦ 0.5AL,
It has been found that satisfying the above relationship is necessary to form a driving pulse signal for a small ball.

  By configuring the drive pulse signal so as to satisfy the above range, it is possible to suppress a decrease in ejection speed even when the ink droplet is ejected in a small size. Therefore, it is possible to realize an ink jet head capable of controlling droplets without damaging landing accuracy (recording accuracy) even for small droplets.

  Next, the present inventor also evaluated the combination shown in FIG. 10 as another experiment for optimizing Tp, Tm, Ts, Wp, and Wm. Since the experiment shown in FIG. 10 is an experiment conducted for the purpose of shortening the driving cycle in ejecting small ink droplets (small balls), the evaluation items were set to “stability” and “density”.

  “Stability” indicates whether there is any splash or ink mist in the ejection state, and “○” indicates the most stable state with no splash or ink mist. “△” indicates that there is no problem, and “×” indicates that the stability is poor and impractical. Further, the “density” was evaluated by evaluating the volume of the ink droplets by ejecting the ink in a dot matrix form on a predetermined area on the recording medium and observing the density. That is, if an appropriate volume of ink droplets is ejected continuously and stably, an image having a predetermined density is formed. “○” indicates that the density is within the appropriate range, and “△” indicates that the density is slightly lighter or darker than that, but there is no practical problem, and the density is too low or too high. Those which were too small or too large were marked with “x”.

  In this experiment, the driving pulse signals optimum for the formation of the small balls were those of numbers (1) to (6) described on the right side of FIG. The numerical values in FIG. 10 are actual numerical values (μsec) of the pulse width and interval. In the ink jet head used in this experiment, as the volume of the pressure chamber 36 is increased, the pressure wave generated in the ink in the ink flow path including the pressure chamber 36 propagates in one direction in the longitudinal direction in the ink flow path ( That is, 1/2 of the ink pressure fluctuation period AL = 4 μsec. The following are actual values (μsec) of pulse width and interval, and AL converted values.

(1) Tp = 1.5 μsec Tp = 0.375AL
Wp = 0.5μsec Wp = 0.125AL
Tm = 2.5μsec Tm = 0.625AL
Wm = 2.5μsec Wm = 0.625AL
Ts = 1.5μsec Ts = 0.375AL
(2) Tp = 1.5 μsec Tp = 0.375AL
Wp = 0.5μsec Wp = 0.125AL
Tm = 2.5μsec Tm = 0.625AL
Wm = 2.7μsec Wm = 0.675AL
Ts = 1.5μsec Ts = 0.375AL
(3) Tp = 1.5 μsec Tp = 0.375AL
Wp = 0.5μsec Wp = 0.125AL
Tm = 2.5μsec Tm = 0.625AL
Wm = 2.5μsec Wm = 0.625AL
Ts = 0.7μsec Ts = 0.175AL
(4) Tp = 1.5 μsec Tp = 0.375AL
Wp = 0.5μsec Wp = 0.125AL
Tm = 2.5μsec Tm = 0.625AL
Wm = 2.5μsec Wm = 0.625AL
Ts = 1.1μsec Ts = 0.275AL
(5) Tp = 1.5 μsec Tp = 0.375AL
Wp = 0.5μsec Wp = 0.125AL
Tm = 2.5μsec Tm = 0.625AL
Wm = 2.5μsec Wm = 0.625AL
Ts = 1.3μsec Ts = 0.325AL
(6) Tp = 1.7 μsec Tp = 0.425AL
Wp = 0.5μsec Wp = 0.125AL
Tm = 2.5μsec Tm = 0.625AL
Wm = 2.5μsec Wm = 0.625AL
Ts = 1.5μsec Ts = 0.375AL
That is, (1) to (6)
1.5 μsec ≦ Tp ≦ 1.7 μsec 0.375 AL ≦ Tp ≦ 0.425 AL
Wp = 0.5μsec Wp = 0.125AL
Tm = 2.5μsec Tm = 0.625AL
2.5 μsec ≦ Wm ≦ 2.7 μsec 0.625 AL ≦ Wm ≦ 0.675 AL
0.7 μsec ≦ Ts ≦ 1.5 μsec 0.175 AL ≦ Ts ≦ 0.375 AL
It is in the range.

  These AL converted values coincided with the range obtained by the experiment of FIG. 9, and it was confirmed that the range obtained by the experiment of FIG. 9 was correct.

  The pulse width Tm of the main pulse Pm has sufficient time to allow the voltage applied to the actuator to reach the voltage V2 from the voltage V1 as in the case of the pulse Pm in FIG. In the range where the pulse widths Ts and Tp are short, the stabilization pulse Ps and the pre-pulse Pp do not reach the voltage V2 from the voltage V1 as in the case of the pulse Ps in FIG. Note that, in the region where the pulse widths Ts and Tp are close to 2 μsec, the voltage V2 is almost reached, but a practically satisfactory result can be obtained.

  Further, in the experiment of FIG. 10, good results could be obtained even when Tp exceeding Tp <0.44AL in the experiment of FIG. 9 was 1.9 μsec (0.475AL). This is considered to be due to a combination of a low range and Tp with respect to the range of Wp in the experiment of FIG. 9 of 0.3 μsec <Wp <0.9 μsec (0.125AL <Wp <0.225AL).

  That is, as a result of the experiment of FIG. 10, the range suitable for practical use in consideration of the margin and the like was as follows.

1 μsec <Tp <2 μsec 0.25AL <Tp <0.50AL
0.3 μsec ≦ Wp <1 μsec 0.08AL ≦ Wp <0.25AL
2 μsec <Tm <3.2 μsec 0.50AL <Tm <0.80AL
2μsec <Wm <3.2μsec 0.50AL <Wm <0.80AL
0.7 μsec ≦ Ts <2 μsec 0.18AL <Ts <0.50AL
In the drive pulse signal, the volume of the ink droplet when ejected by only the combination of the pre-pulse Pp and the main pulse Pm is 2 pl (picoliter), but the ink droplet when the stabilization pulse Ps is added thereafter is used. The volume was 1.5 pl, and the ink droplets could be miniaturized. In addition, as apparent from the above-described experiment, the residual vibration of the ink after each ejection can be suppressed and the ink can be ejected continuously and stably over a predetermined area.

It is a perspective view of the inkjet head applied to the ink droplet ejection apparatus of the present invention. It is a disassembled perspective view of an inkjet head. It is an expansion disassembled perspective view of a cavity unit. FIG. 4 is an enlarged sectional view taken along line IV-IV in FIG. 1. FIG. 5 is an enlarged sectional view taken along line VV in FIG. 1. It is a block diagram of a control apparatus. (A) And (b) is a schematic diagram which shows the relationship between the pulse width and voltage in a drive pulse. It is a schematic diagram which shows a drive pulse signal. It is a figure which shows the experimental result which optimizes a drive pulse signal. It is a figure which shows another experimental result of optimizing a drive pulse signal.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cavity unit 2 Piezoelectric actuator 3 Flexible flat cable 4 Nozzle 7 Common ink chamber 36 Pressure chamber 41-43 Piezoelectric sheet 44 Individual electrode 46 Common electrode 48 Surface electrode 50 LSI chip 100 Inkjet printer

Claims (10)

In an ink droplet ejection apparatus that ejects ink droplets onto a recording medium by applying a drive pulse signal to an actuator that changes the volume of a pressure chamber filled with ink,
The drive pulse signal for forming one dot has a main pulse for ejecting ink, a pre-pulse output before the main pulse, and a stabilization pulse output after the main pulse,
A pulse in which the main pulse is shifted with respect to the time AL in which the pressure wave generated in the ink in the ink flow path including the pressure chamber propagates in one direction in the longitudinal direction in the ink flow path as the volume of the pressure chamber changes. The width is set and output, the pre-pulse is output so as not to eject the ink droplet, and the stabilization pulse is output so as to pull back a part of the ink droplet started to be ejected by the main pulse. Ink droplet ejection apparatus characterized by the above. The pre-pulse is a pulse that vibrates the ink in the pressure chamber by expanding the volume of the pressure chamber and then reducing the volume.
The main pulse is a pulse for discharging ink in the pressure chamber by expanding the volume of the pressure chamber at an interval from the pre-pulse and then reducing the volume.
The stabilizing pulse is spaced from the main pulse, and the volume of the pressure chamber is enlarged at a timing before the ink started to be ejected by the main pulse leaves the nozzle, and then a part of the ink is reduced. 2. The ink droplet ejection apparatus according to claim 1, wherein the ink droplet ejection device pulls back toward the nozzle and suppresses residual vibration of ink in the pressure chamber.   Of the two voltage values set for driving the actuator, the main pulse has a pulse width sufficient for the voltage applied to the actuator to reach from one voltage value to the other voltage value. 3. The pulse according to claim 1, wherein the stabilization pulse is a pulse whose pulse width is set short so that the voltage does not reach from one voltage value to the other voltage value. 4. Ink droplet ejection device.   The ink droplet according to claim 3, wherein a pulse width of the stabilization pulse and a pulse interval between the main pulse and the stabilization pulse are each less than 1 AL with respect to the one-way propagation time AL. Discharge device.   5. The pulse width of the pre-pulse, the main pulse, and the stabilization pulse, and the pulse interval therebetween are respectively less than 1 AL with respect to the one-way propagation time AL. Ink droplet ejection device. The pulse width Tp of the prepulse, the pulse width Tm of the main pulse, the pulse width Ts of the stabilization pulse, the interval Wp between the end of the prepulse and the start of the main pulse, and the end of the main pulse and the stabilization pulse The distance Wm from the beginning of each is one-way propagation time AL,
0.1AL ≦ Tp <0.44AL, 0.1AL ≦ Wp ≦ 0.5AL,
0.5AL ≦ Tm ≦ 0.8AL, 0.4AL ≦ Wm ≦ 0.8AL,
0.1AL ≦ Ts ≦ 0.5AL,
The ink droplet ejection apparatus according to claim 1, wherein the relationship is satisfied. The pulse width Tp of the prepulse, the pulse width Tm of the main pulse, the pulse width Ts of the stabilization pulse, the interval Wp between the end of the prepulse and the start of the main pulse, and the end of the main pulse and the stabilization pulse The distance Wm from the beginning of each is one-way propagation time AL,
0.20AL ≦ Tp ≦ 0.33AL, 0.11AL ≦ Wp ≦ 0.44AL,
0.56AL ≦ Tm ≦ 0.78AL, 0.56AL ≦ Wm ≦ 0.78AL,
0.11AL ≦ Ts ≦ 0.44AL,
The ink droplet ejection apparatus according to claim 1, wherein the relationship is satisfied. The pulse width Tp of the prepulse, the pulse width Tm of the main pulse, the pulse width Ts of the stabilization pulse, the interval Wp between the end of the prepulse and the start of the main pulse, and the end of the main pulse and the stabilization pulse The distance Wm from the beginning of each is one-way propagation time AL,
0.25AL <Tp <0.50AL, 0.08AL ≦ Wp <0.25AL,
0.50AL <Tm <0.80AL, 0.50AL <Wm <0.80AL,
0.18AL <Ts <0.50AL,
The ink droplet ejection apparatus according to claim 2, wherein the relationship is satisfied. The pulse width Tp of the prepulse, the pulse width Tm of the main pulse, the pulse width Ts of the stabilization pulse, the interval Wp between the end of the prepulse and the start of the main pulse, and the end of the main pulse and the stabilization pulse The distance Wm from the beginning of each is one-way propagation time AL,
0.375AL ≦ Tp ≦ 0.425AL, Wp = 0.125AL,
Tm = 0.625AL, 0.625AL ≦ Wm ≦ 0.675AL,
0.175AL ≦ Ts ≦ 0.375AL,
The ink droplet ejection apparatus according to claim 1, wherein the relationship is satisfied.   The ink droplet ejection apparatus according to claim 1, wherein the actuator is a piezoelectric element that is displaced with respect to the pressure chamber by application of a voltage.

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