METHOD FOR DRIVING DROPLET-DISCHARGING HEAD AND DROPLET-DISCHARGING DEVICE

Abstract

 The present invention addresses the problem of efficiently and stably forming, when continuously discharging droplets from the same nozzle to coalesce the droplets and form large droplets, large droplets of increased liquid volume with a short drive cycle. When applying a drive signal to a pressure-generating means for expanding or contacting the volume of a pressure chamber, applying pressure on a liquid in the pressure chamber by driving said pressure-generating means, and discharging droplets from a nozzle, the above problem is solved by: a first drive signal PA1 comprising a first expanding pulse Pa1 for expanding the volume of the pressure chamber and contracting same after a specified period, a first contracting pulse Pa2 for contracting the volume of the pressure chamber and expanding same after a specified period, a second expanding pulse Pa3 for expanding the volume of the pressure chamber and contracting same after a specified period, and a second contracting pulse Pa4 for contracting the volume of the pressure chamber and expanding same after a specified period, in said order; and the pulse width PAW1 of the first expanding pulse Pa1 being greater than 2AL and less than 4 AL (AL being ½ of the acoustic resonance period of the pressure wave in the pressure chamber).

Description

Technical Field

[0001] The present invention relates to a method for driving a droplet ejecting head and a droplet ejecting apparatus, and particularly, to a method for driving a droplet ejecting head and a droplet ejecting apparatus that can stably form a large droplet with a shorter driving period.

Background Art

[0002] Conventionally, as a type of droplet ejecting apparatuses, an inkjet recording apparatus adapted to print by ejecting ink (liquid) as ink droplets (droplets) from an inkjet head (droplet ejecting head) to a medium (medium) and adhering the ink droplets to the medium is known.

[0003] Furthermore, demands for such an inkjet recording apparatus include, for example, not only the ejection of small droplets for image quality improvement, but also the formation of large dots on the medium through the ejection of as large droplets as possible from a nozzle. The formation of the large dots can be used not only for gradation expression, but also, for example, for efficient high-speed printing with large droplets. Furthermore, in the case of single pass printing, nozzle failure can be complemented by ejecting a large droplet from a nozzle adjacent to a nozzle from which no droplet is ejected due to nozzle clogging or the like.

[0004] Methods of changing the dot diameter include a method of changing the number of droplets to be ejected from an identical nozzle within one pixel cycle, a method of changing driving signals according to the dot size, and the like. Among these methods, the method of changing the number of droplets has an advantage that gradation can be expressed easily just by changing the number of driving signals applied within one pixel cycle. However, when the number of driving signals is increased to form a large dot, the pixel cycle is prolonged. This is problematic for high-frequency driving. Accordingly, a refinement for stably forming a large dot with a shorter driving period is desired.

[0005] Conventionally, there have been methods described in Patent Literature 1 to 3 as methods for driving a droplet ejecting head.

[0006] According to Patent Literature 1, when at least two droplets are successively ejected at different velocities from an identical nozzle, a pixel is formed such that a droplet at a low velocity is ejected ahead of a droplet at a high velocity and the droplets are superposed and adhered within one pixel.

[0007] In addition, according to Patent Literature 2, a driving signal that is formed of square waves and sequentially generates a first pulse, a second pulse, a third pulse, and a fourth pulse is applied. The first pulse expands the volume of a pressure chamber. The second pulse contracts the volume of the pressure chamber. The third pulse expands the volume of the pressure chamber. The fourth pulse contracts the volume of the pressure chamber. The pulse width of the third pulse is shorter than that of the first pulse and the pulse width of the fourth pulse is shorter than that of the second pulse. In addition, pressure waves generated by the first pulse and the second pulse are canceled by the third pulse and the fourth pulse, by setting the time difference between the center of the pulse width of the first pulse and the center of the pulse width of the third pulse to 1 AL, and the time difference between the center of the pulse width of the second pulse and the center of the pulse width of the fourth pulse to 1 AL, and defining a ratio between the pulse width of the first pulse and the pulse width of the third pulse and a ratio between the pulse width of the second pulse and the pulse width of the fourth pulse according to a damping rate of the residual vibration of ink in a pressure chamber.

[0008] In contrast, according to Patent Literature 3, when the time taken for a pressure wave to propagate one way in an ink channel is T, the pulse width of a first jetting pulse signal to be applied first is set to 0.35 T to 0.65 T, the pulse width of second and succeeding jetting pulse signals to be applied is set to approximately T, and the time interval between the first jetting pulse signal and the subsequent jetting pulse signal is set to T. In this way, a droplet by the second jetting pulse signal is jetted from a nozzle before a droplet jetted from the nozzle by the first jetting pulse signal leaves the nozzle.

[0009] Actuator walls deformed by each jetting pulse signal increase the volume of the ink channel. After a certain period of time, the actuator walls return to the state before the deformation, and the pressure applied to ink jets an ink droplet. A droplet ejected by the second jetting pulse signal catches up with a droplet ejected by the first jetting pulse signal and is combined together to eject a large droplet.

Citation List

Patent Literature

[0010] 

Patent Literature 1: JP 3530717 B2

Patent Literature 2: JP 4247043 B2

Patent Literature 3: JP 3551822 B2

Summary of Invention

Technical Problem

[0011] Any of these techniques of Patent Literature 1 to 3 have issues that it is difficult to efficiently and stably form a large droplet of more increased liquid volume with a short driving period when ejecting a droplet from a nozzle by expanding and contracting the volume of a pressure chamber.

[0012] Note that such a situation is not limited to inkjet recording apparatuses, but is generally common to any of droplet ejecting apparatuses that eject liquid as a droplet.

[0013] Therefore, an object of the present invention is to provide a method for driving a droplet ejecting head and a droplet ejecting apparatus that can efficiently and stably form a large droplet of more increased liquid volume with a short driving period when ejecting a droplet from a nozzle by expanding and contracting the volume of a pressure chamber.

[0014] Other objects of the present invention will be apparent from the following description.

Solution to Problem

[0015] The object described above is achieved by each of the following inventions.

1. A method for driving a droplet ejecting head, including applying a driving signal to a pressure generation means to expand or contract a volume of a pressure chamber, applying pressure to liquid in the pressure chamber by driving the pressure generation means, and ejecting a droplet from a nozzle,
wherein a first driving signal is included as the driving signal,
the first driving signal includes
a first expansion pulse that expands the volume of the pressure chamber and contracts the volume of the pressure chamber after a certain period of time,
a first contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time,
a second expansion pulse that expands the volume of the pressure chamber and contracts the volume of the pressure chamber after a certain period of time, and
a second contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time, in this order, and
a pulse width of the first expansion pulse is greater than 2 AL and less than 4 AL (where AL is 1/2 an acoustic resonance period of a pressure wave in the pressure chamber).

2. The method for driving a droplet ejecting head described in 1 above, wherein in the first driving signal, the pulse width of the first expansion pulse is 2.5 AL or more and less than 3.8 AL.

3. The method for driving a droplet ejecting head described in 1 or 2 above, wherein in the first driving signal, a pulse width of the first contraction pulse is 0.4 AL or more and 0.7 AL or less, a pulse width of the second expansion pulse is 0.8 AL or more and 1.2 AL or less, and a pulse width of the second contraction pulse is 1.8 AL or more and 2.2 AL or less.

4. The method for driving a droplet ejecting head described in 1, 2, or 3 above, wherein in the first driving signal, a voltage value of the first expansion pulse and a voltage value of the second expansion pulse are equal, and a voltage value of the first contraction pulse and a voltage value of the second contraction pulse are equal.

5. The method for driving a droplet ejecting head described in 4 above, wherein in a case where a viscosity of the liquid is greater than 5 mPa·s, when the voltage values of the first expansion pulse and the second expansion pulse are set to VH2 and the voltage values of the first contraction pulse and the second contraction pulse are set to VH1, the first driving signal is |VH2| / |VH1| = 2/1.

6. The method for driving a droplet ejecting head described in 4 above, wherein in a case where a viscosity of the liquid is 5 mPa·s or less, when the voltage values of the first expansion pulse and the second expansion pulse are set to VH2 and the voltage values of the first contraction pulse and the second contraction pulse are set to VH1, the first driving signal is |VH2|/|VH1| = 1/1.

7. The method for driving a droplet ejecting head described in 1 or 2 above,
wherein the first driving signal further includes a third contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time,
a pulse width of the second contraction pulse is 0.3 AL or more and 0.7 AL or less,
a pulse width of the third contraction pulse is 0.8 AL or more and 1.2 AL or less, and
the third contraction pulse is applied following a pause period of 0. 3 AL or more and 0.7 AL or less after the application of the second contraction pulse ends.

8. The method for driving a droplet ejecting head described in 7 above, wherein in the first driving signal, a pulse width of the first contraction pulse is 0.4 AL or more and 0.7 AL or less, and a pulse width of the second expansion pulse is 0.8 AL or more and 1.2 AL or less.

9. The method for driving a droplet ejecting head described in 7 or 8 above, wherein in the first driving signal, a voltage value of the first expansion pulse and a voltage value of the second expansion pulse are equal, and a voltage value of the first contraction pulse and voltage values of the second contraction pulse and the third contraction pulse are equal.

10. The method for driving a droplet ejecting head described in 9 above, wherein in a case where a viscosity of the liquid is greater than 5 mPa·s, when the voltage values of the first expansion pulse and the second expansion pulse are set to VH2 and the voltage values of the first contraction pulse, the second contraction pulse, and the third contraction pulse are set to VH1, the first driving signal is VH2| / |VH1| = 2/1.

11. The method for driving a droplet ejecting head described in 9 above, wherein in a case where a viscosity of the liquid is 5 mPa·s or less, when the voltage values of the first expansion pulse and the second expansion pulse of the first driving signal are set to VH2 and the voltage values of the first contraction pulse, the second contraction pulse, and the third contraction pulse are set to VH1, the first driving signal is |VH2|/|VH1| = 1/1.

12. The method for driving a droplet ejecting head described in any one of 1 to 11 above, further including
a second driving signal as the driving signal upon forming a small droplet by ejecting a single droplet from the nozzle,
wherein the second driving signal includes
a first expansion pulse that expands the volume of the pressure chamber and contracts the volume of the pressure chamber after a certain period of time, and
a first contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time, in this order,
a pulse width of the first expansion pulse of the second driving signal is identical to the pulse width of the second expansion pulse of the first driving signal,
a pulse width of the first contraction pulse of the second driving signal is identical to the pulse width of the second contraction pulse of the first driving signal, and
depending on image data, a large droplet by the first driving signal and a small droplet by the second driving signal are selectively ejected from the identical nozzle.

13. A droplet ejecting apparatus including
a droplet ejecting head configured to apply pressure for ejection to liquid in a pressure chamber by driving a pressure generation means, and eject a droplet from a nozzle, and
a driving control means configured to output a driving signal that drives the pressure generation means,
wherein the driving signal includes a first driving signal,
the first driving signal includes
a first expansion pulse that expands a volume of the pressure chamber and contracts the volume of the pressure chamber after a certain period of time,
a first contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time,
a second expansion pulse that expands the volume of the pressure chamber and contracts the volume of the pressure chamber after a certain period of time, and
a second contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time, in this order, and
a pulse width of the first expansion pulse is greater than 2 AL and less than 4 AL (where AL is 1/2 an acoustic resonance period of a pressure wave in the pressure chamber).

14. The droplet ejecting apparatus described in 13 above, wherein in the first driving signal, the pulse width of the first expansion pulse is 2.5 AL or more and less than 3.8 AL.

15. The droplet ejecting apparatus described in 13 or 14 above, wherein in the first driving signal, a pulse width of the first contraction pulse is 0.4 AL or more and 0. 7 AL or less, a pulse width of the second expansion pulse is 0.8 AL or more and 1.2 AL or less, and a pulse width of the second contraction pulse is 1.8 AL or more and 2.2 AL or less.

16. The droplet ejecting apparatus described in 13, 14, or 15 above, wherein in the first driving signal, a voltage value of the first expansion pulse and a voltage value of the second expansion pulse are equal, and a voltage value of the first contraction pulse and a voltage value of the second contraction pulse are equal.

17. The droplet ejecting apparatus described in 16 above,
wherein a viscosity of the liquid is greater than 5 mPa·s, and
when the voltage values of the first expansion pulse and the second expansion pulse are set to VH2 and the voltage values of the first contraction pulse and the second contraction pulse are set to VH1, the first driving signal is |VH2| / |VH1| = 2/1.

18. The droplet ejecting apparatus described in 16 above,
wherein a viscosity of the liquid is 5 mPa·s or less, and
when the voltage values of the first expansion pulse and the second expansion pulse are set to VH2 and the voltage values of the first contraction pulse and the second contraction pulse are set to VH1, the first driving signal is |VH2|/|VH1| = 1/1.

19. The droplet ejecting apparatus described in 13 or 14 above,
wherein the first driving signal further includes a third contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time,
a pulse width of the second contraction pulse is 0.3 AL or more and 0.7 AL or less,
a pulse width of the third contraction pulse is 0.8 AL or more and 1.2 AL or less, and
the third contraction pulse is applied following a pause period of 0. 3 AL or more and 0. 7 AL or less after the application of the second contraction pulse ends.

20. The droplet ejecting apparatus described in 19 above, wherein in the first driving signal, a pulse width of the first contraction pulse is 0.4 AL or more and 0.7 AL or less, and a pulse width of the second expansion pulse is 0.8 AL or more and 1.2 AL or less.

21. The droplet ejecting apparatus described in 19 or 20 above, wherein in the first driving signal, a voltage value of the first expansion pulse and a voltage value of the second expansion pulse are equal, and a voltage value of the first contraction pulse and voltage values of the second contraction pulse and the third contraction pulse are equal.

22. The droplet ejecting apparatus described in 21 above,
wherein a viscosity of the liquid is greater than 5 mPa·s, and
when the voltage values of the first expansion pulse and the second expansion pulse are set to VH2 and the voltage values of the first contraction pulse, the second contraction pulse, and the third contraction pulse are set to VH1, the first driving signal is |VH2|/|VH1| = 2/1.

23. The droplet ejecting apparatus described in 21 above,
wherein a viscosity of the liquid is 5 mPa·s or less, and
when the voltage values of the first expansion pulse and the second expansion pulse of the first driving signal are set to VH2 and the voltage values of the first contraction pulse, the second contraction pulse, and the third contraction pulse are set to VH1, the first driving signal is |VH2| / | VH1| = 1/1.

24. The droplet ejecting apparatus described in any one of 13 to 23 above, further including
a second driving signal as the driving signal upon forming a small droplet by ejecting a single droplet from the nozzle,
wherein the second driving signal includes
a first expansion pulse that expands the volume of the pressure chamber and contracts the volume of the pressure chamber after a certain period of time, and
a first contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time, in this order,
a pulse width of the first expansion pulse of the second driving signal is identical to the pulse width of the second expansion pulse of the first driving signal,
a pulse width of the first contraction pulse of the second driving signal is identical to the pulse width of the second contraction pulse of the first driving signal, and
the driving control means outputs the first driving signal or the second driving signal to the pressure generation means so as to selectively eject a large droplet by the first driving signal and a small droplet by the second driving signal from the identical nozzle depending on image data.

25. The droplet ejecting apparatus described in any one of 13 to 24 above, wherein the droplet ejecting head is a shear-mode type droplet ejecting head.

Advantageous Effects of Invention

[0016] According to the present invention, it is possible to provide a method for driving a droplet ejecting head and a droplet ejecting apparatus that can efficiently and stably form a large droplet of more increased liquid volume with a short driving period when ejecting a droplet from a nozzle by expanding and contracting the volume of a pressure chamber.

Brief Description of Drawings

[0017] 

Fig. 1 is a schematic configuration view illustrating an example of an inkjet recording apparatus according to the present invention.

Fig. 2 is a view illustrating an example of an inkjet head, where (a) is a perspective view illustrating an external appearance in cross section and (b) is a cross-sectional view seen from a side surface.

Fig. 3 is a diagram for describing a first embodiment of a first driving signal, which is generated in a driving control unit, for forming a large droplet.

Fig. 4 is a view and includes (a) to (c) for describing ejection operations of the inkjet head.

Fig. 5 is a conceptual diagram of the large droplet ejected by the first driving signal.

Fig. 6 is a diagram for describing a second embodiment of a first driving signal, which is generated in a driving control unit, for forming a large droplet.

Fig. 7 is a diagram and includes (a) and (b), each describing an embodiment of a second driving signal for ejecting a small droplet.

Fig. 8 is a graph illustrating a relationship between a first expansion pulse width and a liquid volume at a droplet velocity of 6 m/s.

Fig. 9 is a pair of graphs of pressure in a channel measured with passage of time where (a) is an ink viscosity of 10 mPa·s and (b) is an ink viscosity of 4 mPa·s, in both of which a driving voltage ratio is set to |VH2|/ / |VH1| = 2/1 and |VH2| / | VH1| = 1/1.

Description of Embodiments

[0018] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0019] Hereinafter, an embodiment of an inkjet recording apparatus (an example of a droplet ejecting apparatus) that ejects ink (an example of liquid) as an ink droplet (an example of a droplet) will be described together with a method for driving an inkjet head (a method for driving a droplet ejecting head) in the inkjet recording apparatus with reference to the drawings.

[0020] Fig. 1 is a schematic configuration view illustrating an example of the inkjet recording apparatus according to the present invention.

[0021] In the inkjet recording apparatus 1, a conveyance mechanism 2 sandwiches a medium 7 made of paper, a plastic sheet, a cloth, or the like with a conveyance roller pair 22, and conveys the medium 7 in the Y direction (sub-scanning direction) in the figure by the rotation of a conveyance roller 21 driven by a conveyance motor 23. An inkjet head (hereinafter, simply referred to as a head) 3 is provided between the conveyance roller 21 and the conveyance roller pair 22. The head 3 is arranged and mounted on a carriage 5 in such a way that a nozzle surface side thereof faces a recording surface 71 of the medium 7. The head 3 is electrically connected via a flexible cable 6 to a driving control unit 8 constituting a driving control means according to the present invention.

[0022] The carriage 5 is provided so as to be reciprocally movable along guide rails 4 by a driving means, which is not illustrated, in the X-X' direction (main-scanning direction) in the figure substantially orthogonal to the sub-scanning direction. The guide rails 4 are bridged over the width direction of the medium 7. The head 3 moves along the recording surface 71 of the medium 7 in the main-scanning direction along with the reciprocating movement of the carriage 5. In the course of this movement, the head 3 ejects droplets from the nozzles according to image data to record an inkjet image.

[0023] Fig. 2 is a view illustrating an example of the head 3, where (a) is a perspective view illustrating an external appearance in cross section and (b) is a cross-sectional view seen from a side surface.

[0024] In the head 3, 30 represents a channel substrate. In the channel substrate 30, a large number of narrow groove channels 31 and partition walls 32 are alternately arranged side by side. On an upper surface of the channel substrate 30, a cover substrate 33 is provided so as to close up upper portions of all the channels 31. A nozzle plate 34 is joined to end surfaces of the channel substrate 30 and the cover substrate 33. One end of each channel 31 communicates with the outside via a corresponding nozzle 341 formed in this nozzle plate 34.

[0025] The other end of each channel 31 is formed such that the groove gradually shallows relative to the channel substrate 30. A common channel 331 common to each channel 31 is formed in the cover substrate 33. The other end of each channel 31 communicates with this common channel 331. The common channel 331 is closed up by a plate 35. An ink supply port 351 is formed in the plate 35. Through this ink supply port 351, ink is supplied from an ink supply tube 352 to the common channel 331 and each channel 31.

[0026] The partition walls 32 are formed of piezoelectric elements such as PZT which are electromechanical conversion means. These partition walls 32 exemplified are the ones in which upper wall portions 321 and lower wall portions 322 are formed of piezoelectric elements subjected to polarization processing in directions opposite to each other. However, the portions formed of the piezoelectric elements in the partition walls 32 may only be the upper wall portions 321, for example. The partition walls 32 and the channels 31 are alternately arranged side by side. Thus, one partition wall 32 is shared by the channels 31 and 31 on both sides thereof.

[0027] Driving electrodes (not illustrated in Fig. 2) are each formed in inner surfaces of the respective channels 31 from wall surfaces to bottom surfaces of both the partition walls 32 and 32. When a driving signal at a predetermined voltage is applied from the driving control unit 8 to each of the two driving electrodes arranged with the partition wall 32 interposed therebetween, the partition wall 32 is shear-deformed along a joint surface as a boundary between the upper wall portion 321 and the lower wall portion 322. When two of the adjacent partition walls 32 and 32 are shear-deformed in directions opposite to each other, the volume of the channel 31 sandwiched between these partition walls 32 and 32 expands or contracts, generating a pressure wave inside. This applies the pressure for ejection to the ink in the channel 31.

[0028] This head 3 is a shear-mode type head which ejects ink in the channel 31 from the nozzle 341 by the shear-deformation of the partition walls 32. This is a preferable mode in the present invention. The shear-mode type head can efficiently eject a droplet by preferably using square waves to be described later as a driving signal.

[0029] Note that in this head 3, the channels 31 surrounded by the channel substrate 30, the partition walls 32, the cover substrate 33, and the nozzle plate 34 constitute pressure chambers in the present invention. The partition walls 32 and the driving electrodes on the surfaces thereof constitute pressure generation means in the present invention.

[0030] The driving control unit 8 generates a driving signal for ejecting a droplet from the nozzle 341. The generated driving signal is output to the head 3, which is applied to each driving electrode formed on the corresponding partition wall 32.

[0031] Next, a description will be given of a first driving signal which is an example of the driving signal according to the present invention.

[0032] Fig. 3 is a diagram for describing the first embodiment of the first driving signal, which is generated in the driving control unit 8, for forming a large droplet.

[0033] A first driving signal PA1 is a driving signal for forming a large droplet by ejecting at least two droplets from the identical nozzle 341 and combining them together mid-flight immediately after the ejection. This first driving signal PA1 includes a first expansion pulse Pa1, a first contraction pulse Pa2, a second expansion pulse Pa3, and a second contraction pulse Pa4 in this order. The first expansion pulse Pa1 expands the volume of the channel 31 and contracts the same after a certain period of time. The first contraction pulse Pa2 contracts the volume of the channel 31 and expands the same after a certain period of time. The second expansion pulse Pa3 expands the volume of the channel 31 and contracts the same after a certain period of time. The second contraction pulse Pa4 contracts the volume of the channel 31 and expands the same after a certain period of time.

[0034] The first expansion pulse Pa1 of the first driving signal PA1 illustrated in the present embodiment is a pulse that rises from a reference potential and falls to the reference potential after a certain period of time. The first contraction pulse Pa2 is a pulse that falls from the reference potential and rises to the reference potential after a certain period of time. The second expansion pulse Pa3 is a pulse that rises from the reference potential and falls to the reference potential after a certain period of time. The second contraction pulse Pa4 is a pulse that falls from the reference potential and rises to the reference potential after a certain period of time. Note that although the reference potential is set to 0 here, the reference potential is not limited thereto.

[0035] This first driving signal PA1 includes the expansion pulses that rise from the reference potential and fall to the reference potential after a certain period of time and the contraction pulses that fall from the reference potential and rise to the reference potential after a certain period of time. This can keep the driving voltage low, compared to the case where unipolar pulses are used. Accordingly, the circuit load and the power consumption can be suppressed.

[0036] The first contraction pulse Pa2 continuously falls from the end of the fall of the first expansion pulse Pa1 without a pause period. In addition, the second expansion pulse Pa3 continuously rises from the end of the rise of the first contraction pulse Pa2 without a pause period. Furthermore, the second contraction pulse Pa4 continuously falls from the end of the fall of the second expansion pulse Pa3 without a pause period.

[0037] Then, by applying the first contraction pulse Pa2 to the driving electrode following the application of the first expansion pulse Pa1, the first large droplet is ejected from the nozzle 341. Immediately after that, by applying the second expansion pulse Pa3 and the second contraction pulse Pa4, the second droplet is ejected from the identical nozzle 341. The ejected droplets combine together immediately after the ejection and form a large droplet, which then lands on the medium 7.

[0038] Note that although a large droplet is formed by combining at least two droplets together as described above, the timing of the combination may be at any time as long as it is before the droplets land on the medium 7 at the latest. For example, the large droplet may be formed and then land on the medium 7 by ejecting the first droplet and the second droplet so as to become a continuous liquid column by narrowing the interval between the ejection timing of the first droplet and the ejection timing of the second droplet. According to this method, it is easier to control the landing position than landing the second droplet on the medium 7 following the landing of the first droplet on the medium 7 and superposing the first droplet and the second droplet on the medium 7.

[0039] In this first driving signal PA1, a pulse width PAW1 of the first expansion pulse Pa1 is set to greater than 2 AL and less than 4 AL. By setting the pulse width PAW1 of the first expansion pulse Pa1 within this range, not only can the liquid volume of a large droplet including at least two droplets be increased, but also the large droplet can be stably ejected and high-frequency and high-quality image recording can be performed.

[0040] Normally, the ejection efficiency is highest when the pulse width PAW1 is set to around 1 AL. According to the present invention, therefore, the ejection efficiency is decreased since this pulse width PAW1 is set to greater than 2 AL and less than 4 AL. However, compared to the velocity of the first large droplet ejected by this, the velocity of the second droplet ejected immediately after that by the application of the second expansion pulse Pa3 and the second contraction pulse Pa4 is fast by setting a pulse width PAW3 of the second expansion pulse Pa3 closer to 1 AL than the pulse width PAW1 of the first expansion pulse Pa1. As a result, the second droplet can be combined together with the first droplet and form a large droplet.

[0041] As long as at least two droplets combine together and form a large droplet mid-flight immediately after the ejection, the droplets may be ejected in a partially connected state, or may be ejected in a state separated from each other.

[0042] Here, the large droplet in the present invention indicates a droplet with a larger liquid volume than a single droplet ejected by a draw-release-reinforce (DRR) waveform (see Fig. 7(a)) at the droplet velocity identical to the velocity of the droplet ejected by the first driving signal PA1. The DRR waveform is a basic waveform formed of the second expansion pulse Pa3 and the second contraction pulse Pa4. Specifically, it is preferred that the droplet has a liquid volume ratio of 2.8 or more to the droplet ejected by the DRR waveform (a liquid volume of a droplet by the driving signal according to the present invention/a liquid volume of a droplet by a DRR waveform). The liquid volume can be measured, for example, by measuring the droplet velocity and then weighing an arbitrary number of droplets ejected.

[0043] In addition, AL is an abbreviation for acoustic length, which is 1/2 an acoustic resonance period of a pressure wave in the channel 31. AL is obtained as a pulse width with which the flight velocity of a droplet becomes maximum when the flight velocity of the droplet, which is ejected when the square-wave driving signal is applied to the driving electrode, is measured, and the pulse width of the square wave is changed while the voltage value of the square wave is kept constant.

[0044] In addition, when 0 V is assumed to be 0% and the peak value voltage is assumed to be 100%, the pulse width is defined as the time between 10% rise from 0 V of the voltage and 10% fall from the peak value voltage.

[0045] Furthermore, the square wave indicates a waveform in which both of the rise time and the fall time between 10% and 90% of the voltage are within 1/2, preferably within 1/4 of AL.

[0046] When the pulse width PAW1 of the first expansion pulse Pa1 becomes 2 AL or less, the liquid volume extruded from the nozzle 341 by the first expansion pulse Pa1 becomes insufficient to form a large droplet that achieves the object of the present invention. In addition, when the pulse width PAW1 of the first expansion pulse Pa1 becomes 4 AL or more, the liquid volume can be increased. However, since the driving waveform length is increased, the ejection pause time in the driving period becomes shorter. Accordingly, a large pressure wave reverberation vibration remains, and thus the flight stability significantly decreases. Therefore, this is not suitable for high-frequency driving of a large droplet.

[0047] The pressure generated in the channel 31 due to the expansion of the volume of the channel 31 is inverted at every 1 AL from negative to positive and from positive to negative. Accordingly, when the pulse width PAW1 of the first expansion pulse Pa1 is an even number AL, the pressure in the channel 31 is inverted to negative, which therefore cancels out the positive pressure each other generated when the volume of the channel 31 is contracted due to the end of the application of the first expansion pulse Pa1. This results in deterioration in the ejection efficiency. For this reason, the pulse width PAW1 of the first expansion pulse Pa1 is greater than 2 AL and less than 4 AL.

[0048] When the velocity of the first droplet ejected by the application of the first expansion pulse Pa1 and the first contraction pulse Pa2 is low and the liquid volume thereof is large, even if the droplet is combined with the second droplet ejected immediately after that, the droplet velocity of the large droplet formed therefrom is decreased and the ejection efficiency declines. Accordingly, a need to increase the driving voltage value arises. Therefore, it is preferred that the pulse width PAW1 of the first expansion pulse Pa1 is set to around an odd number AL, specifically, 2.5 AL or more and less than 3.8 AL, where the pressure waves do not cancel out each other.

[0049] When ejecting the second droplet at a higher droplet velocity than that of the first droplet following the ejection of the first droplet and combining both of the droplets together mid-flight to form a large droplet by the first driving signal PA1, a pulse width PAW2 of the first contraction pulse Pa2 is preferably set to 0.4 AL or more and 0.7 AL or less, and most preferably 0.5 AL, from a viewpoint of allowing the large droplet to be formed more efficiently. In addition, from a similar viewpoint, the pulse width PAW3 of the second expansion pulse Pa3 is preferably set to 0.8 AL or more and 1.2 AL or less, and most preferably 1 AL. Furthermore, from a similar viewpoint, a pulse width PAW4 of the second contraction pulse Pa4 is preferably set to 1.8 AL or more and 2.2 AL or less, and most preferably 2 AL.

[0050] Next, a description will be given of an example of the ejection operation of the head 3 when this first driving signal PA1 is applied with reference to Fig. 4. Fig. 4 illustrates a part of a cross section of the head 3 which is cut in a direction orthogonal to the longitudinal direction of the channels 31. Here, droplets are assumed to be ejected from a channel 31B in the center of Fig. 4. In addition, a conceptual diagram of a large droplet ejected when the first driving signal PA1 is applied is illustrated in Fig. 5.

[0051] First, when driving electrodes 36A and 36C are grounded and the first expansion pulse Pa1 of the first driving signal PA1 is applied to a driving electrode 36B from the neutral state of partition walls 32B and 32C illustrated in Fig. 4(a), the partition walls 32B and 32C are bent and deformed outward against each other as illustrated in Fig. 4(b), and the volume of the channel 31B sandwiched between the partition walls 32B and 32C expands. As a result, a negative pressure is generated in the channel 31B, and ink flows thereinto.

[0052] After the first expansion pulse Pa1 is maintained for a period greater than 2 AL and less than 4 AL, the application of the first expansion pulse Pa1 ends. Accordingly, the volume of the channel 31B contracts from the expanded state and the partition walls 32B and 32C return to the neutral state illustrated in Fig. 4(a). Then, when the first contraction pulse Pa2 is subsequently applied without a pause period, the volume of the channel 31B is immediately brought into the contracted state illustrated in Fig. 4(c). At this time, pressure is applied to the ink in the channel 31B, by which the ink is pushed out and ejected as the first large droplet from the nozzle 341.

[0053] When the application of the first contraction pulse Pa2 ends, the volume of the channel 31B expands from the contracted state, and the partition walls 32B and 32C return to the neutral state illustrated in Fig. 4(a). Then, when the second expansion pulse Pa3 is subsequently applied without a pause period, the volume of the channel 31B is immediately brought into the expanded state illustrated in Fig. 4(b), and a negative pressure is generated in the channel 31. Therefore, the velocity of the first large droplet ejected earlier is suppressed. Furthermore, the negative pressure generated in the channel 31B causes ink to flow thereinto again.

[0054] When the application of the second expansion pulse Pa3 ends, the volume of the channel 31B contracts from the expanded state, and the partition walls 32B and 32C return to the neutral state illustrated in Fig. 4(a). Then, when the second contraction pulse Pa4 is subsequently applied without a pause period, the volume of the channel 31B is immediately brought into the contracted state illustrated in Fig. 4 (c). At this time, a large pressure is applied to the ink in the channel 31B, and the ink is further pushed out following the first large droplet ejected by the first expansion pulse Pa1 and the first contraction pulse Pa2. The ink that has been pushed out is eventually torn, and the second droplet at a high droplet velocity is ejected.

[0055] As for the droplet ejected by the first driving signal PA1, as illustrated in Fig. 5, a first droplet 101 at a low droplet velocity is formed by the first expansion pulse Pa1 and a first contraction pulse Pa2, following which a second droplet 102 at a high droplet velocity is formed by the second expansion pulse Pa3 and the second contraction pulse Pa4. The formation of the first droplet 101 and the second droplet 102 are continuous in a droplet 100 at the beginning of the ejection. However, since the ejection velocity of the second droplet 102 is sufficiently higher than that of the first droplet 101, they combine together mid-flight immediately after the ejection and become the single large droplet 100.

[0056] When the application of the second contraction pulse Pa4 ends, the volume of the channel 31B expands from the contracted state, and the partition walls 32B and 32C return to the neutral state in Fig. 4(a).

[0057] Since this droplet 100 is a combination of the first droplet 101 at a low droplet velocity and the second droplet 102 at a high droplet velocity, the droplet velocity is decreased and the satellite volume is also suppressed compared to the case where a single large droplet with the same liquid volume is ejected from the nozzle 341.

[0058] That is, in general, a satellite is generated when a tail which accompanies an ejected main droplet and is formed so as to extend rearward is separated from the main droplet. As long as the satellite is separated while being close to the main droplet, both land in an approximately identical position and thus the image quality is less likely to be affected. However, when the satellite is separated at a position away from the main droplet, the landing position is also significantly away from that of the main droplet, thus causing the deterioration of the image quality. The faster the droplet velocity is, the longer the tail becomes and the more likely the satellite is separated into multiple pieces at positions away from the main droplet. With the first driving signal PA1, ejection can be performed at a low velocity even if the droplet volume is increased. Thus, the tail accompanying the droplet 100 (main droplet) can be decreased in length, while the satellite is reduced in number and separated at positions close to the main droplet. Therefore, the influence of the satellite can be suppressed while the large droplet 100 is ejected.

[0059] Note that in the present invention, the droplet velocity is calculated by causing a droplet observing apparatus to recognize a droplet as an image and obtaining the time elapsed from the ejection and a position coordinate at which the droplet exists at that time. Specifically, the droplet velocity is calculated from the distance that the droplet flies within a period of 50 µs from a position 500 µm away from the nozzle surface. The time elapsed from the ejection can be calculated by synchronizing an ejection signal of the inkjet head with a strobe for observation. Additionally, the position coordinate of the droplet can be calculated by subjecting a flight image to image processing.

[0060] The first driving signal PA1 is preferably of square waves. The first expansion pulse Pa1, the first contraction pulse Pa2, the second expansion pulse Pa3, and the second contraction pulse Pa4 constituting the first driving signal PA1 are square waves as illustrated in Fig. 3. In particular, the shear-mode type head 3 can generate pressure waves with the phases aligned in response to the application of the driving signal formed of the square waves. Accordingly, not only can a large droplet be efficiently ejected, but also the driving voltage can be further kept low. In general, voltage is applied to the head 3 all the time irrespective of ejection or non-ejection. Thus, low driving voltage is important for the suppression of the heat generation of the head 3 and stable ejection of droplets.

[0061] Moreover, since the square waves can be easily generated using a simple digital circuit, the circuit configuration can also be simplified, compared to the case where trapezoidal waves having inclined waves which necessitate an analog circuit is used.

[0062] In the first driving signal PA1, it is preferred that a voltage value of the first expansion pulse Pa1 and a voltage value of the second expansion pulse Pa3 are equal, and a voltage value of the first contraction pulse Pa2 and a voltage value of the second contraction pulse Pa4 are equal. Since at least two power supplies suffice, the number of power supplies can be reduced. As a result, the circuit configuration of the driving control unit 8 can be simplified.

[0063] Moreover, in cases where the viscosity of the liquid to be used is greater than 5 mPa·s, when the voltage values of the first expansion pulse Pa1 and the second expansion pulse Pa3 are set to VH2 and the voltage values of the first contraction pulse Pa2 and the second contraction pulse Pa4 are set to VH1, |VH2|/|VH1| = 2/1 is preferred. This accelerates the attenuation of the pressure wave reverberation vibration in the channel 31 and makes high-frequency driving possible. Furthermore, it is also possible to stabilize the flight in the case of using high-viscosity ink in particular.

[0064] However, in cases where the viscosity of the liquid to be used is 5 mPa·s or less, |VH2| / |VH1| = 1/1 is preferable from a viewpoint of obtaining a similar effect to above. This is because the pressure wave is difficult to attenuate, compared to the high-viscosity ink. Therefore, the driving voltage ratio of VH1 to VH2 needs to be increased to cancel the pressure generated by the first expansion pulse Pa1 and the second expansion pulse Pa3.

[0065] Next, a description will be given of the second embodiment of a first driving signal which is another example of the driving signal according to the present invention.

[0066] Fig. 6 is a diagram for describing the second embodiment of the first driving signal for forming a large droplet to be generated in a driving control unit 8.

[0067] Similarly to the first driving signal PA1, this first driving signal PA2 is also a driving signal for forming a large droplet by ejecting at least two droplets from the identical nozzle 341 and combining them together mid-flight immediately after the ejection. The first driving signal PA2 includes a first expansion pulse Pa1, a first contraction pulse Pa2, a second expansion pulse Pa3, a second contraction pulse Pa4, and a third contraction pulse Pa5 in this order. The first expansion pulse Pa1 expands the volume of the channel 31 and contracts the same after a certain period of time. The first contraction pulse Pa2 contracts the volume of the channel 31 and expands the same after a certain period of time. The second expansion pulse Pa3 expands the volume of the channel 31 and contracts the same after a certain period of time. The second contraction pulse Pa4 contracts the volume of the channel 31 and expands the same after a certain period of time. The third contraction pulse Pa5 contracts the volume of the channel 31 and expands the same after a certain period of time.

[0068] The waveform configuration of the first driving signal PA2 is a driving signal having the second expansion pulse Pa3 and the second contraction pulse Pa4 as a basic waveform (DRR waveform), and is different from the first driving signal PA1 only in that the third contraction pulse Pa5 is applied following an interval after the end of the application of the second contraction pulse Pa4. This third contraction pulse Pa5 is a pulse that falls from a reference potential and rises to the reference potential after a certain period of time. Note that although the reference potential is also set to 0 here, the reference potential is not limited thereto.

[0069] In this first driving signal PA2, a pulse width PAW1 of the first expansion pulse Pa1 is also set to greater than 2 AL and less than 4 AL. Then, immediately after the first droplet is ejected from the nozzle 341 by the application of the first expansion pulse Pa1 and the first contraction pulse Pa2, the second droplet is ejected by the application of the second expansion pulse Pa3 and the second contraction pulse Pa4. As a result, a similar effect to the first driving signal PA1 is attained.

[0070] Furthermore, a pulse width PAW4 of the second contraction pulse Pa4 is set to 0.3 AL or more and 0.7 AL or less. A pulse width PAW5 of the third contraction pulse Pa5 is set to 0.8 AL or more and 1.2 AL or less. The third contraction pulse Pa5 is set to be applied following an interval, that is a pause period PAW6, in which the reference potential is maintained for 0.3 AL or more and 0.7 AL or less after the end of the application of the second contraction pulse Pa4. This can promote the tearing of a tail accompanying a main droplet, reducing the influence of a satellite further. Furthermore, the pressure wave reverberation vibration in the channel 31 can also be effectively canceled by the third contraction pulse Pa5.

[0071] In order to achieve this effect, the pulse width PAW4 of the second contraction pulse Pa4 is most preferably 0.5 AL, the pulse width PAW5 of the third contraction pulse Pa5 is most preferably 1 AL, and the third contraction pulse Pa5 is most preferably applied following an interval of 0.5 AL after the end of the application of the second contraction pulse Pa4.

[0072] Furthermore, from a viewpoint of achieving the effect above, a pulse width PAW2 of the first contraction pulse Pa2 and a pulse width PAW3 of the second expansion pulse Pa3 are preferably set identically to those of the first contraction pulse Pa2 and the second expansion pulse Pa3 in the first driving signal PA1, respectively.

[0073] Next, a description will be given of an example of the ejection operation of the head 3 when this first driving signal PA2 is applied, with reference to Fig. 4, as with the first driving signal PA1. Since the first expansion pulse Pa1 to the second expansion pulse Pa3 are similar to those in the first driving signal PA1, these descriptions are as described in the descriptions of the first driving signal PA1 and omitted here.

[0074] When the application of the second expansion pulse Pa3 in this first driving signal PA2 ends, the volume of a channel 31B sandwiched between partition walls 32B and 32C contracts from the expanded state, and the partition walls 32B and 32C return to the neutral state illustrated in Fig. 4(a). Then, when the second contraction pulse Pa4 is subsequently applied to a driving electrode 36B without a pause period, the volume of the channel 31B is immediately brought into the contracted state illustrated in Fig. 4(c). At this time, a large pressure is applied to the ink in the channel 31B, which causes additional ink to be ejected following the ink ejected by the first expansion pulse Pa1 and the first contraction pulse Pa2. Then, a large droplet 100 formed of a first droplet 101 and a second droplet 102 is ejected as in Fig. 5.

[0075] After the second contraction pulse Pa4 is maintained for 0.3 AL or more and 0.7 AL or less, the volume of the channel 31B expands from the contracted state, and the partition walls 32B and 32C return to the neutral state illustrated in Fig. 4(a). At this time, since a negative pressure is generated in the channel 31, the ink meniscus is pulled back relatively early by the negative pressure generated in the channel 31. As a result, a tail of the ejected ink droplet is torn early, and the tail accompanying the ejected droplet 100 (main droplet) is shortened. Therefore, the influence of the satellite can be further reduced as compared to the first driving signal PA1.

[0076] Moreover, when the third contraction pulse Pa5 is applied following an interval of 0.3 AL or more and 0.7 AL or less after the partition walls 32B and 32C return to the neutral state illustrated in Fig. 4(a) following the end of the application of the second contraction pulse Pa4, the volume of the channel 31B becomes the contracted state illustrated in Fig. 4 (c) again. Then, after 0.8 AL or more and 1.2 AL or less has elapsed, the volume of the channel 31B is expanded and the partition walls 32B and 32C return to the neutral state illustrated in Fig. 4(a) again, while the positive pressure remains in the channel 31. This generates a negative pressure in the channel 31, and cancels the pressure wave reverberation vibration.

[0077] This first driving signal PA2 is also preferably of square waves for a similar reason to the first driving signal PA1. That is, the first expansion pulse Pa1, the first contraction pulse Pa2, the second expansion pulse Pa3, the second contraction pulse Pa4, and the third contraction pulse Pa5 constituting the first driving signal PA2 are also composed of square waves as illustrated in Fig. 6.

[0078] In the first driving signal PA2, for a similar reason to the first driving signal PA1, it is also preferred that a voltage value of the first expansion pulse Pa1 and a voltage value of the second expansion pulse Pa3 are equal, and a voltage value of the first contraction pulse Pa2, a voltage value of the second contraction pulse Pa4, and a voltage value of the third contraction pulse Pa5 are equal.

[0079] Furthermore, at this time, for a similar reason to the first driving signal PA1, in cases where the viscosity of the liquid to be used is greater than 5 mPa·s, when the voltage values of the first expansion pulse Pa1 and the second expansion pulse Pa3 are set to VH2 and the voltage values of the first contraction pulse Pa2, the second contraction pulse Pa4, and the third contraction pulse Pa5 are set to VH1, |VH2| / |VH1| = 2/1 is preferred. In cases where the viscosity of the liquid to be used is 5 mPa·s or less, |VH2|/|VH1| = 1/1 is preferred.

[0080] Incidentally, a small droplet can be formed by ejecting a single droplet from the nozzle 341 using a driving signal having a shape excluding the first expansion pulse Pa1 and the first contraction pulse Pa2 in the first driving signals PA1 and PA2 described above. Figs. 7(a) and 7(b) illustrate diagrams, each illustrating an embodiment of a second driving signal for ejecting a small droplet in this way.

[0081] A second driving signal PB1 illustrated in Fig. 7 (a) includes a first expansion pulse Pb1 and a first contraction pulse Pb2 in this order. The first expansion pulse Pb1 expands the volume of the channel 31 and contracts the same after a certain period of time. The first contraction pulse Pb2 contracts the volume of the channel 31 and expands the same after a certain period of time.

[0082] A pulse width PBW1 of the first expansion pulse Pb1 of this second driving signal PB1 is identical to the pulse width PAW3 of the second expansion pulse Pa3 of the first driving signal PA1. A pulse width PBW2 of the first contraction pulse Pb2 of the second driving signal PB1 is set identically to the pulse width PAW4 of the second contraction pulse Pa4 of the first driving signal PA1.

[0083] This second driving signal PB1 has a general draw-release-reinforce (DRR) waveform, and this is a driving signal having a shape excluding the first expansion pulse Pa1 and the first contraction pulse Pa2 in the first driving signal PA1. This makes it possible to eject a small droplet with liquid volume smaller than that of a large droplet ejected by the first driving signal PA1.

[0084] In addition, a second driving signal PB2 illustrated in Fig. 7(b) includes a first expansion pulse Pb1, a first contraction pulse Pb2, and a second contraction pulse Pb3 in this order. The first expansion pulse Pb1 expands the volume of the channel 31 and contracts the same after a certain period of time. The first contraction pulse Pb2 contracts the volume of the channel 31 and expands the same after a certain period of time. The second contraction pulse Pb3 contracts the volume of the channel 31 and expands the same after a certain period of time. The second contraction pulse Pb3 is applied following a predetermined pause period after the end of the application of the first contraction pulse Pb2.

[0085] A pulse width PBW1 of the first expansion pulse Pb1 of this second driving signal PB2 is identical to the pulse width PAW3 of the second expansion pulse Pa3 of the first driving signal PA2. A pulse width PBW2 of the first contraction pulse Pb2 of the second driving signal PB2 is identical to the pulse width PAW4 of the second contraction pulse Pa4 of the first driving signal PA2. A pulse width PBW3 of the second contraction pulse Pb3 of the second driving signal PB2 is set identically to the pulse width PAW5 of the third contraction pulse Pa5 of the first driving signal PA2. In addition, a pause period PBW4 of the second driving signal PB2 is set identically to the pause period PAW6 of the first driving signal PA2.

[0086] In other words, the waveform configuration of the second driving signal PB2 is the driving signal having a shape excluding the first expansion pulse Pa1 and the first contraction pulse Pa2 in the first driving signal PA2. This makes it possible to eject a small droplet with liquid volume smaller than that of a large droplet ejected by the first driving signal PA2.

[0087] Note that the second contraction pulse Pb3 of the second driving signal PB2 may be omitted.

[0088] Then, by applying these second driving signals PB1 or PB2 according to image data, a small droplet can be ejected from the nozzle 341 identical to the nozzle 341 from which a large droplet is ejected by the first driving signal PA1 or PA2. A large droplet by the first driving signal PA1 or PA2 and a small droplet by the second driving signal PB1 or PB2 can be selectively ejected from the identical nozzle 341.

[0089] Since the second driving signal PB1 or PB2 has a waveform configuration in which the first expansion pulse Pa1 and the first contraction pulse Pa2 are simply excluded from the first driving signal PA1 or PA2, the second driving signal PB1 or PB2 can be formed easily just by using the second expansion pulse Pa3 and subsequent waveform portions of these first driving signals PA1 or PA2. Therefore, although a large droplet and a small droplet are selectively ejected from the identical nozzle 341, it is only necessary to prepare the first driving signal PA1 or PA2 as the driving signal. This has an effect of simplifying the circuit configuration of the driving control unit 8.

[0090] In the embodiments described above, the droplet ejecting apparatus may be a droplet ejecting apparatus that ejects liquid other than ink. Furthermore, the liquid described herein may be any material that can be ejected from the droplet ejecting apparatus. For example, the liquid may be any material that is in a liquid phase and includes liquid-state materials with high or low viscosities, sol, gel water, and other fluid-state materials such as inorganic solvent, organic solvents, solutions, liquid resin, and liquid metal (metallic melt). Furthermore, not only liquid as a state of materials, but also materials in which particles of functional materials made of solid such as pigment or metallic particles are dissolved, dispersed, or mixed with the solvent are included. Representative examples of liquid include ink, liquid crystal, and the like described in the embodiments described above. Here, the ink includes various kinds of liquid compositions such as general water-based ink, oil-based ink, gel ink, hot-melt ink, and the like.

[0091] Specific examples of the droplet ejecting apparatus include, for example, droplet ejecting apparatuses that eject, as droplets, liquid containing dispersed or dissolved materials such as electrode materials or coloring materials used for manufacturing liquid crystal displays, electroluminescence (EL) displays, surface light emission displays, color filters, and the like. Furthermore, the droplet ejecting apparatus may be a droplet ejecting apparatus for ejecting biological organic materials used for manufacturing biochips, a droplet ejecting apparatus which is used as a precision pipette and ejects sample liquid, or the like. Furthermore, the droplet ejecting apparatus may be a droplet ejecting apparatus for ejecting lubricating oil to precision instruments such as watches or cameras with pinpoint precision, or a droplet ejecting apparatus for ejecting transparent resin solution such as ultraviolet curable resin on substrates to form micro-hemispherical lenses (optical lenses) or the like used for optical communication elements or the like. Furthermore, the droplet ejecting apparatus may be a droplet ejecting apparatus for ejecting etchant such as acidic or alkaline for performing etching on substrates or the like.

[0092] Furthermore, although the head 3 that shear-deforms the partition wall 32 between adjacent channels 31 and 31 has been exemplified as the head 3 in the description above, the head 3 is not limited thereto. For example, the head 3 may use an upper wall or a lower wall of the channel as a pressure generation means including piezoelectric elements such as PZT and shear-deform this upper wall or lower wall.

[0093] Still further, the droplet ejecting head according to the present invention is not limited to the shear-mode type at all. For example, the droplet ejecting head may be of a type in which one wall face of a pressure chamber is formed by a vibration plate, and this vibration plate is vibrated by a pressure generation means including piezoelectric elements such as PZT to apply pressure for ejection to ink in the pressure chamber.

Examples

[0094] Hereinafter, the effects of the present invention will be illustrated by examples.

(Example 1)

[0095] The shear-mode type inkjet head (nozzle diameter = 24 µm, AL = 3.7 µs) illustrated in Fig. 2 was used. For ink, UV curable ink was used at 40°C. The viscosity of the ink at this time was 0.01 Pa·s.

[0096] When the pulse width PAW1 of the first expansion pulse Pa1 was changed from 1.6 AL to 4.5 AL as illustrated in Table 1 using the first driving signal PA1 having the square waves illustrated in Fig. 3 as the first driving signal, the liquid volume (ng) of each large droplet ejected was measured.

[0097] Note that the droplets were ejected so as to have a droplet velocity of 6 m/s with the pulse width PAW2 of the first contraction pulse Pa2 = 0.5 AL, the pulse width PAW3 of the second expansion pulse Pa3 = 1 AL, the pulse width PAW4 of the second contraction pulse Pa4 = 2 AL, and the driving period being 9 AL.

[0098] In addition, the liquid volume ratio (liquid volume according to the present invention/liquid volume according to the DRR waveform) of a droplet according to the present invention was calculated relative to the liquid volume (6.1 ng) of a droplet ejected so as to have a driving period of 5 AL and a droplet velocity of 6 m/s using the DRR waveform illustrated in Fig. 7(a).

[0099] Furthermore, the flight determination was evaluated according to the following evaluation criteria by observing the ejection state for five consecutive minutes via strobe measurement using a CCD camera while changing the driving voltage (VH2, VH1), and measuring the droplet velocity when nozzle failure or ejection bending phenomenon occurred. That is, it was determined that the higher the droplet velocity when nozzle failure or ejection bending phenomenon occurred, the higher the flight stability.

⊙: Droplet velocity when nozzle failure or ejection bending occurred ≥ 11 m/s

○: 11 m/s > Droplet velocity when nozzle failure or ejection bending occurred ≥ 9 m/s

Δ: 9 m/s > Droplet velocity when nozzle failure or ejection bending occurred ≥ 7 m/s

×: 7 m/s > Droplet velocity when nozzle failure or ejection bending occurred

[0100] The results are illustrated in Table 1. In addition, a graph illustrating a relationship between the pulse width of the first expansion pulse and the liquid volume is illustrated in Fig. 8.

[Table 1]

FIRST EXPANSION PULSE (AL)	LIQUID VOLUME (ng)	LIQUID VOLUME RATIO	FLIGHT STABILITY	REMARKS
1.6	16.2	2.7	○
1.9	16.4	2.7	○
2.2	16.8	2.8	○	PRESENT INVENTION
2.5	16.8	2.8	○	PRESENT INVENTION
2.8	17.0	2.8	○	PRESENT INVENTION
3.1	17.2	2.8	⊙	PRESENT INVENTION
3.3	17.7	2.9	⊙	PRESENT INVENTION
3.5	18.1	3.0	⊙	PRESENT INVENTION
3.7	18.5	3.0	○	PRESENT INVENTION
3.9	18.9	3.1	Δ	PRESENT INVENTION
4.2	19.6	3.2	×
4.5	21.3	3.5	×

[0101] As illustrated in Table 1, when the pulse width PAW1 of the first expansion pulse Pa1 was greater than 2 AL and less than 4 AL, a large droplet was successfully ejected in a stable manner. When the pulse width PAW1 was 4 AL or more, the liquid volume was increased, but the flight stability was deteriorated.

[0102] Note that even when the first driving signal PA2 illustrated in Fig. 6 was used, it was also confirmed that a large droplet was successfully ejected in a stable manner as described above when the pulse width PAW1 of the first expansion pulse Pa1 was greater than 2 AL and less than 4 AL as described above. This was confirmed by calculating the liquid volume ratio with the liquid volume of a droplet ejected by the DRR waveform as a reference. The DRR waveform includes the second expansion pulse Pa3 and the second contraction pulse Pa4 of the first driving signal PA2 as the basic waveform.

(Example 2)

[0103] The shear-mode type inkjet head (nozzle diameter = 24 µm, 1 AL = 4.8 µs) illustrated in Fig. 2 was used. Ink A (solvent-type, a viscosity of 10 mPa·s) and ink B (aqueous-type, a viscosity of 4 mPa·s) were used as ink.

[0104] Using the first driving signal PA2 with the square waves illustrated in Fig. 6 as the first driving signal, the pressure in the channel was measured for each ink A and B with the passage of time during the application of the first driving signal PA2 for each of the cases where the driving voltage value ratio was set to |VH2| / |VH1| = 2/1 and |VH2|/|VH1| = 1/1.

[0105] Note that the droplets were ejected so as to have a droplet velocity of 6 m/s while the first driving signal PA2 was set to the pulse width PAW1 of the first expansion pulse Pa1 = 3.5 AL, the pulse width PAW2 of the first contraction pulse Pa2 = 0.5 AL, the pulse width PAW3 of the second expansion pulse Pa3 = 1 AL, the pulse width PAW4 of the second contraction pulse Pa4 = 0.5 AL, the pulse width PAW5 of the third contraction pulse Pa5 = 1 AL, the pause period PAW6 = 0.5 AL, and the driving period being 11 AL.

[0106] The results were illustrated in Figs. 9 (a) and 9 (b). (a) is the case where the ink A was used, and (b) is the case where the ink B was used.

[0107] As a result, in the case of high-viscosity ink, the attenuation (the area surrounded by a broken line) of the pressure wave in the channel was faster in the case of |VH2|/|VH1| = 2/1 than in the case of |VH2|/|VH1| = 1/1. On the other hand, in the case of low-viscosity ink, the attenuation of the pressure wave in the channel was faster in the case of |VH2| / | VH1| = 1/1 than in the case of |VH2| / |VH1| = 2/1. That is, it was confirmed that a large droplet can be ejected stably at high frequency.

Reference Signs List

[0108] 

1

inkjet recording apparatus

2

conveyance mechanism

21

conveyance roller

22

conveyance roller pair

23

conveyance motor

3

inkjet head

30

channel substrate

31

channel

32

partition wall

321

upper wall portion

322

lower wall portion

33

cover substrate

331

common channel

34

nozzle plate

341

nozzle

35

plate

351

ink supply port

352

ink supply tube

4

guide rail

5

carriage

6

flexible cable

7

medium

71

recording surface

8

driving control unit

100

droplet

101

first droplet

100

second droplet

PA1, PA2

first driving signal

Pa1

first expansion pulse

Pa2

first contraction pulse

Pa3

second expansion pulse

Pa4

second contraction pulse

Pa5

third contraction pulse

PAW1 to PAW5

pulse width

PAW6

pause period

PB1, PB2

second driving signal

Pb1

first expansion pulse

Pb2

first contraction pulse

Pb3

second contraction pulse

PBW1

to PBW3 pulse width

PBW4

pause period

Claims

1. A method for driving a droplet ejecting head, comprising:

applying a driving signal to a pressure generation means to expand or contract a volume of a pressure chamber;

applying pressure to liquid in the pressure chamber by driving the pressure generation means; and

ejecting a droplet from a nozzle,

wherein a first driving signal is included as the driving signal,

the first driving signal includes:

a first expansion pulse that expands the volume of the pressure chamber and contracts the volume of the pressure chamber after a certain period of time;

a first contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time;

a second expansion pulse that expands the volume of the pressure chamber and contracts the volume of the pressure chamber after a certain period of time; and

a second contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time, in this order, and

a pulse width of the first expansion pulse is greater than 2 AL and less than 4 AL (where AL is 1/2 an acoustic resonance period of a pressure wave in the pressure chamber).

2. The method for driving a droplet ejecting head according to claim 1,
wherein in the first driving signal, the pulse width of the first expansion pulse is 2.5 AL or more and less than 3.8 AL.

3. The method for driving a droplet ejecting head according to claim 1 or 2,
wherein in the first driving signal, a pulse width of the first contraction pulse is 0.4 AL or more and 0.7 AL or less, a pulse width of the second expansion pulse is 0.8 AL or more and 1.2 AL or less, and a pulse width of the second contraction pulse is 1.8 AL or more and 2.2 AL or less.

4. The method for driving a droplet ejecting head according to claim 1, 2, or 3,
wherein in the first driving signal, a voltage value of the first expansion pulse and a voltage value of the second expansion pulse are equal, and a voltage value of the first contraction pulse and a voltage value of the second contraction pulse are equal.

5. The method for driving a droplet ejecting head according to claim 4,
wherein in a case where a viscosity of the liquid is greater than 5 mPa·s, when the voltage values of the first expansion pulse and the second expansion pulse are set to VH2 and the voltage values of the first contraction pulse and the second contraction pulse are set to VH1, the first driving signal is |VH2| / |VH1| = 2/1.

6. The method for driving a droplet ejecting head according to claim 4,
wherein in a case where a viscosity of the liquid is 5 mPa·s or less, when the voltage values of the first expansion pulse and the second expansion pulse are set to VH2 and the voltage values of the first contraction pulse and the second contraction pulse are set to VH1, the first driving signal is |VH2| / |VH1| = 1/1.

7. The method for driving a droplet ejecting head according to claim 1 or 2,
wherein the first driving signal further includes a third contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time,
a pulse width of the second contraction pulse is 0.3 AL or more and 0.7 AL or less,
a pulse width of the third contraction pulse is 0.8 AL or more and 1.2 AL or less, and
the third contraction pulse is applied following a pause period of 0. 3 AL or more and 0. 7 AL or less after the application of the second contraction pulse ends.

8. The method for driving a droplet ejecting head according to claim 7,
wherein in the first driving signal, a pulse width of the first contraction pulse is 0.4 AL or more and 0.7 AL or less, and a pulse width of the second expansion pulse is 0.8 AL or more and 1.2 AL or less.

9. The method for driving a droplet ejecting head according to claim 7 or 8,
wherein in the first driving signal, a voltage value of the first expansion pulse and a voltage value of the second expansion pulse are equal, and a voltage value of the first contraction pulse and voltage values of the second contraction pulse and the third contraction pulse are equal.

10. The method for driving a droplet ejecting head according to claim 9,
wherein in a case where a viscosity of the liquid is greater than 5 mPa·s, when the voltage values of the first expansion pulse and the second expansion pulse are set to VH2 and the voltage values of the first contraction pulse, the second contraction pulse, and the third contraction pulse are set to VH1, the first driving signal is |VH2| / |VH1| = 2/1.

11. The method for driving a droplet ejecting head according to claim 9,
wherein in a case where a viscosity of the liquid is 5 mPa·s or less, when the voltage values of the first expansion pulse and the second expansion pulse of the first driving signal are set to VH2 and the voltage values of the first contraction pulse, the second contraction pulse, and the third contraction pulse are set to VH1, the first driving signal is |VH2| / |VH1| = 1/1.

12. The method for driving a droplet ejecting head according to any one of claims 1 to 11, further comprising:

a second driving signal as the driving signal upon forming a small droplet by ejecting a single droplet from the nozzle,

wherein the second driving signal includes:

a first expansion pulse that expands the volume of the pressure chamber and contracts the volume of the pressure chamber after a certain period of time; and

a first contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time, in this order,

a pulse width of the first expansion pulse of the second driving signal is identical to the pulse width of the second expansion pulse of the first driving signal,

a pulse width of the first contraction pulse of the second driving signal is identical to the pulse width of the second contraction pulse of the first driving signal, and

depending on image data, a large droplet by the first driving signal and a small droplet by the second driving signal are selectively ejected from the identical nozzle.

13. A droplet ejecting apparatus comprising:

a droplet ejecting head configured to apply pressure for ejection to liquid in a pressure chamber by driving a pressure generation means, and eject a droplet from a nozzle; and

a driving control means configured to output a driving signal that drives the pressure generation means,

wherein the driving signal includes a first driving signal,

the first driving signal includes:

a first expansion pulse that expands a volume of the pressure chamber and contracts the volume of the pressure chamber after a certain period of time;

a first contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time;

a second expansion pulse that expands the volume of the pressure chamber and contracts the volume of the pressure chamber after a certain period of time; and

a second contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time, in this order, and

a pulse width of the first expansion pulse is greater than 2 AL and less than 4 AL (where AL is 1/2 an acoustic resonance period of a pressure wave in the pressure chamber).

14. The droplet ejecting apparatus according to claim 13,
wherein in the first driving signal, the pulse width of the first expansion pulse is 2.5 AL or more and less than 3.8 AL.

15. The droplet ejecting apparatus according to claim 13 or 14,
wherein in the first driving signal, a pulse width of the first contraction pulse is 0.4 AL or more and 0.7 AL or less, a pulse width of the second expansion pulse is 0.8 AL or more and 1.2 AL or less, and a pulse width of the second contraction pulse is 1.8 AL or more and 2.2 AL or less.

16. The droplet ejecting apparatus according to claim 13, 14, or 15,
wherein in the first driving signal, a voltage value of the first expansion pulse and a voltage value of the second expansion pulse are equal, and a voltage value of the first contraction pulse and a voltage value of the second contraction pulse are equal.

17. The droplet ejecting apparatus according to claim 16,
wherein a viscosity of the liquid is greater than 5 mPa·s, and
when the voltage values of the first expansion pulse and the second expansion pulse are set to VH2 and the voltage values of the first contraction pulse and the second contraction pulse are set to VH1, the first driving signal is |VH2|/|VH1| = 2/1.

18. The droplet ejecting apparatus according to claim 16,
wherein a viscosity of the liquid is 5 mPa·s or less, and
when the voltage values of the first expansion pulse and the second expansion pulse are set to VH2 and the voltage values of the first contraction pulse and the second contraction pulse are set to VH1, the first driving signal is |VH2|/|VH1| = 1/1.

19. The droplet ejecting apparatus according to claim 13 or 14,
wherein the first driving signal further includes a third contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time,
a pulse width of the second contraction pulse is 0.3 AL or more and 0.7 AL or less,
a pulse width of the third contraction pulse is 0.8 AL or more and 1.2 AL or less, and
the third contraction pulse is applied following a pause period of 0.3 AL or more and 0.7 AL or less after the application of the second contraction pulse ends.

20. The droplet ejecting apparatus according to claim 19,
wherein in the first driving signal, a pulse width of the first contraction pulse is 0.4 AL or more and 0.7 AL or less, and a pulse width of the second expansion pulse is 0.8 AL or more and 1.2 AL or less.

21. The droplet ejecting apparatus according to claim 19 or 20,
wherein in the first driving signal, a voltage value of the first expansion pulse and a voltage value of the second expansion pulse are equal, and a voltage value of the first contraction pulse and voltage values of the second contraction pulse and the third contraction pulse are equal.

22. The droplet ejecting apparatus according to claim 21,
wherein a viscosity of the liquid is greater than 5 mPa·s, and
when the voltage values of the first expansion pulse and the second expansion pulse are set to VH2 and the voltage values of the first contraction pulse, the second contraction pulse, and the third contraction pulse are set to VH1, the first driving signal is |VH2|/|VH1| = 2/1.

23. The droplet ejecting apparatus according to claim 21,
wherein a viscosity of the liquid is 5 mPa·s or less, and
when the voltage values of the first expansion pulse and the second expansion pulse of the first driving signal are set to VH2 and the voltage values of the first contraction pulse, the second contraction pulse, and the third contraction pulse are set to VH1, the first driving signal is |VH2|/|VH1| = 1/1.

24. The droplet ejecting apparatus according to any one of claims 13 to 23, further comprising:

a second driving signal as the driving signal upon forming a small droplet by ejecting a single droplet from the nozzle,

wherein the second driving signal includes:

a first expansion pulse that expands the volume of the pressure chamber and contracts the volume of the pressure chamber after a certain period of time; and

a first contraction pulse that contracts the volume of the pressure chamber and expands the volume of the pressure chamber after a certain period of time, in this order,

a pulse width of the first expansion pulse of the second driving signal is identical to the pulse width of the second expansion pulse of the first driving signal,

a pulse width of the first contraction pulse of the second driving signal is identical to the pulse width of the second contraction pulse of the first driving signal, and

the driving control means outputs the first driving signal or the second driving signal to the pressure generation means so as to selectively eject a large droplet by the first driving signal and a small droplet by the second driving signal from the identical nozzle depending on image data.

25. The droplet ejecting apparatus according to any one of claims 13 to 24,
wherein the droplet ejecting head is a shear-mode type droplet ejecting head.

Drawing