阅读说明：本技术 喷墨头的驱动方法以及喷墨记录装置 (Ink jet head driving method and ink jet recording apparatus ) 是由 X.李 末富靖彦 于 2019-03-29 设计创作，主要内容包括：提供可以有效地抑制画质的降低的喷墨头的驱动方法以及喷墨记录装置。在喷墨头的驱动方法中,所述喷墨头具有：喷嘴；以及压力产生部,根据驱动信号的施加,对与喷嘴连通的压力室的墨水提供压力变化而从喷嘴喷出墨水,喷墨头通过使根据一连串的驱动信号的施加而从喷嘴喷出的多个墨水的液滴着落在记录介质上而形成一个像素,驱动方法将包含具有第1电压振幅的第1驱动信号、和具有大于第1电压振幅的第2电压振幅的第2驱动信号的一连串的驱动信号对压力产生部施加,一连串的驱动信号中的最后的驱动信号是第2驱动信号,在将第1电压振幅设为Va,将第2电压振幅设为Vb的情况下,以比率Va/Vb成为与从喷嘴喷出的墨水的比重相应的值的方式,来决定第1电压振幅以及第2电压振幅。(Provided are a method for driving an ink jet head and an ink jet recording apparatus, wherein the deterioration of image quality can be effectively suppressed. In a driving method of an inkjet head, the inkjet head includes: a nozzle; and a pressure generating section for applying a pressure change to the ink in a pressure chamber communicating with the nozzle in accordance with application of a driving signal to discharge the ink from the nozzle, wherein the inkjet head forms one pixel by landing droplets of a plurality of inks discharged from the nozzle in accordance with application of a series of driving signals on a recording medium, the driving method applies a series of driving signals including a 1 st driving signal having a 1 st voltage amplitude and a 2 nd driving signal having a 2 nd voltage amplitude larger than the 1 st voltage amplitude to the pressure generating section, a last driving signal in the series of driving signals is the 2 nd driving signal, when the 1 st voltage amplitude is Va and the 2 nd voltage amplitude is Vb, the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so that the ratio Va/Vb is a value corresponding to the specific gravity of the ink discharged from the nozzles.)

1. A method of driving an ink jet head, the ink jet head having: a nozzle for ejecting ink; and a pressure generating section that provides a pressure change to ink in a pressure chamber communicating with the nozzle in accordance with application of a driving signal to discharge the ink from the nozzle, wherein the inkjet head forms one pixel by landing droplets of a plurality of inks discharged from the nozzle in accordance with application of a series of the driving signal on a recording medium,

applying the series of driving signals including the 1 st driving signal having the 1 st voltage amplitude and the 2 nd driving signal having the 2 nd voltage amplitude larger than the 1 st voltage amplitude to the pressure generating portion,

the last drive signal in the series of drive signals is the 2 nd drive signal,

when the 1 st voltage amplitude is Va and the 2 nd voltage amplitude is Vb, the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so that a ratio Va/Vb is a value corresponding to a specific gravity of the ink discharged from the nozzle.

2. The driving method according to claim 1,

the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so that the ratio Va/Vb decreases as the specific gravity of the ink ejected from the nozzles increases.

3. The driving method according to claim 1 or 2,

the specific gravity of the ink ejected from the nozzle is 1.0g/cm³Above, 1.9g/cm³In the following cases, the ratio of Va/Vb to Vb is 0.75 & lt, 0.86The 1 st voltage amplitude and the 2 nd voltage amplitude are determined.

4. The driving method according to claim 3,

the specific gravity of the ink ejected from the nozzle is 1.2g/cm³Above, 1.4g/cm³In the following cases, the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so as to satisfy 0.76 < Va/Vb < 0.80.

5. A method of driving an ink jet head, the ink jet head having: a nozzle for ejecting ink; and a pressure generating section that provides a pressure change to ink in a pressure chamber communicating with the nozzle in accordance with application of a driving signal to discharge the ink from the nozzle, wherein the inkjet head forms one pixel by landing droplets of a plurality of inks discharged from the nozzle in accordance with application of a series of the driving signal on a recording medium,

applying the series of driving signals including the 1 st driving signal having the 1 st voltage amplitude and the 2 nd driving signal having the 2 nd voltage amplitude larger than the 1 st voltage amplitude to the pressure generating portion,

the last drive signal in the series of drive signals is the 2 nd drive signal,

when the 1 st voltage amplitude is Va and the 2 nd voltage amplitude is Vb, the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so that a ratio Va/Vb is a value corresponding to the viscosity of the ink discharged from the nozzles.

6. The driving method according to claim 5,

the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so that the ratio Va/Vb decreases as the viscosity of the ink ejected from the nozzle decreases.

7. The driving method according to claim 5 or 6,

when the viscosity of the ink ejected from the nozzle is 8cP or more and 16cP or less, the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so as to satisfy 0.60 < Va/Vb < 0.91.

8. The driving method according to claim 7,

the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so as to satisfy 0.74 < Va/Vb < 0.84 when the viscosity of the ink ejected from the nozzles is 10cP or more and 14cP or less.

9. The driving method according to any one of claims 1 to 8,

the drive signal includes an expansion pulse signal that expands the pressure chamber and a contraction pulse signal that contracts the pressure chamber to which the expansion pulse signal is applied,

when 1/2 of an acoustic resonance period of the pressure wave in the pressure chamber is set to AL, the pulse width of the expansion pulse signal in the 1 st drive signal is equal to or greater than AL and equal to or less than 1.4 AL.

10. The driving method according to claim 9, wherein,

the pulse width of the expansion pulse signal in the 1 st drive signal is 1.2AL or more and 1.4AL or less.

11. The driving method according to any one of claims 1 to 8,

the drive signal includes an expansion pulse signal that expands the pressure chamber and a contraction pulse signal that contracts the pressure chamber to which the expansion pulse signal is applied,

the pulse width of the expansion pulse signal in the 1 st drive signal is different from 1/2 of the acoustic resonance period of the pressure wave in the pressure chamber.

12. The driving method according to any one of claims 1 to 11,

when 1/2 indicating the acoustic resonance period of the pressure wave in the pressure chamber is set to AL, the series of drive signals are applied after a wait time of 4AL or more has elapsed from the end of application of any other drive signal.

13. An inkjet recording apparatus having an inkjet head, the inkjet head having: a nozzle for ejecting ink; and a pressure generating section that provides a pressure change to ink in a pressure chamber communicating with the nozzle in accordance with application of a drive signal to discharge the ink from the nozzle, wherein the inkjet recording apparatus forms one pixel by landing droplets of a plurality of inks discharged from the nozzle in accordance with application of a series of the drive signal on a recording medium,

the inkjet recording apparatus includes: a driving section for applying the series of driving signals including the 1 st driving signal having the 1 st voltage amplitude and the 2 nd driving signal having the 2 nd voltage amplitude larger than the 1 st voltage amplitude to the pressure generating section,

the last drive signal in the series of drive signals is the 2 nd drive signal,

when the 1 st voltage amplitude is Va and the 2 nd voltage amplitude is Vb, the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so that a ratio Va/Vb is a value corresponding to a specific gravity of the ink discharged from the nozzle.

14. An inkjet recording apparatus having an inkjet head, the inkjet head having: a nozzle for ejecting ink; and a pressure generating section that provides a pressure change to ink in a pressure chamber communicating with the nozzle in accordance with application of a drive signal to discharge the ink from the nozzle, wherein the inkjet recording apparatus forms one pixel by landing droplets of a plurality of inks discharged from the nozzle in accordance with application of a series of the drive signal on a recording medium,

the inkjet recording apparatus includes: a driving section for applying the series of driving signals including the 1 st driving signal having the 1 st voltage amplitude and the 2 nd driving signal having the 2 nd voltage amplitude larger than the 1 st voltage amplitude to the pressure generating section,

the last drive signal in the series of drive signals is the 2 nd drive signal,

when the 1 st voltage amplitude is Va and the 2 nd voltage amplitude is Vb, the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so that a ratio Va/Vb is a value corresponding to the viscosity of the ink discharged from the nozzles.

Technical Field

The invention relates to a driving method of an ink jet head and an ink jet recording apparatus.

Background

Conventionally, there is an ink jet recording apparatus that forms an image by ejecting ink from a nozzle provided in an ink jet head and landing the ink on a desired position. The ink jet head is provided with a pressure chamber communicating with the nozzle, and a pressure generating unit (for example, a piezoelectric element) for applying a pressure change to the ink in the pressure chamber in response to application of a drive signal.

In an inkjet recording apparatus, a series of drive signals are applied to a pressure generating section, and droplets of a plurality of inks ejected from nozzles in response to the application of the series of drive signals are landed on a recording medium in a united manner, thereby forming one pixel (for example, patent documents 1 and 2). According to this technique, the amount of ink droplets landed can be adjusted by changing the number of drive signals to be applied.

Documents of the prior art

Patent document

Patent document 1 Japanese patent laid-open publication No. 2017-202588

Patent document 2 Japanese patent laid-open publication No. 2018-176457

Disclosure of Invention

Problems to be solved by the invention

However, the amount of ink droplets discharged from the nozzles and the speed of the discharged ink droplets can vary depending on the behavior of the ink in the nozzles. Since the behavior of ink in the nozzles generally differs depending on the characteristics (for example, specific gravity and viscosity) of the ink, if the same drive signal is applied to inks of various characteristics in a unified manner, the amount of droplets of the ejected ink and the speed of the droplets of the ejected ink may deviate from desired values. As a result, the droplets of the plurality of inks discharged in accordance with the series of drive signals are not properly merged, and the landing position on the recording medium is shifted from a desired position, which leads to a problem of deterioration in image quality.

The invention aims to provide a driving method of an ink jet head and an ink jet recording apparatus, which can effectively inhibit the reduction of image quality.

Means for solving the problems

In order to achieve the above object, the invention of claim 1 is a method of driving an ink jet head, the ink jet head including: a nozzle for ejecting ink; and a pressure generating section that provides a pressure change to ink in a pressure chamber communicating with the nozzle in accordance with application of a drive signal to discharge the ink from the nozzle, wherein the inkjet head forms one pixel by landing droplets of a plurality of inks discharged from the nozzle in accordance with application of a series of the drive signal on a recording medium,

the driving method applies the series of driving signals including the 1 st driving signal having the 1 st voltage amplitude and the 2 nd driving signal having the 2 nd voltage amplitude larger than the 1 st voltage amplitude to the pressure generating portion,

the last drive signal in the series of drive signals is the 2 nd drive signal,

when the 1 st voltage amplitude is Va and the 2 nd voltage amplitude is Vb, the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so that a ratio Va/Vb is a value corresponding to a specific gravity of the ink discharged from the nozzle.

The invention described in claim 2 is the driving method described in claim 1,

the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so that the ratio Va/Vb decreases as the specific gravity of the ink ejected from the nozzles increases.

The invention described in claim 3 is the driving method described in claim 1 or 2,

the specific gravity of the ink ejected from the nozzle is 1.0g/cm³Above, 1.9g/cm³In the following cases, the 1 st voltage amplitude and the 2 nd voltage are determined so as to satisfy 0.75 < Va/Vb < 0.86Amplitude of vibration.

The invention described in claim 4 is the driving method described in claim 3,

the specific gravity of the ink ejected from the nozzle is 1.2g/cm³Above, 1.4g/cm³In the following cases, the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so as to satisfy 0.76 < Va/Vb < 0.80.

In order to achieve the above object, the invention of a driving method described in claim 5 is a driving method of an ink jet head including: a nozzle for ejecting ink; and a pressure generating section that provides a pressure change to ink in a pressure chamber communicating with the nozzle in accordance with application of a driving signal to discharge the ink from the nozzle, wherein the inkjet head forms one pixel by landing droplets of a plurality of inks discharged from the nozzle in accordance with application of a series of the driving signal on a recording medium,

applying the series of driving signals including the 1 st driving signal having the 1 st voltage amplitude and the 2 nd driving signal having the 2 nd voltage amplitude larger than the 1 st voltage amplitude to the pressure generating portion,

the last drive signal in the series of drive signals is the 2 nd drive signal,

when the 1 st voltage amplitude is Va and the 2 nd voltage amplitude is Vb, the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so that a ratio Va/Vb is a value corresponding to the viscosity of the ink discharged from the nozzles.

The invention described in claim 6 is the driving method described in claim 5,

the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so that the ratio Va/Vb decreases as the viscosity of the ink ejected from the nozzle decreases.

The invention described in claim 7 is the driving method described in claim 5 or 6,

when the viscosity of the ink ejected from the nozzle is 8cP or more and 16cP or less, the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so as to satisfy 0.60 < Va/Vb < 0.91.

The invention described in claim 8 provides the driving method described in claim 7,

the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so as to satisfy 0.74 < Va/Vb < 0.84 when the viscosity of the ink ejected from the nozzles is 10cP or more and 14cP or less.

The invention described in claim 9 provides the driving method described in any one of claims 1 to 8,

the drive signal includes an expansion pulse signal that expands the pressure chamber and a contraction pulse signal that contracts the pressure chamber to which the expansion pulse signal is applied,

when 1/2 of an acoustic resonance period of the pressure wave in the pressure chamber is set to AL, the pulse width of the expansion pulse signal in the 1 st drive signal is equal to or greater than AL and equal to or less than 1.4 AL.

The invention described in claim 10 provides the driving method described in claim 9,

the pulse width of the expansion pulse signal in the 1 st drive signal is 1.2AL or more and 1.4AL or less.

The invention described in claim 11 provides the driving method described in any one of claims 1 to 8,

the drive signal includes an expansion pulse signal that expands the pressure chamber and a contraction pulse signal that contracts the pressure chamber to which the expansion pulse signal is applied,

the pulse width of the expansion pulse signal in the 1 st drive signal is different from 1/2 of the acoustic resonance period of the pressure wave in the pressure chamber.

The invention described in claim 12 provides the driving method described in any one of claims 1 to 11,

when 1/2 indicating the acoustic resonance period of the pressure wave in the pressure chamber is set to AL, the series of drive signals are applied after a wait time of 4AL or more has elapsed from the end of application of any other drive signal.

In order to achieve the above object, the invention of an inkjet recording apparatus according to claim 13 is an inkjet recording apparatus including an inkjet head, the inkjet head including: a nozzle for ejecting ink; and a pressure generating section that provides a pressure change to ink in a pressure chamber communicating with the nozzle in accordance with application of a drive signal to discharge the ink from the nozzle, wherein the inkjet recording apparatus forms one pixel by landing droplets of a plurality of inks discharged from the nozzle in accordance with application of a series of the drive signal on a recording medium,

the inkjet recording apparatus includes: a driving section for applying the series of driving signals including the 1 st driving signal having the 1 st voltage amplitude and the 2 nd driving signal having the 2 nd voltage amplitude larger than the 1 st voltage amplitude to the pressure generating section,

the last drive signal in the series of drive signals is the 2 nd drive signal,

when the 1 st voltage amplitude is Va and the 2 nd voltage amplitude is Vb, the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so that a ratio Va/Vb is a value corresponding to a specific gravity of the ink discharged from the nozzle.

In order to achieve the above object, the invention of an ink jet recording apparatus according to claim 14 is an ink jet recording apparatus including an ink jet head, the ink jet head including: a nozzle for ejecting ink; and a pressure generating section that provides a pressure change to ink in a pressure chamber communicating with the nozzle in accordance with application of a drive signal to discharge the ink from the nozzle, wherein the inkjet recording apparatus forms one pixel by landing droplets of a plurality of inks discharged from the nozzle in accordance with application of a series of the drive signal on a recording medium,

the inkjet recording apparatus includes: a driving section for applying the series of driving signals including the 1 st driving signal having the 1 st voltage amplitude and the 2 nd driving signal having the 2 nd voltage amplitude larger than the 1 st voltage amplitude to the pressure generating section,

the last drive signal in the series of drive signals is the 2 nd drive signal,

when the 1 st voltage amplitude is Va and the 2 nd voltage amplitude is Vb, the 1 st voltage amplitude and the 2 nd voltage amplitude are determined so that a ratio Va/Vb is a value corresponding to the viscosity of the ink discharged from the nozzles.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, the image quality can be effectively prevented from being degraded.

Drawings

Fig. 1 is a schematic configuration diagram of an inkjet recording apparatus.

Fig. 2 is a schematic diagram showing the structure of the head unit.

Fig. 3 is an exploded perspective view showing the structure of the ink jet head.

Fig. 4A is a schematic cross-sectional view illustrating a driving operation of the pressure generating section.

Fig. 4B is a schematic cross-sectional view illustrating a driving operation of the pressure generating portion.

Fig. 4C is a schematic cross-sectional view illustrating a driving operation of the pressure generating portion.

Fig. 5 is a diagram showing a state of ink ejection from the nozzles in accordance with the driving operation.

Fig. 6 is a block diagram showing a functional configuration of the inkjet recording apparatus.

Fig. 7 is a diagram showing an example of the composite drive signal.

Fig. 8 is a diagram showing the transition of the position of the meniscus according to the characteristics of the ink.

Fig. 9A is a diagram showing an example of the position of the meniscus after ink is ejected.

Fig. 9B is a diagram showing an example of the position of the meniscus after the ejection of the ink having a higher specific gravity or the ink having a lower viscosity.

Fig. 10 is a diagram showing experimental conditions showing effects obtained by the adjustment of the pulse width PWa and the pulse period SDPa, and evaluation results.

Detailed Description

Hereinafter, an embodiment of an inkjet head driving method and an inkjet recording apparatus according to the present invention will be described with reference to the drawings.

Fig. 1 is a diagram showing a schematic configuration of an inkjet recording apparatus 1 according to an embodiment of the present invention.

The inkjet recording apparatus 1 includes a conveyance unit 2, a head unit 3, and the like.

The conveying unit 2 has a wheel-shaped conveying belt 2c supported on the inner side by 2 conveying rollers 2a and 2b that rotate about a rotation axis extending in the X direction in fig. 1. In the transport unit 2, the transport roller 2a rotates in accordance with the operation of a transport motor (not shown) in a state where the recording medium M is placed on the transport surface of the transport belt 2c, and the transport belt 2c moves around, thereby transporting the recording medium M in the moving direction (transport direction; Y direction in fig. 1) of the transport belt 2 c.

The recording medium M may be set as a printing paper cut to a certain size. The recording medium M is fed onto the transport belt 2c by a paper feeding device, not shown, and is discharged from the transport belt 2c to a predetermined paper discharge portion after an image is recorded by discharging ink from the head unit 3. Further, as the recording medium M, roll paper may be used. As the recording medium M, various media capable of fixing ink landed on the surface, such as cloth or sheet-like resin, may be used in addition to paper such as plain paper and coated paper.

The head unit 3 ejects ink at an appropriate timing based on image data to the recording medium M conveyed by the conveying unit 2, and records an image. In the inkjet recording apparatus 1 of the present embodiment, 4 head units 3 corresponding to 4 colors of yellow (Y), magenta (M), cyan (C), and black (K) are arranged in a row at predetermined intervals in the order of Y, M, C, K colors from the upstream side in the conveyance direction of the recording medium M. The head unit 3 is disposed so that the ink discharge direction is directed downward in the vertical direction. The number of head units 3 may be 3 or less or 5 or more.

Fig. 2 is a schematic diagram showing the configuration of the head unit 3, and is a plan view of the head unit 3 as viewed from the side opposite to the conveying surface of the conveying belt 2 c. The head unit 3 includes a plate-like base portion 3a, and a plurality of (here, 8) ink jet heads 10 fixed to the base portion 3a in a state of being fitted in through holes provided in the base portion 3 a. The inkjet head 10 is fixed to the base 3a in a state where a nozzle opening surface provided with an opening of the nozzle 18 is exposed in the-Z direction from the through hole of the base 3 a.

In the inkjet head 10, the plurality of nozzles 18 are arranged at equal intervals in a direction intersecting the conveying direction of the recording medium M (in the present embodiment, in the X direction, which is a width direction orthogonal to the conveying direction). That is, each inkjet head 10 has a row (nozzle row) of nozzles 18 arranged one-dimensionally at equal intervals in the X direction.

The inkjet head 10 may have a plurality of nozzle rows. In this case, the positions of the plurality of nozzle rows in the X direction are arranged to be shifted from each other so that the positions of the nozzles 18 in the X direction do not overlap.

The 8 ink jet heads 10 in the head unit 3 are arranged in a staggered grid pattern so that the arrangement range in the X direction with respect to the nozzles 18 is continuous. The arrangement range of the nozzles 18 included in the head unit 3 in the X direction covers the width of the region in the X direction in which an image can be recorded in the recording medium M conveyed by the conveying belt 2 c. The head unit 3 is used with a fixed position when recording an image, and ejects ink from the nozzles 18 to each position at a predetermined interval (conveyance direction interval) with respect to the conveyance direction in accordance with conveyance of the recording medium M, thereby recording an image in a single pass.

Fig. 3 is an exploded perspective view showing the structure of the ink jet head 10. In fig. 3, the number of nozzles 18 in the inkjet head 10 is not illustrated as 7, but in each inkjet head 10 of the present embodiment, hundreds to thousands or more nozzles 18 are provided.

The inkjet head 10 has a channel substrate 11, and the channel substrate 11 forms a plurality of pressure chambers 19 (channels) communicating with the nozzles 18 in correspondence with the plurality of nozzles 18. A nozzle plate 13 having a plurality of nozzles 18 formed therein is bonded to an end surface of the channel substrate 11. A cover plate 12 is attached to the channel substrate 11 at the upper portion on the nozzle plate 13 side.

The channel substrate 11 has a structure in which 2 substrates 14 and 15 are bonded to each other via a bonding portion 16. The substrates 14 and 15 are made of a piezoelectric material such as lead zirconate titanate (PZT), and are polarized in directions opposite to each other in the thickness direction. A plurality of pressure chambers 19 are formed on the channel substrate 11 at equal intervals, and partition walls 171 (piezoelectric elements) made of a piezoelectric material are formed between the pressure chambers 19. Electrodes 172 (see fig. 4A) are provided on the side walls of the pressure chambers 19 (the surfaces of the partition walls 171), and the partition walls 171 are bent (shear-deformed) around the adhesive portions 16 in accordance with the voltage applied between the electrodes 172 of the adjacent pressure chambers 19. The pressure of the ink in the pressure chamber 19 is varied by the shear deformation of the partition wall 171 in accordance with the application of the voltage signal of the predetermined drive waveform to the electrode 172, and the ink in the pressure chamber 19 is ejected from the nozzle 18 in accordance with the variation. In this way, the electrode 172 and the partition wall 171 constitute a pressure generating unit 17 (actuator) that performs a driving operation for applying a pressure change to the ink in the pressure chamber 19. The operation of the pressure generating section 17 to apply a pressure change to the ink in the pressure chamber 19 will be referred to as a driving operation hereinafter.

As described above, the inkjet head 10 according to the present embodiment is a shear mode inkjet head that ejects ink from the nozzles 18 by shear stress generated by applying an electric field in a direction orthogonal to the polarization direction of the piezoelectric element.

Fig. 4A to 4C are schematic sectional views for explaining the driving operation of the pressure generating section 17.

These fig. 4A to 4C are schematic sectional views of the inkjet head 10 in a plane parallel to the nozzle plate 13. In fig. 4A to 4C, three pressure chambers 19A to 19C adjacent to each other are drawn, and the operation in the case where ink is ejected from the central pressure chamber 19B will be described below. The electrodes 172A to 172C of the pressure chambers 19A to 19C are connected to a head drive unit 20 (see fig. 6), and drive signals are supplied from the head drive unit 20 to the electrodes 172A to 172C.

Fig. 5 is a diagram showing a state where ink is ejected from the nozzles 18 in accordance with a driving operation. Fig. 5 shows the behavior of ink in the nozzle 18 and ejected ink at each of the timings t1 to t 9.

When ink is discharged from the nozzle 18 of the pressure chamber 19B, first, from the neutral state (timing t1 in fig. 5) described in fig. 4A, as shown in fig. 4B, the electrodes 172A and 172C of the pressure chambers 19A and 19C are brought to the ground potential, and a pulse signal of voltage + V is applied to the electrode 172B of the pressure chamber 19B (that is, an expansion pulse signal is applied), so that an electric field is generated in the partition wall 171, and the partition wall 171 is shear-deformed to expand the volume of the pressure chamber 19B. As a result, a negative pressure is generated in the pressure chamber 19B, and ink flows into the pressure chamber 19B from the ink flow path connected to the pressure chamber 19B (timing t2 in fig. 5).

Next, as shown in fig. 4C, by applying a pulse signal of voltage + V to the electrodes 172A, 172C of the pressure chambers 19A, 19C and changing the potential of the electrode 172B of the pressure chamber 19B to the ground potential (that is, by applying a contraction pulse signal), an electric field is generated in the partition wall 171, and the partition wall 171 is shear-deformed to contract the volume of the pressure chamber 19B. As a result, a positive pressure is generated in the pressure chamber 19B, and droplets of ink (hereinafter, referred to as ink droplets) are ejected from the nozzle 18 (timings t3 to t9 in fig. 5).

In this way, in the inkjet head 10, the pressure generating section 17 performs a driving operation of expanding the pressure chamber 19 by the expansion pulse signal and then contracting the pressure chamber 19 by the contraction pulse signal, thereby increasing the internal pressure of the pressure chamber 19 and discharging ink from the nozzle 18. The pressure generating portion 17 corresponding to the pressure chamber 19B is constituted by a pair of partition walls 171 adjacent to the pressure chamber 19B, and electrodes 172A to 173C provided on the pair of partition walls 171.

Fig. 6 is a block diagram showing a functional configuration of the inkjet recording apparatus 1 according to the present embodiment.

The inkjet recording apparatus 1 includes: the inkjet head 10, the head driving unit 20 (driving unit), the control unit 30, the communication unit 41, the operation display unit 42, the conveyance driving unit 43, the temperature detection unit 44, the bus 45, and the like described above.

The head driving unit 20 outputs (applies) an expansion pulse signal and a contraction pulse signal for ejecting ink from each nozzle 18 of the inkjet head 10 at an appropriate timing to the pressure generating unit 17 corresponding to the selected nozzle 18, thereby operating the pressure generating unit 17. The head drive unit 20 includes a drive waveform signal output unit 21, a digital-to-analog converter (DAC) 22, a drive circuit 23, an output selection unit 24, and the like.

The drive waveform signal output unit 21 outputs digital data of a drive waveform corresponding to ejection or non-ejection of ink in synchronization with a clock signal input from an oscillation circuit, not shown. The DAC22 converts the drive waveform of the digital data into an analog signal and outputs the analog signal to the drive circuit 23 as an input signal.

The drive circuit 23 amplifies the input signal to a voltage value corresponding to the drive voltage of the pressure generating unit 17, and then current-amplifies the amplified signal to output the amplified signal as a pulse signal. As will be described later, the drive circuit 23 outputs a pulse signal of a voltage Va and a voltage Vb larger than the voltage Va in accordance with the drive waveform output from the drive waveform signal output unit 21.

The output selection unit 24 outputs a switching signal for selecting the pressure generation unit 17 to which the pulse signal is to be output, based on the pixel data of the image to be formed input from the control unit 30.

The communication unit 41 transmits and receives data to and from an external device in accordance with a predetermined communication standard. The communication unit 41 includes a connection terminal related to a communication standard to be used, and hardware (network card) of a driver related to communication connection.

The operation display section 42 displays status information, menus, and the like relating to image recording, and accepts input operations from a user. The operation display unit 42 includes, for example, a display screen based on a liquid crystal panel, a driver for the liquid crystal panel, a touch panel provided so as to be superimposed on the liquid crystal panel, and the like, and outputs an operation detection signal according to a position touched by a user and a type of operation to the control unit 30.

The conveyance drive unit 43 acquires the recording medium M before image recording from the medium supply unit, and is disposed so as to face the ink discharge surface of the inkjet head 10 at an appropriate position, and discharges the recording medium M on which an image is recorded from a position facing the ink discharge surface. The conveyance driving unit 43 rotates a motor for rotating the conveyance roller 2a at an appropriate speed and timing.

The temperature detector 44 is attached to the inkjet head 10 or disposed near the inkjet head 10, detects the temperature, and outputs the detection result to the controller 30.

The control unit 30 is a processor that collectively controls the overall operation of the inkjet recording apparatus 1. The control Unit 30 includes a CPU31(Central Processing Unit), a RAM32(Random Access Memory), a storage Unit 33, and the like. The CPU31 performs various arithmetic processes related to the overall control of the inkjet recording apparatus 1. The RAM32 provides the CPU31 with a memory space for work, and stores temporary data. The storage unit 33 stores control programs, setting data, and the like executed by the CPU31, and temporarily stores image data to be formed. The storage unit 33 includes a volatile memory such as a DRAM, a Hard Disk Drive (HDD), or a nonvolatile storage medium such as a flash memory, and is used separately according to the application.

The bus 45 is a communication path for transmitting and receiving data by connecting these components.

Next, a method of driving the inkjet head 10 in the inkjet recording apparatus 1 according to the present embodiment and an ink ejection operation according to the driving method will be described.

In the inkjet recording apparatus 1, as shown in fig. 4A to 4C, after the pressure chamber 19 is expanded by the expansion pulse signal, the pressure chamber 19 is contracted by the contraction pulse signal, and the droplet of the ink is ejected from the nozzle 18. The signal composed of the expansion pulse signal and the contraction pulse signal constitutes a drive signal.

In the inkjet recording apparatus 1 of the present embodiment, a plurality of ink droplets are ejected from the nozzles 18 by a series of driving operations in accordance with the application of a series of 2 or more driving signals. Then, the plurality of ink droplets are landed in one pixel range on the recording medium M in unison to form one pixel. By changing the number of ink droplets to be combined, the density (gradation) of the pixel can be expressed. The plurality of ink droplets before being combined may be connected to each other by a columnar ink (ink liquid column) or may be separated from each other.

Hereinafter, for convenience, a series of drive signals for ejecting the merged ink droplets will be referred to as a composite drive signal.

Fig. 7 is a diagram showing an example of the composite drive signal.

The composite drive signal in fig. 7 is a series of drive signals applied to the electrodes 172 of the pressure chambers 19 (the pressure chambers 19B in the examples in fig. 4A to 4C) that eject ink. The composite driving signal includes 7 driving signals a (1 st driving signal), and one driving signal B (2 nd driving signal) applied last after the 7 driving signals a.

The drive signal a includes a pulse signal having a voltage (voltage amplitude) Va. In the drive signal a, a portion from the rise to the fall of the pulse signal corresponds to the expansion pulse signal, and a portion after the fall of the pulse signal corresponds to the contraction pulse signal. Hereinafter, the voltage application time of the expansion pulse signal (the time from the rise of the expansion pulse signal to the fall of the contraction pulse signal) will be referred to as the pulse width PWa of the expansion pulse signal. In addition, the period of the drive signal a is a pulse period SDPa. Therefore, 7 drive signals a are applied 7 times in succession per pulse period SDPa.

The drive signal B includes a pulse signal having a voltage Vb (> Va). In the drive signal B, a portion from the rise to the fall of the pulse signal corresponds to the expansion pulse signal, and a portion after the fall of the pulse signal corresponds to the contraction pulse signal. Hereinafter, the voltage application time of the expansion pulse signal (the time from the rise of the expansion pulse signal to the fall of the contraction pulse signal) will be referred to as the pulse width PWb of the expansion pulse signal. In addition, the period of the drive signal B is set to the pulse period SDPb.

Further, as shown in fig. 4A to 4C, since a voltage is applied to the electrodes 172 of the adjacent pressure chambers 19 in a period complementary to the composite drive signal of fig. 7, the amplitudes of the voltages applied to the partition walls 171 when switching from the expansion pulse to the contraction pulse are 2Va in the drive signal a and 2Vb in the drive signal B, but the following description focuses on the composite drive signal applied to the electrodes 172 of the pressure chambers 19 from which ink is ejected.

The pulse widths PWa and PWb may be set to a Length equal to 1/2 (hereinafter referred to as AL (Acoustic Length)) of the Acoustic (impact) resonance cycle of the pressure wave in the pressure chamber 19, for example. The pulse periods SDPa and SDPb may have a length equal to 2AL, for example. When the pulse widths PWa and PWb are AL and the pulse periods SDPa and SDPb are 2AL, pressure waves having the same phase can be generated in the pressure chamber 19, and therefore, resonance of the pressure waves is utilized to maximize efficiency, and thus, the ink droplets can be ejected at a high speed. However, by deviating the pulse width PWa from AL (ずらす), an effect of stabilizing the flight trajectory of the ink can be obtained. Which will be described later.

Further, the composite drive signal is applied after a wait time of 4AL or more has elapsed from the end of application of any other drive signal.

By applying such a composite drive signal to the pressure generating section 17, 7 ink droplets based on 7 drive signals a and one ink droplet based on one drive signal B are continuously ejected and landed on the recording medium M in combination.

Here, by making the voltage Vb of the last drive signal B of the composite drive signal larger than the voltage Va of the drive signal a, the speed of the ink ejected in response to the drive signal B (the last ink) can be made larger than the speed of the ink ejected in response to the drive signal a. This makes it easier for the last ink to catch up with the preceding ink, and enables the inks to be combined more appropriately.

In this way, in the driving method in which a plurality of droplets are combined, the ratio of the voltage Va and the voltage Vb and the characteristics of the ink tend to cause a problem that the last ink does not catch up with the preceding ink and the inks are not properly combined. Such a problem is likely to occur particularly when the specific gravity of the ink is large or when the viscosity is small. The reason for this will be described below with reference to fig. 8, 9A, and 9B.

Fig. 8 is a diagram showing the transition of the position of the meniscus according to the characteristics of the ink.

Fig. 8 shows the transition of the positional variation (oscillation) of the meniscus of the ink in the nozzle 18 immediately after one ink droplet is ejected. The position of the meniscus corresponds to the lower vertical direction downwards in the figure. Examples of the meniscus position fluctuation when the specific gravity of the ink is 1 or less and/or when the viscosity of the ink is 10cP or more are drawn by solid lines, and examples of the meniscus position fluctuation when the specific gravity of the ink is greater than 1 and/or when the viscosity of the ink is less than 10cP are drawn by broken lines.

As shown by the portion surrounded by the broken line in fig. 8, when the specific gravity of the ink is high and/or the viscosity is low, the position of the meniscus in the nozzle 18 relatively shifts downward with respect to the vertical direction, and the average position of the meniscus shifts downward. For example, at a predetermined timing after the ejection of the normal ink, when the meniscus is located on the same plane as the nozzle opening as shown in fig. 9A, the position of the meniscus at the same timing when the ink having a higher specific gravity or the ink having a lower viscosity is ejected under the same conditions is in a state of protruding from the nozzle opening (a state of protruding from the nozzle opening) as shown in fig. 9B. When the next ink is ejected in the state shown in fig. 9B, the amount of the ejected ink increases compared to the case where the ink is ejected in the state shown in fig. 9A, and the velocity of the ink decreases according to the law of conservation of energy. This makes it easy for the last ink to not catch up with the preceding ink, and thus a failure occurs in which proper alignment cannot be performed.

Therefore, in the composite drive signal of the present embodiment, the voltage Va of the drive signal a and the voltage Vb of the drive signal B are determined so that the inks are appropriately combined even when the specific gravity of the ink is large and/or the viscosity of the ink is small.

In the voltage adjustment according to the specific gravity of the ink, the voltage Va and the voltage Vb are determined so that the ratio Va/Vb becomes a value according to the specific gravity of the ink.

Specifically, the specific gravity of the ink is 1.0g/cm³Above, 1.9g/cm³In the following cases, voltage Va and voltage Vb are determined so as to satisfy 0.75 < Va/Vb < 0.86. Wherein the specific gravity of the ink is 1.2g/cm³Above, 1.4g/cm³In the following case, the condition is satisfied in a range of 0.76 <Va/Vb < 0.80, voltage Va and voltage Vb. Further, the voltage Va and the voltage Vb may be determined so that the ratio Va/Vb decreases as the specific gravity of the ink increases. Here, "the ratio Va/Vb is smaller as the specific gravity of the ink is larger" means that the ratio Va/Vb monotonically increases with an increase in the specific gravity of the ink, and the ratio Va/Vb may change in stages with a change in the specific gravity of the ink.

In addition, in the voltage adjustment according to the viscosity of the ink, the voltage Va and the voltage Vb are determined so that the ratio Va/Vb becomes a value according to the viscosity of the ink.

Specifically, when the viscosity of the ink is 8cP or more and 16cP or less, the voltages Va and Vb are determined so as to satisfy 0.60 < Va/Vb < 0.91. When the viscosity of the ink is 10cP or more and 14cP or less, the voltages Va and Vb are determined so as to satisfy 0.74 < Va/Vb < 0.84. Further, the voltage Va and the voltage Vb may be determined so that the ratio Va/Vb decreases as the viscosity of the ink decreases. Here, the phrase "the ratio Va/Vb is smaller as the viscosity of the ink is smaller" means that the ratio Va/Vb monotonically increases with a decrease in the viscosity of the ink, and the ratio Va/Vb may be changed in stages with a change in the viscosity of the ink.

Thus, the voltage Vb (and thus the voltage Va) is determined such that the ratio Va/Vb converges within a predetermined range according to the specific gravity or viscosity of the ink, and the speed of the ejected ink becomes constant regardless of the number of droplets ejected according to the composite drive signal (i.e., regardless of the gradation of the pixel to be recorded).

The values of the ratio Va/Vb, the voltage Va, and the voltage Vb may be set directly by the user's input operation to the operation display unit 42, or may be set automatically by the user's input of the type of ink to the operation display unit 42, based on data on the specific gravity or viscosity of the ink to be input.

Since the viscosity of the ink changes depending on the temperature, the set values of the voltage Va and the voltage Vb may be adjusted based on the detection result of the temperature detector 44 when the voltage adjustment is performed based on the viscosity. That is, the viscosity of the ink at each temperature may be obtained in advance, and the set values of the voltage Va and the voltage Vb may be adjusted so that the ratio Va/Vb falls within the above range, based on the viscosity of the ink at the detected temperature.

In the present embodiment, the voltage Va and the voltage Vb are determined so that the ratio Va/Vb falls within the above range, and the pulse width PWa of the expansion pulse signal in the drive signal a is adjusted, whereby the stability of the discharged ink can be improved and the ink can be further appropriately combined. Further, the effect of improving the stability of the ink can be also produced by adjusting the pulse period SDPa.

Fig. 10 is a diagram showing experimental conditions showing effects obtained by the adjustment of the pulse width PWa and the pulse period SDPa, and evaluation results.

In this experiment, ink was ejected by a composite drive signal including 2 or more drive signals a of a predetermined number and one drive signal B, and the flight trajectory of the ejected ink was photographed to evaluate the stability of ejection.

Examples 1 to 9 were prepared by changing the pulse width PWa or the pulse period SDPa at 9 levels, and the stability was evaluated for each example.

Specifically, examples in which the pulse width PWa was set to 0.8AL, 1.2AL, 1.4AL, and 1.5AL were set to examples 1 to 5, respectively. In examples 1 to 5, the pulse period SDPa is set to 2 AL.

Examples in which the pulse period SDPa was set to 1.8AL, 2AL, 2.2AL, and 2.4AL were set to examples 6 to 9, respectively. In examples 6 to 9, the pulse width PWa was AL.

In examples 1 to 9, the pulse width PWb of the expansion pulse signal in the drive signal B in any of the examples is AL, and the pulse period SDPb is 2 AL.

The voltage Vb in each example is adjusted so that the ink ejected by the composite drive signal is equal in speed to the ink ejected by the single drive signal B. In addition, the ratio Va/Vb in each example is fixed.

The stability was evaluated at 3 levels of "good", and "Δ".

Specifically, a case where blurring (ブレ) is observed in the ejected ink, that is, a case where a part of the flight trajectory of the ink deviates from the ideal trajectory is assumed to be "Δ".

In addition, although blurring is observed in the discharged ink, the "good quality" is rarely obtained.

In addition, the discharged ink was regarded as "excellent" when no blurring was observed.

In any of "excellent", "good", and "Δ", the ink is appropriately combined by adjusting the ratio Va/Vb.

The evaluation results of stability in each example are as follows.

First, in examples 1 to 5 in which the pulse width PWa was changed, the stability evaluation results became "excellent" in example 3 in which the pulse width PWa was set to 1.2AL and example 4 in which the pulse width PWa was set to 1.4 AL. In example 2 in which the pulse width PWa is set to AL, the evaluation result of stability is evaluated to "good". The evaluation results of stability were "Δ" in example 1 in which the pulse width PWa was 0.8AL and in example 5 in which the pulse width PWa was 1.5 AL. From these results, it was confirmed that blurring of ink can be effectively suppressed by adjusting the pulse width PWa to AL or more and 1.4AL or less. In particular, it was confirmed that the blurring of the ink can be further suppressed by adjusting the pulse width PWa to be 1.2AL or more and 1.4AL or less.

In examples 6 to 9 in which the pulse period SDPa was changed, the evaluation result of stability became "excellent" in example 8 in which the pulse period SDPa was set to 2.2 AL. In addition, the evaluation result of the stability in example 7 in which the pulse period SDPa was set to 2AL became "good". In example 6 in which the pulse period SDPa was set to 1.8AL and example 9 in which the pulse period SDPa was set to 2.4AL, the evaluation result of stability was Δ ". From these results, it was confirmed that blurring of ink can be effectively suppressed by adjusting the pulse period SDPa to 2AL or more and 2.2AL or less. In particular, it was confirmed that the blurring of the ink can be further suppressed by adjusting the pulse period SDPa to 2.2 AL.

In the right 2-end column of the table of fig. 10, the upper limit value of the velocity at which the ink stably flies (stable velocity upper limit) and the voltage at which the velocity of the ink becomes the stable velocity upper limit in each example are shown. The upper limit of the stable speed is a speed at which a large blur of the ink occurs in a case where the voltage Vb is increased to increase the speed of the ink. As a result, the higher the upper limit of the stable speed, the better the evaluation result of the stability of the ink.

The increasing width of the upper limit of the steady speed in the case where the pulse period SDPa is adjusted from 2AL (example 7) to 2.2AL (example 8) is kept at 0.2m/s, while the upper limit of the steady speed is increased by an amount of 1.1m/s by adjusting the pulse width PWa from AL (example 2) to 1.4AL (example 4). From this, it is found that the ink can be stabilized efficiently by adjusting the pulse width PWa, as compared with the case of adjusting the pulse period SDPa.

As is clear from examples 1 to 5 in fig. 10, it is not always preferable to set the pulse width PWa to AL from the viewpoint of blurring of ink, and the stability of ink can be improved by deviating (particularly increasing) the pulse width PWa from AL. This is considered to be because the phases of the pressure waves generated in the respective pressure chambers 19 can be shifted in the rise of the expansion pulse signal and the fall of the contraction pulse signal of the drive signal a. That is, it is considered that this is because, when the phases are aligned, the ink excessively overflows from the nozzle opening portion, and the flight trajectory of the ink tends to become unstable, and the occurrence of such a problem can be suppressed by shifting the phases.

Further, from examples 6 to 9, it is understood that the pulse period SDPa of the drive signal a is not necessarily optimal to be 2AL from the viewpoint of blurring of ink, and the stability of ink can be improved by deviating (particularly increasing) the pulse period SDPa from AL. The reason is considered to be that the phase of the pressure wave generated in the pressure chamber 19 in the rise of the expansion pulse signal of the drive signal a can be shifted from the phase of the pressure wave generated when the ink is ejected most recently, and as a result, the excessive ink overflow can be suppressed.

As described above, in the method of driving the inkjet head 10 according to the present embodiment, the inkjet head 10 includes: a nozzle 18 for ejecting ink; and a pressure generating unit 17 for applying a pressure change to the ink in a pressure chamber 19 communicating with the nozzle 18 in accordance with the application of a drive signal to discharge the ink from the nozzle 18, wherein the inkjet head 10 forms one pixel by landing a plurality of ink droplets discharged from the nozzle 18 on the recording medium M in accordance with the application of a series of drive signals, wherein a composite drive signal, which is a series of drive signals including a drive signal a having a 1 st voltage amplitude of a voltage Va and a drive signal B having a 2 nd voltage amplitude of a voltage Vb larger than the voltage Va, is applied to the pressure generating unit 17, and the last drive signal in the composite drive signal is the drive signal B, and the voltage Va and the voltage Vb are determined so that a ratio Va/Vb becomes a value corresponding to a specific gravity of the ink discharged from the nozzle 18.

According to such a driving method, even when ink having a high specific gravity, which easily overflows from the opening of the nozzle 18 and whose ejection speed is easily slow, is ejected, the speed of the ink droplet to be ejected last in response to the driving signal B can be made to fly at a desired value. This makes it possible to appropriately integrate the last ink with the preceding ink. This effectively suppresses the degradation of image quality caused by the improper integration of the inks.

Further, by determining the voltage Va and the voltage Vb such that the ratio Va/Vb decreases as the specific gravity of the ink ejected from the nozzle 18 increases, the last ink is ejected with a larger energy as the specific gravity increases, and therefore, the occurrence of a problem that the speed of the last ink is insufficient can be effectively suppressed.

In the driving method of the present embodiment, the specific gravity of the ink discharged from the nozzle 18 is 1.0g/cm³Above, 1.9g/cm³In the following cases, voltage Va and voltage Vb are determined so as to satisfy 0.75 < Va/Vb < 0.86.

In addition, the liquid is discharged from the nozzle 18The specific gravity of the ink (2) is 1.2g/cm³Above, 1.4g/cm³In the following cases, voltage Va and voltage Vb are determined so as to satisfy 0.76 < Va/Vb < 0.80.

When the ratio Va/Vb is larger than the lower limit of each of the above ranges, it is possible to suppress a decrease in uniformity due to a low speed of ink discharged in accordance with the drive signal a while suppressing an excessive increase in the voltage Vb. That is, when the voltage Va is increased while maintaining the ratio Va/Vb in order to eject the ink ejected in accordance with the drive signal a at a predetermined speed, it is possible to suppress the occurrence of a problem that the pressure generating unit 17 is deteriorated due to the voltage Vb becoming excessively high.

Further, by setting the ratio Va/Vb to be lower than the upper limit of each of the above ranges, it is possible to suppress the occurrence of a problem that the last ink does not catch up with the preceding ink and does not merge. Thus, by determining the voltage Va and the voltage Vb so that the ratio Va/Vb falls within the above range, the ink can be integrated more reliably.

In addition, the method of driving the inkjet head 10 of the present embodiment, wherein the inkjet head 10 includes: a nozzle 18 for ejecting ink; and a pressure generating section 17 for applying a pressure change to the ink in a pressure chamber 19 communicating with the nozzle 18 in accordance with application of a drive signal to discharge the ink from the nozzle 18, wherein the inkjet head 10 forms one pixel by landing a plurality of ink droplets discharged from the nozzle 18 on the recording medium M in accordance with application of a series of drive signals, and in a driving method of the inkjet head 10, a composite drive signal is applied to the pressure generating section 17 as a series of drive signals including a drive signal a having a 1 st voltage amplitude of a voltage Va and a drive signal B having a 2 nd voltage amplitude of a voltage Vb larger than the voltage Va, a last drive signal in the composite drive signal is the drive signal B, and the voltage Va and the voltage Vb are determined so that a ratio Va/Vb becomes a value according to a viscosity of the ink discharged from the nozzle 18.

According to such a driving method, even when ink having a small viscosity, which easily overflows from the opening of the nozzle 18 and whose ejection speed is easily slow, is ejected, the speed of the ink droplet to be ejected last in response to the driving signal B can be made to fly at a desired value. This makes it possible to appropriately integrate the last ink with the preceding ink. This effectively suppresses the degradation of image quality caused by the failure to properly integrate the inks.

Further, by determining the voltage Va and the voltage Vb such that the ratio Va/Vb decreases as the viscosity of the ink discharged from the nozzle 18 decreases, the ink having a smaller viscosity discharges the last ink with a larger energy, and therefore, the occurrence of a problem in which the speed of the last ink is insufficient can be effectively suppressed.

In the driving method of the present embodiment, when the viscosity of the ink ejected from the nozzle 18 is 8cP or more and 16cP or less, the voltage Va and the voltage Vb are determined so as to satisfy 0.60 < Va/Vb < 0.91.

When the viscosity of the ink discharged from the nozzle 18 is 10cP or more and 14cP or less, the voltage Va and the voltage Vb are determined so as to satisfy 0.74 < Va/Vb < 0.84.

When the ratio Va/Vb is larger than the lower limit of each of the above ranges, it is possible to suppress a decrease in uniformity due to a low speed of ink discharged in accordance with the drive signal a while suppressing an excessive increase in the voltage Vb. That is, in order to eject the ink ejected in accordance with the drive signal a at a predetermined speed, when the voltage Va is increased while maintaining the ratio Va/Vb, it is possible to suppress the occurrence of a problem that the voltage Vb becomes excessively high and the pressure generating unit 17 is deteriorated.

Further, by setting the ratio Va/Vb lower than the upper limit of each of the above ranges, it is possible to suppress the occurrence of a problem that the last ink does not catch up with the preceding ink and does not merge together. Thus, by determining the voltage Va and the voltage Vb so that the ratio Va/Vb falls within the above range, the ground ink can be integrated more reliably.

The drive signal a and the drive signal B include an expansion pulse signal for expanding the pressure chamber 19 and a contraction pulse signal for contracting the pressure chamber 19 to be applied next to the expansion pulse signal, and the pulse width PWa of the expansion pulse signal in the drive signal a is set to AL or more and 1.4AL or less, whereby the phase difference of the pressure wave generated in each pressure chamber 19 is adjusted in the rise of the expansion pulse signal and the fall of the contraction pulse signal in the drive signal a, whereby a problem that ink excessively overflows from the nozzle opening portion can be suppressed, and blurring of ink can be effectively suppressed.

Further, by setting the pulse width PWa of the expansion pulse signal in the drive signal a to 1.2AL or more and 1.4AL or less, the blurring of ink can be further suppressed to be low.

The drive signal a and the drive signal B include an expansion pulse signal for expanding the pressure chamber 19 and a contraction pulse signal for contracting the pressure chamber 19 to which the expansion pulse signal is applied next, and the pulse widths PWa and AL of the expansion pulse signal in the drive signal a are made different from each other, whereby the phase difference of pressure waves generated in the pressure chamber 19 during the rise of the expansion pulse signal and the fall of the contraction pulse signal in the drive signal a is adjusted, whereby the problem that ink excessively overflows from the nozzle opening is suppressed, and the blurring of ink can be effectively suppressed.

When 1/2 indicating the acoustic resonance period of the pressure wave in the pressure chamber 19 is set to AL, the composite drive signal is applied after a waiting time of 4AL or more has elapsed since the end of the application of any other drive signal. This makes it possible to start ink ejection in a state where the pressure wave in the pressure chamber 19 is attenuated and the meniscus is stabilized, and therefore, it is possible to suppress degradation in uniformity due to ink blurring or ink velocity fluctuation.

The inkjet recording apparatus 1 of the present embodiment includes an inkjet head 10, and the inkjet head 10 includes: a nozzle 18 for ejecting ink; and a pressure generating section 17 for applying a pressure change to the ink in a pressure chamber 19 communicating with the nozzles 18 in accordance with the application of a drive signal to discharge the ink from the nozzles 18, wherein the ink jet recording apparatus 1 forms one pixel by landing droplets of a plurality of inks discharged from the nozzles 18 on a recording medium M in accordance with the application of a series of drive signals, and the ink jet recording apparatus 1 includes a head drive section 20 for applying a composite drive signal to the pressure generating section 17, the composite drive signal being a series of drive signals including a drive signal a having a 1 st voltage amplitude of a voltage Va and a drive signal B having a 2 nd voltage amplitude of a voltage Vb larger than the voltage Va, the last drive signal in the composite drive signal being the drive signal B, and the voltage Va and the voltage Vb being determined so that a ratio Va/Vb becomes a value in accordance with the specific gravity of the ink discharged from the nozzles 18.

According to this configuration, even when ink having a high specific gravity, which easily overflows through the opening of the nozzle 18 and whose ejection speed is easily slow, is ejected, the speed of the ink droplet ejected last in response to the drive signal B can be made to fly at a desired value. This makes it possible to appropriately integrate the last ink with the preceding ink. This effectively suppresses the deterioration of the image quality caused by the improper combination of the inks.

The inkjet recording apparatus 1 of the present embodiment includes an inkjet head 10, and the inkjet head 10 includes: a nozzle 18 for ejecting ink; and a pressure generating section 17 for applying a pressure change to the ink in a pressure chamber 19 communicating with the nozzle 18 in response to the application of a drive signal to discharge the ink from the nozzle 18, wherein the inkjet recording apparatus 1 forms one pixel by landing a plurality of droplets of the ink discharged from the nozzle 18 on the recording medium M in response to the application of a series of drive signals. The inkjet recording apparatus 1 includes a head driving unit 20, and applies a composite driving signal to a pressure generating unit 17, the composite driving signal being a series of driving signals including a 1 st driving signal a having a voltage amplitude Va and a 2 nd driving signal B having a voltage amplitude Vb larger than the voltage Va, the last driving signal in the composite driving signal being the driving signal B, and the voltage Va and the voltage Vb being determined such that a ratio Va/Vb is a value according to the viscosity of ink ejected from a nozzle 18.

With this configuration, even when ink having a low viscosity, which easily overflows the opening of the nozzle 18 and whose ejection speed is easily slow, is ejected, the speed of the ink droplet ejected last in response to the drive signal B can be made to fly at a desired value. This makes it possible to appropriately integrate the last ink with the preceding ink. This effectively suppresses the deterioration of the image quality caused by the improper combination of the inks.

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

For example, the composite drive signal may include 3 or more types of drive signals having different voltage amplitudes and a voltage amplitude equal to or smaller than the voltage amplitude of the drive signal B. In this case as well, the last drive signal in the composite drive signal is set as the drive signal B, and the speed of each ink is adjusted so that the last ink catches up with the preceding ink, whereby the inks can be appropriately combined.

The waveforms of the drive signal a and the drive signal B are not limited to those shown in fig. 7, and may be other drive waveforms capable of ejecting ink by providing appropriate pressure changes to the ink in the pressure chamber 19. For example, the rise and fall of the voltage may be asymmetric waveforms, and the voltage change may not be a linear change. Further, the pulse signal may be a signal obtained by combining a positive voltage pulse signal and a negative voltage pulse signal. In this case, the voltage amplitudes of the positive voltage pulse signal and the negative voltage pulse signal may be set to voltage Va and voltage Vb.

In addition, a part of the plurality of driving signals a included in the composite driving signal may be replaced with the driving signal B.

In the above-described embodiment, the example of the inkjet head 10 in the shear mode is described, but the present invention is not limited thereto. For example, the present invention may be applied to a vent (vent) type ink jet head that discharges ink by changing the pressure of ink in a pressure chamber by deforming a piezoelectric element (pressure generating portion) fixed to a wall surface of the pressure chamber.

Alternatively, another pressure generating portion that converts heat, electromagnetism, or the like into spatial deformation and can change the pressure of the ink in the pressure chamber may be used.

In the above-described embodiment, the recording medium M is conveyed by the conveying unit 2 having the conveying belt 2c, but the present invention is not limited to this, and the conveying unit 2 may be a member that conveys the recording medium M while holding it on the outer peripheral surface of a rotating conveying drum, for example.

In the above-described embodiment, the description has been given by taking the example of the inkjet recording apparatus 1 of the one-pass type, but the present invention may be applied to an inkjet recording apparatus that records an image while scanning the inkjet head 10.

Although the embodiments of the present invention have been described, the scope of the present invention is not limited to the above-described embodiments, and includes the scope of the invention described in the claims and the equivalent scope thereof.

Industrial applicability

The present invention can be used for a method of driving an inkjet head and an inkjet recording apparatus.

Description of the reference symbols

1 ink jet recording apparatus

2 conveying part

2a, 2b transport roller

2c conveyor belt

3 head unit

10 ink jet head

11-channel substrate

12 cover plate

13 nozzle plate

14. 15 base plate

16 adhesive part

17 pressure generating part

18 spray nozzle

19 pressure chamber

20 head driving part (driving part)

21 drive waveform signal output section

22D/A converter

23 drive circuit

24 output selection part

30 control part

31 CPU

32 RAM

33 storage unit

41 communication unit

42 operation display part

43 conveyance drive unit

44 temperature detecting part

45 bus

171 partition wall

172 electrode

A drive signal (1 st drive signal)

B drive signal (2 nd drive signal)

M recording medium