EP2905138B1

EP2905138B1 - Driving method of inkjet head, driving apparatus of inkjet head, and inkjet recording apparatus

Info

Publication number: EP2905138B1
Application number: EP13843417.0A
Authority: EP
Inventors: Ryohei Kobayashi
Original assignee: Konica Minolta Inc
Current assignee: Konica Minolta Inc
Priority date: 2012-10-02
Filing date: 2013-10-01
Publication date: 2019-06-05
Anticipated expiration: 2033-10-01
Also published as: JP6202002B2; CN104703801B; WO2014054655A1; EP2905138A4; EP2905138A1; CN104703801A; JPWO2014054655A1

Description

TECHNICAL FIELD

The present invention relates to a driving method of an inkjet head, a driving apparatus of an inkjet head, and an inkjet recording apparatus and, more particularly, to a driving method of an inkjet head, a driving apparatus of an inkjet head, and an inkjet recording apparatus capable of reducing an effect of crosstalk on an inkjet head having two or more rows of pressure chambers communicating with each other through a common ink chamber.

BACKGROUND ART

Higher speed and higher definition recording are demanded for an inkjet head having a pressure in a pressure chamber generated by an operation of a pressure applier to eject ink in the pressure chamber, and the number of nozzles and the number of rows of nozzles tend to further increase. This leads to problems of an increase in crosstalk caused by a pressure wave generated in a pressure chamber at the time of ejection and propagating to another pressure chamber to destabilize a droplet speed (droplet amount) and an increase in driving load due to driving of a large number of pressure chambers at the same time.
The destabilization of the droplet speed due to the crosstalk occurs because a pressure wave generated in a pressure chamber at the time of ejection propagates from an inlet of the pressure chamber to a common ink chamber and affects another pressure chamber through the common ink chamber. Particularly, in the case of an inkjet head having two or more rows of pressure chambers such that the pressure chambers are in communication with each other through a common ink chamber between the rows of the pressure chambers, the pressure wave propagates through the common ink chamber to the pressure chambers in another row of pressure chambers and, therefore, it is important to suppress crosstalk between the rows of the pressure chambers.
With regard to this crosstalk problem, it is described in Patent Document 1 that the common ink chamber is divided into two parts by a division wall along the rows of pressure chambers so as to prevent the pressure wave from propagating from one row of pressure chambers to the other row of pressure chambers.
It is described in Patent Document 2 that a wall surface of the common ink chamber facing the inlets of the pressure chambers is regulated to a predetermined bulk modulus or less to attenuate the pressure wave propagating into the common ink chamber, thereby reducing the crosstalk.
On the other hand, with regard to the driving load problem, it is described in Patent Document 3 that a load of a driving circuit is suppressed by arranging nozzle rows with a nozzle pitch therebetween shifted from integer multiples of the minimum pixel pitch and by shifting a phase of a driving signal in accordance with the shift amount to make an adjustment such that a droplet from each nozzle row impacts at a lattice point of an image.
It is described in Patent Document 4 that electric power saving is achieved by shifting a phase of a driving signal by a predetermined delay time to prevent peak values of a charging current charging, or a discharging current discharging, a piezoelectric element, from overlapping between channels. This delay time is described as being equal to a rising time determined from an electrostatic capacity and a charging resistance of the piezoelectric element.

PRIOR ART DOCUMENT

Patent Documents

Patent Document 1: JP-A-2003-11368
Patent Document 2: JP-A-2007-168185
Patent Document 3: JP-A-2002-137388
Patent Document 4: JP-A-7-125195

EP 163470581 discloses a line type inkjet recording apparatus wherein the actuators of ink chambers are activated at different timings within one printing cycle and the ink is ejected from each of the ink chambers The timings are determined based on the positional relationship between the ink chambers and based on the rigidity of the surroundings.

SUMMARY OF THE INVENTION

Problem to Be Solved by the Invention

The techniques of Patent Documents 1 and 2 have a problem that an inkjet head structure itself must be changed for reducing the crosstalk between rows of pressure chambers. Particularly, if the division wall is disposed in the common ink chamber as in Patent Document 1, the head structure becomes complex, resulting in a problem of cost increase. Moreover, since a division wall must additionally be disposed as the number of rows of pressure chambers increases, an ink supply system becomes complex, resulting in a problem of difficulty of handling of the head.
On the other hand, focusing only on the viewpoint of suppressing the driving load, it is considered effective to shift the phase of the driving signal as described in Patent Documents 3 and 4. However, even when the phase of the driving signal is simply shifted, the pressure wave generated at the time of ejection still affects another pressure chamber through the common ink chamber. A reduction in crosstalk generated between rows of pressure chambers is not mentioned in Patent Documents 3 and 4.
Therefore, as a result of intensive studies about the problems of the crosstalk between rows of pressure chambers communicating through the common ink chamber and the driving load, the present inventor found that the problems of the crosstalk and the driving load can solved at the same time by not only simply shifting the phase of the driving signal between the rows of pressure chambers for reducing the driving load but also applying a predetermined phase difference.
Therefore, the purpose of the present invention is to provide a driving method of an inkjet head, a driving apparatus of an inkjet head, and an inkjet recording apparatus capable of reducing a crosstalk generated between multiple rows of pressure chambers communicating through a common ink chamber and a driving load at the same time without making any changes to head structure.
Other purposes of the present invention will become apparent from the following description.

MEANS FOR SOLVING PROBLEM

1. A driving method of an inkjet head having two or more rows of pressure chambers having a pressure for ejecting internal ink from a nozzle generated by a pressure applier operated by application of a driving signal, the pressure chambers in the rows of the pressure chambers being in communication with each other through a common ink chamber, wherein
the rows of the pressure chambers are divided into N driving groups, wherein N is an integer of two or more, wherein a phase difference of nAL+t between the different driving groups is applied to the driving signals applied to the pressure appliers of the pressure chambers, wherein n is an integer of one or more, wherein AL is 1/2 of an acoustic resonance period of a pressure wave in the pressure chambers, and wherein t is a pressure wave transmission time obtained from an inter-nozzle distance between the driving groups / a speed of sound traveling through the ink.
2. The driving method of an inkjet head of 1, wherein the rows of pressure chambers are adjacent to each other and the rows are divided into different driving groups.
3. The driving method of an inkjet head of 1, wherein
the rows of the pressure chambers are driven by two or more driving circuits, and wherein
the rows of the pressure chambers of the same driving circuit are divided into different driving groups.
4. The driving method of an inkjet head of 1, 2, or 3, wherein the inkjet head is an inkjet head having a head chip with inlets and outlets of the pressure chambers opened in opposite end surfaces such that the pressure chambers are formed straight from the inlets to the outlets, and wherein the common ink chamber is disposed on the inlet side of the pressure chambers in the head chip.
5. A driving apparatus of an inkjet head having two or more rows of pressure chambers having a pressure for ejecting internal ink from a nozzle generated by a pressure applier operated by application of a driving signal, the pressure chambers in the rows of the pressure chambers being in communication with each other through a common ink chamber, wherein
the rows of the pressure chambers of the inkjet head are divided into N driving groups, wherein N is an integer of two or more, wherein a phase difference of nAL+t between the different driving groups is applied to the driving signals applied to the pressure applier of the pressure chambers, wherein n is an integer of one or more, wherein AL is 1/2 of an acoustic resonance period of a pressure wave in the pressure chambers, and wherein t is a pressure wave transmission time obtained from an inter-nozzle distance between the driving groups / a speed of sound traveling through the ink.
6. The driving apparatus of an inkjet head of claim 5, wherein the rows of pressure chambers are adjacent to each other and the rows are divided into different driving groups.
7. The driving apparatus of an inkjet head of 5, wherein
the driving apparatus of an inkjet head has two or more driving circuits driving the rows of the pressure chambers, and wherein
the rows of the pressure chambers of the same driving circuit are divided into different driving groups.
8. The driving apparatus of an inkjet head of 5, 6, or 7, wherein the inkjet head is an inkjet head having a head chip with inlets and outlets of the pressure chambers opened in opposite end surfaces with the pressure chambers formed straight from the inlets to the outlets, and wherein the common ink chamber is disposed on the inlet side of the pressure chambers of the head chip.
9. An inkjet recording apparatus comprising:
- an inkjet head having two or more rows of pressure chambers having a pressure for ejecting internal ink from a nozzle generated by a pressure applier operated by application of a driving signal, the pressure chambers in the rows of the pressure chambers being in communication with each other through a common ink chamber; and
- the driving apparatus of an inkjet head of any one of 5 to 8.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a diagram of a general configuration of an inkjet recording apparatus.
Fig. 2 is an exploded perspective view of a general configuration of an inkjet head.
Fig. 3 is a partial back view of a head chip depicted in Fig. 2.
Fig. 4 is a partial cross-sectional view of the head chip.
Fig. 5 is a diagram of an example of a driving signal used in the present invention.
Fig. 6 is a diagram for explaining a deforming motion of a partition wall due to the driving signal depicted in Fig. 5.
Fig. 7 is a front view of the head chip depicting a divided form of driving groups of two channel rows.
Fig. 8 is a timing chart of the driving signals applied to the driving groups depicted in Fig. 7.
Fig. 9 is a graph for explaining a relationship between variation in droplet speed and Delay [AL].
Fig. 10 is a diagram for explaining driving timing between multiple channel rows.
Fig. 11 is a front view of the head chip depicting a divided form of driving groups of four channel rows.
Fig. 12 is a timing chart of the driving signals applied to the driving groups depicted in Fig. 11.
Fig. 13 is a front view of the head chip depicting a divided form of driving groups of six channel rows.
Fig. 14 is a timing chart of the driving signals applied to the driving groups depicted in Fig. 13.
Fig. 15 is a diagram for explaining a form of a plurality of channel rows driven by a plurality of driving circuits.

MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will now be described with reference to the drawings.
Fig. 1 is a general configuration diagram of an example of an inkjet recording apparatus according to the present invention.
In an inkjet recording apparatus 100, a recording medium P is clamped by a feeding roller pair 201 of a feeding mechanism 200 and is fed in a Y-direction (vertical scanning diction) depicted in Fig. 1 by a feeding roller 203 rotationally driven by a feeding motor 202.
An inkjet head H is disposed between the feeding roller 203 and the feeding roller pair 201 to face a recording surface PS of the recording medium P. The inkjet head H is mounted on a carriage 400 that is disposed along guide rails 300 placed over the recording medium P in the width direction thereof and that can be reciprocated by a driving means not depicted along a X-X' direction (horizontal scanning direction) depicted in Fig. 1 substantially orthogonal to the feeding direction (vertical scanning direction) of the recording medium P, such that a nozzle surface side is located facing the recording surface PS of the recording medium P, and as described in detail later, the inkjet head H is electrically connected via an FPC 4 to a driving apparatus 500.
As the carriage 400 moves in the horizontal scanning direction, the inkjet head H performs scanning movement for the recording surface PS of the recording medium P in the X-X' direction depicted in Fig. 1 and ejects droplets from nozzles in the course of this scanning movement to record a desired inkjet image.
An example of the inkjet head H preferably used in the present invention is depicted in Figs. 2 to 4. Fig. 2 is an exploded perspective view of an inkjet head; Fig. 3 is a partial back view of a head chip thereof; and Fig. 4 is a partial cross-sectional view of the head chip.
In Figs. 2 to 4, reference numerals 1, 2, 3, 4, and 5 denote a so-called harmonica type head chip, a nozzle plate, a wiring substrate, the FPC, and an ink manifold, respectively.
The head chip 1 forms a hexahedron shape and has two channel rows with pluralities of channels arranged. A channel row on the lower side of Fig. 3 is referred to as a row A, and a channel row on the upper side is referred to as a row B. The head chip 1 has driving channels 11 acting as pressure chambers to eject ink and dummy channels 12 without ejection of ink as channels and is a head chip of an independent driving type recording an image with ink ejected only from the driving channels 11.
A driving channel of the row A, a driving channel of the row B, a dummy channel of the row A, and a dummy channel of the row B may hereinafter be denoted by 11A, 11B, 12A, and 12B, respectively.
The respective channel rows have the driving channels 11A, 11B and the dummy channels 12A, 12B arranged alternately. Between the driving channel 11A or 11B and the dummy channel 12A or 12B, a partition wall 13 is formed that is a pressure applier made up of a piezoelectric element such a PZT.
A partition wall of the row A and a partition wall of the row B may hereinafter be denoted by 13A and 13B, respectively.
The driving channels 11A, 11B and the dummy channels 12A, 12B are opened in both a front end surface 1a and a back end surface 1b of the head chip 1 and respective driving electrodes 14 formed in close contact with the inner surfaces thereof. The driving channels 11A, 11B and the dummy channels 12A, 12B have outlets and inlets opened in the front end surface 1a and the back end surface 1b, respectively, which are opposite end surfaces of the head chip 1, and are formed straight from the inlets to the outlets.
In the inkjet head H depicted in Figs. 2 to 4, the "front surface" is the end surface on the side from which ink is ejected from the head chip 1 and the "back end surface" is the end surface on the opposite side.
The back end surface 1b is provided with respective connection electrodes 15A, 15B conducting to the driving electrodes 14 of the driving channels 11A, 11B and the dummy channels 12A, 12B. One end of each of the connection electrodes 15A, 15B conducts to one of the driving electrodes 14 in the corresponding driving channels 11A, 11B or dummy channels 12A, 12B. The other end of each of the connection electrodes 15A corresponding to the driving channels 11A and the dummy channels 12A of the row A is formed from the inside of each of the channels 11A, 12A to one end edge 1c of the head chip 1, while the other end of each of the connection electrodes 15B corresponding to the driving channels 11B and the dummy channels 12B of the row B is formed to extend from the inside of each of the channels 11B, 12B toward the row A to a position before the channel row of the row A. Therefore, all the connection electrodes 15A, 15B extend from the channels 11A, 11B, 12A, and 12B in the same direction.
The nozzle plate 2 is bonded to the front end surface 1a of the head chip 1 by an adhesive. The nozzle plate 2 has nozzles 21 opened only at positions corresponding to the driving channels 11A, 11B.
A wiring substrate 3 is a flat plate-shaped substrate larger than the back end surface 1b of the head chip 1 and, in a bonding region (region indicated by a dashed-dotted line in Fig. 2) 31 bonded to the back end surface 1b of the head chip 1, the wiring substrate 3 has through- holes 32A, 32B individually opened for supplying ink from a common ink chamber 51 in the ink manifold 5 into the driving channels 11A, 11B, only at the positions corresponding to the driving channels 11A, 11B opened in the back end surface 1b of the head chip 1.
The common ink chamber 51 is made up of an internal space of the box-shaped ink manifold 5 bonded on the back surface of the wiring substrate 3 (on the surface opposite to the head chip 1) so as to supply the ink through the through- holes 32A, 32B commonly to the driving channels 11A, 11B. Therefore, the driving channels 11A, 11B are in communication with each other through this common ink chamber 51. In contrast, the dummy channels 12A, 12B are blocked by the wiring substrate 3.
On the surface of the wiring substrate 3, wiring electrodes 33A, 33B are formed by vapor deposition or sputtering and are electrically connected to the connection electrodes 15A, 15B arranged on the back end surface 1b of the head chip 1. The wiring electrodes 33A, 33B extend on the surface of the wiring substrate 3 in the direction orthogonal to the channel rows of the head chip 1.
Each of the wiring electrodes 33A corresponding to the connection electrodes 15A led out from the channels 11A, 12A of the row A has one end located in the vicinity of each of the channels 11A, 12A of the row A in the bonding region 31 and the other end extending from the bonding region 31 in the direction orthogonal to the channel rows of the head chip 1 and reaching an end portion 3a of the wiring substrate 3.
On the other hand, each of the wiring electrodes 33B corresponding to the connection electrodes 15B led out from the channels 11B, 12B of the row B has one end located in the vicinity of each of the channels 11B, 12B of the row B in the bonding region 31 and the other end extending in the same direction as the wiring electrodes 33A and going between the adjacent through-holes 32A of the row A to the end portion 3a of the wiring substrate 3. The wiring electrodes 33A and 33B are alternately arranged at the end portion 3a of the wiring substrate 3.
The wiring substrate 3 is attached to the back end surface 1b of the head chip 1 such that the connection electrodes 15A, 15B of the head chip 1 and the wiring electrodes 33A, 33B of the wiring substrate 3 are electrically connected in a corresponding manner, and is bonded by an adhesive with a predetermined pressing force (e.g., 1 MPa or more). For the adhesive, an adhesive without conducting particles is preferably used partially because of certainly preventing a short circuit although an anisotropic conductive adhesive containing conducting particles can be used.
The inkjet head H is mounted on the carriage 400 of the inkjet recording apparatus 100 such that the row direction of the channel rows extends along the Y-direction of Fig. 1, and is electrically connected via the FPC 4 to the driving apparatus 500 as depicted in Fig. 1. When a driving signal corresponding to image data transmitted from a driving circuit in the driving apparatus 500 is applied via the FPC 4 to the driving electrode 14 of each of the driving channels 11, the shear deformation of the partition wall 13 occurs and changes the capacity of the driving channel 11, applying a pressure for ejection of the ink in the driving channel 11.
Fig. 5 depicts an example of the driving signal applied to the inkjet head H for ejecting the ink. This driving signal is a rectangular wave formed by a positive voltage (+V) of a pulse width PW generating a negative pressure in a channel.
An ink ejection operation corresponding to this driving signal will be described with reference to Fig. 6. Fig. 6 depicts only one of the driving channels 11 and the two dummy channels 12 on both sides thereof in one of the channel rows of the inkjet head H and the two partition walls 13 therebetween.
If the partition walls 13 between the channels 11, 12 are both in a neutral state as depicted in Fig. 6(a) and the driving signal depicted in Fig. 5 is applied to the driving electrode 14 of the driving channel 11 at the center, an electric field is generated in a direction orthogonal to a polarization direction (indicated by arrows in Fig. 6) of the piezoelectric elements making up the partition walls 13, and the shear deformation of the both partition walls 13 occurs toward the outside and expands the capacity of the driving channel 11 as depicted in Fig. 6(b). This deformation of the partition walls 13 causes the ink to flow into the driving channel 11. After this state is maintained for the predetermined pulse width PW, when the driving signal is returned to the zero potential, a pressure is applied to the ink in the driving channel 11 and a droplet is ejected from the nozzle 21.
The pressure in the driving channel 11 generated by the deformation of the partition walls 13 toward the outside is repeatedly inverted from negative to positive and positive to negative for each AL. Therefore, to efficiently eject the droplets, the pulse width PW, i.e., a duration time of the positive voltage of the driving signal, is preferably set to near 1 AL at which the pressure in the driving channel 11 is inverted from negative to positive, or specifically, preferably set within a range from 0.8 AL or more to 1.2 Al or less.
AL (acoustic length) indicative of the duration time of the driving signal means 1/2 of an acoustic resonance period of a pressure wave in the channel 12. AL is obtained by measuring a speed of a droplet ejected when the rectangular-wave driving single is applied to the driving electrode 14, as a pulse width at which a droplet flying speed is maximized when the pulse width PW of the rectangular wave is changed while a voltage value of the rectangular wave is kept constant.
A pulse is a rectangular wave at a constant voltage peak value and, when 0 V is 0 % and a peak-value voltage is 100 %, the pulse width PW is defined as a time between a 10 % rise from the voltage of 0 V and a 10 % fall from the peak-value voltage.
The rectangular wave refers to a waveform having both a rising time and a falling time between 10 % and 90 % of the voltage set within 1/2, preferably 1/4, of AL.
A method of applying the driving signal from the driving apparatus 500 to the inkjet head H in the present invention will be described.
In the present invention, the driving apparatus 500 divides all the channel rows included in the inkjet head H into N driving groups. N is an integer of two or more.
A driving group is a group on the basis of a channel row having the driving channels 11 to which the driving signal is applied at the same timing from the driving apparatus 500 within a driving period T of the inkjet head H. Therefore, the driving channels 11 and the dummy channels 12 in the same channel are included in the same driving group. A single driving group may be made up of a plurality of channel rows. Since the inkjet head H depicted in Figs. 1 and 2 has two channel rows, N=2 is set in this case, and the channel rows are divided into two different driving groups such that the channel row of the row A is defined as a driving group A while the channel row of the row B is defined as a driving group B as depicted in Fig. 7.
The driving channels 11 of the same driving group in the inkjet head H are all driven at the same time.
As depicted in a timing chart of Fig. 8, a phase difference of nAL+t is applied between a driving signal applied to the driving electrodes 14 of the driving channels 11 making up the driving group A and a driving signal applied to the driving electrodes 14 of the driving channels 11 making up the driving group B from the driving apparatus 500 to the inkjet head H. This timing chart represents the case of applying the driving signal for each driving period T of the driving group A such that the driving group A is driven earlier than the driving group B.
It is noted that n is an integer of one or more and that AL is 1/2 of the acoustic resonance period of the pressure wave in the driven channels 11 as described above. Fig. 8 exemplarily illustrates the case of n=1.
It is also noted that t is a pressure wave transmission time obtained from "inter-nozzle distance between driving groups"/"speed of sound traveling through the ink".
The driving groups of the "inter-nozzle distance between driving groups" are two different driving groups to be driven with a phase difference in the present invention. If two channel rows are disposed as depicted in Fig. 7, the inter-nozzle distance between the driving groups is a distance indicated by D in Fig. 7.
The speed C of sound traveling through the ink can be calculated from the following equation. $C = \sqrt{K / ρ}$
K is the bulk modulus of the ink and ρ is the density of the ink. The speed C is a characteristic value of the ink.
When the driving signals are applied to the driving electrodes 14 of the respective driving channels 11A, 11B of the driving groups A, B to cause the ejection of droplets, the droplet speed may significantly fluctuate due to the effect of crosstalk since the driving channels 11A, 11B of the driving groups A, B are in communication with each other through the common ink chamber 51. However, according to experiments of the inventor, it was found that, if the driving groups are driven to eject droplets first from, for example, the driving channels 11A of the driving group A and then eject droplets from the driving channels 11B of the driving group B after the elapse of a predetermined Delay time, the droplet speed of ejection from the driving channels 11B of the driving group B periodically varies to the positive and negative sides relative to the droplet speed of ejection from the driving channels 11A of the driving group A in accordance with the Delay time.
With regard to this period, after a certain time lag from the time of droplet ejection of the driving group A as depicted in Fig. 9, the droplet speed is repeatedly inverted to positive and negative for each AL, and the droplet speed from the driving channels 11B of the driving group B becomes substantially the same as the droplet speed from the driving channels 11A of the driving group A at the time of the inversion. As a result of studies about the time lag of the period, the present inventor found out that the period started after the elapse of the time t described above after the droplet ejection of the driving group A. In particular, it was found that after the elapse of nAL+t from the droplet ejection from the driving channels 11A of the driving group A, the droplet speed from the driving channels 11B of the driving group B becomes substantially the same as the droplet speed from the driving channels 11A of the driving group A performing the ejection first.
Therefore, according to this driving method, the driving apparatus 500, and the inkjet recording apparatus 100 including the driving apparatus 500, when the phase difference of nAL+t is applied to the driving signals applied to the respective driving groups A and B, the effect of the crosstalk between the driving groups A and B having the common ink chamber 51 in common can substantially be ignored without the need of making any changes to the head structure of the inkjet head H, and the fluctuation of the droplet speed between the channel rows can be suppressed. Moreover, since the phase difference is applied to the driving signals between the driving groups A and B, the driving load is also suppressed.
As depicted in Fig. 9, since the droplet speed from the driving channels 11 of the driving group having the driving signal applied later is inverted to positive or negative for each AL after the elapse of the time t, n may be any integer equal to or greater than one. However, the driving for ejection from the driving group driven later must not overlap with the next driving period T. If n is too large, the different driving groups may be separated in terms of the driving timing, resulting in a reduction in print speed, and therefore, n is preferably set to a value as small as possible in terms of speeding up of printing and is most preferably set to n=1.
An inkjet head having a plurality of channel rows typically starts ejection at different temporal timing in advance for impact position adjustment because of a difference of physical nozzle positions between adjacent channel rows. For example, in the case of the inkjet head H having the two channel rows as described above, as depicted in Fig. 10, after a first row (e.g., the driving group A) starts ejection, the recording medium P and the inkjet head H moves relative to each other so that the nozzles 21 of a second row (the driving group B) arrive at the physical position where the nozzles 21 of the first row existed at the start of ejection, and the second row then starts ejection at this moment. Even in this case, the driving channels 11 perform the ejection at the same driving timing and only the starting time and the ending time are different for each channel row.
The phase difference of nAL+t between the driving groups of the present invention is the Delay time not including a difference of the starting time and the ending time due to the impact position adjustment because of a difference of the physical nozzle positions between the driving groups as described above (an impact position adjustment period between the driving groups). Therefore, as depicted in Fig. 10, the phase difference indicates the Delay time provided within a period of driving of both two driving groups that are objects of application of the phase difference, and the droplet ejection timing itself is varied by applying the phase difference of nAL+t to the driving signal application timing between the different driving groups A and B in the period of driving of the both driving groups.
Although the application of the phase difference of nAL+t between the different driving groups A and B produces a problem that the impact position adjustment is required between the driving groups A and B in the strict sense, this problem can be solved by adjusting a relative movement speed of the recording medium P and the inkjet head H.
Although the number of the channel rows is two in the above description, the number of the channel rows may be more than one in the present invention and the configuration as described above can be achieved by dividing a plurality of the channel rows into N driving groups (N is an integer of two or more).
Fig. 11 depicts the case of having four channel rows and, in this case, all the channel rows are divided into two driving groups A and B, and the dividing groups are grouped alternately in the order of A, B, A, B from the upper side of Fig. 11 such that the adjacent channel rows belong to different driving groups.
In such an embodiment, D' is preferably equivalent to D or made larger such that the pressure wave is sufficiently attenuated.
With regard to the driving signal application timing in this case, as depicted in Fig. 12, when the phase difference of 1AL+t (in the case of n=1) is applied between the different driving groups A and B, the fluctuation of the droplet speed can be suppressed between the driving groups A and B and the driving load can be reduced.
Fig. 13 depicts the case of having six channel rows and, in this case, all the channel rows are divided into three driving groups and the dividing groups are grouped alternately in the order of A, B, C, A, B, C from the upper side of Fig. 13 such that the adjacent channel rows belong to different driving groups.
In such an embodiment, D' is preferably equivalent to D or made larger such that the pressure wave is sufficiently attenuated.
With regard to the driving signal application timing in this case, as depicted in Fig. 14, when the phase difference of 1AL+t (in the case of n=1) is applied between the different driving groups A and B and between driving groups B and C adjacent to each other, the fluctuation of the droplet speed can be suppressed among the driving groups A, B, and C and the driving load can be reduced.
If the number of the divided groups is three or more as described above, it is preferable to set all the phase differences nAL+t between the driving groups to the same value in terms of avoidance of print speed reduction.
When the number of the channel rows is three or more, the channel rows are preferably divided such that the adjacent channel rows belong to different driving groups. Since at least one channel row belonging to a different driving group is disposed between the channel rows belonging to the same driving groups, a separation distance between the same driving groups is made larger and the effect of the crosstalk can be reduced between the same driving groups.
In the present invention, the channel rows of the inkjet head H may not all be driven by the common driving circuit in the driving apparatus 500, and two or more driving circuits may be included in the driving apparatus 500 so that the respective channel rows are driven by the two or more driving circuits. In this case, the driving groups are preferably differentiated between the channel rows of the same driving circuit.
Fig. 15 depicts an example of four channel rows driven by two driving circuits 501, 502 each driving two rows in the driving apparatus 500. In this case, the two channel rows driven by the driving circuit 501 are divided into the different groups A, B and the two channel rows driven by the driving circuit 502 are divided into the different groups A, B. As a result, the lowering of the droplet speed can be reduced. This is because a decrease in the number of the driving channels driven by one driving circuit at the same time reduces the load of the driving circuits and the waveform rounding of the driving signals can be reduced.
Although the rectangular wave formed by the positive voltage (+V) of the pulse width PW generating a negative pressure in the channels 12 is exemplified as the driving signal in the above description, the driving signal of the present invention is not limited thereto and may be any driving signal for ejecting a droplet.
In the above description, a so-called harmonica type head forming a hexahedron shape with inlets and outlets of channels disposed on opposite end surfaces is exemplified as the head chip 1 of the inkjet head H. Since the inlets of the driving channels 11 are arranged on the same plane of the back end surface 1b in all the channel rows with the common ink chamber 51 disposed on the inlet side of the driving channels 11, the head chip 1 described above is relatively significantly affected by the crosstalk and likely to cause fluctuation of the droplet speed and is therefore in a preferable form since the remarkable effect is acquired from the application of the present invention. However the head chip structure of the present invention is not limited thereto and may be any structure in which pressure chambers are in communication with each other through a common ink chamber between multiple rows of pressure chambers.
The inkjet recording apparatus of the present invention is not limited to an apparatus ejecting droplets for recording in the course of the scanning movement of the inkjet head H in the width direction (horizontal scanning direction) over the recording medium P as described above and may be an apparatus having the inkjet head H made up of a line-shaped inkjet head fixed in the width direction over the recording medium P to eject droplets from the nozzles 21 for recording in the course of movement of the recording medium P in the Y-direction of Fig. 1. In this case, the channel rows of the inkjet head H is arranged in the X-X' direction of Fig. 1.

Examples

The effect of the present invention will hereinafter be verified with Examples.

(Example 1)

An inkjet head having two channel rows with the structure same as the inkjet head depicted in Fig. 2 was prepared, and one channel row was defined as a driving group A while the other channel row was defined as a driving group B. Each of the channel rows had 256 nozzles, the inter-nozzle distance D of 1.128 mm between the channel rows, and AL=5.0 µs.
The two channel rows of the inkjet head are driven by the same driving circuit.
Ink used for this inkjet head had viscosity of 10 mPa·s and surface tension of 32 mN/m, and the speed of sound traveling through the ink was 1300 m/s.
From the conditions described above, a value of the pressure wave transmission time t obtained from "inter-nozzle distance between driving groups"/"speed of sound traveling through the ink" was 1128 (µm)/1300×10⁶ (µm/s)=0.87×10^-6 (s)=0.87 (µs) and was set to t=0.9 (µs) from this calculated value in this example.
For the driving signals applied to the driving channels from the driving apparatus, a rectangular wave composed only of the positive voltage (+V) depicted in Fig. 5 was used. The pulse width PW was set to 1 AL=5.0 µs and the driving period T was set to 100 µs. The driving signals were applied from the common driving apparatus to all the channel rows.
This inkjet head was mounted on the carriage of the inkjet recording apparatus depicted in Fig. 1 with the phase difference (nAL+t) between the driving groups A, B set by using n=1 to 1×5.0+0.9=5.9 µs, and the inkjet head was driven such that the driving signal is first applied to the driving group A.

The respective droplets ejected from the nozzles of the driving groups A, B were photographed by using a camera to calculate the droplet speed from the image processing of the acquired droplet images, and the average speed of the nozzles was obtained from the result for each of the channel rows. From the acquired average speeds, |the average speed of the driving group A - the average speed of the driving group B| was calculated, and the calculated value was used for obtaining a fluctuation rate to the average speed of the driving group A (=the calculated value/the average speed of the driving group A×100; in %) to evaluate the effect of the crosstalk in accordance with the following criteria. The result is described in Table 1.

⊚: less than 5 %
○: 5 % or more and less than 10 %
Δ: 10 % or more and less than 15 %
×: 15 % or more

For the driving load at the time of driving the inkjet head, it was assumed that a current value is 100 when all the driving channels are driven at nAL+t=0 with a phase difference applied to none of the channel rows, and a proportion (%) to the current value was obtained. A smaller value of the driving load is more preferable. The result is described in Table 1.

(Example 2)

The crosstalk and the driving load were evaluated in the same way as Example 1 except that the phase difference (nAL+t) is set by using n=2 to 2×5.0+0.9=10.9 µs. The result is described in Table 1.

(Comparison Example 1)

The crosstalk and the driving load were evaluated in the same way in the same inkjet head as Example 1 without providing a phase difference between the driving groups A, B by using nAL+t=0. The result is described in Table 1.

(Comparison Example 2)

The crosstalk and the driving load were evaluated in the same way as Example 1 except that the phase difference (nAL+t) is set by using n=0.5 and t=0 to 0.5×5.0=2.5 µs. The result is described in Table 1. [Table 1]

crosstalk driving load

Example 1 ⊚ 50 %

Example 2 ⊚ 50 %

Comparison Example 1 Δ 100 %

Comparison Example 2 × 50 %

(Example 3)

An inkjet head was prepared such that the inkjet head has four nozzle rows as depicted in Fig. 11, and all the channel rows were divided into two driving groups A, B such that the driving groups are differentiated between adjacent channel rows. Each of the channel rows had 256 nozzles, the inter-nozzle distance D of 0.846 mm between the channel rows belonging to the different driving groups, and AL=5.0 µs.
The four channel rows of the inkjet head are driven by two driving circuits as depicted in Fig. 15.
Ink used for this inkjet head had viscosity of 5.7 mPa·s and surface tension of 41 mN/m, and the speed of sound traveling through the ink was 1600 m/s.
From the conditions described above, a value of the pressure wave transmission time t obtained from "inter-nozzle distance between driving groups"/"speed of sound traveling through the ink" was 846 (µm)/1600×10⁶ (µm/s)=0.53×10^-6 (s)=0.53 (µs) and was set to t=0.5 (µs) from this calculated value in this example.
The driving signals were the same as Example 1.
The inkjet head was driven with the phase difference (nAL+t) between the driving groups A, B set by using n=1 to 1×5.0+0.5=5.5 µs such that the driving signal is first applied to the driving group A, and the crosstalk and the driving load were evaluated in the same way as Example 1. The result is described in Table 2.

(Example 4)

The crosstalk and the driving load were evaluated in the same way as Example 3 except that the phase difference (nAL+t) is set by using n=2 to 2×5.0+0.5=10.5 µs. The result is described in Table 2.

(Comparison Example 3)

The crosstalk and the driving load were evaluated in the same way in the same inkjet head as Example 3 without providing a phase difference between the driving groups A, B by using nAL+t=0. The result is described in Table 2.

(Comparison Example 4)

The crosstalk and the driving load were evaluated in the same way as Example 3 except that the phase difference (nAL+t) is set by using n=0.5 and t=0 to 0.5×5.0=2.5 µs. The result is described in Table 2. [Table 2]

crosstalk driving load

Example 3 ⊚ 50 %

Example 4 ⊚ 50 %

Comparison Example 3 Δ 100 %

Comparison Example 4 × 50 %
From the results described above, the fluctuation of the droplet speed was suppressed and the effect of the crosstalk was reduced in all Examples.

Explanations of Letters or Numerals

H:: inkjet head
1:: head chip
1a: front end surface
1b: back end surface
1c: end edge
11, 11A, 11B: driving channel (pressure chamber)
12, 12A, 12B: dummy channel
13, 13A, 13B: partition wall (pressure applier)
14: driving electrode
15A, 15B: connection electrode
2:: nozzle plate
21: nozzle
3:: wiring substrate
3a: end portion
31: bonding region
32A, 32B: through-hole
33A, 33B: wiring electrode
4:: FPC
5:: ink manifold
51: common ink chamber
100:: inkjet recording apparatus
200:: feeding mechanism
201: feeding roller pair
202: feeding motor
203: feeding roller
300:: guide rail
400:: carriage
500:: driving apparatus
501, 502: driving circuit

Claims

A driving method of an inkjet head having two or more rows (a, B) of pressure chambers (11) having a pressure for ejecting internal ink from a nozzle (21) generated by a pressure applier operated by application of a driving signal, the pressure chambers (11) in the rows (A, B) of the pressure chambers being in communication with each other through a common ink chamber (51), wherein
the rows (A, B) of the pressure chambers are divided into N driving groups, wherein N is an integer of two or more,
characterised in that a phase difference of nAL+t between the different driving groups is applied to the driving signals applied to the pressure appliers of the pressure chambers (11), wherein n is an integer of one or more, wherein AL is 1/2 of an acoustic resonance period of a pressure wave in the pressure chambers (11), and wherein t is a pressure wave transmission time obtained from an inter-nozzle distance between the driving groups / a speed of sound traveling through the ink.
The driving method of an inkjet head of claim 1, wherein the rows (A, B) of pressure chambers are adjacent to each other and the rows are divided into different driving groups.
The driving method of an inkjet head of claim 1, wherein
the rows (A, B) of the pressure chambers are driven by two or more driving circuits (501, 502), and wherein

the rows (A, B) of the pressure chambers of the same driving circuit are divided into different driving groups.
The driving method of an inkjet head of claim 1, 2, or 3, wherein the inkjet head is an inkjet head having a head chip with inlets and outlets of the pressure chambers (11) opened in opposite end surfaces such that the pressure chambers (11) are formed straight from the inlets to the outlets, and wherein the common ink chamber (51) is disposed on the inlet side of the pressure chambers (11) in the head chip.
A driving apparatus of an inkjet head having two or more rows (A, B) of pressure chambers (11) having a pressure for ejecting internal ink from a nozzle (21) generated by a pressure applier operated by application of a driving signal, the pressure chambers (11) in the rows (A, B) of the pressure chambers being in communication with each other through a common ink chamber (51), wherein
the rows (A, B) of the pressure chambers of the inkjet head are divided into N driving groups, wherein N is an integer of two or more,
characterised in that the driving apparatus is configured to apply a phase difference of nAL+t between the different driving groups to the driving signals applied to the pressure applier of the pressure chambers (11), wherein n is an integer of one or more, wherein AL is 1/2 of an acoustic resonance period of a pressure wave in the pressure chambers, and wherein t is a pressure wave transmission time obtained from an inter-nozzle distance between the driving groups / a speed of sound traveling through the ink.
The driving apparatus of an inkjet head of claim 5, wherein the rows (A, B) of pressure chambers are adjacent to each other and the rows (A, B) are divided into different driving groups.
The driving apparatus of an inkjet head of claim 5, wherein
the driving apparatus of an inkjet head has two or more driving circuits (501, 502) driving the rows of the pressure chambers (11), and wherein

the rows (A, B) of the pressure chambers of the same driving circuit (501, 502) are divided into different driving groups.
The driving apparatus of an inkjet head of claim 5, 6, or 7, wherein the inkjet head is an inkjet head having a head chip with inlets and outlets of the pressure chambers (11) opened in opposite end surfaces with the pressure chambers (11) formed straight from the inlets to the outlets, and wherein the common ink chamber (51) is disposed on the inlet side of the pressure chambers (11) of the head chip.
An inkjet recording apparatus comprising:
an inkjet head having two or more rows (A, B) of pressure chambers (11) having a pressure for ejecting internal ink from a nozzle (21) generated by a pressure applier operated by application of a driving signal, the pressure chambers (11) in the rows (A, B) of the pressure chambers being in communication with each other through a common ink chamber (51); and

the driving apparatus of an inkjet head of any one of claims 5 to 8.