JP2001047621A - Electrostatic ink jet head - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize low voltage driving while optimizing ejection characteristics of ink liquid drop. SOLUTION: The electrostatic ink jet head 1 comprises a diaphragm 51 serving as a common electrode defining the bottom face of a pressure chamber 5, and a segment electrode 10a of individual electrode facing the diaphragm 51 through a specified gap G which is minimized G(min) at the end parts 51a, 51b of the diaphragm on the ink supply opening 7 side and the ink nozzle 11 side and maximized G(max) at a position in the central part of the diaphragm. The gap G increases gradually from the minimum gap part toward the maximum gap part. The diaphragm 51 is attracted to the segment electrode 10a side along the direction for passing ink or released therefrom. Consequently, an ink flow is generated efficiently in the pressure chamber 5. Since the diaphragm 51 is attracted to the segment electrode 10a starting from the position of minimum gap G(min), driving voltage can be lowered.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an electrostatic ink jet head for ejecting ink droplets by displacing a diaphragm by electrostatic force. More specifically, the present invention relates to an electrostatic ink jet head that can improve the ejection characteristics of ink droplets and can be driven at a low voltage.

[0002]

2. Description of the Related Art As disclosed in, for example, Japanese Patent Application Laid-Open No. 2-219351, an electrostatic ink jet head includes a diaphragm serving as a common electrode forming a part of an ink flow path, and a diaphragm as a common electrode. And an electrode plate as an individual electrode facing the diaphragm with a small gap therebetween. A drive voltage is applied between these opposing electrodes to apply an electrostatic force to bend the vibrating plate to change the volume in the flow path. The recording is performed by discharging.

In such an electrostatic ink jet head, in order to realize high quality of an output image and a high output speed, it is desired that the ejection efficiency of ink droplets is high and the ink jet head can be driven at a low voltage. ing.

For this purpose, an ink jet head having a structure in which a gap between a diaphragm on which an electrostatic force acts and an electrode plate is changed is disclosed in Japanese Patent Application Laid-Open Nos. 9-39235 and 9-19.
No. 3,375,387. In the ink jet head having this configuration, the gap between the vibration plate that defines the bottom surface of the ink pressure chamber connected to the ink nozzle and the electrode plate that faces the vibration plate moves in the direction of the ink nozzle. It is set to be large.

[0005]

In recent years, there has been a demand for an electrostatic ink jet head which can further improve the ejection characteristics of ink droplets and can be driven at a lower voltage. In order to improve the ejection characteristics of ink droplets, the driving frequency for repeatedly ejecting ink droplets by stably ejecting main ink droplets at high speed and suppressing the generation of unnecessary ink droplets thereafter It is necessary to increase.

Therefore, in the electrostatic ink jet head, the volume variation of the pressure chamber caused by the deflection of the diaphragm formed at the bottom of the pressure chamber storing the ink causes the efficiency of the pressure chamber to be reduced. It is important that they produce good ink flow. It is also important to efficiently drive the diaphragm at a low voltage for this purpose.

[0007] Variations in head dimensions during manufacturing and changes in the environment at the time of ink ejection have a great effect on the ink ejection conditions.
Due to these factors, the movement of the diaphragm is slightly different,
Variations in ink ejection occurred. That is, the ink flow inside the pressure chamber is not uniquely defined, and the ink flow in the pressure chamber is disturbed, which adversely affects the ink discharge state, and unstable and inefficient discharge occurs. As a result, there have been restrictions such that high-quality ejection cannot be stably obtained, that the driving frequency cannot be increased, and that low-voltage driving cannot be performed.

An object of the present invention is to provide an electrostatic ink jet head capable of stably and optimizing the ejection characteristics of ink droplets, thereby increasing the driving frequency and driving at a low voltage. Is to provide.

[0009]

In order to achieve the above object, the present invention provides a pressure chamber storing ink, an ink supply port for supplying ink to the pressure chamber, and an ink supply port in the pressure chamber. An ink nozzle communicating with a portion opposite to the mouth, a vibrating plate that vibrates in an out-of-plane direction that defines a part of the pressure chamber, and faces the vibrating plate with a predetermined gap. And a gap between the vibration plate and the electrode plate, when viewed along an ink flow direction from the ink supply port toward the ink nozzle, the ink supply port. And gradually increasing from the ink nozzle toward the center of the pressure chamber.

[0010] In the ink jet head of the present invention configured as described above, the gap between the vibration plate and the electrode plate is maximum near the center position of the pressure chamber, and is minimum on the ink nozzle side and the ink supply port side. Therefore, when a voltage is applied between the vibration plate and the electrode plate and the vibration plate is attracted to the electrode plate side, both ends of the vibration plate, that is, the ink nozzle side and the ink supply port side with a small gap are first placed on the electrode plate. These are sequentially sucked toward the center portion of the diaphragm from the electrode plate side.

The volume of the pressure chamber increases due to the suction of the diaphragm,
While the ink flows into the pressure chamber from the ink supply port side, the ink on the ink nozzle side is also drawn to the pressure chamber side. Since each part of the vibration plate is sequentially sucked by the electrode plate in a direction along the flow of the ink, an efficient flow of the ink is formed in the pressure chamber.

Further, the diaphragm is sequentially sucked from the small gap portion toward the electrode plate side toward the large gap portion.
Since the electrostatic force required when the gap is small may be small, the entire diaphragm can be attracted to the electrode plate side only by applying a low driving voltage.

Next, when the vibration plate is separated from the electrode plate and elastically returns, the vibration plate starts to separate from the electrode plate around the maximum gap near the center thereof. The ink inside the pressure chamber is increased by the internal pressure generated by the elastic return operation of the diaphragm,
The ink flows out from the center of the pressure chamber toward the ink nozzle and is ejected from the ink nozzle as ink droplets.

Since the diaphragm elastically returns along the flow of the ink, a smooth flow of the ink in the ink discharge direction is efficiently formed in the pressure chamber even during the ink discharge.

As described above, according to the present invention, the diaphragm can be successively sucked and elastically returned along the direction in which ink flows. Therefore, the ejection efficiency of ink droplets can be improved. In addition, the gap between both ends of the diaphragm and the electrode plate is small, and each part of the diaphragm comes into contact with the electrode plate starting from these portions. For this reason, the voltage required for contacting the portion where the gap becomes maximum can be reduced.

Here, in order to more efficiently form the ink flow in the pressure chamber due to the vibration of the diaphragm in order to optimize the ejection characteristics of the ink droplets, the position where the gap becomes maximum is determined by the pressure It is desirable to set a position where a node of the ink pressure vibration generated in the chamber appears.

The position where the gap becomes maximum can be determined based on inertance. In this case, the position where the gap is the maximum is equal in the inertance of the ink path from the ink supply port through the pressure chamber to the ink nozzle on the ink nozzle side and the ink supply side. It is desirable to set so that

Further, in order to efficiently form the ink flow in the pressure chamber due to the vibration of the vibration plate, the gap between the vibration plate and the electrode plate should be formed in a direction perpendicular to the ink flow direction. It is desirable that the diaphragm be formed so as to gradually increase from both end positions of the diaphragm toward the center position. When a voltage is applied between the diaphragm and the electrode plate to attract the diaphragm to the side of the electrode plate, both ends of the diaphragm having a small gap are firstly attracted to the electrode plate in the orthogonal direction, and the vibration starts from these. It is sequentially sucked toward the electrode plate side toward the center of the plate. The pressure chamber volume increases due to the diaphragm suction,
When ink flows into the pressure chamber from the ink supply port and the ink nozzle side, each part of the vibration plate is sequentially sucked by the electrode plate in the orthogonal direction, so that a stable and efficient ink flow in the pressure chamber is achieved. Is formed. In addition, since the diaphragm is sequentially attracted to the electrode plate side from the small gap portion toward the large gap portion, the required electrostatic force may be small. The whole plate can be sucked to the side of the electrode plate.

Next, when the vibration plate is separated from the electrode plate and elastically returns, the diaphragm starts to separate from the electrode plate around a portion of the maximum gap near the center. The ink inside the pressure chamber is increased by the internal pressure generated by the elastic return operation of the diaphragm,
The flow of the ink in the orthogonal direction flows from both end portions toward the center of the pressure chamber, while flowing from the center portion of the pressure chamber toward the ink nozzle, and is ejected from the ink nozzle as ink droplets. The diaphragm returns elastically along the flow of the ink, and a stable and smooth flow of the ink is efficiently formed in the pressure chamber at the time of ink discharge.

As described above, according to the present invention, the diaphragm can be sequentially sucked and elastically returned along the direction in which the ink should flow. Therefore, the ejection efficiency of ink droplets can be improved.

Here, in order to more efficiently form the ink flow in the pressure chamber due to the vibration of the vibration plate in order to optimize the ejection characteristics of the ink droplet, the position where the gap becomes maximum is determined by the orthogonal position. It is desirable to set a position where a node of the ink pressure vibration generated in the pressure chamber appears in the direction.

The position where the gap becomes maximum can be determined based on inertance. In this case, it is desirable to set the position where the gap becomes maximum such that the inertance of the ink path in the orthogonal direction is equal on both sides of the maximum position. In this case, it can be set so that the gap is maximized at the center position.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an electrostatic ink jet head to which the present invention is applied will be described below with reference to the drawings. FIG. 1 is a cross-sectional view of the electrostatic ink jet head of the present embodiment, and FIG. 2 is a plan view thereof. FIG. 3 is a longitudinal sectional view of a head of another embodiment of the electrostatic ink jet head according to the present invention.

As shown in these figures, an electrostatic ink jet head 1 has a silicon substrate 2 sandwiched therebetween, a nozzle plate 3 also made of silicon on the upper side, and a borosilicate glass substrate 4 whose thermal expansion coefficient is close to that of silicon on the lower side. Are laminated in a three-layer structure.

The central silicon substrate 2 is etched from its surface to form five independent elongated pressure chambers 5, one common ink chamber 6, and the common ink chamber 6 communicating with each pressure chamber 5. Grooves each functioning as an ink supply port 7 are formed. These grooves are closed by the nozzle plate 3, and the portions 5, 6, and 7 are defined.

In order to communicate with the ink supply port 7,
An ink inlet 12 is formed in a portion that defines the lower surface of the common ink chamber 6. Accordingly, ink is supplied from an external ink tank (not shown) to the common ink chamber 6 through the intake port 12, and from here to the independent pressure chambers 5 through the respective ink supply ports 7. You.

In the nozzle plate 3, ink nozzles 11 are formed at positions corresponding to the front end portions of the respective pressure chambers 5, that is, at positions opposite to the ink supply ports 7. It communicates with each corresponding pressure chamber 5.

The bottom surface of each independent pressure chamber 5 is thin and is set so as to function as a diaphragm 51 which can be elastically displaced in an out-of-plane direction, that is, in the vertical direction in FIG.

Next, in the glass substrate 4 located below the silicon substrate 2, on the upper surface joined to the silicon substrate 2, each vibration defining the bottom surface of each pressure chamber 5 of the silicon substrate 2 is provided. A shallowly etched recess 41 is formed at a position facing the plate 51. The individual electrode 10 corresponding to the diaphragm 51 is formed on the bottom surface of the concave portion 41. The individual electrode 10 is a segment electrode 1 made of ITO.
0a and a terminal portion 10b.

By joining the glass substrate 4 to the silicon substrate 2, the diaphragm 51 defining the bottom surface of each pressure chamber 5 and the segment electrode 10a of the individual electrode are
They face each other with a very small gap G therebetween. This gap G is formed by the sealant 6 disposed between the silicon substrate 2 and the glass substrate 4.
It is sealed by 0 and is in a closed state.

A common electrode terminal 22 is formed on the silicon substrate 2, and a driving voltage is applied between the common electrode terminal 22 and each individual electrode 10 by voltage applying means 21. Since the silicon substrate 2 is conductive, each diaphragm 51 functions as a common electrode.

When a voltage is supplied to the common electrode with a lower electric resistance, for example, a thin film of a conductive material such as gold may be formed on one surface of the silicon substrate 2 by vapor deposition or sputtering. In the electrostatic ink jet head 1 of the present embodiment, since anodic bonding is used for connecting the silicon substrate 2 and the glass substrate 4, a conductive film is formed on the surface of the silicon substrate 2 on the side where ink flows.

The diaphragm 51 is driven by an electrostatic force generated by applying a drive voltage between the diaphragm 51 and the individual electrode 10.
Is attracted to the individual electrode side, the diaphragm 51 is elastically deformed and bent toward the segment electrode 10a, and is attracted to the surface of the segment electrode 10a. As a result, the volume of the pressure chamber 5 increases, and ink is supplied from the ink supply port 7 to the pressure chamber 5.

When the electrostatic attraction force is released, the diaphragm 51 is separated from the surface of the segment electrode 10a by the elastic force and returns to the initial state, and the volume of the pressure chamber 5 rapidly decreases. At this time, a part of the ink in the pressure chamber is ejected as ink droplets from the ink nozzle 11 communicating with the pressure chamber 5 due to the ink pressure vibration generated in the pressure chamber. (Gap G between Vibrating Plate and Segment Electrode) As shown in FIG. 1, a sealed space (hereinafter referred to as a vibrating chamber) 41A between the vibrating plate 51 and the segment electrode 10a has an upper surface 41.
a is defined by the flat lower surface of the diaphragm 51, and its side surface 41 b and bottom surface 41 c are defined by the concave portion 41 formed in the glass substrate 4. In this example, the cross section of the bottom surface 41c has an inverted triangular shape as shown in FIG. As described above, the gap G between the diaphragm 51 and the segment electrode 10a is defined by the flat upper surface 41a and the bottom surface 41c etched in an inverted triangular cross section, and is as follows.

That is, when the vibration chamber 41A is viewed along the ink flow direction 70 (the paper surface direction in FIG. 1) from the ink supply port 7 toward the ink nozzle 11, the gap G Plate end 51a and ink nozzle 1
The portion of the diaphragm end portion 51b on the side of No. 1 is the minimum gap G (mi
n), the gap G gradually increases toward the vicinity of the center of the pressure chamber 5 from each of these, and is formed so as to become the maximum gap G (max) at the position A corresponding to the vicinity of the center of the pressure chamber. ing.

In this embodiment, it has been described that the pressure chamber gradually increases toward the center position along the ink flow direction. However, the pressure chamber is located at the center position in a direction orthogonal to the ink flow direction. It may be gradually increasing toward. This example is shown in FIG. 3, and the same function as in the above-described embodiment is denoted by the same reference numeral, and the description is omitted.

In another embodiment of the electrostatic ink jet head according to the present invention, as shown in FIG. 3, even when viewed along a direction orthogonal to the ink flowing direction 70, the gap G The minimum gap G (min) is set at both ends 51 c and 51 d of the plate 51, and gradually increases from these positions toward the position B where the maximum gap G (max) is formed.

The minimum gap G (min) is, for example, 1000 angstroms, and the maximum gap G at positions A and B is
(Max) is set to 2000 angstroms.
Of course, other values may be adopted.

In still another embodiment of the present invention, the bottom surface 41c of the vibration chamber 41A is formed as an inverted quadrangular pyramid so that FIGS. The gap G is gradually increased from the position of the minimum gap G (min) at the four peripheral edges of the elongated diaphragm to a position A near the center, ie, B
May be used as the maximum gap G (max). In order to gradually change the gap G, instead of using a flat surface as shown in FIGS. 1 and 3, it is also possible to combine a convex curved surface and a concave curved surface. Alternatively, the bottom surface of the concave portion 41 may be a flat surface, and the lower surface of the diaphragm 51 facing the concave portion may have a shape in which a central portion is curved upward. In addition, when the gap is actually formed, the gap is formed through a plurality of etching processes using masks of different sizes to form a gap including a plurality of steps, and the gap is changed in a stepwise manner. You can do things. The maximum gap A and / or B may not be formed in a point or linear shape, but may be formed by a flat portion having a corresponding width.

Further, in the present embodiment, the gap between the diaphragm and the segment electrode at the four peripheral edges of the diaphragm is set as the minimum gap. However, for example, the diaphragm end 51b on the ink nozzle side may be set to the minimum gap, and the diaphragm end 51a on the opposite ink supply port side may be set to a larger gap. Conversely, the diaphragm end 51a on the ink supply port side can be made the minimum gap. (Method of Determining Maximum Clearance Position A) Here, in order to efficiently and smoothly form the ink flow in the pressure chamber 5 due to the vibration of the vibration plate 51 and thereby further improve the ink ejection characteristics, the following method is required. It is desirable to determine the position A of the maximum gap as follows.

The first method is to set the position A of the maximum gap to a position where a node P of the ink pressure vibration generated in the pressure chamber 5 due to the vibration of the vibration plate 51 appears. At the position where the node of the ink pressure vibration appears, substantially no ink vibration occurs. Therefore, the ink flow toward the ink nozzle side and the ink supply port side or the ink nozzle side and the ink supply port side From the pressure chamber toward the pressure chamber. Therefore, this position is set to the position A of the maximum gap.
Then, the elastic displacement of the vibration plate 51 occurs along the flow of the ink, so that an efficient and smooth ink flow can be formed in the pressure chamber. As a result, the ink ejection characteristics can be improved.

The second method is to determine the position A of the maximum gap based on the inertance. That is, this position A
From the supply port 7 side of the pressure chamber 5 and the supply port 7
Is set at a position where the inertance value including the ink nozzle 11 side of the pressure chamber 5 and the ink nozzle 11 from the pressure chamber 5 becomes equal to each other. The inertance M is the specific gravity ρ of the ink liquid, the length L of the ink flow path,
When the cross-sectional area S of the ink flow path is obtained, it is obtained by the following equation. M = ρ * L / S Specifically, when the cross-sectional area S (x) of the ink flow path changes with respect to the distance x in the ink flow direction, it can be obtained by the following equation.

[0043]

(Equation 1) Therefore, the position A of the maximum gap G (max) is determined by the inertance Mc of the pressure chamber 5, the inertance M2 of the supply port 7,
Assuming that the inertance M3 of the ink nozzle 11 and the total length L1 (length of the diaphragm) in the longitudinal direction of the pressure chamber 5, the distance L2 from the diaphragm end 51b on the ink nozzle side is determined by the following equation.
May be determined. L2 = (M2 + Mc-M3) / 2Mc * L1 (Operation) As described above, the gap G between the diaphragm 51 and the individual electrode 10 is obtained.
Electrostatic ink jet head 1 of this example in which is set
, A driving voltage pulse is applied between the diaphragm 51 and the segment electrode 10a to move the diaphragm 51 to the segment electrode 1
When sucked to 0a, the diaphragm 51 is sucked by the segment electrode 10a from the end portions 51a and 51b on the ink supply port 7 side and the ink nozzle 11 side, as shown by broken lines in FIG. Similarly, also in the orthogonal cross section shown in FIG. 3, the diaphragm electrode is gradually attracted to the segment electrode 10a from both end positions 51c and 51d. That is,
From the four circumferential ends of the diaphragm 51 where the minimum gap G (min) is formed, the diaphragm 51 is gradually attracted toward the segment electrode 10a toward the center.

As described above, when the diaphragm is sucked, the diaphragm 5
In 1, the elastic displacement gradually progresses from the end 51 a on the ink supply port side to the position A of the maximum gap near the center thereof. Accordingly, the elastic displacement of the vibration plate 51 proceeds along the direction of the ink flow supplied from the ink suction port 7 to the pressure chamber 5.

As a result, diaphragm 51 has minimum gap G (mi
From the position of n) toward the maximum gap G (max), the vibration chamber 41 is sequentially held in contact with the bottom surface 41c of the vibration chamber 41,
The deformation is hindered by the ink pressure generated in the pressure chamber 5, and the compliance of the portion of the vibration plate 51 in the holding state is reduced. That is, the rigidity is high. Therefore, a stable ink flow occurs during ink suction, and the supply of ink into the pressure chamber proceeds efficiently and quickly.

The diaphragm 51 has a minimum gap G (m
In), the suction is performed by the segment electrode 10a from the position of “in”, and the suction state proceeds from this portion toward the portion of the maximum gap G (max), so that the suction force for sucking the diaphragm 51 can be small. The drive voltage to be applied can be reduced.

Next, the diaphragm 51 is connected to the segment electrode 10a.
After the electrostatic attraction force is released after the suction
1 is separated from the segment electrode 10a starting from a portion facing the position A of the maximum gap G (max) near the center thereof. As described above, since the diaphragm gradually separates from the segment electrode 10a from the vicinity of the center of the diaphragm toward the ink nozzle, an ink discharge flow toward the ink nozzle 11 can be efficiently and promptly formed.

The portion of the vibration plate 51 located at the minimum gap G (min) tries to return slowly, but the ink pressure generated in the pressure chamber 5 hinders the return, and the vibration chamber 51 returns. 41 is temporarily held in contact with the bottom surface 41c. In this state, compliance is small. That is, the rigidity of the pressure chamber 5 is high when ejecting ink droplets.

Therefore, at the time of ink ejection, the flow of ink is stable and efficient, and it is generated rapidly, and ink droplets are ejected at high speed. Therefore, the ink ejection characteristics can be improved.

When the diaphragm 51 is completely separated from the segment electrode 10a on the bottom surface 41c, the compliance of the diaphragm 51 is large and the rigidity is low.
Although elastically displaced in proportion to the pressure, the vibration of the ink flow (ink pressure vibration) is reduced by the viscosity of the ink and the flow path resistance, and the vibration of the nozzle meniscus after the ejection of the ink droplet is also suppressed and attenuated.

It should be noted that the present invention is not limited to the above-described embodiment, and various changes can be made.

For example, although the ink has been described as a printing ink, liquids other than the printing ink or a molten liquid can be applied to applications other than printing using the head according to the present invention.

The head has a three-layer structure and has five pressure chambers and one common ink chamber.
The present invention is not limited to the number of pressure chambers and common ink chambers, and the present invention can achieve the same effects.

[0054]

As described above, in the electrostatic ink jet head of the present invention, the gap between the diaphragm and the electrode plate is
Both ends on the ink supply port side and the ink nozzle side are small, and are formed so as to be maximum at a position substantially at the center of the pressure chamber. Alternatively, in the direction perpendicular to the direction connecting the supply port and the nozzle, the gap between both ends of the diaphragm is small,
It is formed so as to be larger at the center.

Therefore, when the diaphragm is attracted to the electrode plate,
The ink flow in the pressure chamber can be generated efficiently and quickly, and the ink discharge flow generated in the pressure chamber can be generated efficiently and quickly even when the diaphragm is released from the electrode plate. Can be. Therefore,
The ink ejection characteristics can be improved. That is, by regulating the movement of the vibration plate so as to generate the movement stably and efficiently, the ink flow in the pressure chamber can be uniquely defined, and the turbulence of the ink flow in the pressure chamber which adversely affects the ink ejection state is eliminated. I can do it. Further, it is possible to suppress variations in ink ejection between the heads due to various variations in head manufacturing.

In addition to this, the operation of attracting the diaphragm to the electrode plate occurs at the end of the diaphragm where the minimum gap is formed. Proceed towards. Therefore, it is possible to reduce the driving voltage for causing the diaphragm to be attracted to the electrode plate.
That is, low voltage driving becomes possible.

[Brief description of the drawings]

FIG. 1 is a schematic vertical sectional view showing an example of an electrostatic inkjet head to which the present invention is applied.

FIG. 2 is a schematic plan view of the electrostatic inkjet head shown in FIG.

FIG. 3 is a schematic cross-sectional view showing another embodiment to which the present invention is applied, in which the inkjet head is cut in a direction orthogonal to a direction in which ink flows.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Electrostatic inkjet head 2 Silicon substrate 3 Nozzle plate 4 Glass substrate 5 Pressure chamber 6 Common ink chamber 7 Ink supply port 10 Individual electrode 10a Segment electrode 10b Terminal part 11 Ink nozzle 12 Ink outlet 21 Voltage applying means 22 Common electrode terminal 41 Concavity 41A Vibration chamber 41a Upper surface of vibration chamber 41b Side surface of vibration chamber 41c Bottom of vibration chamber 51 End of vibration plate 51a, 51b, 51c, 51d End of vibration plate 60 Sealant 70 Direction of ink flow

Claims (6)

[Claims] A pressure chamber storing ink; an ink supply port for supplying ink to the pressure chamber; an ink nozzle communicating with a portion of the pressure chamber opposite to the ink supply port; An electrostatic inkjet head having a vibrating plate capable of vibrating in an out-of-plane direction defining a part of the pressure chamber and an electrode plate facing the vibrating plate at a predetermined gap, The gap between the electrode plate and the electrode plate gradually increases from the ink supply port and the ink nozzle toward the center position of the pressure chamber when viewed along the ink flow direction from the ink supply port toward the ink nozzle. An electrostatic inkjet head. 2. The electrostatic ink jet head according to claim 1, wherein the position where the gap is maximum is a position where a node of ink pressure vibration generated in the pressure chamber appears. 3. The ink nozzle according to claim 1, wherein the position where the gap is maximum is an inertance of an ink path from the ink supply port to the ink nozzle via the pressure chamber and the ink nozzle side. An electrostatic ink jet head, wherein the positions are equal on the ink supply side. 4. A pressure chamber storing ink, an ink supply port for supplying ink to the pressure chamber, an ink nozzle communicating with a portion of the pressure chamber opposite to the ink supply port, An electrostatic inkjet head having a vibrating plate capable of vibrating in an out-of-plane direction defining a part of the pressure chamber and an electrode plate facing the vibrating plate at a predetermined gap, And the gap between the electrode plate and the electrode plate gradually increases from both end positions of the vibration plate toward the center position when viewed along a direction orthogonal to an ink flow direction from the ink supply port toward the ink nozzle. An electrostatic inkjet head characterized by the above-mentioned. 5. The electrostatic device according to claim 4, wherein the gap is maximum at a center position of the diaphragm when viewed along a direction orthogonal to the ink flow direction. Type inkjet head. 6. An electrostatic ink jet head according to claim 1, 2 or 3, which satisfies claim 4 or 5.