JPH07214769A - Inkjet head - Google Patents

Abstract

(57) [Abstract] [Purpose] In an inkjet head that uses electrostatic force for ink ejection, consistent ink ejection is obtained to obtain stable and high print quality, and it is destroyed even when used for a long time. (EN) Provided is an inkjet printer that can obtain a highly reliable inkjet head that does not cause problems such as the above, and can be efficiently driven with a low drive voltage. [Structure] A nozzle 4, a vibrating plate 5 provided in a part of a discharge chamber 6 communicating with the nozzle 4, and an electrode 21 provided so as to face the vibrating plate 5 are provided. The inkjet head 10 in which an oxide film 24 is formed on opposite surfaces, and the material of the electrode 21 is an oxide conductor such as indium, tin, zinc, and cadmium.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an on-demand type ink jet head, and more particularly to an ink jet head which uses electrostatic force as its driving means.

[0002]

2. Description of the Related Art An ink jet recording apparatus has many advantages such as extremely low noise during recording, high speed printing, and use of plain paper which has a high degree of freedom of ink and is inexpensive. Among these, the so-called ink-on-demand method, which ejects ink droplets only when recording is necessary, is becoming mainstream because it does not require recovery of ink droplets unnecessary for recording.

In a conventional method for driving an on-demand type ink jet head, for example, Japanese Examined Patent Publication No. 2-2421.
In the driving method known from No. 8, a piezoelectric element that changes the volume of a pressure chamber that generates ink ejection pressure is provided, and an electric pulse in the same direction as the polarization voltage of the piezoelectric element is applied to the piezoelectric element in a standby state. The element is charged to reduce the volume of the pressure chamber, and when the ink is ejected, the piezoelectric element is gradually discharged to increase the volume of the pressure chamber, and then an electric pulse is applied to the piezoelectric element again to remove the piezoelectric element. A method of ejecting ink from a nozzle by rapidly charging and reducing the volume of a pressure chamber is disclosed. Furthermore, in the same certificate, in order to eject the ink droplets most efficiently at a low voltage, the voltage is applied again to the piezoelectric element in the vicinity of the maximum value of the damping vibration of the ink system that occurs when the ink is sucked into the pressure chamber, and the pressure is rapidly applied. A method of reducing the volume of a chamber is disclosed.

[0004]

However, the above-mentioned conventional method for driving an ink jet head is one of the most suitable methods for using a piezoelectric element as a driving means, but it is provided in a part of the flow path. And a method in which an electric pulse is applied between the vibrating plate and an electrode provided opposite to the vibrating plate to deform the vibrating plate by an electrostatic force and eject ink droplets from the nozzle, that is, for example, Japanese Patent Laid-Open No. 2-289351. However, if it is simply applied to the drive means using electrostatic force as disclosed in the above publication, the following problems occur and it is difficult to put it into practical use.

In the case where a piezoelectric element is used as a driving element, the vibration plate has energy (voltage, energization width) applied to the piezoelectric element.
Since the ink is flexed in proportion to the accuracy, the amount of ink ejected from each nozzle is about the same amount if you keep in mind that constant energy is applied to each piezoelectric element in an inkjet head obtained with the current processing accuracy. And sufficient print quality can be obtained.

On the other hand, in the case where the electrostatic force is used as the driving means, if the distance between the diaphragm and the electrode is made larger than necessary, the electrostatic force generated between the diaphragm and the electrode is 2 times the gap distance. Since it becomes smaller in proportion to the power, the standard voltage used as a printer cannot generate the pressure enough to eject the ink or suck the ink in the flow path. Therefore, the gap is extremely small (about 2 μm). Below)

Further, unlike the case where a piezoelectric element is used as the driving means, if the electrostatic attraction generated between the diaphragm and the electrode is too strong, that is, the voltage of the electric pulse applied between the diaphragm and the electrode is too high or the current is applied. If the width is too long, there is a problem peculiar to inkjet heads that uses electrostatic force, in which the diaphragm and the electrodes come into contact with each other and short-circuit, causing the head to break. It was

Therefore, even if the gap is processed with extremely high accuracy (± 0.05 μ) with the current processing accuracy and a certain amount of energy is applied between the vibrating plate and the electrodes facing the vibrating plate, the strain amount of each vibrating plate is increased. It is difficult to control the
Therefore, the amount of ink ejected from each nozzle per one time is not constant and stable, and defects such as missing dots in the recorded image and uneven dot sizes frequently occur in print quality. Furthermore, especially when the material of the diaphragm is a semiconductor, when charging and discharging are repeated between the diaphragm and the electrodes, a residual electric field is generated in the diaphragm, making it more difficult to always keep the amount of bending of the diaphragm constant. Become.

Therefore, an object of the present invention is to solve the above-mentioned problems, and in an ink jet head of a system utilizing electrostatic force, a stable ink ejection can be always obtained in order to obtain a stable high printing quality, and a long time is required. An object of the present invention is to provide an inkjet printer that can obtain a highly reliable inkjet head that does not cause troubles such as destruction even when used, and that can be efficiently driven with a low drive voltage.

[0010]

An ink jet head according to the present invention includes a nozzle, an ink flow path communicating with the nozzle, a vibration plate provided in a part of the flow path, and a diaphragm facing the vibration plate. An ink jet having a provided electrode, applying an electric pulse between the vibrating plate and the electrode, deforming the vibrating plate by an electrostatic force, and ejecting ink droplets from the nozzle toward recording paper to perform printing. In the head, the vibrating plate is made of a semiconductor in which an insulating film such as an oxide film or a nitride film is formed on a surface facing the electrode, and the electrode is made of an oxide conductor.

According to a preferred aspect of the ink jet head of the present invention, the oxide conductor is indium,
It is characterized in that it is made of an oxide of tin, zinc, cadmium, or a composite of two or more of the oxides, and the resistivity of the oxide conductor is 5 × 10 ⁻⁴ Ωcm or less.

Further, according to a preferred aspect of the ink jet head of the present invention, the insulating film has a thickness of 500 to
It is characterized by being 3000 angstroms.

[0013]

In the ink jet head of the present invention, when an electric pulse is applied to the electrode, an attractive force or repulsive force due to electrostatic force is exerted between the electrode and the vibration plate arranged so as to face the electrode, and the electrostatic force and the vibration. The vibrating plate is vibrated by the elastic force of the plate itself, and ink droplets are ejected from the nozzle. The facing distance between the vibrating plate and the electrode is the lower limit in order to obtain the pixel size required for printing the ink ejection volume. Is about 0.05 μm, and the upper limit is 2.0 from a practical driving voltage.
The distance between the diaphragm and the electrodes is preferably close to the lower limit in order to set the driving voltage low.

At this time, since the vibrating plate is attracted until it comes into contact with the electrodes, the ink ejection amount per ejection from each nozzle is uniquely determined by the facing interval between the vibrating plate and the electrodes. The discharge amount is constant, and stable print quality is obtained.

The diaphragm is a semiconductor such as silicon having an oxide film or a nitride film formed on the surface facing the electrode. Since the diaphragm contacts the electrode through the oxide film which is an insulator, the head is shorted by a short circuit. Since the electrodes are not destroyed, and the material of the electrodes is an oxide conductor such as ITO, the withstand voltage is excellent, and the trouble that the head is destroyed is unlikely to occur even after long-term driving.

[0016]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is an exploded perspective view of an ink jet head according to a first embodiment of the present invention, which is a partial sectional view. This embodiment shows an example of an edge eject type in which ink droplets are ejected from a nozzle provided at the end of the substrate. 2 is a cross-sectional side view of the assembled whole device,
FIG. 3 is a view taken along the line AA of FIG.

The ink jet head 10 of this embodiment is
It has a laminated structure in which three substrates 1, 2, 3 having a structure described in detail below are stacked and joined. Intermediate first substrate 1
Is a silicon substrate, and a plurality of nozzle grooves 11 are formed in the surface of the substrate 1 so as to form a plurality of nozzle holes 4 in parallel at equal intervals from one end, and a bottom wall communicating with each nozzle groove 11 is formed. A concave portion 12 which constitutes the ejection chamber 6 which is the vibrating plate 5, and a narrow groove 13 for an ink inlet which constitutes an orifice 7 provided at the rear portion of the concave portion 12,
It has a recess 14 that will form a common ink cavity 8 for supplying ink to each ejection chamber 6.
Further, in the lower part of the vibration plate 5, there is provided a concave portion 15 which constitutes the vibration chamber 9 for mounting electrodes described later. Further, as shown in the cross-sectional view of the diaphragm 5 in the S portion, the substrate 1 is provided with the thermal oxide film 24 (SiO ₂ ) with a uniform thickness. It has a function of electrically insulating these when they come into contact with each other.

In this embodiment, the facing gap between the diaphragm 5 and the electrode arranged opposite thereto, that is, the length G of the gap 16 is the difference between the depth of the recess 15 and the thickness of the electrode. In addition, the space holding means is constituted by the vibration chamber recess 15 formed on the lower surface of the first substrate 1. Here, the depth of the recess 15 is set to 0.3 μm by etching.
I am trying. The pitch of the nozzle groove 11 is 0.72 mm.
And its width is 70 μm.

Pyrex glass (borosilicate glass) is used as the lower second substrate 2 bonded to the lower surface of the first substrate 1, and the vibration chamber 9 is formed by bonding the substrates 2 together. The substrate 2 is formed at each position corresponding to the diaphragm 5 in a shape similar to the shape of the diaphragm ITO (tin (S
n) added indium oxide (In ₂ O ₃ )) 0.1 μm
m is sputtered, and an ITO pattern is formed in substantially the same shape as the vibration plate 5 to form the electrode 21. The electrode 21 has a lead portion 22 and a terminal portion 23.

The upper third substrate 3 bonded to the upper surface of the first substrate 1 is made of Pyrex glass as in the second substrate 2. The nozzle holes 4, the ejection chambers 6, the orifices 7, and the ink cavities 8 are formed by joining the substrates 3. The substrate 3 is provided with an ink supply port 31 communicating with the ink cavity 8. The ink supply port 31 is connected to an ink tank (not shown) via a connection pipe 32 and a tube 33.

Regarding the bonding of the above-mentioned substrates, the first substrate 1 and the second substrate 2 are anodically bonded at a temperature of 300 ° C. and a voltage of 500 V is applied, and the first substrate 1 and the third substrate are bonded under the same conditions. The substrates 3 are joined and the inkjet head is assembled as shown in FIG.

FIG. 4 mainly shows the diaphragm 5 and the electrodes 2 in FIG.
FIG. 3 is a detailed cross-sectional view of the actuator unit T formed of 1. The gap distance 16 between the diaphragm 5 and the electrode 21 after the anodic bonding is 0.
2 μm, and on the surface of the diaphragm 5 facing the electrode 21,
An insulating film 24 having a thickness of about 0.1 μm is formed. The insulating film 24 is a thermal oxide film of silicon, and the vibrating plate 5 is the electrode 21.
When it comes into contact with, the short circuit is prevented, and the electrode 21 and the diaphragm 5 are prevented from being broken. The insulating film may be formed only on the surface facing the electrode 21, or may be formed on the entire surface of the first substrate 1.

A nitride film may be formed instead of the oxide film. The silicon oxide film can be formed relatively easily and stably by thermal oxidation of silicon at a relatively low temperature of about 900 ° C. The nitride film is formed by heating silicon in a nitrogen atmosphere. However, since it has a higher dielectric constant than the oxygen film, even if it is formed to be relatively thick (about twice), the film between the diaphragm and the electrode is It is possible to prevent a decrease in electric field strength, that is, an increase in drive voltage. Furthermore, if a material having a high electric field strength such as MgO (magnesium oxide or magnesia) is formed as a substitute for the oxide film by sputtering or the like and is used as an insulating film, the durability (lifetime) of the inkjet head is further improved. Is possible.

Further, referring to the thicknesses of the above-mentioned insulating films, the thickness of these insulating films is preferably about 500 to 3000 angstroms. When the thickness is 500 angstroms or less, the Fowler-Nordheim tunnel current is generated, and the durability of the insulating film is significantly reduced. On the contrary, when the thickness is 3000 angstroms or more, the electric field strength between the diaphragm and the electrode is lowered. That is, the driving voltage increases.

The material of the electrode 21 is, in addition to ITO, an FTO film containing tin oxide (SnO ₂ ) as a main component, and AZO containing zinc oxide (ZnO) containing aluminum as a component.
A conductive oxide film containing CdSnO ₃ , Cd ₂ SnO ₄ , or CdIdO ₄ as a component may be formed in addition to the film, but the ITO film has the lowest volume resistivity and is preferable from the viewpoint of safety.

Further, since these oxide films have a property of transmitting light according to the oxygen content, visual detection, optical detection means such as an optical sensor, etc. are used to form a diaphragm, a flow path, etc. There is also an effect that it is easy to detect defects such as holes, distortions, omissions, and foreign matter inclusion, and it is easy to detect the presence or absence of ink in the flow path and the entry of bubbles, foreign matter, and the like even during use.

Here, referring to the resistivity of the electrode 21, it is preferable that the resistivity of the electrode 21 is 5 × 10 ⁻⁴ Ωcm or less. For example, when a high-density wiring having a thickness of about 1000 Å, a pattern width of 80 μm and a wiring length of about 1 cm is made of a material having a resistivity of 5 × 10 ⁻⁴ Ωcm or more, the wiring resistance is 6. If the resistance is 25 Ω or higher, the time constant determined by the wiring resistance and the capacitance formed by the diaphragm-opposite electrode is increased in the case of a resistance higher than 50 Ω, which is usually used for driving a printer.
This is because it is impossible to obtain sufficient deformation of the vibration plate to eject ink at a voltage of V or less.

After the ink jet head has been assembled as described above, the drive circuit 40 is connected between the substrate 1 and the terminal portion 23 of the electrode 21 by the wiring 49 to form an ink jet recording apparatus.

The ink 103 is supplied from an ink tank (not shown) to the inside of the substrate 1 through the ink supply port 31, and the so-called ink flow path formed by the ink cavity 8, the ejection chamber 6, the nozzle groove 11 and the like is filled with the ink 103. To be done.

In FIG. 2, 104 is an ink droplet ejected from the nozzle hole 4, and 105 is a recording paper. And
The basic mechanism of ejection of the ink droplet 104 will be described as follows. When a suitable voltage is applied to the electrode 21 and the surface of the electrode 21 is positively charged, the lower surface of the corresponding diaphragm 5 is negatively charged. Therefore, the diaphragm 5
Bends downward due to the attraction of static electricity. Next, electrode 2
When 1 is turned off, the diaphragm 5 is restored by the elasticity of the diaphragm itself. Therefore, the pressure in the ejection chamber 6 rapidly rises, and the ink droplets 104 are ejected from the nozzle holes 4 toward the recording paper 105. Then, when the vibration plate 5 is bent again downward, the ink 103 is replenished from the ink cavity 8 into the ejection chamber 6 through the orifice 7.

5 and 6 are cross-sectional side views showing a state in which a voltage is applied to the electrode 21 and the diaphragm 5 is bent downward due to the electrostatic attraction. FIG. 5 shows a method of driving the diaphragm 5 by applying a voltage so that the diaphragm 5 does not come into contact with the electrode 21 (hereinafter referred to as non-contact drive), and FIG. 6 shows that the diaphragm 5 and the electrode 21 come into contact with each other. In this case, in each case, in an inkjet head having a plurality of nozzles, ink is ejected from each nozzle by fully charging the same to drive the vibrating plate 5 (hereinafter referred to as contact drive). The amount is
The diaphragm 5 is most flexed, that is, the maximum volume change amount dV of the discharge chamber 6 is determined.

In the case of non-contact driving, since the electrical and mechanical stress applied to the electrode 21 and the diaphragm 5 is small, there is an advantage that the head is relatively hard to break, but the shape of the diaphragm 5 such as the thickness is different. Discharge due to factors such as non-uniformity in spring constant of each diaphragm 5, voltage of energization pulse applied to the electrode 21 and diaphragm 5, and a slight difference in energization width, resulting in non-uniformity in charge amount to be charged. It is very difficult to make the maximum volume change amount dV of the chamber 6, that is, the amount of ink ejected from each nozzle constant, and therefore it is also difficult to make dot dropouts and dot diameters in a recorded image uniform. Further, when the diaphragm 5 is driven continuously for a long time, the diaphragm 5
A residual electric field is gradually formed therein, and even if constant energy (voltage, energization width) is constantly applied to the diaphragm 5, the ink ejection amount changes with time.

On the other hand, in the contact drive, the electric and mechanical stress applied to the electrode 21 and the vibration plate 5 is larger than that in the non-contact drive, so that the head is easily broken. Since a voltage equal to or higher than the voltage for contacting the electrode 21 (contact voltage) is applied, the discharge amount of ink discharged from each nozzle per time, that is, the maximum volume change amount dV of the discharge chamber 6, is determined by the gap length. Since it is uniquely determined by G, if the gap length G and the thickness of the diaphragm are processed with high precision, the ink ejection amount will be stable and good print quality will be obtained.

FIG. 7 shows a second substrate 2 made of Pyrex glass (borosilicate glass) and a layer 2 on which chromium is undercoated.
6 is a detailed cross-sectional view of an actuator portion A in which an individual electrode 25 made of gold is formed on 6 of FIG. As shown in FIGS. 4 and 7, the inkjet head 10 having the actuator unit A was driven by contact driving at a driving voltage of 41 V and a driving cycle of 3 kHz, and the durability was compared and tested. As a result, the individual electrodes shown in FIG. The one formed of ITO did not change the performance of the inkjet head 10 even if a pulse of 200 million pulses or more was applied, whereas the individual electrode shown in FIG. 7 formed of gold has 13 million pulses. Then, the individual electrode 25 and the diaphragm 5 are short-circuited, and the inkjet head 10
Stopped working. In this way, a significant difference in the durability of the material of the individual electrodes occurred because the oxide having conductivity with respect to the metal is molecularly stable and has a high hardness and is resistant to electrical / mechanical stress. It is suitable as an electrode of the actuator having the above-described structure, the contact drive can be practically used, and the advantages of the contact drive described above can be obtained.

A more detailed operation of the present embodiment configured as described above will be described.

FIG. 8 is a block diagram showing a method of driving the ink jet head of the present invention, FIG. 9 is its timing chart, and is an example applied to a serial type printer as shown in FIG.

In FIG. 10, the ink jet head 1
0 is mounted on the carriage 302, and the carriage 302
Is moved in the digit direction by carriage driving means 310 such as a stepping motor, and ink is appropriately ejected from the nozzle row of the inkjet head 10 in synchronization with this movement.
Characters and the like are formed on the recording paper 105. In addition, 300 is a platen, 301 is an ink tank, 302 is a carriage of the inkjet head 10, 303 is a pump, 304 is a cap, 305 is a waste ink reservoir, 306 is an ink supply tube, 307 is an ink discharge tube, and 308 is an ejection port discharge. It is a tube.

In FIG. 8, in synchronization with the reference signal S1 output from the timing circuit 61, the carriage 302 is sent by the carriage drive circuit 62 one dot at a period T in the digit direction. The switching circuit 63 uses the reference signal S
In synchronism with 1, the three-phase switching element 65 composed of a transistor or the like is connected to the charging circuit 66 and the electrode 2 at time t0.
Switch so that 1 connects. The timer 64 measures the charging time, the preset electrode 21 and the diaphragm 5 come into contact with each other, and the switching circuit 63 causes the switching element 65 to discharge the discharging circuit 67 at time t1 when sufficient suction for ink ejection can be performed.
And the electrode 21 are connected to each other, and the electric charge accumulated between the vibrating plate 5 and the electrode 21 is rapidly discharged to eject ink. At time t2 when a period T has elapsed from time t0, the switching circuit 63 switches the switching element 65 so that the charging circuit 66 and the electrode 21 are connected again in synchronization with the reference signal S1, and charging is started. Repeat the above cycle until finished.

FIG. 11 is a drive circuit diagram for electrically controlling the diaphragm 5, and FIG. 12 is an input signal 5 to this drive circuit.
1, 52 and the voltage waveform 53 between the diaphragm 5 and the electrode 21, and the meniscus 10 of the ink 103 formed at the tip of the nozzle 4.
FIG. 13 shows the vibration of No. 2 and the state of each stage in the vicinity of the discharge chamber 6 when the diaphragm 5 is driven.

Before time t0, that is, in the standby state, since the transistors 42 and 45 are both turned off, the vibration plate 5-
No voltage is applied between the electrodes 21, so that the vibrating plate 5 is not displaced and the state of the ejection chamber 6 is a state in which no pressure is applied to the ink 103 as shown in FIG.

At time t0, the transistor 41 is turned on by the rise of the charging signal 51 and the transistor 42 is turned on.
Also, since the voltage is applied between the vibrating plate 5 and the electrode 21, a current flows in the direction of the arrow A, and the static charge acting between the vibrating plate 5 and the electrode 21 is caused by the electric charge charged between the vibrating plate 5 and the electrode 21. Due to the electric attractive force, the diaphragm 5 is connected to the electrode 21 and the diaphragm 5 is connected to the electrode 2.
The ink 103 is attracted until it comes into contact with No. 1, the volume of the ejection chamber 6 increases as shown in FIG. 13B, and the ink 103 near the ejection chamber 6 is attracted in the direction of the arrow. At this time, the diaphragm 5
The voltage 53 between the electrodes 21 changes as shown in part C of FIG. 12 according to the time constant determined from the resistance 43 and the capacitance of the head itself, and the meniscus 102 changes with the displacement of the diaphragm.
Changes like the part E in FIG.

At time t1, the charging signal 51 is turned off.
At the same time, the discharge signal 52 is turned on. At this time, the transistor 41 is turned off and the transistor 42 is turned off, so that the charging between the diaphragm 5 and the electrode 21 is stopped, while the transistor 44 is turned off and the transistor 45 is turned on. The electric charge stored in
6 is discharged in the direction of arrow B. Resistor 46 is resistor 4
Since it is set to be considerably smaller than 3 and the time constant at the time of discharging is small, the battery is discharged in a short time as compared with charging as shown in part D of FIG. At this time, the vibrating plate is released from the electrostatic attraction all at once, and as shown in FIG. 13C, the vibrating plate 5 returns to the standby position by the elastic force of the vibrating plate 5 itself, and suddenly presses the ejection chamber 6, thereby Ink droplets 104 due to the pressure generated on
Is discharged from the nozzle 4. Also, at this time, meniscus 1
The displacement 54 of 02 changes as shown in F part of FIG. 12, and is ejected at a point where the ejection pressure is larger than the force of drawing back into the nozzle 4 due to the viscosity and surface tension of the ink 103. Causes damped oscillation with period τ. Incidentally, FIG.
In FIG. 1, the resistor 47 has a significantly larger value than the resistors 43 and 46, has almost no effect on charge / discharge when the head is driven, and when the power is turned on, the diaphragm 5 and the electrode 2 are not affected.
It is a resistance that slowly releases the initially accumulated electric charge during 1 period. Further, as described above, when discharging the head at the same time as the charging is stopped, only one input signal is required, and the charging signal 51 starts charging at the rising edge and stops charging at the falling edge and starts discharging at the falling edge. The circuit may be simplified.

FIG. 11 shows the ink jet head 10 of the present application.
When a printing test was conducted using the described drive circuit, a drive voltage of 3
0V, drive cycle T 3kHz, resistor 43 50Ω ~ 5k
Ω, pulse width of charging signal 51 15 μsec to 45 μse
Stable ejection could be confirmed under the driving condition of c.

As described above, by applying the electric pulse so as to attract the vibration plate 5 until it comes into contact with the electrode 21, almost the same amount of ink droplets is ejected from each nozzle, so that stable printing quality is obtained. Is obtained.

Next, another embodiment of the present invention will be described.

At time t1 in FIG. 12, the charging signal 5
When only 1 is turned OFF and the discharge signal 52 is kept OFF, the displacement 54 of the meniscus 102 is damped and oscillated as shown by the broken line in FIG. The period τ of these damping vibrations is caused by the viscous resistance of the flow path, the viscosity of the ink, etc., and the suction of the ink is around t2, which is ¼ of the period τ of the damping vibrations, and preferably slightly longer than the quarter period t2. When the ink is pressurized at the same time as when the suction is completed and when the suction is completed, the vibration energy of the ink system can be effectively used for ejecting the ink, and the ink can be ejected at the most efficient and low voltage.

FIG. 14 is a timing chart showing a driving method in the driving circuit of FIG. 11 in which the above-described damping vibration of the ink system is added, and the discharging state is started after the charging state is held for a certain period after the charging is completed. From time t0 to t1, the electric charge charged between the diaphragm 5 and the electrode 21 causes the electrostatic attraction acting between the diaphragm 5 and the electrode 21 to bring the diaphragm 5 into contact with the electrode 21 and then charge at the time t1. OF signal 51
When only F is performed and the discharge signal 52 is kept off, the transistor 41 is turned off and the transistor 42 is turned off, so that the charging between the diaphragm 5 and the electrode 21 is stopped, while the transistor 44 is in the on state. Since the 45 is kept in the ON state, the vibration plate 5-electrode 21
In the meantime, neither charging nor discharging is performed, and the diaphragm 5 is kept in contact with the electrode 21 by the electric charge accumulated from time t0 to t1.

Next, 1 / of the period τ of the damping vibration of the ink system
4, the discharge signal 52 is turned on at a time t3 near 4, preferably at a time t3 slightly after the quarter cycle t2. At this time, the transistor 44 is turned off and the transistor 45 is turned on, so that the electric charge accumulated between the diaphragm 5 and the electrode 21 is discharged in the direction of arrow B in FIG. As described above, the resistor 46 is set to be considerably smaller than the resistor 43, and the time constant during discharging is small, so that the resistor 46 is discharged in a short time compared to charging. At this time, the vibrating plate is released all at once from the electrostatic attractive force, returns to the standby position by the elastic force of the vibrating plate 5 itself, and suddenly presses the ejection chamber 6, and the pressure generated in the ejection chamber 6 attenuates the ink system. The ink droplet 104 is ejected from the nozzle 4 in synchronization with the vibration.

According to the above driving method, the vibrating plate 5 contacts the electrode 21 because the ink system vibrates after the vibrating plate 5 vibrates due to viscous resistance of the ink flow path, inertance, ink viscosity and the like. Time t, which is a little after time t1
If the ink droplets are ejected in 3, the vibration energy of the ink system can be used most efficiently for ejecting the ink. Furthermore, for this purpose, the diaphragm 5-electrode 21 is maintained until time t3.
Although it may be charged during the period, by the time t2, the vibrating plate 5 is bent enough to eject ink droplets.
Essentially, further charging is unnecessary, and there is a concern that the diaphragm 5 and the electrode 21 may be short-circuited or broken due to overcharging.

Therefore, if the ink droplets are ejected by using the driving method of starting the discharge after stopping the charging operation for a certain period from the time t1 when the diaphragm 5 contacts the electrode 21 to the time t3, It is possible to prevent overcharge between the vibration plate 5 and the electrode 21, prevent damage to the head itself, and effectively use the vibration energy of the ink system for ejecting ink, which is the most efficient and ejects ink at a low voltage. it can.

Next, the speed of attracting the diaphragm 5 to the electrode 21 will be described in detail.

FIG. 15 is a graph showing changes in the amount of charge between the electrode 21 and the diaphragm 5 during charging.

In the drive circuit shown in FIG. 11, when the value of the resistor 43 that determines the time constant during charging is small, the change in the charge amount changes as shown by the line 91, and when the value of the resistor 43 is large, the change in the charge amount changes. Changes as indicated by line 93.

If the change in the amount of charge changes slowly as shown by the line 93, sufficient charge for ink suction cannot be obtained at time t1 and therefore the change in volume of the discharge chamber 6 for sufficient ink discharge cannot be obtained. However, if the ink is ejected in this state, the print quality is significantly deteriorated. Therefore, in order to obtain a sufficient charge, the charge pulse width must be extended until time t2, which impairs the responsiveness of the head and makes it impossible to obtain a desired printing speed. According to the values confirmed by the experiment and calculation, if the electric charge q1 of about 90% of the capacity of the capacitor composed of the diaphragm 5-electrode 21 is charged, sufficient ink suction for ejection is performed. Therefore, the time constant of the charging circuit may be 1/2 or less of the charging pulse width P1.

When the change in the amount of electric charge changes rapidly as shown by the line 91, the ink in the flow path is attracted to the vicinities of the vibrating plate, as shown in FIG. Due to the pressure (negative pressure), the air bubble 106 enters from the nozzle communicating with the flow path, or nitrogen or the like existing in the ink dissolved therein is rapidly vibrated in the flow path to cause the air bubble 106 to flow.
And appear in the flow path, the electric charge stored between the diaphragm and the electrode is discharged, and even if the volume of the flow path is returned to the standby state, the pressure acting in the direction of pushing out the generated ink is absorbed by the bubble 106. However, a situation occurs where ink is not ejected. Therefore, it is also necessary to specify the lower limit of the time constant of the charging circuit. According to the value confirmed in the experiment, when the ink containing nitrogen contained in the ink is degassed and the ink is not easily bubbled, when a negative pressure of 2 × 10 ⁵ Pascal or more is applied, bubbles are generated in the ink flow path. Occurs and ink cannot be ejected.

In another experiment, the head applied voltage 30
V, charging pulse width P1 is 15 μsec to 45 μsec,
When the resistance 43 was set to a circuit condition of 50Ω, sufficiently stable ejection was obtained. At this time, the capacitance of the capacitor composed of the electrode 21 and the diaphragm 5 was measured to be about 270.
When the time constant is calculated at about pF, it is 0.135 μsec.
It will be about.

Therefore, in the method of driving the ink jet head of the present invention, the upper limit of the time constant of the charging circuit is specified to be 1/2 or less of the charging pulse width, and the lower limit is 2 × 10 ⁵ negative pressure applied to the ink during ink suction. Stable ink ejection can be obtained by defining the ink to be equal to or less than Pascal.

[0059]

According to the above-described ink jet head of the present invention, when an electric pulse is applied to the electrode, an attractive force or a repulsive force due to an electrostatic force is exerted between the electrode and the vibration plate arranged so as to face the electrode. In an inkjet head that vibrates the vibrating plate by the electrostatic force and the elastic force of the vibrating plate itself to eject ink droplets from the nozzle, an insulating film such as an oxide film or a nitride film is formed on the surface of the vibrating plate facing the electrodes. Since the diaphragm is made of a semiconductor such as silicon and contacts the electrode through the oxide film that is an insulator, the head is not destroyed by a short circuit, and the material of the electrode is ITO.
Since it is an oxide conductor such as, the breakdown voltage is excellent, and the head is less likely to be damaged even during long-term driving.

For this reason, it is possible to drive (contact drive) the diaphragm until it comes into contact with the electrode, and the amount of ink discharged from each nozzle at one time is opposite to that of the diaphragm and the electrode. Since the interval is uniquely determined, the ejection amount of each nozzle is constant, and stable printing quality is obtained.

Further, even if the vibration plate and the electrode come into contact with each other, the head is not broken by a short circuit, so that the gap between the vibration plate and the electrode can be formed small, and the driving can be performed at a relatively low voltage.

[Brief description of drawings]

FIG. 1 is an exploded perspective view showing an inkjet head of the present invention.

FIG. 2 is a sectional side view of FIG.

FIG. 3 is a view taken along the line AA of FIG.

FIG. 4 is a detailed sectional view of the diaphragm / electrode portion of FIG.

FIG. 5 is a cross-sectional side view showing a state in which the diaphragm is bent downward by an electrostatic attraction action.

FIG. 6 is a cross-sectional side view showing a state in which the diaphragm is bent downward by an electrostatic attraction action.

FIG. 7 is a detailed cross-sectional view of a conventional diaphragm / electrode portion.

FIG. 8 is a block diagram showing a method for driving an inkjet head of the present invention.

9 is a timing chart showing an input signal to the drive circuit of FIG. 8 and a voltage waveform of a diaphragm-electrode.

FIG. 10 is a schematic view showing a printer incorporating the inkjet head of the present invention.

FIG. 11 is a drive circuit diagram of the inkjet head of the present invention.

12 is a timing chart showing the input signal to the drive circuit of FIG. 11, the voltage waveform of the diaphragm-electrode, and the vibration of the meniscus of the ink formed at the nozzle tip.

FIG. 13 is a sectional side view showing the operating principle of the inkjet head of the present invention.

14 is a timing chart in another example showing the input signal to the drive circuit of FIG. 8 and the voltage waveform of the diaphragm-electrode.

FIG. 15 is a graph showing a voltage waveform of a diaphragm-electrode of the inkjet head of the present invention.

FIG. 16 is a cross-sectional side view of an inkjet head in which bubbles are generated in a flow path.

[Explanation of symbols]

1 1st substrate 2 2nd substrate 3 3rd substrate 4 Nozzle hole 5 Vibration plate 6 Discharge chamber 9 Vibration chamber 10 Inkjet head 21 Electrode 24 Thermal oxide film 41, 42, 44, 45 Transistor 43, 46, 47 Resistance 51 charge signal 52 discharge signal 53, V ₁ electrode-vibration plate voltage

Front page continuation (72) Inventor Tadaaki Hakata 3-5 Yamato, Suwa, Nagano Seiko Epson Corporation (72) Inventor Shuji Koeda 3-5 Yamato, Suwa, Nagano Seiko Epson Stock In the company

Claims (9)

[Claims] 1. A vibrating plate having a nozzle, an ink flow path communicating with the nozzle, a vibrating plate provided in a part of the flow path, and an electrode provided so as to face the vibrating plate. In an inkjet head that applies an electric pulse between a plate and the electrode to deform the vibrating plate by electrostatic force and ejects ink droplets from the nozzle toward recording paper to perform printing, the vibrating plate faces the electrode. An inkjet head, comprising a semiconductor having an insulating film formed on a surface thereof, and the electrode comprising an oxide conductor. 2. The ink jet head according to claim 1, wherein the oxide conductor is a transparent conductive film. 3. The oxide conductor is indium, tin,
The inkjet head according to claim 1, wherein the inkjet head is made of an oxide of zinc or cadmium, or a composite of two or more of the oxides. 4. The ink jet head according to claim 1, wherein the oxide conductor comprises an ITO film. 5. The resistivity of the oxide conductor is 5 × 10 ⁻⁴
The inkjet head according to claim 1, which has an Ωcm or less. 6. The ink jet head according to claim 1, wherein the insulating film is an oxide film. 7. The inkjet head according to claim 1, wherein the insulating film is a nitride film. 8. The ink jet head according to claim 1, wherein the insulating film is made of magnesium oxide. 9. The ink jet head according to claim 1, wherein the insulating film has a thickness of 500 to 3000 angstroms.