Inkjet printhead and method of removing bubbles in the same

An inkjet printhead includes an ink flow channel including a pressure chamber, a nozzle to communicate with the pressure chamber, an actuator to provide a driving force to eject ink from the pressure chamber, and a plurality of electrodes, a lower voltage is applied to an electrode closer to the nozzle as compared to an electrode farther from the nozzle to form a non-uniform electric field in the ink flow channel, and a method of removing bubbles in the inkjet printhead.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(a) from Korean Patent Application No. 10-2006-0014247, filed on Feb. 14, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present general inventive concept relates to an inkjet printhead to remove bubbles and a bubble removing method of the same.

2. Description of the Related Art

Generally, inkjet printheads are devices for printing a color image on a printing medium by firing droplets of ink onto a desired region of the printing medium. Depending on the ink ejecting method, the inkjet printheads can be classified into two types: thermal inkjet printheads and piezoelectric inkjet printheads. The thermal inkjet printhead generates bubbles in ink to be ejected by using heat, and ejects the ink using an expansion of the bubbles. On the other hand, the piezoelectric inkjet printhead ejects ink using a pressure generated by deforming a piezoelectric material.

An ink flow channel in the printhead, in particular, a pressure chamber, should be filled with ink. Air flows through a nozzle of the printhead during printing, and the air and other gases dissolved in the ink grow into bubbles due to a temperature rise or other factors. The bubbles existing in the ink flow channel in the printhead, in particular, in the pressure chamber, degrades an ejection performance of the printhead. Also, as the temperature increases, the bubbles expand. This upsets a pressure balance of the ink in the printhead, which may cause the ink to leak through the nozzle.

In order to remove the bubbles, a method of forcibly sucking the ink through the nozzle using a vacuum pump has been used. However, the bubbles in a corner of the ink flow channel, in particular, in the pressure chamber, are not easily removed even using this conventional forcible sucking method.

SUMMARY OF THE INVENTION

The present general inventive concept provides an inkjet printhead in which bubbles can be removed and a bubble removing method of the same.

The foregoing and/or other aspects and utilities of the present general inventive concept may be achieved by providing an inkjet printhead, including an ink flow channel including a pressure chamber to contain ink, a nozzle to communicate with the pressure chamber, an actuator to provide a driving force to eject the ink from the pressure chamber, and a plurality of electrodes to receive voltages to form a non-uniform electric field in the ink flow channel such that an electrode closer to the nozzle receives a lower voltage relative to an electrode farther from the nozzle.

The voltages may be variable-frequency, traveling-pulse voltages.

The plurality of electrodes may be disposed on walls forming the ink flow channel.

The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a method of removing bubbles in an inkjet printhead including an ink flow channel having a pressure chamber to contain ink, a nozzle to communicate with the pressure chamber, an actuator to provide an ink ejecting force to the pressure chamber, and a plurality of electrodes disposed in the ink flow channel, the method including applying voltages to the plurality of electrodes such that a lower voltage is applied to an electrode closer to the nozzle relative to an electrode farther from the nozzle, moving bubbles to the nozzle by a non-uniform electric field formed by the plurality of electrodes and dielectrophoresis generated by dipole moments of the bubbles, and discharging the bubbles through the nozzle.

The discharging of the bubbles may include discharging the bubbles together with the ink through the nozzle.

The discharging of the bubbles may include applying a negative pressure to the nozzle to suck the bubbles out of the pressure chamber through the nozzle.

The voltages applied to the plurality of electrodes may be variable-frequency, traveling-pulse voltages to accelerate the bubbles.

The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a printhead, including a chamber layer including an ink chamber to contain ink, a nozzle layer disposed on the chamber layer and including a nozzle in communication with the ink chamber to eject the ink from the ink chamber, an actuator to provide a driving force to eject the ink from the ink chamber through the nozzle, and a plurality of electrodes to move bubbles in the ink contained in the ink chamber to the nozzle.

The plurality of electrodes may move the bubbles in the ink to the nozzle using dielectrophoresis. The plurality of electrodes may generate a non-uniform electric field in the ink chamber to dielectrically polarize the bubbles and applies a force to move the polarized bubbles towards the nozzle.

The plurality of electrodes may be disposed on a wall of the ink chamber. Shapes of at least a portion of the plurality of electrodes may be non-uniform shapes. The non-uniform shapes may include at least one of a flat panel shape extending in a width direction of the ink chamber, and a flat panel shape including branches protruding in a length direction of the ink chamber. The printhead may further include a voltage applying unit to apply voltages to the plurality of electrodes. The voltage applying unit may apply a first voltage to a portion of the plurality of electrodes disposed closer to the nozzle, and may apply a second voltage to a portion of the plurality of electrodes disposed farther from the nozzle. The first voltage may be lower than the second voltage. The voltages applied by the voltage applying unit may be variable-frequency traveling pulse voltages to accelerate the movement of the bubbles towards the nozzle.

The actuator may be selected from a thermal actuator and a piezoelectric actuator. The chamber layer may further include a manifold to supply ink to the ink chamber, and a restrictor to restrict a back flow of ink from the ink chamber to the manifold.

The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a method of removing bubbles from a printhead including an ink chamber to contain ink and a nozzle in communication with the pressure chamber to eject the ink, the method including generating a non-uniform electric field in the ink chamber to dielectrically polarize bubbles in the ink and to apply a force to move the polarized bubbles towards the nozzle, and ejecting the bubbles from the ink chamber through the nozzle by applying a negative pressure to the nozzle.

The generation of the non-uniform electric field may include applying voltages to electrodes disposed in the ink chamber. Shapes of at least a portion of the electrodes may be non-uniform shapes. The applying of the voltages to the electrodes may include applying a lower voltage to a portion of the electrodes closer to the nozzle and applying a higher voltage a portion of the electrodes farther from the nozzle to move the polarized bubbles towards the nozzle.

The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a method of removing bubbles from a printhead, including gathering bubbles around a nozzle of the printhead by dielectrophoresis using a plurality of electrodes, and discharging the bubbles gathered around the nozzle through the nozzle by applying a force at the nozzle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures. In the drawings, thicknesses of layers and regions may be exaggerated for clarity. It will also be understood that when a layer or plate is referred to as being “on” another layer or plate, it can be directly on the other layer or plate, or intervening layers or plates may also be present.

FIG. 1is a plan view illustrating an inkjet printhead according to an embodiment of the present general inventive concept,FIG. 2is a cross-sectional view illustrating the inkjet printhead ofFIG. 2, andFIG. 3is a sectional view taken along line A-A′ ofFIG. 2.

Referring toFIGS. 1 and 2, the inkjet printhead includes a flow channel forming plate110where an ink flow channel is formed, and a piezoelectric actuator140to provide a pressure to eject ink. The flow channel forming plate110includes a pressure chamber111, a manifold113to supply ink to the pressure chamber111, and a restrictor112. A nozzle plate120is bonded to the flow channel forming plate110, and a nozzle122is formed on the nozzle plate120to eject ink from the pressure chamber111. A vibrating plate114is disposed on the pressure chamber111, and is deformable by a driving force provided by the piezoelectric actuator140. The flow channel forming plate110and the nozzle plate120define the ink flow channel.

The piezoelectric actuator140is formed on the flow channel forming plate110, and provides the driving force to eject ink to the pressure chamber111. The piezoelectric actuator140includes a lower electrode141, a piezoelectric layer142, and an upper electrode143stacked sequentially on the flow channel forming plate110. The lower electrode141serves as a common electrode, the piezoelectric layer142is deformable by a voltage applied thereto, and the upper electrode143serves as a driving electrode.

The lower electrode141is formed on the flow channel forming plate110including the pressure chamber111. The flow channel forming plate110may be formed of a silicon wafer, and a silicon oxide layer131may be formed between the flow channel forming plate110and the lower electrode141. The lower electrode141is formed of a conductive metal material. The lower electrode141may include one or more metal layers, such as two metal layers. For example, the lower electrode may include Ti and Pt layers. The lower electrode141including a Ti/Pt layer can serve not only as a common electrode, but also as a diffusion barrier layer to prevent inter-diffusion between the piezoelectric layer142and the flow channel forming plate110that are formed respectively on and under the lower electrode141.

The piezoelectric layer142is formed on the lower electrode141and is located to correspond to the pressure chamber111. The piezoelectric142may be formed of a piezoelectric material, such as lead zirconate titannate (PZT) ceramic material.

The upper electrode143is formed on the piezoelectric layer142and serves as the driving electrode to apply a voltage to the piezoelectric layer142. Wiring151of a drive circuit to apply the voltage, for example, a flexible printed circuit150, may be bonded to an upper surface of the upper electrode143.

When a driving voltage is applied to the upper electrode143, the piezoelectric layer142is deformed and the vibrating plate114bends, thereby changing a volume of the pressure chamber111. Therefore, the pressure to eject the ink is generated in the pressure chamber111, and the ink in the pressure chamber111is ejected through the nozzle122.

Air and other gasses dissolved in the ink grow into bubbles by various factors, such as a temperature rise. Also, air flows in the printhead through the nozzle of the printhead during printing. The bubbles in the printhead lower an ejection performance of the printhead. In addition, as the temperature rises, the bubbles expand, which may upset a pressure balance of the ink in the printhead, thereby causing the ink to leak through the nozzle.

Noncharged bubbles are dielectrically polarized in an electric field. A non-uniform electric field provides a force to move the polarized bubbles. A phenomenon where noncharged particles move in the non-uniform electric field is called dielectrophoresis. A main issue in this case is a moving direction. The moving direction depends on a magnitude of a dipole moment by polarization. In the non-uniform electric field, particles having a large dipole moment move toward an electrode to which a high voltage is applied, whereas particles having a small polarization moment move toward an electrode to which a low voltage is applied. The magnitude of the dipole moment depends on a permittivity of the particles. When a permittivity of a vacuum is defined as 1, a permittivity of air that is a main ingredient of the bubbles in the ink is approximately 1.0005. Also, a permittivity of water is approximately 80, and a permittivity of the ink used to print is approximately 10 to 80. Therefore, the permittivity of the ink is generally higher than that of the bubbles, and the bubbles move towards electrodes to which a lower voltage is applied in the non-uniform electric field. A force (f) applied to the bubbles by the non-uniform electric field can be expressed by Equation 1.

f=2⁢π⁢⁢r3⁢ɛm⁢Re⁡[ɛp-ɛmɛp+2⁢ɛm]⁢∇E2(1)
where εpindicates the permittivity of the bubbles, εmindicates the permittivity of the ink, r indicates a radius of bubbles when the bubbles are considered to have a spherical shape, and Re indicates a real component of

As described above, in order to remove the bubbles using dielectrophoresis, the inkjet printhead according to the present embodiment includes a plurality of electrodes170to form a non-uniform electric field in the ink flow channel, as illustrated inFIGS. 2 and 3. The electrodes170are disposed on a bottom111aof the pressure chamber111facing the piezoelectric actuator140. An insulating layer160may be included to insulate the electrodes170from the flow channel forming plate110and the ink in the pressure chamber111, as illustrated inFIG. 2. However, the insulating layer160may be omitted, as illustrated inFIG. 3. A voltage applying unit180applies a voltage to the electrodes170. The electrodes170may have non-uniform features in order to form the non-uniform electric field. For example, the electrodes170aand170chave a flat panel shape extending in a width direction of the pressure chamber111, and the electrodes170band170dhave a flat panel shape which includes branches protruding in a length direction of the pressure chamber111. Therefore, the non-uniform electric field is formed between the electrodes170. The shape of the electrodes170, a number thereof and an arrangement thereof, is not limited to the example illustrated inFIG. 3. For example, the shape of the electrodes170is not limit to the flat panel shape extending in the width direction of the pressure chamber111and the flat panel shape which includes the branches protruding in the length direction of the pressure chamber111. Furthermore, althoughFIG. 3illustrates pairs of electrodes170in which the electrodes170of each pair have the same shape, the present general inventive concept is not so limited. For example, each of the electrodes170may have a different shape, or more than two of the electrodes170may have the same shape.

The bubbles move towards the electrode(s)170(i.e.,170a-170d) to which a low voltage is applied. The bubbles may be discharged together with the ink by ejecting the ink using the piezoelectric actuator140after moving the bubbles around the nozzle122. Therefore, in applying the voltages to the electrodes170, a higher voltage is applied to a first portion of the electrodes170disposed farther from the nozzle122, whereas a lower voltage is applied to a second portion of the electrodes170disposed closer to the nozzle122. For example, inFIG. 3, a highest voltage is applied to the electrode170a, whereas a lowest voltage is applied to the electrode170d. The number of the electrodes170is not limited. The voltage applying unit180applies the voltages to the electrodes170through the terminals171of the electrodes170.

A method of removing bubbles in a printhead having the aforementioned structure will now be described. When voltages are applied to the electrodes170, a non-uniform electric field is formed between the electrodes170. A force defined by Equation 1 is applied to the bubbles by a dipole moment generated by polarization of the bubbles and by a slope of the non-uniform electric field. The bubbles that have a smaller permittivity than ink move towards a first portion of the electrodes170to which a low voltage is applied. For example, referring toFIGS. 3 and 4, the bubbles sequentially move from the electrode170ato the electrode170dto gather around the nozzle122, as denoted by arrows inFIG. 4. Next, a driving voltage is applied to the upper electrode143through the wires (lines)151of the flexible printed circuit150to eject the ink. Then, the bubbles gathered around the nozzle122are discharged together with the ink through the nozzle122.

Conventionally, negative pressure is provided through a nozzle to forcibly suck bubbles as well as ink through the nozzle. In general, bubbles existing near walls of an inkjet printhead or in a corner of an ink flow channel (a portion denoted by “a” inFIG. 2) of the inkjet printhead are not easily removed with the conventional method. Such bubbles have a great effect on a driving performance of a piezoelectric actuator or a thermal actuator. However, in an inkjet printhead and a method of removing bubbles in the inkjet printhead according to embodiments of the present general inventive concept, since bubbles gather around the nozzle122using dielectrophoresis, the bubbles existing near walls or in a corner of the ink flow channel can be easily removed. Accordingly, a decrease in an ejection speed of ink droplets by the bubbles, a non-uniformity of a volume of the ink droplets, a lowering of an ejection frequency, etc., can be prevented. Also, since the bubbles are discharged by ejecting ink after gathering the bubbles around the nozzle122, an amount of ink consumed to remove the bubbles can be significantly reduced.

As discussed above, and as illustrated inFIG. 3, the electrodes170may be disposed on the bottom111aof the pressure chamber111, but the scope of the present general inventive concept is not limited to this. The electrodes170can be disposed on any walls forming the pressure chamber111, as well as a sidewall111bof the pressure chamber111. However, the electrodes170should not be disposed on a ceiling111cof the pressure chamber111because a piezoelectric actuator140is disposed on the pressure chamber111. Also, the electrodes170may extend toward the restrictor112.

As illustrated inFIG. 5, the nozzle122is capped with a nozzle cap191after gathering the bubbles around the nozzle122by applying the voltages to the electrodes170, and then the bubbles as well as the ink can be sucked out through the nozzle122using a negative pressure providing unit190. The negative pressure providing unit190may be, for example, a vacuum pump. In this case, since most of the bubbles have already gathered around the nozzle122, an amount of the sucked ink can be significantly reduced in comparison to the conventional method of removing the bubbles by suction. Since the negative pressure f to suck the ink and the bubbles can be also lowered, a risk of damaging the ink flow channel due to an excessive negative pressure can be reduced.

Also, a variable-frequency, traveling-pulse voltage may be applied to the electrodes170. Therefore, the bubbles can move around the nozzle122more quickly by accelerating the bubbles moving towards a portion of the electrodes170to which a low voltage is applied.

The structure of the flow channel forming plate110, the nozzle plate120, and the piezoelectric actuator140illustrated inFIGS. 1 and 2is only an example. Therefore, the ink flow channel can be formed in the inkjet printhead to have various structures, and can be formed using a plurality of plates, such as more than two plates (i.e., the flow channel forming plate110and the nozzle plate120illustrated inFIG. 1). Also, the structure of the piezoelectric actuator140and the structure to connect the piezoelectric actuator140with the drive circuit to apply a voltage may be modified. That is, the present general inventive concept is limited to the structure of the ink flow channel, the ink ejecting method, etc., illustrated inFIGS. 1-5.

A method of removing the bubbles using a plurality of electrodes can also be applied to a thermal inkjet printhead employing a thermal actuator that generates bubbles in a pressure chamber using heat and ejects ink by expansion of the bubbles, in addition to being applied to a piezoelectric actuator.

An inkjet printhead and a method of removing the bubbles therein according to embodiments of the present general inventive concept have at least the following advantages.

Since the bubbles are gathered around a nozzle by dielectrophoresis using a plurality of electrodes, the bubbles existing around walls and in a corner of an ink flow channel can be easily removed. Therefore, an optimum ejection performance of the printhead can be maintained. Furthermore, since the bubbles are gathered around the nozzle and then are discharged through the nozzle, an amount of ink consumed to remove the bubbles can be significantly reduced. In addition, a voltage of a variable-frequency traveling pulse may be applied to the plurality of electrodes to accelerate the bubbles, allowing the bubbles to move more quickly around the nozzle.