Ink ejecting method and ink-jet printhead utilizing the method

A method of ejecting ink from a ink-jet printhead includes filling a rear end of a nozzle with ink using a capillary force, the rear end of the nozzle being surrounded by a hydrophilic layer, forming an electric field directed toward an outlet of the nozzle on a front end of the nozzle, the front end of the nozzle being surrounded by a hydrophobic layer, varying a surface tension of ink to separate ink droplets having a predetermined volume from ink and to move the separated ink droplets within the front end of the nozzle toward the outlet of the nozzle, and ejecting the separated ink droplets through the outlet of the nozzle.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ink-jet printhead. More particularly, the present invention relates to an ink ejecting method and an ink-jet printhead utilizing the method.

2. Description of the Related Art

Typically, ink-jet printheads are devices for printing a predetermined image, color or black, by ejecting a small volume droplet of printing ink at a desired position on a recording sheet. Ink-jet printheads are largely categorized into two types depending on which ink droplet ejection mechanism is used. A first type is a thermally driven ink-jet printhead in which a heat source is employed to form and expand bubbles in ink causing ink droplets to be ejected. A second type is a piezoelectrically driven ink-jet printhead in which a piezolectric crystal bends to exert pressure on ink causing ink droplets to be ejected.

FIGS. 1A and 1Billustrate examples of a conventional thermally driven ink-jet printhead.FIG. 1Aillustrates a cutaway perspective view of a structure of a conventional ink-jet printhead.FIG. 1Billustrates a cross-sectional view for explaining an ink droplet ejection mechanism of the conventional ink-jet printhead shown inFIG. 1A.

The conventional thermally driven ink-jet printhead shown inFIGS. 1A and 1Bincludes a manifold22provided on a substrate10, an ink channel24and an ink chamber26defined by a barrier wall14installed on the substrate10, a heater12installed in the ink chamber26, and a nozzle16that is provided on a nozzle plate18and through which ink droplets29′ are ejected. When a pulse-shaped current is supplied to the heater12and heat is generated in the heater12, ink29filled in the ink chamber26is heated, and a bubble28is generated. The formed bubble28continuously expands and exerts pressure on the ink29contained within the ink chamber26. This pressure causes the ink droplets29′ to be expelled through the nozzle16. Subsequently, ink29is absorbed from the manifold22into the ink chamber26through the ink channel24, thereby refilling the ink chamber26with ink29.

However, in the thermally driven ink-jet printhead, when ink droplets are ejected due to the expansion of bubbles, a portion of the ink in the ink chamber26flows backward to the manifold22, and an ink refill operation is performed after ink is ejected. Thus, there is a limitation in implementing high printing speed.

Additionally, a variety of ink droplet ejection mechanisms as well as the two above-described ink droplet ejection mechanisms may be used in the ink-jet printhead and include an ink droplet ejection mechanism using an electrostatic force.

FIGS. 2A and 2Billustrate another example of a conventional ink droplet ejection mechanism and schematically show a principle of ink droplet ejection using an electrostatic force.FIG. 3illustrates a schematic cross-sectional view of a conventional ink-jet printhead adopting the ink ejecting method shown inFIGS. 2A and 2B.

Referring toFIG. 2A, an opposite electrode33is disposed to be opposite to a base electrode32, and ink31is supplied between the two electrodes32and33. A DC power source34is connected to the two electrodes32and33. When a voltage is applied from the power source34between the two electrodes32and33, an electrostatic field is formed between the two electrodes32and33. The electrostatic field causes a Coulomb force toward the opposite electrode33that acts on ink31. At the same time, resistance against the Coulomb force acts on ink31due to the surface tension and viscosity of ink31. Accordingly, ink31is not easily ejected to the opposite electrode33. Thus, a very high voltage should be applied between the two electrodes32and33so that ink droplets are separated from the surface of ink31to be ejected. In this case, ejection of ink droplets occurs irregularly and a predetermined portion of ink31is heated locally. More specifically, temperature T1of ink31′ in a region S1increases to be higher than temperature T0of ink31in another region. Then, ink31′ in the region S1expands, and an electrostatic field is condensed on the region S1, and an electric charge is collected in the electrostatic field. As such, a repulsive force, acting between electric charges, and the Coulomb force, caused by the electrostatic field, act on ink31′ in the region S1. Thus, as shown inFIG. 2B, ink droplets are separated from ink31′ in the region S1and move toward the opposite electrode33.

Referring toFIG. 3, a pair of wall members40and41are spaced apart from each other, and ink43is filled therebetween. An exhaust hole44opposite to a recording paper42is provided on one side end of the wall members40and41. A heating element46is installed at an inner side of the wall member41, and electrodes47and48are connected to both ends of the heating element46. A base electrode49for forming an electric field is provided at an inner side of the wall member40. An opposite electrode51is installed at a rear side of the recording paper42. A power source52for applying a voltage is connected to the opposite electrode51, and the base electrode49is grounded. Another power source53is also connected to the both ends of the heating element46. A control unit54for turning on/off the power sources52and53according to an image signal is connected to the power sources52and53.

When a voltage is applied from the power source52between the base electrode49and the opposite electrode51, ink43near the exhaust hole44is affected by the electric field. If a current is simultaneously applied from the power source53to the heating element46, only ink43around the heating element46is ejected to the recording paper42.

In the aforementioned conventional ink-jet printhead for ejecting ink using an electrostatic force, a very high voltage should be applied between two electrodes or ink should be locally heated by an additional heating element so that ink droplets are separated from the surface of ink to be ejected. These requirements increase power consumption. Due to electric charges irregularly collected on the surface of ink, it is very difficult to precisely control the volume and speed of ejected ink droplets. Thus, it is difficult to implement high-resolution printing.

Accordingly, in order to implement a low power consumption ink-jet printhead having high printing speed and high resolution, a new ink droplet ejection mechanism is needed.

SUMMARY OF THE INVENTION

The present invention provides an ink ejecting method by which ink is previously separated from droplets having a predetermined volume in a nozzle and ink droplets are ejected through the nozzle.

The present invention also provides a low power consumption ink-jet printhead having high integration and high resolution utilizing the ink ejecting method.

According to a feature of an embodiment of the present invention, a method of ejecting ink includes (a) filling a rear end of a nozzle with ink using a capillary force, the rear end of the nozzle being surrounded by a hydrophilic layer, (b) forming an electric field directed toward an outlet of the nozzle on a front end of the nozzle, the front end of the nozzle being surrounded by a hydrophobic layer, (c) varying a surface tension of ink to separate ink droplets having a predetermined volume from ink and to move the separated ink droplets within the front end of the nozzle toward the outlet of the nozzle, and (d) ejecting the separated ink droplets through the outlet of the nozzle.

In the method, forming an electric field directed toward the outlet of the nozzle may include sequentially applying a voltage to a plurality of electrode pads, the plurality of electrode pads being disposed on the front end of the nozzle at predetermined intervals in a lengthwise direction of the nozzle. Varying the surface tension of ink may include lowering the surface tension of ink adjacent to one of the plurality of electrode pads to which the voltage is applied so that a contact angle of ink with respect to the hydrophobic layer is reduced.

In the method, forming the electric field and varying the surface tension of ink may include sequentially applying a voltage to a first electrode pad and a second electrode pad of the plurality of electrode pads to move ink within the front end of the nozzle to a position corresponding to a location of the second electrode pad, and cutting off the voltage applied to the first electrode pad to separate the ink droplets from ink.

The method may further include cutting off the voltage applied to the second electrode pad and sequentially applying a voltage to at least one electrode pad of the plurality of electrode pads disposed after the second electrode pad to move the separated ink droplets toward the outlet of the nozzle, after the separation of the ink droplets from ink.

In the method, an area of each of the plurality of electrode pads is variable so that a volume of the ink droplets is adjustable. A moving speed of the separated ink droplets in the front end of the nozzle is adjusted by a time difference during the sequential application of the voltage to the plurality of electrode pads.

The method may further include cutting off the voltage applied to an electrode pad where the ink droplets are located, prior to ejecting the separated ink droplets. In the method, the ejection of the separated ink droplets may be performed by an electrostatic force or by lowering an atmospheric pressure around the outlet of the nozzle.

According to another feature of an embodiment of the present invention, there is provided an ink-jet printhead including a capillary nozzle, having a rear end being surrounded by a hydrophilic layer, a front end being surrounded by a hydrophobic layer, and an outlet, an insulating layer, which is formed at an external surface of the hydrophobic layer along a lengthwise direction of the nozzle, a plurality of electrode pads disposed at an external surface of the insulating layer at predetermined intervals along the lengthwise direction of the nozzle, an opposite electrode disposed at an external surface of the hydrophobic layer and opposite to the plurality of electrode pads, a voltage applying unit, which sequentially applies a voltage to the plurality of electrode pads and forms an electric field directed toward the outlet of the nozzle to separate ink droplets having a predetermined volume from ink and move the separated ink droplets toward the outlet of the nozzle, and a droplets ejecting unit, which ejects the separated ink droplets through the outlet of the nozzle.

In an embodiment of the present invention, the hydrophobic layer may be a porous layer, and the opposite electrode and the separated ink droplets may be electrically connected via porosities of the porous layer.

In another embodiment of the present invention, the ink-jet printhead may further include a plurality of through holes formed in the hydrophobic layer at a location corresponding to the opposite electrode, wherein the opposite electrode and the separated ink droplets are electrically connected via the plurality of through holes.

In yet another embodiment of the present invention, the ink-jet printhead may further include a plurality of probes provided on the opposite electrode, the plurality of probes perforating the hydrophobic layer, wherein the opposite electrode and the separated ink droplets are electrically connected via the plurality of probes.

In the above embodiments, the nozzle may have a rectangular cross-sectional shape or a circular cross-sectional shape. Further, the plurality of electrode pads may be three electrode pads disposed in a line.

The voltage applying unit may include a first power source connected to each of the plurality of electrode pads, and a control unit, which is provided between the first power source and the plurality of electrode pads, the control unit controlling the first power source so that a voltage is sequentially applied from the first power source to the plurality of electrode pads. Alternately, the voltage applying unit may include a plurality of power sources, each of the plurality of power sources being connected to a corresponding one of the plurality of electrode pads.

The droplets ejecting unit may include an external electrode installed to face the outlet of the nozzle, and a second power source for applying a voltage to the external electrode to form an electric field between the nozzle and the external electrode, wherein the separated ink droplets are ejected through the outlet of the nozzle due to an electrostatic force acting on the separated ink droplets.

DETAILED DESCRIPTION OF THE INVENTION

Korean Patent Application No. 2003-2729, filed on Jan. 15, 2003, and entitled: “Ink Ejecting Method and Ink-Jet Printhead Utilizing the Method,” is incorporated by reference herein in its entirety.

FIG. 4illustrates a schematic cross-sectional view in a lengthwise direction of a nozzle of a structure of an ink-jet printhead according to a first embodiment of the present invention.FIG. 5illustrates a cross-sectional view of the nozzle taken along line A-A′ ofFIG. 4. Although only a unit structure of an ink-jet printhead is shown, a plurality of nozzles are disposed in one row or in two or more rows in an ink-jet printhead manufactured in a chip shape.

Referring toFIGS. 4 and 5, the ink-jet printhead according to the first embodiment of the present invention includes a nozzle110through which ink101supplied from an ink reservoir (not shown) is ejected. A hydrophilic layer120surrounds a rear end of the nozzle110. A hydrophobic layer130surrounds a front end of the nozzle110. More specifically, the hydrophilic layer120forms a wall member of the nozzle110in a predetermined distance along a lengthwise direction of the nozzle110from a nozzle inlet112, and the hydrophobic layer130forms a wall member of the nozzle110from the hydrophilic layer120to an outlet114of the nozzle110. Thus, ink101supplied from the ink reservoir may be filled by a capillary force only in a rear end of the nozzle110, which is surrounded by the hydrophilic layer120. Additionally, ink101has conductivity. For example, a nonpolarity solvent is mixed with a pigment having a predetermined polarity to form ink101.

An insulating layer140is formed at an external surface of the hydrophobic layer130along the lengthwise direction of the nozzle110. As shown inFIG. 5, when the nozzle110has a rectangular cross-sectional shape, the insulating layer140may be formed at one side, for example, on a bottom surface of the hydrophobic layer130.

At least two, and preferably three, electrode pads151,152, and153are disposed at a lower external surface of the insulating layer140in a line at predetermined intervals along the lengthwise direction of the nozzle110. Meanwhile, three or more electrode pads may be disposed at the external surface of the insulating layer140. An opposite electrode160is disposed at an external surface, that is, on an upper surface of the hydrophobic layer130opposite to the three electrode pads151,152, and153.

A voltage applying unit for sequentially applying a voltage to the three electrode pads151,152, and153is provided. A first power source170connected to each of the three electrode pads151,152, and153may be used as the voltage applying unit. In this case, a control unit172is provided between the first power source170and the three electrode pads151,152, and153. The control unit172controls the first power source170so that a voltage is sequentially applied from the first power source170to the three electrode pads151,152, and153. For example, a switching unit may be used as the control unit172.

Additionally, a power source may be provided in each of the three electrode pads151,152, and153.

The opposite electrode160is grounded, and ink101filled in the rear end of the nozzle110is grounded. In addition, the hydrophobic layer130may be a porous layer having a plurality of porosities. Thus, as will be described later, ink droplets102separated from ink101may contact the opposite electrode160via the porosities. Accordingly, the separated ink droplets102are electrically connected to the opposite electrode160.

In the ink-jet printhead having the above structure, when a voltage is sequentially applied to the three electrode pads151,152, and153, an electric field is formed in the nozzle110, and the electric field moves toward the outlet114of the nozzle110. As such, the electric field acts on ink101inside the nozzle110, and the ink droplets102are separated from ink101. The separated ink droplets102move toward the outlet114of the nozzle110. This process will be subsequently described in greater detail with reference toFIGS. 10A through 10E.

A droplets ejecting unit for ejecting the ink droplets102through the outlet114of the nozzle110is provided. The droplets ejecting unit may include an external electrode180installed to be opposite to the outlet114of the nozzle110and a second power source190for applying a voltage to the external electrode180. Thus, the ink droplets102may be ejected from the nozzle110to a recording paper P provided at a front side of the external electrode180. The operation of the droplets ejecting unit will be subsequently described in more detail.

FIGS. 6 through 8illustrate a cross-sectional structure of the nozzle according to second through fourth embodiments of the present invention. Like reference numerals fromFIG. 5denote elements having same functions.

Referring toFIG. 6, a hydrophobic layer230surrounding the nozzle110may not be a porous layer, unlike in the first embodiment. In the second embodiment, a plurality of through holes232is formed in a portion where the opposite electrode160is disposed so that the opposite electrode160and the ink droplets102are electrically connected in the nozzle110. Thus, the ink droplets102contact the opposite electrode160via the plurality of through holes232so that the ink droplets102and the opposite electrode160are electrically connected.

Referring toFIG. 7, if a hydrophobic layer330is not a porous layer as in the second embodiment, a plurality of probes362perforating the hydrophobic layer330may be installed on the opposite electrode360. Thus, in the third embodiment, the opposite electrode360and the ink droplets102are electrically connected via the plurality of probes362.

Referring toFIG. 8, a nozzle410may have a circular cross-sectional shape, unlike in the previous embodiments. Alternately, the nozzle410may have a variety of cross-sectional shapes, such as an oval cross-sectional shape or a polygonal cross-sectional shape, in addition to the rectangular cross-sectional shape and the circular cross-sectional shape.

As shown inFIG. 8, in the fourth embodiment, when the nozzle410has the circular cross-sectional shape, a hydrophobic layer430surrounding the nozzle410has a circular shape. An insulating layer440is provided to a predetermined width at a lower external surface of the hydrophobic layer430, and an electrode pad452is disposed at an external surface of the insulating layer440, and an opposite electrode460is disposed at an upper external surface of the hydrophobic layer430.

Hereinafter, the operation of the ink-jet printhead having the above structure according to the first embodiment of the present invention will be described.

FIG. 9schematically explains the movement of ink in the nozzle ofFIG. 4. Referring toFIG. 9, if a voltage is not applied to an electrode, due to the surface tension of ink, ink contacts the surface of a hydrophobic layer at a relatively large contact angle Θ1. Alternately, if the voltage is applied from a power source to the electrode, an electric field acts on ink having conductivity. As such, electric charges having predetermined polarity, e.g., negative electric charges, are collected at an interface between the electrode and an insulating layer, and electric charges having opposite polarity, e.g., positive electric charges, are collected at an interface between ink and the hydrophobic layer. Since a repulsive force acts between the positive electric charges collected at the interface between ink and the hydrophobic layer, the surface tension of ink is reduced. Thus, as indicated by a dotted line, a contact angle Θ2of ink with respect to the hydrophobic layer is reduced so that a contact area between ink and the hydrophobic layer is increased. In this way, ink reacts as if the property of the hydrophobic layer has been changed to a hydrophilic property. If the voltage applied to the electrode is cut off, due to the surface property of the hydrophobic layer, the surface tension of ink increases, and ink is returned to an original state indicated by a solid line.

Due to the movement of ink in the nozzle, ink droplets are separated from ink, and the separated ink droplets move toward the outlet of the nozzle. This process will now be described in detail with reference toFIGS. 10A through 10E.

FIGS. 10A through 10Esequentially illustrate an ink ejecting method according to an embodiment of the present invention.

Referring toFIG. 10A, ink101supplied from an ink reservoir (not shown) is filled by a capillary force in a rear end of the nozzle110surrounded by a hydrophilic layer120. Ink, however, is not filled in a front end of the nozzle110surrounded by a hydrophobic layer130due to a surface property of the hydrophobic layer130.

Next, as shown inFIG. 10B, when a voltage is sequentially applied from a first power source170to a first electrode pad151and a second electrode pad152, ink101moves a portion of the nozzle110corresponding to a location of the second electrode pad152. The movement of ink101occurs when a voltage is applied to the first and second electrode pads151and152. This application of voltage causes the surface property of the hydrophobic layer130at a location corresponding to the first and second electrode pads151and152to change to a hydrophilic property. More specifically, when the voltage is applied to the first and second electrode pads151and152, the surface tension of ink101is reduced by an electric field acting on ink101. As such, a contact angle of ink101with respect to the hydrophobic layer130is reduced. Thus, ink101moves by a capillary force to the portion of the nozzle110corresponding to the position of the second electrode pad152.

Next, as shown inFIG. 1C, when the voltage applied to the first electrode pad151is cut off, ink droplets102having a predetermined volume are separated from ink101. More specifically, when the voltage is applied to the second electrode pad152and only the voltage applied to the first electrode pad151is cut off, the portion of the hydrophobic layer130corresponding to the location of the first electrode pad151is returned to a hydrophobic property, which is an original surface property. As such, ink101is separated into two parts at the location of the first electrode pad151, and a portion of the ink101adjacent to the second electrode pad152forms a separated ink droplet102having a predetermined volume.

According to the present invention, the ink droplets102having a predetermined volume are separated from ink101in the nozzle110such that the volume of the ink droplets102ejected through the nozzle110becomes uniform. In the present invention, the area of each of the first and second electrode pads151and152may be varied, such that the volume of the ink droplets102may be adjustable, thereby resulting in finer and more uniform separate ink droplets102.

When the length of the nozzle110is relatively short, only two electrode pads151and152are provided and the second electrode pad152is adjacent to the outlet114of the nozzle110. Thus, the ink droplets102are separated from ink101and are ejected through the nozzle110using a predetermined droplets ejecting unit, as shown inFIG. 10E. In this case, when the voltage applied to the second electrode pad152is cut off, the hydrophobic layer130at a position corresponding to the location of the second electrode pad152is returned to a hydrophobic property. Thus, a contact angle of the ink droplets102with respect to the hydrophobic layer130is increased, and the ink droplets102are varied in a shape shown inFIG. 4. Thus, due to a lower driving force, for example, an electrostatic force, ejecting of ink droplets102is performed.

Meanwhile, when the length of the nozzle110is relatively long, as shown inFIG. 10D, the third electrode pad153is provided after the second electrode pad152, and the step of moving the ink droplets102to a portion of the nozzle110corresponding to a location of the third electrode pad153may be performed.

Specifically, after the ink droplets102are separated from ink101, when the voltage applied to the second electrode pad152is cut off and a voltage is applied to the third electrode pad153, the ink droplets102move from a portion corresponding to the location of the second electrode pad152, which has returned to a hydrophobic property, to a portion corresponding to a location of the third electrode pad153, which has changed into a hydrophilic property. In this case, the portion of the nozzle110corresponding to the location of the first electrode pad151maintains a hydrophobic property. Thus, reverse movement of the ink droplets102, i.e., backflow, is prevented.

When the length of the nozzle110is even longer, one or more electrode pad may be provided after the third electrode pad153. If a voltage is sequentially applied to the electrode pads151,152, and153, the ink droplets102consecutively move toward the outlet114of the nozzle110, as described above.

In the case of a plurality of electrode pads, e.g., more than three, the moving speed of the ink droplets102in the nozzle110may be adjusted by a time difference when sequentially applying the voltage to the plurality of electrode pads.

The ink droplets102that have moved toward the outlet114of the nozzle110are ejected through the outlet114of the nozzle110, as shown inFIG. 10E. Specifically, if a predetermined voltage is applied from the second power supply190to an external electrode180, an electric field between the nozzle110and the external electrode180is formed. As such, an electrostatic force, that is, a Coulomb force, acts on the ink droplets102. Accordingly, the ink droplets102may be ejected from the nozzle110to a recording paper P provided at a front side of the external electrode180. If a voltage applied to the third electrode pad153is cut off before the ink droplets102are ejected, the hydrophobic layer130at the location corresponding to the third electrode pad153is returned to having a hydrophobic property. Thus, the ink droplets102may be easily ejected by a lesser electrostatic force.

Meanwhile, a variety of conventional methods, as well as the above-described method using an electrostatic force, may be used to actually eject the ink droplets102from the nozzle110. For example, a fluid-flow may be formed around the outlet114of the nozzle110, and the atmospheric pressure around the outlet114of the nozzle110may be lowered to eject the separated ink droplets102.

As described above, in an ink ejecting method and an ink-jet printhead utilizing the method according to the present invention, since a lower voltage may be used, ink droplets having a predetermined volume are previously separated from ink in a nozzle and are ejected, necessary power consumption to eject the ink droplets may be reduced, and the volume of the ejected ink droplets may become uniform. In addition, the area of the electrode pad may be varied so that a volume of the ink droplets may be finely and precisely adjusted. Accordingly, a low power consumption ink-jet printhead having high resolution can be implemented.

Further, the moving speed of the ink droplets may be adjusted by a time difference when sequentially applying the voltage to a plurality of electrode pads. Additionally, ink in the nozzle may be prevented from flowing backward, and an ink refill operation is not required. Thus, an ink-jet printhead capable of printing at a high speed can be implemented.

Preferred and exemplary embodiments of the present invention have been disclosed herein and, although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. For example, although ink droplets separated from ink are shown and described in the exemplary embodiments of the present invention being ejected by an electrostatic force, the ink droplets may be ejected through the nozzle using different methods. More specifically, the present invention may be characterized in that ink droplets having a predetermined volume are separated from ink in the nozzle and the separated ink droplets are moved toward an outlet of the nozzle. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.