JP2006082264A - Inkjet head, its control method and inkjet recorder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inkjet head which can enhance concentration capability of ink and ejection stability, and to provide its control method. <P>SOLUTION: The inkjet head 10 has an ejection electrode 18 and a guard electrode 20 at a discharge opening substrate 16 where a discharge opening 28 is formed. A guard electrode control part 35 applies a pulse voltage to the guard electrode 20 while controlling the voltage which has the same frequency as that of a driving voltage applied to the ejection electrode and which alternately repeats a positive voltage and a negative voltage. At the time of ejection of the ink, color material particles are migrated and concentrated to an ink liquid level of the discharge opening by applying the negative voltage to the guard electrode 20. At the time of non ejection of the ink, a meniscus of the ink at the discharge opening is suppressed by an electric field directed towards the ejection electrode from the guard electrode 20 by applying the positive voltage to the guard electrode 20. Accordingly, while the concentration capability of ink is kept, an unnecessary overflow of the ink is prevented and the ejection stability is enhanced. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an ink jet head for ejecting ink toward a recording medium to form an image on the recording medium, a control method therefor, and an ink jet recording apparatus. More specifically, the present invention relates to an ink containing charged fine particles. The present invention relates to an ink jet head that discharges ink using the above, a control method thereof, and an ink jet recording apparatus.

  There is known an electrostatic ink jet recording method in which ink is ejected toward a recording medium using an electrostatic force. In the electrostatic ink jet recording method, ink containing charged fine particle components is used, and a drive voltage is applied to the discharge electrodes arranged around the discharge port for discharging the ink, and electrostatic force is applied to the ink at the discharge port. By acting, ink droplets are ejected from the ejection port toward the recording medium. By controlling the drive voltage applied to the ejection electrodes according to the image data, an image corresponding to the image data can be recorded on the recording medium.

  A multi-channel head in which a plurality of ejection openings (channels) are arranged in one head is known as an ink-jet head used in this electrostatic ink-jet recording type recording apparatus. In order to perform recording at a higher resolution using a multi-channel electrostatic inkjet head, it is necessary to dispose the ejection portions at high density and control each ejection port independently. However, in the electrostatic ink jet head, as described above, ink droplets are ejected using electrostatic force generated by applying a voltage to the ejection electrode of each ejection port. Therefore, if the discharge ports are arranged at high density, electric field interference occurs between adjacent discharge ports, and the size of the ink droplets to be ejected and the flying direction of the ink droplets vary, so that accurate recording cannot be performed. was there.

  With respect to such a problem, Patent Document 1 includes a plurality of ejection electrodes (individual electrodes) on a substrate that separates ink guides, and ink droplets are ejected from the ink guides by electrostatic force generated by applying a voltage to the ejection electrodes. An ink jet recording apparatus that discharges an ink jet recording apparatus in which a shield electrode for shielding electric lines of force from adjacent channels is formed between discharge electrodes is disclosed. In this ink jet recording apparatus, interference between adjacent discharge electrodes is suppressed by applying a high voltage to the shield electrode that is lower than the voltage applied to the discharge electrodes but not to discharge between the discharge electrodes. Yes.

However, since the ejection electrode and the shield electrode are provided on the same surface in the ink jet recording apparatus disclosed in Patent Document 1, the shield electrode can shield the lines of electric force generated from the outer peripheral end of the ejection electrode. There is a problem that electric field interference between adjacent ejection ports cannot be effectively prevented.
In order to suppress electric field interference, it is effective to provide a wide shield electrode between adjacent discharge ports. However, in a configuration in which the shield electrode is provided on the same plane as the discharge electrode as in Patent Document 1, the discharge electrode is not provided. If the outlets are arranged with high density, a sufficient width of the shield electrode cannot be secured between the adjacent discharge ports. Further, it is not preferable to increase the interval between the discharge portions in order to provide the shield electrodes widely between the discharge ports, because this reduces the degree of integration of the discharge ports and increases the head size. Therefore, there is a problem that it is difficult to perform accurate recording with a compact head.

JP 2000-25233 A

  In order to solve such a problem, in the Japanese Patent Application No. 2003-90367, the present applicant laminated a discharge electrode and a guard electrode on a substrate on which a discharge port is formed with an insulator interposed therebetween. In addition, an ink jet head in which the inner diameter of the guard electrode is smaller than the outer diameter of the ejection electrode has been disclosed. In the ink jet head disclosed in Japanese Patent Application No. 2003-90367, the inner diameter of the guard electrode is made smaller than the outer diameter of the ejection electrode, so that the electric force from other channels is secured while securing the electric lines of force to the own channel. Lines can be shielded efficiently. Therefore, even in a multi-channel head in which the discharge ports are arranged at a high density, the influence of the electric field of the other channel can be suppressed without weakening the electric field of the own channel. There is no influence on the flying direction of the droplets, the fluctuation of the recording dot diameter and the landing position is prevented, and high-resolution recording can be performed with high accuracy.

  In addition, in the Japanese Patent Application No. 2003-203824, the applicant of the present invention applied an AC bias voltage to a back electrode (counter electrode) disposed on the back side of the recording medium in order to improve the discharge property of ink droplets. A recording device is disclosed. In this ink jet recording apparatus, the AC bias voltage is applied to the back electrode to change the force acting on the ink at the ejection port, thereby promoting the formation of the ink meniscus and improving the ejection response. In addition, by applying an AC bias voltage to the back electrode, the charged particles contained in the ink in the ejection port can always be swung, so even if the ink is not ejected from the ejection port for a long time, It is possible to prevent overconcentration and prevent clogging of the discharge port.

In addition, in order to draw an image with high image quality and high speed on a recording medium using an electrostatic ink jet head, it is necessary to quickly supply a sufficient amount of charged color material particles to the ejection unit. As a method for rapidly supplying the color material particles to the discharge unit, for example, a method of using a liquid flow or moving the color material particles to the discharge port by electrophoresis can be considered.
However, these methods alone are not sufficient to supply a sufficient amount of colorant particles to the ejection portion quickly, clogging of the ejection port of the inkjet head, and the division of dots formed on the recording medium There is a risk of causing problems such as.

The present invention has been made in view of the above circumstances, and an object of the present invention is to supply color material particles to an ejection port quickly and reliably at the time of ejection of ink droplets, and at high speed and stably. Ink jet head capable of drawing and maintaining ink concentration at the ejection port when ink droplets are not ejected, preventing ink from overflowing and being stable against vibration, and control method thereof And providing an inkjet recording apparatus.
Another object of the present invention is to reduce the drive voltage applied to the ejection electrode, to reduce the cost by making the drive circuit a simple configuration, an inkjet recording apparatus including the inkjet head, and an inkjet It is to provide a method for controlling a head.

  In order to solve the above problem, a first aspect of the present invention is a method for controlling an ink jet head that ejects ink droplets by electrostatic force, wherein the ink jet head includes a plurality of ink droplets ejected from the ink droplet. A discharge port substrate in which discharge ports are formed and a discharge electrode that is formed corresponding to each of the plurality of discharge ports of the discharge port substrate and that generates an electrostatic field in each of the plurality of discharge ports. In order to block the electric field generated from the discharge electrode, the discharge port substrate is formed between the adjacent discharge ports at a position closer to the ink discharge side than the discharge electrode and insulated from the discharge electrode. A guard electrode, which applies a drive voltage to the discharge electrode in accordance with a drawing signal, has the same frequency as the drive voltage applied to the discharge electrode, and a positive voltage and a negative voltage are interchanged. To provide a control method for an inkjet head for applying a pulse voltage to the guard electrode to be repeated.

In the inkjet control method of the present invention, it is preferable that the guard electrodes are controlled in common. In addition, it is preferable that the pulse voltage is applied so as to have a phase opposite to that of the drive voltage signal when forming one dot on the recording medium.
Further, it is preferable that the pulse voltage is applied so that at least one of the phase and the pulse width is different from the signal of the driving voltage when forming one dot on the recording medium. In this case, it is preferable to control at least one of the phase and the pulse width of the pulse voltage so that the positive voltage is switched to the negative voltage immediately before the drive voltage is applied to the discharge electrode. It is preferable to control at least one of the phase and the pulse width of the pulse voltage so that the negative voltage is switched to the positive voltage before the application of the driving voltage is completed.

  The ink is preferably an ink obtained by dispersing charged fine particles containing at least a coloring material in an insulating dispersion medium.

  According to a second aspect of the present invention, there is provided an inkjet head that records an image on a recording medium by ejecting ink containing charged fine particles using an electrostatic force, and a plurality of ejections from which the ink droplets are ejected. A discharge port substrate in which an outlet is formed, a discharge electrode that is formed corresponding to each of the plurality of discharge ports of the discharge port substrate and generates an electric field in each discharge port, and generated from the adjacent discharge electrode A guard electrode formed on the discharge port substrate between the adjacent discharge ports at a position closer to the ink discharge side than the discharge electrode and insulated from the discharge electrode, A guard that is connected to the guard electrode, has the same frequency as the drive voltage applied to the ejection electrode, and applies a pulse voltage in which a positive voltage and a negative voltage are alternately repeated while controlling the guard electrode. An inkjet head having a pole controller.

  In the ink jet head according to the second aspect of the present invention, the guard electrodes are preferably controlled in common. Further, it is preferable that the guard electrode control unit applies the pulse voltage so as to have a phase opposite to that of a drive voltage signal applied to the ejection electrode when one dot is formed on the recording medium.

  In the ink jet head according to the second aspect of the present invention, the guard electrode control unit may detect a phase and a pulse with respect to a drive voltage signal applied to the ejection electrode when one dot is formed on the recording medium. The pulse voltage is preferably applied so that at least one of the widths is different. In this case, it is preferable that the guard electrode control unit controls at least one of the phase and the pulse width of the pulse voltage so that the positive voltage is switched to the negative voltage immediately before the drive voltage is applied to the ejection electrode. It is preferable to control at least one of the phase of the pulse voltage and the pulse width so that the negative voltage is switched to the positive voltage before the application of the drive voltage to the ejection electrode is completed.

  The ink is preferably an ink obtained by dispersing charged fine particles containing at least a coloring material in an insulating dispersion medium.

  According to a third aspect of the present invention, there is provided an ink jet recording apparatus comprising the ink jet head according to the second aspect of the present invention, and a moving means for relatively moving the ink jet head and the recording medium.

  In the inkjet head control method of the present invention, a drive voltage is applied to the ejection electrode in accordance with a drawing signal, and the positive voltage and the negative voltage are alternately repeated with the same frequency as the drive voltage applied to the ejection electrode. Since the pulse voltage is applied to the guard electrode, when the drive voltage is applied to the ejection electrode, the colorant particles in the ink are applied to the ink surface of the ejection port by the electric field generated between the guard electrode and the ejection electrode. The ink can be concentrated by aggregation. When no drive voltage is applied to the discharge electrode, the electric field that is opposite to the electric field generated between the guard electrode and the discharge electrode suppresses the ink meniscus at the discharge port and makes it difficult for the ink to be discharged. Thus, it is possible to prevent unnecessary ink overflow from the ejection port while maintaining the ink concentration.

  In the ink jet head and the ink jet recording apparatus of the present invention, the guard electrode control unit uses a pulse voltage having the same frequency as the drive voltage applied to the ejection electrode and a pulse voltage in which a positive voltage and a negative voltage are alternately repeated. As a result, when the drive voltage is applied to the discharge electrode, the ink is concentrated by aggregating the color material particles in the ink onto the surface of the ink at the discharge port by the electric field generated between the guard electrode and the discharge electrode. Can be made. When no drive voltage is applied to the discharge electrode, the electric field that is opposite to the electric field generated between the guard electrode and the discharge electrode suppresses the ink meniscus at the discharge port and makes it difficult for the ink to be discharged. Thus, it is possible to prevent unnecessary ink overflow from the ejection port while maintaining the ink concentration. As a result, an inkjet head and an inkjet recording apparatus that have high drawing stability and are capable of high-speed drawing that is stable against vibration and the like are provided. In addition, the voltage value of the drive voltage applied to the ejection electrode can be made lower than before by simply providing a guard electrode control unit for controlling the pulse voltage applied to the guard electrode, and the drive circuit can be greatly reduced in cost. Can do.

Hereinafter, an ink jet head, a control method thereof, and an ink jet recording apparatus according to the present invention will be described in detail based on preferred embodiments shown in the accompanying drawings.
FIG. 1A schematically shows a cross-section of the schematic configuration of the ink jet head according to the present invention, and FIG. 1B shows a view taken along line IB-IB in FIG. As shown in FIG. 1A, the inkjet head 10 includes a head substrate 12, an ink guide 14, and an ejection port substrate 16 on which ejection ports 28 are formed. On the discharge port substrate 16, discharge electrodes 18 are arranged so as to surround the discharge port 28. A counter electrode 24 that supports the recording medium P and a charging unit 26 of the recording medium P are disposed at a position facing the ink discharge side surface (upper surface in the drawing) of the inkjet head 10.
In addition, the head substrate 12 and the discharge port substrate 16 are arranged at a predetermined interval while facing each other. An ink flow path 30 that supplies ink to each ejection port 28 is formed by a space formed between the head substrate 12 and the ejection port substrate 16.

  The inkjet head 10 has a multi-channel structure in which a plurality of discharge ports (nozzles) 28 are two-dimensionally arranged in order to perform higher-density image recording at high speed. FIG. 2 schematically shows a state in which a plurality of discharge ports 28 are two-dimensionally arranged on the discharge port substrate 16 of the inkjet head 10. In FIGS. 1A and 1B, only one of the plurality of discharge ports is shown for easy understanding of the configuration of the inkjet head.

  In the inkjet head 10 of the present invention, the number of ejection ports 28, the physical arrangement position thereof, and the like can be freely selected. For example, not only a multi-channel structure as shown in FIG. 2 but also a single discharge port may be provided. Further, a so-called (full) line head having a row of ejection openings corresponding to the entire area of the recording medium P may be used, or a so-called serial head (shuttle type) scanned in a direction orthogonal to the direction of the nozzle row. Good. The ink jet head of the present invention can be used for both monochrome and color recording apparatuses.

FIG. 2 shows an arrangement of a part of the multi-channel structure (3 rows and 3 columns), and in a preferred embodiment, in the ink flow direction, the outlets 28 on the downstream side are arranged on the upstream side. The nozzles are arranged so as to be shifted by a predetermined pitch in the direction perpendicular to the ink flow with respect to the ejection ports. In this way, by disposing the discharge ports in the downstream row in a direction perpendicular to the ink flow direction with respect to the discharge ports in the upstream row, it is possible to satisfactorily supply ink to the discharge ports. In the inkjet head of the present invention, n rows and m columns (n and m are positive) are arranged such that the discharge ports in the downstream column are shifted in the direction perpendicular to the ink flow direction with respect to the discharge ports in the upstream column. May be configured such that each of the discharge ports is perpendicular to the ink flow with respect to the discharge ports located on the upstream side. They may be continuously shifted in the direction (downward or upward in FIG. 2). The number, pitch, repetition period, and the like of the discharge ports can be appropriately set according to the resolution and the feed pitch.
In addition, in FIG. 2, as a preferred mode, in the ink flow direction, the discharge ports in the downstream row are shifted from the discharge ports in the upstream row in the direction perpendicular to the ink flow, but the present invention is not limited to this. Instead, the downstream outlet and the upstream outlet may be arranged on the same straight line in the ink flow direction. In this case, it is preferable to dispose each ejection port in each row in the ink flow direction with respect to each ejection port in a row adjacent to the row in a direction perpendicular to the ink flow.

  In such an ink jet head 10, an ink Q is used, which includes a coloring material such as a pigment and is dispersed in an insulating liquid (carrier liquid) with charged fine particles (hereinafter referred to as coloring material particles). . Then, a driving voltage is applied to the ejection electrode 18 provided on the ejection port substrate 16 to generate an electric field at the ejection port 28, and the ink in the ejection port 28 is ejected by electrostatic force. Further, by turning on / off the drive voltage applied to the ejection electrode 18 according to the image data (ejection on / off), the ink droplets are ejected from the ejection port 28 according to the image data, and the recording medium P Record the image on top.

  Hereinafter, the structure of the inkjet head 10 of the present invention shown in FIGS. 1A and 1B will be described in more detail.

  As shown in FIG. 1A, the discharge port substrate 16 of the inkjet head 10 includes an insulating substrate 32, a guard electrode 20, a discharge electrode 18, and an insulating layer 34. A guard electrode 20 and an insulating layer 34 are sequentially stacked on the upper surface of the insulating substrate 32 in the drawing (the surface opposite to the side facing the head substrate 12). A discharge electrode 18 to which a drive voltage for discharging ink is applied is formed on the lower surface of the insulating substrate 32 in the drawing (the surface facing the head substrate 12).

Further, the discharge port substrate 16 is formed with a discharge port 28 for discharging the ink droplet R penetrating the insulating substrate 32. As shown in FIG. 1B, the ejection port 28 is a bowl-shaped opening (slit) elongated in the ink flow direction in which both short sides of the rectangle are semicircular, and has a length L in the ink flow direction. And the length D in the direction perpendicular to the ink flow has an aspect ratio (L / D) of 1 or more.
In the present invention, the ejection port 28 is thus an opening having an aspect ratio (L / D) of 1 or more between the length L in the ink flow direction and the length D in the direction perpendicular to the ink flow. Ink tends to flow to the outlet 28. That is, it is possible to improve the ink particle supply to the ejection port 28, improve the frequency response, and prevent clogging. This will be described in detail later together with the action of discharging ink droplets.

Further, in the present embodiment, the ejection port 28 is formed as a bowl-shaped opening. However, the present invention is not limited to this, and any shape can be used as long as ink can be ejected from the ejection port 28. The discharge port 28 can be formed in any shape such as a square, a rhombus, or a parallelogram.
The shape of the ejection port 28 is preferably an elongated shape in the ink flow direction such that the aspect ratio between the length in the ink flow direction and the length in the direction perpendicular to the ink flow is 1 or more. As a result, the ability to supply ink to the ejection port is improved, clogging can be prevented, and continuous large dots can be stably formed on the image recording medium, and the number of wavenumbers with a higher frequency can be increased. An image of an image quality can be drawn. For example, the ejection port can be formed in a rectangular shape having the long side in the ink flow direction, or an ellipse or rhombus having the long direction in the ink flow direction. Further, the ejection port may be formed in a trapezoidal shape in which the upstream side of the ink flow is the upper bottom and the downstream side is the lower bottom, and the height in the ink flow direction is longer than the lower bottom. In this case, the upstream side may be lengthened or the downstream side may be lengthened. Alternatively, a shape in which a circle having a diameter larger than the short side of the rectangle is connected to both short sides of the rectangle having the long side in the ink flow direction. Thus, by making the ejection port 28 elongated in the ink flow direction, ink supply to the ejection port 28 can be improved and clogging can be prevented. Further, the discharge port 28 may have a symmetric shape or an asymmetric shape on the upstream side and the downstream side with respect to the center thereof.

Next, the discharge electrodes formed on the discharge port substrate 16 of the inkjet head 10 shown in FIG. A discharge electrode 18 as shown in FIG. 1B is formed on the lower surface of the insulating substrate 32 (the surface facing the head substrate 12). The discharge electrode 18 is disposed along the periphery of the discharge port 28 so as to surround the periphery of the discharge port 28. In FIG. 1B, the discharge electrode 18 is formed in a shape similar to the discharge port 28. However, the shape is not limited to this, and the shape is changed to various shapes as long as the shape surrounds the discharge port 28. be able to. For example, the discharge electrode can be formed in a shape such as a circle, a substantially circle, an ellipse, or a substantially ellipse. Moreover, it can change into various shapes according to the shape of the discharge outlet 28. The discharge port 28 may not completely surround the discharge port 28. For example, the discharge port 28 may have a C shape, a U shape, or the like in which a part of the discharge electrode on the upstream side or the downstream side in the ink flow direction is cut off. There may be. Moreover, it is good also as a parallel electrode arrange | positioned so that an ejection port may be pinched | interposed in parallel with an ink flow direction, and a substantially parallel electrode.
As described above, since the inkjet head 10 has a multi-channel structure in which the discharge ports 28 are two-dimensionally arranged, the discharge electrode 18 corresponds to each discharge port 28 as schematically shown in FIG. Are two-dimensionally arranged.

  The ejection electrode 18 is exposed to the ink flow path 30 and is in contact with the ink Q flowing through the ink flow path 30. With the ink jet head shown in FIG. 1A having such a structure, the discharge property of ink droplets can be greatly improved. This point will be described later together with the action of discharge. However, the discharge electrode 18 is not necessarily exposed to the ink flow path 30 and is in contact with the ink, and the discharge electrode 18 may be formed inside the discharge port substrate 16 as shown in FIG. The surface exposed to the ink flow path of the discharge electrode 18 shown may be covered with an insulating layer.

  Further, the ejection electrode 18 is connected to the drive voltage control unit 33 as shown in FIG. The drive voltage control unit 33 can control the drive voltage applied to the ejection electrodes during ink ejection and non-ejection according to the drawing signal.

Next, the guard electrode 20 of the inkjet head 10 shown in FIG. As shown in FIG. 1A, the guard electrode 20 is formed on the surface of the insulating substrate 32, and the surface of the guard electrode 20 is covered with an insulating layer 34. FIG. 3 schematically shows the planar structure of the guard electrode 20. FIG. 3 is a view taken along the line III-III in FIG. 1A and schematically shows a planar structure of the guard electrode 20 of the multi-channel inkjet head 10. As shown in FIG. 3, the guard electrode 20 is a sheet-like electrode common to each discharge electrode such as a metal plate, and the discharge electrode 18 formed around each discharge port 28 arranged two-dimensionally. Has an opening 36 at a position corresponding to. The opening 36 of the guard electrode 20 is formed in a rectangular shape similar to the shape of the discharge port 28. The length and width of the opening 36 of the guard electrode 20 are formed to be larger than the length and width of the discharge port 28.
The guard electrode 20 can shield electric field lines between adjacent ejection electrodes 18 and suppress electric field interference. The guard electrode 20 is connected to the guard electrode control unit 35, and a predetermined pulse voltage synchronized with the frequency of the drawing signal is applied by the guard electrode control unit 35. Then, such a pulse voltage is applied to the guard electrode 20 during the recording operation of the ink jet head, whereby the ink concentration and ejection properties are controlled. A method for controlling the pulse voltage applied to the guard electrode 20 will be described in detail later.

As a preferred embodiment, the guard electrode 20 is formed in a layer different from the ejection electrode 18 and the surface thereof is covered with an insulating layer 34 as shown in FIG. By covering the surface of the guard electrode 20 with the insulating layer 34, the ink leaking from the ejection port is prevented from coming into contact with the guard electrode.
By having such an insulating layer 34, electric field interference between the adjacent ejection electrodes 18 can be suitably prevented, and the color material particles of the ink Q are coated between the ejection electrodes 18 and the guard electrodes 20 to discharge. Can also be prevented.

Here, the guard electrode 20 secures the electric lines of force that act on the corresponding discharge ports 28 (hereinafter referred to as “own channels” for convenience) among the electric lines of force generated from the discharge electrodes 18. It is provided so as to shield the electric lines of force of the discharge electrode 18 provided in the discharge port 28 (also referred to as “other channels”) and the electric lines of force to the other channels.
In the absence of the guard electrode 20, the electric lines of force generated from the end of the discharge electrode 18 on the discharge port side (hereinafter referred to as the inner edge of the discharge electrode) during the discharge of ink droplets are inside the discharge electrode 18, that is, the discharge It converges in a region surrounded by the inner edge of the electrode 18 and acts on its own channel to generate an electric field necessary for discharging ink droplets. On the other hand, electric lines of force generated from the end of the discharge electrode 18 opposite to the discharge port side (hereinafter referred to as the outer edge of the discharge electrode) diverge further outward than the outer edge of the discharge electrode 18 and enter other channels. It affects and causes electric field interference.

Considering the above points, the width and length of the opening 36 of the guard electrode 20 are set so that the electric field lines to the own channel are not shielded when viewed from the substrate plane. It is preferable to make it larger than the width and length. In other words, the end of the guard electrode 20 on the discharge port 28 side is preferably separated (retreated) from the discharge port 28 rather than the inner edge of the discharge electrode 18 of the own channel.
Further, in order to efficiently shield the lines of electric force to other channels, the length and width of the opening 36 of the guard electrode 20 are set so that the outer edge of the discharge electrode 18 of the own channel when viewed in the substrate plane. It is preferable to make it smaller than the interval (outer diameter). That is, it is preferable that the inner edge portion of the guard electrode 20 is closer (advanced) to the discharge port 28 than the outer edge portion of the discharge electrode 18 of the own channel. According to the study of the present inventor, this proximity amount is preferably 5 μm or more, particularly preferably 10 μm or more.
By having the above-described configuration, the ejection stability from the ejection port 28 is sufficiently ensured, and variations in the ink landing position due to electric field interference between adjacent channels are suitably suppressed, and the stability can be increased. It is possible to perform image recording with high image quality.

  The opening 36 of the guard electrode 20 is substantially similar to the shape formed by the inner edge or the outer edge of the ejection electrode 18, and the inner edge of the guard electrode 20 is more ejected than the inner edge of the ejection electrode 18 of its own channel. The guard electrode 20 may be provided so as to be separated from (retracted from) 28 and closer to (advance) the discharge port 28 than the outer edge of the discharge electrode (that is, the opening 36 of the guard electrode 20 may be formed). Good).

  In the above example, the guard electrode 20 is a sheet-like electrode common to the discharge electrodes 18, but the present invention is not limited to this, and the electric lines of force of other channels are shielded between the discharge ports. Any shape or structure may be used as long as it is provided. For example, the guard electrode may be provided in a mesh shape between the discharge ports. In addition, in a plurality of discharge ports arranged in a matrix, for example, when the intervals between adjacent discharge ports in the row direction and the column direction are different, between the discharge ports sufficiently separated so as not to cause electric field interference. May not be provided with a guard electrode, but may be provided only between adjacent outlets. In addition, the guard electrode does not have to have a structure common to the discharge electrodes arranged in a matrix, but has a structure that is common to each column or row of the matrix-like discharge electrodes or each staggered arrangement. May be. In this case, a guard electrode control unit may be provided individually so as to correspond to each guard electrode provided in common for each column or row of the matrix-like ejection electrodes or in a staggered arrangement. You may connect each guard electrode to a guard electrode control part, and may control each guard electrode individually or in common. Here, the term “common” means not only a structural common but also an electric common concept. Therefore, even if the guard electrodes have a structure that is structurally independent from each other, the above-described common is satisfied as long as they are electrically connected and common.

In the case of any of the above-described guard electrodes, as shown in FIG. 1A, the inner edge portion of the guard electrode 20 is larger than the inner edge portion of the discharge electrode 18 with respect to the discharge electrode 18 of its own channel. The guard electrode 20 may be formed so as to be separated from the outer periphery of the discharge electrode 18 and closer to the discharge port 28 than the outer edge of the discharge electrode 18.
Here, the shape of the opening 36 of the guard electrode 20 is substantially the same as the shape of the discharge port 28, but the shape is not limited to this, and the lines of electric force between the adjacent discharge electrodes 18 are shielded. As long as the electric field interference can be prevented, it can be formed into an arbitrary shape. For example, the opening 36 of the guard electrode 20 can be formed in a shape such as a circle, an ellipse, a square, or a rhombus.

  Next, the ink guide 14 of the inkjet head 10 shown in FIG. The ink guide 14 is made of a ceramic flat plate having a predetermined thickness, and is disposed on the head substrate 12 corresponding to each ejection port 28 (ejection unit). The ink guide 14 is formed to be slightly wider according to the length of the ejection port 28 in the long side direction. As described above, the ink guide 14 passes through the ejection port 28, and the tip end portion 14 a projects upward from the surface of the ejection port substrate 16 on the recording medium P side (the surface of the insulating layer 34).

The front end portion 14a of the ink guide 14 is shaped so that the cross-sectional shape parallel to the ink flow direction becomes a substantially triangular shape (or trapezoid) that gradually becomes thinner toward the counter electrode 24 side. The ink guide 14 is disposed such that the inclined surface of the tip portion 14a intersects the ink flow direction. As a result, the ink flowing into the ejection port 28 reaches the apex of the distal end portion 14 a along the inclined surface of the distal end portion 14 a of the ink guide 14, so that an ink meniscus is stably formed at the ejection port 28.
Further, by forming the ink guide 14 wider in the long side direction of the ejection port 28, the width in the direction perpendicular to the ink flow can be shortened, and the influence on the ink flow can be reduced, and A meniscus described later can be formed stably.

The shape of the ink guide 14 is not particularly limited as long as the color material in the ink Q can be concentrated to the tip portion 14a through the discharge port 28 of the discharge port substrate 16, and for example, the tip portion 14a has a shape. The shape does not have to be thinner as it goes toward the counter electrode, and can be changed as appropriate. For example, a cutout serving as an ink guide groove that collects the ink Q in the front end portion 14a may be formed in the center portion of the ink guide 14 in the vertical direction in the drawing by capillary action. In FIG. 1B, a plate-like shape that is long in the ink flow direction is used in accordance with the shape of the ejection port. However, the shape is not limited to this, and a prism may be used.
In addition, the ink guide 14 is preferably formed by depositing metal on the most distal portion thereof, and the dielectric constant of the tip portion 14a of the ink guide 14 is substantially reduced by depositing metal on the most distal portion of the ink guide 14. Become bigger. As a result, when a drive voltage is applied to the ejection electrode, a strong electric field is easily generated in the ink guide 14, and the ink ejection performance can be improved.

As shown in FIG. 1A, the ink jet head 10 of the present embodiment is provided with an ink guide weir 40 that guides ink to the ejection port 28 on the head substrate 12 as a preferred form. The ink guide weir 40 will be described in detail below with reference to FIGS. 4 (A) and 4 (B).
FIG. 4A is a partial cross-sectional perspective view showing the configuration in the vicinity of the ejection portion in the inkjet head 10 of FIG. In the drawing, in order to clearly show the structure of the ink guide weir 40, the discharge port substrate 16 is shown cut along the ink flow direction at a substantially central position of the ink guide 14.

  The ink guide weirs 40 are provided on the upstream side and the downstream side of the ink flow direction (arrow direction) of the ink guide 14 on the ink channel 30 side surface of the head substrate 12, that is, the bottom surface of the ink channel 30. The ink guide weir 40 has a surface inclined so as to gradually approach the ejection port substrate 16 from the vicinity of the position corresponding to the ejection port 28 toward the position corresponding to the center of the ejection port 28 with respect to the ink flow direction. Have. That is, the ink guide weir 40 has a shape that is inclined toward the ejection port 28 along the ink flow direction.

  In addition, the ink guide weir 40 has a shape having a width substantially the same as the ejection port 28 in the direction orthogonal to the ink flow and having a wall surface extending from the bottom surface. In addition, the ink guide weir 40 does not block the discharge port 28 and secures a flow path for the ink Q from the surface on the ink flow channel 30 side of the discharge port substrate 16, that is, from the upper surface of the ink flow channel 30. It is provided at intervals. Such an ink guide weir 40 is provided in each discharge part.

  Thus, by providing the ink guide weir 40 that is inclined toward the ejection port 28 along the ink flow direction on the bottom surface of the ink flow path 30, an ink flow toward the ejection port 28 is formed, and the ink Q The discharge port 28 is guided to the opening on the ink flow path 30 side. Therefore, the ink Q can be suitably flown into the discharge port 28, and the ink particle supply property can be further improved. Furthermore, clogging of the discharge port can be prevented more reliably.

  The length l of the ink guiding weir 40 in the ink flow direction may be set as appropriate so that the ink Q can be suitably guided to the ejection port 28 within a range that does not interfere with the adjacent ejection port. As shown in FIG. 4, the height h of the highest part of the ink guide weir 40 is preferably 3 times or more (l / h ≧ 3), more preferably 8 times or more (l / h ≧ 8). preferable.

  The width of the ink guide weir 40 in the direction perpendicular to the ink flow is preferably equal to or slightly wider than the ejection port 28. Further, the width of the ink guide weir 40 is not limited to a uniform one as shown in the drawing, and may be one in which the width gradually decreases or one in which the width gradually increases. Further, the wall surface is not limited to a vertical surface, and may be an inclined surface or the like.

  The inclined surface (ink guiding surface) of the ink guide weir 40 may be a shape suitable for guiding the ink Q to the ejection port 28, and may be a slope having a certain tilt angle. It may be a changing surface or a curved surface. The surface is not limited to a smooth surface, and one or more wrinkles, grooves, or the like may be formed in the ink flow direction or radially toward the center of the ejection port 28.

  Further, the vicinity of the contact portion of the upper part of the ink guide weir 40 with the ink guide 14 may have a smoothly connected shape without a step as in the illustrated example.

  In the illustrated example, the ink guide weir 40 is arranged on the upstream side and the downstream side of the ink guide 14, but a trapezoidal ink guide weir 40 having slopes on the upstream side and the downstream side of the ejection port 28 is provided. The ink guide 14 may be erected on the upper portion, or the ink guide 14 and the ink guide weir 40 may be integrally formed. As described above, the ink guide weir 40 may be formed separately or integrally with the ink guide 14 and attached to the head substrate 12, or the head substrate 12 is shaved by a conventionally known excavation means. It may also be formed.

  The ink guide weir 40 may be provided on the upstream side of the ejection port 28, but the height in the ejection direction of the ink droplets R is also ejected on the downstream side of the ejection port 28 as shown in the illustrated example. It is preferable to be provided so as to become lower as the distance from the outlet 28 increases. As a result, the ink Q guided toward the discharge port 28 by the upstream ink guide weir 40 flows smoothly to the downstream side, so that the ink Q does not become turbulent and the ink flow can be kept stable. , Discharge stability can be maintained.

In the example shown in FIG. 4, the ink guide weir 40 is disposed on the upper surface of the head substrate 12, but is not limited thereto. For example, an ink flow groove is formed in the head substrate 12, and the ink flow groove is formed inside the ink flow groove. An ink guide weir having a structure as shown in FIG. 4 may be provided.
For example, in FIG. 1A, an ink flow groove having a predetermined groove depth is formed on the upper surface of the head substrate 12 so as to pass through a position corresponding to the ejection port 28 along the ink flow direction. Then, an ink guide weir having a surface inclined toward the discharge port 28 along the ink flow direction is provided at a position corresponding to the discharge port 28 of the ink flow groove. Thus, by forming the ink flow groove on the head substrate, most of the ink flowing through the ink flow path 30 can be selectively flowed to the ink flow groove. Further, by forming the ink guide weir 40 shown in FIG. 4 in the ink flow groove, the ink flowing through the ink flow groove can be suitably flown into the discharge port 28, and the tip portion of the ink guide 14 Ink supply to 14a can be improved.

Next, the counter electrode 24 arranged to face the ink droplet discharge surface of the inkjet head 10 will be described. As shown in FIG. 1A, the counter electrode 24 is disposed at a position facing the tip portion 14a of the ink guide 14, and is grounded, and the lower surface of the electrode substrate 24a in the drawing, It is comprised by the insulating sheet 24b arrange | positioned on the surface at the side of the inkjet head 10.
The recording medium P is held on the lower surface of the counter electrode 24 in the drawing, that is, the surface of the insulating sheet 24b by, for example, electrostatic adsorption. The counter electrode 24 (insulating sheet 24b) functions as a platen of the recording medium P.

  The recording medium P held on the insulating sheet 24b of the counter electrode 24 is charged to a predetermined negative high voltage having a polarity opposite to the drive voltage applied to the ejection electrode 18 by the charging unit 26 at least during recording. As a result, the recording medium P is negatively charged and biased to a negative high voltage, acts as a substantial counter electrode with respect to the ejection electrode 18, and is electrostatically adsorbed to the insulating sheet 24 b of the counter electrode 24.

  The charging unit 26 includes a scorotron charger 26a for charging the recording medium P to a negative high voltage, and a bias voltage source 26b for supplying a negative high voltage to the scorotron charger 26a. The charging means of the charging unit 26 used in the present invention is not limited to the scorotron charger 26a, and various discharging means such as a corotron charger, a solid charger, and a discharge needle can be used.

In the illustrated example, the counter electrode 24 includes an electrode substrate 24a and an insulating sheet 24b, and the recording medium P is charged to a negative high voltage by the charging unit 26, whereby a bias voltage is applied to the counter electrode. However, the present invention is not limited to this, and the counter electrode 24 is constituted only by the electrode substrate 24a, and the counter electrode 24 (electrode substrate 24a itself) is formed. May be connected to a negative high voltage bias voltage source so as to be constantly biased to a negative high voltage so that the recording medium P is electrostatically attracted to the surface of the counter electrode 24.
Further, the electrostatic adsorption of the recording medium P to the counter electrode 24 and the charging of the recording medium P to a negative high voltage or the application of a negative bias high voltage to the counter electrode 24 are separate negative high voltage sources. The holding of the recording medium P by the counter electrode 24 is not limited to the electrostatic adsorption of the recording medium P, and other holding methods and holding means may be used.

  The structure of the inkjet head 10 of the present invention has been described in detail above. Next, the ink discharge operation of the inkjet head having such a structure will be described with reference to FIGS. Although the case where the polarity of the color material particles is positively charged will be described as an example, when the polarity of the color material particles is reversed, that is, in the case of negative charge, the voltage value is reversed.

  FIG. 5 shows a waveform of a drawing signal (discharge data signal) such as image data and print data, a drive voltage waveform (pulse waveform) applied to the discharge electrode, and a voltage waveform A to pulse voltage applied to the guard electrode. D was shown. The drawing signal shown in the upper part of FIG. 5 is used for instructing ink ejection and non-ejection, and ink ejection is controlled based on the drawing signal. The period and pulse width of the drive voltage (pulse voltage) applied to the ejection electrodes are the same as the period and pulse width of the drawing signal as shown in FIG. Further, the drive voltage Vp applied to the ejection electrode 18 is set to +300 [V], for example. When no drive voltage is applied to the ejection electrode 18, the ejection electrode 18 is set to 0 [V], for example. The voltage value of the drive voltage is not limited to the above value, and can be set to an arbitrary voltage value as long as ink can be reliably ejected when the drive voltage is applied to the ejection electrode 18.

In the recording operation of the ink jet head shown in FIG. 1A, a driving voltage is applied to the ejection electrode 18 at a timing as shown in the driving voltage waveform of FIG. That is, a driving voltage for ejecting ink is applied to the ejection electrode 18 in synchronization with the drawing signal. When a drawing signal instructing ink ejection (a signal indicated by “on” in the drawing) is supplied to the drive voltage control unit 33 connected to the ejection electrode 18, the ejection electrode 18 is driven at the same timing as the rendering signal. A voltage is applied. As a result, an electric field that acts on ink ejection from the ejection electrode 18 is generated, and ink is ejected from the ejection port 28.
On the other hand, when a drawing signal instructing non-ejection of ink (a signal indicated by “off” in the drawing) is supplied to the drive voltage control unit, the drive voltage is applied to the ejection electrode 18 during one pulse period. Without being set to 0 [V]. For this reason, since an electric field for ejection is not generated from the ejection electrode 18, ink is not ejected from the ejection port 28. The ink ejection action of the inkjet head 10 will be described in detail later. In FIG. 5, one pulse cycle of the drawing signal corresponds to the time required to form one dot or one pixel on the recording medium.

  In the drive voltage waveform shown in FIG. 5, the pulse width of the drive voltage is set to the same width as the pulse width of the drawing signal. However, the present invention is not limited to this, and it is longer or shorter than the pulse width of the drawing signal. May be. Thus, if the pulse width of the driving voltage applied to the ejection electrode 18, that is, the application time of the driving voltage is adjusted to be longer or shorter, the amount of ink ejected from the ejection port 28 can be adjusted. The gradation of one pixel formed on the recording medium can be adjusted.

  Next, the voltage applied to the guard electrode 20 during such a recording operation will be described with reference to FIGS. 1 (A), 5, 6 (A) and (B). The voltage waveforms A to D shown in FIG. 5 are examples of pulse voltages applied to the guard electrode 20, respectively. FIG. 6A is a diagram schematically illustrating the state of the ejection port 28 when a driving voltage is applied to the ejection electrode 18, and FIG. 6B is a diagram in which no driving voltage is applied to the ejection electrode 18. It is the figure which showed typically the mode of the discharge outlet 28 at the time (or when a low voltage is applied).

First, the operation when a pulse voltage is applied to the guard electrode 10 according to the voltage waveform A will be described. In the following description, the voltage waveform of the pulse voltage applied to the guard electrode 20 is generated in the guard electrode control unit 35 connected to the guard electrode 20. As shown in FIG. 5, according to the voltage waveform A, the pulse voltage having the same frequency as the signal of the drive voltage applied to the ejection electrode 18 and having a phase delayed by 180 degrees from the drive voltage signal is generated by the guard electrode 20. To be applied. That is, a pulse voltage signal having a phase opposite to that of the drive voltage signal applied to the ejection electrode 18 is applied to the guard electrode 10. The positive voltage V _{g +} of the pulse voltage is set to +300 [V], for example, and the negative voltage V _g− is set to −200 [V], for example. Regardless of whether ink is ejected or not, a positive voltage V _{g +} and a negative voltage V _g− are alternately applied to the guard electrode 20 with a constant period.

When the pulse voltage applied to the guard electrode 20 is controlled according to the voltage waveform A shown in FIG. 5, the drive voltage is applied to the ejection electrode 18 during one pulse period, that is, while ink is ejected (hereinafter, referred to as “the discharge voltage”). The negative voltage V _g− of −200 [V] is applied to the guard electrode 20 when the ink is discharged). At this time, as shown in FIG. 6A, an electric field E ₁ from the discharge electrode 18 toward the guard electrode 20 is formed at the discharge port 28. As shown in FIG. 6A, the electrostatic field F _{1 in the} direction toward the ink surface (ink ejection direction) acts on the color material particles in the ink at the ejection port 28 by the electric field E ₁ . As a result, positively charged color material particles migrate to the surface of the ink present at the ejection port 28, and the ink is concentrated at the ejection port 28. The ink at the tip portion 14 a of the ink guide 14 is attracted by the attractive force of the counter electrode 24, and a part of the ink is ejected toward the counter electrode 24 as ink droplets. Thereby, dots of ink droplets are formed on the surface of the recording medium P arranged on the counter electrode 24. As described above, the drive voltage is applied to the ejection electrode 18 and simultaneously, the negative voltage V _g− is applied to the guard electrode 20, the color material particles are aggregated in the ejection port 28, and the concentrated ink is discharged from the ejection port 28. Can be discharged.

On the other hand, as shown in the voltage waveform A of FIG. 5, when the application of the drive voltage to the ejection electrode 18 is completed and the ejection electrode 18 becomes 0 [V] in one pulse period (hereinafter referred to as stop), The pulse voltage changes from the negative voltage V _g− (−200 [V]) to the positive voltage V _{g +} (+300 [V]). At this time, the discharge port 28, as shown in FIG. 6 (B), the electric field E ₂ directed from the guard electrode 20 to the ejection electrode 18 is formed. Then, the electrostatic force F _{2 in the} direction opposite to the direction toward the counter electrode 24 acts on the color material particles contained in the ink at the ejection port 28 by the electric field E ₂ . For this reason, the position of the meniscus M of the ink at the ejection port 28 is suppressed to the ink flow path 30 side (downward in the drawing) as compared with the case where the electrostatic force F ₁ directed toward the counter electrode 24 is acting. The ink concentration at the ejection port 28 is maintained, and unnecessary overflow of the ink is prevented. As described above, the driving voltage applied to the ejection electrode 18 becomes 0 [V] (or a low voltage level), and at the same time, the positive voltage V _{g +} having the same polarity as that of the color material particles is applied to the guard electrode 20, thereby The ink formed at the outlet 28 can be made difficult to be ejected, and even if vibration is given to the ink jet head, unintentional ink is prevented from overflowing from the ejection port 28, and the drawing stability against vibration is improved. Can be increased.

As described above, when a pulse voltage is applied to the guard electrode 20 based on the voltage waveform A in FIG. 5, a drive voltage (positive voltage) is applied to the ejection electrode 18 when ejecting ink. A state in which the negative voltage V _g− is applied to the guard electrode 20 and the color material particles migrate to the ink liquid surface of the ejection port 28 by the electrostatic force based on the electric field from the ejection electrode 18 toward the guard electrode, thereby concentrating the ink. It becomes. On the other hand, a positive voltage V _{g +} is applied to the guard electrode 20 before the ink is discharged, and an electrostatic force in the direction opposite to the direction toward the counter electrode 24 acts on the ink meniscus M of the discharge port 20, and the ink at the discharge port 28 is discharged. Thus, the ink can be prevented from overflowing while maintaining its concentration. By controlling the pulse voltage applied to the guard electrode 20 in this way, the ink does not overflow from the ejection port 28 due to the electrostatic force in the direction opposite to the direction toward the counter electrode until immediately before the drive voltage is applied to the ejection electrode 18. As described above, the ink meniscus can be suppressed by using the electrostatic force. At the same time or substantially the same time as the driving voltage is applied, the color material particles are aggregated on the ink liquid surface of the ejection port 28 and the ink meniscus of the ejection port 28 is released to concentrate the ink from the ejection port 28. Can be discharged quickly.

Further, when a pulse voltage as shown in the voltage waveform A is applied to the guard electrode 20, even if the drive voltage applied to the ejection electrode 18 is low, the ejection and non-ejection of ink can be reliably controlled. . Therefore, it is possible to realize stable and stable ink ejection with a low driving voltage and to further increase the frequency of the driving voltage.
Further, since the pulse voltage is applied to the guard electrode 20 even when ink is not ejected, the color material particles in the ink at the ejection port 28 oscillate, and clogging due to ink overconcentration can be prevented.

  Next, the operation when a pulse voltage is applied to the guard electrode 20 according to the voltage waveform B shown in FIG. 5 will be described. A voltage waveform B shown in FIG. 5 is a case where the phase is different from that of the drive voltage signal applied to the ejection electrode 18, and is a case where the phase is ahead of the voltage waveform A. In the voltage waveform B, the fall of the pulse voltage applied to the guard electrode 20 precedes the rise of the drive voltage. In addition, the rise of the pulse voltage precedes the fall of the drive voltage.

When a pulse voltage is applied to the guard electrode 20 according to the voltage waveform B, the negative voltage V _g− is applied to the guard electrode 20 before the drive voltage is applied to the ejection electrode 18. That is, when the ejection electrode 18 is 0 [V], a negative voltage _Vg− of −200 [V] is applied to the guard electrode. As a result, an electric field from the ejection electrode 18 toward the guard electrode 20 is formed, and the ink in the ejection port 28 is concentrated before ink ejection as described above. That is, the colorant particles in the ink are supplied to the ejection port 28 while the ink is not being ejected. In this state, a driving voltage for causing ink to be ejected is applied to the ejection electrode 18, so that the concentrated ink is ejected from the ejection port 28 at the same time or substantially simultaneously with the application of the driving voltage.

According to the voltage waveform B, a positive voltage V _{g +} of +300 [V] is applied to the guard electrode 20 immediately before the application of the drive voltage to the ejection electrode 18 is completed, that is, immediately before the ejection of ink is stopped. The After the application of the driving voltage to the ejection electrode 18 is completed, a positive voltage _{g +} of +300 [V] is applied to the guard electrode 20 and the ejection electrode 18 is set to 0 [V]. Since the meniscus of the ink at the ejection port 28 is suppressed by the electrostatic force generated based on the electric field between the electrode 20 and the ejection electrode 18, even if the ink jet head 10 vibrates, the ink unnecessarily overflows from the ejection port. It is prevented.

  As can be seen from the above description, if a pulse voltage is applied to the guard electrode 20 in accordance with the voltage waveform B, the color material particles are aggregated at the ejection port 28 to concentrate the ink before the ink is ejected. Concentrated ink can be ejected simultaneously or substantially simultaneously with the application of the driving voltage, and after the application of the driving voltage to the ejection electrode 18 is finished, the ink at the ejection port should not overflow due to vibration or the like. Therefore, it is possible to improve the ink concentration and ejection stability.

Next, the operation when a pulse voltage is applied to the guard electrode 20 according to the voltage waveform C shown in FIG. 5 will be described.
A voltage waveform C in FIG. 5 is an example in which the application time (pulse width) of the positive voltage V _{g +} of the pulse voltage applied to the guard electrode 20 is shorter than the voltage waveform A. In the voltage waveform C, as in the voltage waveform B, the fall of the voltage applied to the guard electrode 20 precedes the rise of the drive voltage. On the other hand, the rise of the voltage applied to the guard electrode 20 coincides with the fall of the drive voltage. In this example, the voltage applied to the guard electrode 20 is switched from the positive voltage V _{g +} to the negative voltage V _g− prior to application of the drive voltage to the ejection electrode 18, that is, prior to ink ejection. As described in the description of the voltage waveform B, the colorant particles can be supplied to the ejection port 28 before the ink is ejected to concentrate the ink at the ejection port 28. Further, while the driving voltage is applied to the ejection electrode, the colorant particles are supplied to the ejection port 28 by the electrostatic force generated based on the electric field between the ejection electrode 18 and the guard electrode 20, thereby enhancing the ink concentration. Ink can be ejected. Further, since the positive voltage V _{g +} is applied to the guard electrode 20 simultaneously with the end of the application of the drive voltage, unnecessary ink is prevented from overflowing from the ejection port after the ink ejection operation.

Next, the operation when a pulse voltage is applied to the guard electrode 20 according to the voltage waveform D shown in FIG. 5 will be described.
The voltage waveform D shown in FIG. 5 is an example in which the application time (pulse width) of the positive voltage V _{g +} of the pulse voltage applied to the guard electrode 20 is longer than that of the voltage waveform A. In this voltage waveform, the falling edge of the pulse voltage coincides with the rising edge of the drive voltage applied to the ejection electrode 18. On the other hand, the rise of the pulse voltage precedes the fall of the drive voltage. In this example, the voltage applied to the guard electrode 20 is switched from the negative voltage V _g− to the positive voltage V _{g +} prior to the end of application of the drive voltage. In addition, since the voltage applied to the guard electrode 20 is switched from the positive voltage V _{g +} to the negative voltage V _g− simultaneously with the application of the driving voltage to the ejection electrode 18, the color material particles are applied to the ejection port 28 before ink ejection. By supplying the ink, the ink at the discharge port 28 is concentrated, and the concentrated ink can be quickly discharged from the discharge port 28. Further, while the driving voltage is applied to the ejection electrode 18, the colorant particles are supplied to the ejection port 28 by the electrostatic force generated based on the electric field between the ejection electrode 18 and the guard electrode 20, thereby improving the ink concentration. Ink can be discharged while increasing.

Next, the operation when a pulse voltage is applied to the guard electrode 20 according to the voltage waveform E shown in FIG. 5 will be described.
A voltage waveform E shown in FIG. 5 is an example in which the falling edge of the pulse voltage applied to the guard electrode 20 is delayed from the rising edge of the driving voltage, and the rising edge of the pulse voltage coincides with the falling edge of the driving voltage. Therefore, the pulse width of the voltage waveform E is the same as the pulse width of the voltage waveform D. In this voltage waveform E, the positive voltage V _{g +} is applied to the guard electrode 20 immediately after the drive voltage is applied to the ejection electrode 18. The voltage applied to the guard electrode 20 is switched from the positive voltage V _{g +} to the negative voltage V _g− after a predetermined time has elapsed since the drive voltage was applied to the ejection electrode 18.

When a pulse voltage is applied to the guard electrode 20 based on the voltage waveform E, a positive voltage V _{g +} of +300 [V] is applied to the guard electrode prior to the application of the drive voltage. Immediately before the drive voltage is applied to the discharge electrode 18, the discharge electrode 18 is set to 0 [V] (or a low voltage level), and a voltage of +300 [V] is applied to the guard electrode 20. Therefore, in this state, an electric field from the guard electrode 20 toward the ejection electrode is formed at the ejection port 28, and the meniscus of the ink is suppressed by the electrostatic force based on the electric field, and unnecessary overflow of the ink is prevented.

Furthermore, when the voltage applied to the guard electrode 20 is switched to a negative voltage _Vg− of −200 [V] after a certain time has elapsed, as described above, the discharge port 28 is directed to the guard electrode 20 from the discharge electrode 18. An electric field is formed. Then, due to the electrostatic force generated based on this electric field, the color material particles migrate to the surface of the ink existing at the ejection port 28, and the ink is concentrated at the ejection port 28. The ink at the tip portion 14 a of the ink guide 14 is attracted by the attractive force of the counter electrode 24, and a part of the ink is ejected toward the counter electrode 24 as ink droplets.

Simultaneously with the end of the application of the drive voltage, the positive voltage V _{g +} is applied to the guard electrode 20, and as described above, the meniscus of the ink at the ejection port 28 is suppressed by the electrostatic force. This prevents unnecessary ink from overflowing from the ejection port after the ink ejection operation.

  Here, an example is shown in which the phase of the voltage waveform D is delayed so that the rising edge of the pulse voltage applied to the guard electrode 20 coincides with the falling edge of the driving voltage. However, the present invention is not limited to this, and is applied to the guard electrode 20. The falling edge of the pulse voltage may be delayed from the rising edge of the driving voltage, and the rising edge of the pulse voltage may be preceded or delayed from the falling edge of the driving voltage.

The ink ejection operation of the ink jet head when a pulse voltage is applied to the guard electrode 20 has been described above. In any case of the voltage waveforms A to E, basically, a positive voltage V _{g +} is applied to the guard electrode during a certain period while the drive voltage is not applied, so that the gap between the discharge electrode and the guard electrode. An electric field is formed in the ink, and the electrostatic force generated by the electric field is used to agglomerate the color material particles at the discharge port to concentrate the ink at the discharge port. During the ink ejection operation, that is, when a driving voltage is applied, the concentrated ink is ejected as ink droplets from the ejection port.

Further, the voltage waveforms A to E of the pulse voltage shown in the above description are only examples, and when the ink discharge operation is performed, the meniscus is opened so that the ink can be easily discharged. If the meniscus can be suppressed so that the ink is difficult to be discharged, various voltage waveforms can be generated. Further, the voltage value of the pulse voltage is not limited to the above voltage value, and can be set to an arbitrary voltage value as long as the ink is not ejected only by applying the pulse voltage to the guard electrode 20. The positive voltage value V _{g +} of the pulse voltage to be applied to the guard electrode 20 is obtained when the pulse voltage is applied to the discharge electrode 18 for the purpose of preventing unnecessary discharge and preventing discharge breakdown with the discharge electrode. It is desirable to set the potential difference between the driving voltage to 300% or less and the off voltage applied to the ejection electrode 18 to 2000 [V] or less, and the negative voltage value V _g− of the pulse voltage is It is preferable to set the potential difference from the on-time voltage applied to the discharge electrode 18 to 2000 [V] or less for the purpose of preventing discharge breakdown between them.

  In addition, when the phase of the pulse voltage signal applied to the guard electrode 20 is shifted with respect to the drive voltage signal applied to the ejection electrode 18, the deviation of the pulse voltage signal applied to the guard electrode 20 is the drive voltage. It is preferable to be in the range of + 40% to −40% with respect to the duty ratio of the signal.

  In the voltage waveform shown in FIG. 5, a rectangular wave is used. However, the present invention is not limited to this, and a sine wave, a triangular wave, a trapezoidal wave, or the like may be used.

According to the voltage waveforms A to E of the pulse voltage shown in FIG. 5, the positive voltage V _{g +} and the negative voltage V _g− are alternately applied to the guard electrode 20 regardless of the drawing signal, that is, regardless of whether ink is ejected or not. Repeatedly applied. That is, even when ink is not ejected, the positive voltage V _{g +} and the negative voltage V _g− are applied to the guard electrode 20. Thereby, when the ink is not ejected, the color material particles and the meniscus in the ink at the ejection port are swung, and clogging due to ink overconcentration can be prevented.
In the method disclosed in Japanese Patent Application No. 2003-203824, a pulse voltage is applied to the counter electrode 24 shown in FIG. 1A, and the ink color material particles and the meniscus in the ejection port 24 are swung to prevent clogging. is doing. However, since the counter electrode 24 is relatively distant from the discharge electrode 18, when the frequency of the pulse voltage applied to the counter electrode 24 is increased, that is, when the drive frequency is increased, the counter electrode 24 and the discharge electrode 18 are discharged. There is a possibility that the electric field generated between the electrodes 18 cannot follow the frequency and the color material particles and the meniscus in the ink at the ejection port cannot be sufficiently swung. On the other hand, in the present invention, since the guard electrode 20 to which the pulse voltage is applied is close to the ejection electrode 18, even if the frequency of the pulse voltage is increased, the colorant particles and the meniscus in the ink at the ejection port 28 are surely obtained. Can be swung, and clogging can be more effectively prevented.
In addition, since it is only necessary to provide a guard electrode control unit for driving the guard electrodes, the cost of the entire drive circuit can be greatly reduced.

The operation when the pulse voltage is applied to the guard electrode 20 of the inkjet head has been described above.
Next, the present invention will be described in more detail by describing the ejection action of the ink droplets R in the inkjet head 10.

As shown in FIG. 1A, in the inkjet head 10, a color charged to the same polarity as the voltage applied to the ejection electrode 18 at the time of recording, for example, positive (+) by an ink circulation mechanism including a pump (not shown). Ink Q containing material particles circulates in the ink flow path 30 in the direction of the arrow (from the left to the right in the figure).
On the other hand, at the time of recording, the recording medium P is supplied to the counter electrode 24 and is charged by the charging unit 26 to a polarity opposite to that of the color material particles, that is, negative high voltage (for example, −1500 [V]). In a charged state, it is electrostatically attracted to the counter electrode 24.
In this state, while the recording medium P (counter electrode 24) and the inkjet head 10 are relatively moved, the driving voltage control unit 33 applies a pulse voltage (hereinafter referred to as driving) to the ejection electrode 18 according to the supplied image data. (Referred to as voltage). Basically, the ink droplet R is modulated and ejected according to the image data by ejecting on / off by applying the drive voltage on / off, and an image is recorded on the recording medium P.

  Here, in a state where the drive voltage is not applied to the ejection electrode 18 (or in a state where the applied voltage is at a low voltage level), that is, in a state where a bias voltage is applied to the counter electrode 24, the ink Q Coulomb attractive force acting between the counter electrode 24 and the color material particles (charged particles) of the ink Q, Coulomb repulsive force between the color material particles, viscosity of the carrier liquid, surface tension, dielectric polarization force, and the like are acting. Then, these forces are combined to move the colorant particles and the carrier liquid, and the ink Q has a meniscus shape slightly raised from the discharge port 28 as conceptually shown in FIG. Balanced. At this time, as described above, in the state where the drive voltage is not applied to the ejection electrode 18, a positive pulse voltage is basically applied to the guard electrode 20. That is, an electric field is generated from the guard electrode 20 toward the ejection electrode 18. Therefore, the electrostatic force toward the counter electrode 20 is smaller by the amount of the electric field generated from the guard electrode 20 than when no bias voltage is applied to the guard electrode 20. That is, the overflow of ink from the ejection port 28 is prevented. The coloring material particles move toward the side of the recording medium P charged by the counter electrode 24 by so-called electrophoresis by coupling such as Coulomb attractive force by the electrostatic force toward the counter electrode 20. Therefore, the ink Q is concentrated in the meniscus M formed at the ejection port 28. In this way, the ink is concentrated at the ejection port 28 while preventing overflow of the ink.

  From this state, a driving voltage is applied to the ejection electrode 18. At this time, as described above, a negative voltage is applied to the guard electrode 20. Thereby, the action from the discharge electrode to which the drive voltage is applied and the action from the guard electrode to which the pulse voltage is applied are superimposed on the action from the counter electrode to which the bias voltage is applied. Coupling also causes coupled movement by superposition of their actions. An electrostatic force acts on the colorant particles and the carrier liquid by an electric field generated by applying a driving voltage to the ejection electrode 18 and applying a pulse voltage to the guard electrode 20. The electrostatic force causes the colorant particles and the carrier liquid to be pulled to the bias voltage (counter electrode) side, that is, the recording medium P side, and the meniscus M formed at the ejection port 28 grows upward, and above the ejection port 28. A substantially conical ink liquid column, so-called tailor cone is formed. Similarly to the above, the color material particles are moved to the meniscus by electrophoresis and the electric field from the discharge electrode, and the meniscus ink Q is concentrated and has a large number of color material particles. It is in a state.

  When a finite time has passed after the start of application of the drive voltage to the discharge electrode 18, mainly due to the surface tension of the colorant particles and the carrier liquid at the tip of the meniscus having a high electric field strength due to the movement of the colorant particles, etc. Is lost, the meniscus grows rapidly, and a slender ink liquid column having a diameter of about several μm to several tens of μm is formed.

Further, when a finite time elapses, the silk thread grows, and the growth of the silk thread, vibration caused by Rayleigh / Weber instability, uneven distribution of colorant particles in the meniscus, uneven distribution of electrostatic field on the meniscus, etc. The silk thread is broken by the interaction. Then, the split string is ejected as ink droplets R, flies toward the recording medium P, and is also pulled by the bias voltage of the counter electrode to land on the recording medium P. It should be noted that the growth and splitting of the kite and the movement of the color material particles to the meniscus (spinner) occur continuously during the application of the drive voltage. Therefore, by adjusting the time for applying the driving voltage, the ejection amount of ink droplets per pixel or dot can be adjusted.
When the application of the drive voltage is completed (discharge is turned off), the state returns to the state of the meniscus where the bias voltage is applied to the counter electrode 24 and the guard electrode 20.

  Here, in the ink jet head of this embodiment, as shown in FIGS. 1A and 1B, the ejection electrode 18 is exposed to the ink flow path 30 and is in contact with the ink Q. For this reason, when a drive voltage is applied (discharge on) to the discharge electrode 18 in contact with the ink Q in the ink flow path 30, a part of the charge supplied to the discharge electrode 18 is injected into the ink Q, and the discharge port 28. The conductivity of the ink Q located between the ejection electrodes 18 is increased. Therefore, in the inkjet head 10 of the present embodiment, the ink Q is in a state in which the ink droplet R is easily ejected when the drive voltage is applied to the ejection electrode 18 (when ejection is on) (ejection properties). Has improved). In this state, since ink is ejected according to the ejection principle described above, a high-quality image can be formed on the recording medium.

  Further, in the present embodiment, since the ejection port 28 of the inkjet head 10 has an elongated hole shape that is elongated in the ink flow direction, the ink can easily flow into the ejection port 28, and ink can be supplied to the ejection port 28. Is high. For this reason, the ink particle supply property to the ink guide tip 14a is also improved. Therefore, even if the ejection frequency during image recording is increased and ink droplets are ejected continuously at a high speed, dots of a desired size are stably formed on the recording medium. According to the present invention, when the image output time is taken into consideration, an ejection frequency of 5 kHz, preferably 10 kHz, more preferably 15 kHz can be realized. Furthermore, by setting the aspect ratio of the ejection port 28 to 1 or more, the flow of ink becomes smooth and clogging at the ejection port 28 can be prevented.

Next, the ink used in the inkjet head 10 of the present invention will be described.
Ink Q is obtained by dispersing colorant particles in a carrier liquid. The carrier liquid is preferably a dielectric liquid (nonaqueous solvent) having a high electric resistivity (10 ⁹ Ω · cm or more, preferably 10 ¹⁰ Ω · cm or more). If the electric resistance of the carrier liquid is low, the carrier liquid itself is charged by charge injection due to the drive voltage applied to the control electrode, and the colorant particles do not concentrate. In addition, a carrier liquid having a low electrical resistance is not suitable because there is a concern of causing electrical conduction between adjacent control electrodes.

The relative dielectric constant of the dielectric liquid used as the carrier liquid is preferably 5 or less, more preferably 4 or less, and still more preferably 3.5 or less. By setting the relative dielectric constant in such a range, an electric field effectively acts on the colorant particles in the carrier liquid, and migration easily occurs.
Note that the upper limit value of the specific electric resistance of such a carrier liquid is desirably about 10 ¹⁶ Ω · cm, and the lower limit value of the relative dielectric constant is desirably about 1.9. The reason why it is desirable that the electric resistance of the carrier liquid is in the above range is that if the electric resistance is low, ink ejection under a low electric field is deteriorated, and the reason why the relative dielectric constant is preferably in the above range is the reason. This is because, when the dielectric constant increases, the electric field is relaxed by the polarization of the solvent, and the color of the dots formed thereby becomes thin or causes blurring.

  The dielectric liquid used as the carrier liquid is preferably a linear or branched aliphatic hydrocarbon, alicyclic hydrocarbon, or aromatic hydrocarbon, and halogen-substituted products of these hydrocarbons. For example, hexane, heptane, octane, isooctane, decane, isodecane, decalin, nonane, dodecane, isododecane, cyclohexane, cyclooctane, cyclodecane, benzene, toluene, xylene, mesitylene, Isopar C, Isopar E, Isopar G, Isopar H, Isopar L, Isopar M (isopar: trade name of Exxon), Shellsol 70, Shellsol 71 (shellsol: trade name of Shell Oil), Amsco OMS, Amsco 460 Solvent (trade name of Amsco: Spirits), Silicone oil (for example, KF-96L manufactured by Shin-Etsu Silicone) or the like can be used alone or in combination.

  The colorant particles dispersed in such a carrier liquid may be dispersed in the carrier liquid as the colorant itself as colorant particles, but preferably contain dispersed resin particles for improving fixability. When the dispersed resin particles are included, the pigment is generally coated with the resin material of the dispersed resin particles to form resin-coated particles, and the dye is colored with the dispersed resin particles to form colored particles. Is common.

As the coloring material, any of conventional pigments and dyes used in a flow channel kujet ink composition, a printing (oil-based) ink composition, or an electrophotographic liquid developer can be used.
As the pigment used as the color material, regardless of inorganic pigments or organic pigments, those generally used in the technical field of printing can be used. Specifically, for example, carbon black, cadmium red, molybdenum red, chrome yellow, cadmium yellow, titanium yellow, chromium oxide, viridian, cobalt green, ultramarine blue, Prussian blue, cobalt blue, azo pigment, phthalocyanine pigment Conventionally known pigments such as quinacridone pigments, isoindolinone pigments, dioxazine pigments, selenium pigments, perylene pigments, perinone pigments, thioindigo pigments, quinophthalone pigments and metal complex pigments are used without particular limitation. be able to.
As dyes used as coloring materials, azo dyes, metal complex dyes, naphthol dyes, anthraquinone dyes, indigo dyes, carbonium dyes, quinoneimine dyes, xanthene dyes, aniline dyes, quinoline dyes, nitro dyes, nitroso dyes, benzoquinone dyes, naphthoquinone dyes And oil-soluble dyes such as phthalocyanine dyes and metal phthalocyanine dyes.

Further, as dispersed resin particles, for example, rosins, rosin-modified phenol resins, alkyd resins, (meth) acrylic polymers, polyurethane, polyester, polyamide, polyethylene, polybutadiene, polystyrene, polyvinyl acetate, polyvinyl alcohol, polyvinyl alcohol, acetal-modified Products, polycarbonate and the like.
Among these, from the viewpoint of ease of particle formation, the weight average molecular weight is in the range of 2,000 to 1,000,000 and the polydispersity (weight average molecular weight / number average molecular weight) is 1.0 to 5 Polymers in the range of 0.0 are preferred. Furthermore, from the viewpoint of ease of fixing, a polymer having any one of a softening point, a glass transition point, and a melting point within a range of 40 ° C. to 120 ° C. is preferable.

  In the ink Q, the content of the color material particles (the total content of the color material particles or further dispersed resin particles) is preferably contained in the range of 0.5 to 30% by weight with respect to the whole ink, and more preferably. Is preferably contained in the range of 1.5 to 25% by weight, more preferably 3 to 20% by weight. If the content of the colorant particles is reduced, problems such as insufficient printed image density or difficulty in obtaining a strong image due to difficulty in obtaining the affinity between the ink Q and the surface of the recording medium P are likely to occur. On the other hand, when the content increases, it becomes difficult to obtain a uniform dispersion, or the ink Q is easily clogged with an inkjet head or the like, and it is difficult to obtain stable ink discharge. .

  The average particle diameter of the colorant particles dispersed in the carrier liquid is preferably 0.1 to 5 μm, more preferably 0.2 to 1.5 μm, and still more preferably 0.4 to 1.0 μm. . This particle size is determined by CAPA-500 (trade name, manufactured by Horiba, Ltd.).

After the colorant particles are dispersed in the carrier liquid (a dispersant may be used if necessary), the chargeant is added to the carrier liquid to charge the colorant particles, and the charged colorant The ink Q is obtained by dispersing particles in a carrier liquid. When dispersing the colorant particles, a dispersion medium may be added as necessary.
As an example of the charge control agent, various materials used in electrophotographic liquid developers can be used. Also, “Recent development and commercialization of electrophotographic development systems and toner materials”, pages 139 to 148, “The Basics and Applications of Electrophotographic Technology” edited by Electrophotographic Society, pages 497 to 505 (Corona Inc., published in 1988), Yuji Harasaki Various charge control agents described in “Electrophotography” 16 (No. 2), p. 44 (1977) can also be used.

The color material particles may be positively charged or negatively charged as long as they have the same polarity as the drive voltage applied to the control electrode.
The charge amount of the color material particles is preferably in the range of 5 to 200 μC / g, more preferably 10 to 150 μC / g, and still more preferably 15 to 100 μC / g.

In addition, since the electric resistance of the dielectric solvent may change due to the addition of the charge control agent, the distribution ratio P defined below is preferably 50% or more, more preferably 60% or more, and even more preferably 70% or more. And
P = 100 × (σ1−σ2) / σ1
Here, σ1 is the electrical conductivity of the ink Q, and σ2 is the electrical conductivity of the supernatant obtained by applying the ink Q to the centrifuge. The electrical conductivity was measured using an LCR meter (AG-4311 manufactured by Ando Electric Co., Ltd.) and an electrode for liquid (LP-05 type manufactured by Kawaguchi Electric Manufacturing Co., Ltd.), applied voltage 5 [V], frequency 1 kHz. It is the value which measured on condition of this. Centrifugation was performed for 30 minutes using a small high-speed cooling centrifuge (Tomy Seiko Co., Ltd. SRX-201) under conditions of a rotational speed of 14500 rpm and a temperature of 23 ° C.
By using the ink Q as described above, migration of charged particles is likely to occur, and concentration is facilitated.

The electrical conductivity of the ink Q is preferably 100 to 3000 pS / cm, more preferably 150 to 2500 pS / cm, and still more preferably 200 to 2000 pS / cm. By setting the electric conductivity in the above range, the voltage applied to the control electrode does not become extremely high, and there is no fear of causing electrical continuity between adjacent recording electrodes.
The surface tension of the ink Q is preferably in the range of 15 to 50 mN / m, more preferably 15.5 to 45 mN / m, and still more preferably 16 to 40 mN / m. By setting the surface tension within this range, the voltage applied to the control electrode does not become extremely high, and the ink does not leak around the head to be contaminated.
Furthermore, the viscosity of the ink Q is preferably 0.5 to 5 mPa · sec, more preferably 0.6 to 3.0 mPa · sec, and still more preferably 0.7 to 2.0 mPa · sec.

As an example, such an ink Q can be prepared by dispersing color material particles in a carrier liquid to form particles, and adding a charge adjusting agent to the dispersion medium to cause the color material particles to be charged. Specific methods include the following methods.
(1) A method in which a color material or further dispersed resin particles are mixed (kneaded) in advance, and then dispersed in a carrier liquid using a dispersant as required, and a charge adjusting agent is added.
(2) A method in which a coloring material, or further dispersed resin particles and a dispersing agent are simultaneously added to a carrier liquid, dispersed, and a charge adjusting agent is added.
(3) A method in which a coloring material and a charge adjusting agent, or further dispersed resin particles and a dispersing agent are simultaneously added to a carrier liquid and dispersed.

FIG. 7 shows a conceptual diagram of an embodiment of an ink jet recording apparatus of the present invention using an ink jet head for carrying out the ink jet head control method of the present invention.
An ink jet recording apparatus 60 (hereinafter referred to as a printer 60) shown in the figure is an apparatus that performs four-color printing on one side of a recording medium P, and includes a conveying means for the recording medium P, an image recording means, and a solvent recovery means. Yes, these are housed in a housing 61.
The conveying means includes a feed roller pair 62, a guide 64, rollers 66 (66a, 66b and 66c), a conveying belt 68, a conveying belt position detecting means 69, an electrostatic adsorption means 70, a static eliminating means 72, a peeling means 74, and a fixing. -It has the conveyance means 76 and the guide 78. The image recording forming unit includes a head unit 80, an ink circulation system 82, a head driver 84, and a recording medium position detecting unit 86. Further, the solvent recovery means includes a discharge fan 90 and a solvent recovery device 92.

  In the conveyance means for the recording medium P, the feed roller pair 62 is a conveyance roller pair provided adjacent to the carry-in port 61 a provided on the side surface of the housing 61. The feed roller 62 feeds the recording medium P supplied from a stocker (not shown) to the transport belt 68 (portion supported by the roller 66a). The guide 64 is provided between the feed roller pair 62 and a roller 66 a that supports the conveyance belt 68, and guides the recording medium P to the conveyance belt 68.

It should be noted that a foreign matter removing means for removing foreign matter such as dust and paper dust adhering to the recording medium P is preferably provided in the vicinity of the feed roller pair 62.
As the foreign matter removing means, one or more of non-contact methods such as known suction removal, blow-off removal, electrostatic removal, and contact methods using brushes, rollers, etc. may be used in combination. Alternatively, the feed roller pair 62 may be a slightly adhesive roller, and a cleaner for the feed roller pair 62 may be provided to remove foreign matters such as dust and paper powder when the recording medium P is fed by the feed roller pair 62.

The conveyor belt 68 is an endless belt stretched around the three rollers 66. At least one of the rollers 66a, 66b, and 66c is connected to a drive source (not shown), and rotates the conveyor belt 68.
The conveyance belt 68 functions as a platen for holding the recording medium P in addition to the scanning conveyance means for the recording medium P during image recording by the head unit 80, and further conveys the image to the fixing / conveyance means 76 after image recording. Therefore, the transport belt 68 is preferably formed of a material having excellent dimensional stability and durability. For example, the transport belt 68 is formed of a metal, a polyimide resin, a fluororesin, another resin, or a composite thereof.

In the illustrated example, since the recording medium P is held on the conveyance belt 68 by electrostatic attraction, the conveyance belt 68 is insulative on the side (front surface) that holds the recording medium P and is in contact with the roller 66 (back surface). ) Has conductivity. In the illustrated example, the roller 66a is a conductive roller, and the back surface of the transport belt 68 is grounded via the roller 66a.
That is, the conveyance belt 68 functions as the counter electrode 24 composed of the electrode substrate 24a and the insulating sheet 24b shown in FIG. 1A when holding the recording medium P.

As such a transport belt 68, a metal belt coated with a fluororesin on the surface side of the metal belt, a belt coated with any of the above resin materials on a metal belt, a resin sheet and a metal belt are bonded together with an adhesive or the like. A belt having a metal layer and an insulator layer manufactured by various methods such as a metal belt or a belt formed by metal vapor deposition on the back surface of a belt made of the above resin may be used.
Further, it is preferable that the surface of the conveying belt 68 that contacts the recording medium P is smooth, and thereby, good adsorbability of the recording medium P can be obtained.

  The conveyor belt 68 is preferably suppressed from meandering by a known method. As a meandering suppression method, for example, the roller 66c is a tension roller, and the shaft of the roller 66c is adjusted between the roller 66a and the roller 66b in accordance with the output of the conveying belt position detecting means 69, that is, the detecting position in the width direction of the conveying belt 68. A method of suppressing meandering by changing the tension at both ends in the width direction of the conveyor belt by tilting with respect to the axis is exemplified. Further, the meandering may be suppressed by forming the roller 66 in a tapered shape, a crown shape, or other shapes.

  Here, as described above, the conveying belt position detecting unit 69 suppresses the meandering of the conveying belt and regulates the position of the recording medium P in the scanning conveying direction at the time of image recording to a predetermined position. 68 is used to detect the position in the width direction, and known detection means such as a photosensor is used.

The electrostatic attraction means 70 applies a predetermined bias voltage to the head unit 80 (the ink jet head of the present invention) to the recording medium P and attracts it to the conveying belt 68 by electrostatic force to hold it. Is charged to a predetermined potential.
In the illustrated example, the electrostatic attraction unit 70 includes a scorotron charger 70a for charging the recording medium P, and a negative high-voltage power supply 70b connected to the scorotron charger 70a. While the recording medium P is conveyed by the feed roller pair 62 and the conveying belt 68, the recording medium P is charged with a negative bias voltage by the scorotron charger 70a connected to the negative high voltage power source 70b, and is applied to the insulating layer of the conveying belt 68. It is electrostatically attracted.

In addition, the conveyance speed of the conveyance belt 68 when charging the recording medium P may be in a range that can be stably charged, and may be the same as or different from the conveyance speed at the time of image recording. Alternatively, the recording medium P may be rotated a plurality of times so that the electrostatic adsorption means acts on the same recording medium P a plurality of times to perform uniform charging.
In the illustrated example, the electrostatic adsorption unit 70 performs electrostatic adsorption and charging of the recording medium P. However, the electrostatic adsorption unit and the charging unit may be provided separately.

  The electrostatic attraction means is not limited to the illustrated scorotron charger 70a, and various other means and methods such as a corotron charger, a solid charger, and a discharge needle can be used. Further, as will be described in detail later, at least one of the rollers 66 is a conductive roller, or a conductive platen is provided on the back surface side (opposite side of the recording medium P) of the conveying belt 68 at the recording position on the recording medium P. By arranging and connecting this conductive roller or conductive platen to a negative high voltage power source, the electrostatic attraction means 70 may be configured, or the conveying belt 68 is an insulating belt and the conductive roller is grounded. The conductive platen may be connected to a negative high voltage power source.

The recording medium P charged by the electrostatic attraction means 70 is transported to the position of the head unit 80 described later by the transport belt 68.
The head unit 80 records an image on the recording medium P by ejecting ink droplets according to image data using an inkjet head that performs the inkjet head control method of the present invention. Here, the ink jet head of the present invention discharges the ink droplet R by superimposing the drive voltage on the bias voltage by applying the drive voltage to the discharge electrode 18 with the charging potential of the recording medium P as the bias voltage, The image is recorded on the recording medium P as described above. At this time, by providing a heating means for the conveying belt 68 and increasing the temperature of the recording medium P, fixing of the ink droplets R on the recording medium P can be promoted, and bleeding is further suppressed and image quality is improved. Can be achieved.
The image recording by the head unit 80 or the like will be described in detail later.

The recording medium P on which the image is recorded is discharged by the discharging unit 72, peeled off from the transport belt 68 by the peeling unit 74, and transported to the fixing / transporting unit 76.
In the illustrated example, the static elimination unit 72 is a so-called AC corotron static eliminator having a corotron static eliminator 72a, an AC power source 72b, and a DC high-voltage power source 72c grounded at one end. In addition to the above, as the static elimination means, various means and methods such as a scorotron static eliminator, a solid charger, a discharge needle, etc. can be used, and a conductive roller, A configuration using a conductive platen is also preferably used.
As the peeling means 74, a known technique such as a peeling blade, a reverse rotation roller, an air knife or the like can be used.

The recording medium P peeled off from the conveying belt 68 is sent to the fixing / conveying means 76, and the image formed by ink jet is fixed. A roller pair including a heat roller 76a and a conveyance roller 76b is used as the fixing / conveying means 76, and the recorded image is heated and fixed while the recording medium P is nipped and conveyed.
The recording medium P on which the image is fixed is guided by a guide 78 and discharged to a discharge stocker (not shown).

Examples of the heat fixing means include general heat fixing such as irradiation with infrared rays or a halogen lamp or a xenon flash lamp, or hot air fixing using a heater, in addition to the heat roll fixing described above. In the heat fixing / conveying means 76, the heating means performs only heating, and the conveying means and the heat fixing means may be provided separately.
In the case of heat fixing, when coated paper or laminated paper is used as the recording medium P, a phenomenon called blistering occurs in which the water inside the paper rapidly evaporates due to a rapid temperature rise and the paper surface is uneven. May occur. In order to prevent this, it is preferable to arrange a plurality of fixing devices and change one or both of the power supply of each fixing device and the distance to the recording medium P so that the temperature of the recording medium P gradually increases.

The printer 60 is preferably configured so that nothing touches the image recording surface of the recording medium P from at least the image recording by the head unit 80 until the fixing by the fixing / conveying means 76 is completed.
Further, the moving speed of the recording medium P at the time of fixing in the fixing / conveying means 76 is not particularly limited, and may be the same as or different from the conveying speed by the conveying belt 68 at the time of image formation. . When the conveyance speed is different from that at the time of image formation, it is preferable to provide a speed buffer for the recording medium P immediately before the fixing / conveyance means 76.

Hereinafter, image recording in the printer 60 will be described in detail.
As described above, the image recording means of the printer 60 includes the head unit 80 that discharges the ink jet, the ink circulation system 82 that supplies and recovers the ink Q to the head unit 80, a computer (not shown), a RIP (Raster Image Processor), and the like. A head driver 84 that drives the head unit 80 by an output drawing signal from an external device, and a recording medium position detecting unit 86 that detects the recording medium P in order to determine an image recording position on the recording medium P are configured.

FIG. 7B is a perspective view schematically showing the head unit 80 and the conveying means for the recording medium P around it.
The head unit 80 has four inkjet heads 80a corresponding to ink ejection of four colors of cyan (C), magenta (M), yellow (Y), and black (K) for recording a full color image. Then, in accordance with a signal from the head driver 84 supplied with the image data, the ink Q supplied by the ink circulation system 82 is ejected as an ink droplet R, and the recording medium P being conveyed by the conveying belt 68 at a predetermined speed. Record an image on The inkjet heads 80 a for the respective colors are arranged in the conveyance direction of the conveyance belt 68.
In addition, the inkjet head 80a of each color of the head unit 80 is an inkjet head according to the present invention.

In the illustrated example, the inkjet heads 80a are line heads in which the ejection ports 28 are arranged in the entire width direction of the recording medium P, and are preferably arranged in a staggered manner as shown in FIG. And a multi-channel head having a plurality of nozzle rows.
Accordingly, in the illustrated example, the recording medium P is held by the transport belt 68 and the recording medium P is simply passed through the head unit 80 once by transport, that is, only one scanning transport is performed. An image is formed on the entire surface of the recording medium P. Therefore, image recording (drawing) can be performed at a higher speed than when the ejection head is serially scanned.

The ink jet head of the present invention can also be used for a so-called serial head (shuttle type). Therefore, the printer 60 may also be in this mode.
In this case, the head unit 80 is configured by aligning the row of the ejection ports 28 of each inkjet head (which may be single row or multi-channel) with the carrying direction of the carrying belt 68, and the head unit 80 is carried by the recording medium P. Scanning means for scanning in a direction orthogonal to the direction is provided. As the scanning means, known scanning means can be used.
The image recording may be performed in the same manner as a normal shuttle type ink jet printer. The recording medium P is intermittently conveyed by the conveying belt 68 according to the length of the row of the ejection ports 28, and is synchronized with the intermittent conveyance. Then, at the time of stop, the head unit 80 is scanned to record an image on the entire surface of the recording medium P.
Thus, the image formed on the entire surface of the recording medium P by the head unit 80 is fixed by the fixing / conveying means 76 by the recording medium P being nipped and conveyed by the fixing / conveying means 76 as described above. Is done.

The head driver 84 receives image data from an external device, receives image data from a system control unit (not shown) that performs various processes, and drives the head unit 80 based on the image data.
This system control unit performs color separation, division into appropriate numbers of pixels and gradations, etc. on image data received from external devices such as computers, RIPs, image scanners, magnetic disk devices, and image data transmission devices. Thus, the head driver 84 serves as image data for driving the head unit 80 (inkjet head). Further, the system control unit controls the ink ejection timing by the head unit 80 in accordance with the conveyance timing of the recording medium P by the conveyance belt 68. The discharge timing is controlled by using an output from the recording medium position detecting unit 86 and an output signal from the encoder disposed on the conveying belt 68 or the driving unit of the conveying belt 68.
The recording medium position detection means 86 is for detecting the recording medium P conveyed to the ink droplet ejection position by the head unit 80, and an already known detection means such as a photosensor is used. it can.
Here, the head driver 84 divides the drawing when there are a large number of ejection units (number of channels) to be controlled, such as when a line head is applied, and a known resistance matrix driving method or resistance diode matrix driving method. May be used. Thereby, the number of ICs used by the head driver 84 can be reduced, and the cost can be reduced and the control circuit size can be suppressed.

  The ink circulation system 82 is for flowing the ink Q through the ink flow paths 30 (see FIG. 1A) of the ink jet heads 80a of the respective colors of the head unit 80, and is for four colors (C, M, Y, K). An ink circulation device 82a having an ink tank of each color, a pump, a replenishment ink tank (not shown), and the like, and an ink of each color from the ink tank of the ink circulation device 82a to the ink flow path 30 of the ink jet head of each color of the head unit 80 An ink supply system 82b that supplies Q and an ink collection system 82c that collects ink from the ink flow path 30 of each color inkjet head of the head unit 80 to the ink circulation device 82a are provided.

The ink circulation system 82 supplies ink Q for each color from the ink tank to the head unit 80 via the ink supply system 80b by the ink circulation device 82a, and for each color from the head unit 80 via the ink supply system 80c. Any ink Q can be used as long as it can be collected and circulated in the ink tank.
The ink tank stores ink Q of each color, and the ink Q is pumped out by a pump and sent to the head unit 80. As the ink is discharged from the head unit 80, the density of the ink circulating in the ink circulation system 82 is decreased. In the ink circulation system 82, the ink density is detected by the ink density detector and replenished accordingly. It is desirable to appropriately replenish ink from the ink tank and maintain the ink density within a predetermined range.

Further, the ink tank is preferably provided with a stirring device for suppressing precipitation / concentration of the solid component of the ink and an ink temperature management device for suppressing temperature change of the ink. This is because if the temperature is not controlled, the ink temperature changes due to changes in the environmental temperature, etc., and the dot diameter changes due to changes in the physical properties of the ink, making it impossible to stably form high-quality images. Because there is.
As the stirring device, a rotary blade, an ultrasonic vibrator, a circulation pump, or the like can be used.
As the ink temperature control device, a known method such as a method in which a heating element such as a heater or a Peltier element or a cooling element is arranged in the head unit 80, ink tank, ink piping system, etc., and control is performed by a temperature sensor, for example, a thermostat. Can be used. When the temperature control device is arranged in the ink tank, it is preferable to arrange it together with the stirring device so as to make the temperature distribution constant. Further, the stirring device for keeping the concentration distribution in the tank constant may be shared with the stirring device for suppressing the precipitation and concentration of the solid component of the ink.

As described above, the printer 60 has the solvent recovery means including the exhaust fan 90 and the solvent recovery device 92. The solvent recovery means recovers the carrier liquid that evaporates from the ink droplets ejected from the head unit 80 onto the recording medium P, particularly the carrier liquid that evaporates from the recording medium P when fixing the image formed by the ink droplets. To do.
The discharge fan 90 is for sucking air inside the casing 61 of the printer 60 and sending it to the solvent recovery device 92.
The solvent recovery device 92 includes a solvent vapor absorber, and adsorbs the solvent component of the gas containing the solvent vapor sucked by the exhaust fan 90 to the solvent vapor absorber, and the gas after the solvent is adsorbed and recovered. The paper is discharged out of the casing 61 of the printer 60. As the solvent vapor absorbing material, various activated carbons are preferably used.

  In the above description, an electrostatic ink jet recording apparatus that records a color image using four color inks of C, M, Y, and K has been described. However, the present invention is not limited to this, and a monochrome recording apparatus Alternatively, recording may be performed using an arbitrary number of inks of other colors, for example, light colors or special colors. In that case, the number of head units 80 and ink circulation systems 82 corresponding to the number of ink colors are used.

  In each of the above examples, the ink jet discharges ink droplets R by charging the color material particles in the ink positively and setting the opposite electrode on the back of the recording medium or recording medium P to a negative high voltage. However, the present invention is not limited to this, and conversely, the color material particles in the ink may be negatively charged, and the recording medium or the counter electrode may be set to a positive high voltage to perform image recording by inkjet. . Thus, when the polarity of the colored charged particles is reversed from the above example, the polarity of the voltage applied to the electrostatic adsorption means, the counter electrode, and the drive electrode of the inkjet head may be reversed from the above example. .

  The inkjet head, the inkjet head control method, and the inkjet recording apparatus according to the present invention have been described in detail above. However, the present invention is not limited to the above-described embodiment, and various improvements can be made without departing from the gist of the present invention. Of course, you may make changes.

(A) is a cross-sectional view schematically showing an ink jet head according to the present invention, and (B) is a view schematically showing a planar configuration of an ejection electrode. It is the figure which showed typically a mode that the several discharge outlet was arranged in two-dimensionally on the discharge outlet board | substrate of an inkjet head. It is the figure which showed typically the planar structure of the guard electrode of the inkjet head of a multichannel structure. It is a typical perspective view for demonstrating the structure of the ink guide weir shown to FIG. 1 (A). It is a figure which shows typically the relationship between a drawing signal, the voltage waveform of the drive voltage applied to a discharge electrode, and the voltage waveform of the pulse voltage applied to a guard electrode. (A) is a diagram schematically showing the state of the discharge port 28 when a drive voltage is applied to the discharge electrode 18, and (B) is a view when no drive voltage is applied to the discharge electrode 18 (or low). It is the figure which showed typically the mode of the discharge outlet 28 when a voltage is applied. It is a conceptual diagram of an example of the inkjet recording device of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Inkjet head 12 Head substrate 14 Ink guide 14a Tip part 16 Discharge port substrate 18 Discharge electrode 20 Guard electrode 24 Counter electrode 24a Electrode substrate 24b Insulating sheet 26a Scorotron charger 26b Bias voltage source 28 Each discharge port 30 Ink flow path 32 Insulating substrate 33 Drive voltage controller 34 Insulating layer 35 Guard electrode controller 36 Opening 40 Ink guide weir 60 Inkjet recording apparatus (inkjet printer)
62 Feed roller 64 Guide 66 Roller 66a, 66b, 66c Roller 68 Conveying belt 69 Conveying belt position detecting means 70 Electrostatic adsorption means 70a Scorotron charger 70b High voltage power supply 72 Discharge means 72a Corotron static remover 72b AC power supply 72c DC high voltage power supply 76 Fixing Transport means 76a Heat roller 78 Guide 80 Head unit 80a Inkjet heads 80b, 80c Ink supply system 82 Ink circulation system 82a Ink circulation device 82b Ink supply system 82c Ink collection system 84 Head driver 86 Recording medium position detection means 90 Discharge fan E ₁ , E ₂ electric field F ₁ , F ₂ electrostatic force P recording medium Q ink R ink droplet

Claims (15)

A method for controlling an inkjet head that ejects ink droplets by electrostatic force,
The inkjet head includes a discharge port substrate on which a plurality of discharge ports from which the ink droplets are discharged is formed;
A discharge electrode that is formed corresponding to each of the plurality of discharge ports of the discharge port substrate and generates an electrostatic field in each of the plurality of discharge ports;
In order to block the electric field generated from the adjacent discharge electrodes, the discharge port substrate is formed between the adjacent discharge ports at a position closer to the ink discharge side than the discharge electrodes and insulated from the discharge electrodes. And a guard electrode to be
A drive voltage is applied to the ejection electrode in accordance with a drawing signal, and a pulse voltage having the same frequency as the drive voltage applied to the ejection electrode and alternately repeating a positive voltage and a negative voltage is applied to the guard electrode. Control method for inkjet head.   The inkjet head control method according to claim 1, wherein the guard electrodes are controlled in common.   The method of controlling an ink jet head according to claim 1 or 2, wherein the pulse voltage is applied so as to have a phase opposite to the signal of the driving voltage when forming one dot on the recording medium.   The pulse voltage is applied according to any one of claims 1 to 3, wherein at least one of a phase and a pulse width is different from a signal of the drive voltage when forming one dot on the recording medium. Ink jet head control method.   The inkjet head control method according to claim 4, wherein at least one of a phase and a pulse width of the pulse voltage is controlled so that the voltage is switched from a positive voltage to a negative voltage immediately before the drive voltage is applied to the ejection electrode.   6. The inkjet head control according to claim 4, wherein at least one of a phase and a pulse width of the pulse voltage is controlled so as to switch from a negative voltage to a positive voltage before the application of the drive voltage to the ejection electrode is completed. Method.   The ink jet head control method according to claim 1, wherein the ink is an ink obtained by dispersing charged fine particles including at least a coloring material in an insulating dispersion medium. An ink jet head that records an image on a recording medium by discharging ink containing charged fine particles using electrostatic force,
A discharge port substrate on which a plurality of discharge ports from which the ink droplets are discharged is formed;
A discharge electrode formed to correspond to each of the plurality of discharge ports of the discharge port substrate, and for generating an electric field at each of the discharge ports;
In order to block the electric field generated from the adjacent discharge electrodes, the discharge port substrate is formed between the adjacent discharge ports at a position closer to the ink discharge side than the discharge electrodes and insulated from the discharge electrodes. Guard electrode to be
Guard electrode control for applying to the guard electrode a pulse voltage that is connected to the guard electrode and has the same frequency as the drive voltage applied to the ejection electrode and in which a positive voltage and a negative voltage are alternately repeated. An inkjet head.   The inkjet head according to claim 8, wherein the guard electrode is controlled in common.   The said guard electrode control part applies the said pulse voltage so that it may become a reverse phase with respect to the signal of the drive voltage applied to the said discharge electrode, when forming 1 dot in the said recording medium. The inkjet head as described.   The guard electrode control unit applies the pulse voltage so that at least one of a phase and a pulse width differs from a signal of a drive voltage applied to the ejection electrode when forming one dot on the recording medium. Item 10. The inkjet head according to any one of Items 8 to 10.   The said guard electrode control part controls at least one of the phase of the said pulse voltage, and a pulse width so that it may switch from a positive voltage to a negative voltage just before the said drive voltage is applied to the said discharge electrode. Inkjet head.   The said guard electrode control part controls at least one of the phase and pulse width of the said pulse voltage so that it may switch from a negative voltage to a positive voltage before completion | finish of the application of the said drive voltage to the said discharge electrode. The inkjet head described in 1.   The inkjet head according to any one of claims 8 to 13, wherein the ink is an ink obtained by dispersing charged fine particles including at least a coloring material in an insulating dispersion medium. The inkjet head according to any one of claims 8 to 14,
An ink jet recording apparatus comprising: a moving means for relatively moving the ink jet head and the recording medium.

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