JP2005297295A - Inkjet recording method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inkjet recording method of an electrostatic system excellent in discharge stability such as ink impact position and density. <P>SOLUTION: In inkjet recording of discharging ink with charged color material particles by an electrostatic force, a plurality of ink liquid droplets are discharged by application of a driving voltage of one pulse. Moreover, the first ink liquid droplet by this application of the driving voltage of one pulse is made the maximum size and/or the maximum density, or pulse width modulation is further carried out. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrostatic ink jet recording method in which an ink composition is discharged using electrostatic force.

Electrostatic ink jet recording uses, for example, an ink composition (hereinafter referred to as ink) that includes a color material and in which charged fine particles (hereinafter referred to as color material particles) are dispersed in a dispersion medium. By applying a predetermined voltage to each ejection part of the inkjet head according to the data, the electrostatic force is used to eject and control the ink, and an image corresponding to the image data is recorded on the recording medium.
As this electrostatic ink jet recording apparatus, for example, an ink jet recording apparatus disclosed in Patent Document 1 is known.

  FIG. 4 shows a conceptual diagram of an electrostatic inkjet head disclosed in Patent Document 1. As shown in FIG.

The inkjet head 80 includes a head substrate 82, an ink guide 84, an insulating substrate 34, an ejection electrode 88, a counter electrode 90, a DC bias voltage source 92, and a pulse voltage source 94.
A nozzle (through hole) 96 for ejecting ink is formed in the insulating substrate 34. A head substrate 82 is provided extending in the arrangement direction of the nozzles 96, and an ink guide 84 is disposed on the head substrate 82 at a position corresponding to the through hole. The ink guide 84 passes through the nozzle 86, and the tip end portion 84 a protrudes above the surface of the insulating substrate 34 on the recording medium P side.

The head substrate 82 and the insulating substrate 34 are arranged at a predetermined interval, and a flow path 98 for the ink Q is formed between them.
The ink Q is formed by dispersing color material particles (charged to the same polarity as the driving voltage applied to the ejection electrode 88) in a carrier liquid (dispersion medium) as described above, and circulation of ink (not shown). The mechanism circulates in the ink flow path 98 from the right side to the left side in the drawing, for example, and ink is supplied to each nozzle 96.
The discharge electrode 88 is provided in a ring shape so as to surround the periphery of the nozzle 96 on the surface of the surface of the insulating substrate 34 on the recording medium P side. The ejection electrode 88 is connected to a pulse voltage source 94 that generates a pulse voltage in accordance with image data. The pulse voltage source 94 is grounded via a DC bias power source 92.

  In such electrostatic ink jet recording, the recording medium P is preferably charged (bias voltage is applied) to a high voltage opposite to the discharge electrode by a charging device using scorotron band electricity or the like. The insulating layer 44 of the grounded electrode substrate 90 is held.

  When no voltage is applied to the ejection electrode 88, the bias voltage charged on the recording medium P and the Coulomb attractive force between the charged particles (coloring material particles) in the ink, the viscosity of the ink (dispersion medium), the surface tension, and the charged particles As shown in FIG. 4, the ink repulsion force, the fluid pressure of the ink supply, and the like are coupled to form a meniscus shape in which the ink is slightly raised from the nozzle 96 and is balanced.

When a drive voltage is applied to the ejection electrode 88, the drive voltage is superimposed on the bias voltage, and as a result, the ink Q is attracted to the recording medium P (opposite electrode) side to form a substantially conical so-called Taylor cone. The
When time elapses after the start of application of the drive voltage, the balance between the Coulomb attractive force acting on the colorant particles and the surface tension of the dispersion medium is lost, and an elongated ink liquid column having a diameter of about several μm to several tens of μm, called a kite string Is formed. When the time further elapses, as disclosed in Patent Document 2 and the like, the leading end of the kite string is cut, and ink droplets are ejected toward the recording medium P, and then fly by suction.

  In the electrostatic ink jet, by applying a pulsed drive voltage to each discharge electrode 88 according to the recorded image, each discharge electrode 88 is modulated and driven, and ink droplet discharge is turned on / off. On-demand ink droplets are ejected according to the recorded image.

JP 10-138493 A U.S. Pat. No. 4,314,263

Such an electrostatic ink jet has a so-called nozzleless structure that does not require an independent ink flow path or partition wall for separating each discharge part if a discharge electrode can be created corresponding to the discharge part. Therefore, the cost of the inkjet head can be reduced, and the yield is high. Furthermore, because of the above structure, even when an ink clogging or the like occurs in the ejection unit, it is possible to recover with a simple measure.
On the other hand, with electrostatic inkjets, the ejection and landing position of ink droplets due to splitting of the kite string, etc., and the concentration of ink (the amount of colorant particles relative to the carrier liquid) are unstable. There is also a problem that it cannot be obtained stably.

  An object of the present invention is to solve the problems of the prior art described above. In electrostatic ink jet recording, the landing position of an ink droplet is stabilized, the dot diameter on the recording medium and the density of each dot are stabilized. Electrostatic discharge that can stabilize the discharge of ink droplets and achieve high gradation resolution and can stably record high-quality images. Another object of the present invention is to provide an ink jet recording method.

In order to achieve the above object, according to a first aspect of the ink jet recording method of the present invention, the ink liquid is obtained by applying an electrostatic force to ink formed by dispersing charged particles containing a coloring material in a dispersion medium. In ink jet recording in which droplets are ejected, a plurality of ink droplets that form one dot on a recording medium are ejected by applying a pulse of driving voltage for ejecting the ink droplets, and the driving of the one pulse is performed. Provided is an ink jet recording method characterized by first ejecting a maximum size ink droplet in ejecting a plurality of ink droplets by applying a voltage.
In the first aspect of the ink jet recording method of the present invention, it is preferable that the first ink droplet ejected has a volume 1.5 times or more that of the ink droplet ejected later.

  The second aspect of the ink jet recording method of the present invention is an ink jet recording in which ink droplets are ejected by applying an electrostatic force to ink obtained by dispersing charged particles containing a colorant in a dispersion medium. In the above, a plurality of ink droplets that become one dot are ejected on the recording medium by applying one pulse of driving voltage for ejecting the ink droplets, and a plurality of ink droplets are applied by applying this one pulse of driving voltage. In the ink droplet ejection, an ink jet recording method is characterized in that an ink droplet having the highest concentration of the charged particles is first ejected.

Furthermore, the third aspect of the ink jet recording method of the present invention is a method for ejecting ink droplets by applying an electrostatic force to ink obtained by dispersing charged particles containing a colorant in an insulating dispersion medium. In inkjet recording, a plurality of inks that form a string by the ink by application of the ink droplet and one pulse of driving voltage for ejection, and become one dot on the recording medium by the division of the string In ejecting a plurality of ink droplets by ejecting the droplets and applying this one-pulse driving voltage, first, the ink droplet having the highest size and the highest concentration of the charged particles is ejected. An ink jet recording method characterized by adjusting the number of ink droplets ejected by applying the driving voltage of one pulse by adjusting a pulse width of the driving voltage. To provide.
In the third aspect of the ink jet recording method of the present invention, it is preferable that the first ink droplet ejected has a volume 1.5 times or more that of the ink droplet ejected later.

  According to the present invention having the above configuration, in electrostatic ink jet recording, the landing position of the ink droplet on the recording medium is stabilized, and the dot diameter and density (color material particle amount) on the recording medium are stabilized. Can be adjusted to stabilize the discharge of ink droplets, and with the target ink density, dots of the target diameter can be formed and image recording can be performed, so high-quality images can be recorded stably. can do. Further, according to the present invention, if necessary, density control and dot diameter control by pulse width modulation can be performed, and a high-quality image with higher gradation resolution and higher density stability can be obtained. It is also possible to record.

  Hereinafter, the ink jet recording method of the present invention will be described in detail based on preferred embodiments shown in the accompanying drawings.

  The ink jet recording method of the present invention uses an ink containing a coloring material and in which charged fine particles (hereinafter referred to as coloring material particles) are dispersed in a carrier liquid, and discharges the ink by applying an electrostatic force. , By electrostatic ink jet.

The carrier liquid used for such an ink is preferably a dielectric liquid (nonaqueous solvent) having a high electrical 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 ejection electrode, and the colorant particles do not concentrate. Further, a carrier liquid having a low electrical resistance is not suitable for the present invention because there is a concern that electrical conduction between adjacent ejection electrodes may occur.

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.
The upper limit value of the specific electric resistance of such a carrier liquid is preferably about 10 ¹⁶ Ωcm, and the lower limit value of the relative dielectric constant is preferably 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 color material, any pigments and dyes that have been conventionally used in inkjet ink compositions, printing (oil-based) ink compositions, or electrophotographic liquid developers 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, metal complex pigments, and the like are not particularly limited. Can be used.
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 phenolic 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 to 120 ° C. is preferable.

  In the ink, the content of the color material particles (the total content of the color material or further dispersed resin particles) is preferably in the range of 0.5 to 30% by weight, more preferably 1%. It is desirable to contain in the range of 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 print image density or difficulty in obtaining a strong image due to difficulty in obtaining the affinity between the ink and the recording medium surface are likely to occur. On the other hand, when the content is increased, it becomes difficult to obtain a uniform dispersion, or the ink Q is likely to be clogged in the inkjet head 10 or the like, and stable ink ejection is difficult to obtain. .

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.).
The colorant particles preferably have a spherical or elliptical shape in order to cause stable electrophoresis of the colorant particles and to prevent unnecessary solidification (aggregation) of the colorant particles. Further, the colorant particles preferably have a relatively smooth surface.

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 a driving voltage applied to the discharge electrode 18 described later.
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.) under the conditions of an applied voltage of 5 V and a frequency of 1 kHz. This is the measured value. 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 ejection 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 ejection electrode does not become extremely high, and the ink does not leak around the head and become 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. As a concrete method,
(1) A method in which a coloring 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; and
(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 is exemplified.

As described above, the ink jet recording method of the present invention relates to electrostatic ink jet recording in which ink droplets are ejected by applying static electricity to such ink obtained by dispersing colorant particles in a carrier liquid. It is.
FIG. 1 conceptually shows an example of an electrostatic ink jet recording apparatus for carrying out the ink jet recording method of the present invention. In FIG. 1, (a) is a (partial cross section) perspective view, and (b) is a partial cross sectional view.

An ink jet recording apparatus 10 (hereinafter referred to as a recording apparatus 10) shown in FIG. 1 includes an ink jet head 12 (hereinafter referred to as a head 12), a holding means 14 for a recording medium P, and a charging unit 16. Is done.
In the recording apparatus 10, the recording medium P is charged with a predetermined high potential by the charging unit 16, and the head 12 and the holding unit 14 are relatively moved with the head 12 and the recording medium P facing each other (an example). In addition, the recording medium P is conveyed by moving the holding means 14, and the ink droplets R are on-demand by applying a driving voltage to each ejection electrode of the head 12 in accordance with the recorded image to perform modulation driving. The target image is recorded on the recording medium P.

The head 12 discharges ink particles R as ink droplets R by applying an electrostatic force to the ink Q formed by dispersing the color material particles (including the color material and charged fine particles) as described above in a carrier liquid. This is an electric ink jet head, and includes a head substrate 20, a discharge substrate 22, and an ink guide 24.
In the head 12, the head substrate 20 and the discharge substrate 22 are arranged facing each other and spaced apart from each other by a predetermined distance, and an ink channel 26 for supplying ink Q to each discharge port 48 described later is formed therebetween. The The ink Q is circulated through a predetermined path including the ink flow path 26 by an ink circulation means (not shown), and at the time of recording, the ink flow path 26 has a predetermined speed (for example, an ink flow of 200 mm / s) in a predetermined direction. For example, it flows in the direction of arrow a in the figure.

  In the illustrated example, the head 12 has a multi-channel structure in which a large number of ejection portions are two-dimensionally arranged as shown in FIG. 2, but for ease of explanation, FIG. FIG. 1A shows only one ejection unit, and FIG. 1B shows only two ejection units.

The head substrate 20 is a sheet-like insulative substrate common to all ejection units, and a floating conductive plate 28 that is in an electrically floating state is provided on the surface thereof.
The floating conductive plate 28 generates an induced voltage that is induced in accordance with a drive voltage applied to a first discharge electrode 36 and a second discharge electrode 38 to be described later when an image is recorded. In addition, the voltage value of the induced voltage automatically changes according to the number of operating channels. By this induced voltage, the color material particles contained in the ink Q in the ink flow path 26 are energized and migrate to the discharge substrate 22 side, that is, the concentration of the ink Q at the discharge port 48 described later is more preferably performed. Is called.

  The floating conductive plate 28 is not an essential component and is preferably provided as necessary. Further, the floating conductive plate 28 may be disposed on the head substrate 20 side with respect to the ink flow path 26, and may be disposed inside the head substrate 20, for example. In addition, the floating conductive plate 28 is preferably disposed on the upstream side of the ink flow path 26 from the position where the ejection unit is disposed. Further, a predetermined voltage may be applied to the floating conductive plate 28.

On the other hand, the ejection substrate 22 is a sheet-like insulating substrate that is common to all ejection sections, like the head substrate 20, and includes an insulating substrate 34, a first ejection electrode 36, a second ejection electrode 38, and a guard. An electrode 40 and insulating layers 42, 44 and 46 are provided. The ejection substrate 22 has an ink ejection port 48 penetratingly opened at a position corresponding to each ink guide 24, and the ink guide 24 protrudes from the surface of the ejection substrate 22 by the ink guide 24. Is inserted. As described above, the head substrate 20 and the discharge substrate 22 are spaced apart, and the ink flow path 26 is formed therebetween.
In the head 12, a corresponding first discharge electrode 36, second discharge electrode 38, discharge port 48, ink guide 24, and the like constitute one ink discharge portion.

The first discharge electrode 36 and the second discharge electrode 38 are circular electrodes provided in a ring shape on the surfaces of the upper surface and the lower surface of the insulating substrate 34 in the drawing so as to surround the periphery of the corresponding discharge ports 48, respectively. Yes (see FIG. 2).
The surfaces of the insulating substrate 34 and the first discharge electrode 36 are covered with an insulating layer 44 that protects and planarizes the surface. Similarly, the surfaces of the insulating substrate 34 and the second discharge electrode 38 are covered with the insulating layer 44. An insulating layer 42 for planarizing the surface is covered. Further, a guard electrode 40 is formed on the insulating layer 44, and an insulating layer 46 for planarizing the surface is formed on the guard electrode 40 and the insulating layer 44.

In the head 12, when the driving voltage is applied (driven (on)) to the second ejection electrode 38 and the first ejection electrode 36 is on, the ink Q is ejected as ink droplets R (ejection on). In the case where the driving voltage is not applied to at least one of the first ejection electrode 36 and the second ejection electrode 38 (non-driving (off)), the ink is not ejected (ejection off).
Then, the ink droplets R ejected from each ejection unit are attracted to the recording medium P charged with a negative bias voltage, and adhere to a predetermined position of the recording medium P to form an image.

Note that the first discharge electrode 36 and the second discharge electrode 38 are not limited to ring-shaped circular electrodes, and may be substantially circular electrodes, divided circular electrodes, parallel electrodes, for example, as long as the electrodes are disposed so as to face the ink guide 24. Any shape such as a substantially parallel electrode may be used.
Further, the discharge electrode is not limited to the two-layer electrode structure of the first discharge electrode 36 and the second discharge electrode 38, but may be a single-layer electrode structure or an electrode structure having three or more layers.

The guard electrode 40 is a sheet-like electrode that is common to all the discharge portions, and a portion corresponding to the first discharge electrode 36 and the second discharge electrode 38 formed around each discharge port 48 is opened in a ring shape. (See FIG. 2 (a)). That is, the guard electrode 40 is an electrode disposed between the ejection electrodes.
The surfaces of the insulating layer 44 and the guard electrode 40 are covered with an insulating layer 46 that protects and flattens the surfaces.

A predetermined voltage is applied to the guard electrode 40 and serves to suppress electric field interference generated between the ink guides 24 of the adjacent ejection portions.
The guard electrode 40 is not an essential component. Further, on the ejection substrate 22, a shield electrode is provided on the ink channel 26 side from the second ejection electrode 38 in order to shield a repulsive electric field from the first ejection electrode 36 or the second ejection electrode 38 toward the ink channel 26. May be provided.

  The ink guide 24 is a ceramic flat plate having a convex tip portion 30 and having a predetermined thickness. In the illustrated example, the ink guides 24 of the ejection units in the same row are disposed at a predetermined interval on the same support 47 disposed on the floating conductive plate 28 on the head substrate 20. The ink guide 24 passes through the ejection port 48 opened in the ejection substrate 22, and the tip portion 30 is located above the outermost surface of the ejection substrate 22 on the recording medium P side (the upper surface in the drawing of the insulating layer 46). It protrudes.

The tip portion 30 of the ink guide 24 is formed in a substantially triangular shape (or trapezoidal shape) that gradually becomes thinner toward the holding means 14 of the recording medium P.
In addition, it is preferable that the metal is vapor-deposited in the front-end | tip part 30 (most advanced part). Although the metal vapor deposition of the tip portion 30 is not an essential element, there is an effect that the dielectric constant of the tip portion 30 is substantially increased and a strong electric field can be easily generated.

  The shape of the ink guide 24 is not particularly limited as long as the colorant particles in the ink Q can migrate toward the tip portion 30 (that is, the ink Q is concentrated). For example, the tip portion 30 is convex. It may be changed freely such as not having to. Further, in order to promote the concentration of ink, a notch serving as an ink guide groove for collecting the ink Q in the tip portion 30 by capillary action may be formed in the central portion of the ink guide 24 along the vertical direction in the drawing. .

  As shown in FIG. 2, in the head 12, each ejection unit including the ink guide 24, the first ejection electrode 36, the second ejection electrode 38, the ejection port 48, and the like is two-dimensionally arranged in a matrix. Yes.

Specifically, as shown in FIGS. 2B and 2C, the head 12 arranges the columns of the ejection units arranged in the row direction (sub-scanning direction (lateral direction in FIG. 2)) in the column direction (main direction). 3 rows (A row, B row, C row) in the scanning direction (vertical direction in FIG. 2). Note that FIG. 2 shows a total of 15 ejection units arranged in a matrix of 5 (1 column, 2 columns, 3 columns, 4 columns, 5 columns) in the row direction.
The ejection units in each row are arranged at a predetermined interval in the column direction. In addition, each row is arranged so that the ejection units are shifted from each other by 1/3 pitch in the row direction so that their ejection units are located between the ejection units of other rows (between the row directions).

As an example, the head 12 is a line head having a row of ejection units corresponding to one side of the maximum size recording medium P in the row direction, and the recording medium P held by the holding unit 14 is orthogonal to the row direction. Conveyed in the column direction (main scanning), the entire surface of the recording medium P is scanned by the ejection unit, and ink droplets land and an image is recorded.
The ink jet recording method of the present invention is not limited to the one using such a line head, and the recording head is intermittently transported while the recording head is in a direction orthogonal to the transport direction in synchronization with the transport. Ink jet recording using a so-called shuttle type recording head that scans in the direction of recording may be used.

Here, as shown in FIG. 2B, the first ejection electrodes 36 of the ejection sections arranged in the same column are connected to each other. In addition, as shown in FIG. 2C, the second ejection electrodes 38 of the ejection sections arranged in the same row are connected to each other.
Further, although not shown, the first ejection electrode 36 and the second ejection electrode 38 are connected to a pulse power source (not shown) for each row and each column, respectively. Both the first ejection electrode 36 and the second ejection electrode 38 are applied with a driving voltage (pulse voltage) having a pulse width corresponding to a predetermined duty or the like corresponding to a pulse signal having a predetermined frequency by a pulse power source. Ink droplets R are ejected.
Accordingly, the first ejection electrodes 36 arranged in the same column are simultaneously applied with a drive voltage of the same voltage level (on (drives the ejection electrodes)). Similarly, the drive voltage is simultaneously applied (on) to the second ejection electrodes 38 arranged in the same row at the same voltage level.

In the illustrated example, one row recorded on the recording medium P is divided into three groups corresponding to the number of rows of the second ejection electrodes 38 in the column direction, and each row of A row, B row, and C row Record sequentially by dividing.
That is, the A line, the B line, and the C line of the second ejection electrode 38 are sequentially turned on at a predetermined timing according to the conveyance speed of the recording medium P. Corresponding to the on of each row, the first ejection electrodes 36 of each column are turned on according to the image data (recorded image), so that one row is recorded on the recording medium P by three time-divided image recording. Record images. In this way, each ejection electrode is modulated and driven (on / off) to turn on / off the ejection of the ink droplet R, thereby recording an image on demand.
As described above, the positions of the ejection units in each row are different in the row direction, and the recording medium P is conveyed in the column direction, so that the density of the ejection units in each row is three times in the row direction (sub-scanning direction). It is possible to perform image recording with a recording density of.

  As described above, when the rows of the lower second discharge electrodes 38 are sequentially turned on and the upper first discharge electrodes 36 are turned on / off according to the image data, the first discharge electrodes 36 are turned on according to the image data. Therefore, the first discharge electrode 36 frequently changes to the high voltage level or the ground level in the discharge portions on both sides of the respective discharge portions in the column direction. In this case, the influence of the electric field of the adjacent ejection part can be eliminated by biasing the guard electrode 40 to a predetermined guard potential, for example, the ground level, at the time of image recording.

  In the head 12, it is not limited at all whether one or both of the first ejection electrode 36 and the second ejection electrode 38 perform the ink ejection on / off control. That is, ink is ejected when the difference between the voltage value when the ink is on / off on the ejection electrode side and the voltage value on the recording medium P side is larger than a predetermined value, and ink is ejected when the difference is smaller than the predetermined value. What is necessary is just to set suitably the voltage by the side of a discharge electrode and the recording medium P so that it may not discharge.

Accordingly, in the head 12, in contrast to the illustrated example, that is, the first ejection electrodes 36 are sequentially turned on for each column, and the second ejection electrodes 38 are turned on in accordance with the image data, whereby the ink ejection is turned on / off. It is also possible to turn it off.
In this case, the column direction is turned on for each column of the first discharge electrodes 36, and the first discharge electrodes 36 of the discharge units in the columns on both sides of the discharge direction in the column direction are always at the ground level. The first discharge electrodes 26 of the discharge portions in the rows on both sides serve as the guard electrode 40. In this manner, when each column is sequentially turned on by the upper first discharge electrode 36 and the lower second discharge electrode 38 is driven according to the image data, the adjacent discharge is not required even if the guard electrode 40 is not provided. It is possible to improve the recording quality by eliminating the influence of the part.

  In this example, the color material particles in the ink Q are positively charged and the recording medium side is charged to a negative high voltage. However, the present invention is not limited to this. Conversely, the color material particles in the ink are negatively charged. The recording medium P side may be charged to a positive high voltage. As described above, when the polarity of the color material particles is reversed from that in this mode, the counter electrode 18, the charging unit 16 of the recording medium P, and the first discharge electrode 36 and the second discharge electrode 38 of each discharge portion are connected. What is necessary is just to make an applied voltage polarity etc. reverse to said example.

In the recording apparatus 10, the holding means 14 for the recording medium P includes an electrode substrate 50 and an insulating sheet 52, and a predetermined interval (for example, with respect to the tip portion 30 of the ink guide 24 so as to face the head 12. , 200-1000 μm).
The electrode substrate 50 is grounded, and the insulating sheet 52 is disposed on the surface of the electrode substrate 50 on the ink guide 24 side. At the time of recording, the recording medium P is held on the surface of the insulating sheet 52, that is, the holding means 14 (insulating sheet 52) functions as a platen of the recording medium P.
The holding unit 14 is engaged with a moving unit (not shown), and is moved in the row direction (main scanning direction) of the head 12 by the moving unit during image recording. In the recording apparatus 10, the recording medium P is conveyed (scanned) by this as described above. The moving means of the holding means is not particularly limited, and for example, a known plate-like member conveying means may be used.

The charging unit 16 includes a scorotron charger 60 for charging the recording medium P to a negative high voltage, and a bias voltage source 62 that supplies the scorotron charger 60 with a negative high voltage.
The scorotron charger 60 is arranged at a predetermined interval at a position facing the surface of the recording medium P. The negative terminal of the bias voltage source 62 is connected to the scorotron charger 60, and the positive terminal is grounded.

  The charging means of the charging unit 16 is not limited to the scorotron charger 60, and various conventionally known charging means such as a corotron charger and a solid charger can be used.

Prior to image recording, the surface of the insulating sheet 52, that is, the recording medium P, is a predetermined negative high voltage having a polarity opposite to the drive voltage applied to the first ejection electrode 36 and the second ejection control electrode 38 by the charging unit 16. For example, it is charged to -1500V. As a result, the recording medium P is biased to a negative high voltage with respect to the first ejection electrode 36 or the second ejection electrode 38, and electrostatically attracted to the insulating sheet 52 of the holding unit 14 by this bias voltage. .
That is, in the illustrated recording apparatus 10, the recording medium P functions as a counter electrode in electrostatic ink jet recording.

In the illustrated example, the holding means 14 is composed of an electrode substrate 50 and an insulating sheet 52, and the recording medium P is electrostatically attracted to the surface of the insulating sheet 52 by being charged to a negative high voltage by the charging unit 16. However, the present invention is not limited to this, and the holding means 14 is composed only of the electrode substrate 50, and the holding means (electrode substrate 50 itself) 14 is connected to the bias power source 62 to be constantly biased to a negative high voltage. The recording medium P may be electrostatically attracted to the surface of the counter electrode.
Further, the electrostatic adsorption to the holding means 14 of the recording medium P and the application of the negative bias high voltage to the recording medium P or the application of the negative bias high voltage to the holding means 14 are separate negative high voltage sources. The support of the recording medium P by the holding means 14 is not limited to electrostatic adsorption of the recording medium P, and other support methods and support means may be used.

Hereinafter, the electrostatic ink jet recording method of the present invention will be described in detail by explaining the operation of discharging the ink droplet R in the recording apparatus 10.
As described above, in this example, the color material particles of the ink Q are positively charged, and a positive drive voltage is applied to the first discharge electrode 36 and the second discharge electrode 38, so that the recording medium P is applied. Charges a negative bias voltage.

Prior to recording, the charging unit 16 charges the recording medium P with a negative high potential (for example, −1500 V) bias voltage, and electrostatically attracts to the insulating sheet 52 of the holding unit 14 by this bias voltage.
At the time of image recording, the ink Q is circulated in the ink flow path 26 from the right side in the drawing to the left side (in the direction of arrow a in FIG. 1) at a predetermined speed by an ink circulation mechanism (not shown). Further, the moving means (not shown) moves the holding means 14 in the main scanning direction (the column direction), and transports (scans) the recording medium P in the main scanning direction orthogonal to the sub-scanning direction (the row direction). To do.

In a state where only this bias voltage is applied, the ink Q has a Coulomb attractive force between the bias voltage and the charge of the color material particles of the ink Q, a Coulomb repulsion between the color material particles, a viscosity of the carrier liquid, a surface tension, Dielectric polarization force and the like act, and these are coupled to move the colorant particles and the carrier liquid. As shown conceptually in FIG. 3A, the meniscus shape slightly raised from the nozzle 48 is balanced. Is removed.
In addition, the colorant particles move toward the recording medium P charged with a bias voltage by so-called electrophoresis due to the Coulomb attractive force or the like. That is, the ink Q is concentrated in the meniscus of the nozzle 48.

From this state, a driving voltage for ejecting the ink droplet R is applied. That is, in the illustrated example, a driving voltage of about 400 V to 600 V is applied to the first ejection electrode 36 and the second ejection electrode 38 from the corresponding pulse power supply (ejection on).
As a result, the drive voltage is superimposed on the bias voltage, and the movement coupled by the superposition of the drive voltage further occurs in the previous coupling, and the color material particles and the carrier liquid are moved to the bias voltage (counter electrode) side by electrophoresis. That is, it is pulled to the recording medium P side, and as shown conceptually in FIG. 3B, a meniscus grows to form a substantially conical ink liquid column so-called tailor cone from its upper part. Similarly to the above, the color material particles move to the meniscus surface by electrophoresis, and the meniscus ink Q is further concentrated.

When a finite time has elapsed after ejection on (start of application of drive voltage), the color material particles move mainly at the tip of the meniscus with high electric field strength due to the movement of the color material particles. The balance is lost and the meniscus grows abruptly, and as shown conceptually in FIG. 3C, an elongated ink liquid column having a diameter of several μm to several tens of μm, which is called a kite string, is formed.
Furthermore, after a finite time has passed, interactions such as the growth of the silk thread, vibration caused by Rayleigh / Weber instability, uneven distribution of the colorant particles in the meniscus, non-uniform distribution of the electrostatic field on the meniscus, etc. Are ejected / flyed as ink droplets R, and are also pulled by the bias voltage to land on the recording medium P.

The growth and splitting of the kite, and the movement of the color material particles to the meniscus (punch) occur continuously during the application of the drive voltage.
When the application of the drive voltage in one pulse is finished, the state returns to the meniscus state in FIG. 3A in which only the bias voltage is applied after a finite time has elapsed.

  In such an electrostatic ink jet, a plurality of ink droplets R separated from the string formed by applying a driving voltage once (one pulse) land substantially at the same position on the recording medium P, One dot of ink on the recording medium P is formed by the plurality of ink droplets R landed by the application of the driving voltage of one pulse.

Here, in the ink jet recording method of the present invention, in the electrostatic ink jet recording using such color material particles, the largest ink liquid is first discharged in the discharge of the ink droplet R by the application of the driving voltage of one pulse. The droplet R is discharged. That is, the ink droplet R that is first separated from the string by applying one pulse of driving voltage is made larger than the ink droplet R that is ejected later.
In another aspect of the present invention, similarly, in the ejection of the ink droplet R by applying one pulse of the driving voltage, the ink droplet R that is ejected first has the highest concentration (the amount of the color material particles relative to the carrier liquid ( For example, the weight%) is large). In other words, the ink droplet R that is first separated from the string by applying one pulse of driving voltage is made to have a higher density than the ink droplet R that is ejected later.
In the ink jet recording method of the present invention, preferably, the number of ink droplets R to be ejected is adjusted by adjusting the pulse width when one drive voltage is applied and both conditions are satisfied.

  As described above, in the electrostatic ink jet, a plurality of ink droplets R are ejected by applying one pulse of driving voltage, and one dot on the recording medium P is formed by the plurality of ink droplets R. As a matter of course, each ink droplet R has a very fine, so-called mist shape, and its flight, that is, the landing position on the recording medium P is affected by various external forces, and the landing position is As a result, the image quality is degraded. Further, the influence of external force increases as the size of the ink droplet R decreases.

On the other hand, the first ink droplet R ejected by applying one pulse of driving voltage is made larger than the subsequent ink droplet R. Preferably, the size of the ink droplet R is gradually reduced. As a result, the large-sized ink droplet R ejected and flying first reduces the air resistance and the like applied to the small ink droplet R ejected later, so that the small ink droplet R whose flight is more unstable can be reduced. Flying can be stabilized.
As a result, it is possible to stabilize the ejection of the ink droplets as an appropriate position of the landing positions of all the ink droplets R in one dot formed by applying a driving voltage of one pulse, that is, a plurality of droplets. In addition, since variations in the landing positions can be suppressed, the carrier liquid can be efficiently absorbed by the recording medium P, blurring between adjacent dots can be prevented, and productivity can be improved.
In particular, by making the volume of the ink droplet R ejected first to be 1.5 times or more the volume of the subsequent ink droplet, the above effects can be expressed more favorably and higher quality drawing can be achieved. Become.

  Further, as described above, in the electrostatic ink jet using the ink Q containing the color material particles, a string is formed by applying a driving voltage of one pulse, and a plurality of ink liquids are formed by dividing the string. With the droplet R, one dot by the ink Q on the recording medium P is formed. Preferably, the kite string is split at a predetermined splitting frequency according to Rayleigh / Weber instability. Further, the ink Q is concentrated by a thread (meniscus) and discharged.

Accordingly, in such an electrostatic ink jet, the density of the first ink droplet R ejected by applying one pulse of driving voltage is set to be higher than that of the subsequent ink droplets, whereby the duty and By selecting / adjusting the pulse width, the number of ink droplets R ejected by applying a drive voltage of one pulse is adjusted, and the dot diameter and dot density (amount of color material particles) on the recording medium P are adjusted. Can be adjusted. That is, since the ink droplets have a lower concentration after the first, if the number of droplets is reduced, dots that are smaller and have a higher concentration than the amount of color material particles can be formed. In addition, it is possible to form dots with a low density compared to the color material particle amount. Preferably, the above-described effect can be more suitably exhibited by gradually reducing the concentration of the ink droplet R ejected by applying one pulse of driving voltage.
Furthermore, by recording an image by performing pulse width modulation, it is possible to use not only the area modulation by the number of dots, which is carried out by a normal ink jet, but also the density modulation by the dot diameter and the dot density, and more gradation. High resolution image recording can be performed.

In electrostatic ink jet recording using color material particles, the size of ink droplets and the density of the thread, that is, the concentration of ink droplets R (the degree of ink concentration) are affected by various conditions. The present invention can be implemented by appropriately selecting / controlling.
Specifically, the characteristics of the head 12 and the ink Q are exemplified as static control factors. The characteristics of the head 12 include the shape and configuration of various members such as the ink guide 24, the ejection port 48, the first ejection electrode 36, and the second ejection electrode 38, the wettability of the inner wall of the ejection port 48, and the ink Q. The temperature of each part, the distance from the ejection part (tip) to the recording medium P, etc. are exemplified. Further, as the characteristics of the ink Q, various physical property values and the like exemplified above in the description of the ink, such as electric conductivity, surface tension, viscosity, charge amount of the color material particles, content of the color material particles, and the like are exemplified. .

  On the other hand, as dynamic control factors, drive voltage applied to the discharge electrode, drive voltage application waveform, drive voltage application timing, pulse width, discharge frequency (drive pulse frequency), ink Q supply amount, floating conductive plate The voltage applied to the nozzle 28, the waveform of the same voltage, the application timing of the same voltage, the flow state of the ink Q at the ejection port 48, etc. are illustrated.

By appropriately selecting / controlling these, the ink jet recording method of the present invention can be carried out.
As a specific example, the base portion has a quadrangular prism shape with a rectangular bottom surface of 75 × 75 μm, the two opposite surfaces have a quadrangular pyramid shape forming an angle of 60 degrees, and protrudes from the surface of the insulating layer 46 to the tip end. A recording head 12 having an ink guide 30 made of silicon ceramic having an amount of 150 μm and a discharge substrate 22 having a cylindrical shape having an inner diameter of φ150 μm and formed with discharge ports 48 on the inner wall surface of which ink-repellent processing has been performed with a fluorine-based resin. In addition, the bias voltage for charging the recording medium P is − using the ink Q in which the carrier liquid is Isopar G, the electrical resistivity is adjusted to 1.5 × 10 ¹⁰ Ω, and the content of the colorant particles is adjusted to 7% by weight. The inkjet recording method of the present invention is carried out by applying a rectangular wave 600 V drive voltage of 1.5 kV, an applied frequency of 15 kHz, and a duty of 50% to each discharge electrode. Rukoto can.

Such an inkjet recording method of the present invention may be a color image recording or a monochrome image recording as long as the above conditions are satisfied.
In addition, when recording a color image, it is of course preferable to carry out the present invention with all colors. However, the present invention is not limited to this, and the inkjet recording method of the present invention is performed with only one color. You may implement.

  As mentioned above, although the inkjet recording method of the present invention has been described in detail, the present invention is not limited to the above-described embodiments, and various improvements and modifications may be made without departing from the gist of the present invention. Of course.

(A) And (b) is a conceptual diagram of an example of the inkjet recording device which implements the inkjet recording method of this invention. (A)-(c) is a conceptual diagram for demonstrating the discharge electrode of the inkjet recording device shown in FIG. (A)-(c) is a conceptual diagram for demonstrating the inkjet recording method of this invention. It is a conceptual diagram for demonstrating the conventional electrostatic inkjet recording.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Inkjet recording device 12,80 (Inkjet) Head 14 Holding means 16 Charging unit 20,82 Head substrate 22 Nozzle substrate 24,84 Ink guide 26 Ink flow path 28 Floating conductive plate 30 Tip portion 34,86 Insulating substrate 36 1st Discharge electrode 38 Second discharge electrode 40 Guard electrode 42, 44, 46 Insulating layer 48, 96 Nozzle 50 Electrode substrate 52 Insulating sheet 60 Scorotron charger 62 Bias voltage source

Claims (5)

In inkjet recording in which droplets of the ink are ejected by applying an electrostatic force to ink formed by dispersing charged particles containing a colorant in a dispersion medium,
By applying one pulse of driving voltage for discharging the ink droplets, a plurality of ink droplets forming one dot on the recording medium are discharged, and a plurality of ink liquids are generated by applying this one pulse of driving voltage. An ink jet recording method characterized by ejecting ink droplets of a maximum size first in ejecting droplets.   The ink jet recording method according to claim 1, wherein the first ink droplet ejected has a volume 1.5 times or more that of the ink droplet ejected later. In inkjet recording in which droplets of the ink are ejected by applying an electrostatic force to ink formed by dispersing charged particles containing a colorant in a dispersion medium,
By applying one pulse of driving voltage for discharging the ink droplets, a plurality of ink droplets forming one dot on the recording medium are discharged, and a plurality of ink liquids are generated by applying this one pulse of driving voltage. An ink jet recording method characterized by ejecting an ink droplet having the highest concentration of the charged particles first in ejecting the droplet. In inkjet recording in which droplets of the ink are ejected by applying an electrostatic force to ink formed by dispersing charged particles containing a coloring material in an insulating dispersion medium,
By applying a drive voltage of one pulse for discharging with the ink droplets, a string is formed by the ink, and a plurality of ink droplets that become one dot on the recording medium is discharged by the division of the string. In the discharge of a plurality of ink droplets by applying the driving voltage of one pulse, first, the ink droplet having the maximum size and the highest concentration of the charged particles is discharged, and the pulse width of the driving voltage is further discharged. Adjusting the number of ink droplets to be ejected by applying the one-pulse driving voltage.   The ink jet recording method according to claim 4, wherein the first ink droplet ejected has a volume 1.5 times or more that of the ink droplet ejected later.

Images