JP2004066765A - Electrostatic inkjet recorder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrostatic inkjet recorder capable of high speed printing by preventing ink from blurring as much as possible thereby forming a clear dot. <P>SOLUTION: Density of ink being supplied from an ink tank to a print head is set not higher than 10% and the magnitude of a bias voltage or a pulse voltage from a bias voltage source or a signal voltage source is controlled thus condensing the ink to have a density not lower than 15%. Furthermore, the ink being supplied from the ink tank to the print head satisfies a relation of 0.8×k×10<SP>6</SP>≤Q/M≤3.5×k×10<SP>6</SP>, wherein k is the conductivity of ink and Q/M is the quantity of charges. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an electrostatic ink jet recording apparatus that performs printing by applying an electric field to ink in which charged toner is dispersed and applying a Coulomb force to eject ink droplets containing the toner onto recording paper. .
[0002]
[Prior art]
Japanese Patent Application Laid-Open No. H11-34338 discloses that an ink is ejected toward a recording paper by generating an electric field around the leading end of a print head and applying an electrostatic force to ink staying at the leading end of the print head. An electrostatic ink jet recording apparatus for performing printing is described.
[0003]
As shown in FIG. 9, this electrostatic ink jet recording apparatus includes a plurality of print heads 100 arranged in one direction (in FIG. 9, a direction perpendicular to the paper surface), and a set plate 101 for the print heads 100 and recording paper P. (Intermediate electrode 102) between the print head 100 and the set plate 101 to generate an electric field around the print head 100. A voltage is applied to the intermediate electrode 102 to generate a higher potential gradient between the print head 100 and the intermediate electrode 102. As a result, the Coulomb force is applied to the ink by a stronger electric field, and the ink is ejected from the leading end of the print head 100 toward the recording paper P, so that dots forming a print are formed on the recording paper P.
[0004]
[Problems to be solved by the invention]
By the way, the ink used in this type of recording apparatus is composed of a liquid component and a solid component called a toner, of which the liquid component evaporates when the ink droplet reaches the recording paper. However, when the ratio of the liquid component is large (the concentration of the toner is small), as shown in FIG. 8B, the ink droplets are diffused outward and the dots are blurred. In particular, before the liquid component evaporates, the ink droplet contacts the undried ink droplet ejected from the adjacent print head 100 on the recording paper, or the ink droplet continuously moves to the same position before evaporation. Is ejected, dot bleeding becomes noticeable. Therefore, in order to prevent dot bleeding from occurring, it is necessary to increase the density of the ink ejected on the recording paper.
[0005]
However, if the density of the ink supplied to the ejection unit is increased to prevent bleeding of the dots, since the viscosity of the ink is relatively high, the toner precipitates in the circulation path of the ink to the ejection unit, or Problems such as toner stagnation at the tip of the discharge unit and subsequent toner clogging may occur.
[0006]
Further, in order to prevent the bleeding of the dots, if the apparatus is configured to wait for the ejection of the ink droplets from the same or adjacent print head 100 until the liquid component of the ink droplets evaporates, dot formation, As a result, the printing speed cannot be increased.
[0007]
The present invention has been made in view of such circumstances, and an electrostatic ink jet recording apparatus capable of preventing bleeding of ink as much as possible, forming clear dots, and thereby speeding up printing. The purpose is to provide.
[0008]
[Means for Solving the Problems]
According to the first aspect of the invention, the charged ink supplied through the flow path is discharged from the discharge electrode by an electric field generated by applying a potential difference of a predetermined level or more between the discharge electrode and the external electrode in a pulsed manner. In an electrostatic ink jet recording apparatus that discharges droplets toward a recording medium surface, the discharge is performed during a period excluding a period in which a potential difference of the predetermined level or more is applied between the discharge electrode and an external electrode in a pulsed manner. A bias supply unit configured to supply the potential difference at the predetermined level as a bias voltage between the electrode and the external electrode, wherein the ink temporarily stored in the discharge electrode is used for the ink including the conductivity and the charge amount of the ink. It is characterized in that the concentration is increased to a predetermined concentration based on the relationship between physical properties and the predetermined level of the potential difference.
[0009]
According to the present invention, an electric field is generated by applying a potential difference of a predetermined level or more between the ejection electrode and the external electrode in a pulsed manner, and the electric field causes charged ink supplied through the flow path to be ejected. The droplets are ejected from the electrodes toward the recording medium surface.
[0010]
In this case, a potential difference of a predetermined level is supplied as a bias voltage between the ejection electrode and the external electrode during a period except for a period in which the potential difference of the predetermined level or more is applied between the ejection electrode and the external electrode in a pulsed manner. Bias supply means, and the ink temporarily stored in the ejection electrode is reduced to a predetermined concentration based on the relationship between the physical properties of the ink including the conductivity and the charge amount of the ink and the potential difference at the predetermined level. Since the ink is concentrated, the ink having a density lower than the predetermined density is supplied to the discharge electrode, and the ink discharged from the discharge electrode is concentrated to the predetermined density, so that toner clogging or the like in the flow path can be prevented. It is possible to prevent bleeding of dots while avoiding the above.
[0011]
According to a second aspect of the present invention, in the electrostatic inkjet recording apparatus according to the first aspect, the external electrode is an intermediate electrode provided between a discharge electrode and a recording medium surface. Is what you do. According to the present invention, since the external electrode is an intermediate electrode provided between the ejection electrode and the recording medium surface, the electric field around the ejection electrode is increased, and the responsiveness of ink ejection from the ejection electrode is improved. Straightness is improved.
[0012]
In this case, when the weight of the ink is M (g), the total charge amount of the ink is Q (C), and the conductivity is k (S / cm), the charge amount (Q / M) and the conductivity k are calculated. Equation (1)
0.8 × k × 10 ⁶ ≦ Q / M ≦ 3.5 × k × 10 ⁶ ... (1)
Is supplied to the discharge electrode (claim 3), the concentration of the ink can be reliably concentrated to the predetermined concentration regardless of the voltage of the discharge electrode or the intermediate electrode, the shape of the discharge electrode, and the like. .
[0013]
According to a fourth aspect of the present invention, in the electrostatic inkjet recording apparatus according to any one of the first to third aspects, the potential difference of a predetermined level or more between the ejection electrode and the external electrode is applied in a pulsed manner. It is performed at a predetermined cycle. According to the present invention, the invention according to any one of claims 1 to 3 is employed in which a pulse-like application of a potential difference of a predetermined level or more between the discharge electrode and the external electrode is performed in a predetermined cycle. Thus, before the liquid component evaporates, the ink droplets come into contact with the undried ink droplets ejected from the adjacent ejection electrodes on the recording paper, causing dot bleeding, or the same position before evaporation. In this way, it is possible to prevent the occurrence of dot bleeding due to the continuous ejection of ink droplets, thereby realizing high-speed printing.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a schematic configuration diagram showing an embodiment of an electrostatic ink jet recording apparatus according to the present invention.
[0015]
As shown in FIG. 1, the electrostatic inkjet recording apparatus 1 includes a paper feed unit 2, a transport unit 3, a print unit 4, a discharge unit 5, and a control unit 6.
[0016]
The paper feed unit 2 includes a paper feed roller 21 that feeds a plurality of recording papers P stacked in the cassette 20 one by one, and a roller pair 22 that separates the fed recording papers P. The recording paper P is placed on an elevating table 23 which is moved up and down by driving a motor (not shown). The recording paper P at the uppermost position is pressed against the paper feed roller 21 via the elevating table 23 by the motor (not shown). . As a result, when the paper feed roller 21 rotates, a frictional force acts between the paper feed roller 21 and the uppermost recording paper P, and the recording paper P is fed out of the cassette 20. It should be noted that a biasing means such as a spring may be used in place of the motor described above to bias the lift 23 upward.
[0017]
The transport unit 3 is provided on the downstream side of the paper feed unit 2, and has an endless transport belt 33 stretched between a driving roller 31 and a driven roller 32. And a roller 34. The recording paper P conveyed onto the conveyor belt 33 by the rotation of the drive roller 31 is conveyed on the conveyor belt 33 downstream at a constant speed, and is printed by the printing unit 4 during the conveyance. The transport belt 33 functions as a counter electrode and is grounded.
[0018]
The printing unit 4 includes printing units 41 to 43 arranged along the direction of the recording paper transport path. The printing unit 41 corresponds to each color of C (cyan), M (magenta), and Y (yellow), and the printing units 41 to 43 have the same structure.
[0019]
The discharge unit 5 includes a discharge roller pair 51 that discharges the recording paper P discharged from the transport unit 3 and a discharge tray 52 on which the recording paper P discharged by the discharge roller pair 51 is placed. Become.
[0020]
The control unit 6 controls the entire operation of the electrostatic ink jet recording apparatus 1, and controls the paper feed operation, the conveyance operation of the recording paper P at a constant speed by the driving roller 31, and the printing units 41 to 43 during conveyance. , And the like. To the printing units 41 to 43, image data of each color is sequentially output line by line in consideration of the conveying speed from an image storage unit (not shown), and the image data of each color is controlled. The output is shifted by the unit 6 by a time corresponding to the arrangement distance of the printing units 41 to 43.
[0021]
FIG. 2 is a configuration diagram of a main part of the printing units 41 to 43. Since the printing units 41 to 43 have the same structure as described above, the printing unit 41 will be described.
[0022]
The printing unit 41 includes a print head 411 that discharges the charged ink droplets Q, and an intermediate electrode 412 that is disposed between the print head 411 and the counter electrode (conveying belt) 33 and has a gap 412a through which the ink droplets Q pass. A first voltage source 10 for giving a potential difference between the print head 411 and the intermediate electrode 412, and a second voltage source 13 for giving a potential difference between the intermediate electrode 412 and the counter electrode 33. Here, it is assumed that the ink droplet Q is positively charged.
[0023]
The first voltage source 10 includes a bias voltage source 11 that applies a predetermined bias voltage E11 to the print head 411 with reference to the intermediate electrode 412, and one line that forms an image signal to each print head 411 with reference to the intermediate electrode 412. (Hereinafter referred to as a pulse voltage) E12 corresponding to the pixel signal of FIG. The bias voltage source 11 has, for example, a power supply voltage E11 of 300 V, and the pulse generator 12 has, for example, a power supply voltage E12 of 200 V. The second voltage source 13 is a bias voltage source that applies a predetermined bias voltage E2 to the intermediate electrode 412 based on the counter electrode 33, and has a power supply voltage of 500 V, for example. Here, it is assumed that the threshold value ESH at which the potential of the print head 411 with respect to the intermediate electrode 412 for discharging ink from the print head 411 is 400V. In the present embodiment, the bias voltage source 11 and the second voltage source 13 constitute a bias supply unit.
[0024]
FIG. 3 is a perspective view showing the structure of the ink discharge portion, and FIG. 4 is a side view showing the structure of the ink discharge portion.
[0025]
As shown in FIG. 3, the print head 411 has a plurality of head portions 411b arranged in a line at a predetermined pitch on a head body 411a, and the head portion 411b is made of a ceramic such as zirconia or a polymer material. The tip made of an insulating material has a triangular structure. The ink droplet Q is configured to be guided to the front end by using a capillary phenomenon in an unillustrated liquid path passing through the front end formed on the peripheral surface of the head section 411b. The charged ink droplet Q supplied from the ink tank (not shown) to the tip of the head portion 411b is discharged by an electric field formed by the first voltage source 10. In this case, an electric field having a predetermined strength or more is continuously applied for a predetermined time or more so that the ink droplet Q is ejected from the print head 411. Further, as the electric field becomes stronger, the time required for the ink droplet Q to start discharging from the print head 411 after the electric field is generated becomes shorter, and the responsiveness is improved. When the ink droplet Q ejected from the head section 411b reaches the recording paper, a dot is formed on the recording paper.
[0026]
In order to improve the response of ink ejection from the print head 411, the intermediate electrode 412 sandwiches an ink ejection area from the tip of the head portion 411b to the electrode surface of the counter electrode 33 (up and down in FIGS. 2 to 4). ). That is, the intermediate electrode 412 is disposed so that the distance between the print head 411 and the intermediate electrode 412 is smaller than the distance between the intermediate electrode 412 and the counter electrode 33, and the potential difference between the print head 411 and the intermediate electrode 412 is reduced. By making the potential difference between the electrode 412 and the counter electrode 33 approximately the same (for example, 500 V), the print head 411 and the intermediate electrode 412 are kept under the condition that the potential difference between the print head 411 and the counter electrode 33 is constant (for example, 1000 V). The electric field between them is strengthened.
[0027]
A taper 412 b is formed on the surface of the intermediate electrode 412 facing the print head 411 to suppress the occurrence of electric field concentration near the gap and the occurrence of leakage between the intermediate electrode 412 and the print head 411. I have. The taper 412b is formed to be gentler than the inclination of the print head 411 so that the electric field near the ink ejection position of the print head 411 becomes stronger, and the distance between the print head 411 and the intermediate electrode 412 is reduced. It is configured to be minimum near the ink ejection position.
[0028]
The counter electrode 33 has the recording paper P set on one surface thereof, keeps a constant interval between the intermediate electrode 412 and the recording paper P, and functions as an electrode that is grounded and faces the print head 411.
[0029]
FIG. 5 is an explanatory diagram showing the relationship between the image signal and the potential of the print head 411 based on the intermediate electrode 412. (A) is a change in the main scanning direction of the potential of the print head 411 with respect to the intermediate electrode 412, and (b) is a change in the main scanning direction of the image signal. That is, the horizontal axes of (a) and (b) are both positions in the main scanning direction, the vertical axis of (a) is the potential of the print head 411 with respect to the intermediate electrode 412, and the horizontal axis of (b) is The vertical axis is the image signal. The potential of the print head 411 with reference to the intermediate electrode 412 is a potential obtained by adding the bias voltage (300 V) from the bias voltage source 11 and the pulse voltage (0 V or 200 V) corresponding to the image signal from the pulse generator 12. I have. In addition, since the threshold value ESH at which the ink droplet Q is ejected from the print head 411 at the potential of the print head 411 with respect to the intermediate electrode 412 is 400 V, when the potential is 400 V or more, the ink droplet Q Will be reached.
[0030]
Next, the ink used in the present electrostatic ink jet recording apparatus will be described.
[0031]
The ink used in the present electrostatic ink jet recording apparatus includes a liquid component and a solid component. The liquid component is mainly composed of a substance called a dispersion medium made of, for example, a hydrocarbon, and occupies 90% or more of the weight of the ink.
[0032]
On the other hand, the solid component is mainly composed of a mixture of a resin (eg, acrylic or epoxy) constituting a solid particle called a toner and a pigment (eg, copper phthalocyanine or carbon black). Occupy the weight.
[0033]
Further, the ink contains a charge control agent made of, for example, metal soap at a ratio of 2 to 3% (weight). The charge control agent dissolves in the dispersion medium to give a charge to the dispersion medium, or disperses the toner in the dispersion medium by being present in combination with the toner particles and giving a charge to the toner. Is a substance for causing the Coulomb force of the toner to act on the toner particles. Therefore, when the amount of the charge control agent dissolved in the dispersion medium increases, the conductivity of the ink increases. As the amount of the charge control agent bound to the toner particles increases, the charge amount of the ink increases. In the present embodiment, as will be described later, the conductivity and the charge amount of the ink are set to suitable values in order to improve the ejection property of the ink.
[0034]
As described above, since the toner particles in the ink are charged by the charge control agent, when the bias voltage is applied by the bias voltage source 11, the toner particles mainly move toward the front end side of the head portion 411b ( On the other hand, when the toner particles are held on the liquid surface by the surface tension of the dispersion medium, the ratio of the toner in the ink staying in the head portion 411b increases, that is, the ink is concentrated. In the present electrostatic ink jet recording apparatus, the ink supplied from the ink tank (not shown) to the print head 411 has a density of 10% or less, and the ink is concentrated to 15% or more. As described above, when the ink supplied from the ink tank to the print head 411 has a density of 10% or less, if the ink having the density of 10% or more is used, the toner is circulated in the ink circulating fluid path to the print head 411. Is settled, or the toner stays in the head portion 411b, and the subsequent toner is likely to be clogged. Concentration to a concentration of 15% or more means that in the case of a concentration lower than that (for example, a concentration of 10% or less), when the ink droplet Q reaches the recording paper because the ratio of the dispersion medium is large, the ink droplet Q This is because the dots are diffused to the side and the dots are blurred, and the image is deteriorated. On the other hand, when the density of the ink is 15% or more, clear dots are formed.
[0035]
When a pulse voltage is applied by the pulse generation unit 12, the concentrated ink staying in the head unit 411b is further concentrated, so that the Coulomb force acting on the concentrated ink becomes larger than the surface tension. Therefore, the ink droplet Q flies toward the recording paper. At this time, the charge controlling agent is dissolved in the dispersion medium as the liquid component, and the Coulomb force acts on the dispersion medium. Therefore, the flying ink droplet Q includes not only the toner but also the dispersion medium. Since the density of the ink changes according to the magnitude of the Coulomb force acting on the ink, the density of the ink is adjusted according to the magnitude of the bias voltage and the pulse voltage. In addition, by controlling the application time of the pulse voltage, the ejection amount of the ink is adjusted.
[0036]
FIG. 6 shows one mode of an electric field generated near the head section 411b. In FIG. 6, EVL indicates an equipotential line. The interval between the equipotential lines EVL means the strength of the electric field, and the electric field is stronger as the interval between the equipotential lines EVL is smaller.
[0037]
Before the pulse voltage is applied, an electric field due to a potential difference of 300 V is generated between the print head 411 and the intermediate electrode 412, and at the time of application, an electric field due to the potential difference of 500 V, that is, a stronger electric field is generated as compared to before the pulse voltage is applied ( The potential gradient increases). That is, assuming that FIG. 6 shows the electric field before the pulse voltage is applied, the interval between the equipotential lines EVL between the print head 411 and the intermediate electrode 412 during the pulse voltage application is narrower than that shown in FIG. The ink droplet Q is ejected from the print head 411 toward the recording paper P when a voltage (200 V) corresponding to the image signal is applied to the pulse generator 12.
[0038]
The ejected ink droplet Q is accelerated by receiving Coulomb force from an electric field generated by a potential difference between the print head 411 and the intermediate electrode 412 applied by the first voltage source 10. The ink droplet Q that has flown to the intermediate electrode 412 passes through a gap 412a formed in the intermediate electrode 412. The ink droplet Q that has passed through the intermediate electrode 412 is accelerated by a Coulomb force from an electric field generated by a potential difference between the intermediate electrode 412 and the recording paper P given by the second voltage source 13 and reaches the recording paper P. .
[0039]
The intermediate electrode 412 is disposed so that the distance between the print head 411 and the intermediate electrode 412 is smaller than the distance between the intermediate electrode 412 and the counter electrode 4. The potential difference between the print head 411 and the intermediate electrode 412 is reduced. And the potential difference between the counter electrode 33 and the counter electrode 33 (e.g., 500 V), the equipotential lines EVL in the space between the print head 411 and the intermediate electrode 412 are different from each other in the space between the intermediate electrode 412 and the counter electrode 33. It becomes narrower than the interval between the equipotential lines EVL.
[0040]
The taper 412b of the intermediate electrode 412 is formed so as to be gentler than the inclination of the print head 411, and the distance between the print head 411 and the intermediate electrode 412 is minimized near the ink discharge position of the print head 411. In the vicinity of the ink ejection position of the print head 411, the interval between the equipotential lines EVL becomes the narrowest.
[0041]
Since the print head 411 and the intermediate electrode 412 are disposed so as to be line-symmetric with respect to the line of symmetry CL, the electric field between them is also line-symmetric with respect to the line of symmetry CL, and is close to the ink discharge position. Thus, the intensity of the electric field is maximum. Therefore, the ink droplet Q ejected from the ink ejection position goes straight toward the counter electrode 33 along the symmetry line CL. Accordingly, regardless of the mounting direction of the printing unit 41, the ink droplet Q can be attached to an appropriate position without substantially affecting the ejection position of the ink droplet Q due to gravity acting on the ink droplet Q.
[0042]
Next, the relationship between the electrical conductivity and the amount of charge of the ink and the ejection property of the ink will be described.
[0043]
As described above, when the bias voltage (E11 + E2) is applied (including when the pulse voltage E12 is applied), the Coulomb force acts on the ink, particularly the toner particles, so that the ink is concentrated in the head portion 411b. The degree of concentration changes according to the conductivity and the amount of charge of the ink. That is, when the conductivity of the ink is high (the amount of the charge control agent dissolved in the dispersion medium is large, that is, the charge amount of the toner is small), a relatively large Coulomb force acts on the dispersion medium, so that Since a large amount of the dispersion medium moves to the head unit 411b, even if an applied voltage (bias voltage E11 and pulse voltage E12) is applied by the first voltage source 10, the ink staying in the head unit 411b has a toner ratio (density). ) Does not rise so much. Therefore, if the conductivity of the ink is excessively increased, the ink droplets are discharged with insufficient concentration, the ink bleeds on the recording paper, and the dots are sharp even if the voltage applied by the bias voltage source 11 or the pulse generator 12 is increased. Is not formed.
[0044]
Conversely, when the conductivity of the ink is low (the amount of the charge control agent bound to the toner particles is large, that is, the amount of charge of the toner is large), concentration is promoted, and the toner in the ink staying in the head portion 411b is accelerated. Ratio increases. Therefore, if the conductivity of the ink is excessively reduced, the ink is excessively concentrated because a large Coulomb force acts on the toner particles even if the voltage applied by the bias voltage source 11 or the pulse generator 12 is reduced. At 411b, a phenomenon such as clogging of the ink in the circulating liquid path due to the stagnation or solidification of the toner occurs, and the ink cannot be ejected.
[0045]
As described above, depending on the values of the conductivity and the amount of charge of the ink, the concentration of the ink may not be concentrated to a predetermined value (15% in the present embodiment).
[0046]
Thus, in the present embodiment, the physical properties of the ink are set as follows.
[0047]
The ink used in the present electrostatic ink jet recording apparatus has an unillustrated shape when the weight of the ink is represented by M (g), the total charge amount of the ink is Q (C), and the conductivity of the ink is k (S / cm). The conductivity k and the charge amount Q / M of the ink supplied from the ink tank to the print head 411 are expressed by the following equations (1) and (2).
Q / M ≧ 150 × 10 ^-6 (C / g) ... (1)
100 × 10 ^-12 (S / cm) ≦ k
≦ 1500 × 10 ^-12 (S / cm) ... (2)
Are set in the range that satisfies.
[0048]
Further, the conductivity k and the charge amount Q / M of the ink were arbitrarily set to values within the ranges satisfying the above formulas (1) and (2), and the ejection property of the ink was examined. FIG. 7 shows the result.
[0049]
FIG. 7 shows the ink ejection properties with the charge amount Q / M (C / g) of the toner on the horizontal axis and the conductivity k (S / cm) of the ink on the vertical axis.
[0050]
As shown in FIG. 7, in the area indicated by the arrow A, the amount of charge of the toner is insufficient, and even if the applied voltage by the bias voltage source 11 or the pulse generation unit 12 is increased, the ink droplet Q flies from the head unit 411b. It is an area that does not. In the region indicated by the arrow B, the charge amount Q / M is relatively large with respect to the conductivity k, and the ink is excessively concentrated in the head portion 411b, so that toner clogging or the like occurs, and the ink droplet Q from the head portion 411b. Is an area where no flight takes place. In the region indicated by the arrow C, since the conductivity k is large, that is, the amount of the charge controlling agent dissolved in the dispersion medium is large, not only the toner particles but also the dispersion medium is applied by the bias voltage source E1 or the voltage applied by the pulse generator 12. A large Coulomb force acts on the head portion 411b to move the dispersion medium along with the toner particles in a relatively large amount. As a result, the concentration of the ink in the head portion 411b is insufficient, so that the dot is not clearly formed.
[0051]
On the other hand, in the area indicated by the arrow D, dots were clearly formed without clogging of the toner or the like. The area of this arrow D is
0.8 × k × 10 ⁶ ≦ Q / M ≦ 3.5 × k × 10 ⁶ … (3)
Is represented by
[0052]
In the range of the above formula (3), the conductivity k and the charge amount Q / M of the ink are particularly
Q / M = 1.8 × k × 10 ⁶ … (4)
Is most preferable. However, even when the magnitude of the voltage applied by the bias voltage source 11 or the pulse generator 12 or the shape of the print head 411 is changed, the ink satisfying the expression (3) is satisfied. Then, the concentration of the ink can be concentrated to 15% or more.
[0053]
Then, the ink to be supplied to the print head 411 has a density of 10% or less and satisfies the above equations (1) to (3). It was confirmed that clear dots were obtained as shown in FIG. 8 (a) by concentrating to 15% or more.
[0054]
As described above, in the present embodiment, the concentration of the ink supplied from the ink tank to the print head 411 is set to 10% or less, and the physical properties of the ink including the conductivity k and the charge amount Q / M of the ink and the bias voltage source When the ink is ejected from the print head 411 based on the relationship between the bias voltage and the pulse voltage by the pulse generator 11 and the pulse generator 12, the ink is concentrated to a density of 15% or more. Can be prevented as much as possible, and as a result, high-speed printing can be realized.
[0055]
In particular, by using the ink in which the conductivity k of the ink and the charge amount Q / M have the relationship of the formula (3), the magnitude of the bias voltage and the pulse voltage, the shape of the print head 411, and the like can be controlled. Regardless, the ink can be reliably concentrated to 15% or more in the head portion 411b.
[0056]
The present invention is not limited to the above embodiment, and the following modifications (1) to (7) can be adopted.
[0057]
(1) In the above embodiment, the density of the ink is set to 15% or more in the head portion 411b, but the threshold value for this density may be changed as appropriate.
[0058]
(2) In the above embodiment, the charge amount Q / M of the ink is set to 150 × 10 ^-6 (C / g) or more, but 200 × 10 ^-6 It is more preferable to set (C / g) or more.
[0059]
(3) The average particle diameter of the toner particles is 1.0 × 10 ^-6 (M) or less, smaller dots can be formed, and toner sedimentation in the circulating fluid path hardly occurs.
[0060]
(4) When the viscosity of the ink is set to 3 (mPa · s) or less, the ink can be smoothly circulated with almost no precipitation of the toner in the circulating liquid path.
[0061]
(5) It is preferable that the toner particles are positively charged. Hydrocarbons constituting the dispersion medium are easily negatively charged, and when an electric field is applied so as to negatively charge the toner particles, a repulsive force is generated between the toner particles and the hydrocarbons when they are charged to the same polarity. This is because the ink easily scatters from the print head 411, and the recording paper becomes dirty.
[0062]
(6) Although it is preferable to provide the counter electrode for improving the ink ejection property, it is not always necessary to provide the counter electrode. That is, in the above embodiment, the transport belt 33 functions as a counter electrode by grounding the transport belt 33, but the transport belt 33 does not have to be grounded.
[0063]
(7) The present invention is applicable to an electrostatic ink jet recording apparatus in which the distance between the print head 411 and the intermediate electrode 412 is larger than the distance between the intermediate electrode 412 and the transport belt 33.
[0064]
【The invention's effect】
According to the first aspect of the present invention, during a period excluding a period in which the potential difference of the predetermined level or more is applied between the discharge electrode and the external electrode in a pulsed manner, the predetermined level is maintained between the discharge electrode and the external electrode. Bias supply means for supplying the potential difference of the ink as a bias voltage, the ink temporarily stored in the ejection electrode, the conductivity of the ink and the physical properties of the ink including the amount of charge and the relationship between the potential difference of the predetermined level and On the basis of the concentration to a predetermined concentration, by supplying an ink having a concentration smaller than the predetermined concentration to the discharge electrode, by concentrating the ink discharged from the discharge electrode to the predetermined concentration, It is possible to prevent dot bleeding while avoiding toner clogging in the flow path.
[0065]
According to the second aspect of the present invention, since the external electrode is an intermediate electrode provided between the ejection electrode and the recording medium surface, the electric field around the ejection electrode becomes strong, and the ink ejection from the ejection electrode is performed. Responsiveness and straightness can be improved.
[0066]
According to the third aspect of the present invention, when the weight of the ink is M (g), the total charge amount of the ink is Q (C), and the conductivity is k (S / cm), the charge amount (Q / M) And the conductivity k is expressed by the following equation (1).
0.8 × k × 10 ⁶ ≦ Q / M ≦ 3.5 × k × 10 ⁶ ... (1)
Is supplied to the ejection electrode, so that the concentration of the ink can be surely concentrated to a concentration larger than the predetermined threshold value regardless of the voltage of the ejection electrode or the intermediate electrode, the shape of the ejection electrode, and the like. it can.
[0067]
According to a fourth aspect of the present invention, in the method, a pulse-like application of a potential difference of a predetermined level or more between the discharge electrode and the external electrode is performed in a predetermined cycle. Before the liquid component evaporates, the ink droplets come into contact with the undried ink droplets ejected from the adjacent ejection electrode on the recording paper, causing dot bleeding or evaporation. It is possible to prevent the occurrence of dot bleeding by successively ejecting ink droplets at the same position before, and as a result, it is possible to realize high-speed printing.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing an embodiment of an electrostatic ink jet recording apparatus according to the present invention.
FIG. 2 is a configuration diagram of a main part of a printing unit.
FIG. 3 is a perspective view illustrating a structure of an ink discharge portion.
FIG. 4 is a side view showing a structure of an ink ejection portion.
FIG. 5 is an explanatory diagram showing a relationship between an image signal and a potential of a print head based on an intermediate electrode.
FIG. 6 is a diagram showing one mode of an electric field generated in the vicinity of a head unit.
FIG. 7 is a diagram illustrating the ink discharge properties with the charge amount Q / M (C / g) of the toner as the horizontal axis and the conductivity k (S / cm) of the ink as the vertical axis.
8A is a diagram showing a dot formation state by the electrostatic ink jet recording apparatus according to the present invention, and FIG. 8B is a view showing a dot formation state by the conventional electrostatic ink jet recording apparatus. is there.
FIG. 9 is an explanatory diagram of an ink ejection unit in a conventional electrostatic ink jet recording apparatus.
[Explanation of symbols]
10 1st voltage source
11 Bias voltage source (bias supply means)
12 pulse generator
13 Second voltage source (bias supply means)
411 print head
411b Head section
412 Intermediate electrode

Claims (4)

The charged ink supplied through the flow path is directed to the recording medium surface in the form of droplets from the discharge electrode by an electric field generated by applying a potential difference of a predetermined level or more between the discharge electrode and the external electrode in a pulsed manner. In an electrostatic ink jet recording apparatus that discharges
A bias for supplying the potential difference of the predetermined level as a bias voltage between the ejection electrode and the external electrode during a period excluding a period of applying the potential difference of the predetermined level or more in a pulsed manner between the ejection electrode and the external electrode. A supply unit that concentrates the ink temporarily stored in the ejection electrode to a predetermined concentration based on a relationship between the physical properties of the ink including the conductivity and the amount of charge of the ink and the potential difference at the predetermined level. An electrostatic ink jet recording apparatus characterized by the above. 2. The electrostatic ink jet recording apparatus according to claim 1, wherein the external electrode is an intermediate electrode provided between a discharge electrode and a recording medium surface. Assuming that the weight of the ink is M (g), the total charge amount of the ink is Q (C), and the conductivity is k (S / cm), the relationship between the charge amount (Q / M) and the conductivity k is expressed by the following equation: 3. The electrostatic ink jet recording apparatus according to claim 1, wherein an ink satisfying 1) is supplied to the discharge electrode.
0.8 × k × 10 ⁶ ≦ Q / M ≦ 3.5 × k × 10 ⁶ (1) 4. The electrostatic ink jet recording apparatus according to claim 1, wherein a pulse-like application of a potential difference of a predetermined level or more between the ejection electrode and the external electrode is performed in a predetermined cycle.

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