JP2000043272A - Ink-jet image formation apparatus and image formation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ink-jet image formation apparatus which has a superior stability for discharging ink and can record high-gradation images at a high speed. SOLUTION: A pulse decreasing a pulse top stepwise is applied to a discharge electrode of this electrostatic ink-jet image formation apparatus in correspondence with each dot. A pulse base of this pulse is set close to a ground potential. The pulse top is a first potential from a rise time point t1 to an intermediate time point t2 which sufficiently exceeds an ink fly-starting potential whereat an electrostatic force starting a fly of ink liquid drops is generated, and a second potential after the intermediate time point t2 which is a transient potential larger than an ink fly-stopping potential whereat the ink fly is stopped and smaller than the ink fly-starting potential i.e., which generates an electrostatic force that can continue the ink fly. A time interval t1-t2 while the first potential is kept is set to be a time required for a leading end of the discharge electrode to be wet with the ink.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrostatic ink jet recording apparatus, and more particularly to control of a recording voltage supplied to a recording head.

[0002]

2. Description of the Related Art There are various ink ejection systems of an ink jet recording apparatus for drawing characters and images by directly spraying atomized ink onto a recording medium. Among them, there are various methods of applying an applied voltage to an ejection electrode. An electrostatic system capable of controlling an ink ejection amount by control (pulse width modulation, electric field intensity control) has attracted attention as a device that can realize a high-definition inkjet printer. For example, the following techniques are known in connection with an electrostatic ink jet recording apparatus. Japanese Patent Application Laid-Open No. 55-154169 discloses an ink jet printer in which an ejection electrode is arranged in an orifice provided in an ink chamber, and the tip of the ejection electrode is easily wetted with ink by a capillary phenomenon.
Japanese Patent Application Laid-Open No. 56-28867 discloses a technique for stabilizing the flight of ink from an ejection electrode by vibrating the ejection electrode. Further, Japanese Patent Application Laid-Open No. Sho 60-165253 discloses an ink dot printer in which the supply of ink to the tip of the discharge electrode is stabilized by disposing a discharge electrode in a slit for sucking ink from an ink tank. .

During recording of a recorded image, a pulse is applied to the ejection electrode in accordance with a recording signal. However, if the pulse base, which is the ink non-flying potential (potential that does not generate an electrostatic force enough to cause ink droplets to fly from the ink liquid surface), is set to a too low potential or ground potential,
Since the time from the rising edge of the pulse to the time when the ink droplet flies becomes longer, the pulse base usually generates the ink flying potential (the electrostatic force that causes the ink droplet to continue to fly stably from the ink liquid surface). Potential to be applied). However, when a pulse base close to the ink flying potential is continuously applied to the ejection electrode, the ink continues to aggregate near the tip of the ejection electrode, so that the ink is fixed near the tip of the ejection electrode, and clogging of the ink is likely to occur. . Therefore, during standby, a ground potential or a potential close thereto is applied to the ejection electrode, and at the start of recording, a pulse base close to the ink flying potential is applied to the ejection electrode to prevent ink clogging. There is also.

[0004]

However, it is difficult to completely prevent the ink from being fixed in the vicinity of the discharge electrode only by the conventional pulse-based control. For example, when the average density of the recorded image is sufficiently large, the ink consumption is large, and the ink is unlikely to stay near the ejection electrode. However, when the average density of the recorded image is small, the ink consumption is small. In addition, the ink stays in the vicinity of the ejection electrode, and the ink easily adheres to the vicinity of the ejection electrode. When a pigment-based ink is used, the viscosity of the ink increases near the tip of the discharge electrode.

[0005] In general, as the performance of an ink jet recording apparatus, gradation and high speed of a recorded image are desired.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an ink jet recording apparatus which has a high stability of ink ejection and can perform high gradation recording at high speed.

[0007]

According to the present invention, a pulse train is applied to a conductive member in contact with ink or an electrode provided in the vicinity of the conductive member. An image forming method for forming an image on a recording medium by flying the ink toward a counter electrode provided on a tip side of the image forming apparatus, wherein a pulse top of each of the pulses is respectively formed from the conductive member. Provided is an image forming method, wherein an ink is dropped within a voltage range in which ink can fly toward a counter electrode.

Further, according to the present invention, as an apparatus for carrying out this image forming method, there is provided a conductive member with which an ink comes in contact, a counter electrode provided on the tip side of the conductive member, and a counter electrode provided from the conductive member. A pulse generating means for generating a pulse train for causing the ink to fly toward the ink jet printer, wherein the pulse generating means moves the pulse top of each pulse from the conductive member to Provided is an ink-jet image forming apparatus characterized in that ink is caused to fall within a voltage range in which ink can fly toward an electrode.

According to the present invention, even if the pulse base of the pulse train is not set close to the ink flying voltage, the leading edge of the conductive member is promptly wetted with the ink by setting the rising of the pulse to a potential higher than in the prior art. The ink ejection frequency from the tip of the conductive member can be increased. Therefore, it is possible to improve the recording speed while preventing the ink from staying near the conductive member. Then, by lowering the pulse top to a potential smaller than the conventional one after the start of the ink flight to suppress the amount of ink ejection from the conductive member, the gradation by pulse width modulation can be further improved.

[0010]

Embodiments of the present invention will be described below with reference to the accompanying drawings. Here, a low-viscosity petroleum-based solvent (for example,
10mPa · s isoparaffin, etc.) and charged pigment (positive)
The case where an ink in which the toner and the charge control agent are dispersed is used will be described.

First, the internal configuration of the electrostatic ink jet image forming apparatus according to the present embodiment will be described.

As shown in FIG. 1, a counter electrode 10 grounded by a bias, a head unit 11 for ejecting ink toward the counter electrode 10, and a head unit 11 are provided inside the housing of the electrostatic ink jet image forming apparatus. A recording medium transport mechanism (not shown) that allows the recording medium to pass through a gap of about 1 mm provided between the ink ejecting section of the recording medium and the counter electrode 10, and a scanning mechanism (not shown) that moves the head unit 11 in a direction across the recording medium. And a controller (not shown) for controlling the entire apparatus.

The head unit 11 moves from the orifice of the ink chamber where the ink 13 is stored to the tip 1 of the ejection electrode 12.
A bias power source 1 for applying a bias voltage to the recording head 15 and the ejection electrode 12 in which the second electrode 2a is projected toward the counter electrode 10 side.
6. A voltage generator 1 for superimposing a pulse for drawing ink droplets forming one dot of a recorded image on a bias voltage
4. A drive circuit 17 for supplying a pulse width modulated recording signal to the voltage generator 14 is provided. As shown in FIG. 2, the voltage generating device 14 includes two voltage generating circuits 14a and 14b (first voltage generating circuit 14a and second voltage generating circuit 14b) for generating mutually different potentials, and each of the voltage generating circuits 14a and 14b. And a voltage superimposing circuit 14c that superimposes the pulse voltages from 14a and 14b on the bias voltage (details of the circuit configuration will be described later). The pulse top as shown in FIG. It outputs a high voltage signal with a pulse falling to (here, one stage). Pulse top of this pulse, the rise time t ₁ to the intermediate time t _2, the minimum potential for generating electrostatic force flight of the ink droplets starts (hereinafter, referred to as ink flying start potential) sufficiently greater than the potential (hereinafter the , we have a first called the potential), the intermediate time t ₂ later, potential flight of the ink is stopped (hereinafter, ink ejecting stop potential as hereinafter) smaller than transient potential than the ink flying start potential large That is, a potential that generates an electrostatic force that can keep the ink flying
(Hereinafter, referred to as a second potential). When such a potential is applied to the ejection electrode 12, even if the bias voltage is set to be close to the ground potential, the bias voltage is set within the duration of the first potential.
The ink 13 quickly creeps up to the tip 12a of the ejection electrode 12 (see FIG. 4), and starts flying from there. That is, it is possible to increase the ink ejection frequency while preventing the ink from staying near the ejection electrode 12. In such a state, even if the pulse top is subsequently lowered to the second potential, the ink continues to fly by reducing the ejection amount without stopping. By modulating the duration of the second potential at which the ink ejection amount is suppressed as described above, a higher-gradation recording image can be recorded.

In order to confirm such an effect, a prototype of an electrostatic ink-jet image forming apparatus in which the distance between the tip 12a of the discharge electrode 12 and the counter electrode 10 is about 1 mm is manufactured, and the recording medium is formed on a recording medium using a pigment-based ink. The recorded image was recorded. In addition,
Under these conditions, the ink jet start voltage is about 1.7 kV and the ink jet stop voltage is about 1.5 kV. Therefore, 2 kV was applied as the first voltage, and 1.6 kV was applied as the second voltage. Since the duration of the first voltage is at most about 10 μs, the duration of the second potential can be modulated up to the time required for forming one dot (about 1 ms), and extremely high gradation recording can be performed. Has been confirmed to be realized. Further, it was also confirmed that the response frequency of ink ejection was improved as compared with the related art even when the pulse base was set near the ground potential so that ink did not stay near the tip 12a of the ejection electrode 12.

Next, a specific circuit configuration of the voltage generator 14 will be described.

FIG. 5 shows a first configuration example of the voltage generator 14,
That is, a configuration is shown in which a pulse generation circuit is used as the first voltage generation circuit 14a for a fixed time and a delay circuit is used as the second voltage generation circuit 14b. The fixed time pulse generation circuit 14a has a circuit configuration as shown in FIG.
A pulse having a pulse width T corresponding to the RC is supplied to the driving circuit 17.
(FIG. 10A)
(b)). The pulse width T is theoretically sufficient to set the time until the tip 12a of the ejection electrode 12 becomes wet with the ink. However, in practice, it is sufficient that the ink flying from the tip 12a of the ejection electrode 12 starts to adhere to the recording medium. It is desirable to set the time. The value is the tip 12 of the discharge electrode 12.
Although it depends on the distance between “a” and the counter electrode 10, the type of ink, and the like, specifically, it is about several μs to several ms. On the other hand, the delay circuit 14b has the circuit configuration shown in FIG. 7, and delays the recording signal from the drive circuit 17 by a delay time corresponding to the RC (see FIG. 10C). Here, the pulse width of the pulse generation circuit 14a for a fixed time and the delay circuit 1
4b is set substantially equal to the delay time. And
The high voltage superposition circuit 14c includes a voltage superposition circuit having the circuit configuration shown in FIG. 8 and the amplifier circuit shown in FIG. The voltage superimposing circuit superimposes the pulse from the pulse generating circuit 14a and the delay signal from the delay circuit 14b on the bias voltage for a certain period of time, and outputs a low voltage signal on which the pulse whose pulse top gradually decreases is placed. (Figure 1
0 (d)). Then, the amplification circuit amplifies the low voltage signal from the voltage superposition circuit and outputs a high voltage signal (see FIG. 3) applied to the ejection electrode 12 of the recording head.

FIG. 11 shows a second configuration example of the voltage generator 14, that is, a configuration in which a pulse generation circuit is used as the first voltage generation circuit 14a for a fixed time and a gamma correction circuit is used as the second voltage generation circuit 14b. Is shown. The fixed time pulse generation circuit 14a has a circuit configuration similar to that shown in FIG. 6, and generates a pulse having a pulse width T according to the RC with a recording signal from the drive circuit 17 as a trigger ( 14 (a) and 14 (b): an output signal of the pulse generation circuit 14a for a fixed time is hereinafter referred to as a first potential enable signal.) The gamma correction circuit 14b includes a gamma curve 120 as shown in FIG.
Outputs the gradation data corrected according to the gamma curve 120 to which the offset until the ink is wetted (see FIG. 14C: hereinafter, the output signal of the gamma correction circuit 14b is referred to as a second potential enable signal). Call). And
As shown in FIG. 13, the high-voltage superimposing circuit 14c is configured by superimposing switching elements (power FETs) 130 and 131 that are driven in response to a predetermined enable signal input in multiple stages. A high-voltage signal with a stepwise falling pulse is output (Fig. 1
4 (d)). Specifically, the upper switching element 13
0 is driven by the first potential enable signal, and the lower switching element 131 is driven by both the first potential enable signal and the second potential enable signal. When the output voltages of the two switching elements 130 and 131 are superimposed and output and only the second voltage enable signal is input, the lower switching element 1
31 output voltages are output. For example, when the output voltage of the upper switching element 130 is 0.5 kV and the output voltage of the lower switching element 131 is 1.5 kV, the high voltage superimposing circuit 14 c outputs a pulse whose pulse top drops from 2 kV to 1.5 kV. Output a high voltage signal.
When the high voltage superimposing circuit 14c has such a circuit configuration, for example, even if the TTL level (5 V, ground potential, etc.) is used for the first enable signal and the second enable signal, the amplification factor of each amplifier circuit , The potential difference of the pulse top of the high voltage signal applied to the ejection electrode 12 can be changed. Note that the high voltage superposition circuit 14c in FIG. 11 does not necessarily need to be configured as shown in FIG. For example, an amplifier circuit for amplifying each enable signal (see FIG. 9) may be configured by overlapping in multiple stages.
For example, if the first potential is 2 kV and the second potential is 1.5 kV, switching between 0.5 kV and 1.5 kV is required. Switching at 0.5 kV is performed according to the first enable signal. The switching of 1.5 kV may be performed according to the second enable signal.

It is possible to generate a pulse whose pulse top drops stepwise without employing the circuit configuration described here. For example, by changing a capacitive load for suppressing an impulse voltage generated by switching, an impulse voltage higher than the ink flying start potential may be generated for a predetermined time.

In the above description, the pulse top is lowered by one stage. However, if the pulse top is lowered more finely in multiple steps, the ink discharge amount from the tip 12a of the discharge electrode 12 becomes finer. Since the control is performed, an image with a higher gradation can be recorded. Hereinafter, the circuit configuration of the voltage generator when the pulse top is attenuated will be described. As shown in FIG. 15, the voltage generator 14 in this case includes a delay circuit 150, a gamma correction circuit 151, a charging circuit 152, a discharging circuit 153, and the like. When the gamma-corrected gradation data is output from the gamma correction circuit 151, the charging circuit 152 starts to charge the electric charge accordingly (FIGS. 16A and 16B).
reference). Then, when the delay circuit 150 generates a pulse delayed by a predetermined time from the recording signal from the drive circuit 17, the pulse is used as an enable signal and the discharge circuit 15
3 discharges the charge of the charging circuit 152 (FIGS. 16C and 16D).
reference). The output of the discharge circuit 153 is
Is output as a high voltage signal applied to the ejection electrode 12 of the recording head (FIG. 16).
(See (e)).

Finally, an extended example of the recording head will be described.

In FIG. 1, a voltage generating circuit 14 and a bias power supply 16 are connected to the ejection electrode 12, but as shown in FIG. 17, peripheral electrodes 18 are provided on both sides of a needle 19 for ejecting ink. , These surrounding electrodes 1
8, a voltage generating circuit 14 and a bias power supply 16 may be connected.

In FIG. 17, the needle 19 is provided in the orifice of the ink chamber in which the ink 13 is stored. However, as shown in FIG. 18, the needle 19 may be provided in the ink flow path through which the ink flows. Absent. Of course, the ejection electrode 12 to which the voltage generation circuit 14 and the bias power supply 16 are connected may be provided in the ink flow path through which the ink flows.

In FIGS. 1, 17 and 18, only one ejection electrode 12 and the like are provided on the recording head. However, as shown in FIG. 19, a plurality of ejection electrodes 12 and the like may be provided. Absent. Further, as shown in FIG. 20, if the discharge electrode row is sandwiched between two plate electrodes 200a and 200b,
The electric field intensity near each ejection electrode 12 can be enhanced.
In the case of such a multi-head, by connecting the voltage generator 14 shown in FIG. 15 to each ejection electrode 12, the charging circuit of the voltage generation circuit 14 connected to a certain ejection electrode 12 While the battery 152 is being charged, the voltage generation circuit 1 connected to another ejection electrode 12
4 can perform time-sharing control such as discharging the discharge circuit 153. In FIG. 20, reference numeral 201 denotes a power supply unit accommodating the voltage generator 14 and the like.
Reference numeral 2 denotes a reverse polarity power supply unit.

[0024]

According to the present invention, it is possible to stabilize the ink ejection from the needle and to record a high-gradation recording image at a high speed.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an internal configuration of a head unit of an electrostatic inkjet image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a circuit configuration diagram of a voltage generator according to one embodiment of the present invention.

FIG. 3 is an output waveform diagram from the voltage generator according to the embodiment of the present invention.

FIG. 4 is a diagram showing a state in which the tip of a discharge electrode is wet with ink.

FIG. 5 is a diagram showing an example of a circuit configuration of the voltage generator according to the embodiment of the present invention.

FIG. 6 is a circuit configuration diagram of a pulse generator for a fixed time shown in FIG. 5;

FIG. 7 is a circuit configuration diagram of the delay circuit of FIG. 5;

8 is a circuit configuration diagram of the voltage superposition circuit in FIG.

9 is a circuit configuration diagram of an amplifier circuit that amplifies a low voltage signal output from the voltage superposition circuit in FIG.

FIG. 10 is a time chart of signals generated by each circuit of FIG. 5;

FIG. 11 is a diagram showing an example of a circuit configuration of the voltage generator according to the embodiment of the present invention.

12 is a diagram showing a gamma correction curve of the gamma correction circuit of FIG.

FIG. 13 is a circuit configuration diagram of the voltage superposition circuit of FIG. 11;

FIG. 14 is a time chart of signals generated by each circuit of FIG. 11;

FIG. 15 is a diagram showing an example of a circuit configuration of the voltage generator according to the embodiment of the present invention.

FIG. 16 is a time chart of signals generated by each circuit of FIG. 15;

FIG. 17 is a diagram showing an internal configuration of a head unit of the electrostatic inkjet image forming apparatus according to one embodiment of the present invention.

FIG. 18 is a schematic diagram of an internal configuration of a head unit of the electrostatic inkjet image forming apparatus according to one embodiment of the present invention.

FIG. 19 is a schematic diagram of an internal configuration of a head unit of the electrostatic inkjet image forming apparatus according to one embodiment of the present invention.

FIG. 20 is a schematic diagram of an internal configuration of a head unit of the electrostatic inkjet image forming apparatus according to one embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Counter electrode 11 ... Head unit 12 ... Discharge electrode 13 ... Ink 14 ... Voltage generator 14a ... First voltage generation circuit (pulse generation circuit for a fixed time) 14b ... Second voltage generation circuit (delay circuit, gamma correction circuit) 14c ... voltage superimposing circuit 15 ... recording head 16 ... bias power supply 17 ... driving circuit 18 ... peripheral electrode 19 ... needle 130 ... switching element (power FET) 131 ... switching element (power FET) 150 ... delay circuit 151 ... gamma correction circuit 152 ... Charge circuit 153 ... Discharge circuit

Continued on the front page (72) Inventor Tatsuo Igawa 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture F-term in Hitachi Research Laboratory, Hitachi Ltd. 2C057 AF72 AG21 AG90 AM03 AM18 AM22 AR06 BD06

Claims (5)

[Claims] An electroconductive member that comes into contact with ink; a counter electrode provided on a tip side of the conductive member; and a pulse generator that generates a pulse train for causing the ink to fly from the conductive member toward the counter electrode. Means, wherein the pulse generating means lowers the pulse top of each of the pulses within a voltage range in which ink can fly from the conductive member toward the counter electrode. An inkjet image forming apparatus, comprising: 2. The ink jet image forming apparatus according to claim 1, wherein the pulse base of the pulse train is a potential at which ink does not fly from the conductive member. 3. The ink-jet image forming apparatus according to claim 1, wherein a pulse top of each of the pulses has a stepped waveform or an attenuated waveform. 4. A pulse train is applied to a conductive member to which ink is supplied or an electrode provided near the conductive member, and the ink is directed from the conductive member to a counter electrode provided on the tip side of the conductive member. An image forming method for forming an image on a recording medium by causing the ink to fly, wherein a pulse top of each of the pulses falls within a voltage range in which ink can fly from the conductive member toward the counter electrode. An image forming method comprising: 5. The image forming method according to claim 4, wherein the pulse top of each of the pulses has a stepped waveform or an attenuated waveform.