Electrostatic ink jet recording apparatus

Abstract: An electrostatic ink jet recording apparatus includes an ink chamber (2) for accommodating ink (1) in which charged toner particles are contained, a discharging exit (4) communicated with the ink chamber (2) and having an opening directed to the outside, an ejection electrode (5) disposed in the discharging exit (4) and having an end portion directed to the outside, an electrophoretic electrode (8) mounted on side walls (2b) of the ink chamber (2) such that the ejection electrode (5) is located therein, and an opposing electrode (10) disposed in an opposing relationship to the discharging exit (4) with a paper travel path (9) interposed therebetween. The electrostatic ink jet recording apparatus further includes a migration voltage source (11) for applying a migration voltage to the electrophoretic electrode (8), and a discharging voltage source (12) for applying a driving pulse voltage to the ejection electrode (5). The opposing electrode (10) is formed from a conductive rubber roller having a high resistance value.

Description: This invention relates to an electrostatic ink jet recording apparatus, and more particularly to an electrostatic ink jet recording apparatus in which a countermeasure against a voltage leak which may occur between an ejection electrode and an opposing electrode is provided. Description of the Related Art Various electrostatic ink jet recording apparatus of the type mentioned are conventionally known, and one of such electrostatic ink jet recording apparatus is disclosed in JP-A-5212867 and shown in FIG. 6. Referring to FIG. 6, the electrostatic ink jet recording apparatus includes an ink chamber 52 in which conductive ink is filled, a plurality of ink jetting elements 56a communicated with the ink chamber 52, an ultrasonic oscillation application unit 57 for applying ultrasonic oscillations to the ink in the ink chamber 52, a first electrode 56 provided for each of the ink jetting elements 56a, a second electrode (back electrode) 60 located outside the ink chamber 52 in an opposing relationship to ink jet exits of the ink jetting elements 56a, and a high voltage pulse application unit 55 for applying a high voltage pulse corresponding to a recording signal between the first electrode 56 and the second electrode 60 while the ultrasonic oscillation application unit 57 is in an energized state to produce, between the first electrode 56 and the second electrode 60, an electrostatic field for causing ink drops to fly from the ink jet exits. The ink in the ink chamber 52 is oscillated by the ultrasonic oscillation application unit 57 to render unstable the configuration of an ink meniscus formed at each of the ink jet exits while the ink flying electric field is concentrated upon a local portion of the meniscus in order that the voltage required to be applied to the first electrode to cause the ink to fly may be reduced comparing with an ordinary application voltage thereby to prevent a voltage leak which may otherwise occur between the first electrode and the second electrode. However, the conventional electrostatic ink jet recording apparatus described above is disadvantageous in that, since an ultrasonic oscillator is required, the apparatus has a large size and a complicated structure accordingly and a high cost is required. The electrostatic ink jet recording apparatus is disadvantageous also in that, since an ink meniscus is rendered unstable in an unpredictable situation, stabilized ink discharging cannot be achieved. Further, even if an ultrasonic oscillator is used, the extent of reduction of a high voltage pulse to be applied to the first electrode is approximately 100 V to the utmost as apparent also from an example of experiment disclosed in the document mentioned above. Thus, the electrostatic ink jet recording apparatus is further disadvantageous in that, where the distance between the first electrode and the second electrode must be set short depending, for example, upon the thickness of a recording medium to be used, there is a fixed limitation to prevention of a voltage leak. A further ink jet recording apparatus according to the preamble of claim 1 is disclosed in JP-A-61057343. It is an object of the present invention to provide an electrostatic ink jet recording apparatus of a small size and a simple construction wherein a voltage leak between an ejection electrode and an opposing electrode can be prevented. In order to attain the object described above, there is provided an electrostatic ink jet recording apparatus as defined in claim 1. With the electrostatic ink jet recording apparatus, since the opposing electrode being formed from a conductive rubber roller having a high resistance value, even if the potential difference between the ejection electrode and the opposing electrode exceeds the dielectric strength of the space between the ejection electrode and the opposing electrode by some cause, an excessive electric current which may otherwise be produced by a spark can be suppressed by the resistance of the opposing electrode. Consequently, the ejection electrode and the opposing electrode can be protected effectively from otherwise possible destruction by a spark with a comparatively inexpensive construction free from increase in size of the apparatus. Further, since only toner particles are discharged from within the liquid ink to effect recording, blurring on a recording medium or a like defect in terms of which conventional ink jet printing should be improved does not occur, and a high printing quality similar to that achieved by electrophotographic recording can be achieved. The electrostatic ink jet recording apparatus further comprises a resistor switch and a plurality of high resistors for being selectively connected to the opposing electrode by the resistor switch, the resistor switch and the plurality of high resistors being interposed between the opposing electrode and the ground. In this instance, the plurality of high resistors may be at least three high resistors having different resistance values equal to or higher than 1 MΩ. In the electrostatic ink jet recording apparatus, one of the plurality of resistors is selectively connected to the ground side of the opposing electrode in accordance with the thickness of the recording medium to be used. Consequently, production of an excessive electric current upon leaking can be prevented effectively irrespective of the thickness of the recording medium to be used. The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference characters. FIG. 1 is a schematic plan view, partly in section, of an electrostatic ink jet recording apparatus; FIG. 2 is a schematic side elevational view, partly in section, of the electronic ink jet recording apparatus shown in FIG. 1; FIGS. 3 to 5 are similar views but showing an electronic ink jet recording apparatus to which the present invention is applied; FIG. 6 is a schematic sectional view showing a conventional electrostatic ink jet recording apparatus. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGS. 1 and 2, there is shown an electrostatic ink jet recording apparatus to which part of the present invention is applied. The electrostatic ink jet recording apparatus shown includes a recording head body 100 in which an ink chamber 2 in which ink 1 in the form of liquid which contains charged toner particles is accommodated is provided. The ink chamber 2 has a front wall 2a in which a discharging exit 4 is formed such that it is communicated with the ink chamber 2 and opened to the outside. The discharging exit 4 is formed as a very small slit. An ink supply port 14 is formed in one of the opposite side walls 2b of the ink chamber 2, and an ink emission port 13 is formed in the other side wall 2b. An ejection electrode 5 is provided in the ink chamber 2 and has an end portion projecting to the outside through the discharging exit 4. An electrophoretic electrode 8 is located on the outer side of the ink chamber 2 along outer faces of the opposite side walls 2b and a rear wall 2c of the ink chamber 2. The electrophoretic electrode 8 is connected to an electrophetic voltage source 11 outside the ink chamber 2, and the ejection electrode 5 is connected to a discharging voltage source 12 outside the ink chamber 2. An opposing electrode 10 is located outside the ink chamber 2 in an opposing relationship to the discharging exit 4 with a paper travel path 9 interposed therebetween. In the electrostatic ink jet recording apparatus, the ink 1 is composed of fine particles of a colored thermoplastic resin called toner dispersed together with a charging control agent in a petroleum organic solvent (isoparaffin). The toner powder is apparently charged to the positive polarity by a zeta (ζ) potential. The ink chamber 2 is connected to an ink tank (not shown) via tubes (not shown) connected to the ink emission port 13 and the ink supply port 14 such that a negative pressure of approximately 1 cmH 2 O is applied to the ink 1 in the inside of the ink chamber 2 and the ink 1 is forced to circulate between the ink chamber 2 and the ink tank. The ejection electrode 5 is formed from a conductive material such as Cu or Ni by electroforming and has a width of approximately 50 µm. The ejection electrode 5 is positioned so that the end portion thereof projects over approximately 80 µm to 100 µm from the discharging exit 4 to the outside. Consequently, an ink meniscus 6 is formed between the projecting ink chamber 2 and the discharging exit 4 by a surface tension of the ink 1. Here, a meniscus formation section 7 is formed from the discharging exit 4 and the ejection electrode 5. However, such meniscus formation section may otherwise be formed from a wire or a like element. The electrophoretic voltage source 11 applies a voltage having a same polarity as that of the zeta potential of the toner powder to the electrophoretic electrode 8. The discharging voltage source 12 applies a high voltage pulse having a same polarity as that of the toner powder to the ejection electrode 5 at a predetermined timing based on a recording signal. The opposing electrode 10 is formed from a conductive rubber roller having a high resistance value and is grounded. Here, the resistance of the opposing electrode 10 is set to a value equal to or higher than 1 MΩ. Further, though not shown, a mechanism is provided for adjusting the distance between the opposing electrode 10 and the ejection electrode 5 among several stages in accordance with the thickness of a recording medium disposed along the paper travel path 9. For the mechanism, for example, a head-platen distance adjustment mechanism which is normally employed in a dot impact printer may be employed. General operation of the electrostatic ink jet recording apparatus described above will be described below. When a voltage is applied from the electrophoretic voltage source 11 to the electrophoretic electrode 8, an electric field is formed between the electrophoretic electrode 8 and the end portion of the ejection electrode 5 and causes toner particles in the ink chamber 2 to electrically migrate by electrophoretic phenomenon so that they concentrate upon the discharging exit 4. Then, when a voltage pulse is applied from the discharging voltage source 12 to the ejection electrode 5, the toner particles concentrated upon the discharging exit 4 are discharged in a group from the ink meniscus 6 toward the opposing electrode 10. Then, the discharged toner group 3 sticks to a recording medium P. On the other hand, after the amount of toner particles in the proximity of the discharging exit 4 is reduced as a result of the discharging, toner particles are supplemented toward the discharging exit 4 by electric migration from the interior side of the ink chamber 2 so as to prepare for successive discharging. Meanwhile, an excessive amount of the ink 1 is discharged from the ink emission port 13. Those steps of operation are repeated to form a toner image on the recording medium P transported along the paper travel path 9. Then, the recording medium P having the toner image formed thereon is transported to and subject to thermal fixation by a fixing unit (not shown) similar to a fixing unit for electrophotographic recording. In this manner, since only toner particles are discharged from within a liquid ink to effect recording, blurring on a recording medium or a like defect in terms of which conventional ink jet printing should be improved does not occur, and a high printing quality similar to that achieved by electrophotographic recording can be achieved. Here, if otherwise the opposing electrode 10 is a complete conductor and is grounded as in the conventional electrostatic ink jet recording apparatus described hereinabove, then if the distance between the ejection electrode 5 and the opposing electrode 10 is reduced shorter than a certain distance, a spark (voltage leak) is produced between the ejection electrode 5 and the opposing electrode 10. If a spark is produced, then metal portions of the ejection electrode 5 and the opposing electrode 10 may be melted by heat by the spark, resulting in failure in function as a head. However, since the opposing electrode 10 is formed from a conductive rubber roller having a high resistance value, even if the potential difference between the ejection electrode 5 and the opposing electrode 10 exceeds the dielectric strength of the space between the ejection electrode 5 and the opposing electrode 10 by some cause, an excessive electric current which may otherwise be produced by a spark can be suppressed by the resistance of the opposing electrode 10. Consequently, the ejection electrode 5 and the opposing electrode 10 can be protected effectively from otherwise possible destruction by a spark with a comparatively inexpensive construction free from increase in size of the apparatus. Referring now to FIGS. 3 to 5, there is shown an electrostatic ink jet recording apparatus to which the present invention is applied. The electrostatic ink jet recording apparatus shown is a modification to and has a common construction to that of the electrostatic ink jet recording apparatus described hereinabove with reference to FIGS. 1 and 2. Thus, overlapping description of the common construction is omitted herein to avoid redundancy. The electrostatic ink jet recording apparatus of the present embodiment is different from the electrostatic ink jet recording apparatus described hereinabove with reference to FIGS. 1 and 2 only in that it includes, between the opposing electrode 10 and the ground, a resistor switch 15 and a plurality of high resistors which are selectively connected to the opposing electrode 10 by the resistor switch 15. In the arrangement shown in FIGS. 3 to 5, three high resistors R1, R2 and R3 are provided and have different resistance values equal to or higher than 1 MΩ. In particular, the resistors R1, R2 and R3 have resistance values of, for example, R1 = 1 MΩ, R2 = 2 MΩ and R3 = 3 MΩ, respectively. Further, the resistor switch 15 is switched in response to the thickness of a printing vehicle to be used for recording. For example, the thickness of paper to be recorded is discriminated based on a result of adjustment of the gap or distance between the recording head body 100 and the opposing electrode 10 performed by an automatic gap adjustment mechanism (not shown), which is usually employed in a dot impact printer as a distance adjustment mechanism between a head body and an opposing electrode, and a main controller (not shown) of the electrostatic ink jet recording apparatus operates the resistor switch 15 based on the discrimination to automatically select an optimum one of the resistors R1, R2 and R3. Here, the reason why the resistors are selectively used in response to the thickness of the recording medium is described. In particular, in order to form normally fixed recording dots on a recording medium, the distance from the extremity of the ejection electrode 5 to the surface of the recording medium must be set fixed, and since the recording medium is disposed in contact with the opposing electrode, it is necessary to adjust the distance between the ejection electrode 5 and the opposing electrode 10 based on the thickness of the recording medium to be used. However, when the magnitude of the high voltage pulse signal applied to the ejection electrode 5 is constant, if the distance between the ejection electrode 5 and the opposing electrode 10 becomes shorter than a fixed distance, then the possibility occurs that a spark may be produced between the ejection electrode 5 and the opposing electrode 10. Therefore, the electrostatic ink jet recording apparatus of the present embodiment adopts the construction wherein the resistor to be used is switched in response to the thickness of the printing medium to prevent production of a spark. Thus, in the electrostatic ink jet recording apparatus, after a recording medium having a certain thickness is set in position into the paper travel path and the distance between the extremity of the ejection electrode 5 and the surface of the recording medium is adjusted to a certain fixed distance, the resistor switch 15 is operated in accordance with the amount of the adjustment then, for example, by the main controller of the apparatus mentioned hereinabove so that one of the three resistors R1, R2 and R3 is selectively connected. For example, when, from among three recording media P1, P2 and P3 which are different in thickness from one another, the comparatively thick recording medium P1 is to be used as seen in FIG. 3, since the distance between the ejection electrode 5 and the opposing electrode 10 is comparatively large, the resistor R1 of 1 MΩ is selectively connected. In contrast, when the recording medium P2 having a middle thickness is to be used as seen in FIG. 4, the resistor R2 of 2 MΩ is selectively connected. On the other hand, when the comparatively thin recording medium P3 is to be used as seen in FIG. 5, since the distance between the ejection electrode 5 and the opposing electrode 10 is comparatively short, the resistor P3 of 3 MΩ is selectively connected. Here, the resistance values of the resistors R1, R2 and R3 must be set to optimum values for recording media of different thickness values in advance. With the electrostatic ink jet recording apparatus of the present embodiment having the construction described above, similar advantages to those achieved by the electrostatic ink jet recording apparatus of the preceding embodiment are achieved, and in addition, there is another advantage that production of an excessive electric current upon leaking can be prevented effectively irrespective of the thickness of a recording medium to be used. In the meantime, it is otherwise possible to vary the voltage to be applied to the ejection electrode 5 in response to the distance between the ejection electrode 5 and the opposing electrode 10 to prevent an excessive electric current upon leaking. Further, the resistor switch 15 may alternatively be operated manually by an operator in accordance with the thickness of a recording medium to be used. In this instance, no particular measure is required for detecting the thickness of a recording medium.

Claims: An electrostatic ink jet recording apparatus which includes an ink chamber (2) for accommodating ink (1) in which charged toner particles are contained, a discharging exit (4) communicated with said ink chamber (2) and having an opening directed to the outside, an ejection electrode (5) disposed in said discharging exit (4) and having an end portion directed to the outside, an electrophoretic electrode (8) mounted on side walls (2b) of said ink chamber (2) such that said ejection electrode (5) is located therein, an opposing electrode formed by a conductive roller (10) disposed in an opposing relationship to said discharging exit (4) with a paper travel path (9) interposed therebetween, an electrophoretic voltage source (11) for applying an electrophoretic voltage to said electrophoretic electrode (8), and a discharging voltage source (12) for applying a driving pulse voltage to said ejection electrode (5), characterized in that said conductive roller (10) is made from rubber and has a high electrical resistance value, equal to or higher than 1 MΩ, and in that the recording apparatus further comprises a resistor switch (15) and a plurality of high resistors (R1, R2, R3) having different resistance values equal to or higher than 1 MΩ for being selectively connected to said opposing electrode (10) by said resistor switch (15), said resistor switch (15) and said plurality of high resistors (R1, R2, R3) being interposed between said opposing electrode (10) and the ground. An electrostatic ink jet recording apparatus as set forth in claim 1, characterized in that said plurality of high resistors are at least three high resistors (R1, R2, R3).

Inventors: HAGIWARA YOSHIHIRO, MINEMOTO HITOSHI, SHIMA KAZUO, SUETSUGU JUNICHI, UEMATSU RYOSUKE, HAGIWARA, YOSHIHIRO, MINEMOTO, HITOSHI, SHIMA, KAZUO, SUETSUGU, JUNICHI, UEMATSU, RYOSUKE
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1996
Main classification: B41J2
IPC codes: B41J2