INK JET RECORDING APPARATUS

Abstract: An ink nozzle plate (14) has ink discharge ports (4) for discharging ink, and an air nozzle plate (2) has air discharge ports(1), facing the ink discharge ports (4), for discharging air to jet out the ink, and an electric source (22,24) produces electric field operating the ink of the ink discharge ports, thereby to prolong the ink meniscus, and the ink is jetted out from the air discharge ports by the air-flow and the electric field, and a heating means increases a temperature of the air nozzle plate,thereby stabilising the jetted ink volume.

Description: The present invention relates to an ink jet recording apparatus for recording characters, figures or images on a recording medium by jetting ink droplets, utilizing electrostatic force and air streams. Hitherto, an ink jet recording apparatus utilizing electrostatic force and air streams, is known in USP 4,403,234(EP 63,853), USP 4,437,076(EP 61,327), USP 4,736,212(EP 212 943) and so on. The conventional ink jet recording apparatus utilizing electrostatic force and air streams is described as follows referring to FIG.1. In the FIG.1, a body 13 is provided with an air nozzle plate 2 of insulation. The air nozzle plate 2 has a plurality of air discharge ports 1. An ink nozzle plate 14 is provided in parallel with the air nozzle plate 2 and has a plurality of ink discharge ports 4 which are arranged facing the plurality of air discharge ports 1 respectively. Convex part 17 projecting in direction of the air discharge port 1, is formed around the ink discharge port 4. Between the convex parts17 neighbouring each other, a projection 5 is formed on the ink nozzle plate 14 in order to stabilize an air flow. Air flows into an air supply passage 8 from an air source 3 and is uniformed in a circular air chamber 9 and further flows into an air passage 7 between the air nozzle plate 2 and ink nozzle plate 14 and finally flows out from the air discharge ports 1. The air expands at the air discharge ports 1 and therefore sharp air pressure gradient appears in a space from the ink discharge port 4 to the air discharge port 1. An ink chamber 10 neighbouring the ink discharge port 4 is connected to an ink tank 11 through an ink supply passage 6. Ink in the ink tank 11 is applied with a pressure by the air pressure of the air source 3, the air pressure regulated by an air pressure regulating valve16. The reason for the airpressure regulation is that it is necessary to maintain static a meniscus formed at the ink of the ink discharge port 4, by almost equalizing the air pressure near the ink discharge port 4 to the ink pressure of the ink discharge port 4 or the ink chamber 10 when the ink jet recording apparatus is not driven. Bias electric source 24 is connected to a common electrode 12 provided around the air discharge ports 1 and a plurality of signal sources 22 are connected to control electrodes 15 provided on the surface facing the ink chamber 10, of the ink nozzle plate 14 around the ink discharge ports 4. Since the ink is conductive, sum of the bias voltage and the signal voltage is applied between the common electrode 12 and ink of the ink discharge port 4. The meniscus of the ink formed at the ink discharge port 4 is projected to the air discharge port 1 by electro static force produced by the voltage. Further, since there is the sharp pressure gradient in the space from the ink discharge port 4 to the air discharge port 1, when the produced ink meniscus of the ink discharge port 4 is prolonged or elongated some length, the ink meniscus jets out from the air discharge port 1. The conventional ink jet recording apparatus has a problem in that ink jet volume decreases when atmosphere (room)temperature is 25°C and relative humidity is 60% or more. The reasons for the ink jet volume decrease are described as follows. Fig.2(a) shows electric field when ink normally jets. The air nozzle plate 2 is made of photosensitive glass and the permittivity of the plate 2 is 6.54. On the other hand permittivity of air is about 1 and therefore the strength of the electric field in the air is larger than that of plate 2. The electric field which is strong and therefore particularly contributes to the elongation of the ink meniscus 21, is shown by electric line of force 18. The electric line of force 18 is produced through the air discharge port 1, from the common electrode 12 to ink meniscus 21. The lines of force are directed such that negatively charged particles would be projected in the direction from the ink nozzle ports towards the air nozzle ports. Since the ink is electrically connected to the negative supply, the electric field effectively projects the ink meniscus 21 towards the air nozzle ports. Fig. 2(b) shows electric field when ink jet volume decreases. When the relative humidity is 60% or more, water (moisture, vapor) 20 is adsorbed onto wall of the air discharge port 1. Specific resistance of the water is 2.5 x 10⁷Ω cm , namely, conductive, the voltage of the water 20 is equal to that of the common electrode 12. In the FIg. 2(b), the electric line of force 19 which indicates the place of strong electric field, is produced between the ink meniscus 21 and the water 20 which is nearer to the ink meniscus 21. In such mechanism, the electric field attracts the meniscus towards the edge of the water 20 rather than the electrode 12. In other words the meniscus is not attracted to the outside of the holes 1. As a result, the ink meniscus 21 is not efficiently elongated and the ink jetting volume decreases. Further, since the electric field is not stable, the ink volume varies. The reason why the water 20 is adsorbed, is described as follows. Considering the air discharge port 1 flowing the air, the air pressure decreases in the air discharge port 1 and air volume expands, when the air of 0.12kg/cm² in the air passage 7 away from the ink discharge port 4, comes out in the atomosphere (0 kg/cm²). When the air rapidly expands, the air absorbs heat of the wall of the air discharge port 1, and therefore a temperature of the wall of the air discharge port 1 decreases. Since the temperature of the wall of the air discharge port 1, is lower or equal to the flowing air, the wall of the air discharge port 1 tends to adsorb or condense water in the atmosphere. Table 1 shows conditions of temperature and humidity that the ink decreases, varies and becomes unstable in a thermo-hygrostat. atomosphere temparature relative humidity in the room 20°C 65% or more 25°C 60% or more 30°C 55% or more 40°C 60% or more The air flowing out from the air discharge port 1 is such air in the room which is supplied from the air source 3, for example, a diaphragm type air pump. Therefore, when the relative humidity of room is 55%∼65% or more, the volume of the absorbed water 20 increases and the ink jet volume decreases and the ink jetting becomes unstable. SUMMARY OF THE INVENTION The present invention intends to make the ink jet volume not vary even when the room humidity increases. That is, the present invention intends to provide an ink jet recording apparatus which comprises: an ink nozzle plate having ink discharge ports for discharging ink, an electric source for producing electric field operating the ink existing at the ink discharge ports, an air nozzle plate having air discharge ports, facing the ink discharge ports,for discharging air to jet out the ink, and a heating means for increasing a temperature of the air nozzle plate. Further,the present invention intends to provide an ink jet recording apparatus which comprises: an ink nozzle plate having ink discharge ports for discharging ink, an electric source for producing electric field operating the ink existing at the ink discharge ports, an air nozzle plate having air discharge ports, facing the ink discharge ports,for discharging air to jet out the ink, and an air supply system, for supplying the air to the air discharge ports, having a humidity decreasing apparatus for decreasing water in the air. Further, the present invention intends to provide an ink jet recording apparatus which comprises: an ink nozzle plate having rowed-up ink discharge ports for discharging ink, an air nozzle plate having rowed-up air discharge ports, facing the ink discharge ports respectively, for discharging air to jet out the ink, an air passage formed between the ink nozzle plate and the air nozzle plate, a common first electrode formed on an outside face of the air nozzle plate, surrounding the air discharge ports, a plurality of second electrodes formed on an ink-side face of the ink nozzle plate, each second electrode surrounding the ink discharge port, an electric source for producing electric field between the common first electrode and the plurality of second electrodes, and a heating means for heating the common first electrode. Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Fig. 1 is a sectional view of the conventional ink jet recording apparatus. FIG.2(a) is a partly enlarged sectional view of the conventional ink jet recording appararus. FIG.2(b) is a partly enlarged sectional view of the conventional ink jet recording appararus. FIG.3(a) is a plan view of an ink jet recording apparatus of an embodiment of the present invention. FIG.3(b) is a sectional view of an ink jet recording apparatus of an embodiment of the present invention. FIG.4 is a graph of a relation of temperature of an air port plate and an ink jet volume of an embodiment of the present invention. FIG.5(a) is a front view of an ink jet recording apparatus and a warm current device of another embodiment of the present invention. FIG.5(b) is a sectional view of an ink jet recording apparatus and a warm current device of another embodiment of the present invention. FIG.6 is a block diagram of an ink jet recording apparatus of still another embodiment of the present invention. FIG.3(a) is a plan view of an ink jet recording apparatus of an embodiment of the present invention. FIG.3(b) is a sectional view of an ink jet recording apparatus of the embodiment of the present invention. A common electrode 23 serving also as a heat device (abbreviated to common electrode), comprising a rectangular resistance device is attached on an air nozzle plate 2, surrounding a plurality of air discharge ports 1 disposed in a straight line. A terminal of the common electrode 23 is connected to a positive terminal of a bias electric source 24 and to a positive terminal of a heat electric source 25. The other terminal of the common electrode 23 is connected to a negative terminal of the heat electric source 25. The common electrode 23 serves as a common electric electrode for applying a bias voltage and as a heating device. The common electrode 23 is connected to the electric electrodes 24,25 utilizing silver paste 26. Other parts of the ink jet recording apparatus of the present invention are similar to the conventional ink jet recording apparatus as shown in FIG.1. The common electrode 23 is formed by depositing Cr of 1000A thick on the air nozzle plate 2 by utilizing an electron beam vapour method. The width of the common electrode 23 is formed 2 mm and the lenghth of the common electrode is formed 19 mm by using a vapour mask and thereby 30 Ω resistance is obtained. The common electrode 23 is applied with a voltage by the heat electrode 25, thereby to heat. The heat increases the temperature of the air nozzle plate 2. For example, when a room temperature is 25°C and air flows, 3.5v is necessary to make the temperature of the air nozzle plate 2 32°C. FIG.4 is a graph of the relation between the temperature of the air nozzle plate 2 and the ink jet volume. As shown in FIG.4, under a room temperature of 25°C and a relative humidity of 65%, the ink jet volume decreases and becomes unstable. But when the temperature increases, the ink jet volume increases. When the temperature is 32°C or more, the similar stable ink jet volume as under low relative humidity, is obtained. The reason of the stable ink jet volume is as follows. That is, a water adsorbed in the air discharge ports 1 is vapoured by heating the temperature of the air nozzle plate 2. Therefore, an electric potential is not applied to adsorbed water and on an electric field is not unstable. Then the problem that the ink meniscus is efficiently not prolonged because of the divergent electric field is solved. When the voltage applied to the common electrode 23 is 3.5v or more,for example, 5v, that is ,when the temperature of the air nozzle plate 2 is made high, the stable ink jet volume is obtained even when the relative humidity is 65% or more. Even when the room temperature varies, the temperature of the air nozzle plate 2 increases on the basis on the room temperature, applied with the heat by the common electrode 23. Thus, stable ink jet volume is obtained. Since the common electrode 23 is provided neighbouring the air discharge ports 1, the place near the air discharge ports 1 is efficiently heated. Then, Cr is used as the material of the common electrode 23 in the above mentioned embodiment, but other material can be used,that is, material of specific resistance of several tensµ Ω cm∼100µ Ω cm, for example, Ti(50µ Ω cm), Hf(29.6µ Ω cm), Ni-Cr(100µ Ω cm) are suitable. The specific resistance of Cr is 18.9µ Ω cm. The shape of the common electrode 23 is not limited to the rectangular type and other shape can be used, considering resistance or temperature distribution. FIG.5(a) is a front view of the ink jet recording apparatus of another embodiment of the present invention. FIG.5(b) is a sectional view of the ink recording apparatus of the embodiment of the present invention. A warm current device 30 comprises a fan 27, a heater 28 and a nozzle 31. The fan 27 flows air and the heater 28 heats the air and the warmed air 29 is jetted out from the nozzle 31. The warm current device 30 is disposed so as to blow the warmed air 29 on the air nozzle plate 2 of the ink jet recording apparatus. Other constitution of the ink jet recording apparatus is similar to the conventional ink jet recording apparatus of FIG.1. When the ink jet volume is unstable because of the high relative humidity (large water(moisture) volume in the atmosphere),the water adsorbed near the air discharge ports 1 is vapoured by increasing the tempareture of the air nozzle plate 2 by utilizing the warmed air 29. Therefore, the electric field is formed so as to efficiently prolong the ink meniscus and the stable ink jet volume is obtained irrespective of the humidity in the atmosphere. The common electrode 23 formed on the air nozzle plate 2 or the warm current device are used as the method for increasing the temperature of the air nozzle plate 2 in the above-mentioned embodiments. Other methods for increasing the temperature of the air nozzle plate 2 can be used, that is, for example, a resistance device for heating, a ceramic heater, an infrared lamp or a band heater covered with insulation material, may be attached on the air nozzle plate 2. FIG.6 is a perspective view of an air supply system of another embodiment of the present invention. A humidity decreasing apparatus 50 is provided between the air source 3 and a three way conduit 40. Other constitution is similar to that of FIG.1. The humidity decreasing apparatus 50 comprises a humidity decreasing system and a reclamation system. The humidity decreasing apparatus 50 interchanges the two systems, thereby successively decreasing the water volume in the atmosphere and decreasing the relative humidity. Referring to the humidity decreasing system, the air source 3 sucks the air of the room and increases pressure and blows the air into an absorbent filler case 41 through a four way valve 37. A casing of the absorbent filler case 41 has a filter 51,52 at the inlet and outlet to prevent flowing out of the absorbent 53. The absorbent 53 absorbs water of the air, thereby decreasing the relative humidity. A heater 33 for reclamation is buried in the absorbent 53. When the moderate size globule of silica gel is used as the absorbent 53 filled in the absorbent filler case 41 of 500cm³, the relative humidity of the air of room temperature 25°C and humidity 60%, is reduced to 20 %. The humidity reduced air passes through the four way valve 36 and enters into the three way conduit 40. Referring to the reclamation system, the air source for reclamation 39 sucks the air of the room and increases pressure and blows the air into a absorbent filler case 42 through the four way valve 36. A casing of the absorbent filler case 42 has a filter 54,55 at the inlet and outlet to prevent flowing out of an absorbent 56 of silica gel. A heater 34 is buried in the absorbent 56. By driving a heater switch 38 of the heater 34, the voltage of an electric source 57 is applied to the heater 34. Then the absorbent 56 is heated to 100°C or more and the air passes through the absorbent 56. Thus the absorbent 56 whose capability of absorbing water is reduced on account of water, is reclaimed. The air passed through the absorbent filler case 42, flows out into the room, through the four ways valve 37. The air is reclaimed for about 10 minutes. Then, after 10 minutes the heater switch 38 is made off and the air source for reclamation 39 stops the air supply. When the temperature of absorbent 56 falls down to room temperature by natural cooling, the reclamation of the absorbent 56 is completed. While the absorbent filler case 41 operates as the the humidity decreasing system, the absorbent filler case 42 operates as a reclamation system. And while the absorbent filler case 42 operates as the humidity decreasing system, the absorbent filler case 41 operates as a reclamation system. The interchange of the humidity decreasing system and a reclamation system is executed by exchanging the four way valves 36,37 with the valve switch 35 and, the heater switch 38 among heater 33, 34. Thus the absorbent 53,56 repeat the absorbing and the discharge of water. The air having the reduced humidity flows into the air supply passage 8 as shown in FIG.1 through the three ways conduit 40 and is uniformed in the circular air chamber 9 and enters into the air passage 7 and finally is jetted out from the air discharge port 1. The humidity decreasing apparatus 50 reduces the water in the air, thereby to make the 20% relative humidity air of 60% relative humidity air. Thus the ink jet volume is stable. As a result, by making the relative humidity of the supplying air to about 50% or less, the water of air is not adsorbed to the air discharge port 1. Then electric field is formed so as to efficiently prolong the ink meniscus and the stable ink jet volume is obtained irrespective of the humidity in the atmosphere of the room. In the above-mentioned embodiments, silica gel is used as the absorbent 53,56, but other material can be used, for example, alumina gel or zeolite.

Claims: An ink jet apparatus having: an ink nozzle plate (14) having at least one ink discharge port (4) for discharging ink; an air nozzle plate (2) having at least one air discharge port (1) which faces a corresponding one of said ink discharge ports (4) and is for discharging air; electric field means for providing said ink with an electric field so as to cause said ink to be projected toward said air discharge ports (1); air supplying means for supplying air so as to transport ink from said ink discharge ports (4) toward said air discharge ports (1) and so as to eject said ink from said air discharge ports (1) by cooperating with said electric field means; first electrode means (15) disposed on said ink nozzle plate (14) and at least surrounding each of said ink discharge ports (4); and second electrode means (23) disposed on said air nozzle plate (2) and at least surrounding each of said air discharge ports (1), said first and second electrode means (15,23) being used at least for generating said electric field, characterized in that a power source means (25) is provided so as to be electrically connected between both ends of said second electrode means (23) for applying a voltage to generate resistance heating in said second electrode means (23) to raise the temperature of said air discharge ports (1). An ink jet apparatus according to claim 1, wherein said air discharge ports (1) and said ink discharge ports (4) respectively form at least one row so that each of said air discharge ports (1) is substantially coaxially aligned with an associated one of said ink discharging ports (4), and wherein said second electrode means (23) comprises a band-type electrode having a resistivity within the range of approximately 18.9 to 100 µΩcm. An ink jet apparatus according to claim 2, wherein said band-type electrode is formed of chromium. An ink jet apparatus according to claim 1, wherein said first electrode means (15) comprises a plurality of electrodes each of which surrounds a corresponding one of said ink discharge ports (4). An ink jet apparatus having: an ink nozzle plate (14) having at least one ink discharge port (4) for discharging ink; an air nozzle plate (2) having at least one air discharge port (1) which faces a corresponding one of said ink discharge ports (4) and is for discharging air; electric field means for providing said ink with an electric field so as to cause said ink to be projected toward said air discharge ports (1); air supplying means for supplying air so as to transport ink from said ink discharge ports (4) toward said air discharge ports (1) and so as to eject said ink from said air discharge ports (1) by cooperating with said electric field means; first electrode means (15) disposed on said ink nozzle plate (14) and least surrounding each of said ink discharge ports (4); and second electrode means (23) disposed on said air nozzle plate (2) and at least surrounding each of said air discharge ports (1), said first and second electrode means (15,23) being used at least for generating said electric field, characterized by humidity absorbing means (50) for reducing the humidity of said air to be discharged by said air discharge ports (1), said humidity absorbing means (50) comprising at least two containers each of which has an absorbent material and a heater, and further has control means for controlling said containers so that one container is used for reducing said humidity of said air and the other container is used for reclaiming said absorbent material. An ink jet apparatus according to claim 5, wherein said air discharge ports (1) and said ink discharge ports (4) respectively form at least one row so that each of said air discharge ports (1) is substantially coaxially aligned with an associated one of said ink discharging ports (4), and wherein said first electrode means (15) comprises a plurality of electrodes each of which surrounds a corresponding one of said ink discharge ports (4) and said second electrode means (23) comprises a band-type electrode.

Inventors: AKAMI, KENJI, IWASAWA, TOSHIYUKI, MIURA, MASAYOSHI, ODA, GEN
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: B41J2
IPC codes: B41J2