Image forming apparatus

Abstract

An electrophotographic image forming apparatus of the present invention deposits charged toner on a latent image formed on a photoconductive drum or similar image carrier to thereby produce a corresponding toner image. The apparatus includes a toner flying device for electrostatically conveying the toner along the conveying surface of a conveyance board to one end of the conveyance board. The toner is caused to fly toward the image carrier from the end of the conveyance board.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an image forming apparatus for depositing toner, or developer, on a latent image formed on a photoconductive drum or similar image carrier to thereby form a corresponding toner image and transferring the toner image to a paper sheet or similar recording medium.

[0003] 2. Description of the Background Art

[0004] An image forming apparatus includes a developing device for developing a latent image formed on an image carrier. It is a common practice with a developing device to deposit toner agitated in the apparatus on a developing roller or developer carrier. The developing roller is rotated to convey the toner to a position where the roller faces the image carrier, so that the toner is transferred to a latent image formed on the image carrier to thereby develop the latent image. The toner left on the developing roller after development is collected in the apparatus due to the rotation of the developing roller. Fresh toner is charged by agitation and again deposited on the developing roller.

[0005] Japanese Patent Laid-Open Publication No. 5-19615 discloses another type of developing device in which toner is electrostically conveyed on the surface of a developing roller and then transferred to the surface of an image carrier by attraction, which acts between the developing roller and the image carrier. Japanese Patent Laid-Open Publication No. 59-181375,for example, proposes a developing device including a conveyance board for electrostatically conveying toner to a position where the board faces an image carrier. The toner is then separated from the conveying surface of the conveyance board by attraction acting between the board and an image carrier and transferred to the image carrier.

[0006] Further, Japanese Patent Laid-Open Publication Nos. 11-170591, 11-115235 and 11-179951, for example, each teach an image forming apparatus of the type causing toner to fly from a developing roller to a recording medium. This type of apparatus includes control electrodes arranged between a developing roller and a recording medium and counter electrodes located at the rear of the recording medium. Electric fields are generated between the developing roller and the counter electrodes, so that toner can fly toward the recording medium. The control electrodes selectively control the flight of the toner to thereby form an image on the recording medium.

[0007] The image forming apparatus of the type transferring toner from the developing roller to the image carrier or causing toner to fly by controlling the electric fields has the following problem. The developing roller essential with such a type of image forming apparatus increases the overall size and cost of the apparatus. Further, the problem with the developing device using the developing roller is that toner enters a gap between the roller and side walls and coheres due to friction, degrading image quality. With the developing device of the type electrostatically conveying toner, it is impracticable to surely convey the toner.

[0008] When toner is charged by friction or corona discharge, toner particles reached saturation charge and toner particles not reached it exist together, resulting in a broad charge distribution. Assume that such toner is forcibly transferred to an image carrier by, e.g., a magnet brush or a transfer roller. Then, the toner particles with low charge and deposited on the image carrier are apt to leave the it due to the developing speed of the state-of-the-art developing roller, i.e., about 100 cm/sec in terms of linear velocity. The toner particles left the image carrier fly about or deposit on the background of an image.

[0009] Moreover, the particle size of toner for development or image formation should preferably be uniform as far as possible. The conventional image forming apparatuses or developing devices thereof feed toner particles sized less than 5 μm to a developing roller together with the other toner particles. Let toner particles, or powder, sized less than 5 m be referred to as extremely fine toner particles hereinafter.

SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to provide an image forming apparatus and a developing device that are simple in construction, low cost, and high in image quality.

[0011] It is another object of the present invention to provide a powder jetting device feasible for the developing device.

[0012] It is still another object of the present invention to provide a toner feeding device capable of uniforming the particle size of toner to be fed to a developing device to thereby enhance image quality.

[0013] It is yet another object of the present invention to provide a toner conveying device for conveying toner to an image carrier or a recording medium.

[0014] It is a further object of the present invention to provide a classifying device for classifying powder.

[0015] In accordance with the present invention, an image forming apparatus includes an image carrier for forming a latent image thereon, and a developing device for developing the latent image with charged toner to thereby form a corresponding toner image. The developing device electrostatically conveys the charged toner toward one end of a conveyance board along the conveying surface of the board and causes it to fly toward the image carrier from the one end.

[0016] Also, in accordance with the present invention, in a toner conveying device for electrostatically conveying toner, a conveyance board includes a first and a second board stacked on each other. The first board includes a substrate and a plurality of substantially parallel electrodes arranged at a preselected distance in the direction of toner conveyance and each extending in the direction crossing the above direction. The second board includes an insulative substrate and a surface layer formed on the surface of the insulative substrate and having low contact resistance with respect to the toner. The surface layer forms a conveying surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:

[0018] FIG. 1 is a view showing the general construction of an image forming apparatus with which preferred embodiments of the present invention are practicable;

[0019] FIG. 2 is a view showing a developing device included in the apparatus of FIG. 1;

[0020] FIG. 3 is a section showing a conveyance board included in a toner flying device representative of a first embodiment of the present invention;

[0021] FIG. 4 is a plan view of the conveyance board;

[0022] FIG. 5 is a section showing path members and return members included in the toner flying device;

[0023] FIG. 6 is a schematic block diagram showing drive circuitry included in the toner flying device;

[0024] FIG. 7 is a view for describing the principle of toner conveyance unique to the conveyance board;

[0025] FIG. 8 is a table listing a specific pattern in which drive waveforms applied to the conveyance board vary;

[0026] FIG. 9 is a table listing another specific pattern of variation;

[0027] FIG. 10 is a view for describing another system for driving the toner flying device;

[0028] FIG. 11 is a table showing a specific pattern of drive waveforms applicable to the system of FIG. 10;

[0029] FIG. 12 is a graph showing field strengths and distribution thereof measured with the conveyance board;

[0030] FIG. 13 is a graph similar to FIG. 12, showing the results of measurement effected with a comparative example;

[0031] FIG. 14 is a view for describing the principle of development unique to the illustrative embodiment;

[0032] FIG. 15 is a view for describing the collection of toner effected in the illustrative embodiment;

[0033] FIG. 16 is a schematic block diagram showing a second embodiment of the present invention;

[0034] FIG. 17 is a schematic block diagram showing a third embodiment of the present invention;

[0035] FIG. 18 is a schematic block diagram showing a fourth embodiment of the present invention;

[0036] FIG. 19 is a schematic block diagram showing a fifth embodiment of the present invention;

[0037] FIG. 20 is a view showing part of a modification of the fifth embodiment;

[0038] FIG. 21 is a view showing a sixth embodiment of the present invention;

[0039] FIG. 22 is a view showing a seventh embodiment of the present invention;

[0040] FIG. 23 is a view showing an eighth embodiment of the present invention;

[0041] FIG. 24 is a view showing a ninth embodiment of the present invention;

[0042] FIG. 25 is a table listing a specific pattern in which drive voltages vary in the ninth embodiment;

[0043] FIG. 26 is a view showing a tenth embodiment of the present invention;

[0044] FIG. 27 is a view for describing the flight of toner particular to the tenth embodiment;

[0045] FIG. 28 is a view for describing the circulation of toner particular to the tenth embodiment;

[0046] FIG. 29 is a view for describing the conveyance of toner available with any one of the illustrative embodiments;

[0047] FIG. 30 is a view showing another specific arrangement of electrodes applicable to any one of the illustrative embodiments;

[0048] FIG. 31 is a view showing an eleventh embodiment of the present invention;

[0049] FIG. 32 is an isometric view of the eleventh embodiment;

[0050] FIG. 33 is a perspective view showing a control board included in the eleventh embodiment;

[0051] FIG. 34 is a view showing an image forming apparatus including a toner feeding device in accordance with the present invention;

[0052] FIG. 35 is a view demonstrating how the toner feeding device of FIG. 34 classifies toner;

[0053] FIG. 36 is a section in the direction of toner conveyance, showing a first specific configuration of a toner conveying device representative of a thirteenth embodiment of the present invention;

[0054] FIG. 37 is a section in a direction perpendicular to the direction of toner conveyance;

[0055] FIG. 38 is a plan view of an electrode board included in the thirteenth embodiment;

[0056] FIG. 39 is a plan view showing a conveyance board included in the thirteenth embodiment;

[0057] FIG. 40 is a section in the direction of toner conveyance, showing a second specific configuration of the toner conveying device;

[0058] FIG. 41 is a section in a direction perpendicular to the direction of toner conveyance, also showing the second specific configuration;

[0059] FIG. 42 is a plan view of a conveyance board included in the second specific configuration;

[0060] FIG. 43 is an enlarged plan view of part of FIG. 42;

[0061] FIG. 44 is a view showing a third specific configuration of the toner conveying device;

[0062] FIG. 45 is a plan view showing a fourth specific configuration of the toner conveying device;

[0063] FIG. 46 is a section in the direction of toner conveyance, showing a fifth specific configuration of the toner conveying device;

[0064] FIG. 47 is a section in the direction perpendicular to the direction of toner conveyance, also showing the fifth specific configuration;

[0065] FIG. 48 is a schematic block diagram showing drive circuitry included in the fifth specific configuration;

[0066] FIG. 49 is a view for describing the principle of operation of the fifth specific configuration;

[0067] FIGS. 50 through 53 are tables each showing a particular specific pattern in which drive waveforms vary in the fifth specific configuration;

[0068] FIG. 54 is a view showing path members and return members included in the developing device;

[0069] FIG. 55 is a view for describing the principle of development unique to the image forming apparatus;

[0070] FIG. 56 is a view for describing toner collection practicable with the image forming apparatus;

[0071] FIG. 57 is a view showing a fourteenth embodiment of the present invention;

[0072] FIG. 58 is a section in the direction of toner conveyance, showing a fifteenth embodiment of the present invention;

[0073] FIG. 59 is a plan view of the fifteenth embodiment;

[0074] FIG. 60 is a schematic block diagram showing drive circuitry included in the fifteenth embodiment;

[0075] FIG. 61 is a section showing a sixteenth embodiment of the present invention;

[0076] FIG. 62 is a section showing a seventeenth embodiment of the present invention;

[0077] FIG. 63 is a view showing an eighteenth embodiment of the present invention;

[0078] FIG. 64 is an isometric view showing a toner conveying device included in the eighteenth embodiment;

[0079] FIG. 65 is an isometric view showing a control board included in the eighteenth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0080] Preferred embodiments of the image forming apparatus in accordance with the present invention will be described hereinafter.

First Embodiment

[0081] Referring to FIG. 1 of the drawings, an image forming apparatus embodying the present invention is shown. It is to be noted that other embodiments to be described later are also practicable with the construction shown in FIG. 1. As shown, the image forming apparatus includes a photoconductive drum or image carrier 1. The drum 1 is implemented by OPC (Organic PhotoConductor) by way of example and rotatable clockwise, as viewed in FIG. 1. When the operator of the apparatus lays a document on a glass platen 2 and then pushes a print start switch, not shown, optics 5 including a light source 3 and a mirror 4 and optics 8 including mirrors 6 and 7 move while scanning the document.

[0082] An imagewise reflection from the document is incident to an image sensor 10 located at the rear of the lens 9. The image sensor 10 outputs an image signal corresponding to the incident reflection. The image signal is digitized and then subjected to image processing. A laser diode emits a laser beam in accordance with the image signal. A polygonal mirror 13 steers the laser beam toward the drum 1, which is uniformly charged beforehand, via a mirror 14. The laser beam forms a latent image on the charged surface of the drum 1.

[0083] A developing device 16 develops the latent image formed on the drum 1 with toner to thereby produce a corresponding toner image. A paper sheet or similar recording medium is fed from a sheet feed section 17a or 17b toward the drum 1. A corona charger 20 transfers the toner image from the drum 1 to the paper sheet by corona discharge. A separation charger 21 separates the paper sheet with the toner image from the drum 1. A belt conveyor 22 conveys the paper sheet separated from the drum 1 to a fixing device 23. The fixing device 23 fixes the toner image on the paper sheet with a pair of rollers 23. The paper sheet coming out of the fixing device 23 is driven out of the apparatus to a tray 24.

[0084] A drum cleaner 25 removes the toner left on the drum 1 after the image transfer. Subsequently, a discharge lamp 26 dissipates charge left on the surface of the drum 1.

[0085] The developing device or developing means 16, which characterizes the illustrative embodiment, will be described in detail with reference to FIG. 2. As shown, the developing device 16 includes a toner flying device 31 for electrostatically conveying charged toner toward one end or outlet along the conveying surfaces of conveyance boards 30. The toner reached the outlet flies toward the drum 1. The toner flying device 31 is representative of a powder jetting device of the present invention.

[0086] More specifically, path members 33 deliver charged toner from a toner box 32 to a path between the conveyance boards 30. A charge roller 34 charges the toner to be fed into the toner box 32. A doctor blade 35 is held in contact with the circumference of the charge roller 34. An agitator 37 conveys the toner stored in a toner hopper 36 toward the charge roller 34. Gutters or collecting means 38 collect part of the toner flown out of the toner flying device 31, but not used for development. Return members 39 electrostatically return the toner collected by the gutters 38 to the toner box 32. The conveyance boards 30 each have a respective conveying surface 30a.

[0087] Reference will be made to FIGS. 3 and 4 for describing the conveyance boards 30 specifically. As shown, each conveyance board 30 includes an elongate, flat substrate 41 on which a number of electrodes 42 are arranged. The electrodes 42, each three of which make a set, extend in the direction perpendicular to the direction in which the toner is conveyed (direction of toner conveyance hereinafter). A member 43 for forming a conveying surface is stacked on the electrodes 42. A coating layer or film 44 is formed the surface of the member 43 in order to reduce contact resistance between the member 43 and the toner. With this configuration, the conveyance board 30 conveys the toner along its conveying surface while accelerating it with an electrostatic force.

[0088] The substrate 41 may be formed of glass, resin, ceramics or similar insulating material or SUS or similar conductive material coated with SiO2 (silicon dioxide) or similar insulating film. In the illustrative embodiment, the substrate 41 is formed of glass and 0.9 mm to 1.1 mm thick.

[0089] To form the electrodes 42, a film of Al (aluminum), Ni—Cr (nickel-chromium) or similar conductive material is formed on the substrate 41 and then patterned by photolithography or similar semiconductor technology. The electrodes 42 should preferably be arranged in density that is one-third to 100 times, particularly one-half to five times, as great as the particle size of toner in terms of lines per space (L/S). This makes toner conveyance desirable in speed and amount. Each electrode 42 should preferably have a width that is one time to three times as great as the particle size of toner in order to guarantee desirable toner conveyance.

[0090] In light of the above, as shown in FIG. 4, the density of the electrodes 42 is sequentially increased stepwise in the direction of toner conveyance. For example, assume consecutive zones A, B, C and D beginning at the toner inlet and ending at the toner outlet. Then, the electrodes 42 are arranged at intervals of 100 μm in the zone A, at intervals of 20 μm in the zone B, at intervals of 10 μm in the zone C, and at intervals of 5 μm in the zone D. Assuming that the mean particle size of toner is 8 μm, the illustrative embodiment provides each electrode 42 with a width of 10 μm.

[0091] The member 43 for forming the conveying surface is implemented as an insulating film having a specific inductive capacity, e.g., SiO2 or Ta2O5 (tantalum pentoxide) and 0.5 μm to 1 μm thick. Ta2O3 has a specific inductive capacity of 28. Alternatively, use may be made of a film of polyimide whose specific inductive capacity is about 3.6. The apparent, specific inductive capacity of polyimide can be increased to 10 to 12 if about 1% of fine BaTiO3 (barium metasilicate) is added in the event of coating.

[0092] By increasing the specific inductive capacity of the member 43, it is possible to lower required drive voltage and to promote the bounce of the toner particles and conveying speed. Therefore, to surely convey the charged toner by low voltage at a speed that allows the toner to fly, the member 43 should preferably be formed of a material whose specific inductive capacity is 10 or above. It was experimentally found that a 1 μm thick polyimide film required a drive voltage of DC 500 V, but derived an acceptable result with a drive voltage of 300 V when about 1% of fine BaTiO3 particles were added.

[0093] The coating layer 44, which reduces contact resistance between the conveying surface and the charged toner, may be formed of PTFE (polytetrafluoroehtylene), PFA (perfluoroalcoxy alkane) or similar fluorine-containing resin and 0.1 μm to 0.3 μm thick.

[0094] Even a single conveyance board 30 suffices for conveying the charged toner. In the illustrative embodiment, two conveyance boards 30 are positioned with their conveying surfaces 30a, FIG. 2, facing each other. The conveyance boards 30 are spaced from each other by 30 μm to 200 μm. A plurality of conveyance boards 30 increase the amount of toner to fly for a unit period of time. Further, the conveyance boards 30 can be selectively driven in order to control the amount of toner to fly.

[0095] The path members 33 and return members 39 will be described specifically with reference also made to FIG. 5. As shown, the path members 33 and return members 39 each are basically identical in configuration with the conveyance boards 30. Specifically, the path members 33 and return members 39 each include an insulative FPC substrate 48 on which a number of electrodes 42 are arranged; each three of the electrodes 42 make a set. The substrates 48 extend in the direction perpendicular to the direction of toner conveyance. A member 43 for forming a conveying surface is formed on the electrodes 42. A coating layer or film 44 is formed on the surface of the member 43 in order to reduce contact resistance between the member 43 and the toner. The electrodes 42 are positioned at substantially identical intervals on both of the members 33 and 39.

[0096] Referring to FIGS. 6 through 11, how the toner flying device 31 conveys the toner and causes it to fly will be described. As shown in FIG. 6, the toner flying device 31 includes two drivers 50 each for applying three-phase drive waveforms or voltages Va, Vb and Vc to each group of three electrodes 42 of one conveyance board 30. Some delay is provided between the drive waveforms Va, Vb and Vc output from each driver 50. The drive waveforms Va, Vb and Vc each selectively take a positive potential, a negative potential and zero potential (not applied).

[0097] As shown in FIG. 7, assume that a charged toner particle T is positioned on the conveyance board 30. Also, assume that the potentials “+”,“−”, “0”, “+” and “−” are respectively applied to the consecutive electrodes 42 on the board 30, as indicated by row [1]. Further, assume that the toner particle T is positioned on the “0” electrode 42. Then, repulsion acts between the toner particle T and the “−” electrode 42 positioned at the left-hand side of the “0” electrode 42, as viewed in FIG. 7. At the same time, attraction acts between the toner particle and the “+” electrode 42 positioned at the right-hand side of the “0” electrode 42. As a result, the toner particle T moves to the “+” electrode 42.

[0098] Assume that the driver 50 applies the three-phase drive voltages Va, Vb and Vc to each three electrodes 42 in a specific pattern shown in FIG. 8. Then, the voltages indicated in row [1] of FIG. 7 are replaced with voltages “0”, “+”, “−”, “0” and “+” indicated in row [2]. Consequently, no force acts between the toner particle T and the electrode 42 changed from “+” to “0”. At the same time, repulsion acts between the toner particle T and the electrode 42 changed from “0” to “−” while attraction acts between the toner particle T and the electrode 42 changed from “−” to “+”. The toner particle T therefore moves further to the “+” electrode 42.

[0099] As stated above, the potentials of the drive waveforms applied to the electrodes 42 are varied to apparently move the drive waveforms, so that the toner particle T sequentially moves toward the “+” electrode 4. That is, the toner particle T is conveyed along the conveying surface of the conveyance board 30. The pattern shown in FIG. 8 will be reversed when the toner particle is charged to positive polarity. Let the following description concentrate on toner charged to negative polarity.

[0100] The distance between nearby electrodes 42 is reduced from the inlet toward the outlet stepwise, as stated earlier. Therefore, repulsion and attraction acting on the negatively charged toner particle T sequentially increase toward the outlet stepwise (region A→region→B region C→region D). Finally, the toner particle T flies from one end of the conveyance board 30.

[0101] FIG. 9 shows another specific pattern of the three-phase drive voltages Va, Vb and Vc that does not include “0”. As shown, a “+” drive waveform is applied to two adjoining ones of three electrodes 42 while a “−” drive waveform is applied to the remaining electrode 42. This is also successful to convey the negatively charged toner T along the surface of the conveyance board 30 while accelerating it.

[0102] Further, as shown in FIG. 10, the driver 50 may be replaced with a six-phase driver 51 that applies drive voltages Va, Vb and Vc and drive waveforms Vd, Ve and Vf to each three electrodes 42. FIG. 11 shows a specific pattern in which the drive voltages va, Vb and Vc and drive waveforms Vd, Ve and Vf are applied in the configuration shown in FIG. 10.

[0103] Reference will be made to FIGS. 12 and 13 for describing a relation between the configuration of the conveyance board 30 and the field strength. FIG. 12 shows field strengths and the distribution of electric fields unique to the illustrative embodiment. FIG. 13 shows field strengths and the distribution of electric fields measured with a comparative conveyance board. In the illustrative embodiment, the electrodes 10 were 10 μm wide each and spaced from each other by 100 μm while the member 43 was 10 μm thick and formed of SiO2. In the comparative conveyance board, the electrodes were 10 μm wide each and spaced from each other by 100 μm while the member 43 was 100 μm thick and formed of organic resin as conventional.

[0104] FIGS. 12 and 13 show electric fields formed by the consecutive electrodes 42 in y direction (perpendicular to the conveying surface; solid lines) and electric fields in x direction (parallel to the conveying surface). It will be seen that the conveyance board 30 of the illustrative embodiment implements electric fields in y direction that is about three times as great as the electric fields of the comparative conveyance board. Further, the conveyance board 30 includes flat regions where the electric field is substantially zero between nearby electrodes.

[0105] The great field intensity and the digital field distribution including flat regions between electrodes allow the toner to move at high speed on the conveyance board 30. The toner T therefore surely flies from the outlet at higher speed. Experiments showed that the conveyance board 30 caused the toner to fly at a speed of 0.5 m/sec to 5 m/sec although dependent on voltage and frequency.

[0106] Development using the developing device 6 will be described hereinafter with reference also made to FIG. 14. As shown, the toner particles with negative charge fly from the toner flying device 31 toward the drum 1 by being conveyed and accelerated by the conveyance boards 30. The toner particles then deposit on a latent image, i.e., the positively charged portions of the surface of the drum 1, thereby developing the latent image. An AC power supply 52 may be connected to the outlet end of the conveyance boards 33 so as to form an AC electric field between the boards 33. The electric field causes the toner T flying toward the drum 1 to form a toner cloud 53 and evenly deposit on the latent image. The toner cloud 53 is successful to enhance image quality.

[0107] As stated above, the developing means (toner flying device or powder jetting device) electrostatically conveys the charged toner toward one end thereof along the conveying surfaces of the conveyance boards and then causes it to fly. Such developing means directly deposits the toner on the drum 1 without contacting the drum 1 and is therefore simple and low cost.

[0108] Further, the developing means of the illustrative embodiment deteriorates the toner less than conventional developing means using a developing roller. Specifically, it is a common practice with developing means to deposit frictionally charged toner on a developing roller together with a carrier, cause the toner electrostatically deposited on the carrier to form a magnet brush, and then bring the magnet brush into contact with an image carrier for thereby developing a latent image. The developing roller, however, kneads the toner or smashes it into fine powder. As a result, SiO2, TiO2 or similar additive is rubbed into the resin of the toner to thereby deteriorate the characteristics of the toner. The developing means of the present invention solves this problem.

[0109] The collection of the toner will be described with reference also made to FIG. 15. While the toner flies out of the conveyance boards 30 to deposit on the latent image formed on the drum 1, not all toner particles are used for development, as stated earlier. As shown in FIG. 15, toner gutters 38 are positioned outside of the conveyance boards 30 and cause the toner particles not used for developing and tending to be scattered around to deposit on the toner gutters 38. The return members 39 convey the above toner particles toward the inlet in the same manner as the conveyance boards 30. Consequently, such toner particles are collected in the toner box 32 and used again.

[0110] As stated above, returning means collects the toner not used for development and thereby prevents it from being scattered around. Further, the collecting means includes means for electrostatically returning the collected toner toward the inlet. This successfully promotes the reuse of the toner and thereby reduces the cost.

[0111] If desired, a bias voltage of the same polarity as the toner and a bias voltage opposite in polarity to the toner may be alternately applied to each toner gutter 38 in order to selectively attract or repulse the toner. In such a case, the toner will be collected and then returned to the outlet of the conveyance board 30 to be reused thereby. This configuration makes it needless to return the collected toner to the toner box 32.

Second Embodiment

[0112] FIG. 16 shows a second embodiment of the present invention, particularly the toner flying device 31 included therein. As shown, each conveyance board 30 has a number of electrodes 42 arranged at substantially the same interval in the direction of toner conveyance. Drivers 55 each apply to the associated electrodes 42 three-phase drive waveforms Va1, Vb1 and Vc1 having a frequency f1, three-phase drive waveforms Va2, Vb2 and Vc2 having a frequency f2, and three-phase drive waveforms Va3, Vb3 and Vc3 having a frequency f3 (f1>f2>f3).

[0113] More specifically, each driver 55 applies the drive waveforms Va1, Vb1 and Vc1 to part of the electrodes 42 that lie in a preselected zone of the conveyance board 30 adjoining the outlet. The driver 55 applies the drive waveforms Va2, Vb2 and Vc2 to the electrodes 42 lying in the intermediate zone of the conveyance board 30. Further, the driver 55 applies the drive voltages Va3, Vb3 and Vc3 to part of the electrodes 42 that lie in a preselected zone adjoining the inlet. As for the rest of the construction, the illustrative embodiment is identical with the first embodiment.

[0114] In the illustrative embodiment, although the electrodes 42 are arranged on the conveyance board 30 at substantially the same interval, the frequencies of the drive waveforms Va, Vb and Vc sequentially increase from the inlet toward the outlet stepwise, i.e., from the frequency f3 to the frequency f1. Attraction and repulsion to act on the charged toner vary in a shorter period of time as the frequency of the drive waveform increases. Therefore, the toner entered the space between the conveyance boards 30 via the inlet is sequentially accelerated as the frequency of the drive waveform increases. The toner is therefore conveyed along the conveying surfaces of the conveyance boards 30 while being accelerated.

[0115] The illustrative embodiment, which varies the frequency of the drive waveform stepwise, is advantageous over the first embodiment in that the electrodes 42 can be arranged at substantially the same interval. The illustrative embodiment, however, makes the configuration of each driver 55 slightly sophisticated. Either one of the two embodiments may be selected in consideration of the production cost of the drivers and conveyance boards. The two embodiments may be combined, if desired.

Third Embodiment

[0116] FIG. 17 shows a third embodiment of the present invention, particularly the toner flying device 31 thereof. As shown, the electrodes 42 are arranged on each conveyance board 30 at substantially the same interval in the direction of toner conveyance. Drivers 56 each applies to the electrodes 42 three-phase drive waveforms Vap1, Vbp1 and Vpc1 having a crest value Vp1, three-phase drive waveforms Vap2, Vbp2 and Vcp2 having a crest value Vp2, and three-phase drive waveforms Vap3, Vbp3 and Vcp3 having a crest value Vp3 (Vp1>Vp2>Vp3).

[0117] More specifically, the driver 56 applies the drive waveforms Vap1, Vbp1 and Vcp1 to part of the electrodes 42 lying in a preselected zone that adjoins the outlet. The driver 56 applies the drive waveforms Vap2, Vbp2 and Vcp2 to the electrodes 42 lying in the intermediate zone of the conveyance board 30. Further, the driver 56 applies the drive waveforms Vap3, Vbp3 and Vcp3 to part of the electrodes 42 that lie in a preselected zone adjoining the inlet. As for the rest of the construction, this embodiment is identical with the previous embodiments.

[0118] In the illustrative embodiment, although the electrodes 42 are arranged on the conveyance board 30 at substantially the same interval, the crest values of the drive waveforms Va, Vb and Vc sequentially increase from the inlet toward the outlet stepwise, i.e., from the crest value Vp3 to the crest value Vp1. Attraction and repulsion to act on the charged toner vary in a shorter period of time as the crest value of the drive waveform increases. Therefore, the toner entered the space between the conveyance boards 30 via the inlet is sequentially accelerated as the crest value of the drive waveform increases. The toner is therefore conveyed along the conveying surfaces of the conveyance boards 30 while being accelerated.

Fourth Embodiment

[0119] FIG. 18 shows a fourth embodiment of the present invention, particularly the toner flying device 31 thereof. As shown, the electrodes 42 are arranged at substantially the same interval in the direction of toner conveyance. The illustrative embodiment includes drivers 57 in addition to the drivers 50 stated earlier. The drivers 57 each apply drive waveforms Vad, Vbd and Vcd higher in duty ratio than the drive waveforms Va, Vb and Vc to preselected ones of the electrodes 42, e.g., the electrodes 42 adjoining the outlet.

[0120] The drive waveforms Vad, Vbd and vcd applied to the electrodes 42 provide the toner with initial speed high enough for the toner to fly. Subsequently, the toner is sequentially conveyed by the electrodes 42 to which the drive waveforms Va, Vb and Vc are applied at substantially the initial speed. The toner then flies toward the drum 1 from the outlet of the conveyance board 30.

Fifth Embodiment

[0121] FIG. 19 shows a fifth embodiment of the present invention, particularly the toner flying device 31 thereof. As shown, the illustrative embodiment includes two conveyance boards 30A and an additional conveyance board 30B intervening between the conveyance boards 30A. While the conveyance boards 30A has a single conveying surface as in the previous embodiments, the conveyance board 30B has a conveying surface on opposite sides thereof. The conveying surfaces of the boards 30A and those of the board 30B cooperate to convey the toner.

[0122] The illustrative embodiment with the above configuration conveys the toner by an amount two times as great as the amount available with the previous embodiments for a unit time. The illustrative embodiment can therefore sufficiently cope with high-speed recording. Specifically, the amount of toner consumption increases with an increase in recording speed with the result that toner replenishment sometimes becomes short. The illustrative embodiment can convey more toner with a simple configuration and successfully copes with high-speed recording.

[0123] FIG. 20 shows a modification of the illustrative embodiment. As shown, the outlet of the intermediate conveyance board 30B is set back relative to the outlets of the conveyance boards 30A. The AC power supply applies an AC voltage between the conveyance boards 30A in order to form an AC electric field. The AC electric field causes the toner T to oscillate at the outlet side of the conveyance boards 30A and easily form a toner cloud.

Sixth Embodiment

[0124] FIG. 21 shows a sixth embodiment of the present invention. As shown, the AC power source 52 applies an AC voltage between the conveyance boards 30 as in the previous embodiment. In addition, a DC power supply 60 applies a DC voltage between the drum 1 and one of the conveyance boards 30 (lower conveyance board 30 in the illustrative embodiment).

[0125] The AC power supply 52 and DC power supply 60 respectively apply AC±300 V and DC 500 V by way of example. In this condition, +500 V is constantly applied between the drum 1 and the lower conveyance board 30 while 200 V (=500−(+300)) and 800 V (=500−(−300)) are alternately applied to the upper conveyance board 30. The alternating electric field generated between the upper conveyance board 30 and the drum oscillates. Consequently, the toner flown out of the conveyance boards 30 is scattered by the oscillation of the electric field and therefore deposits or leaves the drum 1 more frequently. It follows that the toner accurately deposits on the charge pattern of the drum 1 and enhances image quality.

Seventh Embodiment

[0126] FIG. 22 shows a seventh embodiment of the present invention. As shown, the AC power supply applies an AC voltage between the conveyance boards 30 as in the previous embodiment. In addition, a DC power supply 60 applies a DC voltage between the drum 1 and electrode portions 61 provided on the conveyance boards 30. In this configuration, DC bias electric fields are formed between the conveyance boards 30 and the drum 1. The DC bias electric fields confine the toner forming a toner cloud due to the AC electric field therebetween. The illustrative embodiment therefore prevents the toner from being scattered and further improves image quality.

Eighth Embodiment

[0127] FIG. 23 shows an eighth embodiment of the present invention, particularly the toner flying device 31 thereof. As shown, the conveyance boards 30 each are configured to form a storing portion or recess 70. The storing portions 70 can store the toner fed from the toner box 32 and allow the toner to be continuously conveyed and flown without interruption. Specifically, the toner must be continuously conveyed and flown when consumed in a great amount. If the toner is not replenished to the conveyance boards 30 in good time, then the toner becomes short. In this respect, the storing portions 70 allow the toner to be conveyed while being stored therein and insure the stable conveyance and flight of the toner, preventing image quality from being degraded.

Ninth Embodiment

[0128] FIG. 24 shows a ninth embodiment of the present invention, particularly the toner flying device 31 thereof. As shown, the conveyance boards 30 each include the previously stated storing portion 70. A drive circuit 71 applies the drive waveforms Va, Vb and Vc, which vary in the pattern shown in FIG. 9, to the upper conveyance board 30. A drive circuit 72 applies the drive waveforms Va, Vb and Vc, which vary in a pattern shown in FIG. 25, to the lower conveyance board 30.

[0129] In the configuration shown in FIG. 24, the upper conveyance board 30 conveys the toner in the same manner as in the previous embodiments and causes it to fly toward the drum 1. On the other hand, the lower conveyance board 30 opposite in the pattern of the drive waveforms to the upper conveyance board 30 conveys the toner in the reverse direction, i.e., from the outlet toward the inlet. Therefore, the toner bouncing back without depositing on the drum 1 is conveyed from the outlet to the storing portion 70 by the lower conveyance board 30 and reused. This not only promotes the efficient use of the toner, but also prevents the bounced toner from flying about and thereby improves image quality.

Tenth Embodiment

[0130] FIG. 26 shows a tenth embodiment of the present invention, particularly the toner flying device 31 thereof. As shown, the driver 71 applies the drive waveforms Va, Vb and Vc varying in the pattern of FIG. 9 to the upper conveyance board 30. A driver 73 is selectively operable in two different modes. Specifically, the driver 73 applies the drive waveforms Va, Vb and Vc varying in the pattern of FIG. 9 to the lower conveyance board 30 or applies the drive waveforms Va, Vb and Vc varying in the pattern of FIG. 25 opposite to the pattern of FIG. 9 to the same.

[0131] A controller, not shown, sends a mode switching signal to the driver 73. When the apparatus is, e.g., in a standby state, the mode switching signal causes the driver 73 to output the drive waveforms Va, Vb and Vc in the pattern of FIG. 25.

[0132] While the apparatus forms images in the usual manner, the controller causes the lower conveyance board 30, as well as the upper conveyance board 30, to apply the drive voltages Va, Vb and Vc varying in the pattern of FIG. 9. As a result, the upper and lower conveyance boards 30 both convey the toner toward the outlet and therefore by an amount two times as great as the amount available with a single conveyance board 30.

[0133] When the apparatus is in a standby state, the controller causes the lower conveyance board 30 to apply the drive voltages Va, Vb and Vc varying in the pattern of FIG. 25. The lower conveyance board 30 therefore conveys the toner in the reverse direction. In this case, as shown in FIG. 28, the toner is simply circulated in the space between the two conveyance boards 30 without flying toward the drum 1. This prevents the toner from being scattered around more positively and thereby enhances image quality. In addition, the lower conveyance board 30, which selectively conveys the toner from the inlet to the outlet or from the outlet to the inlet, simplifies the configuration of the toner flying device 31.

[0134] In any one of the previous embodiments including at least two conveyance boards 30, the drive voltages and the gap between the conveying surfaces may be so selected as to synchronize the drive waveforms to be applied to the electrodes of the conveyance boards 30. In such a case, as shown in FIG. 29, electric field curtains formed by the upper conveyance board 30 and those formed by the lower conveyance board 30 join each other. In this condition, the toner does not move at the positions where the electric field curtains join each other, while forming layers at the other positions. The toner can therefore be intermittently conveyed in the form of consecutive layers if the curtains 81 joining each other are moved toward the outlet with a preselected time constant.

[0135] FIG. 30 shows another modification of any one of the illustrative embodiments. As shown, each conveyance board includes electrodes 42a extending in x direction and electrodes 42b extending in y direction. The electrodes 42a and electrodes 42b are arranged in a lattice pattern and isolated from each other by an insulating film. A driver 85 applies three-phase drive waveforms to the electrodes 42a while a driver 86 applies three-phase drive waveforms to the electrodes 42b.

[0136] In operation, the toner charged by the electrodes 42a extending in x direction are conveyed in a direction indicated by an arrow in FIG. 30. At the same time, the drive waveforms applied to the electrodes 42b, which extend in y direction, subject the toner to its electric field also. Consequently, the toner is conveyed while oscillating itself. Toner particles are therefore separated from each other and surely fly independently of each other, thereby enhancing image quality.

Eleventh Embodiment

[0137] Referring to FIGS. 31 through 33, an eleventh embodiment of the present invention will be described. FIG. 31 is a fragmentary view of an image forming apparatus. FIG. 32 shows a toner jet head included in the apparatus while FIG. 33 shows a control board included in the toner jet head. As shown, the apparatus includes a toner jet head 100 for causing charged toner to fly in accordance with an image signal. The toner jet head 100 is generally made up of a conveyance board 101 and a control board 102 facing each other.

[0138] As shown in FIG. 32 also, the conveyance board 101, like the conveyance board 30, includes a substrate 111 and a number of electrodes 112 arranged on the substrate 111. The electrodes 112, each three of which make a set, each extend in the direction perpendicular to the direction of toner conveyance. A member 113 for forming a conveying surface is formed on the electrodes 112. A coating layer or film 114 is formed on the surface of the member 113 in order to reduce contact resistance between the member 113 and the toner.

[0139] As shown in FIG. 33 also, the control board 102 includes a substrate 121. A number of first electrodes 122 are arranged on the substrate 121 for conveying the toner in the reverse direction, and each extend in the direction perpendicular to the direction of toner conveyance. An insulative protection film 123 is formed on the first electrodes 122. A number of second electrodes or pixel electrodes 124 are arranged on the protection film 123, and each extend in the direction perpendicular to the direction of toner conveyance. Further, a protection film 125 is formed on the second electrodes 124

[0140] Drive waveforms are applied to the electrodes 112 of the conveyance board 101 in the same manner as in any one of the previous embodiments, causing the board 101 to covey the toner in the same manner as the conveyance board 30. On the other hand, drive waveforms are applied to the first electrodes 122 of the control board 102 in a pattern opposite to the electrodes 122, so that the toner is conveyed from the outlet to the inlet, as stated earlier with reference to FIG. 28.

[0141] Assume that a drive waveform for generating an electric field that repulses the charged toner is applied to any one of the second electrodes 124 of the control board 102 in accordance with a pixel signal. Then, the toner being conveyed by the conveyance board 101 flies away from the board 101. On the other hand, when a drive waveform for generating an electric field that attracts the charged toner is applied to the electrode 124, the toner conveyed by the conveyance board 101 as far as the outlet of the board 101 is attracted by the control board 102. More specifically, by controlling the drive waveforms to be applied to the second electrodes 124 in accordance with a pixel signal, it is possible to implement an on-demand type of toner jet head that controls the flight of the toner from the conveyance board 101 pixel by pixel.

[0142] The toner T jetted from the toner jet head deposits on a recording medium 130, forming a toner image in accordance with an image signal. Subsequently, the toner image is fixed on the recording medium 130. The illustrative embodiment obviates the need for an image carrier and is therefore simple in construction. Moreover, the illustrative embodiment controls the flight of the toner on a pixel basis to thereby circulate part of the toner that did not fly. This successfully maintains the saturation charge of the toner and thereby causes a minimum of toner to be scattered around.

Twelfth Embodiment

[0143] Reference will be made to FIGS. 34 and 35 for describing a developing device representative of a twelfth embodiment of the present invention. As shown in FIG. 34, the developing device includes a developing roller or developing means 141 for causing toner to deposit on a latent image formed on the drum 1. A toner feeding device or classifying device 142 feeds the toner delivered from a toner hopper to the developing roller 141. The developing roller 141 maybe replaced with the toner flying device 31 stated earlier, if desired.

[0144] The toner feeding device 142 includes a number of conveyance boards 30 (including 30A and 30B) described in relation to the previous embodiments. The conveyance boards 30 each convey the toner, which is charged by a charge roller or similar charging means 143, and cause it to fly toward the developing roller 141. A return conveyance board 144 is positioned below the conveyance boards 30 in order to convey the toner toward the inlet in the reverse direction.

[0145] With the toner feeding device 142, the developing device is capable of feeding to the developing roller 141 the toner that is substantially uniform in charge and mass. In addition, the developing device is capable of collecting extremely fine toner particles without feeding them to the developing roller 141.

[0146] As shown in FIG. 35, when the conveyance boards 30 convey and fly the toner, the conveying speed and flying speed of the toner depend on the amount of charge q and mass m of the toner. Further, the distance of flight of the toner varies in dependence on the initial speed of flight, as represented by toner particles T1, T2 and T3. The toner feeding device 142 therefore causes the toner to fly toward the developing roller 41 to thereby select only toner particles that can fly a required distance (classification).

[0147] More specifically, the toner feeding device 142 collects toner particles unable to fly the required distance and returns them to the inlet side or stores them in a storing portion. At this instant, the distance of flight of small toner particles is relatively short, so that such toner particles are not fed to the developing roller 141. It is therefore possible to exclude extremely small toner particles sized less than 5 μm and therefore to uniform the particle size of toner particles expected to contribute to development. The toner feeding device 142 is similarly applicable to any one of the previous embodiments.

[0148] It is to be noted that the powder jetting device and classifying device of the present invention are applicable not only to a developing device, but also to any other device required to jet fine particles or to exclude extremely fine particles.

Thirteenth Embodiment

[0149] Hereinafter will be described a thirteenth embodiment of the present invention, particularly specific configurations of each conveyance board 30 included in a toner conveying device 31, which corresponds to the toner flying device 31.

[0150] FIGS. 36 through 39 show a first specific configuration of the conveyance board 30 included in the toner conveying device 31. As shown, the conveyance board 30 is generally made up of an electrode board or first board 91 and a path board or second board 92 stacked on the conveyance board 91. The electrode board 41 includes an elongate, flat substrate 95 and a number of electrodes 96 arranged on the substrate 95. The electrodes 96, each three of which make a set, each extend in the direction substantially perpendicular to the direction of toner conveyance. The path board 92 includes an insulative substrate 97 formed with channel-like paths 98 and forming a conveying surface. The paths 98 linearly extend in the direction of toner conveyance and are positioned at preselected intervals in the direction perpendicular to the direction of toner conveyance. A coating layer 99 is formed on the surface of the substrate 97 and exerts smaller contact resistance than the substrate 97 with respect to the toner.

[0151] The substrate 95 may be formed of glass, resin, ceramics or similar insulating material or SUS or similar conductive material coated with SiO2 or similar insulating film.

[0152] To form the electrodes 96, a film of Al, Ni—Cr, TiN or polysilicon or similar conductive material or Ti, W, Mo or similar high-temperature metal is formed on the substrate 95 and then patterned by photolithography or similar semiconductor technology. The electrodes 96 should preferably be arranged in a density that is one-third to 100 times, particularly one-half to five times, as great as the particle size of toner in terms of lines per space (L/S). This makes toner conveyance desirable in speed and amount. Each electrode 96 should preferably have a width that is one time to three times as great as the particle size of toner in order to guarantee desirable toner conveyance.

[0153] The insulative substrate 97 of the path board 92 is implemented as a film of polyimide, SiO2 or Ta2O5 by way of example and 0.5 μm to 1 μm thick. Ta2O3 has a specific inductive capacity of 28. When use is made of polyimide whose specific inductive capacity is about 3.6, the apparent, specific inductive capacity of polyimide can be increased to 10 to 12 if about 1% of fine BaTiO3 is added in the event of coating.

[0154] By increasing the specific inductive capacity of the substrate 97, it is possible to lower required drive voltage and to promote the bounce of the toner particles and high-speed conveyance. Therefore, to surely convey the charged toner by low voltage at a speed that allows the toner to fly, the substrate 97 should preferably be formed of a material whose specific inductive capacity is 10 or above. It was experimentally found that a 1 μm thick polyimide film required a drive voltage of DC 500 V, but derived an acceptable result with a drive voltage of 300 V when about 1% of fine BaTiO3 particles were added.

[0155] The paths 98 are implemented as channels capable of efficiently guiding the toner in the direction of toner conveyance. Further, the paths 98 and electrodes 96 should only cross each other. While the paths 98 and electrodes 96 are shown as being substantially perpendicular to each other, the crux is that the paths 98 and electrodes 96 be not parallel to each other. However, the paths 98 and electrodes 96 should preferably cross each other at an angle of 450° to 900° in order to promote efficient toner conveyance.

[0156] The coating layer 99 reduces contact resistance between the conveying surface, i.e., the bottoms of the paths 98 and the charged toner. For this purpose, the coating layer 99 should preferably be formed of a material whose critical surface tension is 30 dyne/cm or below, e.g., PTFE, PFA or similar fluorine-containing resin. Fluorine-containing resin implements the coating layer 49 at low cost.

[0157] FIGS. 40 through 43 show a second specific configuration of the conveyance board 30. As shown, the insulative substrate 97 of the path board 92 is formed with two groups of channel-like paths 98a and 98b. The paths 48a linearly extend in the direction of toner conveyance and are formed at preselected intervals in the direction perpendicular to the above direction. The paths 98b intersect the paths 98a at an angle of 45° and are also arranged at preselected intervals. As for the rest of the configuration, the second specific configuration is identical with the first specific configuration.

[0158] FIG. 44 shows a third specific configuration of the conveyance board 30. As shown, the insulative substrate 97 of the path board 92 is formed with the paths 98 extending in the direction of toner conveyance as in the first specific configuration. The surface of the substrate 97 is covered with the coating layer 99. In this specific configuration, the paths 98 are arranged more densely at opposite zones B in the direction perpendicular to the direction of toner conveyance than at the intermediate zone A (B>A). As for the rest of the configuration, this specific configuration is identical with the first specific configuration.

[0159] FIG. 45 shows a fourth specific configuration of the conveyance board 30. As shown, the insulative substrate 97 of the path board 92 is formed with the paths 98 extending in the direction of toner conveyance as in the first specific configuration. The surface of the substrate 97 is covered with the coating layer 99. In this specific configuration, the paths 98 are arranged more densely at the intermediate zone A in the direction perpendicular to the direction of toner conveyance than at opposite zones B (A>B). As for the rest of the configuration, this specific configuration is identical with the first specific configuration.

[0160] FIGS. 46 and 47 show a fifth specific configuration of the conveyance board 30. As shown, the electrode board 91 includes an insulation film 62 formed on a substrate 61 and the electrodes 96 arranged on the insulation film 62. An insulation film 63 covers the entire surface of the insulation film 62 inclusive of the electrodes 96. Further, a semiconductor film or shield layer 64 is formed on the insulation film 63. The conveyance board 92 is bonded to the semiconductor film 64, so that the semiconductor film 64 intervenes between the electrodes 96 and the path board 92. The semiconductor film 64 is connected to ground although not shown specifically. The semiconductor film 64 playing the role of a shield layer may be replaced with a conductor film, if desired.

[0161] Referring to FIGS. 48 through 53, how the conveyance boards 30 of the toner conveying device 31 conveys the toner will be described. As shown in FIG. 48, the toner conveying device 31 includes two drivers 80 each for applying three-phase drive waveforms Va, Vb and Vc to each group of three electrodes 96 of one conveyance board 30. Some delay is provided between the drive waveforms Va, Vb and Vc output from each driver 80. The drive waveforms Va, Vb and Vc each selectively take a positive potential, a negative potential and zero potential (not applied).

[0162] As shown in FIG. 49, assume that a toner particle T charged to positive polarity is positioned on the conveyance board 30. Also, assume that the potentials “−”, “+”, “0”,“−” and “+” are respectively applied to the consecutive electrodes 96 on the board 30, as indicated by row [1]. Further, assume that the toner particle T is positioned on the “0” electrode 96. Then, repulsion acts between the toner particle T and the “+” electrode 96 positioned at the left-hand side of the “0” electrode 96, as viewed in FIG. 49. At the same time, attraction acts between the toner particle and the “−” electrode 96 positioned at the right-hand side of the “0” electrode 96. As a result, the toner particle T moves to the “−” electrode 96.

[0163] Assume that the driver 80 applies the three-phase drive voltages Va, Vb and Vc to each three electrodes 96 in a specific pattern shown in FIG. 50. Then, the voltages indicated in row [1] of FIG. 49 are replaced with voltages “0”, “−”, “+”, “0” and “−” indicated in row [2] Consequently, no force acts between the toner particle T and the electrode 96 changed from “−” to “0”. At the same time, repulsion acts between the toner particle T and the electrode 96 changed from “0” to “−” while attraction acts between the toner particle T and the electrode 42 changed from “+” to “−”. The toner particle T therefore moves further to the “−” electrode 96.

[0164] As stated above, the potentials of the drive waveforms applied to the electrodes 96 are varied to apparently move the drive waveforms, so that the toner particle T sequentially moves toward the “−” electrode 96. That is, the toner particle T is conveyed along the conveying surface of the conveyance board 30. The pattern shown in FIG. 50 will be reversed when the toner particle is charged to negative polarity. Let the following description concentrate on toner charged to positive polarity.

[0165] FIG. 51 shows another specific pattern of the three-phase drive voltages Va, Vb and Vc that does not include “0”. As shown, a “−” drive waveform is applied to two adjoining ones of three electrodes 96 while a “+” drive waveform is applied to the remaining electrode 96. This is also successful to convey the positively charged toner T along the surface of the conveyance board 30.

[0166] FIG. 52 shows a Va, Vb and Vc pattern opposite to the pattern of FIG. 50. This pattern allows the positively charged toner T to be conveyed in the direction opposite to the direction shown in FIG. 49. Further, FIG. 53 shows a pattern opposite to the pattern of FIG. 51 and causing the positively charged toner T to be conveyed in the reverse direction.

[0167] The paths or channels 98 each including a conveying surface are formed in the insulative substrate 97 and reduce contact resistance, as stated earlier. The paths 98 therefore allow a sufficient conveying force to act on the toner and thereby insure sure conveyance of a great amount of toner.

[0168] The first to fourth specific configurations of the conveyance board 30 will be described more specifically hereinafter.

[0169] The conveyance board 30 of the first specific configuration had the following configuration. The substrate 95 of the electrode board 91 was formed of low-expansion glass and etched to form recesses (grooves) for forming the electrodes 96. Subsequently, an Al, Ni—Cr or similar film was formed on the entire substrate 95 and then patterned to form the stripe-like electrodes 96 by photolithography. On the other hand, the insulative substrate 92 of the other conveyance board 92 was implemented by a 150 μm thick, polyimide film and formed with a 50 μm wide pattern having a pitch of 40 μm and perpendicular to the electrodes 96 of the conveyance board 91. Subsequently, the polyimide film was etched to a depth of 100 μm to 140 μm in an oxygen environment to thereby form the paths 98.

[0170] Thereafter, the substrate or polyimide film 97 and substrate or low-expansion glass 95 were laid on each other such that the paths 98 and electrodes 96 are substantially perpendicular to each other, and then bonded together by heat and pressure. Subsequently, PFA, PTFE or similar fluorine-containing resin whose critical surface tension was 30 dyne/cm or below was coated on the entire surface of the conveyance board 92 in order to reduce contact resistance.

[0171] Experiments showed that when the driver 80, FIG. 48, applied the drive waveforms to the conveyance board 30, charged toner moved in the expected direction.

[0172] The conveyance board 30 of the second specific configuration had the following configuration. The substrate 95 of the electrode board 91 was formed of low-expansion glass and etched to form recesses (grooves) for forming the electrodes 96. Subsequently, a film of Al, Ni—Cr or similar electrode material was formed on the entire surface of the substrate 95 and then etched to form the stripe-like electrodes 96 by photolithography. A 0.1 μm to 0.2 μm thick, insulation film was formed on the entire substrate 95 over the electrodes 96 except for the lead portions of the electrodes 96. Dry film resist was bonded to the insulation film in order to form the insulative substrate 97. The paths or channels 98a and 98b were formed in the dry film resist, as shown in FIG. 42. Thereafter, PFA, PTFE or similar fluorine-containing resin was coated on the entire dry film resist to thereby form the coating layer 99. The gap between the bottoms of the paths 98a and 98b and the electrodes 96 was 0.3 μm to 2.2 μm.

[0173] When the driver 80, FIG. 48, applied the drive waveforms to the conveyance electrode 30, charged toner successfully moved in the expected direction.

[0174] The third specific configuration was identical with the first specific configuration except that the paths 98 of the conveyance board 30 was arranged at a density of 150 dpi (dots per inch) at the intermediate zone A and a density of 300 dpi at the opposite side zones B. The fourth specific configuration was identical with the first specific configuration except that the paths 98 had a density of 300 dpi at the intermediate zone A and a density of 150 dpi at the opposite side zones B.

[0175] When the driver 80, FIG. 48, applied the drive waveforms to the conveyance boards 30 of the third and fourth specific configurations, charged toner successfully moved in the preselected direction.

[0176] Experiments were conducted with the specific configurations described above in order to estimate the scattering of toner and the conveyance of toner toward the outlet facing the drum 1 by video observation. The scattering of toner was not observed in any one of the specific configurations. This is presumably because the conveyance board 30 intensely retained toner with an electrostatic force. As for the conveyance of toner, the second specific configuration was most desirable; the third, fourth and first specific configurations derived better results in this order. This is presumably because conveyance depends on the density of the paths 98 to the end, which faced the drum 1, and the gap between the electrodes 96 and the bottoms of the paths 98 (98a and 98b). It follows that the gap between the electrodes 96 and the paths 98 should preferably be between 0.1 μm and 50 μm, more particularly between 0.5 μm and 10 μm.

[0177] In the fifth specific configuration, the insulator forming the paths 98 are apt to bring about residual charge. However, the semiconductor film 64 or similar shield layer releases the residual charge and therefore prevents the electrostatic force from decreasing due to the residual charge to thereby enhance reliability.

[0178] Even a single conveyance board 30 suffices for conveying the charged toner. In the illustrative embodiment, too, two conveyance boards 30 are positioned with their conveying surfaces facing each other. The conveyance boards 30 are spaced from each other by 30 μm to 200 μm. A plurality of conveyance boards 30 increase the amount of toner to fly for a unit period of time.

[0179] Reference will also be made to FIG. 54 for describing the path members 33 and return members 39. As shown, the path members 33 and return members 39 each include an insulative FPC substrate 71 on which a number of electrodes 76 are arranged; each three electrodes 76 make a set. The substrates 71 extend in the direction substantially perpendicular to the direction of toner conveyance. A member 77 for forming a conveying surface is formed on the electrodes 72. A coating layer or film 78 is formed on the surface of the member 77 in order to reduce contact resistance between the member 73 and the toner.

[0180] The path members 33 and return members 39, like the conveyance board 30, electrostatically convey charged toner when drive voltages are applied to the electrodes 76. The electrodes 76, member 77 and coating layer 78 may be respectively formed of the materials described in relation to the electrodes 96, insulative substrate 97, and coating layer 99. Further, the members to be formed with such electrodes each may be formed of a deformable material in order to facilitate arrangement, if desired. This is also true with any one of the conveyance boards shown and described.

[0181] Development using the developing device 16 will be described hereinafter with reference also made to FIG. 55. As shown, the toner particles with positive charge fly from the toner conveying device 31 toward the end that faces the drum 1 by being conveyed by the conveyance boards 30. The toner particles are then jetted toward the drum 1 and deposit on a latent image, i.e., the negatively charged portions of the surface of the drum 1, thereby developing the latent image. An AC power supply 87 may be connected to the outlet end of the conveyance boards 30 so as to form an AC electric field between the boards 30. The electric field causes the toner T flying toward the drum 1 to form a toner cloud 88 and evenly deposit on the latent image. The toner cloud 88 is successful to enhance image quality.

[0182] As stated above, the developing device directly deposits the toner on the drum 1 without contacting the drum 1 and is therefore simple and low cost.

[0183] Further, the illustrative embodiment deteriorates the toner less than conventional developing means using a developing roller. Specifically, it is a common practice with developing means to deposit frictionally charged toner on a developing roller together with a carrier, cause the toner electrostatically deposited on the carrier to form a magnet brush, and then bring the magnet brush into contact with an image carrier for thereby developing a latent image. The developing roller, however, kneads the toner or smashes it into fine powder. As a result, SiO2, TiO2 or similar additive is rubbed into the resin of the toner to thereby deteriorate the characteristics of the toner. The illustrative embodiment solves this problem.

[0184] Furthermore, in the illustrative embodiment, the toner is conveyed to the image carrier without a developing roller or similar developer carrier being rotated. This obviates the cohesion of the toner in the developing device and electrostatically deposits the toner to thereby reduce the scattering of the toner via seals around the developing section and enhance image quality. In addition, the developing device, which does not need any special material, is compact and low cost.

[0185] The collection of the toner will be described with reference also made to FIG. 56. While the toner flies out of the conveyance boards 30 to deposit on the latent image formed on the drum 1, not all toner particles are used for development, as stated earlier. As shown in FIG. 56, the toner gutters 38 are positioned outside of the conveyance boards 30 and cause the toner particles not used for developing and tending to fly about to deposit on the toner gutters 38. The return members 39 convey the above toner particles toward the inlet in the same manner as the conveyance boards 30. Consequently, such toner particles are collected in the toner box 32 and used again.

[0186] As stated above, returning means collects the toner not used for development and thereby prevents it from being scattered around. Further, the collecting means includes means for electrostatically returning the collected toner toward the inlet. This successfully promotes the reuse of the toner and thereby reduces the cost.

[0187] If desired, a bias voltage of the same polarity as the toner and a bias voltage opposite in polarity to the toner may be alternately applied to each toner gutter 38 in order to selectively attract or repulse the toner. In such a case, the toner will be collected and then returned to the outlet of the conveyance board 30 to be reused thereby. This configuration makes it needless to return the collected toner to the toner box 32.

Fourteenth Embodiment

[0188] FIG. 57 shows a fourteenth embodiment of the present invention, particularly the toner conveying device 31 thereof. As shown, drivers 93 each apply to the electrodes 96 three-phase drive waveforms Va1, Vb1 and Vc1 having a frequency f1, three-phase drive waveforms Va2, Vb2 and Vp2 having a frequency f2, and three-phase drive waveforms Va3, Vb3 and Vc3 having a frequency f3 (f1>f2>f3).

[0189] More specifically, the driver 93 applies the drive waveforms Va1, Vb1 and Vc1 to part of the electrodes 96 lying in a preselected zone that adjoins the outlet. The driver 93 applies the drive waveforms Va2, Vb2 and Vc2 to the electrodes 96 lying in the intermediate zone of the conveyance board 30. Further, the driver 96 applies the drive waveforms Va3, Vb3 and Vc3 to part of the electrodes 96 that lies in a preselected zone adjoining the inlet. As for the rest of the construction, this embodiment is identical with the thirteenth embodiment.

[0190] In the illustrative embodiment, the frequencies of the drive waveforms Va, Vb and Vc sequentially increase from the inlet toward the outlet stepwise, i.e., from the frequency f3 to the frequency f1. Attraction and repulsion to act on the charged toner vary in a shorter period of time as the frequency of the drive waveform increases. Therefore, the toner entered the space between the conveyance boards 30 via the inlet is sequentially accelerated as the frequency of the drive waveform increases. The toner is therefore conveyed along the conveying surfaces of the conveyance boards 30 while being accelerated.

Fifteenth Embodiment

[0191] FIGS. 58 through 60 show a fifteenth embodiment of the present invention, particularly the toner conveying device 31 thereof. As shown, the density of the electrodes 96 is sequentially increased stepwise in the direction of toner conveyance. For example, as shown in FIG. 59, assume consecutive zones C, D, E and F beginning at the toner inlet and ending at the toner outlet. Then, the electrodes 96 are arranged at intervals of 100 μm in the zone C, at intervals of 20 μm in the zone D, at intervals of 10 μm in the zone E, and at intervals of 5 μm in the zone F. Assuming that the mean particle size of toner is 8 μm, the illustrative embodiment provides each electrode 42 with a width of 10 μm. The driver 80 of the thirteenth embodiment applies the drive voltages to the electrodes 96.

[0192] In the illustrative embodiment the distance between nearby electrodes 96 sequentially decreases from the inlet toward the outlet. Attraction and repulsion to act on the charged toner vary in a shorter period of time as the above distance decreases. Therefore, the toner entered the space between the conveyance boards 30 via the inlet is sequentially accelerated stepwise as the distance decreases. The toner therefore flies (jetted) from the outlet of the conveyance boards 30.

Sixteenth Embodiment

[0193] FIG. 61 shows a sixteenth embodiment of the present invention, particularly the toner conveying device 31 thereof. As shown, the conveyance boards 30 each include a storing portion 90. A drive circuit 103 applies the drive waveforms Va, Vb and Vc, which vary in the pattern shown in FIG. 51, to the upper conveyance board 30. A drive circuit 194 applies the drive waveforms Va, Vb and Vc, which vary in a pattern shown in FIG. 53, to the lower conveyance board 30.

[0194] In the configuration shown in FIG. 61, the upper conveyance board 30 conveys the toner in the same manner as in the previous embodiments and causes it to fly toward the drum 1. On the other hand, the lower conveyance board 30 opposite in the pattern of the drive waveforms to the upper conveyance board 30 conveys the toner in the reverse direction, i.e., from the outlet toward the inlet. Therefore, the toner bouncing back without depositing on the drum 1 is conveyed from the outlet to the storing portion 90 by the lower conveyance board 30 and reused. This not only promotes the efficient use of the toner, but also prevents the bounced toner from being scattered around and thereby improves image quality.

Seventeenth Embodiment

[0195] FIG. 62 shows a seventeenth embodiment of the present invention, particularly the toner conveying device 31 thereof. As shown, the driver 71 applies the drive waveforms Va, Vb and Vc varying in the pattern of FIG. 51 to the upper conveyance board 30. A driver 105 is selectively operable in two different modes. Specifically, the driver 105 applies the drive waveforms Va, Vb and Vc varying in the pattern of FIG. 51 to the lower conveyance board 30 or applies the drive waveforms Va, Vb and Vc varying in the pattern of FIG. 53 opposite to the pattern of FIG. 51 to the same.

[0196] A controller, not shown, sends a mode switching signal to the driver 105. When the apparatus is, e.g., in a standby state, the mode switching signal causes the driver 105 to output the drive waveforms Va, Vb and Vc in the pattern of FIG. 53.

[0197] While the apparatus forms images in the usual manner, the controller causes the lower conveyance board 30, as well as the upper conveyance board 30, to apply the drive voltages Va, Vb and Vc varying in the pattern of FIG. 51. As a result, the upper and lower conveyance boards 30 both convey the toner toward the outlet and therefore by an amount two times as great as the amount available with a single conveyance board 30.

[0198] When the apparatus is in a standby state, the controller causes the lower conveyance board 30 to apply the drive voltages Va, Vb and Vc varying in the pattern of FIG. 53. The lower conveyance board 30 therefore conveys the toner in the reverse direction. In this case, the toner is simply circulated in the space between the two conveyance boards 30 without flying toward the drum 1. This prevents the toner from being scattered around more positively and thereby enhances image quality. In addition, the lower conveyance board 30, which selectively conveys the toner from the inlet to the outlet or from the outlet to the inlet, simplifies the configuration of the toner conveying device 31.

Eighteenth Embodiment

[0199] Referring to FIGS. 63 through 65, an eighteenth embodiment of the present invention will be described. As shown, the illustrative embodiment includes a toner jet head 200 for causing charged toner to fly in accordance with an image signal. The toner jet head 200 is generally made up of a conveyance board 201 and a control board 202 facing each other.

[0200] As shown in FIG. 64 also, the conveyance board 201, like the conveyance board 30, includes an electrode board or first board 218 and a path board or second board 212 bonded to each other. The electrode board 218 includes an elongate, flat substrate 215 and a number of electrodes 216 arranged on the substrate 215 at preselected intervals. The electrodes 216, each three of which make a set, each extend in the direction perpendicular to the direction of toner conveyance. The path board 212 includes an insulative substrate 212 formed with paths 218, which form a conveying surface, on a pixel basis. A coating layer or film 219 is formed on the substrate 212 in order to reduce contact resistance between the substrate and toner.

[0201] As shown in FIG. 65 also, the control board 202 includes a substrate 221. A number of first electrodes 222 are arranged on the substrate 221 for conveying the toner in the reverse direction, and each extend in the direction perpendicular to the direction of toner conveyance. An insulative protection film 223 is formed on the first electrodes 222. A number of second electrodes or pixel electrodes 224 are arranged on the protection film 223 on a pixel basis, and each extend in the direction perpendicular to the direction of toner conveyance. Further, a protection film 225 is formed on the second electrodes 224.

[0202] Drive waveforms are applied to the electrodes 216 of the conveyance board 201 in the same manner as in any one of the previous embodiments, causing the board 201 to covey the toner in the same manner as the conveyance board 30. On the other hand, drive waveforms are applied to the first electrodes 222 of the control board 202 in a pattern opposite to the electrodes 216, so that the toner is conveyed from the outlet to the inlet and simply circulated.

[0203] Assume that a drive waveform for generating an electric field that repulses the charged toner is applied to any one of the second electrodes 224 of the control board 202 in accordance with a pixel signal. Then, the toner being conveyed by the conveyance board 201 flies away from the board 201. On the other hand, when a drive waveform for generating an electric field that attracts the charged toner is applied to the electrode 224, the toner conveyed by the conveyance board 201 as far as the outlet of the board 201 is attracted by the control board 202. More specifically, by controlling the drive waveforms to be applied to the second electrodes 224 in accordance with a pixel signal, it is possible to implement an on-demand type of toner jet head that controls the flight/non-flight of the toner from the conveyance board 201 pixel by pixel.

[0204] The toner T jetted from the toner jet head 200 deposits on a recording medium 130, forming a toner image in accordance with an image signal. Subsequently, the toner image is fixed on the recording medium 130. The illustrative embodiment obviates the need for an image carrier and is therefore simple in construction. Moreover, the illustrative embodiment controls the flight/non-flight of the toner on a pixel basis to thereby circulate part of the toner that did not fly. This successfully maintains the saturation charge of the toner and thereby causes a minimum of toner to be scattered around.

[0205] Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.

Claims

1. An image forming apparatus comprising: an image carrier for forming a latent image thereon; and developing means for developing the latent image with charged toner to thereby form a corresponding toner image; wherein said developing means electrostatically conveys the charged toner toward one end of a conveyance board along a conveying surface of said conveyance board and causes said charged toner to fly toward said image carrier from said one end.

2. The apparatus as claimed in claim 1, wherein said developing means sequentially accelerates the charged toner while conveying said charged toner.

3. The apparatus as claimed in claim 1, wherein said conveyance board comprises a flat substrate, a plurality of electrodes arranged on said substrate in a direction of toner conveyance, in which the charged toner is conveyed, and each extending in a direction perpendicular to said direction of toner conveyance, and a member formed on said substrate over said plurality of electrodes for forming the conveying surface.

4. The apparatus as claimed in claim 3, wherein a distance between nearby ones of said electrodes sequentially decreases toward the one end of said conveyance board.

5. The apparatus as claimed in claim 3, further comprising means for applying to said electrodes drive waveforms sequentially increasing in frequency toward one or more of said electrodes adjoining the one end of said conveyance board.

6. The apparatus as claimed in claim 3, further comprising means for applying to said electrodes drive waveforms sequentially increasing in crest value toward one or more of said electrodes adjoining the one end of said conveyance board.

7. The apparatus as claimed in claim 3, further comprising means for applying to part of said electrodes adjoining the other end of said conveyance guide drive waveforms that provide the charged toner with an initial speed high enough for said charged toner to fly from the one end of said conveyance board.

8. The apparatus as claimed in claim 1, wherein said developing means further comprises another conveyance board whose conveying surface faces the conveying surface of said conveyance board.

9. The apparatus as claimed in claim 1, wherein said developing means further comprises at least two other conveyance boards, an intermediate conveyance board having the conveying surface at both sides thereof.

10. The apparatus as claimed in claim 1, further comprising a return conveyance board for electrostatically conveying the charged toner along the conveying surface in a direction opposite to a direction of toner conveyance in which said charged toner is conveyed.

11. The apparatus as claimed in claim 1, further comprising collecting means for collecting the charged toner flown from the one end of said conveyance board, but not used for development.

12. The apparatus as claimed in claim 11, wherein said collecting means comprises means for electrostatically conveying the charged toner collected along the conveying surface in a direction opposite to a direction of toner conveyance in which the toner is conveyed.

13. The apparatus as claimed in claim 1, wherein said developing means further comprises at least another conveyance board whose conveying surface faces the conveying surface of said conveyance board, said apparatus further comprising means for forming an AC electric field at the one end of said conveyance boards.

14. The apparatus as claimed in claim 1, wherein said developing means further comprises at least another conveyance board whose conveying surface faces the conveying surface of said conveyance board, said apparatus further comprising means for forming a DC electric field between said image carrier and said conveyance boards.

15. The apparatus as claimed in claim 1, wherein said developing means further comprises at least another conveyance board whose conveying surface faces the conveying surface of said conveyance board, said apparatus further comprising means for forming an AC electric field at the one end of said conveyance boards, and means for forming a DC electric field between said image carrier and said conveyance boards.

16. The apparatus as claimed in claim 15, wherein said developing means further comprises at least another conveyance board whose conveying surface faces the conveying surface of said conveyance board, one of said conveyance boards comprising means for applying waveforms that cause the charged toner to be conveyed toward the other end.

17. The apparatus as claimed in claim 16, wherein said conveyance boards each include a storing portion for storing the charged toner at the other end.

18. In a developing device for developing a latent image formed on an image carrier with charged toner to thereby produce a corresponding toner image, a conveyance board electrostatically conveys said charged toner along a conveying surface thereof toward one end and causes said charged toner to fly toward said image carrier.

19. The device as claimed in claim 18, wherein said conveyance board sequentially accelerates the charged toner while conveying said charged toner.

20. The device as claimed in claim 18, wherein said conveyance board comprises a flat substrate, a plurality of electrodes arranged on said substrate in a direction of toner conveyance, in which the toner is conveyed, and each extending in a direction perpendicular to said direction toner conveyance, and a member formed on said substrate over said plurality of electrodes for forming the conveying surface.

21. The device as claimed in claim 20, wherein said conveyance board further comprises a coating layer formed on a surface of said member for reducing contact resistance between said surface and the charged toner.

22. The device as claimed in claim 20, wherein said member is formed of a material having a specific inductive capacity of 2 or above.

23. The device as claimed in claim 22, wherein the material comprises either one of SiO2 and Ta2O5.

24. The device as claimed in claim 20, wherein a distance between nearby ones of said electrodes sequentially decreases toward the one end.

25. The device as claimed in claim 18, further comprising another conveyance board whose conveying surface faces the conveying surface of said conveyance board.

26. The device as claimed in claim 18, further comprising at least two other conveyance boards, an intermediate conveyance board having the conveying surface at both sides thereof.

27. The device as claimed in claim 18, wherein said conveyance boards each include a storing portion for storing the charged toner at the other end.

28. In a toner feeding device for feeding charged toner to developing means that causes said charged toner to deposit on either one of an image carrier and a recording medium, a conveyance board electrostatically conveys the charged toner along a conveying surface thereof toward one end while accelerating said charged toner and causes said charged toner to fly toward said image carrier or said recording medium.

29. The device as claimed in claim 28, wherein said conveyance board comprises a flat substrate, a plurality of electrodes arranged on said substrate in a direction of toner conveyance, in which the charged toner is conveyed, and each extending in a direction perpendicular to said direction of toner conveyance, and a member formed on said substrate over said plurality of electrodes for forming the conveying surface.

30. The device as claimed in claim 29, wherein said conveyance board further comprises a coating layer formed on a surface of said member for reducing contact resistance between said surface and the charged toner.

31. The device as claimed in claim 29, wherein a distance between nearby ones of said electrodes sequentially decreases toward the one end.

32. The device as claimed in claim 28, further comprising another conveyance board whose conveying surface faces the conveying surface of said conveyance board.

33. In an image forming apparatus for depositing charged toner on a recording medium to thereby form a toner image, means is provided for causing a conveyance board to electrostatically convey said charged toner along a conveying surface of said conveyance board toward one end and then cause said charged toner to fly toward said recording medium from said one end.

34. The apparatus as claimed in claim 33, wherein said conveyance board accelerates the charged toner while conveying said charged toner.

35. The apparatus as claimed in claim 33, further comprising a control board facing the conveying surface of said conveyance board for controlling flight/non-flight of the charged toner from said conveyance board.

36. The apparatus as claimed in claim 35, wherein said control board conveys the charged toner not flown from the one end toward the other end of said conveyance board.

37. The apparatus as claimed in claim 33, wherein said conveyance board comprises a flat substrate, a plurality of electrodes arranged on said substrate in a direction of toner conveyance, in which the charged toner is conveyed, and each extending in said direction of toner conveyance, and a member formed on said substrate over said plurality of electrodes for forming the conveying surface.

38. The apparatus as claimed in claim 37, wherein said conveyance board further comprises a coating layer formed on a surface of said member for reducing contact resistance between said surface and the charged toner.

39. The apparatus as claimed in claim 37, wherein a distance between nearby ones of said electrodes sequentially decreases toward the one end.

40. The apparatus as claimed in claim 37, further comprising means for applying to said electrodes drive waveforms sequentially increasing in crest value toward one or more of said electrodes adjoining the one end of said conveyance board.

41. The apparatus as claimed in claim 37, further comprising means for applying to said electrodes drive waveforms that provide the charged toner with an initial speed high enough for said charged toner to fly from the one end of said conveyance board.

42. In a powder jetting device for jetting charged powder, means is provided for causing a conveyance board to electrostatically convey said charged powder along a conveying surface of said conveyance board toward one end while accelerating said charged powder and then jet said charged from said one end.

43. The device as claimed in claim 42, wherein said conveyance board comprises a flat substrate, a plurality of electrodes arranged on said substrate in a direction of toner conveyance, in which the charged toner is conveyed, and each extending in a direction perpendicular to said direction of toner conveyance, and a member formed on said substrate over said plurality of electrodes for forming the conveying surface.

44. The device as claimed in claim 43, wherein a distance between nearby ones of said electrodes sequentially decreases toward the one end.

45. In a classifying device for classifying charged powder, means is provided for causing a conveyance board to electrostatically convey said charged powder along a conveying surface of said conveyance board toward one end while accelerating said charged powder and then jet said charged from said one end.

46. The device as claimed in claim 45, wherein said conveyance board comprises a flat substrate, a plurality of electrodes arranged on said substrate in a direction of toner conveyance, in which the charged toner is conveyed, and each extending in a direction perpendicular to said direction of toner conveyance, and a member formed on said substrate over said plurality of electrodes for forming the conveying surface.

47. The device as claimed in claim 46, wherein a distance between nearby ones of said electrodes sequentially decreases toward the one end.

48. In a toner conveying device for electrostatically conveying toner, a conveyance board comprising: a first board comprising a substrate and a plurality of substantially parallel electrodes arranged at a preselected distance in a direction of toner conveyance, in which the toner is conveyed, and each extending in a direction crossing said direction of toner conveyance; and a second board comprising an insulative substrate and a surface layer formed on a surface of said insulative substrate and having low contact resistance with respect to the toner, said surface layer forming a conveying surface; wherein said first board and said second board are stacked on each other.

49. The device as claimed in claim 48, wherein said electrodes are arranged at a pitch that is one-half to 100 times as great as a particle size of the toner.

50. The device as claimed in claim 48, wherein said surface layer is formed of a material having a critical surface tension of 30 dyne/cm or below.

51. The device as claimed in claim 50, wherein the material comprises fluorine-containing resin.

52. The device as claimed in claim 48, wherein said conveyance board further comprises either one of a semiconductor film and a conductor film intervening between said electrodes of said first board and said insulative substrate of said second board and isolated from said electrodes by an insulation film.

53. The device as claimed in claim 48, wherein said second board is formed with channels constituting paths each including said conveying surface.

54. The device as claimed in claim 53, wherein part of said channels extends substantially perpendicularly to said electrodes while the other part of said channels extends substantially at an angle of 45° relative to said electrodes.

55. The device as claimed in claim 48, wherein said channels are arranged in a density varying in a direction substantially perpendicular to the direction of toner conveyance.

56. The device as claimed in claim 48, wherein surfaces of said electrodes and said conveying surface are spaced from each other by a gap lying in a range of from 0.5 μm to 10 μm.

57. The device as claimed in claim 48, wherein at least two conveyance boards are provided.

58. The device as claimed in claim 57, wherein said at least two conveyance boards having respective conveying surfaces facing each other.

59. In a developing device for depositing toner on a latent image formed on an image carrier to thereby develop said latent image and including a toner conveying device for electrostatically conveying said toner, said toner conveying device comprising a conveyance board comprising: a first board comprising a substrate and a plurality of substantially parallel electrodes arranged at a preselected distance in a direction of toner conveyance, in which the toner is conveyed, and each extending in a direction crossing said direction of toner conveyance; and a second board comprising an insulative substrate and a surface layer formed on a surface of said insulative substrate and having low contact resistance with respect to the toner, said surface layer forming a conveying surface; wherein said first board and said second board are stacked on each other.

60. The device as claimed in claim 59, wherein a distance between nearby ones of said electrodes sequentially decreases toward a downstream side in the direction of toner conveyance.

61. The device as claimed in claim 60, wherein said conveyance board includes a storing portion for storing the toner at a toner inlet side thereof.

62. In an image forming apparatus comprising a developing device for depositing toner on a latent image formed on an image carrier to thereby develop said latent image, said developing device comprising a toner conveying device for electrostatically conveying said toner, said toner conveying device comprising: a first board comprising a substrate and a plurality of substantially parallel electrodes arranged at a preselected distance in a direction of toner conveyance, in which the toner is conveyed, and each extending in a direction crossing said direction of toner conveyance; and a second board comprising an insulative substrate and a surface layer formed on a surface of said insulative substrate and having low contact resistance with respect to the toner, said surface layer forming a conveying surface; wherein said first board and said second board are stacked on each other.

63. The apparatus as claimed in claim 62, wherein said toner conveying device further comprises means for applying to downstream ones of said electrodes in the direction of toner conveyance drive voltages higher in frequency than drive voltages applied to upstream ones of said electrodes.

64. In an image forming apparatus for depositing toner on a latent image formed on an image carrier to thereby develop said latent image and comprising a toner conveying device for electrostatically conveying said toner, said toner conveying device comprising: a first board comprising a substrate and a plurality of substantially parallel electrodes arranged at a preselected distance in a direction of toner conveyance, in which the toner is conveyed, and each extending in a direction crossing said direction of toner conveyance; and a second board comprising an insulative substrate and a surface layer formed on a surface of said insulative substrate and having low contact resistance with respect to the toner, said surface layer forming a conveying surface; wherein said first board and said second board are stacked on each other.

65. The apparatus as claimed in claim 64, wherein a distance between nearby ones of said electrodes sequentially decreases toward a downstream side in the direction of toner conveyance.

66. The apparatus as claimed in claim 65, wherein said toner conveying device further comprises means for applying to downstream ones of said electrodes in the direction of toner conveyance drive voltages higher in frequency than drive voltages applied to upstream ones of said electrodes.

67. The apparatus as claimed in claim 64, wherein said toner conveying device further comprises a control board facing the conveying surface of said conveyance board for controlling flight/non-flight of the charged toner from said conveyance board.

68. In a toner feeding device for electrostatically conveying toner to developing means with a toner conveying device, said toner conveying device comprising: a first board comprising a substrate and a plurality of substantially parallel electrodes arranged at a preselected distance in a direction of toner conveyance, in which the toner is conveyed, and each extending in a direction crossing said direction of toner conveyance; and a second board comprising an insulative substrate and a surface layer formed on a surface of said insulative substrate and having low contact resistance with respect to the toner, said surface layer forming a conveying surface; wherein said first board and said second board are stacked on each other.

69. The device as claimed in claim 68, wherein a distance between nearby ones of said electrodes sequentially decreases toward a downstream side in the direction of toner conveyance.

70. The apparatus as claimed in claim 68, wherein said toner feeding device further comprises means for applying to downstream ones of said electrodes in the direction of toner conveyance drive voltages higher in frequency than drive voltages applied to upstream ones of said electrodes.