Abstract:
An image forming apparatus uses an electrode member with apertures therein through which toner is passed according to image data to form an image on a supporter. The electrode member has an insulative inorganic substance formed thereon to protect a surface of the electrode member from wear. Thus, the supply of toner through the apertures in the electrode is stable. Further, since the protection layer of inorganic substance is arranged on the toner carrying roller side of the aperture electrode member, the image forming apparatus has an excellent image forming property.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus such as a copier, a printer, a plotter or a facsimile machine. 
     2. Description of Related Art 
     One type of image forming apparatus that has been conventionally proposed is an image forming apparatus using an electrode having plural opening portions (hereinafter referred to as “apertures”) disclosed in U.S. Pat. No. 3,689,935. In this image forming apparatus, a voltage is applied to the electrode while being modulated in accordance with image data to thereby control the passage of toner through the apertures and form an image on a supporter with the passed toner. 
     This image forming apparatus includes an insulating flat plate, a continuous reference electrode formed on one surface of the flat plate, and plural control electrodes formed on the other surface of the flat plate and electrically insulated from one another. The apparatus further includes an aperture electrode member having at least one array of apertures, each of which is provided in correspondence with each control electrode so as to penetrate through the flat plate, the reference electrode and the control electrode. Also provided are means for selectively applying potential between the reference electrode and each of the control electrodes, means of supplying charged toner to modulate the flow of toner that has been just passed through the apertures in accordance with the supplied potential and means for positioning the supporter in a toner-particle flowing passage such that the supporter and the aperture electrode member are movable relatively to each other. 
     However, in the above-mentioned image forming apparatus, because an electric field in the apertures is used to control the flow of the charged toner particles, an electric field outside the apertures is weakened significantly. Therefore, the amount of the toner particles passing through the apertures in a predetermined time is slow. Thus, the image forming speed is slow, since the electric force for drawing toner particles supplied under the apertures into the apertures is weak. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image forming apparatus having a stable and excellent image forming property. 
     To improve the strength of the electric force drawing toner particles through apertures in an aperture electrode member, the toner particle carrying roller is arranged to contact the aperture electrode member. By this arrangement, an image is formed by controlling the toner particle flow with a strong electric field formed between the control electrode of the contacting aperture electrode member and a toner particle carrying roller. 
     The aperture electrode member is preferably formed with an insulative sheet such as a polyimide film. However, such an insulative sheet will become charged and worn off due to its sliding operation with the toner carrying roller. Further, toner particles will become fixed on the insulative sheet, and, therefore, a proper control electric field cannot be formed between the control electrodes and the toner carrying roller since the insulative sheet is charged by the sliding operation with a toner particle layer formed on the toner carrying roller. Moreover, the life of the insulative sheet will be shortened since the insulative sheet will wear off due to the sliding operation and toner particles that become fixed on the insulative sheet. Therefore, to prevent these problems, an insulative inorganic substance is provided on at least a surface of the insulative sheet facing the carrying roller. 
     To achieve the above and other objects, the image forming apparatus of the present invention comprises a supporting member for supporting and supplying charged toner particles, and toner flow control means having openings and control electrodes around the openings. The toner flow control means sandwiches the charged particles with the supporting member and controls a flow of the charged particles through the openings by an electric field. A back electrode is provided that confronts the toner flow control means sandwiching an image receiving medium therebetween. Additionally, an insulative inorganic substance is provided on at least a surface of the toner flow control means facing the supporting member. 
     In the image forming apparatus of the above-described structure, the electrodes themselves are not charged and not worn off because the insulative inorganic substance is provided on the electric field control means. Therefore, toner particles are always supplied stably, and an unnecessary electric field is not formed when toner particles are transferred. As a result, the image forming apparatus has an excellent image forming property and excellent product stability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention are described in detail referring to the following figures wherein: 
     FIG. 1 is a schematic side view in partial section of the image forming apparatus of the preferred embodiment. 
     FIG. 2 is a perspective view in partial section showing the construction of the aperture electrode member of the preferred embodiment. 
     FIG. 3 is a partial cross-sectional view of the aperture electrode member of the preferred embodiment. 
     FIG. 4 is a partial cross-sectional view of the aperture electrode member of another embodiment. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereafter, preferred embodiments that illustrate the present invention are explained referring to the drawings. 
     FIG. 1 shows an image forming apparatus according to the preferred embodiment. A back electrode roller  22  of a column shape is arranged pivotally on a chassis (not shown) having approximately a one millimeter space between the back electrode roller  22  and an upper surface of the aperture electrode member  1 . The aperture electrode member  1  comprises an insulative sheet  2 , preferably a polyimide film. An image supporting medium  20  is inserted in the space and transported. A toner particle supply device  10  is arranged under the aperture electrode member  1  along the longitudinal direction of the aperture electrode member  1 . In addition, a fixing device  26  is arranged in the transporting path of the image supporting medium  20 , which is transferred by the back electrode roller  22 . 
     The toner particle supply device  10  comprises a toner particle casing  11 , which is a housing of the apparatus, toner particles  16  stored in the toner particle casing  11 , a supplying roller  12 , a toner particle carrying roller  14 , which is a supporting medium, and a toner particle layer trimming blade  18 . The toner particle carrying roller  14  supports toner particles  16  and transfers the toner particles  16  toward the aperture electrode member  1 . The supplying roller  12  supplies toner particles  16  to the toner particle carrying roller  14 . The supplying roller  12  and the toner particle carrying roller  14  are supported by the toner particle casing  11 . The rollers  12  and  14  are rotatable in the direction of the arrows shown in FIG.  1 . The rollers  12  and  14  are arranged in parallel and contact each other. 
     The toner particle layer trimming blade  18  contacts the toner particle carrying roller  14  under pressure. The toner particle layer trimming blade  18  adjusts the amount of toner particles  16  carried on the toner particle carrying roller  14  so that the amount of toner particles  16  is kept uniform on the roller  14  and the blade  18  charges the toner particles  16  uniformly. 
     A row of apertures  6  is formed in the polyimide insulative sheet  2  of the aperture electrode member  1 , as shown in FIG.  2 . The diameter of each aperture  6  is preferably approximately 100 μm, and the insulative sheet preferably has a thickness of approximately 25 μm. Control electrodes  4  of preferably approximately 1 μm thickness are formed around the apertures  6  on the insulative sheet  2 . The aperture electrode member  1  is arranged so that the control electrodes  4  face the image supporting medium  20  and the insulative sheet  2  contacts toner particles  16  carried on the toner particle carrying roller  14  in its aperture position, as shown in FIG.  3 . 
     The modulating electrode member  1  of this embodiment is further explained referring to FIG.  3 . 
     FIG. 3 is a cross-sectional view of the aperture electrode member  1 . The protection layer  7  is covered over the carrying roller side of the aperture electrode member  1 . Preferably, the protection layer  7  is an inorganic substance, which does not have a charging property. For example, alumina, zirconia, nitriding silicon, carbonization silicon, oxidation silicon, oxidation chrome, oxidation tantalum, nitriding tantalum, titanium oxide and glass can be used as the inorganic substance. The protection layer  7  can be formed by a thin film formation method such as sputtering, evaporation and CVD, the sol-gel method, or the screen print method. 
     A control voltage applying circuit  8  is connected between the control electrodes  4  and the toner particle carrying roller  14 . The control voltage applying circuit  8  applies a voltage of 0 volt or +50 volts to the control electrodes  4  based on the image signal. Moreover, a DC power supply  24  is connected between the back electrode roller  22  and the toner particle carrying roller  14 . The DC power supply  24  applies a voltage of +1kV to the back electrode roller  22 . 
     In operation, the toner particles  16  are sent from the supplying roller  12  by rotation of the toner particle carrying roller  14  and the supplying roller  12  in the direction of the arrow shown in FIG.  1 . When the toner particles  16  are rubbed with the toner particle carrying roller  14 , the toner particles  16  are negatively charged and are supported on the toner particle carrying roller  14 . After the supported toner particles  16  are formed as a thin layer by the layer trimming blade  18  and charged, the charged toner particles  16  are transferred toward the aperture electrode member  1  by the rotation of the toner particle carrying roller  14 . The toner particles  16  supported on the toner particle carrying roller  14  are supplied under the apertures  6 . 
     The image forming apparatus of this embodiment does not experience problems caused from the charge of the insulative sheet  2  or the fixing of toner particles on the insulative sheet  2  since the protection layer  7  is formed on the insulative sheet  2 . That is, since the protection layer  7  is formed with an insulative inorganic substance, the protection layer  7  is not charged and few toner particles are fixed on the layer  7 , even if the layer  7  slides with the toner particles  16  or the carrying roller  14 . Therefore, toner particles are supplied quite properly, and the apparatus has an excellent image forming property in this embodiment. This is in sharp distinction from the prior art in which the toner layer is disordered due to the charge of the insulative layer or the fixing of toner particles on the insulative layer in the conventional apparatus. 
     The voltage of +50 volts is applied from the control voltage applying circuit  8  to the control electrodes  4 , which correspond to the image portion according to the image signal. As a result, an electric force line forwarding to the toner particle carrying roller  14  from the control electrodes  4  is formed adjacent to the apertures  6  corresponding to the image portion by the potential difference between the control electrodes  4  and the toner particle carrying roller  14 . Accordingly, since the negatively charged toner particles receive an electrostatic force toward the direction of a higher potential, the negatively charged toner particles pass through the apertures  6  from the toner particle carrying roller  14  to the control electrode  4  side. The toner particles  16  drawn out to the control electrode  4  side are attracted toward the image supporting medium  20  by the electric field formed between the image supporting medium  20  and the aperture electrode member  1  by the voltage applied to the back electrode  22 . The toner particles  16  accumulate on the image supporting medium  20  to form a pixel. 
     The voltage of 0 volt is applied to the control electrodes  4  that correspond to the non-image portion from the control voltage applying circuit  8 . As a result, the toner particles  16  supported on the toner particle carrying roller  14  do not pass through the apertures  6  since the electric field is not formed between the toner particle carrying roller  14  and the control electrodes  4 . Thus, the toner particles  16  on the roller  14  do not have an electrostatic force. 
     While a row of pixels are formed on the image supporting medium  20  with toner particles  16 , the image supporting medium  20  is fed one pixel at a time in the direction perpendicular to the row of apertures. The toner particle image is formed over the image supporting medium  20  by repeating the above-mentioned process. Afterwards, the formed toner particle image is fixed on the image supporting medium  20  by the fixing device  26 . 
     It is to be understood that the present invention is not restricted to the particular forms shown in the foregoing embodiment. Various modifications and alterations can be made thereto without departing from the scope of the invention encompassed by the appended claims. 
     For example, while the control voltage of 0 volt is applied to the apertures  6 , which correspond to the non-image portion in the above-mentioned embodiment, a negative voltage can be applied. If a negative voltage is applied, an image with very little fogging can be obtained. Also, while the aperture electrode member is used as the electric field control means in the above-embodiment, a mesh electrode member as disclosed in U.S. Pat. No. 5,036,341 can be used. Further, while the protection layer  7  is formed on the whole surface of the aperture electrode member in the above-embodiment, the layer  7  can be arranged around the apertures  6  as shown in FIG.  4 .