Patent Publication Number: US-7593675-B2

Title: Color image forming apparatus having a discharging unit to discharge an electrical charge of a toner image transferred on an intermediate transfer member

Description:
This application is based on Japanese Patent Application No. 2005-371677 filed on Dec. 26, 2005, which is incorporated hereinto by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a copier, a printer, a facsimile machine and an image forming apparatus using an electro photography method having the functions of the copier, the printer and facsimile machine. Particularly, the present invention relates to a color image forming apparatus including an intermediate transfer member for superimposing plural color toner images onto the intermediate transfer member to form an image. 
   In the electro photography method color image forming apparatus using the intermediate transfer member, known is an image forming apparatus arranged to transfer a toner image formed on an image carrier, which is a photoreceptor onto the intermediate transfer member (primary transfer), then the toner image on the intermediate transfer member is transferred onto a transfer material (secondary transfer). In this type of color image forming apparatus, the color image forming apparatus is designed to superimpose an electro-static toner image, which has been simultaneously formed on the image carrier with a predetermined polarity, onto the intermediate transfer member by using static electricity. Then static electricity transfers the toner image on the intermediate transfer member onto the transfer material at once. 
   The color image forming apparatus using the intermediate transfer member can superimpose the toner image formed on the image carrier onto the intermediate transfer member. Thus, the color image forming apparatus using the intermediate transfer member is widely applied to a color image forming apparatus. In this color image forming apparatus, after the toner image of each color formed on the image carrier is superimposed onto the intermediate transfer member, the superimposed toner images are transferred onto the transfer material at once by static electricity. 
   Since an electrostatic charge amount per a toner particle is substantially uniform, the toner layer voltage on the intermediate transfer member is determined by the toner adhesion amount in a predetermined area. In the color image forming apparatus, the electrostatic charge amount of the portion where toners of plural colors are superimposed among the toner images of the intermediate transfer member becomes larger than that of the portion where one color toner adheres. And for example, when there are a toner image of a solid portion and a toner image of a halftone portion on the intermediate transfer member, the electrostatic charge voltage of the solid portion is higher than that of halftone portion. 
   As described above, when toner image voltage dispersion on the intermediate transfer member is large, portions where transfer characteristics are different each other exist in the same toner image. When transferring all the portions where the transfer characteristics are different each other onto the transfer material under the same transfer condition, various poor quality images tend to appear when transferring the toner images from the intermediate transfer member to the transfer member. 
   In recent years, in the copier, the printer, the facsimile machine and the image forming apparatus such as a multifunctional product having the function thereof, the ratio of the machines having color capability has become high. At the same time, due to the adoption of polymerization toner and toner having a small diameter, the requirements for high quality images in a transfer process has become strong. Further, a high-speed process trend proceeds in the image forming apparatus. In response to these trends described above, in order to obtain a high quality image, it is necessary to correct the toner voltages on the intermediate transfer member, which vary according to the number of times of the first transfer and environment, so as to be substantially uniform, and to improve the second transfer performance. 
   Japanese Patent Application Publication No. 10-274892 discloses an image forming apparatus including a pre-transfer charging unit for charging a toner image onto an intermediate transfer member before conducting the second transfer to a transfer member. 
   Japanese Patent Application Publication No. 11-143255 discloses a potential difference controller to control a direct current voltage source of a secondary pre-transfer charging unit and a direct current voltage source of a secondary transfer device so that the difference between a toner image voltage of the secondary pre-transfer charging unit and a voltage of the second transfer device is substantially constant. 
   Japanese Patent Application Publication No. 06-236116 discloses an electro photography apparatus comprises a discharging unit for discharging a toner charge transferred onto an intermediate transfer member and a charging unit for charging the toner image on the intermediate transfer member with the same polarity when developing and to charge the toner image on the intermediate member with a reverse polarity against the charged polarity right before starting a second transfer. 
   In the color image forming apparatus for conducting the second transfer of a toner image from the intermediate transfer member to the transfer member after superimposing the toner image of each color formed on the surface of a photoreceptor onto the intermediate transfer member by using the first transfer unit, in order to prevent the occurrence of density unevenness due to the transfer charge deficit caused when the toner adhesive amount is large and the toner layer voltage is high, a secondary pre-transfer charging unit having a scorotron electrode is disposed on the upper stream of the second transfer unit to discharge the electrical charges of the toner image formed on the intermediate transfer member. 
   In this case, following is going to be a problem. Namely, when discharging the electrical charges of the toner image on the intermediate transfer member, the upper layer of the toner image turns to reverse electrical charge toner. As a result, floating toner adheres on a grid electrode having the same polarity of the toner before discharging the electrical charge of the toner image and the toner adhered on the grid deteriorates the dischargation control capability. 
   The color image forming apparatuses disclosed in Japanese Patent Application Publication No. 10-274892 and Japanese Patent Application Publication No. 11-143255 comprise a scorotron charging unit for charging electrical charges to form a toner image and removing the electrical charges, the scorotron charging unit being disposed in the upper stream of the second transfer unit. However, there is a possibility that floating toner adheres the grid electrode of the scorotron charging unit and deteriorates the control performance for charging the electrical charges. 
   The electro photography apparatus disclosed in Japanese Patent Application Publication No. 06-236116 is an apparatus wherein the electrical charges of a toner image is removed to zero by applying AC voltage, then toner image is recharged again. It is not an image forming apparatus in which a scorotron charging unit prevents the dirt of the grid in the scorotron charging unit. The charging unit disclosed here is a scorotron charging unit having no grid. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention is to provide a color image forming apparatus for preventing the dirt on a grid electrode by the adhesion of floating toner in a pre-secondary-transfer discharging unit, to attain the better performance of dischargation of electrical charges and to obtain a high quality secondary transfer image. 
   An object of the present invention will be attained by any one of following configurations. 
   1. A color image forming apparatus comprises a plurality of image carriers, a plurality of a primary transfer units for transferring a toner image formed on the plurality of image carriers onto an intermediate transfer member, a discharging unit for discharging an electrical charge of a toner image transferred by the intermediate transfer member, the discharging unit being disposed between two adjoining image carriers placed along a moving direction of the intermediate transfer member and a secondary transfer unit for transferring a plurality colors of toner images superimposed on the intermediate transfer member onto a transfer material. 
   2. A color image forming apparatus comprises n units of image carriers, an intermediate transfer member, n units of primary transfer units for transferring toner images of n colors formed-on the image carriers onto the intermediate transfer member, a discharging unit for discharging an electric charge of a toner image transferred on the intermediate transfer member, the discharging unit being disposed at a position just after a first transfer process by any one of the primary transfer units for a first, second, . . . (n−1)th color and a secondary transfer unit for transferring a plurality colors of toner images superimposed on the intermediate transfer member onto a transfer material. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  illustrates a cross sectional view of a total configuration of a color image forming apparatus; 
       FIG. 2  illustrates a cross sectional view of a main portion of the color image forming apparatus; 
       FIG. 3  illustrates a schematic diagram of the main portion of an example 1 of the color image forming apparatus; 
       FIG. 4  illustrates a schematic diagram of the main portion of an example 2 of the color image forming apparatus; 
       FIG. 5  illustrates a schematic diagram of the main portion of an example 3 of the color image forming apparatus; 
       FIG. 6  illustrates a schematic diagram of the main portion of a comparison example of the color image forming apparatus; and 
       FIGS. 7(   a )- 7 ( n ) illustrate schematic diagrams of various disposal examples of discharging units. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the present invention will be described below. However, the present invention is not limited to the embodiment to be described below. 
   &lt;A Color Image Forming Apparatus&gt; 
     FIG. 1  illustrates a cross sectional view showing a total configuration of an embodiment of a color image forming apparatus A of the present invention. 
   This color image forming apparatus A is called a tandem type color image forming apparatus. The color image forming apparatus A comprises a plurality of image forming sections  10 Y,  10 M,  10 C and  10 K, an intermediate transfer member  7 , a primary transfer units  5 Y,  5 M,  5 C and  5 K, an intermediate transfer unit configured by a secondary transfer unit  8 , a fixing device  11  and a sheet feeding device  20 . 
   An optical system scans and exposes the document placed on a document table provided upper portion of the color image forming apparatus A. Then a line image sensor reads the image on the document. The line sensor converts the optical image into analog electric signals, which will be inputted into exposure units  3 Y,  3 M,  3 C and  3 K after being processed by an analog process, an A/D conversion, a shading correction and an image compression process in an image processing section. 
   An image forming section  10 Y for forming a yellow (Y) colored image comprises a charge unit  2 Y disposed on the circumference of an image carrier  1 Y, an exposing unit  3 Y, a developing unit  4 Y and a cleaning unit  6 Y. 
   An image forming section  10 M for forming a magenta (M) colored image comprises an image carrier  1 M, a charging unit  2 M, an exposing unit  3 M, an exposing unit  4 M and a cleaning unit  6 M. 
   An image forming section  10 C for forming a cyan (C) colored image comprises an image carrier  1 C, a charging unit  2 C, an exposing unit  3 C, an exposing unit  4 C and a cleaning unit  6 C. 
   An image forming section  10 K for forming a black (K) colored image comprises an image carrier  1 K, a charging unit  2 K, an exposing unit  3 K, an exposing unit  4 K and a cleaning unit  6 K. 
   A latent image forming unit comprises the charging unit  2 Y, the exposing unit  3 Y, the charging unit  2 M, the exposing unit  3 M, the charging unit  2 C, the exposing unit  3 C, the charging unit  2 K and the exposing unit  3 K. 
   With regard to the Image carriers  1 Y,  1 M,  1 C and  1 K, it is preferable that OPC photosensitive material or aSi photosensitive material, which is well known is used. In the embodiment of the present invention, negatively charged OPC is used. 
   With regard to the charging units  2 Y,  2 M,  2 C and  2 K, a corona charging unit such as a scorotron and a corotron is used. It is preferable that the scorotron charging unit is used. 
   With regard to the exposing units  3 Y,  3 M,  3 C and  3 K, a light emitting element, such as a LED array for emitting lights according to image data is used. 
   An intermediate transfer member  7  structured in a belt shape is configured by semiconductor. The intermediate transfer member  7  is wound around a plurality of support rollers  71 ,  72 ,  73 ,  74  and a backup roller  75 , and is supported so that the intermediate transfer member  7  can circularly move thereabout. In this embodiment, the intermediate transfer member  7  is flatly supported between support rollers  73  and  74 . 
   The first transfer units  5 Y,  5 M,  5 C and  5 K simultaneously transfer each color image formed by the image forming units  10 Y,  10 M,  10 C and  10 K onto the intermediate transfer member  7  rotating around the support rollers to synthesize a color image on the intermediate transfer member  7  (the primary transfer). 
   A transfer material P stored in a sheet feeding cassette  21  of a sheet feeding apparatus  20  is fed by a sheet feeding unit (a first sheet feeding section)  22 . Then a color image is transferred onto the transfer material P after the transfer material P is passed through feeding rollers  23 ,  24  and  25 , and a registration roller  26  (secondary transfer). 
   A fixing apparatus  11  applies heat and pressure onto the transfer material P to fix the color toner image (or a monocolor toner image) on the transfer material P. The transfer material onto which the color toner image has been fixed is ejected from a sheet eject roller  27  and placed on the sheet eject tray  28  provided outside on the color image forming apparatus A. 
   On the other hand, after the second transfer unit  8  has transferred the color image onto the transfer material P, the intermediate transfer member  7  separates the transfer material P with separation by curvature. Then the residual toner left on the intermediate transfer member  7  is removed by a cleaning unit  6 A. 
   &lt;Primary Transfer Unit&gt; 
     FIG. 2  illustrates a cross sectional view of the main portion of the color image forming apparatus A. 
   The first transfer unit  5 Y for transferring a yellow colored image, which comprises a first transfer roller  5 YA and a direct current voltage source  5 YE for supplying voltage to the first transfer roller  5 YA. The first transfer roller  5 YA is opposed to the image carrier  1 Y through the intermediate transfer member  7  and contacting to the inside of the intermediate transfer member  7 . The direct current voltage source  5 YE is grounded. 
   The first transfer unit  5 M for transferring a magenta colored image, which comprises a first transfer roller  5 MA and a direct current voltage source  5 ME for supplying voltage to the first transfer roller  5 MA. The first transfer roller  5 MA is opposed to the image carrier  1 M through the intermediate transfer member  7  and contacting to the inside of the intermediate transfer member  7 . The direct current voltage source  5 ME is grounded. 
   The first transfer unit  5 C for transferring a cyan colored image, which comprises a first transfer roller  5 CA and a direct current voltage source  5 CE for supplying voltage to the first transfer roller  5 CA. The first transfer roller  5 CA is opposed to the image carrier  1 C through the intermediate transfer member  7  and contacting to the inside of the intermediate transfer member  7 . The direct current voltage source  5 CE is grounded. 
   The first transfer unit  5 K for transferring a black colored image, which comprises a first transfer roller  5 KA and a direct current voltage source  5 KE for supplying voltage to the first transfer roller  5 KA. The first transfer roller  5 KA is opposed to the image carrier  1 K through the intermediate transfer member  7  and contacting to the inside of the intermediate transfer member  7 . The direct current voltage source  5 KE is grounded. 
   Each direct current voltage sources  5 YE,  5 ME,  5 CE and  5 KE respectively supply current of 40 μA and voltage of 1.5 kV to the first transfer units  5 Y,  5 M,  5 C and  5 K. 
   The first transfer units  5 Y,  5 M,  5 C and  5 K are arranged to move away from the inside surface of the intermediate transfer member  7  by a driving unit (not shown) while the first transfer units are not used for the first transfer operation. 
   &lt;Secondary Transfer Unit&gt; 
   A secondary transfer unit  8  comprises a backup roller  75 , a secondary transfer roller  8 A and a direct current voltage source  8 E. The backup roller  75  structured by a conductive member opposes to the secondary transfer roller  8 A through the intermediate transfer member  7  and contacts with the internal surface of the intermediate transfer member  7 . 
   The secondary transfer roller  8 A is connected with a direct current voltage source  8 E for inputting direct current voltage to the secondary transfer roller  8 A. The direct current voltage source  8 E inputs current 50 μA and voltage +3 kV onto the secondary transfer unit  8 . The direct current voltage source  8 E applies reverse bias voltage to move the residual toner adhered on the secondary transfer roller  8 A contacting with the intermediate transfer member  7  to the intermediate transfer member  7  to clean the secondary transfer roller  8 A. 
   The backup roller  75  of the secondary transfer unit  8  has substantially the same configuration of the first transfer rollers  5 YA,  5 MA,  5 CA and  5 KA, and contacts with the inside surface of the intermediate transfer member  7  with pressure. The backup roller  75  having a conductive characteristic comprises a main body of a roller and an elastic layer formed on the surface of the main body of the roller. 
   A single layer or a multiple layer belt having a material such as polyamide or polyimide structures the intermediate transfer member  7 . The single layer or a multi layer belt has a volume resistivity of 10 7 -10 12  Ωcm. 
   The intermediate transfer member  7  is cleaned while passing through the cleaning unit  6 A after the secondary transfer unit  8  has transferred the image onto the transfer material P. 
   The secondary transfer roller  8 A is moved away from the outer surface of the intermediate transfer member  7  by a driving unit (not shown) while the secondary roller is not used for the secondary transfer operation. 
   &lt;Pre-Secondary-transfer Discharging Unit&gt; 
   As illustrated in  FIG. 2 , a pre-secondary-transfer discharging unit  9  is provided at the position where the intermediate transfer member  7  is supported with a flat surface shape between the first transfer unit  5 K and a support roller  74 , which are provided along with the intermediate transfer member  7 . 
   The pre-secondary-transfer discharging unit  9  comprises a discharger  9 A provided in the image carrier side of the intermediate transfer member  7  and an opposite electrode  9 B provided the inside surface side of the intermediate transfer member  7  shaped in an endless belt. 
   In the color image forming apparatus of an intermediate transfer method, even though the first transfer performance is good, there is a case that a high quality image cannot be obtained when the secondary transfer is not good in the second color. The reason why a high quality image cannot be obtained is that the toner image formed on the intermediate transfer member  7  has toner widely spread over layers from the first layer to the fourth layer in maximum and the optimization of each secondary transfer condition corresponding to the adhesion amount of each layer becomes unbalance. 
   In response to this problem, it becomes possible to satisfy the secondary transfer performance by discharging the toner image formed on the intermediate transfer member  7  and adjusting the electrical charge amount to satisfy the secondary transfer performance against the toner adhesive amount widely spread over the layers from the first layer to the fourth layer. 
   However, as the process speed of the color image forming apparatus has been improved, in order to secure the dischargation, the length of the discharger  9 A of the pre-secondary-transfer dischargation unit  9  in the secondary scanning direction (the running direction of the intermediate transfer member  7 ) must be extended. Accordingly, the length of the opposite electrode  9 B must be extended. 
   A roller has been adopted for the opposite electrode  9 B for many cases. In order to improve the process speed of the color image forming apparatus, it is necessary not only to widen the contact length with the intermediate transfer member  7  but also to set the optimum distance between the intermediate transfer member  7  and the pre-secondary-transfer discharging unit  9 . 
   In order to solve these two problems, it is necessary to extend the outer diameter of the support roller  74  and to widen the winding angle of the intermediate transfer member  7  having a belt shape. However, there are problems that the size of the apparatus becomes large and the manufacturing cost goes up. 
   In order to improve these problems, the opposed electrode  9 B of a conductive blush or a conductive forming material is arranged to be grounded while contacting with the surface of the intermediate transfer member  7 . Based on these solutions, the improvement of the dischargation efficiency, which is better than that of conventional solution has been attained. 
   &lt;Discharger  9 A&gt; 
   The discharger  9 A is a scorotron discharger configured by a discharging electrode, a grid electrode and a case. 
   The discharging electrode is connected to a direct current voltage source E 1 . The grid electrode is so disposed as to oppose to the belt surface of the intermediate transfer member  7  with keeping a predetermined distance. The grid electrode is connected to the direct current voltage source E 2 . The case is arranged to keep the same voltage as the grid electrode via a circuit (not shown). 
   A wire material of tungsten, stainless steal and gold having a diameter of 20-150 μm may configure the discharging electrode. However, a wire material having the surface covered by gold preferably configures the discharging electrode. The wire itself may be structured by gold or may be structured by a base member of stainless steal or tungsten, which is covered with gold thereon. The thickness of the gold membrane is preferably 1 μm-5 μm in average thickness of the membrane from the viewpoint of the removal efficiency of substance generated by discharging such as ozone and a manufacturing cost. 
   With regard to the grid electrode, a wire type grid, a plate shaped grid formed from a pattern shape into which a metal plate is processed by an etching and a plate type grid onto which gold plating has been applied are used. 
   The discharger  9 A is arranged so that the direct current voltage of 0 to +5 kV, which causes reverse polarity discharge of the toner is applied to the discharging electrode, and direct current voltage of 0 to −300 V is applied to the grid electrode. As an example, voltage of +5 kV is applied to the discharging electrode and voltage of −100 V is applied to the grid electrode. 
   In the example of the present invention, the direct current voltage of 0 to +5 kV, which causes reverse polarity discharge of the toner is applied to the discharging electrode, and direct current voltage of 0 to −300 V is applied to the grid electrode. 
   In the example, which will be described later, voltage of +4 kV is applied to the discharging electrode of the pre-second-transfer discharging unit  9  and voltage of −50 V is applied to the grid electrode. 
   &lt;Opposite Electrode  9 B&gt; 
   An opposite electrode  9 B configured by a conductive blush and a pressure contact release mechanism for releasing pressure contact of the conductive blush is provided inside surface of the intermediate transfer member  7  opposed to the pre-secondary-transfer discharging unit  9 . The conductive blush is contacted with the inside surface of the intermediate transfer member  7  with pressure and grounded. 
   It is preferable that the conductive blush comprises a conductive resin material such as acryl, nylon and polyester. It is also preferable that the wire diameter 0.111 tex to 0.778 tex, where tex is proposed by ISO for the unit of measurement of the diameter of wire by representing the number of the length, which can be prolonged from a predetermined fixed weight material of the wire, the blush density is 12000 pieces of hair/cm 2  to 7700 pieces of wire/cm 2  and the original sting resistivity is 10 0  to 10 5  Ωcm. 
   EXAMPLES 
   Examples of the present invention will be described below. However, the present invention is not limited to the examples. In this example, an image has been formed by the color image forming apparatus A including the first transfer units  5 Y,  5 M,  5 C and  5 K, and the secondary transfer unit  8  illustrated in  FIG. 8 . 
   [Image Forming Condition] 
   Image forming apparatus: A tandem type full color copier (Konica Minolta 8050 (Trademark of Konica Minolta Co., Ltd) with some modifications), the continuous copy speed in full color corresponds to the speed of 51 piece of paper sheets (A4 size) per minute. 
   Image carrier  1 Y,  1 M,  1 C and  1 K: The outer diameter is φ60 mm. 
   Transfer member conveyance line speed: 220 mm/sec 
   Developing agent: Average particle diameter of the carrier; 20-60 μm, average particle diameter of the polymerized toner; 3-7 μm 
   Charging unit  2 Y,  2 M,  2 C and  2 K: electrostatic charge voltage VO is −700 V (variable: the number in the left is a nominal value) 
   Exposing unit  3 Y,  3 M,  3 C and  3 K: semiconductor laser (wavelength 780 nm), surface voltage of an image forming member when being exposed Vi is −50 V. 
   Developing unit  4 Y,  4 M,  4 C and  4 K: Developing sleeve voltage Vdc is −500 V (variable: the number in the left is a nominal value), Developing bias voltage alternate voltage element Vac is 1 kvp-p with a rectangular waveform of frequency 5 kHz. 
   First transfer rollers  5 YA,  5 MA,  5 CA and  5 KA: Conductive rollers are used, roller pressure 10 N, transfer current 40 μA, and transfer voltage +1.5 kV is applied. 
   In this example, the color image forming apparatus A including the secondary transfer unit  8  illustrated in  FIGS. 1-2  forms the image. 
   The secondary transfer unit: A configuration for putting the intermediate transfer unit  8  between the backup roller  75  and the secondary transfer roller  8 A is adopted; Electrical resistances are both 1×10 7 Ω; apply a predetermined current value selected from a current value table into which a matrix formed by temperature/humidity and a counter. 
   Pressure force F: 50 N (Newton), Nip width in a transfer material conveyance direction: 3 mm 
   Elastic layer of secondary transfer roller  8 A: Semi-conductive NBR solid rubber (acrylonitrile•butadiene-rubber), volume resistance 4×10 7 Ω, and outer diameter φ40 mm. 
   Length in the axis direction of elastic layer of secondary transfer roller  8 A: LA=150 mm, LB=250 mm, LC=330 mm 
   Intermediate transfer member  7 : Polyimide, seamless semiconductive belt (volume resistivity 10 9  Ωcm), tightly stretched tension 50N, line velocity 220 mm/sec 
   Adhesion amount of toner on the intermediate transfer member  7  from right after passing through the image carrier  1 K to the secondary transfer unit  8 : 10 g/M 2    
   Height of toner on the intermediate transfer member  7  from right after passing through the image carrier  1 K to the secondary transfer unit  8 : 30 μm 
   Reverse-bias-applying-cleaning-control against secondary transfer roller  8 A: The cleaning of secondary transfer roller  8 A is performed by charging + polarity electric charge against the secondary roller  8 A for 1 second while conducting transfer to the transfer material P (when setting the normal conveyance line speed V 1  of transfer material P to 220 mm/sec, (V 1 =V 2 ), time period corresponding to rotate the secondary roller  8 A having the outer diameter of 40 mm twice), when conducting continuous print operation, a toner image is transferred onto the secondary roller  8 A from the back edge of the transfer material P. 
   &lt;Discharging Unit&gt; 
   A discharging unit  90  comprises a discharger disposed in the image carrier side of the intermediate transfer member  7  and an opposite electrode disposed internal surface side of the intermediate transfer member  7  having an endless belt shape. 
   &lt;Discharger&gt; 
   A discharger is a scorotron discharger having a discharging electrode  91 , a grid electrode and a side plate  93 . 
   The discharging electrode  91  is connected to a direct current voltage source E 3 . The grid electrode  92  opposes to the belt surface of the intermediate transfer member  7  with a predetermined distance and is connected with the direct current voltage source E 4 . The side plate  93  is connected with the grid  92  through a circuit, which is not shown to keep the same voltage of the gird. 
   A wire material of tungsten, stainless steal and gold having a diameter of 20-150 μm may configure the discharging electrode  91 . However, a wire material having the surface covered by gold preferably configures the discharging electrode. The wire material itself may be structured by gold or may be structured by a base member of stainless steal or tungsten, which is covered with gold thereon. The thickness of the gold membrane is preferably 1 μm-5 μm in average thickness of the membrane from the viewpoint of the removal efficiency of substance generated by discharging such as ozone and a manufacturing cost. 
   With regard to the grid electrode  92 , a wire type grid, a plate shaped grid formed from a pattern shaped into which a metal plate is processed by an etching and a plate type grid onto which a gold has been flashed are used. 
   The discharging unit  90  is arranged so that the direct current voltage of 0 to +5 kV, which causes reverse polarity discharge of the toner is applied to the discharging electrode  91 , and direct current voltage of 0 to −300 V is applied to the grid electrode  92 . As an example, voltage of +5 kV is applied to the discharging electrode  91  and voltage of −100 V is applied to the grid electrode. 
   In the example of the present invention, the direct current voltage of 0 to +5 kV, which causes reverse polarity discharge of the toner is applied to the discharging electrode  91 , and direct current voltage of 0 to −300 V is applied to the grid electrode  92 . 
   In the example, voltage of +4 kV is applied to the discharging electrode  91  of the discharging unit  90  and voltage of −50 V is applied to the grid electrode  92 . 
   The voltage of a side plate  93  has been set at the same voltage of the grid  92 . The distance between the grid electrode  92  and the intermediate transfer member  7  is set at 1 mm and arrange to be parallel. 
   The width of the discharging electrode  91  (the length of the intermediate transfer member  7  in the running direction) is set 30 mm and the length in the longitudinal direction (a length crossing at right angles with the intermediate transfer member  7 ) is set 320 mm. 
   &lt;Opposite Electrode  9 B&gt; 
   An opposite electrode  9 B configured by a conductive blush  94  and a pressure contact release mechanism for releasing pressure contact of the conductive blush  94  is provided inside surface of the intermediate transfer member  7  opposed to the discharging unit  90 . The conductive blush  94  is contacted with the inside surface of the intermediate transfer member  7  with pressure and grounded. 
   The conductive blush comprises an original wire having resistance of 10 2 Ω, a diameter of 3 deniers (1 denier is a unit denoting a wire diameter, the wire having a length of 4560 m and weight is 50 mg), a density of 200 kF/inch 2  (F denotes filament number, 1 inch denotes 25.4 mm) and the length of the wire being 4 mm. The conductive blush  94  is grounded. 
   The width of the conductive blush  94  of the opposite electrode  9 B (the length in the running direction of the intermediate transfer member  7 ) is set at 30 mm and the longitudinal length (the length crossing at right angles with the running direction of the intermediate transfer member  7 ) is set at 320 mm. 
   &lt;Experimental Conditions&gt; 
   With regard to the method for confirming the effect of the present invention, a solid image onto which a magenta toner image and a cyan toner image have been superimposed has been outputted. When the dischargation effect is insufficient, transfer unevenness of the rear surface occurs when the solid image onto which a magenta toner image and a cyan toner image have been superimposed has been outputted under the condition of low temperature and low humidity. 
   With regard to the method for confirming the effect of countermeasures against the dirt of the grid electrode  92 , 1000 pieces of paper sheet onto which a character (6 point character) image onto which a magenta toner image and a cyan toner image are superimposed are continuously outputted. Then a microscope has observed the adhesive status of the floating toner to the grid electrode  92  after the print output has been completed. 
   Example 1 
   The color image forming apparatus of example 1 is an apparatus being equal to the color image forming apparatus illustrated in  FIG. 2 , from which the image forming section  10 M, which is the second step from the top has been removed and the discharging unit  90  is disposed instead of the image forming section  10 M. A magenta colored developing agent was inputted to the image forming section  10 Y located in the first stage from the top and a cyan colored developing agent was inputted to the image forming section  10 C located in the third stage from the top. 
   +200 μA current was inputted to the discharging electrode  91  of the discharging unit  90  and −50 V voltage was applied to the grid electrode  92 . 
   The transfer image evaluation results of the color image forming apparatus having the configuration described above will be shown in Table 1. The toner adhesive ratio to the grid electrode  92  was observed right under the downstream of the discharging electrode  91 . 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Magenta + Cyan 
               Toner adhesive 
                 
             
             
                 
               Transfer 
               rate to Grid 
               Magenta halftone 
             
             
                 
               unevenness 
               Electrode 92 
               Image Roughness 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
               Example 1 
               Non-occurrence 
               About 30% 
               Non-occurrence 
             
             
               Example 2 
               Non-occurrence 
               About 30% 
               Non-occurrence 
             
             
               Example 3 
               Non-occurrence 
               About 15% 
               Non-occurrence 
             
             
               Comparison 
               Non-occurrence 
               About 60% 
               Image roughness 
             
             
               Example 
                 
                 
               occurs 
             
             
                 
             
          
         
       
     
   
   According to the example 1 in Table 1, transfer unevenness of magenta plus cyan did not concur. The toner adhesion area ration to the grid electrode  92  (toner adhesive coverage ratio) was about 30% and the image roughness of the magenta halftone image did not occur. As a result, good result has been obtained. 
   Example 2 
     FIG. 4  illustrates a schematic diagram of the main section of the color image forming apparatus of the Example 2. 
   The color image forming apparatus of example 2 is an apparatus being equal to the color image forming apparatus illustrated in  FIG. 2 , from which the image forming section  10 C, which is the third step from the top has been removed and the discharging unit  90  is disposed instead of the image forming section  10 C. A magenta colored developing agent was inputted to the image forming section  10 Y located in the first stage from the top and a cyan colored developing agent was inputted to the image forming section  10 K located in the fourth stage from the top. 
   +200 μA current was inputted to the discharging electrode  91  of the discharging unit  90  and −50 V voltage was applied to the grid electrode  92 . 
   The transfer image evaluation results of the color image forming apparatus having the configuration described above will be shown in Table 1. The toner adhesive ratio to the grid electrode  92  was observed right under the downstream of the discharging electrode  91 . 
   According to the example 2 in Table 1, transfer unevenness of magenta plus cyan did not concur. The toner adhesion area ratio to the grid electrode  92  was about 30% and the image roughness of the magenta halftone image did not occur. As a result, good result has been obtained. 
   Example 3 
     FIG. 5  illustrates a schematic diagram showing the main section of the color image forming apparatus of the example 3. 
   The color image forming apparatus of example 3 is an apparatus being equal to the color image forming apparatus illustrated in  FIG. 2 , from which the image forming section  10 M, which is the second step from the top has been removed and the discharging unit  90 A is disposed instead of the image forming section  10 M and further the image forming section  10 K, which is the fourth step from the top has been removed and the discharging unit  90 B is disposed instead of the image forming section  10 K. The first discharging unit  90 A and the second discharging unit  90 B have the same configuration elements. Thus the configuration elements have the same code. 
   The magenta color developing agent was inputted to the image forming section  10 Y of the first step from the top and cyan color developing agent was inputted to the image forming section  10 C of the third steps from the top. 
   +200 μA current was inputted to the discharging electrode  91  of the first discharging unit  90 A and −50 V voltage was applied to the grid electrode  92 . +200 μA current was inputted to the discharging electrode  91  of the second discharging unit  90 B and −50 V voltage was applied to the grid electrode  92 . 
   The transfer image evaluation of the configuration of the color image forming apparatus described above will be described in Table 1. 
   According to the example 3 in Table 1, the transfer unevenness with magenta plus cyan color did not occur. The toner adhesive ratio to the grid electrode  92  was 15%. The image roughness with a magenta color halftone image did not occur. As a result, good result has been obtained. 
   Comparative Example 
     FIG. 6  illustrates the schematic diagram of the main portion of the color image forming apparatus of the comparative example. 
   The color image forming apparatus of the comparative example is an apparatus being equal to the color image forming apparatus illustrated in  FIG. 2 , from which the image forming section  10 K, which is the fourth step from the top has been removed and the discharging unit  90  is disposed instead of the image forming section  10 K. 
   The magenta color developing agent was inputted to the image forming section  10 M of the second step from the top and cyan color developing agent was inputted to the image forming section  10 C of the third steps from the top. 
   +200 μA current was inputted to the discharging electrode  91  of the discharging unit  90  and −50 V voltage was applied to the grid electrode  92 . 
   The transfer image evaluation of the configuration of the color image forming apparatus described above will be described in Table 1. 
   In the comparative example of Table 1, the transfer unevenness of magenta color plus cyan color did not occur. However, the toner adhesive ratio to the grid electrode is 60%, which is quite large number and the magenta color halftone image roughness occurred. 
   Experimental Results 
   A toner image on the lower layer of superimposed image can be discharged by conducting dischargation just after the primary transfer other than the transfer operation at the most downstream. The total electric charge of toner layer can be suppressed by charging the toner image formed on the most upper side layer with reverse polarity while preventing the grid electrode  92  of the discharging unit  90  from getting dirt. As a result, the secondary transfer capability has been improved. 
   Accordingly, satisfactory second transfer capability can be attained and high quality color image can be obtained by disposing the discharging unit  90  at least at one position in any one of places being downstream of the image carriers  1 Y,  1 M and  1 C of the color image forming apparatus illustrated in  FIG. 1 , which are the places as shown in examples 1, 2 and 3. 
   Further, the total electric charges of the toner images, which have been superimposed, can be suppressed. Thus floating toner image adhesion to the grid electrode has been lowered. 
   Disposing Examples of Discharging Unit  90   
     FIGS. 7(   a )- 7 ( n ) illustrate schematic diagrams of various disposing examples of the discharging unit  90 . 
     FIGS. 7(   a )- 7 ( c ) illustrate disposing examples, in which the discharging unit  9  is disposed downstream of any one location of the image carriers  1 Y,  1 M and  1 C. 
     FIGS. 7(   d )- 7 ( i ) illustrate disposing examples, in which the discharging units  90 A and  90 B are disposed downstream of any two locations of the image carriers  1 Y,  1 M and  1 C. 
     FIGS. 7(   j )- 7 ( m ) illustrate disposing examples, in which the discharging units  90 A,  90 B and  90 C are disposed downstream of any three locations of the image carriers  1 Y,  1 M and  1 C. 
     FIGS. 7(   n ) illustrates disposing example, in which the discharging units  90 A,  90 B,  90 C and  90 D are disposed downstream of all four locations of the image carriers  1 Y,  1 M,  1 C and  1 K. 
   In the embodiments of the present invention, with regard to the intermediate transfer member  7 , the example having an intermediate transfer belt was described. However, the present invention can apply to apparatuses having other types intermediate transfer members such as an intermediate transfer drum.