JP2003255633A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of preventing defective electrification and irregularities in a toner image at the end part of a recording paper due to the outflow of an electrifying current to a non-paper passing part and obtaining a good image, in a method for enhancing the attracting force of the toner image to the recording paper by re-electrifying the recording paper by a re-electrifying unit after the paper is separated from a transfer belt in order to prevent the irregularities in the toner image. <P>SOLUTION: A conductive electrode arranged facing the re-electrifying device 8 across the recording paper P is grounded through a resistance element 17 or a varistor 18, alternatively, the conductive electrode is grounded and a resistance is arranged between the conductive electrode and the recording paper P. <P>COPYRIGHT: (C)2003,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus such as a copying machine or a printer.
The present invention relates to an image forming apparatus provided with a charging unit for charging a toner image transferred to a recording material. 2. Description of the Related Art Conventionally, a plurality of image forming units using an electrophotographic method are provided, and toner images of different colors are formed in each image forming unit, and the toner images are sequentially superimposed on the same recording paper. An image forming apparatus that forms a color image by transferring a color image has been proposed. In addition, in order to perform high-speed recording, a transfer belt in the form of a rotating endless belt is provided, a plurality of image forming units are linearly arranged along the belt, and the recording paper is electrostatically attracted onto the belt to separate the recording paper. There has been proposed a color copying machine which conveys a toner image of each color onto a recording paper by being conveyed to an image forming unit. In such a color copying machine, after the transfer of the toner images of all colors is completed, the charge is removed by a corona charger applying an AC bias to the recording paper at a separation position on the transfer belt, and the recording paper and the transfer belt are removed. And the recording paper is separated from the transfer belt. The separated recording paper is sent to a fixing device, where the heated toner pair fuses and presses the toner image on the recording paper to fix the toner image. In the above-mentioned separation step, since the charge is removed from the surface of the recording paper carrying the toner image, the charge of the toner particles is also removed, and the charge drops to almost zero. Since the toner particles having a reduced charge amount have a weak electrostatic attraction force to the recording paper, if the fixing is performed as it is, the toner particles move on the recording paper due to the steam flow released from the recording paper at the time of fixing, and the toner image is disturbed. I will. To prevent this, Japanese Patent Application Laid-Open
As described in JP-A-39644, the recording paper immediately after separation is recharged by a corona charger to which a DC voltage of the same polarity as that of the toner is applied, thereby increasing the charge amount of the toner particles and increasing the static charge of the toner particles. The toner image is prevented from being disturbed by increasing the electroadhesive force. At this time, in order to efficiently perform recharging, a grounded conductive electrode having a length substantially equal to the width of the recording paper is provided at a position opposite to the recharging device on the opposite surface side of the recording paper, and between the corona charger and the recording paper. An electric field is formed to make it easier for the charging current to flow into the recording paper. [0005] However, the above configuration has the following problems. The recording paper is moved in the width direction (perpendicular to the paper passing direction).
Charge width of the recharger,
The length of the conductive electrode in the width direction is set longer than the maximum recording paper width. Since the electric resistance of the conductive electrode is lower than that of the recording paper, a part of the charging current flows to the conductive electrode at both ends of the recording paper during recharging, and the toner image can be sufficiently charged at both ends of the recording paper. However, there is a problem that toner image disturbance occurs. This disturbance of the toner image occurred remarkably in a low humidity environment around 5 g of water where the resistance of the recording paper was increased. In a further low-humidity environment where the water content is 5 g or less, the amount of charge of the toner in the developing device increases, so that the electrostatic attraction force of the toner particles to the recording material increases, and the toner image is less likely to be disturbed. SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has as its object to provide an image forming apparatus provided with a recharging means which does not cause disturbance of a toner image at an end of a recording sheet. . [0008] In order to solve the above-mentioned problems, the present invention is provided with the following structure to suppress the flow of the charging current to the non-sheet passing portion and suppress the toner image on the edge of the recording sheet. Is sufficiently charged to prevent toner image disturbance. An image includes a charging means for charging the recording material onto which the toner image has been transferred to the same polarity as the toner from the toner image surface side, and a conductive member disposed at a position opposite to the charging means with the recording material interposed therebetween. An image forming apparatus, wherein the conductive member is grounded via a resistive element or a varistor, or the conductive member is grounded and a resistor is provided between the conductive member and the recording material. (First Embodiment) A first embodiment of the present invention will be described below with reference to the drawings. FIG. 3 shows an example of a color copying machine to which the present invention is applied. First, second, third, and fourth image forming units Pa, Pb, Pc, and Pd are arranged in the apparatus, and latent images, development,
Through the transfer process, toner images of different colors are formed. The photosensitive drums 2a, 2b, 2c and 2d are rotatable, and their chargers 3a, 3a
b, 3c, 3d, developing devices 4a, 4b, 4c, 4d, transfer chargers 6a, 6b, 6c, 6d and cleaner 7a,
7b, 7c and 7d are provided, and a light source device and a polygon mirror 11 (not shown) are further provided above the device. After the photosensitive drums 2a, 2b, 2c and 2d are charged to predetermined potentials by the chargers 3a, 3b, 3c and 3d, the laser light emitted from the light source device is scanned by rotating the polygon mirror 11. , The light beam of the scanning light is deflected by a reflection mirror, and the photosensitive drums 2a, 2b, 2c,
By condensing and exposing on the generating line of d, a latent image corresponding to the image signal is formed on the photosensitive drums 2a, 2b, 2c, and 2d. The developing devices 4a, 4b, 4c, and 4d are filled with a predetermined amount of cyan, magenta, yellow, and black toners as developers, respectively.
4b, 4c and 4d are photosensitive drums 2a and 2b, respectively.
The latent images on 2c and 2d are developed and visualized as cyan, magenta, yellow, and black toner images. The recording paper P is accommodated in a recording paper cassette 30, and is supplied to the transfer belt 1 via a registration roller 13 through a conveyance path by a plurality of conveyance rollers, is conveyed by the transfer belt 1, and is conveyed by the transfer belt 1. 2b, 2
The sheet is sequentially sent to the transfer section facing c and 2d. The transfer belt 1 is made of a sheet in which a conductive material such as carbon is added to a base material of a dielectric resin such as polyimide, polycarbonate, polyethylene terephthalate, polyvinylidene fluoride or the like to have a medium resistance. Whether they are overlapped and joined together to form an endless shape,
Alternatively, a seamless belt having no joint is used. Now, the drive roller 14 and the tension roller 1
The transfer belt 1 supported by the roller 2 is rotated by the driving of the drive roller 14, the recording paper P is sent out from the registration roller 13 to the transfer belt 1, and the recording paper P is transferred to the first image forming unit Pa.
Is conveyed toward the transfer section. At the same time, the image writing signal is turned on, and an image is formed on the photosensitive drum 2a of the first image forming unit Pa at a certain timing based on the signal. Then, the transfer charger 6a applies a charge to the lower transfer section of the photosensitive drum 2a, so that the photosensitive drum 2a
The first color toner image formed thereon is transferred onto the recording paper P. By this transfer, the recording paper P is held on the transfer belt 1 by electrostatic attraction, and is conveyed to the second image forming portion Pb and thereafter. The transfer chargers 6a, 6b, 6c and 6d are contact chargers which come into contact with the transfer belt 1, and use blades made of conductive rubber. Transfer chargers 6a, 6b, 6
A high-voltage power supply (not shown) is connected to c and 6d, and a constant current is applied to obtain a stable image even when the volume resistance of the recording material P changes due to the thickness, material, moisture absorption conditions, etc. of the recording material P. Next, constant current control is performed. Image formation in the second to fourth image forming units Pb to Pd is performed in the same manner as in the first image forming unit Pa. Then 4
The recording paper P to which the color toner image has been transferred is separated from the end of the transfer belt 1 by removing the charge by the separation charger 9 at the downstream portion of the transfer belt 1 in the transport direction and attenuating the electrostatic attraction force. The separated recording paper P is supported by the guide member 2,
After the toner image on the recording paper P is charged by the recharger 8, the toner image is further sent to the fixing device 16 by the transport belt 15. On the recording paper P, the color mixture of the toner image and fixing to the recording paper P are performed by fusing, a full-color image is formed, and the recording paper P is discharged out of the apparatus. The transferred photosensitive drums 2a, 2b, 2
c, 2d are the respective cleaners 7a, 7b, 7c,
7d cleans and removes the untransferred toner and subsequently prepares for the formation of the next latent image. Further, the transfer belt 1 that has completed the transfer to the recording paper P and the separation therefrom is cleaned by a cleaning blade 10 in contact with the transfer belt 1 to remove toner, paper dust, and other deposits remaining on the belt. The residual charge is removed by the charge removing roller 20 to prepare for the next transfer step. Next, the configuration of a portion related to the present invention will be described. FIG. 1 is a peripheral view of the recharger 8. The recording paper P is discharged by the separation charger 9 and the leading end of the paper is transferred to the transfer belt 1.
After receiving the transfer force from the transfer belt 1, the sheet is conveyed in the direction of arrow C. A recharger 8 is provided at a position immediately after separation along the transport path, and the toner image on the recording paper P is charged by the recharger 8. The recharger 8 is a discharge wire 8
1 and a grounded metal shield 82 surrounding the discharge wire 81
The discharge wire 81 is connected to a high voltage power supply 84 that outputs the same polarity voltage as the toner. An insulating resin plate 83 is attached to a part of the inner surface of the shield 82 to reduce the current flowing into the shield 82 and increase the charging efficiency of the toner image.
The high voltage power supply 84 uses constant current control to keep the charging current constant. The conductive wire 21 is stretched at a position facing the discharge wire 81 with the recording paper P interposed therebetween. By forming an electric field between the conductive wire 21 and the discharge wire 81, the charging current is caused to flow intensively to the recording paper P side to form a toner image. Increases charging efficiency. Conventionally, the conductive wire 21 is directly grounded. However, in the present invention, the conductive wire 21 is grounded via the resistor 17 for the reason described later. The material of the conductive wire 21 has a diameter of 60 to
A 100 μm tungsten or stainless steel wire or a wire obtained by plating a tungsten wire with gold is used. If the conductive wire 21 is not provided, the entire current flows to the shield 82 and the toner image cannot be charged. The conductive wire 21 is supported by a guide member 2 for guiding the conveyance of the recording paper P. The guide member 2 is fixed to the guide 22 made of an insulating resin in contact with the recording paper P and to the guide 22 to prevent the guide 22 from bending. It is composed of a sheet metal 23 formed. The recording paper P that has been recharged is delivered to the conveyor belt 15 and sent to the fixing device 16. FIG. 2 shows a view of the guide member 2 viewed from the recharger 8 side. The guide 22 is provided with a plurality of elongated ribs 25 having a width of about 1 mm protruding toward the recording paper P, and supporting the recording paper P with the ribs 25 to reduce the contact area between the recording paper P and the guide 22 and reduce frictional resistance. The transfer performance has been improved. To prevent the conductive wire 21 from falling off when the recording paper P comes into contact with the conductive wire 21, the conductive wire 21 is provided with a tension spring 24 directly below the downstream end of the rib 25.
And is supported at a position where it does not contact the recording paper P. Next, the effect of the present invention will be described. As shown in FIG. 2, the charging width R of the recharger 8 is
(= 34 cm) is set longer than the maximum sheet passing width L (= 29.7 cm) in order to charge the entire area of the recording paper. The length of the conductive wire 21 is set to the same length as the charging width R in order to receive the charging current over the entire area of the recording paper. For this reason, when the recording paper P passes over the conductive wire 21, the portion of the conductive wire 21 having the maximum paper passing width L is covered with the high-resistance recording paper P, and the portions of the non-paper passing widths S ₁ and S ₂ outside L are provided. Means that the conductive wire 21 is exposed. Conventionally, since the conductive wire 21 is grounded, the charging current is easily drawn by the low-resistance conductive wire 21 at the end of the recording paper and easily flows into S ₁ and S ₂ . FIG. 9 is a diagram showing the distribution of current flowing into the conductive wire 21 when the conductive wire 21 is grounded.
The current density is low at both ends of the maximum sheet passing width L, that is, both ends of the recording paper P, and is high at the non-sheet passing widths S ₁ and S ₂ . The minimum current density required to prevent toner image disturbance is J ₀ = 1.5 μA / cm or more. Since the current density at both ends of the recording paper P is smaller than J ₀ , the toner image can be sufficiently formed at both ends. Charging could not be performed and toner image disturbance occurred. In order to prevent the current from flowing out to the non-sheet passing widths S ₁ and S ₂ , the length of the conductive wire 21 may be made equal to the maximum sheet passing width L by eliminating the S ₁ and S ₂ portions. However, when the recording paper P having a small size width shorter than the maximum width is used, the toner image is disturbed for the same reason as described above. On the other hand, in the present invention, the conductive wire 21
Is connected to the resistor 17 to prevent the current from flowing out to the non-sheet passing widths S ₁ and S ₂ . As the resistor 17, a resistor having a resistance value close to the dynamic resistance of the recording paper P during paper passing is used.
The dynamic resistance can be obtained as a ratio between the output current of the high voltage power supply 84 connected to the recharger 8 and the applied voltage. The value obtained by subtracting the dynamic resistance is the dynamic resistance of the recording paper. According to an experiment, when the resistance was measured in a humidity environment with a moisture content of 5 g using a device having a paper passing speed of 150 mm / sec, using plain paper having a basis weight of 80 g, which is likely to cause toner image disturbance, and measuring the resistance during the paper passing. 3.7 × 10 ⁶ Ω, the resistance during non-paper passing is 3.
0 × 10 ⁶ Ω, from which the dynamic resistance of the paper is 7 × 10 ⁶ Ω.
Ω. However, the output current of the high voltage power supply 84 is 300
μA, the applied voltage is 9-11 kV. From this resistance 1
7 was selected to be 7 × 10 ⁶ Ω. The resistance 1
As 7, an optimum one is selected according to the paper passing speed. By connecting the resistor 17 having substantially the same resistance as the recording paper P, there is no difference in resistance between the recording paper P and the conductive wire exposed portions (non-sheet passing width) S ₁ and S ₂ as viewed from the recharger 8. And the outflow of current to the non-sheet passing widths S ₁ and S ₂ can be reduced. FIG. 4 shows a distribution of a current flowing into the conductive wire 21 when the resistor 17 is connected. In FIG. 4, b indicates a high-voltage power supply 84.
9 is a case where the resistor 17 is connected without changing the output current of FIG. The outflow of current to the non-sheet passing widths S ₁ and S ₂ is reduced, and the decrease in current at the end of the sheet passing width L is suppressed. However, since the resistor 17 is connected, it is difficult for the current to flow in the portion of the paper passing width L, and a required current density cannot be obtained. The required current density can be obtained by increasing the output current of the recharger 8. FIG. 5 shows the relationship between the output current of the recharger 8 and the current flowing into the conductive wire 21 (total current flowing into the wire), where d indicates the case where the conductive wire 21 is grounded, and e indicates the resistance. 17 is connected. In the case of e, the wire inflow current is reduced, and
Is flowing into. The wire inflow current corresponding to the required current density is I ₀ = 50 μA, the higher current I ₀ =
By setting it to 60 μA, it was possible to prevent toner image disturbance, and in the case of grounded d, the output current was set to 300 μA to prevent toner image disturbance in the paper passing section (however,
(Current drop occurs at the end of paper passing.) On the other hand, in the case of e in which the resistor 17 is connected, a sufficient current I ₀ = 60 μA can be obtained by setting the output current to 400 μA. In FIG. 4, a shows the current distribution when the output current is set to 400 μA, and it can be seen that the required current density J ₀ or more is obtained over the entire area of the sheet passing width L. As described above, by connecting the resistor 17 to the conductive wire 21 and adjusting the output current of the recharger 8, the toner images at both ends of the recording paper can be sufficiently charged and the toner image can be prevented from being disturbed. Was. Similar effects were obtained when using small-size paper. (Second Embodiment) Instead of the resistance element 17, a potential generation element such as a varistor may be used. FIG. 6 shows an example in which the conductive wire 21 is grounded via the varistor 18.
When a current flows from the conductive wire 21 to the varistor 18, a potential having the same polarity as that of the toner is generated in the varistor 18, that is, the conductive wire 21. The current from the recharger 8 is repelled, and a current flows to the non-sheet passing widths S ₁ and S ₂ . It becomes difficult. As a result, the same effect as when the resistance element 17 is inserted can be obtained. The varistor 18 preferably has a generated voltage of the same level as that of the resistance element 17, and the voltage of the resistance element 17 in the first embodiment is 420V (resistance × 7 × 10 ⁶ Ω) × inflow current (60 μA). Therefore, the varistor 18 having a voltage of 450 V was used.
In addition, a constant voltage power supply having the same polarity as the toner may be connected to the conductive wire 21. (Third Embodiment) In addition to the above example, the same effect can be obtained by providing a resistor between the conductive wire 21 and the recording paper P or on the surface of the conductive wire 21 over the entire area of the conductive wire 21. can get. FIG. 7 shows a structure in which a resistor sheet 27 is provided on a guide 22 so as to cover the conductive wires 21.
As the material of the resistor sheet 27, a sheet in which carbon black is dispersed in polyethylene or urethane to adjust the resistance is used. FIG. 8 shows a structure in which a resistance layer 26 is provided on the surface of the conductive wire 21. As the material of the resistance layer 26, a material obtained by dispersing carbon black in a resin such as epoxy or the like and adjusting the resistance is adhered to the conductive wire 21. The resistance value (stationary resistance) of these resistors is a dynamic resistance of the recording paper P of 7 × 10 ^6.
Use something approximately equal to Ω. For example, a sheet 21 having a thickness of 50 μm, a volume resistance of 4.8 × 10 ¹⁰ Ω · cm, a width of 34 cm, and a width of 1 cm in the paper passing direction was used. Even when such a resistor was used, it was possible to suppress the current from flowing out to the non-sheet passing portion, and to prevent the toner image from being disturbed at the end. As described above, according to the present invention, after the recording paper on which the toner image has been transferred is separated from the transfer belt by static elimination, the recording paper is recharged to transfer the toner image onto the recording paper. In an image forming apparatus in which the attraction force is increased to prevent toner image disturbance at the time of fixing, a conductive electrode provided at a position facing the recharger with the recording paper interposed therebetween is grounded via a resistor. , Suppresses the flow of charging current to the non-paper passing area,
The toner image at the edge of the recording paper was sufficiently charged, and the toner image was prevented from being disturbed at the edge, and a good image was obtained.

BRIEF DESCRIPTION OF THE DRAWINGS [FIG. 1] Peripheral view of a recharger [FIG. 2] Relationship between conductive wire length and paper passing width [FIG. 3] Overall view of a copying machine [FIG. 4] Distribution of current flowing into conductive wires FIG. 5 shows the relationship between the output current of the recharger and the inflow current of the wire. FIG. 6 is a connection diagram of a varistor (second embodiment). FIG. 7 is a configuration diagram of a guide member (second embodiment). 3) FIG. 8 is a configuration diagram of a conductive wire (Example 3). FIG. 9 is a diagram showing a distribution of current flowing into a conductive wire (conventional example). Explanation of reference numerals P Recording paper (recording material) Pa, Pb, Pc, Pd Image forming section L Maximum paper passing width R Charging width S ₁ , S ₂ Non-paper passing width 1 Transfer belt 2 Guide members 2a, 2b, 2c, 2d Photosensitive drums 3a, 3b, 3c, 3d Chargers 4a, 4b, 4c, 4d Developing devices 6a, 6b, 6c, 6d Transfer chargers 7a, 7b, 7c, 7d Cleaner 8 Charger 9 Separate charger 10 Cleaning blade 11 Polygon mirror 12 Stretch roller 13 Registration roller 14 Drive roller 15 Conveyor belt 16 Fixing device 17 Resistance element 18 Varistor 20 Static elimination roller 21 Conductive wire 22 Guide 23 Sheet metal 24 Tension spring 25 Rib 26 Resistance Layer (Example 3) 27 Resistor sheet (Example 3) 30 Recording paper cassette 81 Discharge wire 82 Metal shield (shield) 83 Insulating resin plate 84 High voltage power supply

Claims (1)

Claims: 1. A charging device for charging a recording material onto which a toner image has been transferred to the same polarity as toner from a toner image surface side, and a charging device disposed at a position opposed to the charging device with the recording material interposed therebetween. An image forming apparatus provided with a conductive member, wherein the conductive member is grounded via a resistive element or a varistor, or the conductive member is grounded and a resistor is provided between the conductive member and the recording material. Image forming apparatus.