JP2015011062A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure excellent primary transferability in primary transfer sections N1a-N1d.SOLUTION: An image forming apparatus includes an intermediate transfer belt 10, a photoreceptor drum 1, charging means 2, exposure means 3, a developing unit 4, a secondary transfer roller 20, and a secondary transfer power source 21. The secondary transfer power source 21 applies a voltage to the secondary transfer roller 20, to generate a potential difference between a surface potential of the photoreceptor drum 1 in a primary transfer section N1 and a surface potential of the intermediate transfer belt 10. A toner image is primarily transferred from the photoreceptor drum 1 to the intermediate transfer belt 10. Multiple pieces of exposure means 3a-3d expose surfaces of the photoreceptor drums 1a-1d at different exposure amounts, on the basis of distances from a secondary transfer section N2 to each of the primary transfer sections N1a-N1d in the photoreceptor drums 1a-1d, on a moving direction of the intermediate transfer belt 10.

Description

  The present invention relates to an image forming apparatus.

2. Description of the Related Art Conventionally, an image forming apparatus configured to use an intermediate transfer member is known as an image forming apparatus such as a copying machine or a laser beam printer. In this image forming apparatus, as a primary transfer process, a voltage is applied to a primary transfer member disposed on a surface of a photosensitive drum so that a toner image formed on the surface of a photosensitive drum as an image carrier is applied to the intermediate transfer body. Transfer on top. This primary transfer process is repeatedly performed for a plurality of color toner images, thereby forming a plurality of color toner images on the surface of the intermediate transfer member. Subsequently, as a secondary transfer process, the toner images of a plurality of colors formed on the surface of the intermediate transfer member are collectively transferred onto the surface of a recording material such as paper by applying a voltage to the secondary transfer member. The batch-transferred toner image is then permanently fixed on a recording material by a fixing unit, thereby forming a color image.
Patent Document 1 discloses an image forming apparatus that can be reduced in cost and size by eliminating a high-voltage power supply dedicated to primary transfer. In the configuration disclosed in Patent Document 1, a belt-like member (hereinafter referred to as an intermediate transfer belt) is used as an intermediate transfer member, and a voltage is applied to a current supply member that contacts the intermediate transfer belt. Then, a current is supplied from the current supply member to the plurality of photosensitive drums via the intermediate transfer belt, whereby the toner image is primarily transferred to the intermediate transfer belt.

JP 2012-98709 A

However, in the configuration in which current is passed in the circumferential direction of the intermediate transfer belt and primary transfer is performed from a plurality of photosensitive drums, the primary transfer potential is set to an appropriate value between the current supply member and the near image formation station and the far image formation station. In some cases, it could not be maintained. If the primary transfer potential cannot be maintained, the necessary toner amount cannot be transferred onto the intermediate transfer belt, which may cause problems such as low density.
Specifically, in a configuration in which primary transfer is performed by passing a current in the circumferential direction of the intermediate transfer belt, the primary transfer voltage at each image forming station (each primary transfer unit) varies depending on the circumferential resistance of the intermediate transfer belt. Resulting in. Therefore, the voltage becomes smaller as the image forming station is farther from the current supply member, and a large potential difference is generated between the upstream image forming station and the downstream image forming station in the moving direction of the intermediate transfer belt. As a result, in each image forming station, it becomes impossible to perform transfer at a voltage appropriate for primary transfer, which may cause transfer failure. This phenomenon occurs remarkably particularly when the resistance of the intermediate transfer belt in the circumferential direction is high because the potential difference between the image forming stations (primary transfer portions) becomes large.

  In view of the above problems, an object of the present invention is to ensure good transferability in each primary transfer portion.

In order to achieve the above object, an image forming apparatus according to the present invention includes:
An endless intermediate transfer belt that circulates,
A plurality of image carriers provided in contact with the outer peripheral surface of the intermediate transfer belt;
A plurality of charging means for charging the surfaces of the plurality of image carriers, respectively.
Exposure means for forming electrostatic latent images by exposing the surfaces of the plurality of image carriers, respectively;
Developing means for developing the electrostatic latent image as a toner image;
A current supply member that contacts the intermediate transfer belt and supplies current to the intermediate transfer belt;
A power source for applying a voltage to the current supply member;
Have
When the power supply applies a voltage to the current supply member, the surface potential of the image carrier and the surface of the intermediate transfer belt in the primary transfer portion which is a contact portion between the image carrier and the intermediate transfer belt In the image forming apparatus for providing a potential difference to the potential and primarily transferring the toner images respectively formed on the surfaces of the plurality of image carriers from the image carrier to the intermediate transfer belt.
The exposure means has a plurality of the plurality of the plurality of the plurality of the plurality of exposure units on the basis of the distances from the contact portion between the current supply member and the intermediate transfer belt to the primary transfer portion of each image carrier, with respect to the moving direction of the intermediate transfer belt. The surface of the image carrier is exposed with different exposure amounts.

An image forming apparatus according to the present invention is
An endless intermediate transfer belt that circulates,
A plurality of image carriers provided in contact with the outer peripheral surface of the intermediate transfer belt;
A plurality of charging means for charging the surfaces of the plurality of image carriers, respectively.
Exposure means for forming electrostatic latent images by exposing the surfaces of the plurality of image carriers, respectively;
Developing means for developing the electrostatic latent image as a toner image;
A current supply member that contacts the intermediate transfer belt and supplies current to the intermediate transfer belt;
A power source for applying a voltage to the current supply member;
Have
When the power supply applies a voltage to the current supply member, the surface potential of the image carrier and the surface of the intermediate transfer belt in the primary transfer portion which is a contact portion between the image carrier and the intermediate transfer belt In the image forming apparatus for providing a potential difference to the potential and primarily transferring the toner images respectively formed on the surfaces of the plurality of image carriers from the image carrier to the intermediate transfer belt.
The plurality of charging units respectively apply the surfaces of the plurality of image carriers based on respective distances from a contact portion between the current supply member and the intermediate transfer belt to the primary transfer portion of each image carrier. It is characterized by charging with different charge amounts.

  According to the present invention, good transferability can be ensured in each primary transfer portion.

1 is a schematic sectional view of an image forming apparatus according to a first embodiment. The figure explaining the structure which stretches an intermediate transfer belt The figure explaining the measurement of the resistance value of an intermediate transfer belt The figure for demonstrating the detail of the structure of the intermediate transfer belt vicinity of Example 1. FIG. FIG. 6 is a diagram for explaining control of weak exposure in the first embodiment. Circuit diagram for calculating primary transfer potential Schematic sectional view of an image forming apparatus according to a modification Schematic sectional view of an image forming apparatus according to Embodiment 2

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be exemplarily described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions. Therefore, unless specifically stated otherwise, the scope of the present invention is not intended to be limited thereto.

Example 1
First, the configuration and operation of a color image forming apparatus according to the first embodiment (hereinafter simply referred to as an image forming apparatus) will be described with reference to FIG. FIG. 1 is a schematic cross-sectional view of the image forming apparatus according to the first embodiment. The image forming apparatus according to the first embodiment is a so-called tandem type printer including image forming stations a to d. The first image forming station a is yellow (Y), the second image forming station b is magenta (M), the third image forming station c is cyan (C), and the fourth image forming station d is black (Bk). ) For each color. Since the configuration of each image forming station is the same except for the color of the toner to be accommodated, description will be given below using the first image forming station a, and description of the other image forming stations will be omitted.

  The first image forming station a includes an electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 1a as a drum-shaped image carrier, a charging roller 2a as a charging unit, a developing device 4a as a developing unit, and a cleaning unit. And a device 5a. The photosensitive drum 1a is provided so as to be rotationally driven at a predetermined peripheral speed (process speed) in the direction of arrow A in FIG.

  The developing device 4a that stores yellow (Y) toner (developer) is a device for developing yellow toner on the photosensitive drum 1a. The cleaning device 5a includes a cleaning blade 51a as a cleaning member that comes into contact with the photosensitive drum 1a, and a waste toner box 52a that stores toner collected by the cleaning blade 51a.

  An image forming operation is started when a control unit (not shown) such as a controller receives an image signal, and the photosensitive drum 1a is rotationally driven. In the rotating process, the photosensitive drum 1a is uniformly charged to a predetermined potential with a predetermined polarity (negative polarity in this embodiment) by the charging roller 2a, and is subjected to exposure according to the image signal by the exposure means 3a. Thereby, an electrostatic latent image corresponding to the yellow component image of the target color image is formed.

  Next, the electrostatic latent image is developed at the developing position by the developing device (yellow developing device) 4a and visualized as a yellow toner image. Here, in this embodiment, the normal charging polarity of the toner stored in the developing device 4a is negative. In this embodiment, the electrostatic latent image is reversely developed with toner charged to the same polarity as the charging polarity of the photosensitive drum 1 by the charging roller 2. However, the present invention is not limited to this, and can be applied to an electrophotographic apparatus in which an electrostatic latent image is positively developed with toner charged to a polarity opposite to the charged polarity of the photosensitive drum 1.

  The intermediate transfer belt 10 is stretched by a plurality of stretching rollers 11, 12, and 13 and is moved in the same direction (in the direction of arrow B in FIG. 1) in the primary transfer portion N 1 a that is in contact with the photosensitive drum 1 a. It is provided so as to be able to circulate (rotatably drive) at the same peripheral speed as the drum 1a. The yellow toner image formed on the photosensitive drum 1a is transferred onto the intermediate transfer belt 10 in the process of passing through a contact portion (hereinafter referred to as a primary transfer portion) N1a between the photosensitive drum 1a and the intermediate transfer belt 10. (Primary transfer). In the present embodiment, during primary transfer, a current is passed in the circumferential direction of the intermediate transfer belt 10 from the secondary transfer roller 20 that is in contact with the outer peripheral surface of the intermediate transfer belt 10, and the primary transfer portions N1a to N1d of the intermediate transfer belt 10 perform primary transfer. A transfer potential is formed.

The primary transfer residual toner remaining on the surface of the photosensitive drum 1a after the primary transfer is cleaned and removed by the cleaning device 5a, and then subjected to an image forming process below charging. Similarly, the second, third, and fourth image forming stations b, c, and d form a second color magenta toner image, a third color cyan toner image, and a fourth color black toner image. Then, the toner images are sequentially transferred onto the intermediate transfer belt 10 and a combined color image corresponding to the target color image is obtained.

The four color toner images primarily transferred onto the intermediate transfer belt 10 pass through the secondary transfer portion N2 formed by the intermediate transfer belt 10 and the secondary transfer roller 20 as a secondary transfer member, and in the process of feeding, 50 is collectively transferred onto the surface of the recording material P as a transfer material fed by 50 (secondary transfer). The secondary transfer roller 20 is a nickel-plated steel rod having an outer diameter of 8 mm, covered with a foamed sponge body mainly composed of nitrile butadiene rubber (NBR) and epichlorohydrin rubber adjusted to a volume resistance of 10 ⁸ Ω · cm and a thickness of 5 mm. The one with a diameter of 18 mm is used.

  The secondary transfer roller 20 abuts on the outer peripheral surface of the intermediate transfer belt 10 with a pressure of 50 N to form a secondary transfer portion N2. The secondary transfer roller 20 is provided so as to rotate in the direction of arrow C in FIG. 1, that is, to rotate following the intermediate transfer belt 10. The secondary transfer roller 20 is applied with a voltage of 2500 [V] from the secondary transfer power source (power source) 21 when the toner on the intermediate transfer belt 10 is secondarily transferred to the recording material P such as paper. ing.

  The secondary transfer power source 21 is connected to the secondary transfer roller 20 and supplies a secondary transfer voltage output from a transformer (not shown) to the secondary transfer roller 20. The secondary transfer voltage is fed back by a CPU (not shown) that is a control IC of the image forming apparatus by feeding back a difference between a preset control voltage and a monitor voltage that is an actual output value to the transformer. Is controlled to be substantially constant. The secondary transfer power source 21 can output in the range of 100 [V] to 4000 [V].

  Thereafter, the recording material P carrying the four-color toner images is introduced into the fixing device 30 where the four-color toners are melted and mixed and fixed to the recording material P by being heated and pressurized. The toner remaining on the intermediate transfer belt 10 after the secondary transfer is cleaned and removed by a cleaning device 16 provided in contact with the intermediate transfer belt 10. With the above operation, a full-color print image is formed.

  Next, referring to FIG. 1 and FIG. 2, the intermediate transfer belt 10 and a stretch member 11 as a support member, which are necessary for forming a primary transfer potential in each of the primary transfer portions N1a to N1d, 12, 13, the metal roller 14, and the voltage maintaining element 15 will be described. FIG. 2 is a diagram illustrating a configuration in which the intermediate transfer belt is stretched. FIG. 2A is a diagram illustrating the arrangement of the metal rollers, and FIG. 2B is a diagram illustrating a state where the intermediate transfer belt is stretched.

  An intermediate transfer belt 10 is disposed as an intermediate transfer member at a position facing each of the image forming stations a, b, c, and d (respective photosensitive drums 1a, 1b, 1c, and 1d). The intermediate transfer belt 10 is an endless belt in which a conductive agent is added to a resin material to impart conductivity. The tension roller 12 is stretched (supported) by three axes of a driving roller 11, a tension roller 12, and a secondary transfer counter roller (secondary transfer counter member) 13, and the tension roller 12 is tensioned with a total pressure of 60N. It is built.

  The intermediate transfer belt 10 is driven by a driving roller 11 that is rotated by a driving source (not shown) in a direction in which the intermediate transfer belt 10 moves in the same direction at the primary transfer portion N1 in contact with the photosensitive drums 1a, 1b, 1c, and 1d. 1c and 1d are rotated at substantially the same peripheral speed. In this embodiment, a primary transfer surface on which a toner image is primarily transferred from the photosensitive drums 1 a, 1 b, 1 c, 1 d is formed by two stretching rollers, a secondary transfer counter roller 13 and a driving roller 11.

  As shown in FIG. 2A, in the moving direction of the intermediate transfer belt 10, a metal roller that is a contact member that contacts the inner peripheral surface of the intermediate transfer belt 10 is positioned between the photosensitive drum 1b and the photosensitive drum 1c. 14 is arranged. The metal roller 14 is disposed at an intermediate position between the second image forming station b and the third image forming station c. The metal roller 14 is lifted with respect to the horizontal plane formed by the photosensitive drums 1b and 1c and the intermediate transfer belt 10 so as to ensure the amount of winding of the intermediate transfer belt 10 around the photosensitive drums 1b and 1c. Hold both ends.

  The metal roller 14 is formed of a straight nickel-plated SUS (stainless steel) round bar having an outer diameter of 6 mm, and is rotated following the rotation of the intermediate transfer belt 10. The metal roller 14 is in contact with a predetermined region in the longitudinal direction perpendicular to the moving direction of the intermediate transfer belt 10.

  2A, the distance between the photosensitive drums 1b and 1c is W, the distance between the photosensitive drums 1b and 1c and the metal roller 14 is T, and the lifting height of the metal roller 14 with respect to the intermediate transfer belt 10 is as shown in FIG. Is defined as H1. The distance W and the distance T are distances between adjacent axis centers in the moving direction of the intermediate transfer belt. In Example 1, W = 50 mm, T = 25 mm, and H1 = 2 mm.

  Further, in Example 1, in order to secure the amount of winding of the intermediate transfer belt 10 around the photosensitive drums 1a and 1d, as shown in FIG. It is lifted from the horizontal plane formed by 1d and the intermediate transfer belt 10. By securing the amount of winding of the intermediate transfer belt 10 around the photosensitive drums 1a and 1d, there is an effect of suppressing transfer defects caused by unstable contact between the photosensitive drums 1a and 1d and the intermediate transfer belt 10. .

  Further, as shown in FIG. 2B, the distance between the secondary transfer counter roller 13 and the photosensitive drum 1a is D1, and the distance between the driving roller 11 and the photosensitive drum 1d is D2. Further, the lifting height of the secondary transfer counter roller 13 with respect to the intermediate transfer belt 10 is H2, and the lifting height of the stretching roller 11 is H3. At this time, in Example 1, D1 = D2 = 50 mm and H2 = H3 = 2 mm.

  As the material of the intermediate transfer belt 10 of Example 1, an endless polyimide resin having a circumferential length of 700 mm and a thickness of 90 μm mixed with carbon as a conductive agent was used. The intermediate transfer belt 10 used in Example 1 is characterized by an electronic conductivity characteristic as an electrical characteristic and a small resistance value variation with respect to temperature and humidity in the atmosphere. In Example 1, polyimide resin is used as the material of the intermediate transfer belt 10, but other materials may be used as long as they are thermoplastic resins. For example, a material such as polyester, polycarbonate, polyarylate, acrylonitrile-butadiene-styrene copolymer (ABS), polyphenylene sulfide (PPS), polyvinylidene fluoride (PVdF), or a mixed resin thereof may be used. In addition to carbon, conductive metal oxide fine particles can be used as the conductive agent.

The intermediate transfer belt 10 of Example 1 has a volume resistivity of 1 × 10 ⁹ Ω · cm. The volume resistivity is measured by using a ring probe type UR (model MCP-HTP12) on a Hiresta-UP (MCP-HT450) manufactured by Mitsubishi Chemical Corporation. The measurement conditions are such that the room temperature is set to 23 ° C., the room humidity is set to 50%, the applied voltage is 100 [V], and the measurement time is 10 seconds. In Example 1, the volume resistivity of the intermediate transfer belt 10 can be in the range of 1 × 10 ⁷ to 10 ¹⁰ Ω · cm.

  Here, the volume resistivity is a measure of conductivity as a material of the intermediate transfer belt, and is a belt (conductive belt) that can actually form a desired primary transfer potential by flowing current in the circumferential direction. Whether the resistance in the circumferential direction is important or not is important.

  The circumferential resistance of the intermediate transfer belt 10 of Example 1 was measured using a circumferential resistance measuring jig shown in FIG. FIG. 3 is a diagram illustrating measurement of the resistance value of the intermediate transfer belt. FIG. 3A is a diagram illustrating a resistance measurement jig, and FIG. 3A is a diagram illustrating an equivalent circuit of the measurement system illustrated in FIG.

  First, the configuration of the measurement system will be described. The intermediate transfer belt 10 to be measured is stretched between the inner roller 101 and the driving roller 102 so that there is no slack. The inner roller 101 made of metal is connected to a high-voltage power source (high-voltage power source: Model_610E manufactured by TREK) 103, and the drive roller 102 is grounded. The surface of the driving roller 102 is covered with a conductive rubber having a sufficiently low resistance with respect to the intermediate transfer belt 10 and rotates so that the intermediate transfer belt 10 becomes 100 mm / sec.

Next, a measurement method will be described. A constant current IL is applied to the inner roller 101 while the intermediate transfer belt 10 is rotated at 100 mm / sec by the driving roller 102, and the voltage [V] L is monitored by the high-voltage power source 103 connected to the inner roller 101. Assuming that the measurement system shown in FIG. 3A is the equivalent circuit shown in FIG. 3B, an intermediate transfer belt having a distance L (300 mm in this embodiment) between the inner roller 101 and the drive roller 102 is shown. 10 circumferential resistances RL = 2 [V] L / IL. The resistance in the circumferential direction is obtained by converting this resistance RL into an intermediate transfer belt circumferential length equivalent to 100 mm of the intermediate transfer belt 10. In order to pass a current from the current supply member to the photosensitive drum 1 through the intermediate transfer belt 10, the circumferential resistance is preferably 1 × 10 ⁸ Ω or less.

In the configuration of Example 1, the intermediate transfer belt 10 having a resistance value in the circumferential direction of 4 × 10 ⁶ Ω obtained by the measurement method described above is used. The intermediate transfer belt 10 of this example was measured with a constant current of IL = 5 μA, and the monitor voltage [VL] at that time was 30 [V]. The monitor voltage [VL] is obtained in one section of the intermediate transfer belt 10 and is obtained from the average value of the section measurement values. Moreover, regarding RL, since RL = 2 [V] L / IL, RL = 2 × 30 / (5 × 10 ⁻⁶ ) = 1.2 × 10 ⁷ Ω, and when this is converted to 100 mm equivalent, The resistance value in the circumferential direction is 4 × 10 ⁶ Ω. In this embodiment, a conductive belt capable of flowing current in the circumferential direction is used as the intermediate transfer belt 10.

  As shown in FIG. 1, in Example 1, the secondary transfer counter roller (connected member) 13 that forms the primary transfer surface of the intermediate transfer belt 10 is connected via a voltage maintaining element (constant voltage element) 15. Grounded. The voltage maintaining element 15 is an element that maintains the potential of the secondary transfer counter roller 13 at a predetermined potential when a current flows from the secondary transfer roller 20 to the voltage maintaining element 15 via the intermediate transfer belt 10. Thus, by maintaining the potential of the secondary transfer counter roller 13 at a predetermined potential, the surface potential of the intermediate transfer belt 10 in the primary transfer portion N1 can be maintained at a predetermined potential or higher.

  In the first embodiment, the potential is set so that the primary transfer potential can be maintained so that desired transfer efficiency can be obtained in each of the primary transfer portions N1a to N1d. In the first embodiment, a Zener diode 15 that is a constant voltage element is used as the voltage maintaining element 15. A voltage maintained on the cathode side when a current exceeding a certain value flows through the Zener diode 15 is defined as a Zener voltage.

  When the Zener diode 15 is used as the voltage maintaining element 15, the Zener voltage of the Zener diode may be set so as to maintain a predetermined potential. In this embodiment, the Zener voltage is set to 300 [V] in order to obtain a desired transfer efficiency.

  Next, a method for forming a primary transfer potential for performing primary transfer will be described in detail. In the configuration of the first embodiment, a secondary transfer power source 21 that applies a voltage to the secondary transfer roller 20 is used as a power source for performing primary transfer. That is, the secondary transfer power source 21 is a common transfer power source for primary transfer and secondary transfer. The secondary transfer roller 20 as a current supply member that contacts the intermediate transfer belt 10 and the primary transfer portion N1 of the intermediate transfer belt 10 are used. This is a power supply that allows current to flow through.

  As described above, the voltage maintaining element 15 is connected to the secondary transfer counter roller 13 that stretches the intermediate transfer belt 10, and the secondary transfer counter roller 13 is connected from the secondary transfer power source 21 via the intermediate transfer belt 10. The primary transfer is performed by passing an electric current toward the roller 13. At this time, the secondary transfer counter roller 13 has a potential corresponding to the voltage maintaining element 15, and this potential is used as a starting point so that a current flows in the circumferential direction of the intermediate transfer belt 10, so that each primary transfer portion N <b> 1 a to N <b> 1. A primary transfer potential is formed at N1d. The toner on the photosensitive drums 1a, 1b, 1c, and 1d is moved onto the intermediate transfer belt 10 by the potential difference between the primary transfer potential, which is the surface potential of the intermediate transfer belt 10, and the surface potential of the photosensitive drum. It is carried out.

  Next, a method of adjusting the surface potentials of the photosensitive drums 1a to 1d with respect to fluctuations in the primary transfer potentials in the primary transfer portions N1a to N1d, which is a feature of this embodiment, will be described with reference to FIG. FIG. 4 is a diagram for explaining details of the configuration in the vicinity of the intermediate transfer belt according to the first exemplary embodiment.

  In the configuration of this embodiment, during primary transfer, current flows from the intermediate transfer belt 10 to the photosensitive drums 1a, 1b, 1c, and 1d in the primary transfer portions N1a, N1b, N1c, and N1d. Therefore, the voltage drops due to the resistance in the circumferential direction of the intermediate transfer belt 10, and the primary transfer potential in the downstream side in the circulation movement direction (arrow B direction) of the intermediate transfer belt 10 is lower than that in the upstream side. The resistance of the intermediate transfer belt 10 changes in proportion to the distance from the secondary transfer portion N2 that is the contact portion between the secondary transfer roller 20 and the intermediate transfer belt 10 to the primary transfer portions N1a, N1b, N1c, and N1d. .

As shown in FIG. 4, in the configuration of the first embodiment, the distances between the primary transfer portions N1 are D1 = D3 = D4 = D5 = 50 mm, and the distances between the respective portions are equal. For this reason, the resistance value of the intermediate transfer belt 10 in each part is 4 × 10 ⁶ Ω in terms of 100 mm, and thus 2 × 10 ⁶ Ω (2 MΩ) in terms of 50 mm.

  As a result, a current of 5 μA flows as an optimum current for transfer at each image forming station, so that the potential that has become 300 V by the zener diode 15 at the secondary transfer counter roller 13 becomes 260 V at the first image forming station a. . Further, the voltage drop is 230 V in the second image forming station b, 210 V in the third image forming station c, and 200 V in the fourth image forming station d. That is, from the secondary transfer counter roller 13 (secondary transfer unit N2) that supplies current to the primary transfer unit N1, each image forming station a, b, c, d (each primary transfer unit N1a, N1b, N1c, The voltage drops according to the distance (power supply distance) to N1d).

Next, a potential control method for the photosensitive drums 1a, 1b, 1c, and 1d will be described with reference to FIG. FIG. 5 is a diagram for explaining the control of weak exposure in the first embodiment. In the first embodiment, as a method for controlling the potential of each photosensitive drum 1, when an electrostatic latent image corresponding to an image signal is formed by the exposure means 3a, 3b, 3c, 3d, the surface of each photosensitive drum 1 is not exposed by weak exposure. The surface potential of the photosensitive drum is controlled by uniformly exposing the image area. In this embodiment, 4
Although a plurality of exposure units 3a to 3d are provided corresponding to one photosensitive drum 1a to 1d, a configuration in which one exposure unit 3 exposes four photosensitive drums 1a to 1d may be used.

  Here, the weak exposure of the non-image area will be described with reference to FIG. 5, taking the exposure means 3a of the first image forming station a as an example. An image signal 60 sent from a controller (not shown) is a multi-value signal (0 to 255) having a depth direction of 8 bits = 256 gradations, and when this signal is 0, the laser beam is turned off. When it is completely on, between 1 and 254, it is assumed that it has a value between the two for a while.

  Here, the non-image area exposure level can be arbitrarily set according to the level of the multilevel signal. In the following description, it is assumed that non-image portion exposure is performed using 32 as the level of the multilevel signal. A non-image portion in which the image signal sent from a controller (not shown) is 0 is converted to 32 by the image signal conversion circuit 68a, and if the image signal has a value of 1 to 255, it is compressed and converted from 33 to 255. ing. Thereafter, the signal is converted into a serial signal in the time axis direction by the frequency modulation circuit 61a, and in this embodiment is used for pulse width modulation of each dot pulse having a resolution of 600 dots / inch.

  By this signal, the laser driver 62a is driven, the laser diode 63a emits light, and the laser light 6a is emitted. The laser beam 6a is irradiated to the photosensitive drum 1a as scanning light through a correction optical system 67a including a polygon mirror 64a, a lens 65a, and a folding mirror 66a. The frequency modulation circuit 61a may be provided on the controller side apart from the laser driver 62a.

  In the first embodiment, the image signal conversion circuit 68a performs compression conversion to 32 when the image signal from a controller (not shown) is 0, and 33 to 255 when the image signal is 1 to 255. It is not something. For example, all the image signals 0 to 32 from the controller (not shown) may be converted to 32, and the image signals 33 to 255 may be left as they are. Alternatively, signal conversion may be performed such that 32 is added to all image signals from a controller (not shown), and all signals exceeding 255 are pasted as 255.

  By performing the above-described method, in Example 1, the charged potential of the non-image portion is decreased from the charged potential before exposure of the non-image portion = −600 V, and the potential of the photosensitive drum 1a is decreased to −500 V by non-image portion exposure. be able to. On the other hand, the exposure potential of the image portion is from −600 V to −150 V due to full light emission of the laser beam 6a. In the first embodiment, the exposure amount for non-image area exposure is changed according to the primary transfer potential at each of the image forming stations a, b, c, and d. Specifically, when the primary transfer potential is high, the laser light amount for non-image area exposure is set high, and when the primary transfer potential is low, the laser light amount for non-image area exposure is set low.

  That is, in the first image forming station a, the surface potential (charging potential) of the photosensitive drum 1a is controlled by −500 V by emitting a small amount of laser light at a ratio of 32/255. Similarly, for the second image forming station b, the third image forming station c, and the fourth image forming station d, the surface potential is controlled according to the surface potential of the intermediate transfer belt 10 in the primary transfer portion N1. That is, in the second image forming station b, the surface potential is controlled to −530 V by emitting a small amount of laser light at a ratio of 26/255. In the third image forming station “b”, the surface potential is controlled to −550 V by minutely emitting laser light at a ratio of 20/255. In the fourth image forming station d, the surface potential is controlled to −560 V by emitting a small amount of laser light at a ratio of 16/255. Accordingly, the surface potentials of the photosensitive drums 1a, 1b, 1c, and 1d can be controlled in accordance with the surface potential value of the intermediate transfer belt 10.

  In the first embodiment, as a method for performing non-image area exposure, a method in which pulse width modulation is performed according to an image signal has been described. However, the present invention is not limited to this. For example, the same effect can be obtained even if an analog non-image portion exposure method in which a laser diode is controlled to emit light by controlling a laser driver so as to be driven by a minute current.

  Next, the operation of the first embodiment will be described. In the configuration of the first embodiment, non-image portions on the surfaces of the photosensitive drums 1a, 1b, 1c, and 1d by weak exposure with respect to fluctuations in the primary transfer potential in the primary transfer portions N1a to N1d due to the resistance (power supply distance) of the intermediate transfer belt. The charging potential is controlled by uniformly exposing the area. As a result, the optimum primary transfer contrast (potential difference) can be maintained in each of the image forming stations a, b, c, and d, so that good primary transfer properties can be secured.

  Details will be described below. Table 1 shows primary transfer potentials at the respective image forming stations a, b, c, and d, surface potentials of the respective photosensitive drums 1a, 1b, 1c, and 1d, and primary transfer contrasts in this embodiment.

  The primary transfer potential at each of the image forming stations a, b, c, and d will be described with reference to FIG. FIG. 6 shows a circuit diagram for calculating the primary transfer potential. The current supplied from the secondary transfer power supply 21 causes the Zener diode 15 current to flow through the secondary transfer counter roller 13, so that the secondary transfer counter roller portion potential Vt becomes 300V.

  Then, with this potential as a starting point, a current flows in the circumferential direction of the intermediate transfer belt 10, whereby the potential drops at each image forming station according to the resistance of each part. Since the current flowing through each image forming station is 5 μA, Ia = 5 μA × 4 = 20 μA flows between the secondary transfer counter roller 13 and the image forming station a. Similarly, between the image forming station a and the image forming station b, Ib = 5 μA × 3 = 15 μA. Similarly, Ic = 5 μA × 2 = 10 μA between the image forming station b and the image forming station c, and Id = 5 μA between the image forming station c and the image forming station d.

  Here, since the resistance value of each part is Ra = Rb = Rc = Rd = 2 MΩ in terms of the distance between stations (between each primary transfer part), the primary transfer potential at the image forming station a is 300V-20 μA. × 2MΩ = 260V. Similarly, when the primary transfer potential is calculated for the image forming stations b, c, and d, they are 230V, 210V, and 200V. Regarding the surface potentials (charging potentials) of the photosensitive drums 1a, 1b, 1c, and 1d, V1a = −500, V1b by controlling the weak exposure amount of the non-image area in accordance with the fluctuation of the primary transfer potential as described above. = −530, V1c = −550, and V1d = −560. As a result, the primary transfer contrast can be maintained at 860 V, which is the same value in each of the image forming stations a, b, c, and d, and good primary transfer properties can be secured.

  In the configuration of this embodiment, even if the potential of the photosensitive drum 1 fluctuates due to changes in the usage environment such as changes in temperature and humidity and the film thickness of the photosensitive drum, the weak exposure of the non-image portion is performed according to the usage environment. By controlling the amount, the potential of the photosensitive drum 1 can be stabilized.

  As described above, in the first embodiment, a voltage is applied from the secondary transfer power source 21 to the secondary transfer roller 20, and the respective photosensitive drums 1 are applied to the primary transfer portions N 1 a to N 1 d via the intermediate transfer belt 10. The configuration is such that primary transfer is performed by passing an electric current. Then, the exposure unit 3 performs photosensitivity so as to control the surface potential of each of the photosensitive drums 1a to 1d based on the distance from the secondary transfer unit N2 to each of the primary transfer units N1a to N1d with respect to the moving direction of the intermediate transfer belt 10. The surface of the drums 1a to 1d is exposed. That is, the exposure amount for each non-image portion of the photosensitive drums 1a to 1d is changed based on the distance from the secondary transfer portion N2 to the primary transfer portions N1a to N1d in the photosensitive drums 1a to 1d. ing. Specifically, the surfaces of the photosensitive drums 1a to 1d are exposed with different exposure amounts so that the primary transfer contrasts in the image forming stations a to d are the same. By adopting such a configuration, the image forming apparatus according to Embodiment 1 can ensure good transferability in each of the primary transfer portions N1a to N1d.

  In Example 1, although SUS plated with nickel is used as the material of the metal roller 14, it is not limited to this, and other metals such as aluminum and iron, or a conductive resin roller, Similar effects can be obtained.

  In the first embodiment, although the Zener diode 15 is used as the voltage stabilizing element, another voltage stabilizing element may be used as long as the element can obtain the same effect, and an element such as a varistor may be used. .

  In the first embodiment, the Zener diode 15 is connected only to the secondary transfer counter roller 13 as a connection configuration. However, the configuration is not limited to this configuration. For example, FIG. 7 shows a configuration of an image forming apparatus according to a modification. As shown in FIG. 7, together with the secondary transfer counter roller 13, the metal roller 14 disposed between the second image forming station b and the third image forming station c, and the driving roller 11 through the zener diode 15. It may be configured to be grounded. With such a configuration, current can be supplied from the vicinity of the image forming stations b, c, and d, so that fluctuations in the primary transfer potential can be further suppressed.

(Example 2)
Next, Example 2 will be described with reference to FIG. FIG. 8 is a schematic cross-sectional view of the image forming apparatus according to the second embodiment. In the configuration of the image forming apparatus applied in the second embodiment, the same members as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted. In the first embodiment, in order to maintain the optimum primary transfer contrast, the surface potential of each of the photosensitive drums 1a, 1b, 1c, and 1d is controlled by controlling the weak exposure amount of the non-image portion with respect to the fluctuation of the primary transfer potential. Controlled. On the other hand, in the second embodiment, the surface potential of each of the photosensitive drums 1a, 1b, 1c, and 1d is controlled by changing the charging voltage (charge amount) of the charging rollers 2a, 2b, 2c, and 2d. Features.

The charging rollers 2a, 2b, 2c, and 2d used in Example 2 use a nitrile butadiene rubber (NBR) having a thickness of 3 mm as an elastic layer and a polyurethane resin having a thickness of 10 μm as a surface layer on a nickel-plated steel rod having an outer diameter of 6 mm. ing. The volume resistivity of the elastic layer is adjusted to 10 ⁴ Ω · cm, and the volume resistivity of the surface layer is adjusted to 10 ¹¹ Ω · cm. Further, the charging roller 2a is brought into contact with the photosensitive drum 1a with a pressure of 6 N and is driven to rotate with respect to the photosensitive drum 1a.

The charging rollers 2a, 2b, 2c, and 2d include charging high-voltage power supplies 22a, 22b, 22c, and 22
A DC voltage is applied by d. For example, when the charging voltage is controlled to be −1100 V, the surface potential Vd of the photosensitive drums 1 a, 1 b, 1 c, and 1 d is charged approximately constant by the charging rollers 2 a, 2 b, 2 c, and 2 d to −600 V. .

  As in the first embodiment, the primary transfer potential is 260 V in the first image forming station a, 230 V in the second image forming station b, 210 V in the third image forming station c, and 210 V in the fourth image forming station d. 200V.

  Next, a method for controlling the charging potential of the photosensitive drums 1a, 1b, 1c, and 1d will be described. In this embodiment, as a method for controlling the surface potential, the surface potential of each of the photosensitive drums 1a, 1b, 1c, and 1d is controlled by changing the charging voltage of the charging rollers 2a, 2b, 2c, and 2d.

  Specifically, when the primary transfer potential is high, the charging voltage value is set low, and when the primary transfer potential is low, the charging voltage value is set high. That is, in the first image forming station a, the surface voltage (charging potential) of the photosensitive drum 1a is controlled by -500V by setting the charging voltage to -1000V. Similarly, for the second image forming station b, the third image forming station c, and the fourth image forming station d, the charging voltage is controlled according to the surface potential of the intermediate transfer belt 10 in the primary transfer portion N1. . In the second image forming station b, the surface potential is controlled to −530 V by setting the charging voltage to −1030 V. In the third image forming station b, the surface potential is controlled to −550 V by setting the charging voltage to −1050 V. In the fourth image forming station d, the surface potential is controlled to −560 V by setting the charging voltage to −1060 V. As a result, the potentials of the photosensitive drums 1a, 1b, 1c, and 1d can be controlled according to the value of the primary transfer potential. Since other image forming apparatus configurations and operations of the primary transfer unit are the same as those in the first exemplary embodiment, description thereof is omitted.

  As described above, in the configuration of the second embodiment, the charging voltage (charge amount) is controlled in accordance with the fluctuation of the primary transfer potential in each image forming station a, b, c, d due to the resistance (power feeding distance) of the intermediate transfer belt 10. doing. By controlling the charging voltage (charging amount) in this way, the surface potential (charging potential) of the photosensitive drums 1a, 1b, 1c, and 1d is controlled. As a result, as in the first embodiment, the optimum primary transfer contrast can be maintained in each of the image forming stations a, b, c, and d, so that a good primary transfer property can be ensured.

  DESCRIPTION OF SYMBOLS 1 ... Photosensitive drum (image carrier), 2 ... Charging roller (charging means), 3 ... Exposure means, 4 ... Developing device (developing means), 10 ... Intermediate transfer belt, 20 ... Secondary transfer roller (Current supply member, Secondary transfer member), 21 ... Secondary transfer power source (power source)

Claims (11)

An endless intermediate transfer belt that circulates,
A plurality of image carriers provided in contact with the outer peripheral surface of the intermediate transfer belt;
A plurality of charging means for charging the surfaces of the plurality of image carriers, respectively.
Exposure means for forming electrostatic latent images by exposing the surfaces of the plurality of image carriers, respectively;
Developing means for developing the electrostatic latent image as a toner image;
A current supply member that contacts the intermediate transfer belt and supplies current to the intermediate transfer belt;
A power source for applying a voltage to the current supply member;
Have
When the power supply applies a voltage to the current supply member, the surface potential of the image carrier and the surface of the intermediate transfer belt in the primary transfer portion which is a contact portion between the image carrier and the intermediate transfer belt In the image forming apparatus for providing a potential difference to the potential and primarily transferring the toner images respectively formed on the surfaces of the plurality of image carriers from the image carrier to the intermediate transfer belt.
The exposure means has a plurality of the plurality of the plurality of the plurality of the plurality of exposure units on the basis of the distances from the contact portion between the current supply member and the intermediate transfer belt to the primary transfer portion of each image carrier, with respect to the moving direction of the intermediate transfer belt. An image forming apparatus that exposes the surface of an image carrier with different exposure amounts.   2. The exposure unit according to claim 1, wherein the exposure unit exposes the surfaces of the plurality of image carriers with different exposure amounts so that the potential differences in the primary transfer portions of the image carriers are all the same. The image forming apparatus described. The exposure means changes an exposure amount for each non-image portion of the plurality of image carriers based on a distance from the contact portion to the primary transfer portion of each image carrier. The image forming apparatus according to claim 1 or 2. An endless intermediate transfer belt that circulates,
A plurality of image carriers provided in contact with the outer peripheral surface of the intermediate transfer belt;
A plurality of charging means for charging the surfaces of the plurality of image carriers, respectively.
Exposure means for forming electrostatic latent images by exposing the surfaces of the plurality of image carriers, respectively;
Developing means for developing the electrostatic latent image as a toner image;
A current supply member that contacts the intermediate transfer belt and supplies current to the intermediate transfer belt;
A power source for applying a voltage to the current supply member;
Have
When the power supply applies a voltage to the current supply member, the surface potential of the image carrier and the surface of the intermediate transfer belt in the primary transfer portion which is a contact portion between the image carrier and the intermediate transfer belt In the image forming apparatus for providing a potential difference to the potential and primarily transferring the toner images respectively formed on the surfaces of the plurality of image carriers from the image carrier to the intermediate transfer belt.
The plurality of charging units respectively apply the surfaces of the plurality of image carriers based on respective distances from a contact portion between the current supply member and the intermediate transfer belt to the primary transfer portion of each image carrier. An image forming apparatus that is charged with different charge amounts.   The plurality of charging units respectively charge the surfaces of the plurality of image carriers with different charge amounts so that the potential differences in the primary transfer portions of the image carriers are all the same. 5. The image forming apparatus according to 4. The current supply member is a secondary transfer member that secondarily transfers a toner image primarily transferred to the intermediate transfer belt to a transfer material in a secondary transfer portion as a contact portion with the intermediate transfer belt. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. A support member for supporting the intermediate transfer belt by contacting the inner peripheral surface of the intermediate transfer belt;
The image forming apparatus according to claim 6, wherein a constant voltage element is connected to the support member. The intermediate transfer belt is supported by a plurality of the support members so as to be able to circulate,
The image forming apparatus according to claim 7, wherein the constant voltage element is connected to one of the plurality of support members.   The image forming apparatus according to claim 7, wherein the support member is a secondary transfer facing member provided to face the secondary transfer member via the intermediate transfer belt.   The image forming apparatus according to claim 7, wherein the constant voltage element is a Zener diode.   The image forming apparatus according to claim 7, wherein the constant voltage element is a varistor.

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