JP2018173592A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation device capable of excellent transfer even if difference in resistance occurs among a plurality of transfer members in a configuration in which power supplies for applying voltage to the transfer members are made common.SOLUTION: An image formation device 100 includes: a plurality of image carriers 1 for carrying a toner image; an intermediate transfer body 10 to which the toner image is transferred from the image carriers; a plurality of transfer members 6 which are provided correspondingly to the respective image carriers, to which voltage is applied, and thereby which transfer the toner image from the image carriers to the intermediate transfer body; a common power supply 8 for applying voltage to the transfer members; and a plurality of resistance elements 7 connected between the respective transfer members and the power supply.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile apparatus using an electrophotographic system or an electrostatic recording system.

  Conventionally, as an image forming apparatus using an electrophotographic method or an electrostatic recording method, after a toner image is primarily transferred from a plurality of image carriers to an intermediate transfer member, the toner image is secondarily transferred from the intermediate transfer member to a recording material. There is a tandem type image forming apparatus that employs an intermediate transfer method to be transferred.

  In the image forming apparatus as described above, generally, primary transfer of a toner image from each image carrier to an intermediate transfer member is performed by applying a voltage to a primary transfer member provided corresponding to each image carrier. Done. Further, the image forming apparatus as described above is often configured such that a voltage is applied to a plurality of primary transfer members from power sources provided independently of each other.

  On the other hand, there is a configuration in which a power source for applying a voltage to a plurality of primary transfer members is shared in order to reduce the size and cost of the image forming apparatus (Patent Document 1).

JP 2014-115484 A

  However, in a tandem-type image forming apparatus that employs an intermediate transfer method, the electrical resistance of a specific primary transfer member (herein simply referred to as “resistance”) may be higher than that of other primary transfer members. At this time, if a power source for applying a voltage to a plurality of primary transfer members is shared, the specified path passes through a current path having a relatively low resistance among the current paths from the power source to each image carrier. A relatively large amount of current flows into the primary transfer member other than the primary transfer member. As a result, good primary transfer may not be performed.

  Accordingly, an object of the present invention is to provide an image capable of performing good transfer even when a difference in resistance occurs between a plurality of transfer members in a configuration in which a power source for applying a voltage to a plurality of transfer members is shared. A forming apparatus is provided.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention corresponds to each of a plurality of image carriers that carry toner images, an intermediate transfer member to which toner images are transferred from the plurality of image carriers, and a plurality of image carriers. A plurality of transfer members, each of which is provided with a voltage to transfer a toner image from the plurality of image carriers to the intermediate transfer member, a common power source for applying a voltage to the plurality of transfer members, and the plurality of the plurality of transfer members. An image forming apparatus comprising: a plurality of resistance elements connected between each of the transfer members and the power source.

  According to the present invention, in a configuration in which a power source for applying a voltage to a plurality of transfer members is shared, it is possible to perform good transfer even when a difference in resistance occurs between the plurality of transfer members.

1 is a schematic sectional view of an image forming apparatus according to Embodiment 1. FIG. It is a schematic perspective view of a primary transfer brush. 3 is a perspective view of the vicinity of an intermediate transfer unit in Embodiment 1. FIG. 6 is an equivalent circuit diagram relating to primary transfer in Embodiment 1. FIG. It is a schematic sectional drawing of the image forming apparatus of a comparative example. FIG. 6 is a perspective view of the vicinity of an intermediate transfer unit in a comparative example. It is an equivalent circuit diagram relating to primary transfer in a comparative example. 5A is a graph illustrating a relationship between an output voltage of a primary transfer power source and a current flowing through each image forming unit in Example 1 and FIG. 6 is a schematic cross-sectional view of an image forming apparatus according to Embodiment 2. FIG. 6 is a perspective view of the vicinity of an intermediate transfer unit in Embodiment 2. FIG. 10 is an equivalent circuit diagram relating to primary transfer in Embodiment 2. FIG. FIG. 10 is an equivalent circuit diagram relating to primary transfer in a modification of Example 2; 6 is a schematic cross-sectional view of an image forming apparatus according to Embodiment 3. FIG. FIG. 10 is a flowchart of control in Embodiment 3. It is a schematic diagram for demonstrating full color mode and mono mode.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

[Example 1]
1. FIG. 1 is a schematic sectional view of an image forming apparatus 100 according to the present embodiment. The image forming apparatus 100 according to the present exemplary embodiment is a tandem type laser beam printer that employs an intermediate transfer method that can form a full-color image using an electrophotographic method.

  The image forming apparatus 100 includes first, second, and second toner images that form yellow (Y), magenta (M), cyan (C), and black (K) toner images, respectively, as a plurality of image forming units (stations). It has 3rd, 4th image formation part Sa, Sb, Sc, Sd. Note that elements having the same or corresponding functions or configurations in each of the image forming units Sa, Sb, Sc, and Sd are a, b, c, and d at the end of the reference numerals indicating that they are elements for any color. Omitted, may be explained comprehensively. In the present embodiment, the image forming unit S includes a photosensitive drum 1, a charging roller 2, an exposure device 3, a developing device 4, a primary transfer brush 6, and a cleaning device 5 which will be described later.

  The image forming apparatus 100 includes a photosensitive drum 1 that is a drum-type electrophotographic photosensitive member (photosensitive member) as an image carrier that supports a toner image. The photosensitive drum 1 is rotationally driven at a predetermined peripheral speed (process speed) in the direction of arrow R1 in the figure. The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential having a predetermined polarity (negative polarity in this embodiment) by a charging roller 2 that is a roller-type charging member as a charging unit. The charging roller 2 is disposed in contact with the surface of the photosensitive drum 1, and is rotated following the rotation of the photosensitive member 1. During the charging process, a predetermined charging voltage (charging bias) is applied to the charging roller 2 from a charging power source (not shown). The surface of the photosensitive drum 1 that has been charged is scanned and exposed with laser light in accordance with image information by an exposure device (laser scanner) 3 serving as an exposure unit, and an electrostatic image (electrostatic latent image) is formed on the photosensitive drum 1. Is formed.

  The electrostatic image formed on the photosensitive drum 1 is developed (visualized) by supplying toner as a developer by a developing device 4 as developing means, and a toner image is formed on the photosensitive drum 1. A predetermined developing voltage (developing bias) is applied from a developing power source (not shown) to the developing roller as a developer carrying member provided in the developing device 4 during the developing process. In this embodiment, the same polarity as the charging polarity of the photosensitive drum 1 (negative polarity in this embodiment) is applied to the exposed portion on the photosensitive drum 1 where the absolute value of the potential has been lowered by being exposed after being uniformly charged. ) Is charged with charged toner. That is, in this embodiment, the normal charging polarity of the toner, which is the charging polarity of the toner at the time of development, is negative.

  An intermediate transfer belt 10 composed of an endless belt as an intermediate transfer member is disposed facing each of the photosensitive drums 1a, 1b, 1c, and 1d. The intermediate transfer belt 10 is stretched around a secondary transfer counter roller 13 which also serves as a plurality of tension rollers 11, tension rollers 11, auxiliary rollers 12 and drive rollers, and is stretched with a predetermined tension. When the secondary transfer counter roller 13 is driven to rotate, the intermediate transfer belt 10 rotates (circulates) in the direction of the arrow R2 in the drawing at a circumferential speed substantially the same as the circumferential speed of the photosensitive drum 1. On the inner peripheral surface side of the intermediate transfer belt 10, a primary transfer brush (conductive brush) 6 that is a brush-like primary transfer member serving as a primary transfer unit is disposed corresponding to each photosensitive drum 1. The primary transfer brush 6 is pressed toward the photosensitive drum 1 via the intermediate transfer belt 10 to form a primary transfer portion (primary transfer nip) T1 where the photosensitive drum 1 and the intermediate transfer belt 10 are in contact with each other. As described above, the toner image formed on the photosensitive drum 1 is transferred to the intermediate transfer belt 10 as a rotating transfer object by the electrostatic force and pressure applied by the primary transfer brush 6 in the primary transfer portion T1. Primary transferred onto. During the primary transfer process, the primary transfer brush 6 receives a primary transfer voltage (positive voltage in this embodiment) from the primary transfer power supply (high voltage power supply circuit) 8 that is a reverse polarity (positive polarity in this embodiment) of the toner. Primary transfer bias) is applied. For example, when forming a full-color image, yellow, magenta, cyan, and black toner images formed on each photosensitive drum 1 are sequentially transferred so as to be superimposed on the intermediate transfer belt 10.

  On the outer peripheral surface side of the intermediate transfer belt 10, a secondary transfer roller 20 that is a roller-type secondary transfer member as a secondary transfer unit is disposed at a position facing the secondary transfer counter roller 13. The secondary transfer roller 20 is pressed toward the secondary transfer counter roller 13 via the intermediate transfer belt 10, and a secondary transfer portion (secondary transfer nip) where the intermediate transfer belt 10 and the secondary transfer roller 20 come into contact with each other. T2 is formed. The toner image formed on the intermediate transfer belt 10 as described above is transferred to the intermediate transfer belt 10 and the secondary transfer roller 20 by the electrostatic force and pressure applied by the secondary transfer roller 20 in the secondary transfer portion T2. Is secondarily transferred onto the recording material P that is nipped and conveyed. During the secondary transfer process, the secondary transfer roller 20 receives a secondary voltage from the secondary transfer power supply (high voltage power supply circuit) 21 that is a DC voltage having a polarity opposite to the normal charging polarity of the toner (positive in this embodiment). A next transfer voltage (secondary transfer bias) is applied. A recording material (transfer material, recording medium, sheet) P such as paper is stacked on a recording material cassette 40, fed by a pickup roller 41, and conveyed to a registration roller 42. After the skew of the recording material P is corrected by the registration roller 42, the recording material P is supplied to the secondary transfer portion T2 in synchronization with the toner image on the intermediate transfer belt 10.

  The recording material P onto which the toner image has been transferred is conveyed to a fixing device 30 as a fixing unit, where the toner image is fixed (melted and fixed) on the surface by being heated and pressed. Thereafter, the recording material P is discharged (output) to the outside of the apparatus main body 110 of the image forming apparatus 100.

  On the other hand, the toner (primary transfer residual toner) remaining on the photosensitive drum 1 during the primary transfer step is removed from the photosensitive drum 1 and collected by the cleaning device 5 as a cleaning unit. The cleaning device 5 scrapes off the primary transfer residual toner from the surface of the rotating photosensitive drum 1 by a cleaning blade disposed in contact with the surface of the photosensitive drum 1 and stores it in a cleaning container. A charging brush (conductive brush) 16 that is a brush-like toner charging member as a toner charging unit is disposed at a position facing the secondary transfer counter roller 13 on the outer peripheral surface side of the intermediate transfer belt 10. The charging brush 16 is pressed toward the secondary transfer counter roller 13 via the intermediate transfer belt 10. The toner (secondary transfer residual toner) remaining on the intermediate transfer belt 10 during the secondary transfer process is opposite to the normal charging polarity of the toner when passing through the contact portion between the charging brush 16 and the intermediate transfer belt 10. It is charged with polarity (positive polarity in this embodiment). At this time, a toner charging voltage (toner charging bias) which is a DC voltage having a polarity opposite to the normal charging polarity of the toner is applied to the charging brush 16 from a toner charging power source (high voltage power circuit) 60. The secondary transfer residual toner charged by the charging brush 16 is transferred to the photosensitive drum 1 by the action of a voltage opposite in polarity to the normal charging polarity of the toner applied to the primary transfer brush 6 in the primary transfer portion T1. . Thereafter, the secondary transfer residual toner is collected by the cleaning device 5. In this embodiment, the secondary transfer residual toner is collected in the first image forming unit Sa. The transfer of the secondary transfer residual toner from the intermediate transfer belt 10 to the photosensitive drum 1 can be performed simultaneously with the primary transfer of the toner image from the photosensitive drum 1 to the intermediate transfer belt 10.

  In this embodiment, the intermediate transfer belt 10, its stretching rollers 11, 12, 13, primary transfer brushes 6 a to 6 d and the like constitute an intermediate transfer unit 14 that can be attached to and detached from the apparatus main body 110 integrally. . The intermediate transfer unit 14 is replaced with a new one when, for example, the intermediate transfer belt 14 reaches the end of its life.

  In the present embodiment, the operation of each unit of the image forming apparatus 100 is comprehensively controlled by a control unit (control circuit) 90 serving as a control unit provided in the apparatus main body 110. The control unit 90 is configured to include a CPU as an arithmetic control unit and a ROM or RAM as a storage unit, and the CPU appropriately uses the RAM as a work area in accordance with a program stored in the ROM. Control each part. In addition, a temperature / humidity sensor 120 is connected to the control unit 90 as an environment detection unit that detects at least one of temperature and / or humidity inside or outside the apparatus main body 110 of the image forming apparatus 100. In this embodiment, the temperature / humidity sensor 120 detects the temperature and humidity inside the apparatus main body 110. The control unit 90 is connected to a counter 130 as a counting unit that counts an index value that correlates with the amount of use of the intermediate transfer unit 14 (intermediate transfer belt 10, primary transfer brushes 6a to 6d, etc.). In the present embodiment, the counter 130 counts the integrated value of the number of printed sheets from when the intermediate transfer unit 14 is new (at the start of use). The index value correlated with the amount of use of the intermediate transfer unit 14 (intermediate transfer belt 10, primary transfer brushes 6a to 6d, etc.) is not limited to the number of printed sheets, but the rotation time and the number of rotations of the intermediate transfer belt 10. It may be.

  Here, the image forming apparatus 100 executes a job (printing operation) that is a series of operations for forming and outputting an image on a single or a plurality of recording materials P, which is started by one start instruction. In general, a job includes an image forming process, a pre-rotating process, a paper-to-paper process when images are formed on a plurality of recording materials P, and a post-rotating process. The image forming process is a period in which an electrostatic latent image of an image actually formed and output on the recording material P, a toner image, a primary transfer and a secondary transfer of the toner image are performed. This is the period. More specifically, the timing at which the image is formed differs depending on the position where the electrostatic latent image formation, the toner image formation, and the primary transfer and secondary transfer of the toner image are performed. The pre-rotation process is a period for performing a preparatory operation before the image forming process from when the start instruction is input until the actual image formation is started. The inter-sheet process is a period corresponding to the interval between the recording material P and the recording material P when the image forming process is continuously performed on the plurality of recording materials P (continuous image formation). The post-rotation process is a period during which an organizing operation (preparation operation) after the image forming process is performed. The non-image forming period is a period other than the image forming time, and is a preparatory operation at the time of turning on the power of the image forming apparatus 100 or returning from the sleep state. This is included during the previous multi-rotation process.

2. Transfer Configuration In this embodiment, the intermediate transfer belt 10 is an endless belt made of a thermoplastic resin having ionic conductivity. In this embodiment, the intermediate transfer belt 10 is given a total pressure of 60 N by a tension roller 11 whose position is variable. In this embodiment, the volume resistivity of the intermediate transfer belt 10 is 1 × 10 ⁹ Ω · cm. In addition, the volume resistivity was measured using a ring probe type UR (model MCP-HTP12) on a Hiresta-UP (MCP-HT450) manufactured by Mitsubishi Chemical Corporation. The measurement was performed under the conditions of an indoor temperature of 23 ° C., an indoor humidity of 50%, an applied voltage of 100 V, and a measurement time of 10 sec.

  Examples of the material of the intermediate transfer belt 10 include polycarbonate, polyvinylidene fluoride (PVDF), polyethylene, polypropylene, polymethylpentene-1, polystyrene, polyamide, polysulfone, polyarylate, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, Examples thereof include thermoplastic resins such as polybutylene naphthalate, polyphenylene sulfide, polyether sulfone, polyether nitrile, thermoplastic polyimide, polyether ether ketone, thermotropic liquid crystal polymer, and polyamic acid. Two or more of these can be mixed and used.

  In this embodiment, the intermediate transfer belt 10 contains an ion conductive conductive agent for imparting conductivity. The ion conductive intermediate transfer belt 10 can suppress the manufacturing tolerance of the resistance value smaller than that of the intermediate transfer belt 10 using the electronic conductive agent. Examples of the ion conductive conductive agent include polyvalent metal salts and quaternary ammonium salts. Quaternary ammonium salts include tetraethylammonium ion, tetrapropylammonium ion, tetraisopropylammonium ion, tetrabutylammonium ion, tetrapentylammonium ion, tetrahexylammonium ion, etc. Examples thereof include fluoroalkyl sulfate ions, fluoroalkyl sulfite ions, and fluoroalkyl borate ions having 1 to 10 carbon atoms in halogen ions and fluoroalkyl groups.

  The intermediate transfer belt 10 as a resin composition can be obtained by melt-kneading each of the material components described above, and then molding by appropriately selecting a molding method such as inflation molding or cylindrical extrusion molding.

In this embodiment, the primary transfer brush 6 is disposed at a position facing the photosensitive drum 1 with the intermediate transfer belt 10 interposed therebetween. FIG. 2 is a schematic perspective view of the primary transfer brush 6. In this embodiment, the primary transfer brush 6 includes a raised portion 6α made of a conductive fiber group and a base fabric portion 6β. The primary transfer brush 6 is supported by fixing the base cloth portion 6β on the support member 61. In the present embodiment, the width W1 of the raised portion 6α in the direction substantially parallel to the moving direction of the intermediate transfer belt 10 is 3 mm, and the width W2 of the base fabric portion 6β in the same direction is 5 mm. In this embodiment, the primary transfer brush 6 pushes up the intermediate transfer belt 10 by pressing the support member 61 against the photosensitive drum 1 by the pressing force of the pressing spring 62 as an urging means, and the intermediate transfer belt 10. Is brought into contact with the photosensitive drum 1 with a contact pressure of 400 gf. The raised portions 6α can be made of conductive fibers such as conductive nylon or polyester in which carbon powder is dispersed as a conductive agent. In this embodiment, the raised portions 6α are made of conductive nylon in which carbon powder is dispersed. In order to obtain good transfer efficiency, it is desirable that the fibers of the raised portions 6α have a single yarn fineness of 2 to 15 dtex and a resistivity of 10 to 10 ⁸ Ωcm. In this example, a conductive fiber having a single yarn fineness of 7 dtex and a resistivity of 1 × 10 ⁶ Ωcm was used as the fiber of the raised portion 6α.

  In the present embodiment, a common primary transfer power supply 8 is connected to the primary transfer brushes 6 a to 6 d of the first to fourth image forming units Sa to Sd, and each primary transfer from the common primary transfer power supply 8. Power is supplied to the raised portions 6α of the brushes 6a to 6d.

In this embodiment, the secondary transfer roller 20 has a thickness mainly composed of NBR and epichlorohydrin rubber whose volume resistivity is adjusted to 1 × 10 ⁸ Ω · cm on the outer periphery of a nickel-plated steel rod having an outer diameter of 8 mm. Is an elastic roller with an outer diameter of 18 mm covered with a foam sponge body of 5 mm. In this embodiment, the secondary transfer roller 20 is driven to rotate as the intermediate transfer belt 10 rotates. In this embodiment, a secondary transfer voltage of +2500 V is applied to the secondary transfer roller 20 during the secondary transfer process.

In this embodiment, the charging brush 16 is made of conductive nylon containing carbon as a conductive agent. In this embodiment, the conductive fiber constituting the charging brush 16 has a resistance value of 1 × 10 ¹² Ω / cm per unit length and a single yarn fineness of 5 dtex. In this embodiment, the density of the charging brush 16 is 100 kF / inch ² .

3. ATVC Control In this embodiment, a thermoplastic resin having ionic conductivity is used as the material of the intermediate transfer belt 10. An ion conductive material has a characteristic that its resistance value is likely to fluctuate greatly depending on temperature and humidity. Therefore, the resistance value of the intermediate transfer belt 10 according to the present exemplary embodiment may fluctuate relatively greatly depending on the environmental temperature and humidity of the image forming apparatus 100. In this embodiment, the primary transfer brush 6 that contacts and rubs the inner peripheral surface of the intermediate transfer belt 10 is used as the primary transfer member. At the tip of the primary transfer brush 6 that comes into contact with the inner peripheral surface of the intermediate transfer belt 10, the intermediate transfer belt 10 or the primary transfer generated by the rubbing between the primary transfer brush 6 and the inner peripheral surface of the intermediate transfer belt 10. There are cases where deposits such as abrasion powder of the transfer brush 6 adhere. Therefore, the resistance value of the primary transfer brush 6 may increase as the number of printed sheets (number of printings) increases.

  The image forming apparatus 100 according to the present exemplary embodiment can perform an ATVC control (Active Transfer Voltage) so that an appropriate primary transfer voltage can be selected at the time of image formation (at the time of the primary transfer process) even when such environmental fluctuations and durability fluctuations occur. Control). That is, before image formation, a test voltage is applied from the primary transfer power supply 8 to the primary transfer brushes 6a to 6d, and information about the resistance values of the intermediate transfer belt 10 and the resistance values of the primary transfer brushes 6a to 6d (voltages). And the relationship between current and current. Then, based on the result, the primary transfer voltage at the time of image formation is determined so that an appropriate primary transfer current is supplied to each primary transfer portion T1a to T1d.

  More specifically, in this embodiment, the ROM of the control unit 90 stores information indicating the relationship between the number of printed sheets of the intermediate transfer unit 14 from the new time and the target value of the total current for each temperature / humidity information of the environment. Is remembered. The total current is a total of primary transfer currents supplied to the first to fourth image forming units Sa to Sd detected by the current detection circuit 9 as a detection unit. In the present embodiment, the current detection circuit 9 detects the current that flows to the primary transfer power supply 8 when the primary transfer power supply 8 applies a voltage to the primary transfer brushes 6a to 6d of the first to fourth image forming units Sa to Sd. It is connected to the primary transfer power supply 8 so that it can be detected. A signal related to the detection result of the current detection circuit 9 is input to the control unit 90. Then, in the ATVC control, the control unit 90 obtains a target value of the total current according to the current detection result of the temperature / humidity sensor 120 and the count value of the counter 130 based on the above relationship. In the ATVC control, the controller 90 adjusts the output value of the primary transfer power supply 8 (constant current control) so that the current detected by the current detection circuit 9 approaches the target value. In addition, the control unit 90 as a detecting unit obtains the output voltage value of the primary transfer power supply 8 (which may be an average value of output values for a certain period) from the output set value of the primary transfer power supply 8 or the like. . Then, the control unit 90 determines the obtained output voltage value as the voltage value of the primary transfer voltage during image formation, and performs constant voltage control on the primary transfer voltage with the voltage value during image formation. In this embodiment, the ATVC control is performed at the time of the pre-rotation process of the job every time as non-image formation. More specifically, in the ATVC control, in the pre-rotation process, an image formation start instruction is input to the image forming apparatus 100 and rotation of the photosensitive drum 1 and the intermediate transfer belt 10 is started, and then the intermediate transfer belt 10 is transferred. This is performed until the primary transfer of the toner image is performed. As described above, in this embodiment, the control unit 90 performs control to determine the voltage value that the primary transfer power supply 8 supplies to the primary transfer brushes 6a to 6d during image formation based on the detection result of the current detection circuit 9. Functions as a means.

4). Resistive element In this embodiment, the primary transfer power supply 8 for applying a voltage to the primary transfer brushes 6a to 6d of the first to fourth image forming portions Sa to Sd is shared. In this embodiment, in this configuration, a resistive element (resistor) 7a is provided between the primary transfer brushes 6a to 6d of the first to fourth image forming units Sa to Sd and the common primary transfer power supply 8 respectively. ~ 7d are connected. In this embodiment, the resistance values of the resistance elements 7a to 7d are substantially the same. When the resistance values of the resistance elements 7a~7d and _{R 7,} in the present embodiment is ^{_{R 7 = 4 × 10 7 Ω}} .

  FIG. 3 is a perspective view showing a configuration for applying a voltage to the primary transfer brushes 6a to 6d in the present embodiment. In FIG. 3, the intermediate transfer belt 10 is not shown. Each of the resistance elements 7 a to 7 d is disposed inside the intermediate transfer unit 14 and is connected between each primary transfer brush 6 a to 6 d and a contact 81 provided on the frame of the intermediate transfer unit 14. In a state in which the intermediate transfer unit 14 is mounted on the apparatus main body 110, the contact 81 is connected to the primary transfer power supply 8 provided on the high voltage substrate 83 via the contact spring 82. The contact spring 82 and the high voltage base 83 are provided in the apparatus main body 110.

FIG. 4 is an equivalent circuit diagram relating to the primary transfer voltage in this embodiment. The current values flowing from the primary transfer power supply 8 to the image forming units Sa to Sd (currents flowing from the primary transfer power supply 8 to the photosensitive drums 1a to 1d) are denoted by Ia, Ib, Ic, and Id, respectively. The value of the current flowing through the current detection circuit 9 (primary transfer power supply 8) is assumed to be I (= Ia + Ib + Ic + Id). That is, Ia, Ib, Ic, and Id are primary transfer current values in the image forming units Sa to Sd, respectively, and I is a total current value that is a sum of primary transfer current values in the image forming units Sa to Sd. . In addition, the resistance values of the image forming units Sa to Sd (resistance values between the primary transfer power source 8 excluding the resistance values of the resistance elements 7a to 7d and the photosensitive drums 1a to 1d) are represented by Ra, Rb, Rc, Let Rd. The resistance values Ra to Rd of the image forming units Sa to Sd are determined by various factors such as the resistance values of the primary transfer brushes 6a to 6d, the resistance value of the intermediate transfer belt 10, and the potentials of the photosensitive drums 1a to 1d. The In the present embodiment, the following relational expression is established in a normal temperature and humidity environment and when the intermediate transfer unit 14 is new (initial state in which there is no use history).
Ra = Rb = Rc = Rd = R _{0
Ia = Ib = Ic = Id =} I 0/4
V ₀ = R ₇ × Ia + Ra × Ia = (R ₇ + R ₀ ) × I _0/4

Here, R ₀ is the resistance value of each of the image forming units Sa to Sd in a normal temperature and normal humidity environment and when the intermediate transfer unit 14 is new. Also, I ₀ is an appropriate total current value (appropriate primary transfer current value) when the intermediate transfer unit 14 is new in a room temperature and humidity environment. V ₀ is the output voltage value of the primary transfer power supply 8 when I ₀ is supplied when the intermediate transfer unit 14 is new in a room temperature and humidity environment.

5. As described above, abrasion powder of the intermediate transfer belt 10 and the primary transfer brush 6 is generated by the rubbing between the primary transfer brush 6 and the inner peripheral surface of the intermediate transfer belt 10 as described above. This wear powder initially has a charge. Therefore, a part of the generated wear powder adheres to the back surface of the intermediate transfer belt 10. However, during the rotation of the intermediate transfer belt 10, the charge of the wear powder adhering to the back surface of the intermediate transfer belt 10 is reduced, and the wear powder is easily separated from the intermediate transfer belt 10. Therefore, the wear powder easily adheres to the primary transfer brush 6a of the first image forming unit Sa, and the primary transfer brush 6a of the first image forming unit Sa has a resistance value of the primary transfer brush of the other image forming units Sb to Sd. It tends to rise from the resistance values of 6b to 6d. As a result, when the number of printed sheets increases, the resistance value Ra of the first image forming unit Sa may be higher than the resistance values Rb to Rd of the other image forming units Sb to Sd.

In this embodiment, it is assumed that the resistance value of the first image forming unit Sa is higher than the resistance values of the other image forming units Sb to Sd and the following relational expression is satisfied.
Ra = 2R ₀
Rb = Rc = Rd = R ₀

Focusing on the first image forming unit Sa, the following relational expression is established.
V _use = R ₇ × Ia + Ra × Ia = (R ₇ + 2R ₀ ) × Ia

Here, V _use is an output voltage value of the primary transfer power supply 8 when the number of printed sheets increases.

Similarly, when attention is focused on the second image forming unit Sb, the following relational expression is established.
V _use = R ₇ × Ib + Rb × Ib = (R ₇ + R ₀ ) × Ib

When the above two expressions are compared, the following relational expression holds.
Ia = {(R ₇ + R ₀ ) / (R ₇ + 2R ₀ )} × Ib

At this time, the resistance value R ₇ of the resistance element 7 is enough large as compared R _0, holds the following relationship.
Ia≈Ib

That is, if the resistance value R ₇ of the resistance element 7 is sufficiently than R ₀ greater, substantially the same as the current flowing through the first image forming unit Sa, the current flowing in the other image forming unit Sb～Sd, the It becomes possible to. In this example, the resistance value R ₇ of the resistance element 7 was set to satisfy the following formula.
R ₇ > R ₀

  In this embodiment, since the ATVC control is performed as described above, an appropriate primary transfer current is applied to each of the image forming units Sa to Sd by setting the output voltage value of the primary transfer power supply 8 to an appropriate value. Can be supplied. That is, in this embodiment, as the number of printed sheets increases, the resistance value of the primary transfer brush 6a of the first image forming unit Sa is higher than the resistance values of the primary transfer brushes 6b to 6d of the other image forming units Sb to Sd. Even if it rises, good primary transfer can be performed.

It should be noted that the resistance value R ₇ of each of the resistance elements 7a to 7d is the resistance value Ra, Rb, It is preferably sufficiently larger than Rc and Rd. For example, as the number of printed sheets increases, the resistance value Ra of some of the image forming units Sa to Sd increases to about twice the resistance value of the other image forming units Sb to Sd. There is a case. Therefore, for example, the resistance value R ₇ of each of the resistance elements _{7 a to 7} d is equal to the resistance value R ₀ (= Ra = Rb = Rc) of each image forming unit Sa to Sd in a normal temperature and humidity environment and when the intermediate transfer unit 14 is new. = Rd) 2 times or more, preferably 3 times or more, more preferably 4 times or more. However, the resistance value R ₇ of each of the resistance elements _{7 a to 7} d is the resistance value R ₀ (= Ra = Rb = Rc = Rd) of the image forming portions Sa to Sd when the intermediate transfer unit 14 is new under a normal temperature and humidity environment. ) Is often sufficient.

6). Comparison between Example and Comparative Example Next, the effect of this example will be further described in comparison with the comparative example.

FIG. 5 is a schematic cross-sectional view of the image forming apparatus 100 of the comparative example. In the image forming apparatus of the comparative example, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the present embodiment will be described with the same reference numerals. In the comparative example, a common resistance element 7 is connected between the primary transfer brushes 6 a to 6 d of the first to fourth image forming units Sa to Sd and the common primary transfer power supply 8. When the resistance value of the resistance element 7 in the comparative example is R ₇ ′, R ₇ ′ = 1 × 10 ⁷ Ω. The configuration of the image forming apparatus 100 of the comparative example is substantially the same as the configuration of the image forming apparatus 100 of the present embodiment except for the above points.

  FIG. 6 is a perspective view showing a voltage application configuration to the primary transfer brushes 6a to 6d in the comparative example. In FIG. 6, the intermediate transfer belt 10 is not shown. In the comparative example, the resistance element 7 is disposed inside the intermediate transfer unit 14 and is connected between the primary transfer brushes 6 a to 6 d and the contacts 81 provided on the frame of the intermediate unit 14.

FIG. 7 is an equivalent circuit diagram relating to the primary transfer voltage in the comparative example. In the comparative example, the following relational expression is established under a normal temperature and humidity environment and when the intermediate transfer unit 14 is new.
Ra = Rb = Rc = Rd = R ₀
Ia = Ib = Ic = Id = I ₀ '/ 4
V ₀ '= R ₇ ' × I ₀ '+ Ra × Ia = (4R ₇ ' + R ₀ ) × I ₀ '/ 4

Here, I ₀ ′ is an appropriate total current value in a normal temperature and humidity environment and when the intermediate transfer unit 14 is new. V ₀ ′ is an output voltage value of the primary transfer power supply 8 when I ₀ ′ is supplied when the intermediate transfer unit 14 is new in a room temperature and humidity environment.

In the comparative example, it is assumed that the resistance value of the first image forming unit Sa is higher than the resistance values of the other image forming units Sb to Sd and the following relational expression is satisfied.
Ra = 2R ₀
Rb = Rc = Rd = R ₀

Focusing on the first image forming unit Sa, the following relational expression is established.
V _use '= R ₇ ' × I _use '+ Ra × Ia

Here, I _use ′ is an appropriate total current value when the number of printed sheets increases. V _use ′ is an output voltage value of the primary transfer power supply 8 when I _use ′ is supplied when the number of printed sheets increases.

Similarly, when attention is focused on the second image forming unit Sb, the following relational expression is established.
V _use '= R ₇ ' × I _use '+ Rb × Ib

When the above two expressions are compared, the following relational expression holds.
Ia = (Rb / Ra) × Ib = (R ₀ / 2R ₀ ) × Ib = Ib / 2

  That is, half the current flowing through the other image forming units Sb to Sd flows through the first image forming unit Sa. At this time, if an appropriate primary transfer current is supplied to the first image forming unit Sa, an excessive primary transfer current is supplied to the other image forming units Sb to Sd. On the other hand, if an appropriate primary transfer current is supplied to the other image forming units Sb to Sd, an excessively small primary transfer current is supplied to the first image forming unit Sa. Therefore, in the comparative example, as the number of printed sheets increases, the resistance value of the primary transfer brush 6a of the first image forming unit Sa increases from the resistance value of the primary transfer brushes 6b to 6d of the other image forming units Sa to Sd. In this case, good primary transfer may not be performed.

Next, in this embodiment, the resistance value Ra of the first image forming unit Sa is 2 × 10 ⁷ Ω, and the resistance values Rb to Rd of the other image forming units Sb to Sd are 1 × 10 ⁷ Ω, respectively. The primary transfer currents flowing in the image forming portions Sa to Sd in the comparative example and the comparative example were compared.

  FIG. 8A shows the output voltage of the primary transfer power supply 8 and the current Ia flowing through the first image forming unit Sa and the currents Ib through Id flowing through the other image forming units Sb through Sd in this embodiment. It is a graph which shows a relationship. The current Ia flowing through the first image forming unit Sa is slightly smaller than the currents Ib through Id flowing through the other image forming units Sb through Sd, but there is no significant deviation. When the appropriate primary transfer current in each of the image forming units Sa to Sd is 10 μA, the output voltage of the primary transfer power supply 8 is about 522 V by performing ATVC control so that a total current of 40 μA flows. At this time, the current Ia flowing through the first image forming unit Sa is about 8.7 μA, and the currents Ib through Id flowing through the other image forming units Sb to Sd are about 10.4 μA.

  On the other hand, FIG. 8B shows the output voltage of the primary transfer power supply 8, the current Ia flowing through the first image forming unit Sa, and the currents Ib through Id flowing through the other image forming units Sb through Sd in the comparative example. It is a graph which shows the relationship. The current Ia flowing through the first image forming unit Sa is half of the currents Ib through Id flowing through the other image forming units Sb through Sd. When the ATVC control is performed so that the total current of 40 μA flows, the output voltage of the primary transfer power supply 8 becomes about 514V. At this time, the current Ia flowing through the first image forming unit Sa is about 5.7 μA, and the currents Ib through Id flowing through the other image forming units Sb to Sd are about 11.4 μA, respectively. When comparing the present example and the comparative example by paying attention to the first image forming portion Sa, in the comparative example, the primary transfer current is about 3 μA less than that of the present example, and it is difficult to perform good primary transfer.

  As described above, when the resistance value of the primary transfer brush 6a of the first image forming unit Sa rises higher than the resistance values of the primary transfer brushes 6b to 6d of the other image forming units Sb to Sd as the number of printed sheets increases. There is a large difference in the primary transfer current of the first image forming portion Sa between the present embodiment and the comparative example. In the present embodiment, the resistance elements 7a to 7d are connected between the primary transfer brushes 6a to 6d of the first to fourth image forming units Sa to Sd and the common primary transfer power supply 8, respectively. The difference in resistance value between one image forming unit Sa and the other image forming units Sb to Sd is reduced. Therefore, in the present embodiment, an appropriate primary transfer current can be passed through the first image forming unit Sa.

  That is, the current detection circuit 9 cannot detect the resistance values of the image forming units Sa to Sd. Therefore, even if ATVC control is performed, in the configuration of the comparative example, the primary transfer current differs according to the resistance values of the image forming units Sa to Sd. An excessive primary transfer current flows in the image forming portions Sb to Sd having a relatively low resistance value, and an excessive primary transfer current flows in the image forming portion Sa having a relatively high resistance value. May not be able to be performed. On the other hand, in the configuration of the present embodiment, the resistance elements 7a to 7d reduce the difference in resistance value of the image forming units Sa to Sd, and the ATVC control is performed appropriately for the image forming units Sa to Sd. Therefore, it is possible to pass a primary transfer current that is good, and good primary transfer can be performed.

  As described above, in the present embodiment, the resistance elements 7a to 7d are connected between the primary transfer brushes 6a to 6d of the first to fourth image forming units Sa to Sd and the common primary transfer power supply 8, respectively. Has been. For this reason, even when the number of printed sheets increases and the resistance values of some of the primary transfer brushes 6a to 6d of the first to fourth image forming units Sa to Sd are higher than others, the resistance elements 7a to 7d can A difference in resistance value between the image forming units Sa to Sd is reduced. Accordingly, it is possible to perform an excellent primary transfer by supplying an appropriate primary transfer current to the first to fourth image forming units Sa to Sc.

[Example 2]
Next, another embodiment of the present invention will be described. In the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted.

FIG. 9 is a schematic cross-sectional view of the image forming apparatus 100 of the present embodiment. In this embodiment, a primary transfer power source (first primary transfer power source) 8 for applying a voltage to the primary transfer brushes 6a to 6c of the first to third image forming units Sa to Sc is shared. The first primary transfer power supply 8 includes a first primary transfer power supply 8 when the first primary transfer power supply 8 applies a voltage to the primary transfer brushes 6a to 6c of the first to third image forming units Sa to Sc. A current detection circuit (first current detection circuit) 9 for detecting a current flowing through the power supply 8 is connected. In the present embodiment, an individual primary transfer power source (second primary transfer power source) 18 is connected to the primary transfer brush 6d of the fourth image forming unit Sd. Further, the second primary transfer power supply 18 includes a current that flows to the second primary transfer power supply 18 when the second primary transfer power supply 18 applies a voltage to the primary transfer brush 6d of the fourth image forming unit Sd. Is connected to a current detection circuit (second current detection circuit) 19. In this embodiment, the resistance elements 7a to 7c are respectively arranged between the primary transfer brushes 6a to 6c of the first to third image forming units Sa to Sc and the common first primary transfer power source 8 in this configuration. 7c is connected. In this embodiment, the resistance values of the resistance elements 7a to 7c are substantially the same. Assuming that the resistance value of each of the resistance elements ₇ a to ₇ c is R ₇ , in this embodiment, R ₇ = 4 × 10 ⁷ Ω.

  FIG. 10 is a perspective view showing a configuration for applying a voltage to the primary transfer brushes 6a to 6d in the present embodiment. In FIG. 10, the illustration of the intermediate transfer belt 10 is omitted. Each of the resistance elements 7 a to 7 c is disposed inside the intermediate transfer unit 14, and contacts provided to the primary transfer brushes 6 a to 6 c of the first to third image forming units Sa to Sc and the frame of the intermediate transfer unit 14. 81 to each other. In a state in which the intermediate transfer unit 14 is mounted on the apparatus main body 110, the contact 81 is connected to the primary transfer power supply 8 provided on the high voltage substrate 83 via the contact spring 82. Further, the primary transfer brush 6d of the fourth image forming unit Sd is connected to an individual contact 181 provided on the frame of the intermediate transfer unit 14 without passing through a resistance element. Then, in a state where the intermediate transfer unit 14 is mounted on the apparatus main body 110, the contact 181 is connected to a second primary transfer power source 18 provided on the high-voltage board 83 via an individual contact spring 182. . The contact springs 82 and 182 and the high voltage base 83 are provided in the apparatus main body 110. FIG. 11 is an equivalent circuit diagram relating to the primary transfer voltage in this embodiment.

  In the present embodiment, ATVC control is performed in each of the first to third image forming units Sa to Sc and the fourth image forming unit Sd, and the first primary transfer power supply 8 at the time of image formation, The output voltage value of the second primary transfer power supply 18 is determined. In the present embodiment, the control unit 90 performs control to determine the voltage value that the first primary transfer power supply 8 supplies to the primary transfer brushes 6a to 6c during image formation based on the detection result of the first current detection circuit 9. It functions as a decision means to perform. In this embodiment, the control unit 90 supplies a voltage supplied from the second primary transfer power source 18 to the primary transfer brush 6d during image formation based on the detection result of the second current detection circuit 19 as another detection unit. It functions as another determining means for performing control for determining the value.

  In the configuration of this embodiment, for example, the image forming apparatus 100 can execute image formation in two image forming modes, a full color mode (first mode) and a mono mode mode (second mode). It can be preferably applied to the case. In the full color mode, the first to fourth image forming portions Sa to Sd can form toner images to form full color images. In the mono mode, a black single color image can be formed by forming a toner image only by the fourth image forming unit Sd among the first to fourth image forming units Sa to Sd. FIGS. 15A and 15B are schematic diagrams illustrating the state of the image forming apparatus 100 in the full color mode and the mono mode, respectively. In the full color mode, as shown in FIG. 15A, the photosensitive drums 1a to 1d and the primary transfer brushes 6a to 6d are brought into contact with the intermediate transfer belt 10 in all the image forming portions Sa to Sd. On the other hand, in the mono mode, as shown in FIG. 15B, in the first to third image forming units Sa to Sc, the primary transfer brushes 6a to 6c are moved to the photosensitive drum 1a by the contact / separation mechanisms 50a, 50b and 50c. Keep away from ~ 1c. Accordingly, in the mono mode, the photosensitive drums 1a to 1c and the primary transfer brushes 6a to 6c are separated from the intermediate transfer belt 10 in the first to third image forming units Sa to Sc. In the mono mode, in the first to third image forming units Sa to Sc, the driving of the photosensitive drum 1 and the developing device 4 and the application of the voltage to the primary transfer brush 6 are stopped. As a result, it is possible to extend the life of components of the image forming units Sa to Sc that are not used in the mono mode.

  As described above, in the present embodiment, the resistance elements 7a to 7c are respectively disposed between the primary transfer brushes 6a to 6c of the first to third image forming units Sa to Sc and the common first primary transfer power source 8. 7c is connected. Therefore, even when the number of printed sheets increases and the resistance values of some of the primary transfer brushes 6a to 6c of the first to third image forming units Sa to Sc increase more than the others, the resistance elements 7a to 7c cause the first change. The difference in resistance value between the first to third image forming units Sa to Sc is reduced. Thereby, an appropriate primary transfer current can be supplied to the first to third image forming units Sa to Sc. In addition, since the primary transfer voltage can be individually set for the fourth image forming unit Sd, an appropriate primary transfer current can be supplied to the fourth image forming unit Sd. Therefore, it is possible to perform good primary transfer.

  In this embodiment, the resistance elements 7a to 7c are provided between the primary transfer brushes 6a to 6c of the first to third image forming units Sa to Sc and the common first primary transfer power source 8, respectively. However, the present invention is not limited to this. A resistance element may be provided between the primary transfer brush having a common primary transfer power source and the primary transfer power source. For example, in a configuration in which only the primary transfer power source that applies a voltage to the primary transfer brushes 6a and 6b of the first and second image forming units Sa and Sb is shared, the primary transfer brushes 6a and 6b and the primary transfer power source thereof are used. A resistive element may be provided between the two.

  In this embodiment, a resistance element is provided between the primary transfer brush 6d of the fourth image forming unit Sd and the second primary transfer power source 18 provided independently of the primary transfer brush 6d. I did not provide it. On the other hand, as shown in the equivalent circuit diagram of FIG. 12, the primary transfer brush 6d of the fourth image forming unit Sd and the second primary transfer power supply 18 provided independently for the primary transfer brush 6d. A resistive element 7d may be provided between the two. As the resistance element 7d, the same resistance elements 7a to 7c as those of the first to third image forming portions Sa to Sc can be used. That is, the resistance value of the resistance element 7d is sufficiently larger than the resistance value between the second primary transfer power supply 19 and the photosensitive drum 1d excluding the resistance value, and typically the first to third values. The resistance values of the resistance elements 7a to 7c of the image forming portions Sa to Sc are substantially the same. The resistance element 7d can suppress the resistance fluctuation of the fourth image forming unit Sd, and the difference in resistance value between the first to fourth image forming units Sa to Sd is reduced. Therefore, for example, the result of ATVC control performed by the fourth image forming unit Sd can be reflected in the first to third image forming units Sa to Sc. Conversely, the results of ATVC control performed in the first to third image forming units Sa to Sc can be reflected in the fourth image forming unit Sd. In this way, output control of the first primary transfer power supply 8 and the second primary transfer power supply 18 can be linked.

  The mono mode is not limited to the one that forms a black single-color image, and may be one that forms a single-color image of another color.

[Example 3]
Next, still another embodiment of the present invention will be described. In the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted.

FIG. 13A is a schematic cross-sectional view of the image forming apparatus 100 of the present embodiment. In the present embodiment, a primary transfer power supply 8 that applies a voltage to the primary transfer brushes 6a to 6d of the first to fourth image forming units Sa to Sd is shared. The primary transfer power supply 8 detects a current that flows through the primary transfer power supply 8 when the primary transfer power supply 8 applies a voltage to the primary transfer brushes 6a to 6d of the first to fourth image forming units Sa to Sd. A detection circuit 9 is connected. In this embodiment, in this configuration, one resistance element 7a to one of the primary transfer brushes 6a to 6c and the common primary transfer power source 8 are respectively provided between the first to third image forming units Sa to Sd. 7c is connected. On the other hand, two resistance elements 7d are connected between the primary transfer brush 6d of the fourth image forming unit Sd and the common primary transfer power supply 8 by a switching element 17 as a switching unit. 7e are provided. In this embodiment, the resistance values of the resistance elements 7a to 7c of the first to third image forming portions Sa to Sc and the one resistance element (first resistance element) 7d of the fourth image forming portion Sd are substantially the same. It is. On the other hand, the resistance value of the other resistance element (second resistance element) 7e of the fourth image forming unit Sd is smaller than the resistance value of the first resistance element 7d (that is, the first to third image formation). Smaller than the resistance values of the resistance elements 7a to 7c of the parts Sa to Sc). Assuming that the resistance values of the resistance elements 7a to 7d are R ₇ and the resistance value of the resistance element 7e is R ₇ e, in this embodiment, R ₇ = 4 × 10 ⁷ Ω and R ₇ e = 1 × 10 ⁷ Ω. Thus, typically, the resistance value of at least one resistance element 7d that is selectively connected to the primary transfer brush 6d of the fourth image forming unit Sd is connected to a primary transfer member other than the primary transfer member. The resistance values of the resistance elements 7a to 7c are substantially the same. Further, the resistance value of at least one other resistance element 7e selectively connected to the primary transfer brush 6d of the fourth image forming unit Sd is the resistance element 7a connected to the primary transfer member other than the primary transfer member. It is made smaller than the resistance value of ˜7c.

  In the present embodiment, the resistance value of the fourth image forming unit Sd rises significantly larger than the resistance values of the first to third image forming units Sa to Sc, and a current flows through the fourth image forming unit Sd. It is assumed that the situation will be difficult. Here, an extremely large resistance value means that the resistance value difference between the image forming portions is sufficiently increased only by providing each image forming portion with a resistance element having substantially the same resistance value as in the first embodiment. It means that the difference between the primary transfer currents is increased to such an extent that it cannot be within an allowable range. Therefore, in this embodiment, in this situation, the primary transfer brush 6d and the primary transfer power supply 8 of the fourth image forming unit Sd are connected via the second resistor element 7e having a resistance value smaller than that of the first resistor element 7d. To connect. As a result, it is possible to facilitate the flow of current to the fourth image forming unit Sd.

  That is, when the frequency of use of a specific image forming unit is higher than that of other image forming units, the resistance value of the specific image forming unit is extremely higher than that of other image forming units, and the specific image forming unit In some cases, it becomes difficult for current to flow through the forming portion, and good primary transfer cannot be performed. Specifically, as described in the second embodiment, the image forming apparatus 100 can execute the full color mode and the mono mode. In such an image forming apparatus 100, the fourth image forming unit Sd that forms a black monochrome image in the mono mode may be used more frequently than the other image forming units Sa to Sc, and the resistance value increases. Cheap. In particular, as described in the second embodiment (FIG. 15), in the configuration in which the photosensitive drum 1d and the primary transfer brush 6d are in contact with the intermediate transfer belt 10 only in the fourth image forming unit Sd in the mono mode, the tendency is remarkable. It is.

  Therefore, in the present embodiment, the counter 130 counts the number of printed sheets for each image forming mode. Thus, in this embodiment, the counter 130 functions as a counting unit that counts an index value that correlates with the number of images formed in the mono mode. Then, until the number of printed sheets in the mono mode reaches a predetermined threshold, the control unit 90 moves the primary transfer brush 6d of the fourth image forming unit Sd to the first resistance as shown in FIG. The switching element 17 is controlled so as to be connected to the primary transfer power supply 8 via the element 7d. As a result, similarly to the first embodiment, it is possible to reduce the difference in resistance value between the image forming units Sa to Sd, which occurs due to an increase in the number of printed sheets. Then, when the number of printed sheets in the mono mode reaches the predetermined threshold, the control unit 90 moves the primary transfer brush 6d of the fourth image forming unit Sd to the first as shown in FIG. The switching element 17 is controlled so as to be connected to the primary transfer power supply 8 through the two resistance elements 7e. In this embodiment, it is possible to determine that the resistance value of the fourth image forming unit Sd is extremely higher than the resistance values of the other image forming units Sa to Sc in this case. In this embodiment, the control unit 90 serves as a switching unit that performs control to switch the resistance element connected to the primary transfer brush 6d of the fourth image forming unit Sd when the count result of the counter 130 reaches a predetermined threshold value. Function. At this time, the control unit 90 changes the resistance element connected to the primary transfer brush 6d of the fourth image forming unit Sd from the resistance element having the first resistance value to a second resistance value smaller than the first resistance value. It controls to switch to the resistance element.

  Since the resistance value of the second resistance element 7e is smaller than the resistance value of the first resistance element 7d, the control increases the resistance value of the fourth image forming unit Sd (more specifically, the primary transfer brush 6d). It is possible to cancel out the minute amount and to facilitate the flow of current to the fourth image forming unit Sd. Thereby, good primary transfer can be performed.

  FIG. 14 is a flowchart showing an outline of a job procedure including a switching operation of the first resistance element 7d and the second resistance element 7e. Here, a full color mode job is taken as an example, and it is assumed that the photosensitive drums 1 a to 1 d and the primary transfer brushes 6 a to 6 d are in contact with the intermediate transfer belt 10 in each of the image forming units Sa to Sd. First, when an image formation start instruction is input (S1), the control unit 90 reads the count value of the counter 130 (S2). Next, the control unit 90 determines whether or not the number of printed sheets in the mono mode is equal to or greater than a predetermined threshold value X (S3). When determining that the threshold value X is not equal to or greater than the threshold value X (less than the threshold value X) in S3, the control unit 90 determines whether the switching element 17 is connected to the first resistance element 7d (S4). When the control unit 90 determines that it is not connected in S4, the control unit 90 connects the switching element 17 to the first resistance element 7d (S5), and maintains the state when connected. Further, when determining that the threshold value X is equal to or greater than the threshold value X in S3, the control unit 90 determines whether or not the switching element 17 is connected to the second resistance element 7e (S6). If the control unit 90 determines that the connection is not made in S6, the control unit 90 connects the switching element 17 to the second resistance element 7e (S7), and maintains the state if connected. Thereafter, the control unit 90 executes ATVC control (S8), and starts image formation as soon as a predetermined pre-rotation process including ATVC control is completed (S9).

  Note that the count value of the counter 130 may be reset to 0 when the intermediate transfer unit 14 is replaced. The controller 90 can recognize that the intermediate transfer unit 14 has been replaced as follows. For example, the attachment / detachment detecting means (photo interrupter, microswitch, etc.) (not shown) for detecting attachment / detachment of the intermediate transfer unit 14 can detect the attachment / detachment of the intermediate transfer unit 14. In addition, for example, information indicating that the intermediate transfer belt 14 has been replaced by an operator such as a user or a service person is input from an operation unit (not shown) provided in the apparatus main body 110. can do. In addition, for example, individual identification information stored in a storage unit (not shown) serving as a new article detection unit provided in the intermediate transfer unit 14 or information indicating that the article is new (or has been used). Can be recognized on the basis. In the mono mode job, the switching element 17 may be connected to either the first resistance element 7d or the second resistance element 7e even when the number of printed sheets exceeds a threshold value. However, in order to keep the output voltage value of the primary transfer power supply 8 low, it is preferable to connect the switching element 17 to the second resistance element 7e when the number of printed sheets exceeds a threshold value.

  As described above, according to the present embodiment, even when the resistance value of the primary transfer brush 6 of a specific image forming unit S may be extremely larger than the others, such as when the number of printed sheets in the mono mode is large. Thus, it is possible to reduce the difference in resistance value of each image forming unit and perform good primary transfer.

  In this embodiment, the number of resistance elements whose connection state is switched by the switching element 17 is two, but may be three or more. In this case, as the resistance value of the primary transfer brush to which a plurality of resistance elements are selectively connected increases, the switching element 17 is set so that the resistance value of the resistance element connected to the primary transfer brush is reduced stepwise. Can be controlled. The number of primary transfer brushes 6 to which a plurality of resistance elements having different resistance values are selectively connected is not limited to one, but a plurality of resistance elements are respectively provided for the plurality of primary transfer brushes 6. May be provided so as to be selectively connected. In addition, for example, a resistance element provided in a specific image forming unit S such as the fourth image forming unit Sd in the present embodiment only needs to be able to selectively change (switch) the resistance value. Container). Also in this case, typically, the resistance value of the resistance element is at least a resistance value substantially the same as the resistance element of the other image forming unit and a resistance value smaller than the resistance value of the resistance element of the other image forming unit. It can be changed to a value. When the count result of the counting unit reaches a predetermined threshold, the control unit 90 as the switching unit reduces the resistance value of the resistance element from the first resistance value to the first resistance value. What is necessary is just to switch to the 2nd resistance value.

[Others]
As mentioned above, although this invention was demonstrated according to the specific Example, this invention is not limited to the above-mentioned Example.

  In the above-described embodiment, an ion conductive belt is used as the intermediate transfer belt. However, a belt containing an electronic conductive conductive agent such as carbon may be used as the conductive agent. For example, an intermediate transfer belt containing carbon becomes brittle as the amount of carbon added increases, and the amount of abrasion powder generated by rubbing with the primary transfer brush increases, so the resistance value of the primary transfer brush increases. It becomes easy. Therefore, it can be said that the present invention is particularly effective when an intermediate transfer belt having a relatively low resistance with a large amount of carbon added is used.

  In the above-described embodiment, the brush is used as the primary transfer member. However, for example, a roller such as a metal roller or a sponge roller may be used. When using a roller, there is little or no friction between the intermediate transfer member and the primary transfer member compared to using a brush, so the amount of abrasion powder generated from the intermediate transfer member or the primary transfer member is Few. However, when the number of printed sheets is relatively large or when a relatively low resistance intermediate transfer belt with a large amount of carbon added is used, even if the primary transfer member is a roller, abrasion powder adheres to the surface. Since the resistance value may fluctuate, the present invention is effective.

  In the above-described embodiment, the voltage value supplied to the primary transfer member at the time of image formation is determined based on the output voltage value when the output of the primary transfer power source is controlled at a constant current with a predetermined current value. Is not limited to this. For example, when the primary transfer voltage at the time of image formation is controlled at a constant current, the primary transfer member at the time of image formation is based on the current value that flows when the output of the primary transfer power source is controlled at a constant voltage by a predetermined voltage value. A current value to be supplied may be determined. Alternatively, a resistance value may be obtained from the relationship between the voltage value and the current value, and the voltage value or current value supplied to the primary transfer member during image formation may be determined based on the resistance value.

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 6 Primary transfer brush 7 Resistance element 8 Primary transfer power supply 9 Current detection circuit 10 Intermediate transfer belt 90 Control part 100 Image forming apparatus

Claims (18)

A plurality of image carriers that carry toner images;
An intermediate transfer member onto which toner images are transferred from the plurality of image carriers;
A plurality of transfer members provided corresponding to each of the plurality of image carriers, each of which is applied with a voltage to transfer a toner image from the plurality of image carriers to the intermediate transfer member;
A common power source for applying a voltage to the plurality of transfer members;
A plurality of resistance elements connected between each of the plurality of transfer members and the power source;
An image forming apparatus comprising: Detecting means for detecting a voltage value or a current value when the power source applies a voltage to the plurality of transfer members;
A determination unit that performs control to determine a voltage value or a current value that the power supply supplies to the plurality of transfer members during image formation based on a detection result of the detection unit;
The image forming apparatus according to claim 1, further comprising:   The image forming apparatus according to claim 1, wherein each of the plurality of transfer members is a conductive brush.   4. The resistance value of each of the plurality of resistance elements is larger than a resistance value between the power source excluding the resistance value and each of the plurality of image carriers. The image forming apparatus according to one item.   The image forming apparatus according to claim 1, wherein resistance values of the plurality of resistance elements are substantially the same. Another image carrier for carrying a toner image;
Another transfer member which is provided corresponding to the other image carrier and applies a voltage to transfer the toner image from the other image carrier to the intermediate transfer member;
Another power source for applying a voltage to the other transfer member;
The image forming apparatus according to claim 1, wherein the image forming apparatus includes: Another detection means for detecting a voltage value or a current value when the other power source applies a voltage to the another transfer member;
Another determination unit that performs control to determine a voltage value or a current value that the another power supply supplies to the another transfer member during image formation based on a detection result of the another detection unit;
The image forming apparatus according to claim 6, further comprising:   The image forming apparatus according to claim 6, wherein the another transfer member is a conductive brush.   The image forming apparatus according to claim 6, further comprising another resistance element connected between the another transfer member and the other power source.   The image forming apparatus according to claim 9, wherein a resistance value of the another resistance element is larger than a resistance value between the another power source excluding the resistance value and the other image carrier.   11. The image forming apparatus according to claim 9, wherein a resistance value of the another resistance element is substantially the same as a resistance value of each of the plurality of resistance elements.   A first mode in which a toner image can be formed on all of the plurality of image carriers and the other image carrier; and the other image carrier among the plurality of image carriers and the other image carrier. The image forming apparatus according to claim 6, wherein the second mode in which only a toner image can be formed is executable.   The plurality of resistance elements having different resistance values are provided so as to be selectively connected between at least one transfer member of the plurality of transfer members and the power source. The image forming apparatus according to claim 1.   The resistance value of at least one resistance element selectively connected between the at least one transfer member and the power source is a transfer member other than the at least one transfer member among the plurality of transfer members and the power source. The resistance value of the at least one other resistance element that is selectively connected between the at least one transfer member and the power source is substantially the same as the resistance value of the resistance element connected between The image forming apparatus according to claim 13, wherein a resistance value of a resistance element connected between a transfer member other than the at least one transfer member and the power source is smaller than the transfer member. A first mode in which a toner image can be formed on all of the plurality of image carriers, and a first mode in which a toner image can be formed only on an image carrier corresponding to the at least one transfer member among the plurality of image carriers. 2 modes can be executed,
Counting means for counting an index value correlated with the amount of the at least one transfer member used in the second mode;
When the counting result of the counting means reaches a predetermined threshold value, a resistance element connected between the at least one transfer member and the power source is changed from a resistance element having a first resistance value to the first resistance. Switching means for performing control to switch to a resistance element having a second resistance value smaller than the value;
The image forming apparatus according to claim 13, further comprising:   4. The resistance element connected between at least one transfer member of the plurality of transfer members and the power source has a variable resistance value. 5. Image forming apparatus.   A resistance value of a resistance element connected between the at least one transfer member and the power source is connected between the power source and the transfer member other than the at least one transfer member among the plurality of transfer members. A resistance value substantially the same as the resistance value of the resistance element, and a resistance value smaller than the resistance value of the resistance element connected between the power supply and the transfer member other than the at least one transfer member among the plurality of transfer members The image forming apparatus according to claim 16, wherein the image forming apparatus can be changed to: A first mode in which a toner image can be formed on all of the plurality of image carriers, and a first mode in which a toner image can be formed only on an image carrier corresponding to the at least one transfer member among the plurality of image carriers. 2 modes can be executed,
Counting means for counting an index value correlated with the amount of the at least one transfer member used in the second mode;
When the counting result of the counting means reaches a predetermined threshold value, the resistance value of the resistance element connected between the at least one transfer member and the power source is changed from the first resistance value to the first resistance value. Switching means for performing control to switch to a second resistance value smaller than the value;
The image forming apparatus according to claim 16 or 17, further comprising:

Images