JP2017125996A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of maintaining performances of a transfer rotating body for a long period of time by effectively reducing polarization of a conducting agent included in the transfer rotating body.SOLUTION: A secondary transfer outer roller 25 for forming a secondary transfer part T2 between itself and an intermediate transfer belt 12 contains an ionic conductive conducting agent in an elastic layer 25b. A feeder roller 26 is pressed to be in contact with the secondary transfer outer roller 25 to form a feeder nip N1 to supply charges to the secondary transfer outer roller 25. A power supply PW forms a potential gradient to give the same polarity to the intermediate transfer belt 12 as the charged polarity of a toner image, based on the potential of the secondary transfer outer roller 25, and to give the opposite polarity to the feeder roller 26 to the charged polarity of the toner image. On a circumference direction of the secondary transfer outer roller 25, a contact width of the feeder nip N1 is set to be equal to or smaller than a contact width of the secondary transfer part T2, which ensures the current density of a current supplied to the elastic layer 25b in the feeder nip N1.SELECTED DRAWING: Figure 2

Description

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

  Conventionally, there has been an intermediate transfer type image forming apparatus in which a toner image formed on a photosensitive drum is primarily transferred to an intermediate transfer belt (intermediate transfer member) at a primary transfer portion, and further secondary transferred onto a recording material at a secondary transfer portion. Are known. Such an image forming apparatus is provided with a transfer rotator such as a transfer roller that is in contact with the outer peripheral surface of the intermediate transfer belt and forms a secondary transfer portion with the intermediate transfer belt. In the secondary transfer section, for example, an electric field is formed to transfer the toner image carried on the intermediate transfer belt to the recording material by applying a bias voltage to the opposite roller that faces the transfer roller across the intermediate transfer belt. Is done.

  By the way, the transfer roller constituting the secondary transfer portion has a conductive shaft core and an outer peripheral layer provided on the outer periphery of the shaft core, and an ion conductive conductive agent (ionic conductive agent) is provided on the outer peripheral layer. Some are given moderate conductivity by being dispersed. In this case, it is conceivable that the electrical resistance increases as the energization time elapses due to the ionic conductive agent being biased (polarized) to the surface side or the axial core side of the outer peripheral layer by the secondary transfer voltage. When the secondary transfer voltage is increased in accordance with the increase in electrical resistance, it is necessary to replace the transfer roller before the power capacity is exceeded. Therefore, the polarization of the ionic conductive agent has been a factor that reduces the life of the transfer roller.

  Therefore, conventionally, in order to prevent an increase in electrical resistance due to the polarization of the conductive agent, a configuration has been proposed in which a power supply roller in contact with the surface of the transfer roller is provided and a bias voltage is applied to the transfer roller via the power supply roller. (See Patent Document 1). In this configuration, the charge supplied from the power supply roller to the transfer roller reaches the secondary transfer portion via the shaft core, thereby reducing the polarization of the ion conductive agent.

JP-A-2005-316200

  However, in the configuration in which the charge is supplied to the secondary transfer outer roller using the power supply roller (power supply member) as in the image forming apparatus described in Patent Document 1, the polarization of the ionic conductive agent may not be sufficiently relaxed. . Such a phenomenon tends to occur when the contact area between the power supply roller and the transfer roller is large, for example, when the pressure contact load of the power supply roller to the transfer roller is large.

  SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an image forming apparatus capable of effectively relaxing the polarization of a conductive agent contained in a transfer rotator and maintaining the performance of the transfer rotator over a long period of time. And

  An image forming apparatus according to the present invention includes an image carrier that moves, a toner image forming unit that forms a toner image on the image carrier, a conductive shaft having conductivity, and an outer periphery of the conductive shaft. A transfer roller having an outer peripheral layer containing an agent and forming a transfer portion for transferring a toner image formed on the image carrier to a recording material; and abutting on the outer peripheral layer of the transfer roller; When a power supply member that forms a power supply unit that supplies electric charge is formed between the transfer roller and the potential of the transfer roller, the potential of the image carrier in the transfer unit is the same as the charging polarity of the toner. And a power supply unit that forms a potential gradient in which the potential of the power supply member has a polarity opposite to the charging polarity of the toner in the power supply unit, and the transfer roller in the power supply unit in the circumferential direction of the transfer roller Said Contact width of the conductive member, the or less contact width between the transfer roller in the transfer unit and the image bearing member, and wherein the.

  According to the present invention, it is possible to effectively relax the polarization of the conductive agent contained in the transfer rotator and maintain the performance of the transfer rotator over a long period of time.

1 is a schematic diagram illustrating a configuration of an image forming apparatus according to a first embodiment. The front view which shows the secondary transfer part which concerns on 1st Embodiment. The schematic diagram which shows the press-contact structure of the electric power feeding roller which concerns on 1st Embodiment. The graph which shows the relationship between the outer diameter of a feed roller, and the contact width in a feed nip part. The graph which shows the relationship between the load amount of a feed roller, and the contact width in a feed nip part. The front view which shows the secondary transfer part which concerns on 2nd Embodiment. The schematic diagram which shows the press-contact structure of the electric power feeding roller which concerns on 2nd Embodiment. The front view which shows the secondary transfer part which concerns on 3rd Embodiment. The front view which shows the secondary transfer part which concerns on 4th Embodiment. The front view which shows the secondary transfer part which concerns on 5th Embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, an image forming apparatus 100 according to the present embodiment will be described with reference to FIG. 1 showing a schematic configuration of the apparatus. An image forming apparatus 100 shown in FIG. 1 is a tandem type intermediate transfer system in which yellow, magenta, cyan, and black image forming units UY, UM, UC, and UK (toner image forming units) are arranged along an intermediate transfer belt 12. It is a full color printer.

[Image forming apparatus]
In the image forming unit UY, a yellow toner image is formed on the photosensitive drum 1Y and transferred to the intermediate transfer belt 12. In the image forming unit UM, a magenta toner image is formed on the photosensitive drum 1M and transferred to the intermediate transfer belt 12. In the image forming units UC and UK, a cyan toner image and a black toner image are formed on the photosensitive drums 1C and 1K, respectively, and transferred (primary transfer) to the intermediate transfer belt 12. The four-color toner images transferred onto the intermediate transfer belt 12 are transferred to the secondary transfer unit 20 and transferred to the recording material S (sheet material such as paper, OHP sheet) at once (secondary transfer). The

  The image forming units UY, UM, UC, and UK are substantially the same except that the toner colors used in the developing devices 4Y, 4M, 4C, and 4K are yellow, magenta, cyan, and black, respectively. In the following, the configuration and operation of the image forming unit U will be described with reference numerals indicating the distinction between the image forming units UY, UM, UC, and UK, with the reference numerals omitting Y, M, C, and K at the end. .

  The image forming unit U includes a charging roller 2, an exposure device 3, a developing device 4, a primary transfer roller 5, and a drum cleaning device 6 that are disposed around the photosensitive drum 1. The photosensitive drum 1 is a drum-shaped photosensitive member formed by forming a photosensitive layer on the peripheral surface of an aluminum cylinder, for example, and rotates in the direction of arrow R1 in the drawing at a predetermined process speed.

  The charging roller 2 is charged with a charging voltage in contact with the photosensitive drum 1 to charge the photosensitive drum 1 to a uniform dark portion potential having the same polarity as the charging polarity of the toner. The exposure device 3 generates a laser beam, which is ON / OFF modulated in accordance with the scanning line image data obtained by developing the separation color image of each color, from the laser light emitting element and scans it with a rotating mirror to convert the image data onto the surface of the photosensitive drum 1. Write the electrostatic image based. The developing device 4 contains a two-component developer containing toner and carrier, and the toner and carrier are frictionally charged by stirring the developer inside the container (in this embodiment, the toner is negative and the carrier is positive). ) The developing device 4 applies a developing voltage to the developing sleeve carrying the developer, thereby supplying toner to the photosensitive drum 1 and developing the electrostatic image into a toner image.

  The primary transfer roller 5 is disposed to face the photosensitive drum 1 with the intermediate transfer belt 12 interposed therebetween, and forms a primary transfer portion T1 as a nip portion between the photosensitive drum 1 and the intermediate transfer belt 12. A primary transfer voltage having a polarity opposite to the charging polarity of the toner is applied to the primary transfer roller 5 by, for example, a high voltage power source (not shown). As a result, the toner image carried on the photosensitive drum 1 is electrostatically attracted to the intermediate transfer belt 12 and primary transfer is performed. The drum cleaning device 6 has a cleaning blade that rubs against the photosensitive drum 1 and collects the primary transfer residual toner that remains slightly on the photosensitive drum 1 after the primary transfer.

  The intermediate transfer belt 12 as an image carrier (intermediate transfer member) that carries and rotates the toner images transferred from the image forming units UY to UK is an endless belt member. The intermediate transfer belt 12 is wound around a driving roller 23, a tension roller 28, idler rollers 22 and 29, a secondary transfer inner roller 24, and the like, and comes into contact with the photosensitive drum 1 at the primary transfer portion T1 of each image forming portion. Yes. The drive roller 23 is connected to a motor (not shown) and is driven to rotate, thereby rotating the intermediate transfer belt 12 in the direction of arrow R2 in the drawing. The tension roller 28 is urged by an elastic member (not shown) such as a spring to press the back surface of the intermediate transfer belt 12 toward the outer peripheral side, and applies a tension of about 10 kgf to the intermediate transfer belt 12. The idler rollers 22 and 29 are arranged on the upstream side and the downstream side of the four primary transfer portions T1 arranged in the rotation direction of the intermediate transfer belt 12, and regulate the rotation trajectory of the intermediate transfer belt 12.

  Specifically, the secondary transfer unit 20 described later includes a secondary transfer portion T2 as a transfer portion that transfers a toner image carried on the intermediate transfer belt 12 to a recording material S (recording medium). In the secondary transfer portion T2, the full-color toner image carried on the intermediate transfer belt 12 is collectively transferred (secondary transfer) to the recording material S. Secondary transfer residual toner remaining on the intermediate transfer belt 12 after the secondary transfer is collected by a belt cleaning device 11 having a blade member that rubs the intermediate transfer belt 12.

  The recording material S on which the full-color toner image is transferred by the secondary transfer unit 20 is conveyed to the fixing device 30. The fixing device 30 includes fixing rollers 31 and 32 that are in contact with each other to form a fixing nip portion T3. The fixing device 30 fixes the toner image onto the recording material S while conveying the recording material S at the fixing nip portion T3. The fixing device 30 is provided with a heating source such as a lamp heater that heats the fixing roller 31 from the inside, and an urging mechanism such as a spring that presses the fixing rollers 31 and 32 together. As a result, the recording material S sandwiched in the fixing nip T3 is heated and pressurized, and the toner image is melted and fixed on the recording material S. The recording material S on which the toner image has been fixed by passing through the fixing device 30 is discharged out of the machine body by a discharge mechanism (not shown).

  In addition, the image forming apparatus 100 includes a transport system including a tray (not shown) on which the recording material S is stacked, a feeding unit (not shown) for feeding the recording material S, and a registration roller pair 66. ing. The feeding unit includes a pickup roller that feeds out the recording material S stacked on the tray, and feeds the recording material S to the apparatus main body in accordance with image formation in the image forming units UY to UK. The registration roller pair 66 abuts against the front end of the recording material S fed from the feeding unit to correct skewing, and transfers the recording material S to the secondary transfer portion T2 in accordance with the transfer timing in the secondary transfer unit 20. Transport to.

[Primary transfer roller]
The primary transfer roller 5 is a metal roller formed into a roller shape using a metal such as SUM (free cutting steel) or SUS (stainless steel). The primary transfer roller 5 is formed in a straight shape in the thrust direction (rotation axis direction), and its diameter (roller diameter) is about 6 to 10 mm.

[Intermediate transfer belt]
The intermediate transfer belt 12 is an endless belt member formed using a resin such as polyimide or polyamide or an alloy thereof, or various rubber materials containing an appropriate amount of an antistatic agent such as carbon black. . The intermediate transfer belt 12 is formed to a thickness of about 0.04 to 0.5 mm, and is given conductivity so that the surface resistivity is 1 × 10 ^{9 to} 5 × 10 ¹³ (Ω / □).

  Hereinafter, the secondary transfer unit of the image forming apparatus according to the first to fourth embodiments will be described in detail. The configuration of the image forming apparatus 100 described above is common to the first to fourth embodiments except for the secondary transfer unit. Therefore, in the following description, members having the same configuration and function are denoted by the same reference numerals and description thereof is omitted.

<First Embodiment>
The secondary transfer unit 20 according to the first embodiment will be described with reference to FIG. The secondary transfer unit 20 includes a secondary transfer outer roller 25 that contacts the outer peripheral surface (toner image carrying surface) of the intermediate transfer belt 12, a secondary transfer inner roller 24 that contacts the inner peripheral surface of the intermediate transfer belt 12, And a power feeding roller 26 that contacts the secondary transfer outer roller 25. The secondary transfer inner roller 24 and the secondary transfer outer roller 25 are arranged to face each other with the intermediate transfer belt 12 interposed therebetween, and are driven to rotate in accordance with the rotation of the intermediate transfer belt 12. The secondary transfer portion T2 is formed as a contact area (transfer nip portion) between the secondary transfer outer roller 25 and the intermediate transfer belt 12.

  The power supply roller 26 arranged in parallel with the secondary transfer outer roller 25 and the shaft center is driven to rotate in accordance with the rotation of the secondary transfer outer roller 25 while being in contact with the outer peripheral portion of the secondary transfer outer roller 25. A power supply nip N1 serving as a power supply unit for transferring charges from the power supply roller 26 to the secondary transfer outer roller 25 is formed as a contact region between the secondary transfer outer roller 25 and the power supply roller 26.

[Secondary transfer outer roller]
The secondary transfer outer roller 25 (transfer roller) as a transfer rotator has a conductive shaft core 25a (conductive shaft) and an elastic layer 25b (outer peripheral layer) formed on the outer periphery of the shaft core 25a. The shaft core 25a is a cylindrical member made of a conductor such as stainless steel, and the elastic layer 25b is formed of a sponge-like or solid elastic material. In the embodiment shown in FIG. 2, the elastic layer 25b is formed as a sponge rubber layer having a thickness of 6 mm, and the entire diameter (roller diameter) of the secondary transfer outer roller 25 is about 24 mm.

  As the elastic material constituting the elastic layer 25b, an elastomer such as NBR (acrylonitrile butadiene rubber) or EPDM (ethylene propylene rubber), or other synthetic resin can be used. An ionic conductive agent such as a metal complex is added to the elastic layer 25b to impart moderate conductivity (semiconductive). Note that a semiconductive polymer such as epichlorohydrin rubber may be kneaded with the base material of the elastic layer 25b as an ion conductive conductive agent, or a semiconductive polymer and a metal complex may be used in combination. Further, an electroconductive conductive agent such as carbon (carbon black) or metal oxide and an ionic conductive agent may be dispersed in the elastic layer 25b. In short, this embodiment can be applied as long as the outer peripheral layer of the secondary transfer outer roller 25 includes an ionic conductive agent.

[Secondary transfer inner roller]
The secondary transfer inner roller 24 as a stretching roller that stretches the intermediate transfer belt 12 includes an elastic layer made of an elastic material such as EPDM and having a thickness of about 0.5 mm, and a metal shaft core that supports the elastic layer. And has a diameter (roller diameter) of 19 mm. The secondary transfer inner roller 24 is pressed against the secondary transfer outer roller 25 with the intermediate transfer belt 12 interposed therebetween by a biasing member 106 that is contracted between the bearing portion and the apparatus main body. As long as the contact pressure is appropriately generated in the secondary transfer portion T2, a biasing member that presses the secondary transfer outer roller 25 against the secondary transfer inner roller 24 may be provided instead of the biasing member 106. . Appropriate conductivity is imparted to the elastic layer of the secondary transfer inner roller 24 by adding a conductive agent such as the above-described ionic conductive agent or carbon. The hardness of the elastic layer is preferably set to be 70 ° using, for example, an Asker C type measuring instrument.

[Power supply roller]
The power supply roller 26 as a power supply member capable of supplying electric charges to the secondary transfer outer roller 25 is a metal roller formed in a roller shape using a metal such as SUM or SUS. The power supply roller 26 has a diameter (roller diameter) of about 10 mm, for example, and is provided so as to be driven to rotate by the secondary transfer outer roller 25 in a state of being in contact with the surface of the elastic layer 25 b of the secondary transfer outer roller 25.

[Secondary transfer power supply and external power supply]
Next, the electrical configuration of the secondary transfer unit 20 will be described. As shown in FIG. 2, the power supply unit PW that supplies power to the secondary transfer unit 20 includes a secondary transfer power supply 40 and an external power supply 50. However, “external” of the external power supply 50 refers to supplying power to members other than the intermediate transfer belt 12, the secondary transfer inner roller 24, and the secondary transfer outer roller 25 that form the secondary transfer portion T2. It does not indicate the outside of the housing of the image forming apparatus 100.

  A secondary transfer power source 40 as a first power source for forming a bias electric field in the secondary transfer portion T2 applies a voltage having the same polarity (negative polarity in this embodiment) as the charging polarity of the toner image to the secondary transfer inner roller 24. A constant voltage source. In other words, the secondary transfer power supply 40 has a potential gradient between the shaft core of the secondary transfer inner roller 24 and the shaft core 25a of the secondary transfer outer roller 25 so that the secondary transfer outer roller 25 side is positive. Form. Note that the maximum output (maximum absolute value of the voltage that can be applied) Vm1 of the secondary transfer power supply 40 is set to 6000 [V]. In other words, the minimum applied voltage of the secondary transfer power supply 40 in this embodiment is −6000 [V].

  The applied voltage of the secondary transfer power supply 40 is controlled (fixed) so that the magnitude of the current (transfer current) flowing between the intermediate transfer belt 12 and the secondary transfer outer roller 25 in the secondary transfer portion T2 approaches the target value. Current control). The control device provided in the apparatus main body is configured such that the magnitude of the transfer current can be measured by an ammeter 80 interposed between the secondary transfer power supply 40 and the secondary transfer inner roller 24, for example. For example, the control device refers to the transfer current measured under the condition that the recording material S does not intervene in the secondary transfer portion T2 before the image forming operation, according to the type (basis weight, material, etc.) of the recording material S. To set the applied voltage during image formation.

  The secondary transfer outer roller 25 is in contact with the intermediate transfer belt 12 at the secondary transfer portion T2, and is in contact with the power supply roller 26 at a position different from the secondary transfer portion T2 in the circumferential direction. The shaft core 25a of the secondary transfer outer roller 25 is connected to the ground potential via a conductive member 60 as a conductive path, and is configured to flow the charge of the shaft core 25a to the ground potential. However, the conductive member 60 is not limited to a conductive wire, and may be any member that has conductivity and grounds the shaft core 25a. For example, the bearing portion of the shaft core 25a may be formed of a conductive material.

  With the above-described configuration, a bias electric field is formed in the secondary transfer portion T2 so that the secondary transfer outer roller 25 side has a polarity opposite to the charging polarity of the toner image. As a result, the toner image is electrostatically attracted to the recording material S sandwiched between the secondary transfer portions T2, and the secondary transfer process is achieved.

  Here, when a transfer current flows through the secondary transfer portion T2, an amount of charge corresponding to the transfer current passes through the elastic layer 25b of the secondary transfer outer roller 25. That is, in the present embodiment, the negative ionic conductive agent moves inward in the radial direction of the elastic layer 25b, and the positive ionic conductive agent moves outward in the radial direction of the elastic layer 25b. If the ionic conductive agent is biased (polarized) toward the inner or outer peripheral side of the elastic layer 25b, the electric resistance of the elastic layer 25b is related to the current in the direction of increasing the degree of polarization of the ionic conductive agent. It will rise. Then, in order to maintain the transfer current, it is necessary to set the secondary transfer voltage to a higher voltage. In this case, since it is necessary to replace the secondary transfer outer roller 25 before the output of the secondary transfer power supply 40 reaches the maximum output, the polarization of the ionic conductive agent is a factor that reduces the life of the secondary transfer outer roller 25. End up. Further, when the secondary transfer voltage is set to a high voltage in order to maintain the transfer current, the toner image is likely to be transferred to the recording material S on the upstream side of the secondary transfer portion T2, and the image quality may be deteriorated. is there.

  Therefore, in the present embodiment, a power supply roller 26 that can supply charges to the secondary transfer outer roller 25 and an external power supply 50 as a second power source that applies a voltage to the power supply roller 26 are provided. The external power supply 50 applies a voltage having a polarity opposite to the charging polarity of the toner image (positive polarity in this embodiment) to the power supply roller 26. In other words, the external power supply 50 forms a potential gradient between the shaft core 25a of the secondary transfer outer roller 25 and the power supply roller 26 so that the secondary transfer outer roller 25 side has a negative polarity. Therefore, the current supplied by the external power supply 50 is in the opposite direction to the direction of the transfer current in the radial direction of the elastic layer 25b. In this embodiment, the transfer current flows radially outward in the elastic layer 25b, while a radially inward current flows between the power supply roller 26 and the shaft core 25a. Note that the maximum output Vm2 of the external power supply 50 (absolute value of the applicable voltage) Vm2 is set to 5000 [V]. That is, in this embodiment, the maximum applied voltage of the external power supply 50 is +5000 [V].

  The external power supply 50 is a constant current source, and supplies a current having a magnitude corresponding to the magnitude of the transfer current to the power supply roller 26. In other words, the output voltage of the external power supply 50 is controlled such that the current flowing between the power supply roller 26 and the shaft core 25a substantially matches the target value for constant current control. For this reason, the ionic conductive agent moved in one radial direction by the transfer current moves to the other radial direction by the current supplied from the power supply roller 26. Thereby, the polarization of the ion conductive material can be relaxed (cancelled), and the performance (electrical resistance) of the elastic layer 25b can be maintained for a long period of time. Hereinafter, of the current flowing through the elastic layer 25b, the current flowing between the feeding roller 26 and the shaft core 25a is referred to as “relaxation current”.

  Note that the secondary transfer power supply 40 and the external power supply 50 may use either a constant voltage source or a constant current source, respectively. However, by using the secondary transfer power supply 40 as a constant voltage source as in the present embodiment, the electric field strength in the secondary transfer portion T2 can be stabilized and good transferability (secondary transferability) can be obtained. . Further, by using the external power supply 50 as a constant current source, a current corresponding to the transfer current can be easily supplied to the power supply roller 26.

  Here, as will be described later as a fifth embodiment, a configuration in which a transfer current and a relaxation current are supplied by a single high-voltage power supply (see FIG. 10) is conceivable. However, in such a configuration, a path through which a current flows is longer than in the present embodiment, and one high-voltage power supply bears the electrical resistance of the entire path from the power supply roller 26 to the secondary transfer inner roller 24. become. Therefore, in order to form a sufficiently strong bias electric field in the secondary transfer portion T2, a large-capacity high-voltage power supply with a large upper limit of the output voltage is required, which causes an increase in cost. On the other hand, in this embodiment, by dispersing the load to the two high-voltage power supplies, it is possible to reduce the polarization of the ion conductive agent while suppressing the cost of the power supply system.

[Pressure contact configuration of power supply roller]
Next, a configuration in which the power feeding roller 26 is pressed against the secondary transfer outer roller 25 will be described with reference to FIG. The secondary transfer outer roller 25 is rotatably supported by a supporting member 103 (supporting portion) attached to a frame or the like of the apparatus main body at shaft portions 25c protruding from both axial ends of the shaft core 25a. . The power supply roller 26 is rotatably supported by a bearing 104 (bearing portion) at shaft portions 26a protruding at both ends in the axial direction. The bearing 104 is disposed in the opening 103 a of the support member 103 and is provided so as to be movable in the direction between the axes of the secondary transfer outer roller 25 and the power supply roller 26.

  Between the bearing 104 and the support member 103, a compression spring as a biasing member that supports the bearing 104 on the support member 103 and presses the power supply roller 26 against the secondary transfer outer roller 25 via the bearing 104. 105 is shrunk. Accordingly, a contact pressure (nip pressure) is generated in the power supply nip portion N1 due to a load applied to the power supply roller 26 by the compression spring 105 via the bearing 104.

  Here, the upper limit of the load applied to the contact power supply roller 26 that defines the contact pressure in the power supply nip portion N1 will be described. When the contact pressure at the power supply nip portion N1 is large, the nip width of the power supply nip portion N1 (the width of the contact surface viewed from the axial direction) increases, and the area of the power supply nip portion N1 (the power supply roller 26 and the secondary transfer outer roller 25). Contact area) increases. If the area of the power feeding nip portion N1 is larger than the area of the secondary transfer portion T2 (the contact area between the intermediate transfer belt 12 and the secondary transfer outer roller 25), the ionic conductive agent is polarized by the relaxation current. The effect of mitigating is impaired. This is because assuming that the amount of current supplied by the power supply roller 26 to the secondary transfer outer roller 25 is equal, the larger the area of the power supply nip portion N1, the smaller the current density in the elastic layer 25b. When the current density of the relaxation current is smaller than the current density of the transfer current, the mobility of the ionic conductive agent in the direction opposite to the polarization direction due to the transfer current is small, and the polarization of the ionic conductive agent is not sufficiently offset It becomes.

  Therefore, in this embodiment, the load amount of the power supply roller 26 is set to a value equal to or less than the upper limit set so that the area of the power supply nip portion N1 does not exceed the area of the secondary transfer portion T2. Accordingly, when viewed from the axial direction of the secondary transfer outer roller 25, the contact width of the power feeding nip portion N1 in the circumferential direction is equal to or smaller than the contact width of the secondary transfer portion T2. However, “so as not to exceed the area of the secondary transfer portion T2” includes a case where the areas can be regarded as being substantially equal in consideration of fluctuations in the contact width due to axial position or roller rotation.

  Further, since the elastic layer 25b of the secondary transfer outer roller 25 is formed of an elastic material, there is a risk of plastic deformation when the contact pressure is excessive. In particular, when the sponge-like elastic layer 25b is used as in the above embodiment, the elastic layer 25b is plastically deformed with a smaller contact pressure than when a solid elastic material is used. Therefore, the contact pressure of the power supply roller 26 is preferably set in a range sufficiently smaller than a value at which plastic deformation of the elastic layer 25b starts.

[Example]
Hereinafter, a specific description will be given using examples to which the present embodiment is applied. The strength of the compression spring 105 in this embodiment is such that the value of the load amount F2 of the power supply roller 26 to the secondary transfer outer roller 25 (hereinafter simply referred to as “the load amount of the power supply roller 26”) is, for example, 1500 [gf]. It is set to become. The outer diameter and width of the secondary transfer outer roller 25 were 24 mm and 300 mm, respectively, and the outer diameter and width of the secondary transfer inner roller 24 were 20 mm and 320 mm, respectively. The power supply roller 26 has an outer diameter and a width of 10 mm and 320 mm, respectively. Note that the load amount F2 of the power supply roller 26 is set in a range (for example, half or less) sufficiently smaller than the load amount 6000 gf at which the elastic deformation of the elastic layer 25b starts.

  As shown in FIG. 4, when the outer diameter L2 of the power supply roller 26 is changed to a value around 10 mm under a load amount F2 = 1500 [gf], the nip width of the power supply nip portion N1 (secondary transfer outer roller 25). The contact width between the power supply roller 26 and the power supply roller 26 is substantially proportional to the outer diameter L2. Further, as shown in FIG. 5, when the outer diameter L2 of the power supply roller 26 is fixed to 10 mm and the load amount F2 is changed before and after 1500 gf, the nip width of the power supply nip portion N1 is substantially proportional to the load amount F2. It became a relationship.

  A similar relationship was also found between the outer diameter L1 and load amount F1 of the secondary transfer inner roller 24 and the nip width of the secondary transfer portion T2. However, the load amount F1 of the secondary transfer inner roller 24 is the magnitude of the load on the secondary transfer outer roller 25 of the secondary transfer inner roller 24 in the secondary transfer portion T2. Further, the nip width of the secondary transfer portion T2 is a contact width between the intermediate transfer belt 12 and the secondary transfer outer roller 25 as viewed from the axial direction. When the outer diameter L1 of the secondary transfer inner roller 24 is changed to a value around 20 mm under a load amount F1 = 3000 [gf], the nip width of the secondary transfer portion T2 is approximately proportional to the outer diameter L1. became. Further, when the outer diameter L1 of the secondary transfer inner roller 24 is fixed to 20 mm and the load amount F1 is changed around 3000 gf, the nip width of the secondary transfer portion T2 is substantially proportional to the load amount F1. It was.

From the above results, it can be seen that the area of the power feeding nip portion N1 is substantially proportional to the product F2 × L2 of the load amount F2 of the power feeding roller 26 and the outer diameter L2. It can also be seen that the area of the secondary transfer portion T2 is approximately proportional to the product F1 × L1 of the load amount F1 of the secondary transfer inner roller 24 and the outer diameter L1. Therefore, in order to prevent the area of the power feeding nip portion N1 from exceeding the area of the secondary transfer portion T2, the following formula (1a) may be satisfied.
F1 × L1 ≧ F2 × L2 (1a)

When formula (1a) is transformed, it becomes as follows.
F2 ≦ (L1 / L2) × F1 (1b)
That is, what is necessary is just to set the load amount F2 of the electric power feeding roller 26 so that it may become below the upper limit prescribed | regulated by Formula (1b). As a result, a current density of relaxation current at least comparable to the current density of the transfer current can be secured, and the polarization of the ionic conductive agent can be efficiently relaxed by the current supplied by the power supply roller 26.

  In this embodiment, the load amount F2 of the power supply roller 26 is adjusted to set the area relationship between the power supply nip portion N1 and the secondary transfer portion T2, but other factors may be adjusted. Absent. For example, the outer diameter L2 of the power supply roller 26, the outer diameter L1 and the load amount F1 of the secondary transfer inner roller 24, or the hardness of the outer peripheral layer of the power supply roller and the secondary transfer inner roller 24 may be adjusted.

[Lower contact pressure]
Next, the lower limit of the contact pressure of the power supply roller 26 will be described. From the viewpoint of simplifying the configuration of the secondary transfer unit 20, a configuration in which the power supply roller 26 is driven and rotated by the secondary transfer outer roller 25 is preferable. However, in this case, a sliding load between the shaft portion 26a of the power supply roller 26 and the bearing 104 (see FIG. 3) (the shaft portion 26a acts on the shaft portion 26a with the shaft portion 26a being stationary with respect to the bearing 104). It is necessary to consider the load torque due to friction). That is, the smaller the contact pressure at the power feeding nip portion N1, the smaller the maximum value of the frictional force that can be transmitted without causing a slip at the power feeding nip portion N1. If the sliding load of the power supply roller 26 exceeds the maximum value of the friction torque in the power supply nip portion N1, the power supply roller 26 does not rotate with the secondary transfer outer roller 25 and slip occurs in the power supply nip portion N1. It becomes the state. In this case, the elastic layer 25b of the secondary transfer outer roller 25 is rubbed against the power supply roller 26, and there is a possibility that the surface of the elastic layer 25b is worn.

Therefore, in the present embodiment, the load amount F of the power supply roller 26 is set to a value larger than the lower limit set so that the maximum value of the friction torque in the power supply nip portion N1 is larger than the sliding load of the power supply roller 26. . That is, Tc [gf · cm] is the torque to the power supply roller 26 when the maximum static frictional force is applied at the power supply nip portion N1, and the maximum value of the sliding load is the sliding torque T [gf · cm] of the power supply roller 26. As
Tc> T (2a)
If it becomes.

Here, when the coefficient of static friction between the power supply roller 26 and the elastic layer 25b is μ, the maximum static frictional force in the power supply nip portion N1 is μ × F2. Then, using the load amount F2 in the inter-axis direction and the outer diameter L2 of the power supply roller 26, the expression (2a) is rewritten as follows.
μ × F2 × (L2 / 2)> T (2b)

The equation (2b) is transformed as follows.
(2 × T) / (μ × L2) <F2 (2c)
That is, the strength of the compression spring 105 may be set so that the power supply roller 26 load amount F2 satisfies the relationship of the expression (2c).

When the coefficient of static friction between the shaft portion 26b and the bearing 104 is μ ′ and the diameter of the shaft portion 26b of the power supply roller is L3, the sliding torque T is approximately T = μ ′ × F2 × (L3 / 2). )It can be expressed as. Substituting this into equation (2b) yields:
μ × F2 × (L2 / 2)> μ ′ × F2 × (L3 / 2)
When this is modified, the following equation (2d) is obtained.
μ × L2> μ ′ × L3 (2d)
That is, in order to satisfy the equation (2c), it is understood that the shape and material of the secondary transfer outer roller 25, the power supply roller 26, and the bearing 104 may be configured so that the relationship of the equation (2d) is satisfied. .

  In the configuration of the above-described embodiment, the sliding start torque T = 10 [gf · cm], the static friction coefficient μ = 5.0, and L2 = 1.0 [cm]. In the other examples, the slip-out torque T = 50 [gf · cm], and the values of μ and L2 were the same as those in the above example. In this case, by setting the load amount F2 to, for example, 1500 [gf], it is possible to suppress the slip at the power feeding nip portion N1 and to ensure the stability of the operation of the secondary transfer unit 20.

To summarize the above description, the load amount F2 applied to the power supply roller 26 may be set so as to satisfy the relationship of the following expression (3).
(2 × T) / (μ × L2) <F2 ≦ (L1 / L2) × F1 (3)
Accordingly, it is possible to configure the secondary transfer unit 20 that operates stably while suppressing the slip in the power feeding nip portion N1 while effectively relieving the polarization of the ionic conductive agent caused by the transfer current.

Second Embodiment
Next, the secondary transfer unit 20A according to the second embodiment will be described with reference to FIG. The secondary transfer unit 20A according to the present embodiment is different from the secondary transfer unit 20 according to the first embodiment (see FIG. 2) in that a Zener diode 61 is provided in the ground portion 60A, and the other configuration. Are the same. About the same structure, the same code | symbol as 1st Embodiment is attached | subjected and description is abbreviate | omitted.

  As shown in FIG. 6, the secondary transfer outer roller 25 according to the present embodiment is connected to the ground potential by a ground portion 60 </ b> A having a conductive member 60 and a Zener diode 61. The conductive member 60 is made of a conductive material such as a conductive wire. The Zener diode 61 is interposed in the conductive path formed by the conductive member 60, and has a breakdown voltage set to a sufficiently small value (for example, 50V) compared to the applied voltage of the secondary transfer power supply 40 and the external power supply 50. Therefore, the Zener diode 61 is an example of an electric element through which a current flows when the potential difference between the potential of the shaft core 25a of the secondary transfer outer roller 25 and the ground potential becomes a predetermined value or more (breakdown voltage or more).

  Further, as shown in FIG. 7, in this embodiment, the contact pressure of the power feeding nip portion N1 is created by using a tension spring 107 as an urging member. The secondary transfer outer roller 25 is rotatably supported by the support member 103A (support portion) at shaft portions 25c protruding from both axial ends of the shaft core 25a. The power supply roller 26 is rotatably supported by a bearing 104A (bearing portion) at a shaft portion 26a that protrudes from both ends in the axial direction. The tension spring 107 is stretched between the bearing 104A and the support member 103A so that the bearing 104A is supported by the support member 103A and the power supply roller 26 is pressed against the secondary transfer outer roller 25. .

  Even in such a configuration, the polarization of the ionic conductive agent contained in the secondary transfer outer roller 25 can be reduced by supplying a current corresponding to the transfer current from the external power supply 50 to the power supply roller 26. . And the effect similar to 1st Embodiment can be acquired by setting the intensity | strength of the tension | pulling spring 107 so that the load amount F2 of the electric power feeding roller 26 may satisfy | fill the relationship of said Formula (3).

<Third Embodiment>
Next, the secondary transfer unit 20B according to the third embodiment will be described with reference to FIG. The secondary transfer unit 20B according to the present embodiment is different from the secondary transfer unit 20 according to the first embodiment (see FIG. 2) in that a varistor 62 is disposed in the grounding portion 60B. Are the same. About the same structure, the same code | symbol as 1st Embodiment is attached | subjected and description is abbreviate | omitted.

  As shown in FIG. 8, the secondary transfer outer roller 25 according to this embodiment is connected to the ground potential by a ground portion 60 </ b> B having a conductive member 60 and a varistor 62. The conductive member 60 is made of a conductive material such as a conductive wire. The varistor 62 is interposed in the conductive path formed by the conductive member 60, and has a varistor voltage set to a sufficiently small value (for example, 50 V) compared to the applied voltage of the secondary transfer power supply 40 and the external power supply 50. However, the varistor voltage refers to a voltage when a current equal to or larger than the transfer current (for example, 10 [uA]) flows as a reference current. Therefore, the varistor 62 is an example of an electrical element through which a current flows when the potential difference between the potential of the shaft core 25a of the secondary transfer outer roller 25 and the ground potential becomes a predetermined value (varistor voltage) or more.

  Even in such a configuration, the polarization of the ionic conductive agent contained in the secondary transfer outer roller 25 can be reduced by supplying a current corresponding to the transfer current from the external power supply 50 to the power supply roller 26. . And the effect similar to 1st Embodiment can be acquired by setting the load amount F2 provided to the electric power feeding roller 26 to the range with which the relationship of said Formula (3) is satisfy | filled.

<Fourth embodiment>
Next, a secondary transfer unit 20C according to the fourth embodiment will be described with reference to FIG. The secondary transfer unit 20C according to the present embodiment is different from the secondary transfer unit 20 according to the first embodiment (see FIG. 2) in that a low-voltage power source 63 is disposed in the grounding portion 60C, and other configurations. Are the same. About the same structure, the same code | symbol as 1st Embodiment is attached | subjected and description is abbreviate | omitted.

  As shown in FIG. 9, the secondary transfer outer roller 25 according to the present embodiment is connected to the ground potential by a ground portion 60 </ b> C having a conductive member 60 and a low-voltage power source 63. The low-voltage power supply 63 applies a voltage whose absolute value is sufficiently smaller (for example, 20 V) than the applied voltage of the secondary transfer power supply 40 and the external power supply 50. Accordingly, the low-voltage power supply 63 is an example of an electrical element through which a current flows when the potential difference between the potential of the shaft core 25a of the secondary transfer outer roller 25 and the ground potential exceeds a predetermined value (voltage applied to the low-voltage power supply). is there.

  Even in such a configuration, the polarization of the ionic conductive agent contained in the secondary transfer outer roller 25 can be reduced by supplying a current corresponding to the transfer current from the external power supply 50 to the power supply roller 26. . And the effect similar to 1st Embodiment can be acquired by setting the load amount F2 provided to the electric power feeding roller 26 to the range with which the relationship of said Formula (3) is satisfy | filled.

<Fifth Embodiment>
Next, a secondary transfer unit 20D according to the fifth embodiment will be described with reference to FIG. Compared to the secondary transfer unit 20 of the first embodiment (see FIG. 2), the secondary transfer unit 20D of the fifth embodiment supplies a transfer current and a relaxation current using one high-voltage power supply. Is different. Other configurations are the same. About the same structure, the same code | symbol as 1st Embodiment is attached | subjected and description is abbreviate | omitted.

  The power supply unit PW <b> 2 according to the present embodiment is configured by a constant voltage source 70 connected to the power supply roller 26. As the constant voltage source 70, for example, one having a maximum applied voltage of +6000 [V] can be used. The secondary transfer inner roller 24 is connected to the ground potential via a conductive member 65 as a ground portion, and the secondary transfer outer roller 25 interposed between the secondary transfer inner roller 24 and the power supply roller 26 is ground potential. Insulated from floating.

  The constant voltage source 70 applies a voltage having a polarity opposite to the toner charging polarity (negative polarity in the present embodiment) to the power supply roller 26. As a result, a potential gradient in which potentials are arranged in this order is formed between the power supply roller 26, the secondary transfer outer roller 25, and the secondary transfer inner roller 24. Due to this potential gradient, a bias electric field that urges the toner image carried on the intermediate transfer belt 12 toward the secondary transfer outer roller 25 is formed in the secondary transfer portion T2. Further, in the power feeding nip portion N <b> 1, a charge having a polarity opposite to that supplied to the secondary transfer outer roller 25 in the secondary transfer portion T <b> 2 is supplied from the power feeding roller 26 to the secondary transfer outer roller 25. Therefore, in the elastic layer 25b of the secondary transfer outer roller 25, currents (transfer current and relaxation current) opposite in the radial direction flow on the power feeding nip portion N1 side and the secondary transfer portion T2 side. It becomes.

  With such a configuration, similarly to the power supply unit PW described in the first to fourth embodiments, the polarization of the ionic conductive agent caused by the transfer current can be relaxed by the relaxation current supplied by the power supply roller 26. In other words, the ionic conductive agent contained in the elastic layer 25b moves alternately to the outer peripheral side and the shaft core side each time the secondary transfer outer roller 25 makes a half rotation, and is less likely to be biased to one side. As a result, an increase in electrical resistance due to polarization can be prevented, and the performance of the secondary transfer outer roller 25 can be maintained over a long period of time. And the effect similar to 1st Embodiment can be acquired by setting the load amount F2 which a biasing member gives to the electric power feeding roller 26 in the range with which the relationship of said Formula (3) is satisfy | filled.

<Other embodiments>
In the first to fifth embodiments described above, the intermediate transfer member has been described as an endless belt member (intermediate transfer belt 12). However, the present invention is not limited to this, and for example, a drum-like intermediate transfer member (intermediate transfer drum) is used. It doesn't matter. In this case, the first power supply (secondary transfer power supply 40) may be connected to the intermediate transfer drum to apply a voltage.

  The configuration of the power supply units (PW, PW2) is not limited to the above-described configuration, and a transfer current is generated in the secondary transfer unit T2 and the transfer current in the thickness direction of the elastic layer 25b in the power feeding nip unit N1. Any device that generates a current in the reverse direction may be used. In short, when the potential of the conductive portion of the transfer rotator is used as a reference, a potential gradient can be formed in which the image carrier has the same polarity as the charging polarity of the toner image and the power supply member has the opposite polarity to the charging polarity of the toner image. Any configuration may be used.

  Further, an endless belt member (transfer conveyance belt) may be stretched around the transfer roller. Even in this case, the polarization of the ionic conductive agent is effectively mitigated by configuring the contact area of the power supply unit (power supply nip unit) so as not to exceed the contact area of the transfer unit (secondary transfer unit). be able to.

  In the first to fifth embodiments, the secondary transfer unit has been described. For example, the first to fifth embodiments are applied as a primary transfer unit that transfers a toner image from a photosensitive drum (photosensitive member) to an intermediate transfer belt (intermediate transfer member). Also good. In this case, the photosensitive drum corresponds to the image carrier, the intermediate transfer belt corresponds to the recording medium, and the primary transfer roller corresponds to the transfer rotator. Even in such a configuration, the power supply unit is configured so that the power supply member can supply a relaxation current, and the contact area of the power supply unit is configured not to exceed the contact area of the transfer unit. Polarization can be effectively relaxed.

  12 ... Intermediate transfer member (intermediate transfer belt) / 24 ... Stretch roller (secondary transfer inner roller) / 25 ... Transfer roller (secondary transfer outer roller) / 25a ... Conductive shaft (axial core) / 25b ... Outer layer ( Elastic layer) / 26 ... Power supply member (power supply roller) / 40 ... First power source (secondary transfer power source) / 50 ... Second power source (external power source) / 60 ... Conductive path (conductive member) / 61, 62 ... Electric Element (Zener diode, varistor) / 63... Power source (low voltage power source) / 70... Power source (constant voltage source) / 100... Image forming apparatus / 103, 103A .. support part (support member) / 104, 104A. ) / 105 ... biasing member (compression spring) / 107 ... biasing member (tensile spring) / N1 ... power feeding part (power feeding nip part) / PW, PW2 ... power source part / S ... recording medium (recording material) / T2 ... Transfer part (secondary transfer part) / UY, UM, UC, UK ... Over image forming unit (image forming unit)

Claims (10)

A moving image carrier;
A toner image forming unit for forming a toner image on an image carrier;
A transfer having a conductive shaft having conductivity and an outer peripheral layer formed on the outer peripheral side of the conductive shaft and including a conductive agent, and forming a transfer portion for transferring a toner image formed on the image carrier to a recording material Laura,
A power supply member that is in contact with the outer peripheral layer of the transfer roller and forms a power supply unit that supplies electric charge to the transfer roller between the transfer roller;
When the potential of the transfer roller is used as a reference, the potential of the image carrier in the transfer portion is the same as the charging polarity of the toner, and the potential of the feeding member in the feeding portion is opposite to the charging polarity of the toner. A power supply unit that forms a potential gradient
In the circumferential direction of the transfer roller, a contact width between the transfer roller and the power supply member in the power feeding unit is equal to or less than a contact width between the transfer roller and the image carrier in the transfer unit.
An image forming apparatus. The power supply member is a power supply roller,
The image carrier is an endless intermediate transfer belt,
A tension roller that contacts an inner peripheral surface of the intermediate transfer belt and is in contact with the transfer roller via the intermediate transfer belt;
The image forming apparatus according to claim 1. When the outer diameter of the transfer roller is L1, the outer diameter of the power supply roller is L2, the load of the stretching roller on the transfer roller is F1, and the load of the power supply roller on the transfer roller is F2.
F2 ≦ (L1 / L2) × F1
It is characterized by satisfying,
The image forming apparatus according to claim 2. A bearing portion for rotatably supporting the power supply roller;
The maximum value of the load torque that can be applied to the power supply roller from the bearing portion in a state where the power supply roller is stationary with respect to the bearing portion is T, and the static friction coefficient between the transfer roller and the power supply roller is μ. If
(2 × T) / (μ × L2) <F2
It is characterized by satisfying,
The image forming apparatus according to claim 3. A support portion for rotatably supporting the transfer roller;
An urging member that supports the bearing portion on the support portion and abuts the power supply roller on the transfer roller via the bearing portion;
The image forming apparatus according to claim 4. The power supply unit includes: a first power source that applies a voltage having the same polarity as the charging polarity of toner to the stretching roller; and a second power source that applies a voltage having a polarity opposite to the charging polarity of toner to the feeding roller. Including
It comprises a conductive path for passing a current from the conductive shaft of the transfer roller to a ground potential.
The image forming apparatus according to claim 2. An electrical element that generates a predetermined voltage when a current flows is provided between the conductive shaft of the transfer roller and a ground potential.
The image forming apparatus according to claim 6. The electrical element is a Zener diode or a varistor,
The image forming apparatus according to claim 7. And a power source for applying a voltage having a small absolute value to the conductive axis as compared with an applied voltage of the first power source and the second power source when a toner image is transferred in the transfer unit. ,
The image forming apparatus according to claim 6. The power supply unit has a power supply that applies a voltage having a polarity opposite to a charging polarity of toner to the power supply roller,
The stretching roller is connected to a ground potential,
The image forming apparatus according to claim 2.

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