JPH09114279A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable stably obtaining an excellent image, by preventing the fluctuation of the current flow in a secondary transferring. SOLUTION: The secondary transferring part of this device cosists of an intermediate transferring belt 1 for carrying on performing a primary transfer of the toner image carried by the image carrier, a bias roll 2 for performing the secondary transfer of the unfixed toner image on the intermediate transferring belt 1 onto a recording medium P, the backup roll 3 for supporting the intermediate transferring belt 1 from its rear surface on a position opposite to the bias roll 2, and the electrode roll 4 for applying a transfer voltage on the bias roll 2 while coming in press contact with the backup roll 3. The backup roll 3, is composed of the insulating rubber roll 3b whose surface is covered by a semiconductive thin layer 3c adopting a material having a large surface energy whose contact angle with a water drop is <=90 deg., or the high polymer material whose glass transition temp. is >=50 deg.C, as a structural component. As the above described material, a block copolymerized body with nylon 12 or its polyester component is suitable therefor.

Description

Detailed Description of the Invention

[0001]

The present invention relates to an electrophotographic copying machine,
The present invention relates to an image forming apparatus using an electrophotographic system such as a printer, a facsimile, and a composite device of these. More specifically, the present invention relates to an image forming apparatus in which a toner image formed on an image carrier is temporarily transferred once to an intermediate transfer belt, and then this is secondarily transferred to a recording medium such as paper to obtain a reproduced image. .

[0002]

2. Description of the Related Art An image forming apparatus using an electrophotographic system is a laser in which a uniform charge is formed on an image carrier composed of a photoreceptor made of an inorganic or organic photoconductive material and an image signal is modulated. After forming an electrostatic latent image with light or the like, the electrostatic latent image is developed with charged toner to obtain a visualized toner image. Then, this toner image is transferred to a recording medium such as a paper through an intermediate transfer body or directly to obtain a required reproduced image. In particular, in the case where the toner image formed on the image carrier is primarily transferred to the intermediate transfer member and the toner image on the intermediate transfer member is secondarily transferred to the recording medium, a conductive bias roll is used. There is known an image forming apparatus of a bias roll type in which a recording medium is pressed against an intermediate transfer body by using the recording medium and the toner image is electrostatically transferred by the action of an electric field. The bias roller transfer method for performing electrostatic transfer while applying a transfer voltage to a conductive bias roll includes a backup roll that receives a pressing force of the bias roll and forms a path for a transfer current.

As a conventional example of an image forming apparatus adopting the above method, for example, Japanese Patent Laid-Open No. 6-95521 is known. FIG. 9 is an explanatory diagram of the secondary transfer portion in the transfer device disclosed in the above publication. In the figure, an intermediate transfer member
01 is stretched by a backup roll 02 and a plurality of support rollers 03, and moves in the direction of the arrow. Backup roll
As 02, a silicone rubber roll coated with a thin layer of carbon black-dispersed polycarbonate is used. Further, in the secondary transfer portion, a bias roll 05 for applying a transfer voltage from the power source 04, and an electrode roll (pressing and rotating on the backup roll 02 to form a transfer current path based on the transfer voltage ( Earth roll) 06. When a transfer voltage is applied between the electrode roll 06 and the backup roll 02 and the bias roll 05 when the toner image on the intermediate transfer member 01 is transferred to the paper P, the backup roll 02 and the intermediate transfer member 01 and the paper are transferred. A transfer current flows to the bias roll 05 through P.

[0004]

As a method of manufacturing and processing a backup roll formed by coating an insulating rubber roll with a semiconductive film, for example, the following methods are known. One is to process a predetermined film material into a tube shape and inflate it by pressurizing a fluid such as air from the inside, insert an insulating roll into it, and then reduce the pressure of the fluid to shrink it. Therefore, there is a processing method for forming a thin surface layer. As another method, a tube material is stretched on the inner surface of a cylinder-shaped mold and attached in a tube shape, and a liquid insulating rubber material is injected thereinto, which is vulcanized and cured to form a film-covered rubber roll. There is a method of molding. The tube (film) material used in both the above-mentioned processing methods is 50 to 20 at the time of processing.
Since 0% elongation is imparted, the tube material must have a deformation amount of 50 to 200% in the elastic region. However, the carbon black-dispersed polycarbonate disclosed in JP-A-6-95521 is
Since the amount of deformation in the elastic region is small, the tube material is plastically deformed during the coating process, and wrinkles, cracks, and the like occur on the surface, which makes it quite difficult to coat the rubber roll.

Fluorine resins such as PFA (perfluoroalkoxy resin), ETFE (ethylene-tetrafluoroethylene resin) and PVDF (polyvinylidene fluoride) are used as the resin having a deformation amount of 50 to 200% in the elastic region. Can be mentioned. However, when a rubber roll coated with a tube material made of carbon black-dispersed PFA is used as the backup roll, the following problems occur. According to the method of expressing the surface energy by the wettability of water, the fluorine resin such as PFA has a contact angle of 100 ° or more between the plane of the test piece and the water drop when water is dropped on the test piece, and the surface energy is small, and the intermolecular The attractive force is also small. Therefore, the force for restraining the conductive filler such as carbon black is small. Therefore, when a voltage is applied, the carbon black easily moves by an electric force, and when a conductive circuit is formed by the movement of the carbon black, the resistance value decreases. This conductive circuit, once formed, exhibits an irreversible property that cannot be restored, so that the resistance value is kept constant at a low value. Further, in the case of a fluororesin, although the molecular segment constituting the polymer does not lose its relative position as a whole molecule, it has a micro-Brownian motion, that is, a glass transition point (Tg: second-order transition point) at which it starts to rotate or vibrate. Is lower than the ambient temperature. Therefore, the ambient temperature (1
At 0 to 50 ° C.), the molecular segment makes a micro Brownian motion, and similarly, the force for restraining the conductive filler such as carbon black is also small. That is, when a fluorocarbon resin such as PFA dispersed in carbon black is used as the tube material forming the surface layer of the backup roll, the resistance changes due to the repeated pressure applied by the electrode roll rotating under pressure and the transfer current. However, there is a problem in that the current flowing in the contact area between the two changes, and hence the transfer electric field changes.

As a measure against this problem, the applicant of the present invention contacts the bias roll with the intermediate transfer member at the time of standby or before the pre-cycle before the copying operation, and the current flowing from the bias roll through the intermediate transfer member and the backup roll is applied. A patent application was filed for an image forming apparatus in which the backup roll resistance is found by detection, the optimum transfer voltage is determined according to the backup roll resistance, and the transfer voltage is applied to the bias roll (Japanese Patent Application No.
-249102). However, in the control method of detecting the current and determining the optimum transfer voltage as described above, it may be difficult to obtain the optimum transfer voltage when the resistance of the backup roll largely changes by one digit (10 times) or more. It was found that there were problems such as high power supply cost.

As described above, the fluororesin which can secure the amount of deformation in the elastic region when the rubber roll is coated has a small surface energy and a Tg lower than the ambient temperature, so that the force for restraining the conductive filler is small, and the carbon black dispersion. When the above fluororesin is used as the surface layer constituent material of the backup roll, the transfer electric field changes during use. Further, in the control method for determining the optimum transfer voltage before the operation of the image forming apparatus, it is difficult to cope with a case where the backup roll resistance changes greatly. That is, in the conventional bias roller transfer method, the transfer electric field fluctuates due to the change in the voltage drop amount due to the change in the transfer current flowing from the backup roll to the bias roll or from the bias roll to the backup roll, and the stable toner is obtained. There was a problem that it could not be transferred. Therefore, the present invention is intended to solve the above-described problems, and prevents fluctuations in the secondary transfer current flowing from the bias roll to the backup roll or in the opposite direction to stably obtain a high quality image. It is to provide an image forming apparatus capable of performing the above.

[0008]

In the conventional bias roller transfer method in which a toner image is secondarily transferred using an intermediate transfer member, the present inventor has room for improvement in the material forming the surface layer of the backup roll. I have reached the point that
As a result of intensive investigations focusing on the constituent material of the surface layer, the above problems can be solved by using a material having a large surface energy or a polymer material having a high glass transition point Tg as the constituent material. Is confirmed. That is, the image forming apparatus of the present invention includes an image carrier that forms an electrostatic latent image according to image information, and a developing device that visualizes the electrostatic latent image formed on the image carrier as a toner image with toner. An intermediate transfer belt that primarily transfers and carries the toner image carried on the image carrier, a bias roll that secondarily transfers the unfixed toner image on the intermediate transfer belt to the recording medium, and an intermediate transfer that faces the bias roll. It consists of a backup roll supported from the back side of the belt and an electrode roll that presses against the backup roll and applies a transfer voltage to the bias roll.The backup roll has a contact angle with a water drop when it is indicated by the wettability of water. Is 90 ° or less and has a high surface energy or a polymer material having a glass transition point of 50 ° C. or more. Characterized by comprising the sexual rubber roll.

The above-mentioned "water wettability" is displayed using the material before the dispersion of the conductive agent that constitutes the semiconductive thin layer as a test piece, and the contact angle between the plane of the test piece and the water droplet as a scale.
Here, when water droplets are placed on the surface of the test piece, the surface tension γs of the test piece, the interfacial tension γi between the liquid and the test piece, and the surface tension γl of the liquid are balanced to form a certain shape as shown in FIG. Form. At this time, if the droplet is small and the influence of gravity can be ignored, the following Young's equation (1) is established. γs = γi + γlcosθ (1) The “material having a large surface energy” in the present invention means a material having the contact angle θ of 90 ° or less.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
First, the surface energy will be further described from the viewpoint of "wetting". From a macro perspective, wetting is a phenomenon in which the contact surface between a solid and a gas spontaneously replaces the contact surface between a solid and a liquid, accompanied by a decrease in the free energy of the system. Also, from a microscopic point of view, this phenomenon is observed when the intermolecular attractive force (adhesive force) between the solid and the liquid is larger than the intermolecular attractive force, that is, the cohesive force of the liquid. It is known that the change in free energy starts from a system in which a solid and a liquid are already in contact with each other, and reverses the sign (±) of the work required to separate the solid and the liquid. Have been. The above work W is the following formula
It is represented by (2). W = γsg + γlg − γsl (2) where γsg, γlg, and γsl are solid / gas, respectively.
It is the interfacial free energy between liquid / gas and solid / liquid, and has the same meaning as γs, γl, and γi in the formula (1). As is apparent from the above equation (2), the change in free energy includes the surface free energy of the solid and the interfacial free energy between the solid and the liquid. Contact angles are used. That is, the above-mentioned Young's equation holds between the above-mentioned γsg, γlg, γsl and the contact angle θ. cosθ = (γsg−γsl) / γlg (1 ′) Therefore, in the present invention, the surface energy of the material forming the semiconductive thin layer is represented by the contact angle θ between the thin layer plane and the water droplet.

Next, the secondary transfer section in the image forming apparatus of the present invention will be described. FIG. 1 is a schematic configuration diagram of the secondary transfer section, in which a backup roll 3 is arranged at a position facing a bias roll 2 via an intermediate transfer belt 1 carrying an unfixed toner image that has been primarily transferred. The backup roll 3 supports the intermediate transfer belt 1 from the back surface. An electrode roll 4 for applying a transfer voltage to the bias roll 2 is pressed against the backup roll 3. The bias roll 2 is grounded, and the electrode roll 4 is connected to the power source 5. As the intermediate transfer belt 1, a belt formed by mixing various resins or rubber materials with an appropriate amount of a conductive agent such as carbon black and molding it to a thickness of 0.05 to 0.15 mm is used, and the surface resistivity thereof is 10 ⁸ to 10 ^{8. 14} Ω / □
The range has been adjusted. Examples of various resins include acrylic resin, polyvinyl chloride, polyester, polycarbonate, and polyimide. Bias roll 2 above
Is made of a rubber material such as silicone rubber or urethane rubber and a conductive agent such as carbon black in an appropriate amount, and has a low resistance, and may be a metal roll in some cases.
The bias roll 2 is separated from the transfer belt 1 while the toner image carried on the image carrier is primarily transferred onto the intermediate transfer belt 1, and the toner image carried on the transfer belt 1 is transferred to, for example, the paper P. When the image is secondarily transferred to a recording medium (typically referred to as a sheet P hereinafter) such as the above, the transfer belt 1 is pressed and pressed against the backup roll 3.

The backup roll 3 has a semiconductive thin layer 3c coated on the surface of a roll body formed of a shaft body 3a and an insulating rubber roll 3b. The insulating rubber roll 3b is made of a rubber material having a volume resistivity of 10 ¹⁴ Ωcm or more, such as silicone rubber, urethane rubber, or EPDM. Further, the semiconductive thin layer 3c constituting the surface layer of the backup roll has a surface resistivity of 10 ⁸ to 10 ¹⁰ Ω /, which is obtained by blending a conductive agent with a material having a large surface energy or a polymer material having a high Tg. It is in the range of □. As described above, the material having a large surface energy means a material having a contact angle θ with a water droplet of 90 ° or less when the wettability is represented by water. A material with a high Tg has a temperature of 50
It means a polymer material having a temperature of ℃ or higher. These materials (hereinafter,
Typical examples of the polymer material) include nylon 11, nylon 12, and block copolymers of these nylons and a polyether component.

As the conductive agent dispersed in the polymer material,
For example, tin oxide (SnO ₂ ), zinc oxide (ZnO), indium oxide (In ₂ O ₃ ), potassium titanate (K ₂ Ti)
O ₄ ), a metal oxide such as SnO ₂ —In ₂ O ₃ solid solution,
Examples include carbon black. The compounding ratio of the conductive agent in the conductive thin layer 3c is, for example, 5 to 15% by weight in the case of carbon black and 5 to 2 in the case of metal oxide.
It is preferably in the range of 5% by weight. When the blending ratio is less than 5% by weight, the surface resistivity of the semiconductive thin layer 3c becomes higher than 10 ¹⁰ Ω / □. On the other hand, when the compounding ratio exceeds the above upper limit, the surface hardness of the semiconductive thin layer 3c increases, and the elastic modulus of the backup roll 3 becomes too high.
As the surface hardness, the Asker C hardness is preferably in the range of 58 to 65 °. The thickness of the semiconductive thin layer 3c is in the range of 10 to 100 μm, preferably 20 to 50 μm. If the thickness is less than 10 μm, a pinhole may be generated during long-term use due to the applied transfer voltage and the pressing force from the electrode roll 4. On the other hand, when the thickness is more than 100 μm, the adhesion with the insulating rubber roll 3b becomes poor, and it becomes difficult to manufacture the backup roll 3 having stable quality.

The material forming the electrode roll 4 is not particularly limited as long as it is a metal or alloy having good electrical conductivity, and for example, stainless steel (SUS), copper, aluminum or the like is used. A transfer voltage of −2 to −5 kV is applied to the bias roll 2 from the electrode roll 4 through the backup roll 3. When the transfer voltage is lower than 2 kV (absolute value), the strength of the electric field for transferring the unfixed toner image on the intermediate transfer belt 1 onto the paper P is insufficient. on the other hand,
If the voltage is higher than 5 kV, it will be
The change in the surface resistivity of the backup roll 3 becomes large, which is not preferable.

The operation of the secondary transcription mechanism in the present invention is as follows. When the primary transfer of the toner image from the image carrier onto the intermediate transfer belt 1 is completed and the intermediate transfer belt 1 carrying the toner image of a desired hue moves to the secondary transfer portion, it is synchronized with this. And the sheet P is conveyed to the secondary transfer portion.
At this time, the bias roll 2 in the retracted position from the transfer belt 1 is in pressure contact with the transfer belt 1 whose back side is supported by the backup roll 3. Then, when the paper P passes through the secondary transfer portion while receiving the pressure contact force between the bias roll 2 and the backup roll 3, a transfer voltage having the same polarity as the charging polarity of the toner image is applied from the electrode roll 4. As a result, the toner image carried on the transfer belt 1 is secondarily transferred to the paper P from the surface of the intermediate transfer belt 1. According to the present invention, the back surface of the intermediate transfer belt 1 is supported by the backup roll 3 at a position facing the bias roll 2, and the semiconductive thin layer 3c on the surface of the backup roll 3 is pressed against the electrode roll 4. Therefore, when a transfer voltage is applied to the electrode roll 4, the bias roll 2
A transfer current path to the semi-conductive thin layer 3c of the backup roll 3 and the electrode roll 4 is formed via the intermediate transfer belt 1. The backup roll 3 has a surface layer composed of a semiconductive thin layer 3c in which a conductive material is mixed with a polymer material having a large surface energy or a high Tg. It is possible,
Since the required secondary transfer current can be obtained stably,
Stable secondary transfer of the toner image is performed on the paper P.

[0016]

EXAMPLES Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited to the following Examples. (Image Forming Apparatus) FIG. 2 is an overall view of a digital color copying machine provided with a transfer belt as the image forming apparatus of the present invention. In FIG. 2, a document illumination lamp 12 that moves along the lower surface of a document (not shown) placed on the platen 11.
The light emitted from the document and reflected by the document is converged on the CCD of the image reading unit via the moving mirror unit 13, the lens 14, and the fixed mirror 15. The CCD converts the original image into an electric signal for each color by using a large number of photoelectric conversion elements and filters of three colors of red (R), green (G) and blue (B). This electric signal is input to the image processing circuit 16, and the image processing circuit 16 has an image memory that converts the original image reading signal input for each color into a digital signal and stores the digital signal.

The optical writing control device 17 reads out the image data of the image processing circuit 16 at a predetermined timing and outputs it to the light beam writing device 18. Light beam writing device 18
Writes an electrostatic latent image corresponding to each of the above colors on the image carrier 19 composed of a photosensitive drum that rotates in the direction of arrow A. Around the image carrier 19, a charging charger 20 for uniformly charging the surface of the image carrier 19, a developing unit 21 for developing the electrostatic latent image written on the image carrier 19 into a toner image of each color, and a developing unit 21 for each color. The intermediate transfer belt 1 onto which the toner image is transferred, a static eliminator, and a cleaner unit 22 having a cleaning blade are arranged. The developing unit 21 is black
(K), yellow (Y), magenta (M), cyan (C) of each color has a developing device containing toner, each of the toner of each color is developed to visualize the electrostatic latent image. The intermediate transfer belt 1 is stretched around the backup roll 3 and the belt conveying rolls 23, 24, 25 which the electrode roll 4 presses, and moves in the tangential direction while contacting the surface of the image carrier 19. In this embodiment, among the rolls (3, 23 to 25) around which the transfer belt 1 is stretched, the transfer belt 1 is the arrow B.
The backup roll 3 driven by the rotation of the shaft body 3a is configured as a drive roll and the other belt transport rolls (23 to 25) are configured as driven rolls so as to move in the direction. Further, in order to prevent the transfer belt 1 from bending, the shaft of the belt transport roll 25 is biased in the direction C by a spring (not shown).

On the back side of the intermediate transfer belt 1, a transfer corotron 26 for transferring toner images of respective colors is provided with an image carrier 19.
And the surface of the transfer belt 1 are in contact with each other. On the other hand, on the surface side of the transfer belt 1 which holds the unfixed toner image, the bias roll 2 is provided so as to face the backup roll 3 and the belt transport roll 23.
And a belt cleaner 27 is arranged. A portion where the bias roll 2 and the backup roll 3 face each other serves as a secondary transfer portion. Further, between the backup roll 3 and the belt transport roll 23, a peeling claw 28 for peeling the sheet P carrying the secondary-transferred toner image from the transfer belt 1 is arranged. A cleaning blade 29 made of polyurethane or the like is constantly in contact with the surface of the bias roll 2 to remove foreign matter such as toner particles and paper dust attached by transfer or the like. An extractable paper feed tray 30 is provided below the main body of the image forming apparatus U, and a pickup roller 31 is arranged above the paper feed tray 30. A pair of registration rolls 32 is arranged downstream of the pickup roller 31. Further, on the downstream side of the secondary transfer portion, a transport belt 33 that sequentially transports the paper P carrying the secondary-transferred toner image, a fixing device 34 that fixes an unfixed toner image on the paper P, A pair of ejection rolls 35 for ejecting the paper P on which the fixed image is formed and a paper ejection tray 36 for placing the ejected paper P are arranged.

(Operation of Image Forming Apparatus) The surface of the image carrier 19 rotating in the direction of arrow A is uniformly charged by the charging charger 20. This uniformly charged image carrier 1
An electrostatic latent image 9 is written by the light beam writing device 18. The electrostatic latent image on the image carrier 19 is developed by the developing unit 21 into an unfixed toner image. In the formation of this toner image, the toner image of the first color is first formed, and thereafter, every time the image carrier 19 makes one rotation, the toner images of the second color to the fourth color are formed. In this embodiment, K, Y, M, C
Color toner images are sequentially formed. The surface of the image carrier 19 is cleaned by the blade of the cleaner unit 22 after the toner image is transferred to the intermediate transfer belt 1. Here, the optical writing control device 17
Then, first, the K color digital signal of the first color is read and output to the light beam writing device 18. The writing device 18 writes an electrostatic latent image corresponding to K color on the surface of the image carrier 19. The electrostatic latent image corresponding to K color is developed into a visualized toner image of K color by the developing device K in the developing unit 21.
Move to the primary transfer part. In the primary transfer portion, the transfer corotron 26 disposed on the back surface side of the intermediate transfer belt 1 applies an electric field having a polarity opposite to the charging polarity to the toner image, so that the K color toner image reaching the primary transfer portion is transferred. While being electrostatically attracted to the intermediate transfer belt 1, the primary transfer is performed by the movement of the intermediate transfer belt 1 in the arrow B direction.

The intermediate transfer belt 1 moves in the same cycle as the image carrier 19 while adsorbing and carrying the K toner image. When the transfer of the K toner image of the first color is completed, the output of the optical writing control device 17 outputs the Y color of the second color until the transfer start position of the K toner image on the transfer belt 1 reaches the primary transfer portion.
Writing of the electrostatic latent image of color is started. When the transfer start position of the transfer belt 1 carrying the K toner image reaches the primary transfer portion, the transfer corotron 26 transfers the second color Y toner image. Then, the transfer of the M toner image and the C toner image is performed in the same manner as the transfer of the Y toner image. In this way, a multiple toner image superimposed on each color is formed on the intermediate transfer belt 1. Until the toner image of each color is primarily transferred onto the intermediate transfer belt 1,
The bias roll 2, the peeling claw 28, and the belt cleaner 27 arranged on the front surface side of the transfer belt 1 are held at a retracted position separated from the intermediate transfer belt 1.

On the other hand, the paper P stored in the paper feed tray 30
Are picked up one by one by a pickup roller 31 at a predetermined timing and fed, and a pair of registration rolls 32
Is temporarily stopped at. The paper P is then the intermediate transfer belt 1
The multiple toner images of the respective colors (K, Y, M, C) transferred on the upper side are conveyed from the registration roll 32 to the secondary transfer portion in synchronization with the movement of the multiple toner images to the secondary transfer portion. In the secondary transfer portion, the bias roll 2 is in pressure contact with the backup roll 3 via the transfer belt 1. Then, the conveyed paper P passes through the secondary transfer portion by the pressure contact conveyance between the rolls 2 and 3 and the movement of the transfer belt 1. At this time, the toner image adsorbed and carried on the transfer belt 1 is secondarily transferred onto the paper P from the surface of the intermediate transfer belt 1 by electrostatic repulsion by applying a transfer voltage having the same polarity as the charging polarity of the toner image from the electrode roll 4. To be done.

The transfer of the full-color image has been described above, but in the case of forming a single-color image, the intermediate transfer belt 1 is used.
When, for example, a K-color toner image primary-transferred above moves to the secondary transfer section, the toner image is immediately transferred to the paper P. In the case of forming an image of a plurality of colors, the desired hue is selected, and when the multicolor toner image superimposed on those colors moves to the secondary transfer section, the toner image may be transferred to the paper P. . In the case of this multicolor image transfer, as described above, the rotation of the image carrier 19 and the movement of the intermediate transfer belt 1 are synchronized with each other so that the toner images of the respective colors accurately coincide with each other at the primary transfer portion. There is. As described above, the sheet P on which the toner image is transferred in the desired hue is peeled off by the operation of the peeling claw 28, is placed on the conveyance belt 33, and is conveyed to the fixing device 45. Then, in the fixing device 34, the unfixed toner image is fixed and fixed to the permanent image, and then the paper P is discharged to the paper discharge tray 36 by the pair of paper discharge rolls 35. When the secondary transfer is completed, the intermediate transfer belt 1
The surface thereof is cleaned by a belt cleaner 27 provided downstream of the secondary transfer portion to prepare for the next transfer.

(Secondary Transfer Member) The specific structure of the secondary transfer member in the image forming apparatus shown in FIG. 2 is as follows. As the intermediate transfer belt (1), a polyimide resin having a thickness of 80 μm and a surface resistivity of 10 ¹² Ω / □ was used. As the bias roll (2), a silicone rubber having an Asker C hardness of 35 ° in which carbon black was dispersed by about 15 wt% was used, and its volume resistivity was 10 ^5.5 Ωcm. As the insulating rubber roll (3b) constituting the backup roll (3), a silicone rubber molded to a thickness of 6.5 mm and having an Asker C hardness of 62 ° is used, and a semiconductive thin layer (3c) for covering the same is used. ) Is a thickness of 50 μm in which about 9 wt% of carbon black is dispersed in the thin layer (3c).
It is made of nylon 12 resin of m. These rolls (2),
The outer diameter of (3) is 28 mm. Electrode roll (4)
As the metal roll, a SUS metal roll having a diameter of 12 mm was used.
The applied voltage to the electrode roll (4) is -2 kV.

(Physical Property Test of Semi-Conductive Material) Further, the surface resistivity of the material constituting the semi-conductive thin layer, its testing method and some physical properties are shown below. In the following physical property tests, the thickness of the tube material and the blending ratio of carbon black were set to the same conditions as those of the semiconductive thin layer (3c) of the above-mentioned example, and the insulating rubber roll (3
The same thing as the Example was used for b). In addition, PFA, E
TFE and polycarbonate are listed as reference examples for comparison. FIG. 3 shows the time-dependent change in the surface resistivity of the backup roll when PFA was used instead of the nylon 12 resin forming the semiconductive thin layer of the backup roll in the secondary transfer member. The method of applying voltage to the backup roll is as shown in FIG.
Two SUS metal rolls each having a length of 330 mm and a length of 330 mm are brought into contact with the surface of the backup roll having a length of 325 mm at a biting amount of 0.2 mm at a circumferential distance of 10 mm, and a voltage of 1 kV between the metal rolls ( V) was applied. Then, the current value (I) 10 seconds after the lapse of a predetermined time from the application of the voltage was read, and the surface resistivity ρs was obtained by the following formula (3). ρs = LV / GI (3) where L: length of backup roll (cm) G: distance between two metal rolls (cm)

The initial surface resistivity ρs ₀ of the backup roll whose surface is coated with the carbon black-dispersed PFA tube material is 10 ^8.74 Ω / □ (log ρs ₀ = 8.74), and [Δlog RS The value of (RS = ρs)] was obtained from the difference in surface resistivity measured by the method shown in FIG. .DELTA.log RS after application of voltage for 2 hours is -0.
97 and the .DELTA.log RS after 20 hours was -1.45. That is, the resistance sharply decreases from the voltage application for 2 hours, and the surface resistivity ρs is about 1. 2 hours after the voltage application.
It is reduced by about 0 digits, and the transfer current is almost 10 times. After that, the resistance decreases slowly, but it continues for 2 more times at a constant voltage.
The decrease in resistance when applied for 0 hours is about 0.5 digit (log
Value). The PFA-coated backup roll has a large change in the surface resistivity immediately after the voltage is applied, so that the change in the transfer current at the secondary transfer portion is large, and therefore stable secondary transfer cannot be performed.

Polyamide elastomer containing nylon 12 as a polyamide component (ΔPAE; Daicel Hüls)
"Product name: Daiamide PAE-E40" manufactured by K.K., nylon 12 (▲ Ny12), polycarbonate (□ P
C), PFA (●), and ETFE (○) are each dispersed with carbon black, and each tube material obtained is 6
FIG. 5 shows changes in surface resistivity when an AC voltage of 2.5 Vrms at 0 Hz was applied for 1 minute, 2 minutes, 5 minutes, 10 minutes, 20 minutes, 30 minutes, and 60 minutes. As shown in FIG. 6, a tube material (film) having a length of 325 mm is placed on an insulating substrate, and two SUS tubes having a diameter of 15 mm and a length of 330 mm are made on the tube material. The metal rolls were brought into contact with each other at a distance of 10 mm, and the alternating voltage (V) was applied between the metal rolls. Further, the surface resistivity ρs of the tube material after a predetermined time has elapsed is obtained by the above formula (3) by applying a voltage of 1 kV and reading the current value (I) after 10 seconds according to the measuring method shown in FIG. It was However, L in the formula is the length (cm) of the tube material. The value of initial surface resistivity [log ρs ₀ ] of each tube material is
"PAE" was 8.6, "Ny12" and "PC" were 8.0, and "PFA" and "ETFE" were 7.8. As shown in FIG. 5, P containing carbon black dispersed therein
The surface resistivity ρs of FA and ETFE immediately after voltage application decreases by one digit or more, but PAE, Ny1
2 and the tube material in which carbon black is dispersed in PC has a small decrease in resistance due to voltage application.

Tensile rupture strength (kg / c) representing the relationship between stress and strain of each of the tube materials in which carbon black is dispersed
The m ² ) -tensile rupture elongation (%) curve is shown in FIG. 7. As shown in FIG. 7, “PC” is a hard and brittle material having large tensile breaking strength and tensile modulus (see Table 1 below), small elongation, and small elongation. That is, since the amount of deformation in the elastic region is as small as 50%, it is not appropriate as a tube material for covering the insulating rubber roll. On the other hand, "Ny12"
In comparison with PC, since the tensile elastic modulus is smaller and the deformation amount in the elastic region is larger than PC, it is possible to secure the possible deformation amount of the tube coating in the elastic region. "P
"AE" is a material that has a small tensile elastic modulus and is soft and tough, and has a large amount of deformation in the elastic region like "PFA" and "ETFE". That is, it is found that the tube material is more appropriate than Ny12.

Table 1 below shows the tensile rupture strength and tensile rupture elongation of the tube material shown in FIG. 7, as well as the tensile elastic modulus, the contact angle of water and Tg described above. The tensile rupture strength, tensile rupture elongation and tensile elastic modulus in Table 1 are “AS
TM D638 ".

[Table 1] In the surface resistivity measurement test, PFA and ETFE, whose resistance decreased by one digit or more immediately after application of voltage, had large contact angles with water droplets of 122 ° and 117 °, respectively, and small surface energy.

As described above, Ny12, PAE can be applied as the tube material forming the semiconductive thin layer. Therefore, in place of the backup roll (semiconductive thin layer; Ny12) and the Ny12 forming the semiconductive thin layer in the secondary transfer member, a change in the surface resistivity of the backup roll when PAE is used is shown. It shows in FIG. This FIG. 8 shows the resistance fluctuation of the backup roll during offline running. The surface resistivity measurement method of the backup roll was the same as the measurement method shown in FIG. 4 except that the voltage (V) applied between the two metal rolls was 2 kV. In this measurement test, the initial current value (I ₀ ) after 10 seconds from the application of voltage and the current value (I) after 90 hours have elapsed are read, and the surface resistivity ρ is calculated by the equation (3).
I asked for s. The decrease in surface resistivity when a constant voltage (2 kV) was applied for 90 hours continuously was about 0.2 for "Ny12".
It is a digit (log value) and is about 0.5 digit in “PAE”.
Therefore, these polymer materials are adequately suitable as materials for forming the semiconductive thin layer.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made within the scope of the claims.
The secondary transfer member in the present invention is not limited to the color image forming apparatus of the embodiment and can be applied to a monocolor image forming apparatus. In the embodiment, the transfer current is made to flow from the bias roll to the backup roll,
The polarity of the voltage applied to the electrode roll may be reversed so that the toner flows in the opposite direction depending on the charging polarity of the toner.

[0031]

The image forming apparatus of the present invention uses a material having a large surface energy or a polymer material having a glass transition point of 50 ° C. or higher as the thin film forming the semiconductive thin layer of the backup roll. Therefore, the decrease in resistance due to voltage application is small, and the change in resistance with time at the secondary transfer portion is small. Therefore, the change with time of the transfer current flowing from the bias roll to the backup roll or in the opposite direction is reduced, and a high-quality image can be stably obtained.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a secondary transfer unit in an image forming apparatus of the present invention.

FIG. 2 is an overall view of an image forming apparatus shown as one embodiment of the present invention.

FIG. 3 is a graph showing the change over time in the surface resistivity of a backup roll whose surface is coated with a carbon black-dispersed PFA tube material.

FIG. 4 is an explanatory diagram showing a method for measuring the surface resistivity of a backup roll.

FIG. 5 is a graph showing changes over time in the surface resistivity of the tube material forming the semiconductive thin layer.

FIG. 6 is an explanatory diagram showing a method for measuring a surface resistivity of a tube material.

FIG. 7 is a graph showing the relationship between stress and strain of a carbon black-dispersed tube material.

FIG. 8 is a graph showing a resistance variation of a backup roll during offline running.

FIG. 9 is an explanatory diagram of a secondary transfer unit in a conventional image forming apparatus.

FIG. 10 is a cross-sectional view of a test piece plane and a water drop showing a contact angle that is a measure of surface energy.

[Explanation of symbols]

U ... Image forming apparatus, P ... Paper (recording medium), 1 ... Intermediate transfer belt, 2 ... Bias roll, 3 ... Backup roll, 3b ... Insulating rubber roll, 3c ... Semi-conductive thin layer, 4
... electrode roll, 19 ... image carrier, 21 ... developing unit (developing device).

Claims (2)

[Claims] 1. An image bearing member that forms an electrostatic latent image according to image information, a developing device that visualizes the electrostatic latent image formed on the image bearing member as a toner image with toner, and an image bearing member. The intermediate transfer belt that primarily transfers the carried toner image and carries it, a bias roll that secondarily transfers the unfixed toner image on the intermediate transfer belt to the recording medium, and a back surface of the intermediate transfer belt that faces the bias roll. Backup roll and an electrode roll that presses the backup roll to apply a transfer voltage to the bias roll, and the backup roll has a contact angle with a water droplet of 90 ° or less when displayed as wettability of water. It consists of an insulating rubber roll whose surface is coated with a semiconductive thin layer having a material having a large surface energy or a polymer material having a glass transition point of 50 ° C. or higher as a constituent component. An image forming apparatus comprising and. 2. The semiconductive thin layer comprises 5-1 in the thin layer.
The image forming apparatus according to claim 1, wherein the image forming apparatus is made of nylon 12 or nylon 12 elastomer in which 5% by weight of carbon black or 5 to 25% by weight of metal oxide is dispersed.