JPH08115002A - Image forming device - Google Patents

Abstract

PURPOSE: To provide an image forming device which prevents excessive current from flowing into a backup roll from a bias roll and ensures steady transfer. CONSTITUTION: The device is equipped with a latent image carrier 1, an intermediate transfer body 2, the bias roll 10, and the backup roll 4 arranged opposite the bias roll 10. The backup roll 4 comprises a roll main body and a covering layer, one of which is made of a conductive or semiconductive material and the other of which is made of an insulated or semiconductive material. Either the roll main body or the covering layer is gounded. The bias roll 10 is provided with a resistance detection means 17 which finds the resistance of the backup roll 4 by detecting a current flowing through the intermediate transfer body 2 and the backup roll 4 from the bias roll 10. The resistance detection means 17 is provided with a control means 18, which determines an optimum transfer voltage for the bias roll 4 and controls the power source of the bias roll 4 so as to apply this voltage to the bias roll 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a laser printer, and more specifically, a toner image formed on a latent image carrier is transferred to a transfer material such as a sheet through an intermediate transfer member. The present invention relates to an image forming apparatus of the type of transfer to a sheet.

[0002]

2. Description of the Related Art Conventionally, as this type of image forming apparatus, those described in JP-B-49-209 and JP-A-62-206567 are known. That is, in the image forming apparatus, as shown in FIG. 7, the toner image T formed on the latent image carrier 100 such as a photosensitive drum is secondarily transferred to a transfer material 102 such as paper via an intermediate transfer body 101. It is generally known that a desired image is formed on the transfer material 102. The image forming apparatus shown in FIG. 7 includes developing devices 103 to 106 corresponding to each color of black (Bk), yellow (Y), magenta (M), and cyan (C), and superimposes toner images of each color. It is of a type that forms a full-color image together, and the toner images of the respective colors formed every one rotation of the latent image carrier 100 are superposed on the intermediate transfer member 101, and this composite image is collectively transferred to the transfer material 102. Is configured.

In such an image forming apparatus, the primary transfer of the toner image T from the latent image carrier 100 to the intermediate transfer member 101 or the secondary transfer of the toner image T from the intermediate transfer member 101 to the transfer material 102 is performed. The transfer is performed by the electrostatic transfer method. That is, a corotron (corona discharger) 107 disposed on the back surface side of the intermediate transfer body 101, or a bias roll 108 disposed so as to face the intermediate transfer body 101.
By applying a voltage having a polarity opposite to that of the toner, the unfixed toner image is transferred from the latent image carrier 100 to the intermediate transfer member 101 or from the intermediate transfer member 101 to the transfer material 102. is there.

By the way, in the color image forming apparatus of the type before that shown in FIG. 7, the toner images of the respective colors are directly and multiple-transferred onto the transfer material to form the color transfer image. The quality of the color image formed on the transfer material is influenced by many factors such as the thickness of the transfer material, its surface characteristics, and the transfer characteristics of the transfer material to the latent image carrier. On the other hand, as shown in FIG. 7, in the color image forming apparatus using the intermediate transfer member 101, since the composite image which has already been subjected to the multiple transfer is transferred to the transfer material 102, the above-mentioned unstable factors are caused. This has the advantage that it can be eliminated, and that the occurrence of image distortion and color misregistration during multiple transfer can be effectively prevented.

However, in such an image forming apparatus, since the bias roll is used as the secondary transfer means of the toner image, there are the following inconveniences. The intermediate transfer member 101 is provided at a position facing the bias roll 108.
A backup roll 109 that serves as a counter electrode sandwiching it
Is provided, the transfer material 102 of a small size, for example,
When the toner image is secondarily transferred onto the sheet, the bias roll 108 protruding from the transfer material 102 has a semi-conductive intermediate transfer member 1.
01, and an excessive current may flow between the bias roll 108 and the backup roll 109. When such an excessive current flows, a sufficient transfer electric field cannot be formed, and as a result, a transfer failure occurs. In addition, when an excessively large current flows, the intermediate transfer body is easily damaged.

As a technique for solving these inconveniences, there is a transfer roller mechanism disclosed in, for example, Japanese Patent Application Laid-Open No. 1-288880. In this transfer roller mechanism, the backup roll is covered with a dielectric film to prevent the current from flowing into the backup roll, and a back electrode is provided on the back side of the intermediate transfer body adjacent to the backup roll to provide a back electrode. The creeping resistance is used to prevent the generation of excessive current.

[0007]

By the way, in this type of image forming apparatus, since the toner image is usually primarily transferred from the latent image carrier to the intermediate transfer member by using a corona discharger, the intermediate transfer member When the resistance is low, the electric charge given by the corona discharger conducts along the surface of the intermediate transfer body, and the toner image carries the latent image before the primary transfer position, that is, in the area where the latent image carrier and the intermediate transfer body are not in close contact. The early transfer that jumps from the body to the intermediate transfer body occurs, and the image quality deteriorates.

However, in the technique disclosed in Japanese Patent Laid-Open No. 1-288880, when a material having a high resistance is selected as a material for the intermediate transfer member in order to solve the problem of such early transfer, the material between the bias roll and the back electrode is selected. In this case, a large voltage drop occurs in the intermediate transfer portion, which causes a new inconvenience that a transfer electric field of sufficient strength cannot be obtained. It is also possible to reduce the voltage drop by bringing the back electrode closer to the bias roll, but the backup roll has a diameter of 25 mm or more, so that the voltage drop at the intermediate transfer member can be ignored. The back electrode cannot be brought close to the bias roll.

In view of the background described above, the present inventors have earnestly studied, and as a result, have found that the technique described below is effective for solving the above-mentioned inconvenience. That is, as the backup roll, a conductive roll whose peripheral surface is covered with a semiconductive or insulating thin layer is used, and the conductive roll is grounded. In addition, as the backup roll, a peripheral surface of the insulating roll coated with a semiconductive thin layer is used, and the semiconductive thin layer is grounded.

According to these techniques, even if the bias roll comes into direct contact with the intermediate transfer member, the backup roll has a high resistance, so that an excessive current does not flow from the bias roll to the backup roll. Therefore, a stable transfer electric field can be formed even on a recording medium of a small size, and damage to the belt can be prevented in advance. Also, because the resistance of the backup roll itself prevents the generation of excessive current, a large voltage drop does not occur here even if a material with a high resistance is selected as the material of the intermediate transfer member, and therefore the toner image is transferred early. It is also possible to prevent deterioration of image quality due to.

However, even with such a technique, the resistance of the backup roll changes due to variations in manufacturing, environmental changes, deterioration over time, etc. Therefore, when the voltage applied to the bias roll is constant, the backup roll is The change in resistance changes the amount of voltage drop here,
As a result, it was found that there is a problem to be improved such that the transfer electric field changes and stable transfer cannot be performed. Incidentally, according to the experiments by the present inventors, it was found that the transfer voltage changes by about one order due to the change of the backup roll resistance. The present invention has been made in view of the above circumstances, and an object of the present invention is to prevent an excessive current from flowing from a bias roll to a backup roll, and to perform stable transfer, thus obtaining a high-quality image. An object is to provide an image forming apparatus.

[0012]

In the image forming apparatus of the present invention, the backup roll has a roll main body and a coating layer that covers the peripheral surface of the roll main body, and one of the roll main body and the coating layer is electrically conductive. Made of a conductive or semi-conductive material, the other made of an insulating or semi-conductive material, and at least one of the roll body and the coating layer is grounded to form the entire backup roll semi-conductive. The bias roll is provided with resistance detection means for determining the resistance of the backup roll by detecting the current flowing from the bias roll through the intermediate transfer body and the backup roll while the bias roll is in contact with the intermediate transfer body. The resistance detecting means includes a bias roll based on the backup roll resistance obtained by the means. To determine the optimum transfer voltage that, that the control means for sending the power control signal of the bias roll to apply a bias roll is provided the determined transfer voltage was means for solving the above-described problem.

[0013]

According to the image forming apparatus of the present invention, since the backup roll resistance is formed sufficiently higher than the bias roll resistance and the intermediate transfer member resistance, the current flowing from the bias roll through the intermediate transfer member and the backup roll by the resistance detecting means. By detecting the bias roll resistance,
The backup roll resistance can be detected stably without being affected by variations in the resistance of the intermediate transfer member. Therefore,
From the obtained backup roll resistance, the control unit determines the optimum transfer voltage according to the backup roll resistance, and by applying the determined transfer voltage to the bias roll, stable transfer becomes possible.

[0014]

EXAMPLES The image forming apparatus of the present invention will be described in detail below with reference to examples. FIG. 1 is a schematic configuration diagram showing a first embodiment when the present invention is applied to a color electrophotographic copying machine. In FIG. 1, reference numeral 1 is a photosensitive drum (latent image carrier).
Is. The photosensitive drum 1 is configured to rotate in the direction of arrow A, and as the photosensitive drum 1 rotates in the direction of arrow A, an electrostatic latent image corresponding to image information is well known in an electrophotographic process. (Not shown) is formed on the surface thereof. In addition, around the photosensitive drum 1,
Developing devices 5 to 8 corresponding to each color of black (Bk), yellow (Y), magenta (M), and cyan (C) are provided, and the electrostatic latent image formed on the photosensitive drum 1 is The toner image T is formed by developing with one developing device. That is, for example, if the electrostatic latent image written on the photosensitive drum 1 corresponds to the image information of yellow, this electrostatic latent image is developed by the developing device 6 containing the yellow (Y) toner, As a result, a yellow toner image is formed on the photosensitive drum 1.

An intermediate transfer member 2 is arranged around the photosensitive drum 1 so as to contact the surface thereof. The intermediate transfer member 2 is in the form of a belt, is stretched around a plurality of rolls, and rotates in the direction of arrow B. Particularly, on the peripheral side of the photosensitive drum 1, it moves in the tangential direction of the photosensitive drum 1. It is configured to do. The intermediate transfer member 2 is made of a resin such as acrylic, vinyl chloride, polyester, or polycarbonate, or various rubbers containing an appropriate amount of an antistatic agent such as carbon black and has a volume resistivity of 10 ⁶ to 10. About 10 ¹⁴ Ω · cm,
It is preferable to use one that is adjusted to 10 ⁶ to 10 ⁹ Ω · cm or less and that is formed to have a thickness of about 0.1 mm. Here, the volume resistivity is 10 ⁹ Ω
The reason why it is preferably equal to or less than cm is that the resistance of the intermediate transfer member 2 is sufficiently smaller than the resistance of the backup roll as described later.

On the back surface side of the intermediate transfer body 2, a primary transfer device 9 composed of a corona discharger (corotron) is arranged at a position where the photosensitive drum 1 and the intermediate transfer device 2 are in contact with each other, that is, the back side of the primary transfer position. It is set up. In addition, the intermediate transfer member 2
A semi-conductive bias roll 10 is disposed around the periphery of the backup roll 4 which is one of the rollers for stretching the roll. The bias roll 10 sandwiches a part of the intermediate transfer body 2 between the backup roll 4 and the backup roll 4, and is arranged so as to come into contact with and separate from the intermediate transfer body 2. By applying a voltage having a polarity opposite to the charging polarity of the toner to the bias roll 10, the toner image T carried on the intermediate transfer body 2 is transferred at the secondary transfer position.
That is, electrostatic transfer is performed between the bias roll 10 and the backup roll 4 on the transfer material 3 such as paper.

As shown in FIG. 2, the backup roll 4 is composed of a grounded conductive roll (roll body) 21 and a semiconductive layer (coating layer) 22 that covers the peripheral surface of the conductive roll 21. is there. The conductive roll 21 is formed by a rubber roller or a metal roll in which conductive carbon is dispersed. The semiconductive layer 22 is made of PVdF (polyvinylidene fluoride), polyester, PFA.
(Tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), acrylic, or silicone rubber in which an appropriate amount of an antistatic agent such as carbon black is dispersed to control the resistance.

The semiconductive layer 22 having a volume resistivity controlled to 10 ⁸ to 10 ¹¹ Ω · cm is preferably used for the following reason. That is, a current flows through the volume resistance from the contact position between the backup roll 4 and the intermediate transfer member 2 to the ground position of the conductive roll 21, so that the conductive roll 21 moves from the contact position between the backup roll 4 and the intermediate transfer member 2. A potential difference is generated between the semiconductive layer 22 and the semiconductive layer 22 is charged by this potential difference. When the volume resistivity of the semiconductive layer 22 exceeds 10 ¹¹ Ω · cm,
This is because even if the backup roll 4 makes one revolution, this potential remains and density unevenness occurs in a cycle of the circumference length of the backup roll 4. The thickness of this semiconductive layer is about 10 μm to 10 mm. That is, the thinner the thickness of the semiconductive layer 22, the closer the conductive roll 21 is to the bias roll 10. Therefore, if the thickness of the semiconductive layer 22 is in the above range, a sufficient transfer electric field can be obtained even when the voltage applied to the bias roll 10 is low. Because it will be obtained.

In this embodiment, the backup roll 4 has a volume resistance from the contact position with the intermediate transfer member 2 to the grounding position of the conductive roll 21 of 10 ⁷ Ω or more per cm of the backup roll 4 in the rotation axis direction. Is preferable for the following reasons. In this type of device, the current capacity of the power source is limited to several mA or less in consideration of safety when touched by a human body and prevention of an accident such as ignition due to a paper jam. When an overcurrent flows in direct contact, the current limiter of the power supply operates to lower the power supply voltage, preventing continuous overcurrent conduction. In addition, such a configuration is also effective in preventing the intermediate transfer member from being damaged or ignited because a scratch or a hole is generated in the intermediate transfer member and the electric current is concentrated on the portion. is there.

Therefore, when transferring to a small-sized transfer material or when there is no transfer material, if the magnitude of the overcurrent is 100 μA or less per unit length, the current limiter operates and the power supply voltage drops sharply. Without doing so, it is possible to prevent transfer failure and damage to the intermediate transfer member. That is, in this embodiment, for example, if the transfer voltage is about 1000 V, the volume resistance from the contact position between the backup roll 4 and the intermediate transfer member 2 to the ground position of the conductive roll is set to a unit length (1c
It is preferable to set 10 ⁷ Ω or more per m). The volume resistivity of the bias roll 10 is preferably 10 ⁷ Ω · cm or less so as to be sufficiently smaller than the resistance of the backup roll.

A power source 16 for applying a voltage to the bias roll 10 is connected to the bias roll 10. Further, the power source 16 and the bias roll 10 function as resistance detecting means of the present invention. An ammeter 17 is provided. That is, according to the ammeter 17, the power source 16
When a predetermined voltage is applied by the bias roll 10, the current flowing through the bias roll 10, the intermediate transfer member 2 and the backup roll 4 can be detected, as will be described later. Therefore, from the detected values, the bias roll 10, the intermediate transfer member 2 and the backup roll 4 can be detected. The resistance value of the entire 4 can be calculated. Therefore, in the present invention, the resistance of the backup roll 4 is
Compared with the respective resistances of the bias roll 10 and the intermediate transfer body 2, by making these large enough to be ignored,
The resistance value obtained and further calculated by the ammeter 17 may be determined as the resistance value of the backup roll 10.

The ammeter 17 and the power supply 16 are also provided.
In addition, the optimum transfer voltage is determined based on the resistance of the backup roll 4 obtained by the ammeter 17, and the determined transfer voltage is supplied to the power source 1 of the bias roll 10.
A control means 18 for sending a control signal to 6 is connected. That is, the control means 18 stores the relationship between the backup roll resistance and the optimum transfer voltage in advance through experiments or the like. When the current value is obtained by the ammeter 17, the resistance of the backup roll 4 is calculated from the voltage at that time. (To be exact, the resistance of the bias roll 10, the intermediate transfer member 2, and the backup roll 4, as described above,
Since the resistance of the backup roll 4 is sufficiently larger than others, it is simply regarded as the resistance of the backup roll 4), and the optimum transfer voltage is determined from the relationship stored in advance, and this is used as a control signal for power supply. 16 to apply the optimum transfer voltage from the power supply 16.

In order to transfer the toner image T to the transfer material 3 by the image forming apparatus having such a structure, first, the bias roll 10 is set to the intermediate transfer member during the standby of the image forming apparatus or the pre-cycle before the copying operation. 2, and the current flowing from the bias roll 10 through the intermediate transfer body 2 and the backup roll 4 is detected by the ammeter 17. Then, the control means 18 receives this detected value to obtain the backup roll resistance by the control means 18, and further determines the optimum transfer voltage according to the backup roll resistance, and the determined optimum transfer voltage is biased. It is applied to the roll 10. Further, if it is detected with the intermediate transfer body 2 stopped, the bias roll 10
And is not affected by the capacity component of the intermediate transfer member 2,
Therefore, the detection accuracy can be further improved.

Under the voltage applied to the bias roll 10 as described above, the toner image T is transferred onto the transfer material 3 by a mechanism similar to that of the conventional image forming apparatus. That is,
As described above, the electrostatic latent image corresponding to the image information is formed on the surface of the photosensitive drum 1, and the electrostatic latent image is developed by the developing devices 5 to 8 to form the toner image T on the photosensitive drum 1. To do. When the toner image T formed on the photoconductor drum 1 reaches the primary transfer position where the photoconductor drum 1 and the intermediate transfer body 2 are in contact with each other due to the rotation of the photoconductor drum 1, the discharge by the primary transfer device 9 is caused. It is received and electrostatically attracted to the intermediate transfer body 2 and is primarily transferred.

When forming a monochromatic image, the toner image T primarily transferred to the intermediate transfer body 2 is secondarily transferred immediately to the transfer material 3 as will be described later, but toner images of a plurality of colors are formed. In the case of forming a color image in which the toner images are overlapped with each other, the steps of forming the toner image on the photosensitive drum 1 and the primary transfer of the toner image are repeated for the number of colors. In this example, a full-color image in which toner images of four colors are superposed is formed, so that the black, yellow, magenta, and cyan unfixed toner images T are formed on the photosensitive drum 1 for each rotation. The unfixed toner image T is sequentially primary-transferred to the intermediate transfer body 2. On the other hand, the intermediate transfer member 2 rotates at the same cycle as the photosensitive drum 1 while holding the black toner image T that has been primarily transferred first, so that the yellow and magenta particles are rotated on the intermediate transfer member 2 at each rotation. The cyan toner image T and the cyan toner image T are sequentially transferred onto the black toner image T.

The toner image T primarily transferred onto the intermediate transfer body 2 in this manner is backed up with the bias roll 10 and the secondary transfer position facing the conveyance path of the transfer material 3 as the intermediate transfer body 2 rotates. It is conveyed to and from the roll 4. Separately from this, the transfer material 3 is the feed roller 1
1 is carried out from the tray 12 at a predetermined timing, is sent between the bias roll 10 and the intermediate transfer body 2, and is sandwiched between them. Then, at this secondary transfer position, the bias roll 10 is in contact with the intermediate transfer member 2, and on the back surface side of the intermediate transfer member 2 at the secondary transfer position, the backup roll 4 forming the counter electrode of the bias roll 10 is provided. Since the bias roller 10 has the polarity opposite to the charging polarity of the toner and the optimum transfer voltage is applied to the bias roller 10 as described above, the toner image T carried on the intermediate transfer body 2 is formed. A high quality image is secondarily transferred to the transfer material 3.

Then, the transfer material 3 on which the toner image T is transferred
Is peeled off from the intermediate transfer body 2 by the peeling claw 13 and is sent to a fixing device (not shown) by the conveyor belt 14 to be subjected to the fixing process of the toner image. On the other hand, the cleaner 15 removes the residual toner from the intermediate transfer body 2 after the secondary transfer of the toner image T is completed. The peeling claw 13 and the cleaner 15 are arranged so as to come into contact with and separate from the intermediate transfer body 2 like the bias roll 10, and when a color image is formed, these members are provided. Are separated from the intermediate transfer body 2 until the final color toner image is primarily transferred to the intermediate transfer body 2.

A specific example of the relationship between the backup roll resistance and the optimum transfer voltage stored in the control means is shown in FIG. FIG. 5 shows the optimum transfer voltage range with respect to the backup roll resistance. The lower limit L in FIG. 5 indicates a transfer voltage capable of generating a transfer electric field required for the toner to overcome the adhesive force to the intermediate transfer body 2 and transfer to the transfer material 3, and the upper limit H indicates the transfer voltage. The electric field is so high that Paschen discharge occurs between the intermediate transfer member 2 and the transfer material 3, and the transfer voltage causes image defects and a decrease in transfer efficiency due to abnormal charging of the toner. It can be seen from FIG. 5 that the higher the resistance of the backup roll, the higher the optimum transfer voltage. In view of this, in this embodiment, the bias roll 10 is brought into contact with the intermediate transfer body 2 during standby or during the pre-cycle before the copying operation, and the current flowing from the bias roll 10 through the intermediate transfer body 2 and the backup roll 4 is detected. Thus, the resistance of the backup roll is obtained, the optimum transfer voltage corresponding to the resistance of the backup roll is determined, and it is applied to the bias roll.

FIG. 6 shows the relationship between the backup roll resistance and the current flowing from the bias roll 10 through the intermediate transfer member 2 and the backup roll 4. In this example, the backup roll 10 has a volume resistivity of 10 ⁹ Ω · cm, the intermediate transfer member 2 has a volume resistivity of 10 ⁹ Ω · cm, and the bias roll 10 has a volume resistivity of 10 ⁷ Ω · cm, the bias roll 10 and the backup roll 4 each have a resistance layer thickness of 5 mm, and the contact width between the bias roll 10 and the intermediate transfer member 2 is 3 mm.
Has a thickness of 0.1 mm. Then, the resistance value (per 1 cm in the rotation axis direction) of the bias roll, the intermediate transfer member, and the backup roll is 1.7 × 10 ⁷ Ω to 3 × 10.
⁷ Ω to 1.7 × 10 ⁹ Ω, and the backup roll resistance is sufficiently higher than the bias roll resistance and the intermediate transfer member resistance. Therefore, the current flowing from the bias roll 10 through the intermediate transfer member 2 and the backup roll 4 is It is determined by the backup roll resistance without being affected by the bias roll resistance and the intermediate transfer member resistance. Therefore, the backup roll resistance can be stably detected from the current value detected by the ammeter 17.

Thus, in the image forming apparatus of this embodiment, the control means 18 determines the optimum transfer voltage according to the backup roll resistance from the backup roll resistance thus determined, and the determined transfer voltage is determined. Is applied to the bias roll 10, stable transfer can be performed, and thereby high quality image can be obtained.

FIG. 3 is a diagram showing a second embodiment of the image forming apparatus of the present invention. The image forming apparatus shown in FIG. 3 is different from the image forming apparatus shown in FIG. The insulating layer 23 is used instead of the semiconductive layer 22. That is, in this example, the backup roll 4 is composed of a grounded conductive roll 21 and an insulating layer (coating layer) 23 covering the peripheral surface of the conductive roll 21. The insulating layer 23 is made of PVdF (polyvinylidene fluoride), polyester, PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), acrylic, or the like, and has a volume resistivity of 10 ⁸ Ω · cm or more. It is controlled by. The thickness of the insulating layer 23 is 10 μm to 10 m.
It is about m. As the insulating layer 23 is thinner, the conductive roll 21 is closer to the bias roll 10, so that a sufficient transfer electric field can be obtained even when the voltage applied to the bias roll 10 is low. However, it is preferable to set the thickness to 10 μm to 100 μm because of the problem of pinholes and manufacturing stability. Also,
Since the higher the dielectric constant of the insulating layer 23 is, the more the same effect as that of the thinner layer can be obtained.
A (dielectric constant of 8) is preferable.

FIG. 4 is a diagram showing a third embodiment of the image forming apparatus of the present invention. The image forming apparatus shown in FIG. 4 is different from the image forming apparatus shown in FIG. 2 mainly in the backup. In the roll 4, an insulating roll 24 is used instead of the conductive roll 21, and a semiconductive thin film 25 is used as a semiconductive layer. The insulating roll (roll body) 24 is formed of the same material as the insulating layer 23, and the peripheral surface thereof is covered with the thin film 25. The thin film 25 is used for the semiconductive layer 22.
It is made of the same material as, and has a thickness of 10 μm to 200 μm.
It has a volume resistivity of 10 ⁴ Ω.
・ It is assumed to be cm or more. Further, an earth roll 26 is connected to the thin layer film 24 at a distance of 20 to 40 mm in the circumferential direction from the contact position with the intermediate transfer member 2, and therefore the thin layer film 24 is located at this position. Is grounded.

In the backup roll 4 having such a structure, the thin layer film 2 for covering the insulating roll 24 is used.
Since the current flows in the creeping resistance direction of 5, the earth roll 2
If the contact point between 6 and the thin film 25 is sufficiently distant from the contact point between the thin film 24 and the intermediate transfer body 2, it is not necessary to set the volume resistivity of the thin film 25 to be so large. The degree of freedom in selecting the material of the thin film 25 becomes great. However, regarding the volume resistance from the contact position between the backup roll 4 and the intermediate transfer member 2 to the grounding position of the thin film 25 (contact point with the earth roll 26), it is 10 for the same reason as in the example shown in FIG. It is preferably ⁷ Ω or more.

The backup roll 4 shown in FIG.
Then, since a current flows in the thickness direction of the semiconductive layer 22 covering the conductive roll 21, it is necessary to set the volume resistivity of the semiconductive image 22 to be large in order to prevent the occurrence of overcurrent. . However, since most of the semiconductive materials have the characteristic that the volume resistivity is significantly reduced in a high electric field, the selection of the material of the semiconductive layer 22 is limited in the configuration of the backup roll 4 shown in FIG. It has become a thing.

In the example shown in FIG. 4, the backup roll 4 has a surface resistivity of 10 ⁸ Ω / □,
The intermediate transfer member 2 has a resistivity of 10 ⁹ Ω · cm, and the bias roll 10 has a volume resistivity of 10 ⁷
If each of the Ω · cm is selected, the resistance value of the bias roll-intermediate transfer member-backup roll (per 1 cm in the rotation axis direction) is 5 mm when the resistance layer of the bias roll 10 is 5 mm. The contact width of the body 2 is 3 mm, the thickness of the intermediate transfer body is 0.1 mm, and the earth roll 26 of the backup roll 4 from the secondary transfer contact position.
If the distance in the circumferential direction up to is 40 mm, then 1.7
Next ^{× 10 7 Ω~3 × 10 7 Ω~4} × 10 8 Ω, the backup roll resistance bias roll resistance, since sufficiently higher than the intermediate transfer member resistance, the intermediate transfer member 2, the backup roll 4 through the bias roll 10 The current flowing therethrough is determined by the backup roll resistance without being affected by the bias roll resistance and the intermediate transfer member resistance.

Therefore, also in this embodiment, the backup roll resistance can be detected stably even with respect to variations in the bias roll resistance and the intermediate transfer member resistance. Further, if the detection is performed in a state where the intermediate transfer body 2 is stopped, it is not affected by the capacitance component of the bias roll 10 or the intermediate transfer body 2, so that the detection accuracy can be further improved.

In the examples shown in FIGS. 2 and 4, the volume resistivity of the intermediate transfer member 2 is measured by using a high resistance resistivity meter (HIRESTA IP, manufactured by Mitsubishi Yuka) and an HR probe (inner electrode diameter 16 mm, ring). Electrode inner diameter 30 mm, made by Mitsubishi Yuka)
Is connected and the voltage of 500 V is applied to the front and back of the intermediate transfer member for 1 minute. Further, the volume resistivity of the bias roll 10 and the backup roll 4 was obtained by winding a 1-inch wide copper tape in the circumferential direction of the roll and applying a voltage V: 50 V between the core metal of the roll and the copper tape (bias roll volume resistivity measurement). Time) or 2000 V (at the time of measuring the volume resistivity of the backup roll) is applied, the current value I after 1 minute is read, and the value is calculated by the following formula.

## EQU1 ## ρv = 2πwV / Iln (r ₂ / r ₁ ) r ₁ : core metal diameter (mm), r ₂ : roll outer diameter (mm), w: copper tape width (2.54 cm) The surface resistivity is measured by contacting two metal rolls with a diameter of 15 mm, which is longer than the backup roll, 10 mm apart in the circumferential direction of the backup roll.
It is a value obtained by applying the applied voltage V: 1000 V between the metal rolls of the book, reading the current value I after 1 minute, and calculating by the following formula.

## EQU2 ## ρ _s = LV / GI L: backup roll length (cm), G: distance between two metal rolls (cm).

In the above embodiment, as the backup roll 4, a conductive roll coated with a semiconductive layer,
The conductive roll is coated with an insulating layer, and the insulating roll is illustrated with a semi-conductive thin layer film, but the present invention is not limited to this, and for example, semi-conductive. The roll may be coated with a semiconductive layer, in which case either the roll body or the coating layer may be grounded. Further, in the above-described embodiments, the present invention is applied to a color electrophotographic copying machine, but the present invention can be applied not only to a monochromatic electrophotographic copying machine, but also to a laser printer or the like.

[0039]

As described above, the image forming apparatus of the present invention prevents the excessive current from flowing from the bias roll to the backup roll by forming the backup roll resistance sufficiently higher than the intermediate transfer member resistance. By detecting the current flowing from the bias roll through the intermediate transfer member and the backup roll by the detection means,
The backup roll resistance can be stably detected without being affected by variations in the bias roll resistance and the intermediate transfer member resistance. Therefore,
From the obtained backup roll resistance, the control unit determines the optimum transfer voltage according to the backup roll resistance, and by applying the determined transfer voltage to the bias roll, stable transfer can be performed. A high quality image can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a first embodiment when an image forming apparatus of the present invention is applied to a color image forming apparatus.

FIG. 2 is an enlarged view of a main part showing a schematic configuration of a backup roll in the first embodiment.

FIG. 3 is an enlarged view of main parts showing a schematic configuration of a backup roll according to a second embodiment.

FIG. 4 is an enlarged view of a main part showing a schematic configuration of a backup roll according to a third embodiment.

FIG. 5 is a graph showing the relationship between backup roll resistance and optimum transfer voltage.

FIG. 6 is a graph showing the relationship between the backup roll resistance and the current flowing from the bias roll through the intermediate transfer member and the backup roll.

FIG. 7 is a schematic configuration diagram showing an example of a conventional image forming apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoconductor drum (latent image carrier) 2 Intermediate transfer body 3 Transfer material 4 Backup roll 5, 6, 7, 8 Developing device 9 Primary transfer device 10 Bias roll 16 Power supply 17 Ammeter (resistance detection means) 18 Control means 21 Conductive roll (roll body) 22 Semi-conductive layer (coating layer) 23 Insulating layer (coating layer) 24 Insulating roll (roll body) 25 Semi-conductive thin layer film (coating layer) 26 Earth roll T Toner image

Claims (2)

[Claims] 1. A latent image carrier that forms a toner image according to image information, an intermediate transfer member to which a toner image is primarily transferred from the latent image carrier, and a contactable and detachable member with respect to the intermediate transfer member. An image including a bias roll that secondarily transfers a toner image from the intermediate transfer member to a transfer material, and a backup roll that is arranged so as to face the bias roll and supports the intermediate transfer member from the back side. In the forming apparatus, the backup roll has a roll main body and a coating layer that covers the peripheral surface of the roll main body, and one of the roll main body and the coating layer is formed of a conductive or semiconductive material, and the other. Is formed of an insulating or semi-conductive material, and at least one of the roll body and the coating layer is grounded to form the entire backup roll semi-conductive. The ass roll is provided with resistance detection means for determining the resistance of the backup roll by detecting a current flowing from the bias roll through the intermediate transfer member and the backup roll in a state where the bias roll is in contact with the intermediate transfer member. The control means sends a control signal to the power source of the bias roll so as to determine the optimum transfer voltage by the bias roll based on the backup roll resistance obtained by the detection means, and to apply the determined transfer voltage to the bias roll. An image forming apparatus comprising means. 2. The volume resistivity of the intermediate transfer member is 10 ⁹ Ω.
The image forming apparatus according to claim 1, wherein the bias roll has a volume resistivity of 10 ⁷ Ω · cm or less.