JP2014228574A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus configured to prevent local discharge in a transfer nip, to prevent a discharge image, such as a white dot.SOLUTION: An image forming apparatus includes: a transfer member 36 which forms a transfer nip N in contact with a surface of a belt-like image carrier 31 where a toner image is carried; and a power source 39 which outputs a bias for transferring the toner image on the image carrier to a recording material P held in the transfer nip. In transferring the toner image on the image carrier to the recording material, a bias in a transfer direction for transferring the toner image from the image carrier side to the recording material side, or a bias having a polarity opposite to a voltage in the transfer direction is alternately selected. At least two belt-like image carriers are layered.

Description

  The present invention relates to an image forming apparatus for transferring a toner image on a surface of an image carrier to a recording material sandwiched in the nip by a contact between a belt-like image carrier and a transfer member.

  To transfer the toner image on the image carrier to a transfer member that forms a transfer nip by contacting the surface of the belt-like image carrier that carries the toner image, and a recording material sandwiched in the transfer nip As an image forming apparatus provided with a power supply that outputs a voltage or current of the above, the one described in Patent Document 1 is known. The image forming apparatus described in Patent Document 1 forms a toner image on the surface of a drum-shaped photoreceptor serving as an image carrier by a known electrophotographic process. An endless intermediate transfer belt, which is an image carrier and an intermediate transfer member, is brought into contact with the photosensitive member to form a transfer nip. In the transfer nip, the toner image on the photoconductor is transferred to the intermediate transfer belt. A nip forming roller (secondary transfer roller) serving as one of the two transfer members is brought into contact with the intermediate transfer belt. A secondary transfer back roller (secondary transfer counter roller) is disposed in the loop of the intermediate transfer belt, and the intermediate transfer belt is sandwiched between the secondary transfer back roller and the nip forming roller so as to form a transfer nip. Forming. Generally, the secondary transfer back roller on the inner side of the loop is connected to the ground, while the nip forming roller outside the loop is supplied with voltage or current from a power source. Thus, a transfer electric field for electrostatically moving the toner image from the secondary transfer back roller side to the nip forming roller side is formed between the secondary transfer back roller and the nip forming roller, that is, in the transfer nip. Then, the toner image on the intermediate transfer belt is transferred to the recording material fed into the transfer nip at a timing synchronized with the toner image on the intermediate transfer belt by the action of a transfer electric field or nip pressure.

In this configuration, when transferring the toner image on the image carrier to the recording material, a bias in the transfer direction for transferring the toner image from the image carrier side to the recording material side, and a bias having a polarity opposite to the voltage in the transfer direction. When using a so-called AC transfer using a bias that alternately switches between the two, a higher voltage is applied than in the case of a DC transfer in which only a direct current bias is applied, so a discharge image such as white spots is generated in the transfer nip. Resulting in.
This white spot image is generated by local discharge in the transfer nip into which the recording material is fed. In general, the base material of the intermediate transfer belt, which is a belt-shaped image carrier, is made of an insulating material. However, in order to apply a transfer electric field to a recording material, it is necessary to lower the electrical resistance to some extent, such as carbon. In many cases. However, in order to disperse the conductive material in the insulative base material, the low resistance portion and the high resistance portion exist on the belt. For this reason, when a high voltage is applied, the surface potential at a location where the resistance is locally low becomes higher than at other locations, and a local discharge is generated therefrom.

  An object of the present invention is to prevent the occurrence of discharge images such as white spots by suppressing local discharge in the transfer nip.

  In order to achieve the above object, an image forming apparatus according to the present invention includes a transfer member that forms a transfer nip in contact with a surface carrying a toner image on a belt-like image carrier, and is sandwiched in the transfer nip. A power source that outputs a bias for transferring the toner image on the image carrier to the recording material, and the bias is used when the toner image on the image carrier is transferred to the recording material. In an image forming apparatus in which a bias in a transfer direction for transferring an image from the image carrier side to a recording material side and a bias having a polarity opposite to the voltage in the transfer direction are switched alternately, at least two layers of belt-like image carriers are provided. It is characterized by having a configuration.

  According to the present invention, when transferring the toner image on the image carrier to the recording material, the bias in the transfer direction for transferring the toner image from the image carrier side to the recording material side, and the polarity opposite to the voltage in the transfer direction When using a bias that alternates with the bias of the current, a higher voltage is applied than when using a direct current bias, and a discharge image such as a white spot appears. However, at least two belt-shaped image carriers are required. By layering, the resistance that passes through the interface of the belt layer increases, so that the discharge can be blocked and local discharge in the transfer nip can be suppressed, and the occurrence of discharge images such as white spots is prevented. be able to.

1 is a schematic configuration diagram of an embodiment of an image forming apparatus according to the present invention. FIG. 3 is an enlarged view showing a configuration in the vicinity of a transfer nip and a configuration of a solid type transfer member in the image forming apparatus. FIG. 3 is an enlarged view showing a configuration in the vicinity of a transfer nip and a configuration of a sponge-type transfer member in the image forming apparatus. (A) is the elements on larger scale which show the structure of the intermediate transfer belt of single layer structure, (b) is the elements on larger scale which show the structure of the intermediate transfer belt of multilayer structure concerning this form. The block diagram which shows the structure of the control system which concerns on this invention. The figure explaining the waveform of the secondary transfer bias which consists of a superposition bias. FIG. 3 is a schematic cross-sectional view showing an embodiment of a cylindrical tube-applying device used for producing the intermediate transfer belt of the present embodiment. FIG. 6 is a diagram illustrating an example of a waveform of a secondary transfer bias output from a power source. The figure which shows the structure of Examples 1-4 and Comparative Examples 1-4 which concern on this invention, and the evaluation result of a white spot and an afterimage. FIG. 6 is a diagram illustrating an example of a waveform for each secondary transfer bias output from a power source.

Hereinafter, embodiments according to the present invention will be described with reference to the drawings. In each drawing, the same member or a member having the same function is basically denoted by the same reference numeral, and redundant description is appropriately omitted.
FIG. 1 is a schematic configuration diagram showing an embodiment of an electrophotographic color printer (hereinafter simply referred to as “printer”) as an image forming apparatus to which the present invention is applied. In the figure, the printer includes four image forming units 1Y, 1M, 1C, and 1K for forming yellow (Y), magenta (M), cyan (C), and black (K) toner images, and a transfer device. As a transfer unit 30, an optical writing unit 80, a fixing device 90, a paper feed cassette 100, a registration roller pair 101, and a control unit 60 serving as control means.

  The four image forming units 1Y, 1M, 1C, and 1K use toners of different colors Y, M, C, and K as image forming substances. Exchanged. The image forming unit 1K for forming a K toner image will be described as an example. This unit includes a drum-shaped photosensitive member 2K as an image carrier, a drum cleaning device 3K, a charge eliminating device (not shown), a charging device 6K, a developing device. The apparatus 8K etc. are provided. The image forming unit 1K is configured such that these components are held in a common casing and can be integrally attached to and detached from the printer body, and these components can be replaced at the same time.

  The photoreceptor 2K has an organic photosensitive layer formed on the surface of a drum-shaped substrate, and is driven to rotate in the clockwise direction in the drawing by a driving means (not shown). The charging device 6K generates a discharge between the charging roller 7K and the photosensitive member 2K while bringing the charging roller 7K to which a charging bias is applied into contact with or in proximity to the photosensitive member 2K, thereby making the surface of the photosensitive member 2K uniform. Charge like this. In this printer, the toner is uniformly charged to the same negative polarity as the normal charging polarity of the toner. More specifically, it is uniformly charged to about −650 [V]. In this embodiment, a charging bias in which an AC voltage is superimposed on a DC voltage is used. The charging roller 7K is formed by coating a metal cored bar with a conductive elastic layer made of a conductive elastic material. Instead of a method of bringing a charging member such as a charging roller into contact with or close to the photosensitive member 2K, a charging method using a charging charger may be adopted.

  The surface of the photoreceptor 2 </ b> K uniformly charged by the charging device 6 </ b> K is optically scanned by a laser beam emitted from the optical writing unit 80 to carry an electrostatic latent image for K. The potential of the electrostatic latent image for K is about −100 [V]. The electrostatic latent image for K is developed by a developing device 8K using K toner (not shown) to become a K toner image. Then, primary transfer is performed on an intermediate transfer belt 31 which is an intermediate transfer body which will be described later and is a belt-like image carrier. The intermediate transfer belt 31 is composed of an endless belt member.

  The drum cleaning device 3K removes transfer residual toner adhering to the surface of the photoreceptor 2K after undergoing a primary transfer process (primary transfer nip described later). The drum cleaning device 3K includes a cleaning brush roller 4K that is driven to rotate, a cleaning blade that abuts the free end of the drum 2K in a cantilevered state, and the like. The drum cleaning device 3K scrapes off the transfer residual toner from the surface of the photoreceptor 2K with the rotating cleaning brush roller 4K, and scrapes off the transfer residual toner from the surface of the photoreceptor 2K with the cleaning blade. The cleaning blade is in contact with the photosensitive member 2K in the counter direction in which the cantilevered support end side is directed downstream of the free end side in the drum rotation direction.

  The static eliminator neutralizes the residual charge on the photoreceptor 2K after being cleaned by the drum cleaning device 3K. By this charge removal, the surface of the photoreceptor 2K is initialized and prepared for the next image formation.

The developing device 8K includes a developing unit that includes a developing roll 9K that serves as a developer carrying member, and a developer transport unit that stirs and transports a K developer (not shown). In the developer conveying portion, a plurality of screw members as conveying members are arranged, and by rotating each screw member, the developer is moved along the rotation axis direction of the developing roll 9K while stirring the K toner. The K developer is supplied to the surface of the developing roll 9K.
The developing device 8K is provided with a toner concentration sensor (not shown) on the lower wall of the casing, and detects the concentration of K toner in the K developer. As the K toner density sensor, a sensor composed of a magnetic permeability sensor is used. Since the magnetic permeability of a so-called two-component K developer containing K toner and a magnetic carrier has a correlation with the K toner concentration, the magnetic permeability sensor detects the K toner concentration.

  In this printer, Y, M, C, and K toner supply units (not shown) for individually supplying Y, M, C, and K toners into the Y, M, C, and K developing devices, respectively. Means are provided. Then, the control unit 60 of the printer stores the Vtref for Y, M, C, and K, which is the target value of the output voltage value from the toner density detection sensor for Y, M, C, and K, in the RAM. Yes. When the difference between each output voltage value from the Y, M, C, and K toner density detection sensors and Vtref for Y, M, C, and K exceeds a predetermined value, the difference is determined according to the difference. The toner supply means for Y, M, C, and K is driven for the time. As a result, Y, M, C, and K toners are supplied into the Y, M, C, and K developing devices.

  The developing roll 9K carries K developer on the sleeve surface by the magnetic force generated by the magnet roller, and conveys it to the developing area facing the photoreceptor 2K as the sleeve rotates. A developing bias having the same polarity as the toner and larger than the electrostatic latent image of the photosensitive member 2K and smaller than the uniform charging potential of the photosensitive member 2K is applied to the developing sleeve. As a result, a developing potential for electrostatically moving the K toner on the developing sleeve toward the electrostatic latent image acts between the developing sleeve and the electrostatic latent image on the photoreceptor 2K. Further, a non-developing potential that moves K toner on the developing sleeve toward the sleeve surface acts between the developing sleeve and the background portion of the photoreceptor 2K. By the action of the developing potential and the non-developing potential, the K toner on the developing sleeve is selectively transferred to the electrostatic latent image on the photoreceptor 2K, and the electrostatic latent image is developed into the K toner image.

In FIG. 1 described above, the Y, M, and C image forming units 1Y, 1M, and 1C also have Y, M on the photoreceptors 2Y, 2M, and 2C in the same manner as the K image forming unit 1K. , C toner images are formed.
Above the image forming units 1Y, 1M, 1C, and 1K, an optical writing unit 80 serving as a latent image writing unit is disposed. The optical writing unit 80 optically scans the photoconductors 2Y, 2M, 2C, and 2K with laser light emitted from a light source such as a laser diode based on image information sent from an external device such as a personal computer. By this optical scanning, electrostatic latent images for Y, M, C, and K are formed on the photoreceptors 2Y, 2M, 2C, and 2K. Specifically, the portion of the uniformly charged surface of the photoreceptor 2Y that has been irradiated with laser light attenuates the potential. Thereby, an electrostatic latent image is obtained in which the potential of the laser irradiation portion is smaller than the potential of the other portion (background portion). The optical writing unit 80 applies the laser beam L emitted from the light source to each photoconductor through a plurality of optical lenses and mirrors while polarizing the laser beam L in the main scanning direction by a polygon mirror that is rotationally driven by a polygon motor (not shown). Irradiation. As the optical writing unit 80, a unit that performs optical writing on the photoreceptors 2Y, 2M, 2C, and 2K by LED light emitted from a plurality of LEDs of the LED array may be employed.

  Below the image forming units 1Y, 1M, 1C, and 1K, there is disposed a transfer unit 30 that moves the endless intermediate transfer belt 31 endlessly in the counterclockwise direction in the drawing. In addition to the intermediate transfer belt 31 as an image carrier, the transfer unit 30 includes a driving roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, and primary transfer rollers 35Y and 35M serving as four primary transfer members. 35C and 35K, a nip forming roller 36 as a transfer member, and a belt cleaning device 37 are provided.

  The endless intermediate transfer belt 31 is stretched by a driving roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, and four primary transfer rollers 35Y, 35M, 35C, and 35K disposed inside the loop. Has been. In this embodiment, the rotation is driven endlessly in the counterclockwise direction in FIG. 1 by the rotational force of the driving roller 32 that is rotated in the counterclockwise direction in the drawing by a driving means (not shown).

The primary transfer rollers 35Y, 35M, 35C, and 35K sandwich the intermediate transfer belt 31 that is moved endlessly between the photoreceptors 2Y, 2M, 2C, and 2K, respectively. As a result, primary transfer nips for Y, M, C, and K where the front surface of the intermediate transfer belt 31 and the photoreceptors 2Y, 2M, 2C, and 2K abut are formed. A primary transfer bias is applied to the primary transfer rollers 35Y, 35M, 35C, and 35K by a primary transfer bias power source (not shown). As a result, transfer electric fields are formed between the Y, M, C, and K toner images on the photoreceptors 2Y, 2M, 2C, and 2K and the primary transfer rollers 35Y, 35M, 35C, and 35K. The Y toner formed on the surface of the Y photoconductor 2Y enters the Y primary transfer nip as the photoconductor 2Y rotates. Then, due to the action of the transfer electric field and the nip pressure, the image is moved from the photoreceptor 2Y onto the intermediate transfer belt 31 to be primarily transferred.
The intermediate transfer belt 31 on which the Y toner image has been primarily transferred in this way then passes sequentially through the M, C, and K primary transfer nips. Then, the M, C, and K toner images on the photoreceptors 2M, 2C, and 2K are sequentially superimposed and sequentially transferred onto the Y toner image. By this superimposing primary transfer, a four-color superposed toner image is formed on the intermediate transfer belt 31.

  The primary transfer rollers 35Y, 35M, 35C, and 35K are constituted by an elastic roller having a metal core and a conductive sponge layer fixed on the surface thereof. The primary transfer rollers 35Y, 35M, 35C, and 35K are shifted about 2.5 [mm] from the axial center of the photoreceptors 2Y, 2M, 2C, and 2K to the downstream side in the belt movement direction. It is arranged to occupy a position. In this printer, a primary transfer bias is applied to such primary transfer rollers 35Y, 35M, 35C, and 35K by constant current control. Instead of the primary transfer rollers 35Y, 35M, 35C, and 35K, a transfer charger, a transfer brush, or the like may be employed as the primary transfer member.

  The nip forming roller 36 is disposed outside the loop of the intermediate transfer belt 31, and the intermediate transfer belt 31 is sandwiched between the secondary transfer back roller 33 inside the loop. As a result, a secondary transfer nip (transfer nip) N in which the front surface of the intermediate transfer belt 31 and the nip forming roller 36 abut is formed. In the example shown in FIGS. 1 and 2, the nip forming roller 36 is grounded (grounded), whereas the secondary transfer back roller 33 is composed of a superimposed bias in which an AC voltage is superimposed on a DC voltage by a power source 39. A secondary transfer bias is applied. As a result, a secondary transfer electric field is formed between the secondary transfer back roller 33 and the nip forming roller 36 to electrostatically move the negative polarity toner from the secondary transfer back roller 33 side toward the nip forming roller 36 side. Is done. The alternating current component and the direct current component of the superimposed bias output from the power source 39 may be subjected to constant current control or constant voltage control, respectively.

  The secondary transfer back roller 33 is configured as a core rubber 33a and a solid rubber roller in which a solid rubber layer 33b is formed on the core metal 33a. Carbon is dispersed in the solid rubber layer 33b. The secondary transfer back roller 33 is provided so as to face the nip forming roller 36 serving as the other transfer member via the intermediate transfer belt 31 and to be in contact with the back surface of the intermediate transfer belt 31. The nip forming roller 36 is configured as a solid rubber roller in which a solid rubber layer is formed on a roller metal core 36a. As shown in FIG. 3, the nip forming roller 36 may be configured as a sponge roller in which a sponge layer is formed on a roller metal core 36a.

In FIG. 2, there is an intermediate between a secondary transfer back roller 33 to which a superimposed bias obtained by superimposing an AC voltage on a DC voltage by a power source 39 is applied as a secondary transfer bias and a grounded nip forming roller 36. A secondary transfer current flows through the transfer belt 31.
A power source 39 that outputs a secondary transfer bias for transferring the toner image on the intermediate transfer belt 31 to the recording material P sandwiched in the secondary transfer nip N has a DC power source and an AC power source. . The power supply 39 is configured to output a superimposed bias obtained by superimposing an AC voltage on a DC voltage as a secondary transfer bias. In this embodiment, as shown in FIG. 1, the nip forming roller 36 is grounded while applying the secondary transfer bias to the secondary transfer back surface roller 33. In this embodiment, the power source 39 outputs a superimposed bias obtained by superimposing an AC voltage on a DC voltage to the secondary transfer back surface roller 33.

  There are various forms of supply of the secondary transfer bias to the secondary transfer nip N. As the form of power supply, the power supply 39 can supply “DC voltage + AC voltage” or “DC voltage” and “AC voltage”. Can be supplied individually, or “DC voltage + AC voltage” and “DC voltage” can be switched and supplied by a single power source.

As shown in FIG. 1, below the transfer unit 30, a paper feed cassette 100 that stores a plurality of recording materials P in a stack of paper sheets is disposed. In the paper feeding cassette 100, a paper feeding roller 100a is brought into contact with the top recording material P of the paper bundle, and the recording material P is directed to the paper feeding path by being driven to rotate at a predetermined timing. And send it out. A registration roller pair 101 is disposed near the end of the paper feed path.
The registration roller pair 101 stops the rotation of both rollers as soon as the recording material P fed from the paper feed cassette 100 is sandwiched between the rollers. Then, rotation driving is resumed at a timing at which the sandwiched recording material P can be synchronized with the four-color superimposed toner image on the intermediate transfer belt 31 in the secondary transfer nip N, and the recording material P is directed to the secondary transfer nip N. And send it out.
The four-color superimposed toner image on the intermediate transfer belt 31 brought into intimate contact with the recording material P at the secondary transfer nip N is collectively transferred onto the recording material P by the action of the secondary transfer electric field or nip pressure, and recorded. A full color toner image is formed in combination with the white color of the material P. When the recording material P having the full-color toner image formed on the surface in this way passes through the secondary transfer nip N, the recording material P is separated from the nip forming roller 36 and the intermediate transfer belt 31 by curvature.

  As shown in FIG. 1, untransferred toner that has not been transferred to the recording material P adheres to the intermediate transfer belt 31 after passing through the secondary transfer nip N. This transfer residual toner is cleaned from the belt surface by a belt cleaning device 37 in contact with the front surface of the intermediate transfer belt 31. A cleaning backup roller 34 disposed inside the loop of the intermediate transfer belt 31 backs up the cleaning of the belt by the belt cleaning device 37 from the inside of the loop.

  A fixing device 90 is disposed on the right side in FIG. 1, which is downstream of the secondary transfer nip N in the recording material conveyance direction. The fixing device 90 forms a fixing nip with a fixing roller 91 containing a heat source such as a halogen lamp and a pressure roller 92 that rotates while contacting with the fixing roller 91 with a predetermined pressure. The recording material P fed into the fixing device 90 is sandwiched between the fixing nips in such a posture that the carrying surface of the unfixed toner image is in close contact with the fixing roller 91. Then, the toner in the toner image is softened by the influence of heating and pressurization, and the full-color image is fixed. The recording material P discharged from the fixing device 90 passes through a post-fixing conveyance path and is then discharged outside the apparatus.

  FIG. 5 is a block diagram showing a part of the control system of the printer shown in FIG. In the figure, a control unit 60 that constitutes a part of the transfer bias output means includes a CPU 60a (Central Processing Unit) as a calculation means, a RAM 60c (Random Access Memory) as a nonvolatile memory, and a ROM 60b (Read Only Memory) as a temporary storage means. And a flash memory 60d. Various components and sensors are electrically connected to the control unit 60 that controls the entire printer so as to be communicable. In FIG. 5, only components related to the characteristic configuration of the printer are shown. Is shown.

  The primary transfer power supply 81 (Y, M, C, K) outputs a primary transfer bias to be applied to the primary transfer rollers 35Y, 35M, 35C, 35K. The power source 39 for secondary transfer outputs a secondary transfer bias supplied to the secondary transfer nip N. In this embodiment, a secondary transfer bias to be applied to the secondary transfer back roller 33 is output. The power supply 39 constitutes a transfer bias output means together with the control unit 60. The operation panel 50 includes a touch panel (not shown) and a plurality of key buttons. The operation panel 50 can display an image on the screen of the touch panel. The operation panel 50 receives an input operation by the operator using the touch panel and the key button, and inputs the input information to the control unit 60. It has a function to send. The operation panel 50 can also display an image on the touch panel based on a control signal sent from the control unit 60.

  The primary transfer power supply 81 (Y, M, C, K) outputs a primary transfer bias to be applied to the primary transfer rollers 35Y, 35M, 35C, 35K. The power source 39 for secondary transfer outputs a secondary transfer bias supplied to the secondary transfer nip N. In this embodiment, a secondary transfer bias to be applied to the secondary transfer back roller 33 is output. The power supply 39 constitutes a transfer bias output means together with the control unit 60. The operation panel 50 includes a touch panel (not shown) and a plurality of key buttons. The operation panel 50 can display an image on the screen of the touch panel. The operation panel 50 receives an input operation by the operator using the touch panel and the key button, and inputs the input information to the control unit 60. It has a function to send. The operation panel 50 can also display an image on the touch panel based on a control signal sent from the control unit 60.

  FIG. 6 is a diagram illustrating an example of a waveform of the secondary transfer bias composed of a superimposed bias applied to the secondary transfer back surface roller 33. In the figure, a time average voltage (hereinafter referred to as “time average value”) Vave [V] represents a time average value of the secondary transfer bias. A time average value is a time average value of a voltage, which is a value obtained by dividing an integrated value over one period of a voltage waveform by the length of one period. As shown in the figure, the secondary transfer bias composed of the superimposed bias has a sinusoidal shape and has a peak value on the return direction side and a peak value on the transfer direction side. Of these two peak values, the reference numeral Vt is the peak value of the direction in which the toner is moved from the belt side to the nip forming roller 36 side (transfer direction side) in the secondary transfer nip N. (Hereinafter referred to as “transfer direction peak value Vt”). Reference numeral Vr denotes a peak value in the direction of returning the toner from the nip forming roller 36 side to the belt side (return direction side) (hereinafter referred to as a return peak value Vr). Further, instead of the superimposed bias as shown in the figure, it is possible to reciprocate the toner between the belt and the recording material in the secondary transfer nip N even when an AC bias consisting only of an AC component is applied. However, with AC bias, the toner cannot be transferred onto the recording material P simply by reciprocating. By applying a superimposed bias including a DC component and setting the time average voltage Vave [V], which is the time average value, to the same negative polarity as that of the toner, recording is performed relatively from the belt side while reciprocating the toner. It is possible to move to the recording material P by moving to the recording material P side.

  In the configuration in which the toner particles are reciprocated, if the return peak value Vr shown in FIG. 6 is not set to a certain large value, the toner particles that have entered the recesses on the surface of the recording material can be sufficiently pulled back to the toner layer on the belt. Inability to do so results in insufficient image density on the recess. If the time average value Vave [V] of the secondary transfer bias is not set to a relatively large value, a sufficient amount of toner cannot be transferred to the convex portion on the surface of the recording material, and an image is formed on the convex portion. Insufficient concentration will occur. In order to obtain a sufficient image density at both the convex and concave portions on the surface of the recording material, the maximum value and the minimum value of the voltage are set so that the time average value Vave [V] and the return peak value Vr are respectively large to some extent. It is necessary to set the voltage (hereinafter referred to as “peak-to-peak voltage”) Vpp from the return peak value Vr that is the value width to the transfer direction peak value Vt to a relatively large value. Then, the transfer direction peak value Vt is inevitably set to a relatively large value. The transfer direction peak value Vt corresponds to the maximum potential difference between the grounded nip forming roller 36 and the secondary transfer back surface roller 33 to which the secondary transfer bias is applied. Is likely to occur.

That is, when AC transfer is performed using a secondary transfer bias in which an alternating current component is superimposed on a direct current component, the alternating current component is shifted to a predetermined polarity as compared with the case where there is no direct current component. For this reason, the absolute value of the peak (current) peak of the secondary transfer bias is larger than in the case of DC transfer in which only a direct current bias is applied, and a high voltage is applied, so that a discharge image such as a white spot appears. . This white spot image is generated by local discharge in the secondary transfer nip N into which the recording material P is fed.
In general, the base material of the intermediate transfer belt 31 is made of an insulating material. However, in order to apply a transfer electric field to the recording material P, it is necessary to lower the electric resistance to some extent, and FIGS. As shown in (b), carbon and the like are often dispersed. However, when conductive material is dispersed in an insulating base material, a low resistance portion and a high resistance portion exist on the belt. For this reason, when a high voltage is applied as in AC transfer, the surface potential at a location where the resistance is locally low becomes higher than at other locations, and a local discharge is generated therefrom. Further, the unevenness of the electric resistance of the intermediate transfer belt can be made uniform by uniformly increasing the resistance, but as the belt resistance increases, a belt afterimage, a decrease in transfer rate, etc. occur. In consideration of this point, a method of uniforming the resistance by uniformly increasing the resistance is not preferable.

  Therefore, in this embodiment, as shown in FIGS. 2 and 3, the intermediate transfer belt 31 is multilayered. In this embodiment, the intermediate transfer belt 31 has a two-layer structure of a first layer 310 and a second layer 311. As described above, when the structure of the intermediate transfer belt 31 is made multi-layered, the resistance passing through the interface of the belt layer is increased, so that the discharge is blocked and local discharge in the secondary transfer nip N can be suppressed. The occurrence of discharge images such as white spots can be prevented.

Hereinafter, the layer structure of the intermediate transfer belt 31 and specific examples and comparative examples will be described.
In Examples 1 and 2 of the intermediate transfer belt 31 according to this embodiment, polyimide (PI) is used as the base material of the belt, and carbon is dispersed in the base material as shown in FIG. In Examples 3 and 4, polyamideimide (PAI) was used as the base material of the belt, and as shown in FIG. 4B, carbon was dispersed in the base material as shown in FIG. The second layer 311 is laminated in two layers.
On the other hand, Comparative Examples 1 and 2 use polyimide (PI) as the base material of the belt, and as shown in FIG. 4 uses polyamideimide (PAI) as the base material of the belt, and as shown in FIG. 4 (a), a layer in which carbon is dispersed in the base material and a carbon in the base material are further dispersed. did.
The intermediate transfer belt 31 has a surface carrying a toner image as a front surface 31A, and an inner surface on the opposite side as a back surface 31B.

  In the embodiment and each example, polyimide is obtained via polyamic acid (polyimide precursor) by reaction of a generally known aromatic polycarboxylic acid anhydride or derivative thereof with an aromatic diamine. It is done. That is, polyimide is insoluble in solvents and the like due to its rigid main chain structure and has an infusible property. Therefore, a polyimide precursor that is soluble in an organic solvent from an acid anhydride and an aromatic diamine ( Polyamic acid or polyamic acid) is synthesized, and at this stage, molding is performed by various methods, and then the polyamic acid is subjected to dehydration reaction by heating or a chemical method to be cyclized (imidized) to obtain polyimide. Polyamideimide is a resin having a rigid imide group and an amide group imparting flexibility in the molecular skeleton, and a generally known structure is used.

  Although the manufacturing method of the intermediate transfer belt 31 according to the present embodiment is not particularly limited, for example, it is preferably manufactured by a spin coating method as shown in FIG. 7 using a polyimide precursor solution. In this method, a cylindrical forming tube 11 having an outer diameter corresponding to the length of the intermediate transfer belt 3 is prepared. A nozzle 15 for discharging the coating liquid 16 onto the outer peripheral surface of the cylindrical molding tube 11 is disposed at a position along the outer peripheral surface of the cylindrical molding tube 11, and the nozzle 15 is connected to the coating liquid container 14 through the pipe. The coating solution container 14 is connected to a pressurizing device 17 through a pipe. A blade 18 for leveling the discharged coating liquid 16 on the outer peripheral surface of the cylindrical tube 11 is disposed below the nozzle 15.

The cylindrical forming tube 11 is rotated in the direction of rotation of the cylindrical forming tube (arrow D), the coating liquid 16 is discharged from the nozzle 15 onto the outer peripheral surface of the cylindrical forming tube 11, and the blade 18 is uniformly applied onto the outer peripheral surface of the cylindrical forming tube 11. The The nozzle 15 and the blade 18 move at a constant speed in the nozzle and blade movement direction (arrow E), and the coating liquid 16 is applied on the outer peripheral surface of the cylindrical tube 11 with a constant thickness. The coating liquid 16 is adjusted so that a predetermined amount is discharged from the nozzle 15 by the pressurizing device 17. Thereby, the coating liquid 16 coating film is formed on the outer peripheral surface of the cylindrical tube 11.
The obtained coating film 16 is dried by heating, and after cooling, is peeled off from the cylindrical tube 11 and cut at a predetermined width, whereby the intermediate transfer belt 31 can be obtained. In addition, when using a polyimide precursor as a resin material of the coating liquid 16, after forming the coating liquid 16 coating film on the outer peripheral surface of the cylindrical tube 11, the solvent is removed by drying at 80 ° C. or higher and 170 ° C. or lower. (Drying step), and further heated to 250 ° C. or higher and 350 ° C. or lower for imide conversion (firing step) to form a polyimide resin film. After cooling, the obtained polyimide resin film is peeled off from the cylindrical tube 11 and cut at a predetermined width, whereby the intermediate transfer belt 31 is obtained.

  The solid content concentration of the coating liquid 16 is, for example, 10 mass% to 40 mass%, and the viscosity is 1 Pa · s to 100 Pa · s. In addition, a predetermined amount of conductive particles such as carbon black is dispersed in the coating liquid 16 in accordance with the required common logarithm of the surface resistivity of the intermediate transfer belt. As a dispersion method, a known method such as a jet mill, a roll mill, a ball mill, a vibrating ball mill, an attritor, a sand mill, a colloid mill, or a paint shaker is used.

  In the method for manufacturing the intermediate transfer belt 31, the drying temperature is desirably 100 ° C. or higher and 170 ° C. or lower. If the drying temperature is too low in the drying step, the solvent in the polyimide precursor does not dry, and even if the drying time is extended, it is difficult to reach a predetermined residual solvent amount. In addition, if the drying temperature is too high, a dry film may be generated on the surface of the polyimide precursor at the very beginning of the drying process, and the solvent inside the polyimide precursor may not be sufficiently dried. The surface of the film is pulled, and wrinkle-like defects are easily generated on the surface of the coating film. By setting the drying temperature to 100 ° C. or more and 170 ° C. or less, a polyimide precursor film having a predetermined residual solvent amount and a better coating film is obtained.

  In the present embodiment, as described above, the intermediate transfer belt 31 has at least two layers. In the case of forming the intermediate transfer belt 31 having two or more layers, the material used is the same as described above. In addition, the multilayer structure film of two or more layers is formed by coating the coating liquid 16 on the outer peripheral surface of the cylindrical tube 11 and drying it to form the innermost peripheral layer, and then repeating the coating and drying process, A desired constituent film is obtained. When a polyimide precursor containing carbon black is used as the coating solution 16, the coating / drying process is repeated, and after the desired configuration is obtained, the desired constituent film is obtained through the firing step.

Next, examples and comparative examples of the intermediate transfer belt 31 according to the present invention will be described.
(Example 1)
Belt material Polyimide Belt structure 2 layers Belt surface resistance 11.3 (logΩ / □)
Belt backside resistance 11.3 (logΩ / □)
Belt volume resistance 9.7 (log Ω / cm)
Transfer Roller Structure (Structure of Secondary Transfer Back Roller 33 and Nip Forming Roller 36) Solid In the first embodiment, the surface resistance and back resistance of the intermediate transfer belt 31 are the same. The belt surface resistance is the electrical resistance on the front surface 31A side, and the belt back surface resistance is the electrical resistance on the back surface 31B side.

(Example 2)
Belt material Polyimide Belt structure 2 layers Belt surface resistance 12.5 (logΩ / □)
Belt backside resistance 11.0 (logΩ / □)
Volume resistance 10.0 (log Ω / cm)
Transfer Roller Structure (Structure of Secondary Transfer Back Roller 33 and Nip Forming Roller 36) Solid In Example 2, the surface resistance of the intermediate transfer belt 31 is made larger than the back surface resistance.

Example 3
Belt material Polyamideimide Belt structure 2 layers Belt surface resistance 11.3 (logΩ / □)
Belt backside resistance 11.3 (logΩ / □)
Belt volume resistance 9.7 (log Ω / cm)
Transfer Roller Structure (Structure of Secondary Transfer Back Roller 33 and Nip Forming Roller 36) Solid In the third embodiment, only the belt material is changed from polyimide to polyamideimide in the first embodiment.

Example 4
Belt material Polyamideimide Belt structure 2 layers Belt surface resistance 11.3 (logΩ / □)
Belt backside resistance 11.3 (logΩ / □)
Volume resistance 9.7 (log Ω / cm)
Transfer roller structure (structure of the nip forming roller 36) Sponge In the fourth embodiment, the nip forming roller 36 is changed from a solid to a sponge roller as compared with the third embodiment. The sponge roller here has the configuration shown in FIG.

(Comparative Example 1)
Belt material Polyimide Belt structure Single layer Belt surface resistance 11.3 (logΩ / □)
Belt backside resistance 11.3 (logΩ / □)
Volume resistance 9.5 (log Ω / cm)
Transfer roller structure (structure of the secondary transfer back roller 33 and the nip forming roller 36) Solid

(Comparative Example 2)
Belt material Polyimide Belt structure Single layer Belt surface resistance 12.5 (logΩ / □)
Belt backside resistance 12.5 (logΩ / □)
Belt volume resistance 11.0 (log Ω / cm)
Transfer Roller Structure (Structure of Secondary Transfer Backside Roller 33 and Nip Forming Roller 36) Solid In Comparative Example 2, the surface resistance and backside resistance of the intermediate transfer belt 31 are made larger than those in Comparative Example 1.

(Comparative Example 3)
Belt material Polyamideimide Belt structure 2 layers Belt surface resistance 11.3 (logΩ / □)
Belt backside resistance 11.3 (logΩ / □)
Belt volume resistance 9.5 (log Ω / cm)
Transfer Roller Structure (Structure of Secondary Transfer Backside Roller 33 and Nip Forming Roller 36) Solid In Comparative Example 3, only the belt material is changed from polyimide to polyamideimide in Comparative Example 1.

(Comparative Example 4)
Belt material Polyamideimide Belt structure 2 layers Belt surface resistance 11.3 (logΩ / □)
Belt backside resistance 11.3 (logΩ / □)
Belt volume resistance 9.7 (log Ω / cm)
Transfer Roller Structure (Structure of Nip Forming Roller 36) Sponge In Comparative Example 4, the nip forming roller 36 was changed from a solid to a sponge roller as compared with Comparative Example 3. The sponge roller here has the configuration shown in FIG.

The transfer conditions in each example and comparative example were all the same as shown below.
Recording material: RESAK 66 (trade name) 175 kg paper manufactured by Tokushu Paper Co., Ltd. Test environment: Temperature 10 ° C Humidity 15%
Transfer current: −40 μA (Voff is −3.5 kV)
AC voltage: peak to peak (Vpp) is 11.0 kV
Waveform: As an AC component, as shown in FIG. 8, the return time B is shorter than the time A on the transfer direction side, and the duty ratio (return time) is 12%, and the waveform is rounded. The duty ratio (return time) is a value indicating the relationship between the center voltage value Voff and the time average value Vave of the voltage, and indicates the ratio of the area on the return direction side relative to the center voltage value Voff in the entire AC waveform. is there.

  FIG. 9 summarizes the evaluation of the occurrence of white spots and afterimages in Examples 1 to 4 and Comparative Examples 1 to 4. In FIG. 9, xx, x, Δ, ◯, and ◎ indicate the evaluation contents. XX indicates a state in which a large amount has occurred, × indicates a state in which a large amount has occurred, and Δ indicates a state in which it has occurred. ○ indicates a slightly generated state, and ◎ indicates a hardly generated state. In this embodiment, ◎ and ○ are acceptable ranges.

First, in Comparative Examples 1 to 4, the occurrence of a white spot image was outside the allowable range regardless of the material of the belt. In particular, when the nip forming roller 36 was a sponge, a large amount of white spots occurred. The afterimage was within the allowable range except for Comparative Example 2. Only in Comparative Example 2, the resistance (log Ω / □) and volume resistance (log Ω / cm) of the front and back surfaces of the belt were set higher than those of the other comparative examples.
On the other hand, in Examples 1 to 4, the occurrence of white spots and afterimages were only ◯ and ◎, and all were within the allowable range. This is because, since the intermediate transfer belt 31 has a two-layer structure, even if conductive carbon is dispersed in the base material, the resistance that passes through the interface between the layers increases, so that the discharge is blocked and local This is thought to be due to the fact that it was possible to suppress general discharge.
Further, if the nip forming roller 36 is a sponge roller as shown in FIG. 3, it can be produced at a lower cost and cleaner-less than the solid roller shown in FIG. However, in the case of the sponge roller, the portion that is in contact with the recording material P inevitably has a portion having high resistance and a portion having low resistance, so that local discharge occurs in the secondary transfer nip N, and the comparative example As shown in FIG. 4, a large amount of white spots occur.
For this reason, when the nip forming roller 36 is used as a sponge roller, as shown in the fourth embodiment, by using the intermediate transfer belt 31 having a two-layer structure, local discharge in the secondary transfer nip N is generated. Is suppressed, and it can be improved from outside the allowable range where a large amount of white spots occur to a slightly generated state.

In consideration of the example, the occurrence of white spots hardly occurred in Example 2 than in Example 1. In Example 2, the surface resistance (logΩ / □) of the intermediate transfer belt 31 is set higher than the back surface resistance (logΩ / □). This is considered to be because local discharge was further suppressed while suppressing the afterimage by increasing the electrical resistance only on the surface 31A side in the two layers of the intermediate transfer belt 31. That is, it can be said that as the surface layer resistance of the intermediate transfer belt 31 is higher, local discharge is suppressed, and a large amount of white spots can be reduced.
As the secondary transfer bias used in each example and each comparative example, the return time B is shorter than the time A on the transfer direction side and the duty ratio (return time) is 12% as the AC component shown in FIG. It is not limited to a round waveform. For example, as shown in FIG. 10, a sinusoidal wave having a duty ratio (return time) of 50% is used as the AC component, and the return time B is the same as the time A on the transfer direction side. (Time) may be 50%.
As described above, even when the sine wave shown in FIG. 10 is used as the secondary transfer bias, the same effect as the above-described embodiment and examples can be obtained by forming the intermediate transfer belt 31 with at least two layers. be able to.

31 Intermediate transfer belt (belt-shaped image carrier)
31A surface (surface carrying a toner image)
31B Back side 36 Nip forming roller (transfer member)
39 Power supply N Transfer nip P Recording material

JP 2006-267486 A

Claims (3)

A transfer member that forms a transfer nip by contacting a surface carrying a toner image on a belt-like image carrier, and a toner image on the image carrier is transferred to a recording material sandwiched in the transfer nip. And a power supply that outputs a bias to
The bias includes a transfer direction bias for transferring the toner image from the image carrier side to the recording material side and a voltage in the transfer direction when transferring a toner image on the image carrier to the recording material. And an image forming apparatus in which a reverse polarity bias is alternately switched,
The image forming apparatus according to claim 1, wherein the belt-shaped image carrier has a layer configuration of at least two layers.   The image forming apparatus according to claim 1, wherein the transfer member forming the transfer nip is a sponge roller.   3. The image forming apparatus according to claim 1, wherein the belt-like image carrier has an electric resistance of a surface carrying a toner image higher than an electric resistance of a back side.

Images