EP3227753A1

EP3227753A1 - Image forming apparatus

Info

Publication number: EP3227753A1
Application number: EP15865806.2A
Authority: EP
Inventors: Tohru Nakaegawa; Yuusuke Torimaru
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2014-12-05
Filing date: 2015-11-17
Publication date: 2017-10-11
Also published as: KR20170088426A; CN107250921A; EP3227753A4; WO2016088316A1

Abstract

An image bearing member, an intermediate transfer member, a transfer member, a constant-voltage element that is connected between an inner circumferential surface of the intermediate transfer member and a ground potential and is configured to generate a predetermined voltage, a first power source that forms a secondary transfer electric field, and a second power source that forms a primary transfer electric field are provided, in which the first power source is controlled in a manner that an electric field having a direction opposite to that of the secondary transfer electric field is formed in a secondary transfer portion during at least a part of a period in which the primary transfer electric field is formed in a primary transfer portion.

Description

IMAGE FORMING APPARATUS

The present invention relates to an image forming apparatus using electrophotography such as a copier or a laser printer.

An intermediate transfer system has been proposed for an image forming apparatus based on an electrophotographic system in which a toner image is transferred from a photosensitive member onto an intermediate transfer member (primary transfer), and the toner image is transferred from the intermediate transfer member onto a recording medium (secondary transfer) to form an image.

However, if a power source for the primary transfer and primary transfer rollers are used in addition to secondary transfer rollers and a power source for the secondary transfer, this configuration may increase costs and increase a size of an intermediate transfer unit. In view of the above, PTL 1 proposes a configuration with which the size of the intermediate transfer unit is intended to be reduced. In the configuration, a resistance of an inner surface of an intermediate transfer belt is decreased, the primary transfer rollers and the power source for the primary transfer are not provided, and the intermediate transfer belt is grounded via a constant-voltage element (hereinafter, such configuration will be referred to as a "primary-transfer-member-less-system").

However, in the above-described primary-transfer-member-less-system, when the photosensitive member is abraded over use and its film thickness is decreased, the amount of a current flowing into a primary transfer portion among currents supplied from the power source may increase, and the amount of a current flowing into the constant-voltage element may decrease in some cases. As a result, there is a problem in that the constant-voltage element does not generate a voltage determined in advance, and a primary transfer failure occurs because of insufficiency of a primary transfer electric field.

To perform cleaning by returning fogging toner or flying toner accumulated on a secondary transfer member back to the intermediate transfer member, an electric field having a direction opposite to that of a secondary transfer electric field may be formed in a secondary transfer portion in some cases. In addition, to suppress adhesion of an adjustment image which is formed between images to the secondary transfer member, an electric field having the opposite direction may be formed in the secondary transfer portion in some cases.

Furthermore, to enhance image productivity, the primary transfer of image toner or adjustment toner may be performed simultaneously at a timing when the cleaning is executed or a timing when the adjustment image passes through the secondary transfer portion.

However, in the above-described primary-transfer-member-less-system, since both the secondary transfer electric field and the primary transfer electric field are generated by causing the current supplied from the power source to the secondary transfer member to flow into the constant-voltage element, there is a problem in that, during a period in which the primary transfer electric field is formed in the primary transfer portion, the electric field having the direction opposite to that of the secondary transfer electric field is not formed in the secondary transfer portion.

Japanese Patent Laid-Open No. 2012-137733

In view of the above, according to an aspect of the present invention, an image forming apparatus includes an image bearing member on which a toner image having a predetermined charging polarity is borne; an intermediate transfer member on which the toner image after primary transfer from the image bearing member in a primary transfer portion is borne; a transfer member that is arranged so as to abut against an outer circumferential surface of the intermediate transfer member and is configured to transfer the toner image from the intermediate transfer member onto a recording medium in a secondary transfer portion; a constant-voltage element that is electrically connected between an inner circumferential surface of the intermediate transfer member and a ground potential and is configured to generate a predetermined voltage by causing a current to flow; a first power source that applies a voltage to the transfer member and forms in the secondary transfer portion a secondary transfer electric field with which the toner image having the charging polarity is moved from the intermediate transfer member toward the recording medium; a second power source that is electrically connected to the inner circumferential surface of the intermediate transfer member to cause a current to flow through the constant-voltage element and forms in the primary transfer portion a primary transfer electric field with which the toner image having the charging polarity is moved from the image bearing member toward the intermediate transfer member; and a control unit configured to control the first power source such that an electric field having a direction opposite to that of the secondary transfer electric field is formed in the secondary transfer portion during at least a part of a period in which the primary transfer electric field is formed in the primary transfer portion.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

Fig. 1 is a diagram describing a basic configuration of the primary-transfer-member-less-system. Fig. 2 is a diagram describing an apparatus configuration according to a first exemplary embodiment. Fig. 3 is a diagram describing an equivalent circuit in the first exemplary embodiment. Fig. 4 illustrates a relationship between a transfer potential and an electrostatic image potential in the first exemplary embodiment. Fig. 5 illustrates a current-voltage characteristic of a Zener diode. Fig. 6 is a block diagram according to the first exemplary embodiment. Fig. 7 is a flowchart according to the first exemplary embodiment. Fig. 8 is a timing chart according to the first exemplary embodiment. Fig. 9 is a diagram describing an apparatus configuration according to a second exemplary embodiment. Fig. 10 is a diagram describing an adjustment toner image (patch image) in the second exemplary embodiment. Fig. 11A is a diagram describing a configuration according to the second exemplary embodiment. Fig. 11B is a diagram describing the configuration according to the second exemplary embodiment. Fig. 12A is a diagram describing control according to a comparative example. Fig. 12A is a diagram describing the control according to the comparative example. Fig. 13 is a block diagram of a control system of a transfer voltage according to the second exemplary embodiment. Fig. 14 is a flowchart of control according to the second exemplary embodiment. Fig. 15 is a timing chart according to the second exemplary embodiment. Fig. 16A is a diagram according to a third exemplary embodiment. Fig. 16B is a diagram according to the third exemplary embodiment. Fig. 17 is a timing chart according to the third exemplary embodiment.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. It should be noted that components assigned with the same reference symbols in the respective drawings have the same configuration or function, and redundant descriptions of these components are appropriately omitted.

First Exemplary Embodiment

Image Forming Apparatus 1

Fig. 2 illustrates an image forming apparatus of a tandem system using an intermediate transfer member according to the first exemplary embodiment.

Image forming units 101a, 101b, 101c, and 101d are configured to respectively form toner images of yellow (Y), magenta (M), cyan (C), and black (K). The image forming units are arranged in the order of the image forming units 101a, 101b, 101c, and 101d, that is, in the order of yellow, magenta, cyan, and black, from an upstream side in a direction of movement of an intermediate transfer belt 7.

The image forming units 101a, 101b, 101c, and 101d are respectively provided with photosensitive members 1a, 1b, 1c, and 1d functioning as image bearing members on which toner images are formed and borne. Charging rollers 2a, 2b, 2c, and 2d are charging units configured to charge surfaces of the respective photosensitive members 1a, 1b, 1c, and 1d. Exposure apparatuses 3a, 3b, 3c, and 3d are provided with laser scanners and expose the photosensitive members 1a, 1b, 1c, and 1d charged by the charging rollers 2a, 2b, 2c, and 2d with light. Power of the laser scanners is turned ON/OFF on the basis of image information, so that electrostatic images corresponding to an image are formed on the respective photosensitive members 1a, 1b, 1c, and 1d. That is, the charging rollers 2a, 2b, 2c, and 2d and the exposure apparatuses 3a, 3b, 3c, and 3d function as electrostatic image forming units configured to form the electrostatic images on the photosensitive members 1a, 1b, 1c, and 1d. Development apparatuses 4a, 4b, 4c, and 4d are development units respectively provided with containers containing toner having the respective colors of yellow, magenta, cyan, and black and configured to develop the electrostatic images on the photosensitive members 1a, 1b, 1c, and 1d by using the toner.

Primary transfers of the toner images formed on the photosensitive members 1a, 1b, 1c, and 1d onto the intermediate transfer belt 7 are performed in primary transfer portions N1a, N1b, N1c, and N1d. Thus, the toner images of the four colors are overlapped with one another and transferred onto the intermediate transfer belt 7.

The intermediate transfer belt 7 is a movable intermediate transfer member onto which the toner images are transferred from the photosensitive members 1a, 1b, 1c, and 1d. The intermediate transfer belt 7 has a two-layer structure including a base layer (on an inner circumferential surface side) and a surface layer (on an outer circumferential surface side). It should be noted that the intermediate transfer belt 7 may have a structure with two or more layers in which another layer is arranged between the base layer and the surface layer.

A layer obtained by adding an appropriate amount of antistatic agent such as carbon black into resin such as polyimide or polyamide, PEN, or PEEK or various rubbers is used as the base layer.

The base layer of the intermediate transfer belt 7 is formed such that surface resistivity of the base layer is set as 10² to 10⁸ Ω/□ (conductive). A film-like endless belt that is formed of polyimide and has a thickness of approximately 45 to 150 um in a central portion is used as the base layer in the present first exemplary embodiment.

The surface layer includes acrylic coating having volume resistivity in a thickness direction of the film including the base layer being set as 10⁹ to 10¹³ Ω*cm through resistance adjustment. That is, resistance of the base layer is lower than resistance of the surface layer.

A thickness of the surface layer is 1 to 10 um. The thickness is not intended to be limited to these numeric values of course.

The intermediate transfer belt 7 is supported by support rollers 10, 11, 12, and 13 as support members abutting against an inner circumferential surface of the intermediate transfer belt 7. The idler roller 12 supports the intermediate transfer belt 7 that extends in the arrangement direction of the respective photosensitive members 1a, 1b, 1c, and 1d.

The tension roller 11 is a roller that applies a certain tension to the intermediate transfer belt 7. Furthermore, the tension roller 11 functions as a correction roller to avoid meandering of the intermediate transfer belt 7. The tension roller 11 is configured such that belt tension thereof is set as approximately 5 to 12 kgf. With the application of the belt tension, nips are formed as the primary transfer portions N1a, N1b, N1c, and N1d between the intermediate transfer belt 7 and the photosensitive members 1a, 1b, 1c, and 1d.

The secondary transfer inner roller 10 functions as a driving roller that is driven by a motor having a satisfactory constant-speed performance and is configured to drive and circulate the intermediate transfer belt 7.

A second power source 21 is a power source that is connected to the tension roller 11 and causes a current to flow in a circumferential direction of the intermediate transfer belt 7. It should be noted that the second power source 21 does not necessarily need to be connected to the tension roller 11, and the second power source 21 may be connected to any one of the support rollers 10, 11, 12, and 13.

A recording medium P is stored in a sheet tray. The recording medium P is picked up from the sheet tray by a pickup roller at a predetermined timing and guided to a registration roller (not illustrated). In synchronous with the toner image on the intermediate transfer belt 7 being conveyed, the recording medium P is conveyed by the registration roller to a secondary transfer portion N2 where the toner image is transferred from the intermediate transfer belt 7 onto the recording medium P.

The secondary transfer outer roller 14 is a secondary transfer member that abuts against the intermediate transfer belt 7 and presses the secondary transfer inner roller 10 via the intermediate transfer belt 7 to constitute the secondary transfer portion N2 together with the secondary transfer inner roller 10.

A first power source 22 is a power source functioning as a voltage application unit that is electrically connected to the secondary transfer outer roller 14 (transfer roller) and applies a voltage to the secondary transfer outer roller 14.

When the recording medium P is conveyed to the secondary transfer portion N2, a secondary transfer voltage having an opposite polarity to that of the toner is applied to the secondary transfer outer roller 14 to form a secondary transfer electric field, and the toner image is transferred from the intermediate transfer belt 7 onto the recording medium P.

The secondary transfer inner roller 10 is made of EPDM rubber. A diameter of the secondary transfer inner roller 10 is set as 20 mm, a thickness of rubber is set as 0.5 mm, and a hardness thereof is set as 70° (Asker-C).

The secondary transfer outer roller 14 is constituted by an elastic layer formed of NBR rubber, EPDM rubber, or the like and a core bar. The secondary transfer outer roller 14 is formed such that its diameter is set as 24 mm.

An intermediate transfer belt cleaning apparatus 15 is provided on a downstream side with respect to the secondary transfer portion N2 in the moving direction of the intermediate transfer belt 7. The intermediate transfer belt cleaning apparatus 15 is configured to remove paper powder and residual toner that remains on the intermediate transfer belt 7 without being transferred onto the recording medium P in the secondary transfer portion N2.

A patch detection sensor (toner image detection unit) 207 is provided at a location where the patch detection sensor 207 faces an intermediate transfer belt portion supported by the support roller 12. The patch detection sensor 207 is configured to optically detect reflected light or scattered light of light incident on an adjustment toner image (patch image) formed on the intermediate transfer belt 7.

Primary Transfer Electric Field Formation

Formation of a primary transfer electric field according to the first exemplary embodiment will be described with reference to the equivalent circuit of Fig. 3. Herein, ITB_b denotes the base layer of the intermediate transfer belt 7, and ITB_s denotes the surface layer of the intermediate transfer belt 7.

A constant-voltage element is arranged between the support rollers 10, 11, 12, and 13 and ground such that the primary transfer electric field is generated by using the current from the first power source 22 for performing the secondary transfer of the toner image from the intermediate transfer belt 7 onto the recording medium.

As a result, as illustrated in Fig. 3, a potential of the intermediate transfer belt 7 is increased, and the primary transfer electric field is realized between the photosensitive members 1a, 1b, 1c, and 1d and the intermediate transfer belt 7.

Next, a primary transfer contrast corresponding to a difference between a potential of the photosensitive member and a potential of the intermediate transfer belt will be described with reference to Fig. 4.

Fig. 4 illustrates a case where a potential of the surface of the photosensitive member is set as Vd (herein, -678 V) while the surface of the photosensitive member is charged by the charging roller, and the potential of the surface of the photosensitive member is then set as Vl (herein, -240 V) while the charged surface of the photosensitive member is exposed by an exposure unit.

The potential Vd is a potential of a non-image part to which the toner is not adhered, and the potential Vl is a potential of an image part to which the toner on the photosensitive member is adhered. Vitb denotes a potential of the intermediate transfer belt.

A surface potential of the photosensitive member is controlled on the basis of a detection result of a potential sensor arranged in the vicinity of the photosensitive member on the downstream side with respect to the charging unit and the exposure unit and on the upstream side with respect to the development unit.

The potential sensor detects the potential of the non-image part and the potential of the image part on the surface of the photosensitive member. The potential sensor controls the charging potential of the charging unit on the basis of the potential of the non-image part and controls the exposure light intensity of the exposure unit on the basis of the potential of the image part.

With regard to the surface potential of the photosensitive member, both the potential of the image part and the potential of the non-image part can be set as appropriate values by the above-described control.

An electrostatic image contrast Vcb corresponding to a difference between the potential Vl of the image part and the potential Vd of the non-image part is set as -240 (V) - (-678 (V)) = 438 (V).

A primary transfer contrast Vtr corresponding to a difference between the potential Vl of the image part of the photosensitive member and the potential Vitb (herein, 300 V) of the intermediate transfer belt is set as 300 (V) - (-240 (V)) = 540 (V).

Voltage-Current Characteristic of Zener Diode

The primary transfer is determined by the primary transfer contrast corresponding to the difference between the potential of the intermediate transfer belt and the potential of the photosensitive member. Accordingly, in order to stably form the primary transfer contrast, the potential of the intermediate transfer belt is desirably maintained to be constant.

In view of the above, according to the first exemplary embodiment, a Zener diode is used as the constant-voltage element arranged between the supporting roller and the ground potential (ground). It should be noted that a varistor may be used instead of the Zener diode.

Fig. 5 illustrates a current-voltage characteristic of the Zener diode. The Zener diode has such a characteristic that a current hardly flows until a voltage higher than or equal to Zener breakdown voltage Vbr, but the current abruptly flows once the Zener breakdown voltage Vbr is generated. That is, when a voltage applied to a Zener diode 16 is higher than or equal to the Zener breakdown voltage, voltage drop of the Zener diode 16 is maintained to be constant at a Zener voltage (predetermined voltage).

The potential of the intermediate transfer belt 7 is maintained to be substantially constant at the predetermined voltage by using the above-described current-voltage characteristic of the Zener diode.

That is, according to the first exemplary embodiment, the Zener diode 16 is electrically connected between all the support rollers 10, 11, 12, and 13 and the ground potential (ground).

According to the exemplified embodiment of the present invention, in order that the voltage generated in the Zener diode 16 can be maintained at the predetermined voltage, a belt potential of the intermediate transfer belt 7 is maintained to be constant by providing the second power source 21.

According to the first exemplary embodiment, a dozen of the Zener diodes 16 having the Zener breakdown voltage Vbr at 25 V are connected in series between the support roller and the ground potential (ground). The potential of the intermediate transfer belt is maintained to be constant at a total of the Zener breakdown voltages of the respective Zener diodes, that is, 25 × 12 = 300 V.

The configuration is not intended to be limited to the configuration using a plurality of Zener diodes of course. A configuration using only the single Zener diode can also be adopted.

The configuration is not intended to be limited to the above-described configuration in which the surface potential of the intermediate transfer belt is set as 300 V of course. The surface potential is desirably set appropriately in accordance with a type of toner to be used and a characteristic of the photosensitive member.

Circuit Detecting Current Flowing through Zener Diode

According to the first exemplary embodiment, a first current detection circuit (first current detection circuit unit, current detection unit) 204 configured to detect a current flowing to ground via the Zener diode 16 is provided.

In a case where the current detected by the first current detection circuit 204 is lower than 5 μA, the voltage generated in the Zener diode 16 is lower than the predetermined voltage. In a case where the detected current is higher than or equal to 5 μA, the voltage generated in the Zener diode 16 is the predetermined voltage.

Controller

A configuration of a controller that performs control of the entire image forming apparatus according to the first exemplary embodiment will be described with reference to Fig. 6. As illustrated in Fig. 6, the controller includes a CPU circuit unit (control unit) 150. A CPU (not illustrated), a ROM 151, and a RAM 152 are built in the CPU circuit unit 150.

The first current detection circuit 204 is a circuit configured to detect a current flowing from the Zener diode 16 to the ground potential. A second current detection circuit (second current detection unit) 205 is a circuit configured to detect a current flowing from the first power source 22 to the secondary transfer portion. A potential sensor 206 that is not illustrated in Fig. 2 is a sensor configured to detect a potential of the surface of the photosensitive member.

The patch detection sensor 207 is a sensor configured to optically detect a patch image on the intermediate transfer belt.

Herein, the patch image includes the following types. (1) Patch image for exposure output setting: electrostatic images having different laser beam intensities in multiple stages are developed to form a patch-like adjustment toner image. (2) Patch image for gradation correction: electrostatic images having different halftone gradations in multiple stages are developed to form a patch-like adjustment toner image. (3) Patch image for refreshing developing agent: a patch-like adjustment toner image is formed at an image interval of every several tens of image formations. (4) Misregistration correction images for respective colors: a plurality of hatched line-like adjustment toner images with respect to a rotation direction of a photosensitive drum which are arranged in the conveyance direction are formed.

The first exemplary embodiment relates to an exemplary embodiment concerning the patch image (a plurality of patches are continuous) which is relatively long in the rotation direction of the photosensitive drum as in (1), (2), and (4).

To adjust image forming conditions such as an image density and an image forming timing, the CPU circuit unit (adjustment toner image forming unit) 150 forms an adjustment toner image (patch image) on the intermediate transfer belt during a period except a period in which the primary transfer of a toner image that is to be subjected to the secondary transfer onto the recording medium is performed.

Information from the first current detection circuit 204, the second current detection circuit 205, the potential sensor 206, and the patch detection sensor 207 is input to the CPU circuit unit 150. The CPU circuit unit 150 controls the second power source 21, the first power source 22, a development high voltage power source 201, an exposure power source 202, and a charging high voltage power source 203 collectively in accordance with a control program stored in the ROM 151.

The CPU circuit unit (adjustment unit) 150 changes image forming conditions such as a charging condition, an exposure condition, and a development condition of the photosensitive member in accordance with a detection result of the patch detection sensor 207.

The RAM 152 temporarily holds control data and is also used as a work area for calculation processing performed with the control.

Control upon Patch Image Detection

Control in a case where patch images are continuously formed on the intermediate transfer belt will be described with reference to Fig. 7.

Herein, the patch image corresponds to a single patch image or a series of patch images divided into plural pieces in a travelling direction of the intermediate transfer belt. A length of the series of patches from a position at the most upstream side to a position at the most downstream side in the travelling direction of the intermediate transfer belt is set to be longer than or equal to a length of a path on the intermediate transfer belt from the primary transfer portion to the secondary transfer portion.

In a case where the patch images are continuously formed, when the patch image is transferred onto the intermediate transfer belt in the primary transfer portion, the patch image that has been previously formed may pass through the secondary transfer portion in some cases. When the patch image passes through the secondary transfer portion, the recording medium does not exist in the secondary transfer portion. Accordingly, a non-transfer electric field that is different from the electric field at the time of the secondary transfer needs to be formed in the secondary transfer portion so that the contacted patch image is not to be electrostatically adhered to the secondary transfer outer roller.

However, the first power source 22 that forms the electric field in the secondary transfer portion is used for forming the electric field in the primary transfer portion in the primary-transfer-member-less-system. Even when the non-transfer electric field is formed in the secondary transfer portion, the appropriate primary transfer electric field needs to be formed in the primary transfer portion.

In view of the above, in S101, the CPU circuit unit 150 controls the second power source 21 such that the detected current of the first current detection circuit 204 is set as +5 μA at least during a period in which the patch images pass through the primary transfer portion. This control is continuously executed in the following S102 and S103.

With this control, the Zener diode can generate +300 V without the unnecessary amount of current flowing, and the appropriate primary transfer electric field can be formed in the primary transfer portion. The voltage of the second power source 21 at this time is substantially equal to +300 V and corresponds to a voltage slightly lower than +300 V.

Next, in S102, the CPU circuit unit 150 controls the first power source 22 such that the second current detection circuit 205 that detects the current flowing through the secondary transfer outer roller indicates -25 μA at least during a period in which the patch images pass through the secondary transfer portion.

Herein, the target current value is not limited to -25 μA. It is possible to previously set an appropriate target current in accordance with a state of the adhered toner or the like. For example, the target current is set as 0 to -5 (μA) in a case where the amount of adhered toner is not high, and the target current is set as -15 to -30 (μA) in a case where the amount of adhered toner is high.

Next, in S103, after the last patch image among the continuously formed patch images passes through the secondary transfer portion, the voltage applied to the secondary transfer outer roller is controlled as follows. That is, the voltage applied to the secondary transfer outer roller is controlled such that the detected current of the second current detection circuit 205 is alternately set as +25 (μA) and -25 (μA) at a time interval equivalent to one rotation of a perimeter of the secondary transfer outer roller.

Herein, the target current value is not limited to this. In addition, the number of alternate applications is not limited to one each for the positive current and the negative current, and it is possible to previously set the number by taking into account the degree of cleaning performance or the like.

With the above-described control, even when the patch image is being transferred in the primary transfer portion, it is possible to avoid the adhesion of the toner to the surface of the secondary transfer outer roller, and furthermore, the toner once adhered on the surface can be returned to the intermediate transfer belt from the secondary transfer outer roller by electrostatic force.

Next, a high voltage switching timing for the second power source 21 and the first power source 22 will be described.

Fig. 8 is a timing chart at the time of the patch image detection. Herein, t0 indicates a timing at which the control of the second power source 21 is started, t1 indicates a timing at which a patch image leading end enters the primary transfer portion, and t2 indicates a timing at which the control of the first power source 22 is started. Then, t3 indicates a timing at which the patch image leading end enters the secondary transfer portion, and t4 indicates a timing at which a patch image trailing end passes through the secondary transfer portion and a polarity of the target current value for the control of the second power source 21 is switched. Then, t5 indicates a timing at which the polarity of the target current value for the control of the second power source 21 is switched, and t6 indicates a timing at which electrostatic cleaning of the secondary transfer outer roller is ended.

First, the control of the second power source 21 is started such that the detected current of the first current detection circuit 204 is set as +5 (μA) at t0 prior to t1 at which the patch image leading end enters the primary transfer portion.

Next, the control of the first power source 22 is started such that the second current detection circuit 205 is set as -25 (μA) at t2 prior to the timing t3 at which the patch image leading end enters the secondary transfer portion. With this control, an electric field having a direction opposite to that of the secondary transfer electric field is formed in the secondary transfer portion.

Subsequently, at t4 at which the patch image trailing end passes through the secondary transfer portion, the polarity of the target value of the current of the second current detection circuit 205 is switched to +25 (μA) at the positive polarity. With this control, an electric field having the same direction as that of the secondary transfer electric field is formed in the secondary transfer portion.

At t5, after an elapse of time during which the secondary transfer outer roller makes one rotation following t4, the polarity of the target value of the current of the second current detection circuit 205 is switched to -25 (μA) at the negative polarity. With this control, an electric field having a direction opposite to that of the secondary transfer electric field is formed in the secondary transfer portion.

At t6, after an elapse of time during which the secondary transfer outer roller makes one rotation following t5, the first power source 22 is turned OFF.

With the above-described configuration, it is possible to avoid the contamination of the secondary transfer outer roller in a case where the continuous patch images are formed in the primary-transfer-member-less-system.

Second Exemplary Embodiment

Image Forming Apparatus 2

Fig. 9 is a diagram describing a configuration of an image forming apparatus. As illustrated in Fig. 9, an image forming apparatus 100 is a full-color printer based on a tandem-type intermediate transfer system in which image forming units Pa, Pb, Pc, and Pd are arranged along a downward facing surface of an intermediate transfer belt 56.

In the image forming unit Pa, a yellow toner image is formed on a photosensitive drum 50a and transferred onto the intermediate transfer belt 56. In the image forming unit Pb, a magenta toner image is formed on a photosensitive drum 50b and transferred onto the intermediate transfer belt 56. In the image forming units Pc and Pd, a cyan toner image and a black toner image are respectively formed on photosensitive drums 50c and 50d and transferred onto the intermediate transfer belt 56.

Image Forming Unit

As illustrated in Fig. 9, the image forming units Pa, Pb, Pc, and Pd have substantially the same configuration except for toner colors respectively used in development apparatuses 53a, 53b, 53c, and 53d being yellow, magenta, cyan, and black. In the following explanation, the image forming unit Pa will be described, and duplicated descriptions will be omitted with regard to the other image forming units Pb, Pc, and Pd.

As illustrated in Fig. 9, the intermediate transfer belt 56 is stretched and supported by a tension roller 63, a secondary transfer inner roller 62, and idler rollers 60 and 61 and driven by the secondary transfer inner roller 62 to be moved in an arrow R2 direction.

The secondary transfer inner roller 62 is arranged so as to also function as a driving motor of the intermediate transfer belt 56 and forms a secondary transfer portion T2 of the toner image with respect to the recording medium P by nipping the intermediate transfer belt 56 with a secondary transfer outer roller 64. The secondary transfer inner roller 62 is driven by a motor having a satisfactory constant-speed performance.

Secondary Transfer Portion

The secondary transfer outer roller 64 arranged on a toner image bearing surface side of the intermediate transfer belt 56 and the secondary transfer inner roller 62 arranged on an inner circumferential surface of the intermediate transfer belt 56 nip the intermediate transfer belt 56 to form the secondary transfer portion T2. The secondary transfer outer roller 64 abuts against the intermediate transfer belt 56 the inner circumferential surface of which is supported by the secondary transfer inner roller 62 and forms the secondary transfer portion T2.

A secondary transfer power source 210 outputs a variable voltage to the secondary transfer outer roller 64. The secondary transfer power source 210 applies to the secondary transfer outer roller 64 a direct-current voltage having a positive polarity with an absolute value higher than that of the secondary transfer inner roller 62, and causes the toner image to be moved from the intermediate transfer belt 56 onto the recording medium P. Accordingly, electric fields are respectively formed in abutting portions against the photosensitive drums 50a, 50b, 50c, and 50d via the conductive intermediate transfer belt 56, and the toner image borne on the intermediate transfer belt 56 is sequentially electrostatically moved to the intermediate transfer belt 56 to realize the primary transfer.

As described above, an exposure apparatus 52a and the development apparatus 53a representing an example of a toner image forming unit form a normal toner image and an adjustment toner image in accordance with image information on the photosensitive drum 50a representing an example of an image bearing member by using the toner charged to have a predetermined charging polarity. The intermediate transfer belt 56 representing an example of a conductive endless belt bears the toner image transferred from the photosensitive drum 50a onto the outer circumferential surface thereof and moves. The secondary transfer outer roller 64 representing an example of a first rotation member forms the secondary transfer portion T2 representing an example of a transfer portion abutting against the outer circumferential surface of the intermediate transfer belt 56. The secondary transfer inner roller 62 representing an example of a second rotation member faces the secondary transfer outer roller 64 and abuts against the inner circumferential surface of the intermediate transfer belt 56.

Adjustment Toner Image (Patch Image)

Fig. 10 is a diagram describing the adjustment toner image according to the second exemplary embodiment. The second exemplary embodiment relates to an exemplary embodiment concerning the patch image having a short length in the rotation direction of the photosensitive drum as in item (3) among the above-described four types of patch images.

An optical sensor 59 is arranged so as to face the intermediate transfer belt 56 supported by the idler roller 61. The optical sensor 59 irradiates the toner image borne on the intermediate transfer belt 56 with infrared light from a light emitting diode 59a, and a photodiode 59b detects reflected light in accordance with a toner adhesion amount of the toner image.

The image forming apparatus 100 forms a patch image between toner images (at image intervals) formed on the photosensitive drum 50a.

After the reflected light is measured by the optical sensor 59, the patch image passes through the secondary transfer portion T2 to be conveyed to a belt cleaning apparatus 65 and removed from the intermediate transfer belt 56 by a cleaning blade. However, when the patch image borne on the intermediate transfer belt 56 passes through the secondary transfer portion T2, if a voltage having the opposite polarity to the charging polarity of the toner has been applied to the secondary transfer outer roller 64, the patch image is moved to the secondary transfer outer roller 64. When the toner is adhered to the secondary transfer outer roller 64, the toner is adhered to a rear surface of the recording medium and fixed at a subsequent time of the image formation, and rear contamination of the recording medium occurs.

In view of the above, according to the second exemplary embodiment, while the patch image passes through the secondary transfer portion T2, the voltage having the opposite polarity to the charging polarity of the toner is applied to the secondary transfer inner roller 62 to avoid the movement of the patch image to the secondary transfer outer roller 64.

Secondary Transfer Power Source

Fig. 11A and Fig. 11B are diagrams describing control according to the second exemplary embodiment. Fig. 11A corresponds to a time when the toner image of the image in the secondary transfer portion T2 is transferred, and Fig. 11B corresponds to a time when the patch image passes through the secondary transfer portion T2.

As illustrated in Fig. 11A, the image forming apparatus 100 does not include a primary transfer roller 73 for each of the photosensitive drums 50a, 50b, 50c, and 50d. While the conductive intermediate transfer belt 56 is set to have a constant potential having the opposite polarity to the charging polarity of the toner by using the idler rollers 60 and 61, the toner images are transferred from the photosensitive drums 50a, 50b, 50c, and 50d onto the intermediate transfer belt 56.

The idler rollers 60 and 61 form a supported surface of the intermediate transfer belt 56 to equally abut against the photosensitive drums 50a, 50b, 50c, and 50d. The idler roller 60 and the tension roller 63 are electrically connected to the idler roller 61 and grounded via a Zener diode circuit 71. Accordingly, when the secondary transfer voltage is applied to the secondary transfer outer roller 64, a potential of a region of the intermediate transfer belt 56, the region being supported by the idler rollers 60 and 61, is maintained to be constant by the Zener diode circuit 71 via the idler rollers 60 and 61.

The Zener diode circuit 71 is configured such that the Zener breakdown voltage Vbr is set as 300 V in total by connecting a plurality, a dozen in this case, of Zener elements each having a breakdown voltage Vbr of 25 V in series. As a result of selection of an element having a transient response characteristic that hardly affects uniformity of the image, SJPZ-N33 manufactured by Sanken Electric Co., Ltd. is used as the Zener diode.

An output of a power source 310 is controlled such that a detected current of a first current detection circuit 510 representing an example of a detection result of a detection unit is set as +5 μA at which the Zener diode circuit 71 can sufficiently maintain the breakdown voltage. A voltage Vc of the power source 310 at this time is set as 300 V that is equal to the Zener breakdown voltage Vbr.

As illustrated in Fig. 11A, the secondary transfer power source 210 applies to the secondary transfer outer roller 64 a transfer voltage Vt used for moving the toner image from the intermediate transfer belt 56 to the recording medium.

A voltage necessary for moving the toner image from the photosensitive drum 50a to the intermediate transfer belt 56 is a substantially constant voltage Vc. On the other hand, since a voltage necessary for moving the toner image from the intermediate transfer belt 56 to the recording medium P greatly changes in accordance with a type of the recording medium and a hygroscopic degree, the secondary transfer power source 210 outputs a variable direct-current constant voltage in a range of 1200 V to 1500 V.

A control unit 110 is configured to control the secondary transfer power source 210. In the case of the recording medium having a high resistance value in the thickness direction, the control unit 110 outputs to the secondary transfer outer roller 64 the transfer voltage Vt higher than that in the case of the recording medium having a low resistance value in the thickness direction. For example, in the case of plain paper (CS-814) having a weight per unit area of 81 [g/m²], the control unit 110 applies a voltage at approximately 1300 V to the secondary transfer outer roller 64 from the secondary transfer power source 210 so that a constant current at 20 μA flows. When the constant voltage at 1300 V is applied to the secondary transfer outer roller 64, a voltage at 1000 V obtained as a result of subtraction of 300 V that is the potential of the secondary transfer inner roller 62 is applied between the secondary transfer inner roller 62 and the secondary transfer outer roller 64.

As illustrated in Fig. 11A, when the toner image of the image passes through the secondary transfer portion T2, the control unit 110 applies the transfer voltage Vt to the secondary transfer outer roller 64 from the secondary transfer power source 210. At this time, since the Zener diode circuit 71 maintains the potential of the intermediate transfer belt 56 supported by the idler rollers 60 and 61 to be at the Zener breakdown voltage Vbr, the toner image is stably transferred from the photosensitive drum 50a onto the intermediate transfer belt 56.

As illustrated in Fig. 11B, when the patch image passes through the secondary transfer portion T2, the control unit 110 stops the output of the secondary transfer power source 210. Subsequently, a switch SW1 is operated to connect the secondary transfer outer roller 64 to the ground potential. At this time also, the control unit 110 controls the power source 310 by using the first current detection circuit 510 such that the constant current flows through the Zener diode circuit 71. As a result, since the constant voltage Vc is applied to the secondary transfer inner roller 62, the difference between the potential of the photosensitive drum 50a and the potential of the intermediate transfer belt 56 is maintained to be substantially the same as that when the toner image of the image passes through the secondary transfer portion T2.

Therefore, during the period in which the primary transfer of the toner image is performed from the photosensitive drum 50a onto the intermediate transfer belt 56, even when the patch image passes through the secondary transfer portion T2 and the secondary transfer outer roller 64 is connected to the ground potential, the primary transfer of the toner image onto the intermediate transfer belt 56 is normally continued.

As described above, the Zener diode circuit 71 representing an example of the constant-voltage element maintains the voltage of the intermediate transfer belt 56 contacted by the photosensitive drum 50a to be at the predetermined voltage.

The idler roller 61 representing an example of an abutting member abuts against the inner circumferential surface of the intermediate transfer belt 56 between the photosensitive drum 50a and the secondary transfer portion T2 in the movement direction of the intermediate transfer belt 56. The Zener diode circuit 71 representing an example of the constant-voltage element is connected to the idler roller 61 to maintain the voltage of the intermediate transfer belt 56 to be at the Zener breakdown voltage Vbr.
(Comparative Example)

Fig. 12A and Fig. 12B are diagrams describing control according to a comparative example. Fig. 12A corresponds to a time when the toner image of the image is transferred in the secondary transfer portion T2, and Fig. 12B corresponds to a time when the patch image passes through the secondary transfer portion T2.

As illustrated in Fig. 12A, in an image forming apparatus 100H according to the comparative example, the secondary transfer inner roller 62, the idler rollers 60 and 61, and the tension roller 63 are connected to the Zener diode circuit 71 to have the potential maintained at the Zener breakdown voltage Vbr.

The secondary transfer power source 210 applies to the secondary transfer outer roller 64 the transfer voltage Vt having the positive polarity necessary to transfer the toner image from the intermediate transfer belt 56 onto the recording medium P. Accordingly, the toner image is transferred from the intermediate transfer belt 56 onto the recording medium and at the same time the potential of the intermediate transfer belt 56 supported by the idler rollers 60 and 61 is maintained at the Zener breakdown voltage Vbr by the Zener diode circuit 71. Thus, while the secondary transfer of the toner image is performed from the intermediate transfer belt 56 onto the recording medium, it is possible to perform the primary transfer of the toner image from the photosensitive drum 50a to the intermediate transfer belt 56.

As illustrated in Fig. 12B, when the patch image passes through the secondary transfer portion T2 in the image forming apparatus 100H according to the comparative example, the polarity of the voltage applied to the secondary transfer outer roller 64 is inverted from the polarity when the toner image is transferred from the intermediate transfer belt 56 onto the recording medium P. When the patch image passes through the secondary transfer portion T2, the control unit 110 applies the direct-current voltage having the negative polarity to the secondary transfer outer roller 64 to avoid the transfer of the patch image borne on the intermediate transfer belt 56 onto the secondary transfer outer roller 64. The patch image passes through the secondary transfer portion T2 to be collected by the belt cleaning apparatus 65 without being transferred onto the secondary transfer outer roller 64.

According to the comparative example, since the charging polarity of the toner is the negative polarity, at the time of a normal transfer, in order to apply a forward voltage to the secondary transfer outer roller 64, the potential of the secondary transfer outer roller 64 needs to be higher than the potential of the secondary transfer inner roller 62. In contrast to this, when the patch image passes through the secondary transfer portion T2, in order to form an electric field having a direction opposite to that at the time of the transfer of the toner image onto the secondary transfer portion T2, the potential of the secondary transfer outer roller 64 needs to be lower than the potential of the secondary transfer inner roller 62.

At this time, since the potential of the intermediate transfer belt 56 supported by the idler rollers 60 and 61 is set as the ground potential because of the characteristic of the Zener diode, the toner image is not transferred from the photosensitive drum 50a onto the intermediate transfer belt 56. Therefore, the patch image is not formed on the interval between the toner images of the image as illustrated in Fig. 10. Thus, before the patch image is formed, the formation of the toner image of the image in the photosensitive drums 50a, 50b, 50c, and 50d and the transfer onto the intermediate transfer belt 56 need to be stopped. Accordingly, the image formation on the photosensitive drum 50a and the transfer onto the intermediate transfer belt 56 need to be started only after the patch image passes through the secondary transfer portion T2.

Control According To Second Exemplary Embodiment

Fig. 13 is a block diagram of a control system for the transfer voltage according to the second exemplary embodiment. Fig. 14 is a flowchart for the control according to the second exemplary embodiment. Fig. 15 is a time chart for the control according to the second exemplary embodiment. With regard to the potential of the support rollers (62, 63, 60, and 61) that support the intermediate transfer belt 56, detailed descriptions are given by individually describing a period in which the toner image of the image passes through the secondary transfer portion and a period in which the patch image passes through the secondary transfer portion according to the second exemplary embodiment.

As illustrated in Fig. 13, the control related to the primary transfer and the secondary transfer of the toner image of the image according to the second exemplary embodiment is executed by using the secondary transfer power source 210 and the power source 310. Digital signals transmitted from a DC controller 111 of the control unit 110 are converted into analog signals by digital-to-analog converters 200 and 300 to be output to the secondary transfer power source 210 and the power source 310. A second current detection circuit 410 outputs a current in accordance with the transfer voltage Vt output from the secondary transfer power source 210. The analog signal obtained by the second current detection circuit 410 is converted into a digital signal by an analog-to-digital converter 400 to be fed back to the control unit 110.

With reference to Fig. 11A and Fig. 11B, as illustrated in Fig. 14, when the operation of the image forming apparatus 100 is started, the control unit 110 operates the image forming units Pa, Pb, Pc, and Pd to start forward rotation of the photosensitive drums 50a, 50b, 50c, and 50d. In the forward rotation, after various adjustments regarding, for example, the charging potential, the development potential, and laser beam intensity are performed, gradation control of the output image is executed (S1).

The control unit 110 executes control to obtain current values at the time of application of a plurality of test voltages, and determines the transfer voltage Vt applied to the secondary transfer outer roller 64 such that a predetermined target current flows through the secondary transfer portion T2 (S2).

The control unit 110 starts continuous image formation as one unit operation received as a job (S3).

The control unit 110 starts formation of the patch image for density control of the output image on the interval between the toner images of the image (S4) to be transferred onto the intermediate transfer belt 56 and detected by the optical sensor 59. A detection result of the patch image is fed back to the control unit 110, and the density adjustment of the output image is executed.

In a case where the image interval is not passing through the secondary transfer portion T2 (S5: NO), that is, during the toner image of the image passing through the secondary transfer portion T2, the control unit 110 connects the secondary transfer inner roller 62 to the secondary transfer power source 210 from which the transfer voltage Vt is output (S6). As illustrated in Fig. 12A, the secondary transfer of the toner image of the image from the intermediate transfer belt 56 onto the recording medium P is performed by a difference voltage between the transfer voltage Vt output from the secondary transfer power source 210 to the secondary transfer outer roller 64 and the output voltage Vc of the power source 310.

While the image interval is passing through the secondary transfer portion T2 (S5: YES), that is, during the patch image passing through the secondary transfer portion T2, the control unit 110 turns the secondary transfer power source 210 OFF to connect the secondary transfer inner roller 62 to the ground potential (S7). As illustrated in Fig. 12B, the voltage Vc is applied from the power source 310 to the secondary transfer inner roller 62 and the intermediate transfer belt 56, and an electric field having a direction opposite to that in a case where the toner image of the image is transferred is formed in the secondary transfer portion T2. Accordingly, the transfer of the patch image on the intermediate transfer belt 56 onto the secondary transfer outer roller 64 is avoided.

When the continuous image formation as the one unit operation received as the image formation job is ended (S8: YES), the control unit 110 executes backward rotation and stops the image forming apparatus 100 (S9). In the backward rotation, gamma correction control or the like is executed. Thus, the operation of the image forming apparatus 100 is ended.

As illustrated in Fig. 15, both the secondary transfer power source 210 and the power source 310 are in an ON state at a timing at which the toner image of the image is located in the secondary transfer portion T2.

At a timing at which the image interval corresponding to an interval between a toner image of the image and another toner image of the image is located in the secondary transfer portion T2, the power source 310 remains in the ON state, but the secondary transfer power source 210 is turned OFF, and the secondary transfer outer roller 64 is connected to the ground potential. Accordingly, the transfer of the patch image onto the secondary transfer outer roller 64 is avoided. At a timing at which the patch image has passed through the secondary transfer portion T2, the secondary transfer outer roller 64 is connected to the secondary transfer power source 210, and the secondary transfer power source 210 is turned ON.

Even when the series of operations is repeated for each image, since the potential of the idler roller 61 is maintained by the Zener diode circuit 71, the potential of the intermediate transfer belt 56 that abuts against the photosensitive drum 50a is regularly maintained at 300 V.

Accordingly, in the configuration of the intermediate transfer system in which the primary transfer roller is not provided, it is possible to perform the image formation in the primary transfer portion even during the cleaning of the secondary transfer outer roller by applying the reverse bias for cleaning the secondary transfer outer roller from the secondary transfer inner roller.

It should be noted that, as represented by a broken line in Fig. 15, the secondary transfer outer roller 64 may be connected to the ground potential in a margin section at the trailing end of the recording medium, and the secondary transfer power source 210 may be connected in a margin section at the leading end of the following recording medium to output the transfer voltage Vt.

As described above, according to the second exemplary embodiment, the secondary transfer power source 210 representing an example of a first power source can apply to the secondary transfer outer roller 64 a first voltage with which the toner image can be transferred from the intermediate transfer belt 56 onto the recording medium while a voltage higher than or equal to the Zener breakdown voltage Vbr is applied to the intermediate transfer belt 56. The power source 310 representing an example of a second power source can apply a second voltage having the same polarity as that of the first voltage and an absolute value lower than the Zener breakdown voltage Vbr and lower than an absolute value of the first voltage to the intermediate transfer belt 56 that is contacted with the secondary transfer outer roller 64 via the secondary transfer inner roller 62. The control unit 110 representing an example of a control unit controls the secondary transfer power source 210 and the power source 310 to form an electric field necessary for the secondary transfer portion T2.

When the normal toner image borne on the intermediate transfer belt 56 passes through the secondary transfer portion T2, the control unit 110 applies a voltage to the intermediate transfer belt 56 and forms an electric field for moving the toner toward the recording medium from the intermediate transfer belt 56. Specifically, the first voltage is applied to the secondary transfer outer roller 64, and the second voltage is applied to the secondary transfer inner roller 62.

When the patch image borne on the intermediate transfer belt 56 passes through the secondary transfer portion T2, the control unit 110 applies a voltage to the intermediate transfer belt 56 and forms an electric field for moving the toner toward the intermediate transfer belt 56 from the secondary transfer outer roller 64. Specifically, the potential of the secondary transfer outer roller 64 is set to a potential toward the charging polarity of the toner as compared to the potential of the secondary transfer inner roller 62, and the second voltage is applied to the secondary transfer inner roller 62.

Effects of Second Exemplary Embodiment

According to the second exemplary embodiment, when the normal toner image borne on the intermediate transfer belt 56 passes through the secondary transfer portion T2, the control unit 110 applies the second voltage to the secondary transfer inner roller 62 such that a current with which a voltage is not lower than the Zener breakdown voltage Vbr flows through a voltage generation unit. Accordingly, even if a resistance value of the secondary transfer portion T2 when the toner image is transferred onto the recording medium P fluctuates, the potential of the intermediate transfer belt 56 does not decrease to be lower than the Zener breakdown voltage Vbr. The potential difference between the photosensitive drum 50a and the intermediate transfer belt 56 is stabled, and disturbance hardly occurs in the transfer of the toner image.

According to the second exemplary embodiment, the power source 310 applies the second voltage to the secondary transfer inner roller 62. Accordingly, it is possible to equally form the electric field over the entire transfer nip facing the secondary transfer outer roller 64.

According to the second exemplary embodiment, since the image forming apparatus 100 is based on the intermediate transfer system without the primary transfer roller, cost saving and reduction in the size of the apparatus are realized as compared with the configuration of the related-art intermediate transfer system because of the omission of the primary transfer roller. In addition, unlike the related-art intermediate transfer system, a transfer failure caused by toner cohesion due to an increase in a local transfer pressure in the center of the toner adhesion region when passing through the primary transfer portion, that is, a so-called hollow image hardly occurs.

According to the second exemplary embodiment, since none of the support rollers (63, 60, and 61) are grounded, the current does not flow from the intermediate transfer belt 56 to the ground potential through the support rollers (63, 60, and 61). Accordingly, the application of the primary transfer voltage from the secondary transfer inner roller 62 can be performed.

According to the second exemplary embodiment, since the support rollers (63, 60, and 61) that support the intermediate transfer belt 56 are grounded via the Zener diode circuit 71 representing an example of a voltage generation unit, wasteful flow of the primary transfer current to the support rollers (63, 60, and 61) is avoided. Since the support rollers (63, 60, and 61) are grounded via the Zener diode circuit 71, the potential of the intermediate transfer belt 56 is maintained at an appropriate potential necessary for the primary transfer of the toner image of the image.

According to the second exemplary embodiment, since the intermediate transfer belt 56 having a resistance value lower than that of the related-art intermediate transfer system is used, the high voltage applied to the secondary transfer portion T2 via the intermediate transfer belt 56 can substantially equally act on the plurality of primary transfer portions. Electric charges supplied from the secondary transfer power source 210 flow into the photosensitive drum 50a via the intermediate transfer belt 56, so that the primary transfer of the toner image of the image is executed similarly as in the related-art intermediate transfer system.

According to the second exemplary embodiment, while the patch image exists in the secondary transfer portion T2, a voltage having the positive polarity is applied to the secondary transfer inner roller 62 to reduce the potential of the secondary transfer outer roller 64 to be lower than that of the secondary transfer inner roller 62 and to form an electric field having the direction opposite to the direction at the time of the transfer onto the secondary transfer portion T2. To clean the secondary transfer outer roller 64 contaminated by being contacted with the patch image, a voltage having the positive polarity is applied from the secondary transfer inner roller 62 by the power source 310. Since the power source 310 continues application of the forward voltage to the primary transfer portion, the formation of the toner image of the image and the primary transfer can be performed in the photosensitive drums 50a, 50b, 50c, and 50d without interruption.

Third Exemplary Embodiment

As illustrated in Fig. 11A, according to the second exemplary embodiment, the power source 310 regularly outputs 300 V to the secondary transfer inner roller 62 also during the toner image of the image passing through the secondary transfer portion T2. In contrast to this, as illustrated in Fig. 16A, according to the third exemplary embodiment, while the toner image of the image passes through the secondary transfer portion T2, the power source 310 is turned OFF, and the Zener diode circuit 71 is connected to the secondary transfer inner roller 62. Accordingly, power consumption of the power source 310 is saved during a period in which the toner image of the image is transferred onto the recording medium.

Control of Third Exemplary Embodiment

Fig. 16A and Fig. 16B are diagrams describing control according to the third exemplary embodiment. Fig. 17 is a time chart of the control according to the third exemplary embodiment. Fig. 16A corresponds to a time when the toner image of the image is transferred in the secondary transfer portion T2, and Fig. 16B corresponds to a time when the patch image passes through the secondary transfer portion T2.

As illustrated in Fig. 16A, the secondary transfer outer roller 64 performs switching between the secondary transfer power source 210 and the ground potential by the switch SW1 similarly as in the second exemplary embodiment. On the other hand, the secondary transfer inner roller 62 performs switching between the power source 310 and the Zener diode circuit 71 by a switch SW2. The switch SW2 representing an example of a switching unit switches a connected state and an interrupted state of an electric connection between the inner circumferential surface of the intermediate transfer belt 56 and the power source 310.

As illustrated in Fig. 16B, while the patch image passes through the secondary transfer portion T2, the control unit 110 connects the secondary transfer outer roller 64 to the ground potential by the switch SW1 and connects the secondary transfer inner roller 62 to the power source 310 by the switch SW2. The control unit 110 controls the power source 310 such that a detected current of the first current detection circuit 510 is set as +5 μA corresponding to a current at which the Zener diode circuit 71 can sufficiently maintain the breakdown voltage. Accordingly, the voltage Vc equal to 300 V of the Zener breakdown voltage Vbr is applied to the secondary transfer inner roller 62, and an electric field having a direction opposite to the direction in a case where the toner image of the image is transferred onto the recording medium is formed in the secondary transfer portion T2. Thus, the toner image on the intermediate transfer belt 56 receives Coulomb force on the intermediate transfer belt 56 side, and the adhesion of the toner image onto the secondary transfer outer roller 64 is avoided.

As illustrated in Fig. 17, after the power source 310 is turned ON when a margin region (5 mm) of the trailing end of the recording medium passes through the secondary transfer portion T2, the secondary transfer power source 210 is turned OFF. Thereafter, the switch SW1 is switched from the secondary transfer power source 210 to the ground potential. Accordingly, the patch image formed so as to correspond to the interval of the recording medium is not transferred onto the secondary transfer outer roller 64.

As illustrated in Fig. 16A, during the transfer of the toner image of the image onto the recording medium, the control unit 110 connects the secondary transfer outer roller 64 to the secondary transfer power source 210 by the switch SW1 and connects the secondary transfer inner roller 62 to the Zener diode circuit 71 by the switch SW2.

As illustrated in Fig. 17, when a margin region (5 mm) of the leading end of the following recording medium passes through the secondary transfer portion T2, the switch SW1 is switched from the ground potential to the secondary transfer power source 210, and the secondary transfer power source 210 is turned ON. Thereafter, the power source 310 is turned OFF. Subsequently, the switch SW2 is switched from the power source 310 to the Zener diode circuit 71 to maintain the voltage of the secondary transfer inner roller 62. Accordingly, the potential of the secondary transfer outer roller 64 is maintained at 300 V also after the power source 310 is turned OFF, and the stable transfer voltage at 1300 V - 300 V = 1000 V is kept applied between the secondary transfer inner roller 62 and the secondary transfer outer roller 64.

Effects of Third Exemplary Embodiment

According to the third exemplary embodiment, as illustrated in Fig. 17, the timing at which the reverse bias for cleaning the secondary transfer outer roller 64 is applied from the secondary transfer inner roller 62 is controlled. Accordingly, the image formation on the photosensitive drum 50a can be performed also at the time of the cleaning of the secondary transfer outer roller 64, and the costs can be further reduced at the same time as compared with the second exemplary embodiment.

According to the second exemplary embodiment, 300 V is regularly output to the secondary transfer inner roller 62 from the power source 310. In this state, when the patch image passes through the secondary transfer portion T2, 1300 V is output to the secondary transfer outer roller 64 from the secondary transfer power source 210. Accordingly, the secondary transfer contrast is set as 1000 V.

According to the third exemplary embodiment, with the provision of the switch SW2, since the time during which the power source 310 is turned OFF is longer than that of the second exemplary embodiment, the power consumption can be accordingly reduced.

Other Embodiments

The present invention is not limited to the configurations and controls according to the first, second, and third exemplary embodiments. The numeric values and the like used for the descriptions according to the first, second, and third exemplary embodiments are examples, and the present invention is not limited by those numeric values.

The control unit controls the second power source such that the detection unit detects a current with which a voltage across the terminals of the constant-voltage element is maintained at a predetermined voltage. The second power source may be an analog power source to which an analog current output from the detection unit is directly fed back and which constantly changes an output voltage. The second power source may also be a digitally controlled analog power source that outputs an output voltage calculated on the basis of a current value measured by the detection unit as a constant voltage. In either case, since the current detected by the detection unit is within an appropriate range, flow of an excess current to the constant-voltage element can be avoided while a necessary and sufficient primary transfer voltage is secured.

Methods for the output voltage control of the first power source and the second power source may be different from each other. The control unit that controls the first power source and the control unit that controls the second power source may be implemented by independent and separate control units.

The image forming apparatus may adopt other image bearing members than the photosensitive drum, and another type of image forming apparatus that adopts an endless belt can also be implemented. The constant-voltage element may have a configuration in which the support rollers (63, 60, 61) are grounded via the constant-voltage element such as the varistor or a resistance higher than or equal to 10⁸ [Ω] other than the use of the Zener diode.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2014-246568, filed December 5, 2014, and Japanese Patent Application No. 2014-247333, filed December 5, 2014, which are hereby incorporated by reference herein in their entirety.

Claims

An image forming apparatus comprising:
an image bearing member on which a toner image having a predetermined charging polarity is borne;
an intermediate transfer member on which the toner image after primary transfer from the image bearing member in a primary transfer portion is borne;
a transfer member that is arranged so as to abut against an outer circumferential surface of the intermediate transfer member and is configured to transfer the toner image from the intermediate transfer member onto a recording medium in a secondary transfer portion;
a constant-voltage element that is electrically connected between an inner circumferential surface of the intermediate transfer member and a ground potential and is configured to generate a predetermined voltage by causing a current to flow;
a first power source that applies a voltage to the transfer member and forms in the secondary transfer portion a secondary transfer electric field with which the toner image having the charging polarity is moved from the intermediate transfer member toward the recording medium;
a second power source that is electrically connected to the inner circumferential surface of the intermediate transfer member to cause a current to flow through the constant-voltage element and forms in the primary transfer portion a primary transfer electric field with which the toner image having the charging polarity is moved from the image bearing member toward the intermediate transfer member; and
a control unit configured to control the first power source such that an electric field having a direction opposite to that of the secondary transfer electric field is formed in the secondary transfer portion during at least a part of a period in which the primary transfer electric field is formed in the primary transfer portion.
The image forming apparatus according to Claim 1, wherein the control unit forms in the secondary transfer portion the electric field having the direction opposite to that of the secondary transfer electric field by performing control to set the first power source to have the ground potential or apply a voltage having the same polarity as the charging polarity to the first power source.
The image forming apparatus according to Claim 1, wherein the control unit forms in the secondary transfer portion the electric field having the direction opposite to that of the secondary transfer electric field at least during a period in which the toner to be subjected to the second transfer onto the recording medium does not exist in the secondary transfer portion.
The image forming apparatus according to Claim 1, wherein the control unit controls the second power source such that the constant-voltage element generates the predetermined voltage in a case where the primary transfer electric field is formed in the primary transfer portion during a period in which the electric field having the direction opposite to that of the secondary transfer electric field is formed in the secondary transfer portion.
The image forming apparatus according to Claim 4, further comprising:
a current detection member configured to detect the current flowing through the constant-voltage element,
wherein the control unit controls the second power source on the basis of a detection result of the current detection member such that the constant-voltage element generates the predetermined voltage.
The image forming apparatus according to Claim 1, further comprising:
an execution unit configured to execute a test mode for forming an adjustment toner image on the intermediate transfer member in a period except a period in which the primary transfer of the toner image which is to be subjected to the secondary transfer onto the recording medium is performed onto the intermediate transfer member;
a toner image detection member configured to detect the adjustment toner image formed on the intermediate transfer member; and
an adjustment unit configured to adjust, on the basis of a detection result of the toner image detection member, an image forming condition of the toner image to be transferred onto the recording medium,
wherein the control unit controls the first power source to form in the secondary transfer portion the electric field having the direction opposite to that of the secondary transfer electric field at least during a period in which the adjustment toner image exists in the secondary transfer portion.
The image forming apparatus according to Claim 6, wherein the control unit controls the second power source to form the primary transfer electric field in the primary transfer portion during a period in which the adjustment toner image exists in the secondary transfer portion and the toner image to be transferred onto the recording medium exists in the primary transfer portion.
The image forming apparatus according to Claim 6, wherein the control unit controls the second power source to form the primary transfer electric field in the primary transfer portion during a period in which a part of the adjustment toner image exists in the secondary transfer portion and another part of the adjustment toner image exists in the primary transfer portion.
The image forming apparatus according to Claim 1, wherein a plurality of image bearing members are arranged along the outer circumferential surface of the intermediate transfer member, and the second power source forms the primary transfer electric field in each of the plurality of primary transfer portions respectively corresponding to the plurality of image bearing members.
The image forming apparatus according to Claim 1, wherein the transfer member includes a transfer roller, and the control unit controls the second power source to form in the secondary transfer portion an electric field having the same direction as that of the secondary transfer electric field for a time when the transfer roller makes at least one rotation during a period in which the toner image to be subjected to the secondary transfer onto the recording medium does not exist in the secondary transfer portion.
The image forming apparatus according to Claim 8, wherein the control unit controls the second power source to form in the secondary transfer portion the electric field having the direction opposite to that of the secondary transfer electric field for a time when a transfer roller makes at least one rotation subsequently after an electric field having the same direction as that of the secondary transfer electric field is formed in the secondary transfer portion.
The image forming apparatus according to Claim 1, further comprising:
a switching unit configured to switch a connected state in which an electric connection between an inner circumferential surface of an endless belt and the second power source is established and an interrupted state in which the electric connection is interrupted,
wherein the control unit causes the switching unit to switch to the connected state during a period in which the electric field having the direction opposite to that of the secondary transfer electric field is formed in the secondary transfer portion and causes the switching unit to switch to the interrupted state during a period in which the secondary transfer electric field is formed in the secondary transfer portion.
The image forming apparatus according to Claim 12, wherein the control unit causes the switching unit to switch to the connected state and sets the first power source to have the ground potential during a period in which the electric field having the direction opposite to that of the secondary transfer electric field is formed in the secondary transfer portion.
The image forming apparatus according to Claim 1, wherein the intermediate transfer member has a structure including two or more layers, and a volume resistivity of a layer on the outer circumferential surface side is higher than a volume resistivity of a layer on the inner circumferential surface side.
The image forming apparatus according to Claim 1, further comprising:
a plurality of support members,
wherein the intermediate transfer member includes an intermediate transfer belt, and the plurality of support members support the intermediate transfer belt by abutting against an inner circumferential surface of the intermediate transfer belt.
The image forming apparatus according to Claim 15, wherein the support member includes a conductive support roller, and the intermediate transfer member and the constant-voltage element are electrically connected to each other when the support roller is electrically connected to the constant-voltage element.
The image forming apparatus according to Claim 1, wherein the constant-voltage element includes a Zener diode or a varistor.