Image forming device

Abstract

To provide an image forming device capable of improving cleaning performance of a transfer member in a case of using toner into which fine powder of strontium titanate is added.SOLUTION: A control unit of an image forming device can execute image forming control for transferring a toner image formed on an image carrier (photosensitive drum) to a sheet, or cleaning control for removing toner adhering to a transfer member (transfer roller) from the transfer member (transfer roller), and controls a transfer power supply to supply a bias of the opposite polarity to the potential polarity of the image carrier (photosensitive drum) not only when the image forming control is being executed, but also when the cleaning control is being executed.SELECTED DRAWING: Figure 8

Description

Technical Field

[0001] The present disclosure relates to an image forming apparatus, and more particularly to an electrophotographic image forming apparatus in which a toner image formed with toner containing strontium titanate fine particles is transferred onto a sheet.

Background Art

[0002] In an electrophotographic image forming apparatus, a toner image is formed on an image carrier, and a sheet is passed through a contact area where a transfer bias voltage is applied to a transfer unit (for example, a transfer roller) and the image carrier come into contact with each other, thereby electrostatically transferring the toner image on the image carrier onto the sheet. In such an image forming apparatus, when electrostatic transfer to the sheet is repeated, toner may adhere to the surface of the transfer unit.

[0003] Patent Document 1 discloses an image forming apparatus that performs a cleaning process for electrostatically returning the toner adhering to the transfer unit to the image carrier side. In the image forming apparatus of Patent Document 1, in the cleaning process, a transfer bias voltage and a bias voltage of opposite polarity are alternately applied to the transfer unit in units of the time it takes for the image carrier to make one rotation. Thereby, even if the toner adhering to the transfer unit contains toner charged to a normal polarity and toner charged to an opposite polarity, the transfer unit can be satisfactorily cleaned.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the power supply for supplying the transfer bias to the transfer unit of the image forming apparatus disclosed in Citation Document 1 is required to be able to selectively supply not only the transfer bias voltage for transferring the toner image to the sheet but also a bias voltage having a polarity opposite to that of the transfer bias voltage for the cleaning process. Therefore, there is a problem that the cost of the power supply for supplying the transfer bias to the transfer unit becomes high. Further, even when a recovery member for recovering the toner attached to the transfer unit by contacting the transfer unit is provided, it is necessary to selectively supply either the transfer bias voltage or a bias voltage having a polarity opposite to that of the transfer bias to the recovery member. Also in this case, there is a problem that the cost of the power supply for supplying the transfer bias to the transfer unit becomes high.

[0006] The present disclosure has been made in view of the above problems. An object of the present disclosure is to provide an image forming apparatus capable of improving the cleaning performance of a transfer member without applying both a transfer bias voltage and a bias voltage having a polarity opposite to that of the transfer bias to the transfer unit.

Means for Solving the Problems

[0007] The image forming apparatus of the present disclosure includes a developing device that charges toner added with fine powder of strontium titanate to a predetermined polarity, and an electrostatic latent image is formed on the surface charged to a predetermined potential having the same polarity as the toner by a charging device by an exposure device, and an image carrier on which a toner image is formed by supplying the toner from the developing device to the electrostatic latent image. A transfer member provided to contact the surface of the image carrier, and transfers the toner image to the sheet by passing the contact portion with the image carrier through the sheet; a transfer power source that supplies a bias, which is a predetermined voltage or current, to the transfer member; and a control unit that controls the transfer power source. The control unit is capable of executing image formation control for transferring the toner image formed on the image carrier to the sheet or cleaning control for removing the toner attached to the transfer member from the transfer member. The control unit controls the transfer power source so as to supply a bias having a polarity opposite to the potential polarity of the image carrier, not only when executing the image formation control but also when executing the cleaning control.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

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Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Mode for Carrying Out the Invention

[0009] Hereinafter, the image forming apparatus according to the embodiment of the present disclosure will be described with reference to the drawings. Regarding the drawings, the same or equivalent elements are denoted by the same reference numerals, and redundant explanations will not be repeated.

[0010] (Embodiment 1) The image forming apparatus 1 according to the first embodiment will be described with reference to FIGS. 1 to 9.

[0011] <Image Forming Apparatus 1> FIG. 1 is a cross-sectional schematic view showing the overall configuration of the image forming apparatus 1. FIG. 2 is a cross-sectional view showing the main part of the image forming apparatus 1. The configuration of the image forming apparatus 1 according to the present embodiment will be described with reference to FIGS. 1 and 2.

[0012] The image forming apparatus 1 of the present embodiment is an electrophotographic monochrome image forming apparatus (printer) having a printing function. The image forming apparatus 1 is connected to a personal computer or the like (not shown) by wire or wirelessly. The image forming apparatus 1 forms a black toner image on a medium, here a predetermined sheet 2, in accordance with print image data sent from a personal computer or the like.

[0013] As shown in FIGS. 1 and 2, the image forming apparatus 1 includes a photosensitive drum (image carrier) 10, a charging device 11, an exposure device 12, and a developing device 13. The image forming apparatus 1 also includes a transfer device 14, a drum cleaner 15, a charge removing device 16, and a fixing device 17. Further, the image forming apparatus 1 includes a paper feeding device 18, a conveying device 19, and a paper discharge tray 20.

[0014] The image forming apparatus 1 can be applied to either a positive development system or a reversal development system. In the image forming apparatus 1 of the present embodiment, the reversal development system is adopted. However, in the image forming apparatus 1 of the present disclosure, the positive development system may be adopted.

[0015] <Photosensitive drum (image carrier) 10> The photosensitive drum 10 is an image carrier that carries a toner image. The photosensitive drum 10 has a cylindrical member formed of aluminum and a photoconductive photosensitive layer formed on the surface of the cylindrical member. The cylindrical member is grounded. The photosensitive layer exhibits insulating properties when not irradiated with light, but the regions irradiated with light exhibit conductive properties. Both axial ends of the photosensitive drum 10 are pivotally supported.

[0016] The photosensitive drum 10 is rotated by a drive source 60 (see FIG. 3) around the central axis of the cylinder (in the direction of arrow F).

[0017] <Charging device 11> The charging device 11 includes a charging roller 111 arranged to face the photoreceptor drum 10 and a charging power source 112. The charging roller 111 is a conductive elastic roller having a conductive elastic layer formed on the outer side of a metal core bar. A negative charging bias, which is a negative voltage or a negative current from the charging power source 112, is supplied to the metal core bar.

[0018] In the present embodiment, the charging roller 111 is configured such that the conductive elastic layer contacts the surface of the photoreceptor drum 10 and rotates with the photoreceptor drum 10 when the photoreceptor drum 10 is rotated by a drive source 60 (see FIG. 3). When a charging bias is supplied to the metal core bar, dielectric breakdown (discharge) occurs in the vicinity of the nip portion where the conductive elastic layer is in contact with the surface of the photoreceptor drum 10 due to the potential difference between the conductive elastic layer and the surface of the photoreceptor drum 10. Thereby, the surface (photosensitive layer) of the photoreceptor drum 10 is uniformly charged to a predetermined potential (for example, about -600 V). The predetermined potential on the surface of the photoreceptor drum 10 charged by the charging roller 111 is the surface potential of the area not exposed by the exposure device 12 described later and is called the "image non-carrying potential" because it is an area where toner for forming an image does not adhere.

[0019] <Exposure device 12> The exposure device 12 has a light emitter (not shown) whose light amount can be changed. The light emitter is, for example, a laser diode. The exposure device 12 exposes the photosensitive layer on the surface of the photoreceptor drum 10 so as to scan based on print image data. Since the resistance value of the area of the photosensitive layer of the photoreceptor drum 10 exposed by the exposure device 12 decreases, the surface potential decreases.

[0020] The potential on the surface of the exposed photoreceptor drum 10 becomes a value close to the ground potential, for example, about -50 V to -80 V. The reason why the ground potential does not become the ground potential (0 V) is that the irradiation time due to exposure is short, so the resistance of the photosensitive layer does not completely become 0.

[0021] In this way, the region on the surface of the photoreceptor drum 10 that is exposed by the exposure device 12 and has a potential lower than the image non-carrying potential is the region where toner adheres, so the potential of that region is called the "image-carrying potential". By performing exposure with the exposure device 12 in this way, an electrostatic latent image for developing toner on the surface of the photoreceptor drum 10 is formed.

[0022] <Developing device 13> The developing device 13 includes a developing tank 131, two stirring members 132a and 132b, a toner supply roller 133, and a developing roller 134. The two stirring members 132a and 132b, the toner supply roller 133, and the developing roller 134 are rotatably supported in the developing tank 131. Also, a developer containing toner is accommodated in the developing tank 131.

[0023] Further, a developer is supplied to the developing device 13 (the developing tank 131 thereof) from a toner cartridge (not shown). The developer in the present embodiment is a two-component developer containing toner and a magnetic carrier. The toner is a fine powder with a particle size of about 7 μm made of a resin (binder resin) such as styrene acrylic or polyester, and strontium titanate with a particle size of less than 1 μm is externally added to the surface. Also, the carrier is a powder composed of a magnetic material such as iron or ferrite with a particle size of about 40 μm. And the mixing ratio (weight ratio) of the toner to the developer (including toner and carrier) is, for example, about 7%.

[0024] The developing tank 131 has a first chamber in which the stirring member 132a is accommodated and a second chamber in which 132b, the toner supply roller 133, and the developing roller 134 are accommodated. The first chamber and the second chamber communicate with each other at the front end and the rear end in the direction perpendicular to the paper surface in FIG. 2, respectively.

[0025] The stirring members 132a and 132b each have spiral blades provided on the outer periphery of a rotating shaft. The stirring members 132a and 132b are rotated by a drive source 60 (see FIG. 3). The stirring member 132a rotates to convey the contained developer while stirring it toward the end on the front side of the paper surface in FIG. 2, and the stirring member 132b conveys the developer while stirring it toward the end on the back side of the paper surface in FIG. 2. That is, the developer is conveyed while being stirred so as to circulate between a first chamber and a second chamber that communicate with each other at both ends in the axial direction of the stirring members 132a and 132b that extend parallel to each other in the developing tank 131. At this time, the toner is charged to a negative predetermined potential by frictional charging with the carrier.

[0026] The charge amount of the toner charged in the developing device 13 in the present embodiment is charged to a value of -20 μC / g to -30 μC / g. If the charge amount exceeds -30 μC / g, it becomes difficult to move from the developing roller 134 to the photosensitive drum 10, the density does not appear, and image quality defects occur. Therefore, in order not to exceed -30 μC / g in charge amount, a charge control agent is added to the toner. In the present embodiment, the developing device charges a toner added with fine powder of strontium titanate (average particle size less than 1 μm) as a charge control agent to a predetermined polarity. The charge control agent containing strontium titanate will be described later.

[0027] The stirring member 132b conveys a part of the developer that is conveyed to the first chamber while stirring the developer to a toner supply roller 133 disposed adjacent to the stirring member 132b. The toner supply roller 133 is rotated by the drive source 60 to convey the developer to the developing roller 134. A part of the developer conveyed by the toner supply roller 133 is carried on the surface of the magnetic developing roller 134.

[0028] The developing roller 134 is rotated by a driving device. A developing bias (for example, about -450 V), which is a predetermined voltage, is supplied from the developing power source 3 to the developing roller 134. Here, the value (voltage value) of the developing bias supplied to the developing roller 134 by the developing power source 3 is the "developing potential".

[0029] Due to the rotation of the developing roller 134, the negatively charged toner among the developer conveyed to the position facing the photoreceptor drum 10 of the developing roller 134 receives a repulsive force from the negative developing bias applied to the developing roller 134. As a result, the toner having a negative polarity adheres (phenomenon) to the exposed area (image-carrying area) of the photosensitive layer on the surface of the photoreceptor drum 10 having an image-carrying potential with an absolute value lower than the value of the developing bias.

[0030] On the other hand, the toner having a negative polarity does not adhere (phenomenon) to the unexposed area (image non-carrying area) of the photosensitive layer on the surface of the photoreceptor drum 10 having an image non-carrying potential with an absolute value higher than the absolute value of the developing bias. In this way, a toner image is formed on the photosensitive layer on the surface of the photoreceptor drum 10 (the electrostatic latent image is visualized).

[0031] As described above, the photoreceptor drum 10 has an electrostatic latent image formed on its surface charged to a predetermined potential of the same polarity (negative) as the toner by the charging device 11, and a toner image is formed by supplying toner from the developing device 13 to the electrostatic latent image.

[0032] <Transfer device 14> The transfer device 14 is a device for transferring the toner image formed on the surface of the photoreceptor drum 10 to a sheet, and has a transfer roller 141 and a transfer power source 30 as a transfer member 140. The transfer roller 141 is an elastic roller provided with a conductive elastic layer on the outer periphery of a metal core, and is arranged so that the elastic layer contacts the photoreceptor drum 10, and forms a transfer nip area NT which is a contact portion between the transfer roller 141 and the photoreceptor drum 10. The sheet 2 is sent to this transfer nip area NT one by one from the paper feeding device 18 described later. The transfer roller 141 rotates passively as the photoreceptor drum 10 rotates.

[0033] A voltage or current of a polarity opposite to the charging polarity of the toner (minus polarity in this embodiment), i.e., a transfer bias of plus polarity in this embodiment, is supplied to the metal core of the transfer roller 141 from the transfer power supply 30. That is, the transfer power supply 30 supplies a bias, which is a predetermined voltage or current, to the transfer member 140 (the transfer roller 141). When the sheet 2 passes through the transfer nip area NT while the bias is being supplied to the transfer member 140, the toner image formed on the surface (photosensitive layer) of the photosensitive drum 10 is electrostatically attracted to the transfer roller 141 side and transferred to the sheet 2.

[0034] That is, the transfer roller 141, which is the transfer member 140, transfers the toner TN from the photosensitive drum 10 to the sheet 2 by cooperating with the photosensitive drum 10 to sandwich and convey the sheet 2 while the transfer bias is supplied from the transfer power supply 30. In other words, the transfer member 140 is provided so as to contact the surface of the photosensitive drum 10, and transfers the toner image (formed on the photosensitive drum 10) to the sheet 2 by passing the sheet 2 through the contact portion (transfer nip area NT) with the photosensitive drum 10. Hereinafter, the transfer member 140 will be described more specifically.

[0035] As described above, the roller type transfer device 14 using the transfer roller 141 as the transfer member 140 has been described. However, there is also a belt type transfer device 14 in the transfer device 14 that uses a transfer belt 142 as the transfer member 140 in addition to the transfer roller 141. Note that even in the belt type transfer device 14, the principle of transferring the toner image on the photosensitive drum 10 to the sheet 2 is the same, and this will be described later. In the case of the belt type transfer device 14, the transfer roller 141 presses the transfer belt 142 toward the photosensitive drum 10 (image carrier).

[0036] <Fixing device 17> The fixing device 17 is a device for fixing the toner image transferred to the sheet by the transfer device 14 onto the sheet. The fixing device 17 includes a heating roller 171 that is heated by a heat source such as a halogen lamp provided inside to have a predetermined temperature on its surface, and a pressure roller 172 provided with a heat-resistant elastic layer on the surface of a metal core.

[0037] The pressure roller 172 is arranged to contact the heating roller 171 with a predetermined pressure, and forms a fixing nip portion which is the contact portion with the heating roller 171. The heating roller 171 and the pressure roller 172 are driven by a drive source 60 to rotate in a predetermined direction, and sandwich and convey the sheet 2 conveyed from the transfer device 14. When the toner image transferred to the sheet 2 passes through the fixing nip portion, it is heated and melted, and is fixed (secured) onto the sheet 2 under pressure.

[0038] Note that the fixing device 17 is not limited to the configuration using the heating roller 171 and the pressure roller 172 as described above. For example, it may irradiate the sheet 2 with flash light. In this case, the toner transferred to the sheet 2 is fixed onto the sheet 2 by being heated by the flash light.

[0039] <Drum cleaner 15> The drum cleaner 15 is provided to remove the residual toner that has not been transferred to the sheet in the transfer nip area NT from the photosensitive drum 10. The drum cleaner 15 is provided to extend along the axial direction of the photosensitive drum 10, and has a cleaning blade 151 arranged to contact the surface of the photosensitive drum 10. The cleaning blade 151 is made of a material such as urethane rubber. The cleaning blade 151 scrapes off the residual transfer toner image remaining on the surface of the photosensitive drum 10 that could not be completely transferred to the sheet 2 from the surface of the photosensitive drum 10.

[0040] As a result, the transfer residual toner remaining on the surface of the photoreceptor drum 10 is removed, and the photoreceptor drum 10 is ready for the next printing. Note that the toner scraped off by the cleaning blade 151 is collected by a discharge screw into a waste toner box (not shown).

[0041] <Charge eliminator 16> The charge eliminator 16 includes an LED (Light Emitting Diode) light source or the like arranged to face the photoreceptor drum 10 along the axial direction of the photoreceptor drum 10. The surface (photosensitive layer) of the photoreceptor drum 10 irradiated with light from the LED light source changes from insulating to conductive. Thereby, the value of the potential of the photosensitive layer on the surface of the photoreceptor drum 10 decreases to a value close to the ground potential. As a result, the electrostatic latent image formed on the surface of the photoreceptor drum 10 is erased. That is, the surface of the photoreceptor drum 10 returns to a non-charged state.

[0042] <Sheet feeder 18> The sheet feeder 18 has a tray on which sheets 2 used for image formation are stacked. When printing is executed, the sheet 2 is sent from the sheet feeder 18 to the transport device 19.

[0043] <Transport device 19> The transport device 19 includes a main path 191 and a reverse path 192 that branches off from the main path 191 and rejoins the main path 191. A pickup roller 193, a registration roller 194, a transfer device 14, a fixing device 17, and a paper discharge roller 195 are arranged on the main path 191. Transport rollers 196 for transporting the sheet 2 are arranged on the main path 191 and the reverse path 192.

[0044] The pickup roller 193 is a roller provided near the end of the sheet feeding device 18. The pickup roller 193 takes out the sheets 2 one by one from the sheet feeding device 18 and supplies them to the conveying device 19. The registration roller 194 temporarily holds the sheet 2 being conveyed from the sheet feeding device 18. Then, the registration roller 194 conveys the sheet 2 to the transfer nip area NT between the transfer roller 141 and the photosensitive drum 10 at the timing when the leading edge of the toner image on the photosensitive drum 10 and the leading edge of the sheet 2 are aligned.

[0045] The paper discharge roller 195 discharges the sheet 2 that has passed through the fixing device 17 onto the paper discharge tray 20. When forming an image on the side of the sheet 2 where no image is formed, the paper discharge roller 195 rotates in the reverse direction. The paper discharge roller 195 has a function of switching back the sheet 2 and sending it to the reverse path 192.

[0046] <Paper discharge tray> The paper discharge tray 20 is a tray on which the sheet 2 with the toner image fixed thereon is placed. By the operation of the paper discharge roller 195, the sheet 2 conveyed by the conveying device 19 is sent out onto the paper discharge tray 20.

[0047] By performing the process described above, the image forming apparatus 1 forms an image on the sheet 2.

[0048] <Control unit 5> Next, a description will be given of a control unit 5 that controls the image forming apparatus 1 to perform a process (image forming process) of forming an image on the sheet 2 described above.

[0049] FIG. 3 is a functional block diagram of the main part of the image forming apparatus 1 according to the present embodiment.

[0050] The control unit 5, also called a controller, has a processor P and a memory M. The control unit 5 is connected to a drive source 60, a charging power source 112, an exposure device 12, a developing power source 3, a transfer power source 30, a fixing device 17, a static eliminator 16, a paper feeding device 18, a conveying device 19, an external terminal, etc. via a bus line, and is also connected to a communication device 70 for communicating with them and an operation unit 80 for causing the image forming apparatus 1 to execute a predetermined operation.

[0051] The memory M includes a non-volatile memory such as a ROM and a non-volatile memory such as a RAM (Random Access Memory), but may be a recording medium such as an HDD (Hard Disk Drive). A control program (software program) for controlling the image forming apparatus 1 is stored in the memory.

[0052] The processor P, also called a CPU (Central Processing Unit), reads out a control program stored in a ROM (Read Only Memory) that constitutes part of the memory M, and controls the above-described respective devices to perform image formation. The control program executed by the processor P may be downloaded from a network such as a LAN (Local Area Network).

[0053] When performing image formation, the processor P of the control unit 5 performs the following series of controls based on the program stored in the memory M. When the processor P of the control unit 5 receives image data and a print command (image formation command) from a personal computer (PC) or the like connected via the communication device 70, it starts image processing to convert the received image data into data for printing processing. At the same time, the processor P of the control unit 5 starts heating the heating roller 171 (turning on the heat source). Then, the processor P of the control unit 5 rotates the drive source 60, outputs a charging bias to the charging power supply 112, and supplies it to the charging member. Also, the processor P of the control unit 5 turns on the light source of the charge removal device 16. That is, the image forming apparatus 1 is set to a state in which a charging process for charging the surface of the photosensitive drum 10 to a predetermined potential and a charge removal process for removing charge from the charged surface of the photosensitive drum 10 are being executed, and the exposure process, development process, and transfer process described later can be executed at any time.

[0054] Next, the processor P of the control unit 5 outputs a developing bias to the developing power supply 3 and starts exposure by the exposure device 12 based on the data for printing processing. As a result, the exposure process and the development process are executed, and a toner image is formed on the photosensitive drum 10. Then, the processor P of the control unit 5 controls the transfer power supply 30 to supply a transfer bias to the transfer member 140. Thereby, the processor P of the control unit 5 drives the paper feeding device 18 and the conveying device 19 to convey the sheet 2 to the transfer nip area NT at a timing when the toner image developed in the transfer nip area NT arrives.

[0055] The toner image on the photosensitive drum 10 is electrostatically transferred to the sheet 2 that has reached the transfer nip area NT by the transfer bias. Then, the processor P of the control unit 5 continues to drive the conveying device 19 to convey the sheet 2 onto which the toner image has been transferred to the fixing device 17 and execute the fixing process in the fixing device 17. After that, the conveying device 19 is continuously driven to discharge the fixed sheet 2 to the paper discharge tray 20. In the present embodiment, the control for transferring the toner image to this sheet 2 is called image formation control.

[0056] When the processor P of the control unit 5 continues printing (image formation), it continues the same control. When the processor P of the control unit 5 finishes printing, with a bias voltage different from that when transferring the toner image to the sheet 2 supplied to the transfer member 140, the photosensitive drum 10 and the transfer member 140 are rotated for a while. Thereby, the processor P of the control unit 5 performs a so-called cleaning process of moving the toner adhering to the transfer member 140 to the photosensitive drum 10. After rotating the photosensitive drum 10 for a while, the processor P of the control unit 5 stops the control of the drive source 60 and other controls, and ends a series of image formation processes. In the present embodiment, the control for performing the cleaning process of moving the toner adhering to the transfer member 140 to the photosensitive drum 10 side for cleaning is referred to as cleaning control. This cleaning process will be described in detail later. As described above, by the control unit 5 controlling each device included in the image forming apparatus 1, an image is formed on the sheet 2.

[0057] As described above, when the printing (image formation process of forming an image on the sheet 2) of the image forming apparatus 1 is completed, a cleaning process for cleaning the toner adhering to the transfer member 140 is performed. On the other hand, in the image forming apparatus 1, there is also one that supplies a cleaning bias voltage, which is a voltage of a polarity opposite to the charging polarity (potential polarity) of the toner charged by the developing device 13, to the transfer member 140 during the execution of the cleaning process. This is because the potential polarity (polarity of the surface potential) of the toner adhering to the surface of the transfer member 140 is the same as that when it is charged by the developing device 13, so it is necessary to supply a bias voltage of the same polarity to the transfer member 140. Thereby, the toner adhering to the surface of the transfer member 140 moves to the photosensitive drum 10 side in the transfer nip region NT and is collected by the drum cleaner 15, so that the transfer member 140 is cleaned.

[0058] However, as described above, since it is necessary to supply a cleaning bias voltage of a polarity opposite to that during the execution of the transfer process of transferring the toner image to the sheet 2, there is a problem that the power cost increases.

[0059] In view of such problems, as a result of intensive studies, the inventors of the present application have found that using strontium titanate as a charge control agent for controlling the charge amount of toner can effectively clean the transfer member 140. Specifically speaking, when strontium titanate is used, the potential polarity of the toner adhering to the transfer member 140 changes to the opposite polarity of the potential polarity when charged by the developing device 13. Further, when a bias (voltage or current) having the same polarity as the transfer bias applied during the transfer process execution to the transfer member 140 and having an absolute value lower than that of the transfer bias is supplied, the toner adhering to the surface of the transfer member 140 can be effectively moved to the side of the photoreceptor drum 10. As a result, the transfer member 140 can be effectively cleaned. That is, the inventors of the present application have found that the transfer member 140 can be effectively cleaned without providing a function of supplying a bias having a polarity opposite to that of the transfer bias to the transfer power supply 30.

[0060] The cleaning process for the transfer member 140 of the image forming apparatus 1 using the toner TN using fine particles of this strontium titanate as a charge control agent will be described in detail.

[0061] <Toner TN> FIG. 4 is a schematic diagram showing the toner TN to which strontium titanate TS is added. FIG. 5 is a diagram comparing the charging characteristics of the toner adhering to the transfer member 140 between the toner TN to which fine particles of strontium titanate TS are externally added and the toner TO to which fine particles of titanium oxide, which has been frequently used as a charge control agent in recent years, are externally added. Note that FIG. 5 describes the respective characteristics of strontium titanate and titanium oxide, the addition amount (external addition amount), and the charge amount of the toner adhering to the transfer member 140. Regarding this charge amount, the value obtained by measuring the charge amount of the toner adhering to the transfer belt 142, which is the transfer member 140, after printing a predetermined number of sheets by the transfer device 14 of the belt transfer method, which will be described later, is described. Details of the transfer device 14 such as the transfer belt 142 will be described later.

[0062] As shown in FIG. 4, the toner TN of the present embodiment is a toner containing a binder resin and a charge control agent, and fine powder of strontium titanate (SrTiO3) ST is externally added as the charge control agent to the binder resin containing polyester. The average particle size of the toner containing the binder resin and the charge control agent is approximately 6 μm to 7 μm, and the number of parts of strontium titanate TS added to the binder resin is 0.4 to 0.5 parts. The number of parts means wt%, which is the weight ratio of the additive to the mass of the toner resin. Specifically, the number of parts of strontium titanate TS with respect to the toner TN means the ratio of the mass of strontium titanate added to the mass of the toner resin.

[0063] Note that both strontium titanate and titanium oxide have conductivity and are externally added to the binder resin to prevent the toner from being overly charged. However, since the charge amount controllability of strontium titanate is inferior to that of titanium oxide, it is necessary to increase the addition amount compared to titanium oxide. Specifically, when the toner TN contains titanium oxide, the externally added amount of titanium oxide may be 0.1 part, but when the toner TN contains strontium titanate, the externally added amount of strontium titanate is required to be 0.45 parts. By setting the externally added amount of strontium titanate to 0.45 parts in this way, it is possible to suppress the increase in the charge amount of the toner even during continuous printing.

[0064] In FIG. 5, the charge amount of the toner TO with externally added titanium oxide attached to the transfer member 140 is -7 μC / g, which is the same as the potential polarity charged by the developing device 13. On the other hand, the charge amount of the toner TN with externally added strontium titanate TS is +5 μC / g, which has a polarity opposite to the potential polarity charged by the developing device 13. This is considered to be due to the fact that the externally added amount of strontium titanate is larger than that of titanium oxide.

[0065] Specifically, as described above, the toner is conveyed while being stirred together with the carrier by the stirring members 132a and 132b in the developing tank 131, and is triboelectrically charged to a predetermined potential (as described above, -20 μC / g to -30 μC / g). The charge control agent externally added to the toner has conductivity and is externally added to prevent the surface potential of the binding member from becoming higher than a predetermined potential (-30 μC / g) by contacting the charge control agents of other toner particles (this effect cannot be exhibited when added internally).

[0066] Since titanium oxide has a better effect of discharging the surface potential of the binding resin than strontium titanate, it is possible to suppress an excessive increase in the surface potential of the toner (binding resin) during stirring and conveyance with a smaller addition (external addition) amount. On the other hand, even when strontium titanate is used for the toner, by increasing the addition (external addition) amount more than that of titanium oxide, an effect similar to the effect obtained by titanium oxide can be obtained.

[0067] Next, a state where the toner attached to the transfer member 140 is transferring an image to the sheet 2, that is, a state where a transfer bias is supplied to the transfer member 140 will be described. Since the charge control agent has conductivity, the potential of the charge control agent in contact with the transfer member 140 becomes the same potential as that of the transfer member 140. This is because a voltage having a polarity different from the potential polarity of the toner charged by development is always applied to the transfer member 140 during the execution of the transfer process or a current having a polarity different from the potential polarity of the toner charged by development always flows. That is, since the charge control agent also has the same polarity potential as the potential of the transfer member 140, the potential of the charge control material acts to lower the surface potential of the binding member.

[0068] Since the toner in this embodiment is negatively charged in the developing device 13, a plus-polarity bias is applied as the transfer bias to the transfer member 140 during the execution of the transfer process. Therefore, the charge amount of the toner adhering to the transfer member 140 decreases so as to approach the plus side during the execution of the transfer process, and in the case of the toner TO with titanium oxide externally added, it decreases to -7 μC / g. Therefore, in the case of the toner TN with a large amount of strontium titanate externally added, it is more strongly affected by the transfer bias, and the charge amount of the toner TN decreases to about +5 μC / g and the potential polarity also changes to the plus side.

[0069] Note that since the fine particles of strontium titanate have an angular shape, there is a problem that the surface of the photoreceptor drum 10 is damaged and a filming phenomenon occurs in which the binder resin of the toner adheres to the damaged position. Therefore, it is desirable that the addition amount of the fine particles of strontium titanate be 0.5 parts or less.

[0070] For the above reasons, when strontium titanate is used as the charge control agent for the toner, a cleaning bias having the same polarity as the transfer bias (during image formation control) is supplied to the transfer member 140 during the cleaning process (during cleaning control). Thereby, the toner TN adhering to the surface of the transfer member 140 can be cleaned. In other words, even if the transfer power supply 30 does not have a function of supplying a bias having a polarity opposite to that of the transfer bias, the toner TN adhering to the transfer member 140 can be cleaned.

[0071] As can be understood from the above, the control unit 5 controls the transfer power supply 30 to supply a bias having a polarity opposite to the charging polarity (potential polarity) of the toner charged in the developing device 13 not only when executing the transfer process (image formation control) but also when executing the cleaning process (cleaning control). As described above, here, the image formation control is control for transferring the toner TN from the photosensitive drum 10 to the sheet 2, and the cleaning control is control for removing the toner TN attached to the transfer member 140 from the transfer member 140. That is, the control unit 5 that controls the transfer power supply 30 can execute image formation control for transferring the toner image formed on the photosensitive drum 10 to the sheet 2 or cleaning control for removing the toner TN attached to the transfer member 140 from the transfer member 140. Further, the control unit 5 controls the transfer power supply 30 to supply a bias having a polarity opposite to the (surface) potential polarity of the photosensitive drum 10 not only when executing the image formation control but also when executing the cleaning control.

[0072] Next, the belt type transfer device 14 of the present embodiment will be described in detail with reference to FIG. 6. The transfer device 14 includes a transfer roller 141 and a transfer belt 142 suspended by a plurality of suspension rollers 143a, 143b, 143c, and 143d. The transfer belt 142 is pressed against the photosensitive drum 10 by the transfer roller 141 from the back side so as to form a transfer nip area NT which is a contact portion with the photosensitive drum 10. The transfer roller 141 is an elastic roller in which a conductive elastic layer is provided on the outer periphery of a metal core, and a bias which is a predetermined voltage or current is applied to the core from the transfer power supply 30, similar to the roller type transfer device 14.

[0073] The transfer belt 142 is configured to rotate when the suspension roller 143a, which is one of the plurality of suspension rollers 143a, 143b, 143c, 143d, receives the rotational force from the drive source 60. Therefore, when the drive source 60 rotates the photosensitive drum 10, the suspension roller 143a is also rotated, so that the toner image on the photosensitive drum 10 can be transferred to the sheet 2 while the sheet 2 is conveyed in the transfer nip region NT. In the present embodiment, the transfer belt 142 uses a resin belt formed of polyimide having a thickness of 80 μm and conductivity, but a rubber belt having conductivity may also be used. Also, the thickness does not have to be 80 μm as long as the necessary flexibility and durability can be ensured.

[0074] Next, with reference to FIGS. 6 to 8, the charging state of the toner TN when the control unit 5 of the present embodiment executes image formation control and cleaning control will be described. In any of FIGS. 6 to 8, in the developing tank 131 of the developing device 13, the toner TN to which strontium titanate TS is added is charged with a charge amount of -25 μC / g.

[0075] FIG. 6 is a schematic diagram showing the first step of the operation of the main part of the image forming apparatus 1 of the present embodiment, in which the control unit 5 is executing image formation control on the first sheet 2. That is, FIG. 6 is a diagram for explaining the state in which the toner image is being transferred to the first sheet 2.

[0076] As shown in FIG. 6, when the toner TN is being transferred to the first sheet 2, the control unit 5 controls the transfer power supply 30 so that a constant current It of +42 μA flows from the transfer power supply 30 to the transfer roller 141. That is, the control unit 5 controls the transfer power supply 30 to supply a transfer bias It to the transfer roller 141. At this time, the toner image (toner TN) on the photosensitive drum 10 is electrostatically transferred to the sheet 2 by the electrostatic force generated when the transfer bias It (+42 μA) supplied to the transfer roller 141 flows into the grounded conductive cylindrical member of the photosensitive drum 10.

[0077] FIG. 7 shows the second step of the operation of the main part of the image forming apparatus 1 according to the present embodiment. Further, FIG. 7 is a schematic diagram showing a state in which the control unit 5 executes image formation control in a so-called inter-sheet (sheet) state before transferring the toner TN to the second sheet 2 after transferring the toner TN to the first sheet 2.

[0078] In FIG. 7, the state during the period from the transfer of the toner TN to the first sheet 2 until the conveyance between the photosensitive drum 10 and the transfer roller 141 of the second sheet 2 is shown. Also in this state, the control unit 5 controls the transfer power supply 30 so that a constant current It of +42 μA flows from the transfer power supply 30 to the transfer roller 141. At this time, there may be toner (referred to as fog toner) adhering to an image non-carrying area on the photosensitive drum 10 where toner should not be developed originally. In this case, the fog toner is attracted to the transfer belt 142 which is the transfer member 140 by the transfer bias and adheres to the transfer belt 142. Although the amount of fog toner generated is very small, as the toner image is transferred to many sheets 2, it gradually accumulates on the transfer belt 142 as shown in FIG. 7. Also, when a paper jam or the like occurs during printing, toner may adhere to the transfer belt 142.

[0079] If the toner TN adhering to the transfer belt 142 is left unattended, the toner adhering to the transfer belt 142 during the transfer process to the sheet 2 will move to the sheet 2 (the surface on the opposite side of the photosensitive drum 10) and cause back contamination. Therefore, it is necessary to clean the transfer belt 142 regularly.

[0080] FIG. 8 is a schematic diagram showing the third step of the operation of the main part of the image forming apparatus 1 according to the present embodiment, which illustrates a state in which the control unit 5 executes cleaning control. That is, it is a diagram explaining a state in which the toner TN adhering to the transfer belt 142 after printing is being cleaned.

[0081] As shown in Fig. 8, after the toner TN is transferred to the second (last) sheet 2, cleaning control is executed. At this time, the control unit 5 controls the transfer power supply 30 so that a constant voltage Vc of +100 V, which is different from the transfer bias, is applied from the transfer power supply 30 to the transfer roller 141. In the present embodiment, when the cleaning control is being executed, the control unit 5 supplies a lower bias to the transfer member 140 than when the image formation control is being executed.

[0082] By performing the cleaning control in this way, the toner TN adhering to the surface of the transfer belt 142 can be moved to the photosensitive drum 10. In the above description, the belt-type transfer device 14 including the transfer belt 142 is used as the transfer member 140. However, even in a roller-type transfer device using the transfer roller 141 as the transfer member 140, the toner TN adhering to the surface of the transfer roller 141 can be moved to the photosensitive drum 10. Also, in the present embodiment, the cleaning bias is a constant voltage of +100 V, but it may be, for example, +2 μA. The reason why it is preferable for the cleaning bias to be such a value is that the absolute value of the charge amount (+5 μC / g) of the toner TN adhering to the transfer belt 142 is smaller than the absolute value of the charge amount (-20 μC / g) of the toner charged in the developing device 13. That is, the toner TN adhering to the transfer member 140 can be moved to the photosensitive drum 10 with a smaller current. In other words, if the value of the cleaning bias is the same as the value of the transfer bias, as described above, the fogged toner is attracted to the transfer belt 142, so that the toner adhering to the transfer belt 142 cannot be moved to the photosensitive drum 10.

[0083] Fig. 9 is a flowchart for explaining the processing executed by the control unit 5 of the image forming apparatus 1 according to the present embodiment.

[0084] As shown in Fig. 9, in step S1, the control unit 5 checks the state of the current image forming apparatus 1. In step S2, the control unit 5 determines whether the state of the current image forming apparatus 1 is a state in which cleaning control should be executed.

[0085] In step S2, if the current state of the image forming apparatus 1 is a state in which cleaning control should be executed, then in step S4, the control unit 5 causes the transfer power supply 30 to execute cleaning control. Further, the control unit 5 causes the drive source 60, the charging power supply 112, the exposure device 12, the developing power supply 3, the transfer power supply 30, the fixing device 17, the charge removing device 16, the paper feeding device 18, and the conveyance device 19 to execute operations corresponding to the cleaning control.

[0086] On the other hand, in step S2, if the current state of the image forming apparatus 1 is not a state in which cleaning control should be executed, then in step S3, the control unit 5 determines whether or not the current state of the image forming apparatus 1 is a state in which image forming control should be executed.

[0087] If it is determined in step S3 that the current state of the image forming apparatus 1 is a state in which image forming control should be executed, then in steps S5 and S6, the control unit 5 causes the image forming control to be executed. That is, the control unit 5 causes the drive source 60, the charging power supply 112, the exposure device 12, the developing power supply 3, the transfer power supply 30, the fixing device 17, the charge removing device 16, the paper feeding device 18, and the conveyance device 19 to execute operations corresponding to the image forming control.

[0088] If it is determined in step S3 that the current state of the image forming apparatus 1 is not a state in which image forming control should be executed, then the control unit 5 executes step S1.

[0089] (Embodiment 2) The image forming apparatus 1 according to Embodiment 2 will be described with reference to FIGS. 10 to 14. Note that descriptions of points that are the same as those of the image forming apparatus 1 according to Embodiment 1 will not be repeated below. The image forming apparatus 1 according to the present embodiment is different from the image forming apparatus 1 according to Embodiment 1 in that it includes a contact member 40 that temporarily collects the toner adhering to the surface of the transfer belt 142, and a contact power supply 50 that supplies a bias, which is a predetermined voltage or current, to the contact member 40. Specifically, the contact member 40 is a contact roller.

[0090] FIG. 10 is a functional block diagram for explaining the main part of the image forming apparatus 1 of the present embodiment.

[0091] In addition to the configuration of the image forming apparatus 1 of the first embodiment, the image forming apparatus 1 of the present embodiment includes a contact member 40 and a contact power source 50 that supplies a bias to the contact member 40. The contact member 40 is a cleaning roller provided with a conductive elastic layer on the outer periphery of a metal core that rotates while contacting the transfer belt 142 (see FIGS. 11 to 13). The conductive elastic layer is formed of a silicon foam, but the outer peripheral portion of the core may be directly brought into contact with the transfer belt 142 without providing this conductive elastic layer.

[0092] The contact power source 50 is constituted by an electric circuit. The contact power source 50 is connected to the contact member 40 and is configured to be able to supply a predetermined bias voltage having a polarity opposite to the polarity of the charge of the toner TN charged by the developing device 13 to the contact member 40 or to ground the contact member 40.

[0093] Note that the electric circuit of the contact power source 50 may be constituted by the same circuit as the electric circuit of the transfer power source 30 or by a separate circuit.

[0094] When the control unit 5 is executing the above-described image formation control, the transfer power source 30 supplies a transfer bias It (+42 μA) to the transfer roller 141, and the contact power source 50 is controlled to set the contact member 40 to the ground potential (ground bias). That is, the contact member 40 in contact with the transfer belt 142 is grounded in a state where the image formation control is being executed and does not directly exert an electrical action on the transfer belt 142. On the other hand, the toner TN attached to the transfer belt 142 is charged to a positive charge amount (+5 μC / g) (as described above). Therefore, the toner TN can be electrostatically moved to the contact member 40.

[0095] During image formation control, no predetermined positive or negative bias is supplied to the contact member 40 in order not to change the charge amount and charge polarity of the toner TN attached to the transfer belt 142. Although the contact member 40 is grounded, since the absolute value of the charge amount of the toner TN is small, the charge polarity of the toner TN that has moved to the contact member 40 does not change. Also, the absolute value of the charge amount does not become 0 μC / g.

[0096] On the other hand, when the control unit 5 executes cleaning control, it controls the contact power supply 50 to supply a moving bias Ci (+2 μA), which is a bias of the same polarity as the transfer bias, to the contact member 40. At the same time, the control unit 5 causes the transfer power supply 30 to supply a cleaning bias Vc (+100 V) to the transfer roller 141. By controlling in this way, the toner TN attached to the contact member 40 is moved to the transfer belt 142 side, and the toner TN that has moved to the transfer belt 142 is moved to the photoreceptor drum 10 at the transfer nip position NT, so that the transfer belt 142 can be cleaned. The reason for this is as described above.

[0097] Next, with reference to FIGS. 11 to 13, the charged state of the toner TN when the control unit 5 of the present embodiment is executing image formation control and cleaning control will be described. In any of FIGS. 11 to 13, in the developing tank 131 of the developing device 13, the toner TN to which strontium titanate ST is added is charged with a charge amount of -25 μC / g.

[0098] FIG. 11 is a schematic diagram showing a state in which the control unit 5 is executing image formation control for transferring the toner TN to the first sheet 2, which is the first step of the operation of the main part of the image forming apparatus 1 of the present embodiment. That is, it is a diagram explaining the state in which the toner image is being transferred to the first sheet 2. As shown in FIG. 11, when the toner TN is being transferred to the first sheet 2, the control unit 5 controls the transfer power supply 30 so that a current of +42 μA flows from the transfer power supply 30 to the transfer roller 141. This is the same as in the first embodiment. Note that the contact member 40 in contact with the transfer belt 142 is in a state of being grounded by the contact power supply 50.

[0099] At this time, the toner image (toner TN) on the photoreceptor drum 10 is electrostatically transferred to the sheet 2 side by the electrostatic force generated when the transfer bias It (+42 μA) supplied to the transfer roller 141 flows into the grounded conductive cylindrical member of the photoreceptor drum 10.

[0100] FIG. 12 is a schematic diagram showing a state in which the control unit 5 executes image formation control in a so-called between-sheets state after transferring the toner TN to the first sheet 2 and before transferring the toner TN to the second sheet 2. In the state of FIG. 12, after the transfer of the toner TN to the first sheet 2, the control unit 5 controls the transfer power supply 30 so that a current of +42 μA also flows from the transfer power supply 30 to the transfer roller 141 during the period until the conveyance between the photoreceptor drum 10 and the transfer roller 141 of the second sheet 2. The contact member 40 is grounded.

[0101] FIG. 12 is a diagram for explaining a state in which the overlapping toner TN existing on the photoreceptor drum 10 moves to the transfer belt 142 and the toner TN that has moved to the transfer belt 142 moves to the contact member 40 in such an inter-sheet state. The movement of the toner TN in this way is as described above.

[0102] FIG. 13 is a schematic diagram showing a state in which the control unit 5 executes cleaning control, which is the third step of the operation of the main part of the image forming apparatus 1 according to the present embodiment. That is, it is a diagram for explaining a state in which the toner TN attached to the transfer belt 142 after printing is being cleaned. As shown in FIG. 13, after the transfer of the toner TN to the second (last) sheet 2, when the cleaning control is being executed, the control unit 5 controls the contact power supply 50 so that a current of +2 μA flows from the contact power supply 50 to the contact member 40. Specifically, the control unit 5 controls the transfer power supply 30 to supply +100 V, which is the cleaning bias Vc, to the transfer roller 141.

[0103] By controlling in this way, the toner TN attached to the contact member 40 is returned from the contact member 40 to the transfer belt 142, and the toner TN returned to the transfer belt 142 is moved to the photoreceptor drum 10 in the transfer nip region NT. As a result, the cleaning of the transfer member 140 is performed well. Note that the bias supplied to the contact member 40 during cleaning control may be, for example, a constant voltage of +100V.

[0104] FIG. 14 is a flowchart for explaining the process executed by the control unit 5 of the image forming apparatus 1 of the present embodiment.

[0105] The process executed by the control unit 5 of the present embodiment is different from the process executed by the control unit 5 of the first embodiment only in the following points.

[0106] In step S4A, the control unit 5 causes the transfer power supply 30 and the contact power supply 50 to execute cleaning control. Further, in step S6A, the control unit 5 causes the transfer power supply 30 and the contact power supply 50 to execute image formation control.

[0107] Also with the image forming apparatus 1 as described above, similar to the image forming apparatus 1 of the first embodiment, the cleaning performance of the transfer member 140 can be improved. As a result, it is possible to suppress the sheet 2 from being soiled by the adhesion of the toner TN to the back surface of the transfer surface of the toner TN from the photoreceptor drum 10. Further, even if the transfer power supply 30 and the contact power supply 50 do not have a function of supplying a bias having the same polarity as the charging polarity of the toner TN charged in the developing device 13, the toner TN attached to the transfer member 140 can be effectively removed.

[0108] Hereinafter, the characteristic configuration of the image forming apparatus 1 of the present disclosure and the effects obtained thereby will be described.

[0109] (1) The image forming apparatus 1 includes a developing device 13, an image carrier (photoconductor drum 10), a transfer member 140, a transfer power supply 30, and a control unit 5. The developing device 13 charges the toner added with fine powder of strontium titanate by stirring. In the image carrier (photoconductor drum 10), an electrostatic latent image is formed on the surface charged to a predetermined potential by a charging power supply 112 by an exposure device 12, and a toner image is formed by supplying toner TN from the developing device 13 to the electrostatic latent image. The transfer member 140 (transfer roller 141) is configured to be able to contact the surface of the image carrier, and transfers the toner image to the sheet by conveying the sheet in a state where the sheet is sandwiched between the image carrier (photoconductor drum 10). The transfer power supply 30 supplies a bias, which is a predetermined voltage or current, to the transfer member 140 (transfer roller 141). The control unit 5 controls the transfer power supply 30. When the control unit 5 executes image formation control for transferring the toner image from the image carrier to the sheet, the control unit 5 controls the transfer power supply 30 to supply a bias having a polarity opposite to the potential polarity of the image carrier. In addition, even when the control unit 5 executes cleaning control for removing the toner TN attached to the transfer member 140 from the transfer member 140, the control unit 5 controls the transfer power supply 30 to supply a bias having a polarity opposite to the potential polarity of the image carrier. According to this, the cleaning performance of the transfer member 140 (transfer roller 141) can be improved without applying both the transfer bias voltage and the bias voltage having a polarity opposite to the transfer bias to the transfer unit.

[0110] (2) The amount of strontium titanate with respect to the toner is preferably 0.4 to 0.5. According to this, it is possible to suppress an increase in the charge amount of the toner even when continuous printing is performed.

[0111] (3) When the control unit 5 executes cleaning control, the control unit 5 controls the transfer power supply 30 to supply a lower bias to the transfer member 140 (transfer roller 141) than when executing image formation control. According to this, the toner TN attached to the transfer member 140 can be moved to the photoconductor drum 10 with a smaller current.

[0112] (4) The transfer member 140 may include a conductive transfer belt 142 and a transfer roller 141. In this case, the conductive transfer belt 142 conveys the sheet by rotating with the image carrier (photoconductor drum 10) sandwiching the sheet. The transfer roller 141 presses the transfer belt 142 toward the image carrier. The control unit 5 controls the transfer power supply 30 so as to supply a bias having a polarity opposite to the potential polarity of the image carrier (photoconductor drum 10) to the transfer roller 141 not only when executing image formation control but also when executing cleaning control. The cleaning performance of the transfer belt 142 and the transfer roller 141 can be enhanced without applying both the transfer bias voltage and the bias voltage having a polarity opposite to the transfer bias to the transfer unit.

[0113] (5) The image forming apparatus 1 may include a contact member 40 and a contact power supply 50. The contact member 40 contacts the transfer belt 142. The contact power supply 50 supplies a bias to the contact member 40. The control unit 5 controls the contact power supply 50 so as to supply a ground potential to the contact member 40 when executing image formation control. On the other hand, the control unit 5 controls the contact power supply 50 so as to supply a bias having a polarity opposite to the potential polarity of the image carrier (photoconductor drum 10) to the contact member 40 when executing cleaning control. According to this, the cleaning performance of the contact member 40 can be improved without applying both the transfer bias voltage and the bias voltage having a polarity opposite to the transfer bias to the transfer unit.

Explanation of Reference Numerals

[0114] 2 Sheet 5 Control unit 10 Photoconductor drum (image carrier) 13 Developing device 30 Transfer power supply 40 Contact member 50 Contact power supply 112 Charging power supply 140 Transfer member 141 Transfer roller 142 Transfer belt TN Toner TS Strontium Titanate

Claims

1. A developing device that charges a toner added with fine powder of strontium titanate to a predetermined polarity, An image carrier on the surface of which an electrostatic latent image is formed by an exposure device and charged to a predetermined potential with the same polarity as the toner by a charging device, and a toner image is formed by supplying the toner from the developing device to the electrostatic latent image, A transfer member provided so as to contact the surface of the image carrier and transferring the toner image to the sheet by passing a contact portion with the image carrier through the sheet, A transfer power source that supplies a bias, which is a predetermined voltage or current, to the transfer member, A control unit that controls the transfer power source, and includes: The control unit is capable of executing image formation control for transferring the toner image formed on the image carrier to the sheet or cleaning control for removing the toner attached to the transfer member from the transfer member, and controls the transfer power source to supply a bias having a polarity opposite to the potential polarity of the image carrier not only when executing the image formation control but also when executing the cleaning control. An image forming apparatus.

2. The number of parts of the strontium titanate with respect to the toner is 0.4 to 0.5, The image forming apparatus according to claim 1.

3. When the control unit executes the cleaning control, the control unit controls the transfer power source to supply a lower bias to the transfer member than when executing the image formation control. The image forming apparatus according to claim 1.

4. The transfer member includes: A conductive transfer belt that conveys the sheet by rotating while sandwiching the sheet with the image carrier, A transfer roller that presses the transfer belt toward the image carrier. The control unit controls the transfer power supply so as to supply a bias having a polarity opposite to that of the image carrier to the transfer roller not only when executing the image formation control but also when executing the cleaning control. The image forming apparatus according to claim 1.

5. A contact member that contacts the transfer belt, and a contact power supply that supplies a bias to the contact member. When executing the image formation control, the control unit supplies a ground potential to the contact member, while when executing the cleaning control, the control unit controls the contact power supply so as to supply a bias having a polarity opposite to that of the image carrier to the contact member. The image forming apparatus according to claim 4.

Images