Image forming apparatus

Abstract

To provide an image forming apparatus that can prevent image unevenness during an image forming operation, and can effectively prevent occurrence of turn-up of a cleaning blade due to attachment of a discharge product on a surface of an image carrier during an image forming preparation operation.SOLUTION: An image forming apparatus 100, during an image forming operation of developing a surface 1a of an image carrier (1) to form a toner image, electrifies the surface 1a of the image carrier (1) in a first condition for supplying an AC superimposition electrification bias Ve1 to an electrifying member (21), and during an image forming preparation operation of making preparation for a condition necessary for image formation while rotating the image carrier (1), electrifies the surface 1a of the image carrier (1) in a second condition for supplying a DC electrification bias Ve2 to the electrifying member (21).SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present disclosure relates to an image forming apparatus such as a copying machine, a multifunction machine, a printer, a facsimile apparatus, etc.

Background Art

[0002] In a conventional image forming apparatus, when a cleaning blade that contacts the surface of an image carrier (for example, a photosensitive drum) is provided, generally, during an image forming operation (during a printing operation on a recording sheet such as paper), the surface of the image carrier is developed while rotating the image carrier with the cleaning blade in contact with the surface of the image carrier. Further, during an image forming preparation operation, the surface of the image carrier is not developed while rotating the image carrier with the cleaning blade in contact with the surface of the image carrier. Here, examples of the image forming preparation operation include toner replenishment to a developing device, transfer cleaning for removing foreign matter such as toner on the surface of a transfer member (for example, an intermediate transfer belt or a transfer roller), stabilization of a toner concentration detection value when detecting the toner concentration (toner density) in a developer, temperature control for a fixing member, and the like.

[0003] Such a conventional image forming apparatus has the following disadvantages with respect to the cleaning blade. This will be described below with reference to FIGS. 6 to 8.

[0004] FIG. 6 is a graph showing a developing bias (developing bias voltage) Vd which is a predetermined voltage supplied to the developing device 4 and a surface potential Vf on the surface 1a of the photosensitive drum 1. FIG. 7 is an explanatory diagram for explaining the disadvantages with respect to the cleaning blade 51 in the conventional image forming apparatus 100X. Further, FIG. 8 is a graph showing an AC superposed charging bias Ve1.

[0005] As shown in FIG. 6, the surface 1a of the photoreceptor drum 1 is charged by a charging device described later to a non-image potential VO which is a predetermined potential and on which no toner image is formed (surface potential Vf in this example). Further, the developing bias Vd is set to a value between the value of the non-image potential VO and the value of an image potential VL (|VL| < |VO|) of an area exposed by an exposure device described later and having a surface potential smaller than the non-image potential VO. In the toner adhesion area βa (development area), which is the area exposed by the exposure device and has a surface potential of VL, since the absolute value of the value of the image potential VL is smaller than the absolute value of the value of the developing bias Vd, toner charged with a predetermined charging polarity (for example, negative polarity) adheres. On the other hand, in the toner non-adhesion area βb (non-development area), the absolute value of the value of the non-image potential VO is larger than the absolute value of the value of the developing bias Vd, and a potential difference ΔV is provided, so that toner charged with a predetermined charging polarity (for example, negative polarity) does not adhere.

[0006] Incidentally, if the surface potential Vf on the surface 1a of the photoreceptor drum 1 (an example of an image carrier) varies, image unevenness may occur. Such variations in the surface potential Vf occur due to variations in the resistance value in the circumferential direction S (and axial direction) of the conductive elastic layer 21b provided on the outer peripheral surface of the rotation shaft 21a of the charging roller 21 and the resistance layer 21c formed on the surface of the elastic layer 21b when the charging member for charging the surface of the photoreceptor drum 1 is the charging roller 21. The image unevenness due to such variations in the surface potential Vf becomes particularly prominent when the surface 1a of the photoreceptor drum 1 is charged by supplying a DC voltage Vdc to the charging roller 21. This is because discharge occurs only in the vicinity space δ1 on the upstream side in the first rotation direction R1 of the photoreceptor drum 1 at the contact portion where the charging roller 21 contacts the photoreceptor drum 1, as shown in FIG. 7. That is, when the surface potential of the photoreceptor drum 1 increases due to the discharge occurring in the vicinity space δ1, the difference from the DC voltage Vdc supplied to the charging roller 21 decreases, so that no discharge occurs in the vicinity space δ2 on the downstream side in the first rotation direction R1 of the photoreceptor drum 1. Therefore, since the discharge for charging the photoreceptor drum 1 occurs only once in the area of the vicinity space δ1, if there are resistance variations in the elastic layer 21b and the resistance layer 21c, the discharge amount changes under the influence of the resistance variations and the surface potential varies.

[0007] In this regard, in the conventional image forming apparatus 100X, from the viewpoint of making the surface potential Vf (see FIG. 6) on the surface 1a of the photosensitive drum 1 uniform, as shown in FIG. 8, an AC superposed charging bias Ve1 (Vdc + Vac) obtained by superposing an AC voltage Vac on a DC voltage Vdc is supplied to the charging roller 21 to charge the surface 1a of the photosensitive drum 1, and the surface 1a of the photosensitive drum 1 may be developed with toner while suppressing variations in the surface potential Vf on the surface 1a of the photosensitive drum 1, and thus image unevenness. In this case, the peak-to-peak voltage Vpp (see FIG. 8) of the AC voltage Vac is several [kV] (for example, 1.8 [kV]), and the frequency f of the AC voltage Vac is about several kHz (for example, 2 kHz). Therefore, the AC voltage Vac of the AC superposed charging bias Ve1 is applied to the surface 1a of the photosensitive drum 1 every 0. several milliseconds (for example, 0.5 ms = 1 / 2 kHz), and discharges (see δ in FIG. 7) due to dielectric breakdown occur every 0. several mS (for example, 0.5 ms) in the vicinity space δ1 on the upstream side and the vicinity space δ2 on the downstream side in the first rotation direction R1 of the photosensitive drum 1 at the contact portion where the charging roller 21 contacts the photosensitive drum 1. More specifically, a discharge occurs every time the difference between the surface potential Vf on the surface 1a of the photosensitive drum 1 and the value of the AC superposed charging bias Ve1 supplied to the charging roller 21 exceeds about 500 V, which is the discharge start voltage (dielectric breakdown of the air layer occurs). Here, since the peak-to-peak voltage Vpp shown in FIG. 8 is larger than the DC voltage Vdc, positive and negative discharges occur repeatedly. While such discharges with changing polarities are repeated in a short period of time, when the photosensitive drum 1 rotates in the first rotation direction R1 and the surface 1a of the photosensitive drum 1 moves to the downstream side in the first rotation direction R1 of the vicinity space δ2, the distance between the charging roller 21 and the surface 1a of the photosensitive drum 1 increases and the discharge becomes weaker. Then, the surface potential Vf of the surface 1a of the photosensitive drum 1 is finally charged so as to converge to the value of the DC voltage Vdc. Therefore, even if there are variations in the elastic layer 21b and the resistance layer 21c of the charging roller 21, uniform charging is possible, but at the same time, a large amount of discharge products N such as nitrogen oxides are also generated. That is, the amount of attachment of the discharge product N to the surface 1a of the photosensitive drum 1 increases.

[0008] In this way, when the amount of the discharge product N adhering to the surface 1a of the photosensitive drum 1 increases by continuously rotating the photosensitive drum 1, the frictional force of the surface 1a of the photosensitive drum 1 increases accordingly. As shown in FIG. 7, the tip 51a of the cleaning blade 51 is compressed and distorted in the rotation direction R of the photosensitive drum 1, and furthermore, the cleaning blade 51 may be curled. During the image forming operation, normally toner adheres to the surface 1a of the photosensitive drum 1 (a toner image is formed), so the increase in the frictional force due to the adhesion of the discharge product N on the surface 1a of the photosensitive drum 1 is suppressed. Therefore, it is difficult for the cleaning blade 51 to be curled. On the other hand, during the image forming preparation operation, normally toner does not adhere to the surface 1a of the photosensitive drum 1, so the frictional force due to the adhesion of the discharge product N on the surface 1a of the photosensitive drum 1 is likely to increase. Therefore, the cleaning blade 51 is likely to be curled.

[0009] Regarding this point, Patent Document 1 discloses a technique of vibrating the image carrier (photosensitive drum) by supplying an alternating voltage to the charging member (charging roller) before the rotation of the image carrier, improving the slipperiness between the image carrier and the cleaning blade, and preventing the occurrence of curling (reversal) of the cleaning blade.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0011] However, even with such a technique, there is no change in the generation of discharge products on the surface of the image carrier, and it is impossible to prevent the occurrence of curling of the cleaning blade due to the adhesion of the discharge products on the surface of the image carrier during the image forming preparation operation.

[0012] Therefore, an object of the present disclosure is to provide an image forming apparatus that can suppress image unevenness during an image forming operation and can effectively prevent the occurrence of curling of a cleaning blade due to adhesion of discharge products on the surface of an image carrier during an image forming preparation operation.

Means for Solving the Problems

[0013] In order to solve the above problems, an image forming apparatus according to the present disclosure includes a rotatable image carrier, a charging member that charges the surface of the image carrier, a charging power source including a DC power source and an AC power source, a developing device that develops the surface of the image carrier with toner, a developing power source that supplies a developing bias, which is a predetermined voltage, to the developing device, and a cleaning blade that contacts the surface of the image carrier. An AC superimposed charging bias obtained by superimposing an AC voltage supplied from the AC power source of the charging power source on a DC voltage supplied from the DC power source of the charging power source, and a DC charging bias obtained by not superimposing the AC voltage on the DC voltage can be switched and supplied to the charging member. During an image forming operation in which the surface of the image carrier is developed to form a toner image, the surface of the image carrier is charged under a first condition in which the AC superimposed charging bias is supplied to the charging member. During an image forming preparation operation in which the conditions necessary for the image forming are adjusted while rotating the image carrier, the surface of the image carrier is charged under a second condition in which the DC charging bias is supplied to the charging member.

Effects of the Invention

[0014] According to the present disclosure, during an image forming operation, image unevenness can be suppressed, and during an image forming preparation operation, the occurrence of curling of the cleaning blade due to adhesion of discharge products on the surface of the image carrier can be effectively prevented.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0016] Hereinafter, embodiments according to the present disclosure will be described with reference to the drawings. In the following description, the same components are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed descriptions thereof will not be repeated.

[0017] [Image Forming Apparatus] FIG. 1 is a cross-sectional view of an image forming apparatus 100 according to the present embodiment as viewed from the front. FIG. 2 is a cross-sectional view of a part of an image forming unit 102 in the image forming apparatus 100 shown in FIG. 1 as viewed from the front. In the figure, reference sign X represents the left-right direction, reference sign Y represents the depth direction (front-rear direction), and reference sign Z represents the up-down direction (vertical direction).

[0018] The image forming apparatus 100 according to the present embodiment is a color image forming apparatus that forms multi-color and monochromatic images on a recording sheet P such as paper. The image forming apparatus 100 performs an image forming process according to image data read by the image reading apparatus 90 or image data transmitted from the outside. Note that the image forming apparatus 100 may be a color image forming apparatus of another form. Further, the image forming apparatus 100 may be a monochrome image forming apparatus.

[0019] The image forming apparatus 100 includes an image reading apparatus 90 and an image forming apparatus main body 101. The image forming apparatus main body 101 is provided with an image forming unit 102 and a sheet conveyance system 103.

[0020] The image reading apparatus 90 is provided above the image forming apparatus main body 101. The image reading apparatus 90 reads an image of a document (not shown), and includes a document conveyance unit 90a and a document reading unit 90b. The image reading apparatus 90 reads the image of the document with the document reading unit 90b while conveying the document with the document conveyance unit 90a, or scans and reads the document placed on the document table of the document reading unit 90b with the document reading unit 90b. The image of the document read by the image reading apparatus 90 is sent to the image forming apparatus main body 101 as image data.

[0021] The image forming unit 102 includes a plurality of photosensitive drums 1 to 1 that act as rotatable image carriers, a plurality of charging devices 2 to 2, an exposure device 3 (exposure unit), a plurality of developing devices 4 to 4 (developing units), a plurality of photosensitive cleaning devices 5 to 5 (photosensitive cleaning units), an intermediate transfer belt device 6 (primary transfer device) including an intermediate transfer belt 61 and a plurality of intermediate transfer rollers 65 to 65, a secondary transfer device 7, a belt cleaning device 8 (belt cleaning unit), a fixing device 9 (fixing unit), and a plurality of toner cartridges 10 to 10. Further, the sheet conveyance system 103 includes a paper feed tray 11, a discharge roller 14, and a discharge tray 15. Since the plurality of photosensitive drums 1 to 1, the plurality of charging devices 2 to 2, the plurality of developing devices 4 to 4, the plurality of photosensitive cleaning devices 5 to 5, and the plurality of intermediate transfer rollers 65 to 65 have substantially the same configuration, they are shown in one figure in FIG. 2. In the following description, they are simply referred to as the photosensitive drum 1, the charging device 2, the developing device 4, the photosensitive cleaning device 5, and the intermediate transfer roller 65.

[0022] The photosensitive drum 1 is a cylindrical body having a photosensitive layer formed on the surface of a grounded aluminum tube, and is provided on the image forming apparatus main body 101 so as to be rotatable in the first rotation direction R1 by a drive source 400 (drive motor) (see FIG. 3 described later).

[0023] The charging device 2 includes a charging member (charging roller 21 in this example) for charging the surface 1a of the photosensitive drum 1. The charging roller 21 contacts the surface 1a of the rotating photosensitive drum 1 and rotates drivenly while charging the surface 1a of the photosensitive drum 1.

[0024] The charging roller 21 has an elastic layer 21b formed on the outer peripheral surface of a rotary shaft 21a (conductive support), and a resistance layer 21c formed on the elastic layer 21b. In the present embodiment, as the rotary shaft 21a of the charging roller 21, for example, a round bar of a metal material such as iron, copper, stainless steel, aluminum, nickel, etc. can be used. The elastic layer 21b has appropriate conductivity and elasticity in order to ensure power supply to the photosensitive drum 1 as a charged object and uniform adhesion of the charging roller 21 to the photosensitive drum 1.

[0025] Specifically, as the elastic layer 21b, for example, natural rubber, ethylene propylene rubber (EPDM), styrene butadiene rubber (SBR), silicone rubber, urethane rubber, epichlorohydrin rubber, isoprene rubber (IR), butadiene rubber (BR), nitrile butadiene rubber (NBR), chloroprene rubber (CR), or other synthetic rubbers, or elastic materials such as polyamide, urethane resin, and silicone resin, a conductive agent having an electronic conduction mechanism such as carbon black, graphite, and conductive metal oxides, a conductive agent having an ionic conduction mechanism such as alkali metal salts and quaternary ammonium salts, etc. can be appropriately added and used.

[0026] Further, the resistance layer 21c is formed in contact with the elastic layer 21b, and is provided to prevent the bleeding out of softening oil, plasticizer, etc. contained in the elastic layer 21b to the surface of the charging roller 21, and to adjust the electric resistance of the entire charging roller 21. As the resistance layer 21c, a material having conductivity or semiconduction can be used.

[0027] The exposure device 3 is a laser scanning device including a laser diode, an optical deflection device, and a reflection mirror (not shown). The exposure device 3 exposes the surface 1a of the charged photoreceptor drum 1 according to image data to form an electrostatic latent image corresponding to the image data on the surface 1a of the photoreceptor drum 1.

[0028] The developing device 4 develops the surface 1a of the photoreceptor drum 1 with toner. Specifically, the developing device 4 develops the electrostatic latent image on the photoreceptor drum 1 with a two-component developer (developer) mainly composed of toner and carrier. As shown in FIG. 2, the developing device 4 includes a developing tank 41, a developing roller 42, a first conveying member 43 (first conveying screw), a second conveying member 44 (second conveying screw), and a regulating blade 45.

[0029] The developing tank 41 extends in the longitudinal direction (depth direction Y) and stores the developer. The developing roller 42 acts as a developer carrier and is arranged along an opening provided at a position facing the photosensitive drum 1 of the developing tank 41. The developing roller 42 has a plurality of axially extending magnets (not shown) arranged around the axis, and the developer is carried on the surface of a metal sleeve arranged outside the plurality of magnets. The metal sleeve rotates in the second rotation direction R2 while carrying the developer on its surface. As will be described in detail later, a developing power source 320, which will be described later, is connected to the metal sleeve. The regulating blade 45 is provided in the developing tank 41 and regulates the developer carried on the surface of the developing roller 42 to a predetermined thickness. In the developing tank 41, the developer is transported while being agitated by the first transport member 43 and the second transport member 44, and the toner is charged to a predetermined charging polarity (negative polarity in this example) and carried on the surface of the developing roller 42. The developer carried on the surface of the developing roller 42 is regulated to a predetermined thickness by the regulating blade 45 and transported to the surface 1a of the photosensitive drum 1 rotating in the first rotation direction R1.

[0030] As shown in FIG. 1, the intermediate transfer belt device 6 further includes an intermediate transfer belt 61. The intermediate transfer roller 65 is provided inside the intermediate transfer belt 61. The intermediate transfer belt 61 moves in a predetermined circulating movement direction M. The intermediate transfer roller 65 transfers the toner images of each color formed on the surface 1a of the photosensitive drum 1 onto the intermediate transfer belt 61 while rotating in a driven manner as the intermediate transfer belt 61 moves in a circulating manner. An intermediate transfer power source 330 (see FIG. 2) is electrically connected to the intermediate transfer roller 65 of the intermediate transfer belt device 6. The intermediate transfer power source 330 supplies a DC intermediate transfer bias Vt (a positive voltage in this example) to the intermediate transfer roller 65.

[0031] The photoreceptor cleaning device 5 includes a cleaning blade 51 (see FIG. 2). The cleaning blade 51 is formed of a flexible member (for example, a rubber member). The cleaning blade 51 abuts on the surface 1a of the photoreceptor drum 1 and scrapes off and removes the toner (waste toner) remaining on the surface 1a of the photoreceptor drum 1 without being transferred to the intermediate transfer belt 61 by the intermediate transfer roller 65.

[0032] The image forming apparatus main body 101 is provided with a sheet conveyance path W1. In the vicinity of the sheet conveyance path W1, a sheet supply unit 11a, a plurality of conveyance rollers 12a to 12a, a registration roller 13, a transfer roller 71, a fixing roller 91 and a pressure roller 92 in the fixing device 9, and a discharge roller 14 are disposed.

[0033] The sheet supply unit 11a supplies the recording sheet P accommodated in the paper feed tray 11 to the sheet conveyance path W1. The sheet conveyance path W1 guides the recording sheet P to the discharge tray 15 via the transfer roller 71 of the secondary transfer device 7 and the fixing device 9.

[0034] The secondary transfer device 7 includes a transfer member (in this example, the transfer roller 71). The transfer roller 71 transfers the toner image transferred onto the intermediate transfer belt 61 to the recording sheet P fed from the paper feed tray 11.

[0035] The belt cleaning device 8 removes the toner (waste toner) remaining on the intermediate transfer belt 61 without being transferred to the recording sheet P by the secondary transfer device 7. The fixing device 9 includes a fixing roller 91 and a pressure roller 92, and heat-fixes the toner image formed on the recording sheet P by the secondary transfer device 7 to the recording sheet P by the fixing roller 91 and the pressure roller 92.

[0036] In the image forming apparatus 100, the recording sheet P supplied from the paper feed tray 11 to the sheet conveyance path W1 by the sheet supply unit 11a is conveyed to the registration roller 13 via the conveyance rollers 12a to 12a. Next, the recording sheet P is conveyed to the transfer roller 71 at a timing when the registration roller 13 aligns the recording sheet P with the toner image on the intermediate transfer belt 61, and the toner image intermediate-transferred to the intermediate transfer belt 61 by the transfer roller 71 is transferred onto the recording sheet P. Thereafter, the recording sheet P passes through the fixing roller 91 and the pressure roller 92 in the fixing device 9, and is discharged onto the discharge tray 15 via the conveyance rollers 12a, 12a and the discharge roller 14. When forming an image not only on the front surface but also on the back surface of the recording sheet P, the recording sheet P is conveyed in the reverse direction from the discharge roller 14 to the reverse sheet conveyance path W2. The recording sheet P passes through the reverse conveyance rollers 12b to 12b, the front and back surfaces of the recording sheet P are reversed, and the recording sheet P is guided again to the registration roller 13. Then, in the same manner as the front surface, after a toner image is formed and fixed on the back surface, the recording sheet P is discharged toward the discharge tray 15.

[0037] FIG. 3 is a block diagram showing an example of the system configuration of the control unit 200 in the image forming apparatus 100. FIG. 4 is a graph showing the DC charging bias Ve2 and the surface potential Vf on the surface 1a of the photosensitive drum 1. Further, FIG. 5 is a graph showing the AC superposed charging bias Ve1, the DC charging bias Ve2, the surface potential Vf on the surface 1a of the photosensitive drum 1, and the developing bias Vd.

[0038] The image forming apparatus 100 further includes a control unit 200. The control unit 200 has a processing unit 210 and a storage unit 220. The processing unit 210 is composed of a microcomputer such as a CPU (Central Processing Unit). The storage unit 220 includes a non-volatile memory such as a ROM (Read Only Memory) and a volatile memory such as a RAM (Random Access Memory). The control unit 200 performs operation control of various components by loading a control program stored in advance in the ROM of the storage unit 220 onto the RAM of the storage unit 220 and executing it. The control unit 200 has a timer function for measuring time.

[0039] The image forming apparatus 100 includes a drive source 400, a charging power source 310, and a developing power source 320.

[0040] The drive source 400 is electrically connected to the output system of the control unit 200. Thereby, the control unit 200 can control the drive source 400 that rotates the photoreceptor drum 1.

[0041] The charging power source 310 includes a DC power source 311 that supplies a DC voltage Vdc to the charging roller 21 and an AC power source 312 that supplies an AC voltage Vac to the charging roller 21. The charging power source 310 (DC power source 311 and AC power source 312) has its input side electrically connected to the output system of the control unit 200 and its output side electrically connected to the rotation shaft 21a (see FIG. 2) of the charging roller 21. Thereby, the control unit 200 can control the charging power source 310 (DC power source 311 and AC power source 312).

[0042] The control unit 200 supplies the charging roller 21 with an AC superposed charging bias Ve1 (Vac + Vdc) obtained by superimposing an AC voltage Vac (see Fig. 8) supplied from the AC power supply 312 of the charging power supply 310 on a DC voltage Vdc (a negative-polarity voltage in this example) supplied from the DC power supply 311 of the charging power supply 310, and a DC charging bias Ve2 (= Vdc) obtained by not superimposing the AC voltage Vac on the DC voltage Vdc (a negative-polarity voltage in this example) supplied from the DC power supply 311 (see Fig. 8), in a switchable manner. In other words, the control unit 200 can switch between an AC superposed charging bias Ve1 obtained by superimposing the AC voltage Vac supplied from the AC power supply 312 of the charging power supply 310 on the DC voltage Vdc supplied from the DC power supply 311 of the charging power supply 310, and a DC charging bias Ve2 obtained by not superimposing the AC voltage Vac on the DC voltage Vdc, and supply the charging roller 21, which is a charging member. Note that the charging power supply 310 may be configured to supply the charging roller 21 with the AC superposed charging bias Ve1 and the DC charging bias Ve2 in a switchable manner.

[0043] Thereby, the control unit 200 can form a surface potential Vf by the AC superposed charging bias Ve1 on the surface 1a of the photosensitive drum 1, or form a surface potential Vf by the DC charging bias Ve2 on the surface 1a of the photosensitive drum 1. For example, the DC voltage Vdc of the AC superposed charging bias Ve1 can be set to -500V to -600V. The AC voltage Vac of the AC superposed charging bias Ve1 can have a frequency f = 2kHz and a peak-to-peak voltage Vpp = 1.8kV. The DC voltage Vdc of the DC charging bias Ve2 can be set to -500V to -600V.

[0044] The developing power supply 320 has its input side electrically connected to the output system of the control unit 200 and its output side electrically connected to the developing roller 42 (see Fig. 2) of the developing device 4. Thereby, the control unit 200 can control the developing power supply 320.

[0045] The developing power source 320 supplies a predetermined DC developing bias Vd (for example, a negative-polarity voltage) to the developing roller 42 (the metal sleeve thereof) of the developing device 4. In other words, the developing power source 320 supplies a developing bias, which is a predetermined voltage, to the developing device 4. That is, when the developing bias Vd is supplied from the developing power source 320 to the developing roller 42, the developing device 4 can develop the electrostatic latent image formed on the surface 1a of the photosensitive drum 1 with toner. For example, the developing bias Vd can be set to -300 to -450 V.

[0046] The control unit 200 performs an image forming operation (during the printing operation on a recording sheet such as paper) and an image forming preparation operation (operations such as toner replenishment, transfer cleaning, stabilization of the toner density detection value, and temperature control for the fixing member) for adjusting the conditions necessary for image formation while rotating the photosensitive drum 1. The image forming operation is an operation of exposing the surface 1a of the charged photosensitive drum 1 to form an electrostatic latent image and developing it with the developing device 4 to form a toner image. During the image forming operation, a developing bias Vd of, for example, -400 is supplied to the developing roller 42. Also, during the image forming preparation operation, no developing bias Vd is supplied to the developing roller 42 (a developing bias Vd of 0 V is supplied).

[0047] (Regarding this embodiment) By the way, if the surface potential Vf on the surface 1a of the photosensitive drum 1 varies, image unevenness may occur. Such variation in the surface potential Vf occurs, as in this embodiment, when the transfer member is the charging roller 21, due to the variation in the resistance values in the circumferential direction S (and the axial direction) of the elastic layer 21b and the resistance layer 21c in the charging roller 21 (it is difficult to make them uniform due to material and manufacturing process constraints).

[0048] In this regard, in the image forming apparatus 100 according to the present embodiment, from the viewpoint of equalizing the surface potential Vf on the surface 1a of the photosensitive drum 1, an AC superimposed charging bias Ve1 (Vdc + Vac) (for example, DC voltage Vdc = -500V to -600V, Vpp = 1.8kV) obtained by superimposing an AC voltage Vac on a DC voltage Vdc is supplied to the charging roller 21 to charge the surface 1a of the photosensitive drum 1, and the surface 1a of the photosensitive drum 1 is developed with toner in a state where the variation in the surface potential Vf on the surface 1a of the photosensitive drum 1 is suppressed.

[0049] However, in this case, as described above, since the number of discharges increases by supplying the AC superimposed charging bias Ve1 to the charging roller 21, the amount of discharge products generated increases, and the amount of attachment of the discharge products to the surface 1a of the photosensitive drum 1 increases. Then, the frictional force on the surface 1a of the photosensitive drum 1 increases, and the tip 51a of the cleaning blade 51 is compressed and distorted in the first rotation direction R1 of the photosensitive drum 1, and furthermore, the cleaning blade 51 may be curled. This phenomenon is that during the image forming operation, the transfer residual toner that has not been transferred to the intermediate transfer belt 61 during intermediate transfer adheres to the surface 1a of the photosensitive drum 1 (a toner image is formed), so the increase in the frictional force due to the attachment of the discharge products on the surface 1a of the photosensitive drum 1 is alleviated, and therefore, the cleaning blade 51 is less likely to be curled. On the other hand, during the image forming preparation operation, etc., since development is not performed, toner does not adhere to the surface 1a of the photosensitive drum 1. Therefore, the frictional force due to the attachment of the discharge products on the surface 1a of the photosensitive drum 1 is likely to increase, and thus, the cleaning blade 51 is likely to be curled.

[0050] Therefore, in this embodiment, during the image formation preparation operation, a DC charging bias Ve2 (= Vdc) that does not superimpose an AC voltage Vac on a DC voltage Vdc (for example, -500V to -600V) is supplied to the charging roller 21. By doing so, the number of discharges can be reduced, and the generation amount of discharge products can be decreased, so that it is difficult to wind the cleaning blade 51. However, since the AC voltage Vac is not superimposed, the surface potential Vf on the surface 1a of the photosensitive drum 1 becomes smaller in absolute value than the DC voltage Vdc. At this time, if the same developing bias Vd (for example, -400V) as during the image formation operation is supplied to the developing roller 42, development is performed based on the difference between the developing bias Vd and the surface potential Vf on the surface 1a of the photosensitive drum 1, and there is a risk that the surface 1a of the photosensitive drum 1 will be developed and image defects will occur. Therefore, in this embodiment, during the image formation preparation operation, the developing bias Vd is not supplied to the developing roller 42 (a developing bias Vd of 0V is supplied). By doing so, even when the DC charging bias Ve2 is supplied to the charging roller 21, a non-developing potential difference ΔVn (for example, 100V to 200V) (see FIG. 5) can be maintained. That is, it becomes possible to prevent the toner from being developed on the surface 1a of the photosensitive drum 1.

[0051] Note that it is also conceivable to stop the discharge that charges the surface 1a of the photosensitive drum 1 and eliminate the generation of discharge products by not supplying the charging bias to the charging roller 21. However, in this case, since the surface potential Vf on the surface 1a of the photosensitive drum 1 becomes smaller in absolute value than the developing bias Vd, the toner adheres to the surface 1a of the photosensitive drum 1. For this reason, in order to make the potential difference, which is the difference between the value of the developing bias Vd supplied to the developing roller 42 and the value of the surface potential Vf of the surface 1a of the photosensitive drum 1, the non-developing potential difference ΔVn (for example, 100V to 200V) at which development is not performed (see FIG. 5), it is necessary to supply the developing roller 42 with a developing bias Vd having a polarity opposite to that of the developing bias Vd supplied during the image formation operation (negative polarity in this example) (positive polarity in this example). In this case, there is a problem that the configuration of the developing power supply 320 becomes complicated and the cost of the developing power supply 320 increases accordingly.

[0052] In order to perform the control described above, in the present embodiment, the control unit 200 includes a first charging control unit Q1, a second charging control unit Q2, and a switching control unit Q3.

[0053] The first charging control unit Q1 charges the surface 1a of the photosensitive drum 1 under a first condition of supplying an AC superposed charging bias Ve1 to the charging roller 21 during the image forming operation. The second charging control unit Q2 charges the surface 1a of the photosensitive drum 1 under a second condition of supplying a DC charging bias Ve2 to the charging roller 21 during the image forming preparation operation. The switching control unit Q3 switches between the first charging control unit Q1 and the second charging control unit Q2. Further, the control unit 200 stops the supply of the developing bias Vd to the developing roller 42 during the image forming preparation operation.

[0054] According to the present embodiment, during the image forming operation, by supplying the AC superposed charging bias Ve1 to the charging roller 21 to charge the surface 1a of the photosensitive drum 1, image unevenness can be suppressed. At this time, even if the amount of discharge products generated increases and the amount of adhesion to the surface 1a of the photosensitive drum 1 increases, toner adheres to the surface 1a of the photosensitive drum 1 (a toner image is formed), so that an increase in the frictional force of the surface 1a of the photosensitive drum 1 can be suppressed. Therefore, it is possible to make it difficult for the cleaning blade 51 to be wound. On the other hand, during the image forming preparation operation, by supplying the DC charging bias Ve2 to the charging roller 21 to charge the surface 1a of the photosensitive drum 1, the amount of discharge products generated can be reduced and the amount of adhesion to the surface 1a of the photosensitive drum 1 can be decreased. At this time, although the surface potential Vf on the surface 1a of the photosensitive drum 1 varies and image unevenness is likely to occur, since no image is formed during the image forming preparation operation, there is no particular problem.

[0055] As described above, when performing an image forming operation of developing the surface 1a of the photoreceptor drum 1 to form a toner image, the control unit 200 charges the surface 1a of the photoreceptor drum 1 under a first condition of supplying an AC superposed charging bias Ve1 to the charging roller 21. When performing an image forming preparation operation of adjusting the conditions necessary for image formation while rotating the photoreceptor drum 1, the control unit 200 charges the surface 1a of the photoreceptor drum 1 under a second condition of supplying a DC charging bias Ve2 to the charging roller 21.

[0056] Therefore, during the image forming operation, image unevenness can be suppressed, and during the image forming preparation operation, the occurrence of curling of the cleaning blade 51 due to the adhesion of discharge products on the surface 1a of the photoreceptor drum 1 can be effectively prevented.

[0057] Also, during the image forming preparation operation, since the DC charging bias Ve2 is supplied to the charging roller 21 to charge the surface 1a of the photoreceptor drum 1, the surface potential Vf on the surface 1a of the photoreceptor drum 1 can be made larger in absolute value than the developing bias Vd. Therefore, without complicating the configuration of the developing power supply 320, and thus, it is possible to prevent the toner from adhering to the surface 1a of the photoreceptor drum 1 while maintaining the cost of the developing power supply 320.

[0058] Further, the control unit 200 stops the supply of the developing bias Vd to the developing roller 42 during the image forming preparation operation, so that it is possible to prevent toner from being supplied to the surface 1a of the photoreceptor drum 1 due to development during the image forming preparation operation.

[0059] By the way, in order to charge the surface potential Vf of the surface 1a of the photoreceptor drum 1 to a specified surface potential Vfd (for example, -500V to -700V), which is the same surface potential Vf as during the image forming operation, by supplying the DC charging bias Ve2 to the charging roller 21 during the image forming preparation operation, as shown in FIG. 4, it is necessary to supply a voltage of, for example, about -1200V or more as the DC charging bias Ve2, which increases the cost of the charging power supply 310 accordingly.

[0060] However, in the present embodiment, during the image formation preparation operation, the value of the developing bias Vd supplied to the developing roller 42 is set to 0V. As a result, even if the value of the DC voltage Vdc supplied to the charging roller 21 is the same as the DC voltage Vdc among the DC charging biases Ve2 during the image formation operation, i.e., -500V to -600V, the potential difference, which is the difference between the value of the developing bias Vd and the surface potential Vf of the surface 1a of the photosensitive drum 1, can be set to a non-developing potential difference ΔVn (for example, 100V to 200V) at which development is not performed. That is, since a charging power supply 310 capable of outputting a DC voltage Vdc with a larger absolute value is not required, the cost can be reduced accordingly.

[0061] <Second Embodiment> By the way, during the image formation preparation operation, if the potential difference between the potential of the surface 1a of the photosensitive drum 1 and the developing bias Vd cannot be set to a non-developing potential difference ΔVn (for example, 100V to 200V) at which development is not performed, there is a risk that the surface 1a of the photosensitive drum 1 will be developed and unnecessary toner will adhere to the surface 1a of the photosensitive drum 1.

[0062] Specifically, during the image formation preparation operation, when the value of the DC charging bias Ve2 is the same as the value of the DC voltage Vdc (for example, -500V to -600V) during the image formation operation, depending on the surrounding environment in which the image forming apparatus 100 is arranged, it may be difficult to set the potential difference, which is the difference between the developing bias Vd and the surface potential Vf of the surface 1a of the photosensitive drum 1, to a non-developing potential difference ΔVn (for example, 100V to 200V) at which development is not performed.

[0063] Therefore, in the present embodiment, during the image formation preparation operation, a DC charging bias Ve2 (=Vdc) that does not superimpose the AC voltage Vac is supplied to the charging roller 21 at a DC voltage Vdc (for example, -500V to -600V + α, for example, α = -300V to -400V).

[0064] In this embodiment, during the image formation preparation operation, the first charging control unit Q1 and the second charging control unit Q2 control the charging power supply 310 so that the value of the direct current voltage Vdc increases by α from that in the first condition, such that the potential difference ΔV, which is the difference between the value of the developing bias Vd and the surface potential Vf of the surface 1a of the photosensitive drum 1 charged by the charging roller 21, maintains the non-developing potential difference ΔVn at which the surface 1a of the photosensitive drum 1 is not developed. The developing power supply 320 is controlled so that no developing bias Vd is supplied to the developing roller 42 (Vd = 0V). By controlling in this way, during the image formation preparation operation, the potential difference, which is the difference between the value of the developing bias Vd and the surface potential Vf of the surface 1a of the photosensitive drum 1, can be maintained at the non-developing potential difference ΔVn at which development does not occur, and the surface 1a of the photosensitive drum 1 can be surely prevented from being developed.

[0065] As described above, during the image formation preparation operation, when the direct current voltage Vdc is the same as the direct current voltage Vdc during the image formation operation (for example, -500V to -600V), it may be difficult to maintain the non-developing potential difference ΔVn (for example, 100V to 200V) at which the surface 1a of the photosensitive drum 1 is not developed.

[0066] In contrast, in this embodiment, the charging power supply 310 is capable of changing the direct current voltage Vdc, and the first charging control unit Q1 and the second charging control unit Q2 make the absolute value of the direct current voltage Vdc in the first condition different from the absolute value of the direct current voltage Vdc in the second condition. By doing so, during the image formation preparation operation, the non-developing potential difference ΔVn at which the surface 1a of the photosensitive drum 1 is not developed can be easily maintained, and the surface 1a of the photosensitive drum 1 can be more surely prevented from being developed.

[0067] In this example, the first charging control unit Q1 and the second charging control unit Q2 make the absolute value of the DC voltage Vdc under the second condition larger than the absolute value of the DC voltage Vdc under the first condition. For example, the DC voltage Vdc under the first condition can be set to -500V to -600V, and the DC voltage Vdc under the second condition can be set to -900V (not exceeding 1kV). In this way, by making the absolute value of the DC voltage Vdc larger than the DC voltage Vdc during the image forming operation, a non-development potential difference ΔVn (for example, 100V to 200V) that does not develop the surface 1a of the photosensitive drum 1 can be surely maintained. Thereby, it is possible to more surely prevent the surface 1a of the photosensitive drum 1 from being developed.

[0068] <Third Embodiment> By the way, when suddenly switching from the DC charging bias Ve2 during the image forming preparation operation to the AC superimposed charging bias Ve1 during the image forming operation at the first switching operation, it is impossible to stably supply the AC superimposed charging bias Ve1 to the charging roller 21 during the image forming operation.

[0069] In this regard, when switching from the DC charging bias Ve2 to the AC superimposed charging bias Ve1, the switching control unit Q3 performs a first switching control for gradually increasing the absolute value of the AC voltage Vac in the AC superimposed charging bias Ve1 and a second switching control for gradually decreasing the absolute value of the DC voltage Vdc in the DC charging bias Ve2. By doing so, it is possible to gently switch from the DC charging bias Ve2 during the image forming preparation operation to the AC superimposed charging bias Ve1 during the image forming operation.

[0070] In this case, it is preferable that the first switching time, which is the time required for the switching of the first switching control, is longer than the second switching time, which is the time required for the switching of the second switching control.

[0071] In this regard, the switching control unit Q3 controls the charging power supply 310 such that the start timing t1 of the first switching control is earlier than the start timing t2 of the second switching control. Further, the switching control unit Q3 controls the charging power supply 310 such that the end timing t3 of the second switching control is earlier than the end timing t4 of the first switching control. By doing so, during the image forming operation, the AC superimposed charging bias Ve1 can be stably supplied to the charging roller 21. In this example, the switching control unit Q3 controls the charging power supply 310 to gradually increase the AC superimposed charging bias Ve1 between the start timing t1 of the first switching control and the end timing t4 of the first switching control (gradually after the start timing t2 and the end timing t3 of the second switching control).

[0072] Also, when transitioning from the image forming preparation operation to the image forming operation, the switching control unit Q3 controls the developing power supply 320 such that the developing bias Vd gradually increases. In this example, the switching control unit Q3 controls the charging power supply 310 and the developing power supply 320 such that the end timing t4 of the first switching control is earlier than the supply start timing t5 of the developing bias Vd.

[0073] <Fourth Embodiment> By the way, when abruptly switching from the AC superimposed charging bias Ve1 during the image forming operation to the DC charging bias Ve2 during the image forming preparation operation in the second switching operation, the DC charging bias Ve2 cannot be stably supplied to the charging roller 21 during the image forming preparation operation.

[0074] In this regard, when switching from the AC superimposed charging bias Ve1 to the DC charging bias Ve2, the control unit 200 performs a third switching control to gradually decrease the absolute value of the AC voltage Vac in the AC superimposed charging bias Ve1 and a fourth switching control to gradually increase the absolute value of the DC voltage Vdc in the DC charging bias Ve2. By doing so, it is possible to gently switch from the AC superimposed charging bias Ve1 during the image forming operation to the DC charging bias Ve2 during the image forming preparation operation.

[0075] In this case, it is preferable that a third switching time, which is the time required for switching the third switching control, is longer than a fourth switching time, which is the time required for switching the fourth switching control.

[0076] In this regard, the switching control unit Q3 controls the charging power supply 310 such that the start timing t7 of the third switching control is earlier than the start timing t8 of the fourth switching control. Further, the switching control unit Q3 controls the charging power supply 310 such that the end timing t9 of the fourth switching control is earlier than the end timing t10 of the third switching control. By doing so, during the image formation preparation operation, the DC charging bias Ve2 can be stably supplied to the charging roller 21. In this example, the switching control unit Q3 controls the charging power supply 310 to gradually decrease the value of the AC voltage Vac in the AC superposed charging bias Ve1 between the start timing t7 and the end timing t10 of the third switching control (gradually at the start timing t8 and the end timing t9 of the fourth switching control) of the fourth switching control.

[0077] Further, in the switching control unit Q3, when shifting from the image formation operation to the image formation preparation operation, the developing power supply 320 is controlled so that the developing bias Vd gradually decreases. In this example, the switching control unit Q3 controls the charging power supply 310 and the developing power supply 320 such that the supply end timing t6 of the developing bias Vd is earlier than the start timing t7 of the third switching control.

[0078] <Fifth Embodiment> Incidentally, as the photosensitive drum 1 rotates with the cleaning blade 51 in contact therewith, the film thickness of the photosensitive film of the photosensitive drum 1 becomes thinner.

[0079] Regarding this point, when the surface potential Vf on the surface 1a of the photoreceptor drum 1 is charged by the AC superposed charging bias Ve1 (during the image forming operation), it is less affected by the film thickness of the photosensitive film of the photoreceptor drum 1. However, when the surface 1a of the photoreceptor drum 1 is charged by the DC charging bias Ve2 (during the image forming preparation operation), it is more easily affected by the film thickness of the photosensitive film of the photoreceptor drum 1. For example, when the film thickness of the photosensitive film is the specified film thickness, where the surface potential Vf should be the desired surface potential when the film thickness of the photosensitive film is the specified film thickness, the surface potential Vf on the surface 1a of the photoreceptor drum 1 tends to be larger in absolute value than the desired surface potential as the film thickness of the photosensitive film of the photoreceptor drum 1 becomes thinner.

[0080] Also, even if the first charging integration time for supplying the AC superposed charging bias Ve1 to the surface 1a of the photoreceptor drum 1 to charge the surface 1a of the photoreceptor drum 1 and the second charging integration time for supplying the DC charging bias Ve2 to the surface 1a of the photoreceptor drum 1 to charge the surface 1a of the photoreceptor drum 1 are the same, the wear amounts of the photosensitive film of the photoreceptor drum 1 may be different between the case of supplying the AC superposed charging bias Ve1 and the case of supplying the DC charging bias Ve2.

[0081] Therefore, the control unit 200 includes a first charging time integration unit Q4, a second charging time integration unit Q5, and a DC charging bias calculation unit Q6.

[0082] The first charging time integration unit Q4 calculates a first charging integration time T1 for integrating the first charging time for charging the surface 1a of the photoreceptor drum 1 with the AC superposed charging bias Ve1 in each image forming job (printing job), and stores the calculated first charging integration time T1 (see FIG. 3) in the storage unit 220. The second charging time integration unit Q5 calculates a second charging integration time T2 for integrating the second charging time for charging the surface 1a of the photoreceptor drum 1 with the DC charging bias Ve2 in each image forming job (printing job), and stores the calculated second charging integration time T2 (see FIG. 3) in the storage unit 220.

[0083] The DC charging bias calculation unit Q6 calculates a DC charging bias Ve2 based on the first charging integration time T1 and the second charging integration time T2.

[0084] Specifically, the DC charging bias calculation unit Q6 includes a first film thickness calculation unit Q61 and a second film thickness calculation unit Q62.

[0085] The first film thickness calculation unit Q61 calculates a first calculated film thickness of the photoreceptor drum 1 when an AC superimposed charging bias Ve1 is supplied to the surface 1a of the photoreceptor drum 1 based on the first charging integration time T1 stored in the storage unit 220. In this example, a first film thickness conversion unit CV1 (see FIG. 3), such as a conversion formula or a conversion table for converting from the first charging integration time T1 to the first calculated film thickness, can be stored in the storage unit 220 in advance. The first film thickness conversion unit CV1 can be set in advance by experiments or the like.

[0086] Also, the second film thickness calculation unit Q62 calculates a second calculated film thickness of the photoreceptor drum 1 when a DC charging bias Ve2 is supplied to the surface 1a of the photoreceptor drum 1 based on the second charging integration time T2 stored in the storage unit 220. In this example, a second film thickness conversion unit CV2 (see FIG. 3), such as a conversion formula or a conversion table for converting from the second charging integration time T2 to the second calculated film thickness, can be stored in the storage unit 220 in advance. The second film thickness conversion unit CV2 can be set in advance by experiments or the like.

[0087] The DC charging bias calculation unit Q6 calculates the DC charging bias Ve2 based on the total film thickness of the first calculated film thickness and the second calculated film thickness. In this example, a charging bias conversion unit CV3 (see FIG. 3), such as a conversion formula or a conversion table for converting from the total film thickness to the DC charging bias Ve2, can be stored in the storage unit 220 in advance. The charging bias conversion unit CV3 can be set in advance by experiments or the like.

[0088] By doing so, the DC charging bias Ve2 is calculated based on the first charging integration time T1 and the second charging integration time T2. Therefore, even if the film thickness of the photosensitive film of the photosensitive drum 1 becomes thinner than the specified film thickness, the surface potential Vf can be maintained at a desired surface potential during the image formation preparation operation.

[0089] The configurations disclosed in any of the above-described embodiments can be applied in combination with the configurations disclosed in other embodiments as long as no contradiction occurs. Also, the embodiments disclosed in this specification are illustrative, and the embodiments of the present disclosure are not limited thereto, and can be appropriately modified within the scope not departing from the object of the present disclosure.

Explanation of Signs

[0090] 1 Photosensitive drum (an example of an image carrier) 1a Surface 21 Charging roller 21a Rotation shaft 21b Elastic layer 21c Resistance layer 2 Charging device 3 Exposure device 4 Developing device 41 Developing tank 42 Developing roller 100 Image forming apparatus 200 Control unit 210 Processing unit 220 Storage unit 310 Charging power source 311 DC power source 312 AC power source 320 Developing power source 330 Intermediate transfer power source 5 Photosensitive drum cleaning device 51 Cleaning blade 51a Tip 6 Intermediate transfer belt device 61 Intermediate transfer belt 65 Intermediate transfer roller 7 Secondary transfer device 71 Transfer roller 8 Belt cleaning device CV1 First film thickness conversion unit CV2 Second film thickness conversion unit CV3 Charging bias conversion unit M Circumferential movement direction N Discharge product P Recording sheet Q1 First charging control unit Q2 Second charging control unit Q3 Switching control unit Q4 First charging time integration unit Q5 Second charging time integration unit Q6 DC charging bias calculation unit Q61 First film thickness calculation unit Q62 Second film thickness calculation unit T1 First charging integration time T2 Second charging integration time Vac AC voltage Vd Development bias Vdc DC voltage Ve1 AC superimposed charging bias Ve2 DC charging bias Vf Surface potential Vfd Specified surface potential Vpp Peak-to-peak voltage Vt Intermediate transfer bias ΔV Potential difference ΔVn Non-development potential difference f Frequency βa Toner adhesion area βb Toner non-adhesion area δ1 Upstream near space δ2 Downstream near space

Claims

1. A rotatable image carrier, a charging member for charging the surface of the image carrier, a charging power source having a DC power source and an AC power source, a developing device for developing the surface of the image carrier with toner, a developing power source for supplying a developing bias, which is a predetermined voltage, to the developing device, a cleaning blade that contacts the surface of the image carrier, comprising: an AC superimposed charging bias obtained by superimposing the AC voltage supplied from the AC power source of the charging power source on the DC voltage supplied from the DC power source of the charging power source, and a DC charging bias obtained by not superimposing the AC voltage on the DC voltage, can be switched and supplied to the charging member, During an image forming operation for developing the surface of the image carrier to form a toner image, the surface of the image carrier is charged under a first condition in which the AC superimposed charging bias is supplied to the charging member, During an image forming preparation operation for adjusting the conditions necessary for the image formation while rotating the image carrier, the surface of the image carrier is charged under a second condition in which the DC charging bias is supplied to the charging member. An image forming apparatus.

2. During the image forming preparation operation, the charging power source and the developing power source are controlled so that a potential difference, which is the difference between the value of the developing bias and the value of the surface potential of the surface of the image carrier charged by the charging member, maintains a non-developing potential difference at which the surface of the image carrier is not developed. The image forming apparatus according to claim 1.

3. The DC power source is capable of changing the DC voltage, The absolute value of the DC voltage in the second condition is greater than the absolute value of the DC voltage in the first condition. The image forming apparatus according to claim 2.

4. When switching from the DC charging bias to the AC superimposed charging bias, a first switching control for gradually increasing the absolute value of the AC superimposed charging bias and a second switching control for gradually decreasing the absolute value of the DC charging bias are performed, and the charging power source is controlled so that the start timing of the first switching control is earlier than the start timing of the second switching control. The image forming apparatus according to claim 1.

5. When switching from the DC charging bias to the AC superimposed charging bias, the image forming apparatus according to claim 1 performs first switching control for gradually increasing the absolute value of the AC superimposed charging bias and second switching control for gradually decreasing the absolute value of the DC charging bias, and controls the charging power source so that the end timing of the second switching control is earlier than the end timing of the first switching control.

6. When switching from the AC superimposed charging bias to the DC charging bias, the image forming apparatus according to claim 1 performs third switching control for gradually decreasing the absolute value of the AC superimposed charging bias and fourth switching control for gradually increasing the absolute value of the DC charging bias, and controls the charging power source so that the start timing of the third switching control is earlier than the start timing of the fourth switching control.

7. When switching from the AC superimposed charging bias to the DC charging bias, the image forming apparatus according to claim 1 performs third switching control for gradually decreasing the absolute value of the AC superimposed charging bias and fourth switching control for gradually increasing the absolute value of the DC charging bias, and controls the charging power source so that the end timing of the fourth switching control is earlier than the end timing of the third switching control.

8. The image forming apparatus according to claim 1 separately calculates a first charging integration time for integrating a first charging time for charging the surface of the image carrier with the AC superimposed charging bias and a second charging integration time for integrating a second charging time for charging the surface of the image carrier with the DC charging bias, and calculates the DC charging bias based on the first charging integration time and the second charging integration time.

Images