Image forming apparatus

Abstract

To suppress the occurrence of density unevenness caused by the failure of scraping developer from a developing member by a supply member while shortening the rotation time of the developing member and the supply member when an image is not formed.SOLUTION: The control unit 60 can control the drive unit 55 to perform image formation in a first mode in which an electrostatic image is developed in a state where the supply member 43 is rotationally driven at a first drive speed, and when the first image formation is performed in the first mode and the second image formation is performed after the first image formation, the control unit 60 rotationally drives the developing member 42 at the first drive speed and rotationally drives the supply member 43 at a second drive speed higher than the first drive speed during a first period after the first image formation and before the second image formation. The driving unit 55 can be controlled so as to perform a preliminary operation of rotationally driving the developing member 42 in a second period after the first period and rotationally driving the supply member 43 at a third driving speed higher than the second driving speed.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a printer, a copying machine, or a facsimile machine that uses an electrophotographic method. [Background technology]

[0002] In electrophotographic image forming apparatuses, an electrostatic image formed on an image carrier is developed by a developing device. Some developing devices have a rotatable developing member that carries and transports developer to the image carrier, a supply member that supplies developer to the developing member and scrapes developer from the developing member, and a regulating member that regulates the amount of developer on the developing member. The supply member is a rotatable supply roller that can contain developer in a foam layer.

[0003] To achieve stable image output using such a developing device, it is necessary to maintain a uniform developer layer thickness on the developing member. However, for example, when the developing device is loaded with a large amount of developer and the driving speed of the supply member during image formation is slow, it can be difficult to form a uniform developer layer on the developing member. When the driving speed of the supply member is slow, the amount of developer discharged from the supply member per unit time decreases, making the developer around the supply member less mobile and compacted. This prevents the developer from being discharged from the supply member, resulting in an excessive amount of developer contained within the supply member. In this state, it becomes difficult for the supply member to scrape the developer, and the amount of developer on the developing member before being regulated by the regulating member increases. As a result, it becomes difficult to form a uniform developer layer on the developing member by the regulating member, which can result in image defects such as uneven density in the developed image. Here, image defects caused by poor developer scraping from the developing member by the supply member are also referred to simply as "density unevenness."

[0004] To address this issue, Patent Document 1 proposes increasing the drive speed of the supply member during non-image formation compared to the drive speed during image formation. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-178113 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the conventional configuration, in order to suppress the occurrence of density unevenness, it may be necessary to rotate the developing member and supply member for a long time when no image is being formed. This is because, in the conventional configuration, the developer discharged from the supply member when no image is being formed gets between the developing member and the supply member, and it takes time to scrape this out.

[0007] Therefore, the object of the present invention is to reduce the rotation time of the developing member and the supplying member when no image is being formed, while suppressing the occurrence of density unevenness caused by the supplying member not being able to properly scrape developer from the developing member. [Means for solving the problem]

[0008] The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides an image forming apparatus including an image carrier on whose surface an electrostatic image is formed, a developing member that rotates to supply toner to the electrostatic image, a supply member that has a foam layer that contacts the developing member to form a nip portion and that rotates to supply toner to the developing member, a drive unit that rotates the developing member and the supply member, and a control unit that can control the drive unit, wherein the control unit can control the drive unit to perform image formation in a first mode in which the electrostatic image is developed by the developing member supplying toner to the electrostatic image to form a toner image on the surface of the image carrier while the supply member is rotationally driven at a first drive speed. and when a first image formation is performed in the first mode and a second image formation is performed following the first image formation, the drive unit can be controlled to perform a preparatory operation of rotationally driving the developing member and rotationally driving the supply member at a second drive speed higher than the first drive speed during a first period after the first image formation and before the second image formation, and rotationally driving the developing member and rotationally driving the supply member at a third drive speed higher than the second drive speed during a second period after the first period and after the first image formation and before the second image formation.

[0009] According to another aspect of the present invention, there is provided an image carrier having an electrostatic image formed on its surface, a developing member which rotates to supply toner to the electrostatic image, a supply member which has a foam layer which contacts the developing member to form a nip portion and which rotates to supply toner to the developing member, a drive unit which drives and rotates the developing member and the supply member, a position movement mechanism which can move the position of the developing member relative to the image carrier between a first position and a second position which is a different distance from the first position to the image carrier, and a control unit which can control the drive unit and the position movement mechanism, wherein the control unit controls the supply member to rotate at a first drive speed, and An image forming apparatus is provided, characterized in that the drive unit can be controlled to perform a first mode in which the electrostatic image is developed while the developing member is rotationally driven, and when a first image formation is performed in the first mode and a second image formation is performed after the first image formation, the drive unit and the position movement mechanism can be controlled to perform a preliminary operation of performing a first position movement operation to move the position of the developing member from the second position to the first position while the developing member is rotationally driven and the supply member is rotationally driven at a drive speed higher than the first drive speed after the first image formation and before the second image formation, and a second position movement operation to move the position of the developing member from the second position to the first position after the first position movement operation. [Effects of the Invention]

[0010] According to the present invention, it is possible to reduce the rotation time of the developing member and the supplying member when no image is being formed, while suppressing the occurrence of density unevenness caused by the supplying member not being able to properly scrape developer from the developing member. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic cross-sectional view showing a general configuration of an image forming apparatus. [Figure 2] FIG. 2 is a schematic cross-sectional view showing the general configuration of a developing device. [Figure 3] FIG. 2 is a block diagram showing an outline of a control configuration of the image forming apparatus. [Figure 4] FIG. 10 is a flowchart of control for determining whether or not to perform a preparatory movement. [Figure 5] FIG. 4 is a timing chart of the preparatory operation in the first embodiment. [Figure 6] FIG. 10 is a timing chart of the preparatory operation in the second embodiment. [Figure 7] FIG. 11 is a timing chart of the preparatory operation in the third embodiment. [Figure 8] FIG. 10 is a timing chart of the preparatory operation in Comparative Example 2. [Figure 9] FIG. 11 is a timing chart of the preparatory operation in Comparative Example 3. [Figure 10] FIG. 10 is a timing chart of the preparatory operation in Comparative Example 4. [Figure 11] FIG. 13 is a timing chart of the preparatory operation in Comparative Example 5. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described in detail by way of example with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of the components described in the embodiments can be changed as appropriate depending on the configuration of the device to which the present invention is applied and various conditions. In other words, the scope of the present invention is not limited to the following embodiments.

[0013] [Embodiment 1] 1. Overview of image forming equipment The overall configuration and image forming operation of an image forming apparatus 100 of this embodiment will be described with reference to Fig. 1. Fig. 1 is a schematic cross-sectional view showing the general configuration of the image forming apparatus 100 of this embodiment.

[0014] The image forming apparatus 100 of this embodiment is a laser beam printer capable of forming a black monochrome image on a sheet-like recording material S using an electrophotographic system. The image forming apparatus 100 forms an image based on image information (image signals) input from an external device such as a host computer. The image forming apparatus 100 mainly includes a photosensitive drum 1, a charging roller 2, an exposure device 31, a developing device 4, a transfer roller 51, a fixing device 52, a control unit 60, etc.

[0015] The photosensitive drum 1, which is a cylindrical (drum-shaped) photosensitive member (electrophotographic photosensitive member) serving as an image carrier, is driven by a drive device 55 to rotate around its axis in the direction of arrow R1 (counterclockwise) in the drawing. In this embodiment, the image forming operation is performed by selecting either a high-speed mode in which the image formation speed is relatively high, or a low-speed mode in which the image formation speed is relatively low, in order to ensure fixation depending on the paper type, etc. In this embodiment, the high-speed mode is the normal operation of the image forming apparatus 100. However, the low-speed mode may also be the normal operation of the image forming apparatus 100. In this embodiment, in the high-speed mode, the photosensitive drum 1 is driven to rotate at a peripheral speed (movement speed of the outer circumferential surface) of 180 mm / sec. In addition, in this embodiment, the photosensitive drum 1 is driven to rotate at a peripheral speed of 60 mm / sec in the low-speed mode.

[0016] The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by a charging roller 2, a roller-type charging member serving as a charging means. In this embodiment, the charging roller 2 is a conductive roller with a conductive elastic layer provided on a metal core. The charging roller 2 is disposed so as to contact the photosensitive drum 1 with a predetermined pressure and rotates in accordance with the rotation of the photosensitive drum 1. During charging, a predetermined charging bias (charging voltage) is applied to the charging roller 2 by a charging power source E1 (FIG. 3) serving as a charging bias application unit. This generates a discharge between the charging roller 2 and the photosensitive drum 1, charging the surface of the photosensitive drum 1 to a predetermined charging potential (dark potential) Vd. In this embodiment, a DC voltage of, for example, −1100 V is applied to the charging roller 2 as the charging bias, and the charging potential Vd of the photosensitive drum 1 after charging is −600 V.

[0017] The surface of the charged photosensitive drum 1 is scanned and exposed by an exposure device 31, which serves as an exposure means, with laser light modulated according to an image signal, and an electrostatic image (electrostatic latent image) is formed on the photosensitive drum 1. The exposure device 31 and a reflecting mirror 32 are arranged so that the laser beam emitted from the exposure device 31 reaches the photosensitive drum 1 via the reflecting mirror 32. The absolute value of the potential of the photosensitive drum 1 in the portion of the surface of the photosensitive drum 1 irradiated with the laser light is reduced, forming a bright area potential Vl, and an electrostatic image is formed on the photosensitive drum 1.

[0018] The electrostatic image formed on the photosensitive drum 1 is developed (visualized) by a developing device 4 serving as a developing means, which supplies toner as a developer, and a toner image (toner image, developer image) is formed on the photosensitive drum 1. Details of the developing device 4 will be described later.

[0019] A transfer roller 51, a roller-type transfer member serving as a transfer means, is disposed opposite the photosensitive drum 1. The transfer roller 51 is pressed against the photosensitive drum 1 with a predetermined pressure, forming a transfer nip TN, which is the contact point between the photosensitive drum 1 and the transfer roller 51. In this embodiment, the transfer roller 51 is driven to rotate by a drive unit 55. At the transfer nip TN, the toner image formed on the photosensitive drum 1 is electrostatically transferred by the transfer roller 51 onto a recording material S, which is sandwiched and transported between the photosensitive drum 1 and the transfer roller 51 and serves as a transfer target. During transfer, a predetermined transfer bias (transfer voltage), which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the transfer roller 51 by a transfer power source E5 (FIG. 3) serving as a transfer bias application unit. The recording material S, such as paper or a plastic sheet (recording medium, transfer material, sheet), is fed from a recording material storage unit (not shown) and transported to a transport roller 50, which serves as a transport member. Then, this recording material S is sent out by a conveying roller 50 at a predetermined control timing synchronized with the position of the toner image on the photosensitive drum 1, and is conveyed to a transfer portion TN.

[0020] The recording material S onto which the toner image has been transferred is conveyed to a fixing device 52 serving as fixing means. The fixing device 52 has a fixing roller 52a equipped with a heat source and a pressure roller 52b that is in pressure contact with the fixing roller 52a. The fixing device 52 applies pressure and heat to the recording material S carrying the unfixed toner image by means of the fixing roller 52a and the pressure roller 52b, thereby fixing (melting and adhering) the toner image onto the recording material S. The recording material S onto which the toner image has been fixed is discharged (output) to the outside of the apparatus main body 110 of the image forming apparatus 100 as an image-formed product.

[0021] Furthermore, the toner remaining on the photosensitive drum 1 after transfer (transfer residual toner) is removed from the photosensitive drum 1 and collected by a cleaning device 7 serving as cleaning means.

[0022] The photosensitive drum 1, the rotating members of the developing device 4 (described later), the conveying roller 50, the transfer roller 51, the pressure roller 52b of the fixing device 52, and the like are rotated by a driving force transmitted from a driving device 55 provided in the device body 110 of the image forming apparatus 100. The driving device 55 may have independent motors (power sources, drive sources) for each of these driven components, or may be configured to transmit driving force from a common motor to multiple (or all) of these driven components. In this embodiment, the photosensitive drum 1 and the developing roller 42 and supply roller 43 of the developing device 4 (described later) are rotated by a driving force transmitted from a common motor. The device body 110 of the image forming apparatus 100 also has a power source for applying a predetermined bias to the charging roller 2, the transfer roller 51, the developing roller 42, supply roller 43, and regulating blade 44 of the developing device 4 (described later).

[0023] In this embodiment, the photosensitive drum 1, the charging roller 2 acting as process means, the developing device 4, and the cleaning device 7 integrally constitute a process cartridge 120 that is detachable from the main body 110 of the image forming apparatus 100. In this embodiment, the main body 110 of the image forming apparatus 100 corresponds to the portion of the image forming apparatus 100 excluding the process cartridge 120.

[0024] 2. Developing device FIG. 2A is a schematic cross-sectional view showing the general configuration of the developing device 4 in this embodiment.

[0025] The developing device 4 supplies toner 90 as a developer to the electrostatic image on the photosensitive drum 1, visualizing it as a toner image. In this embodiment, the developing device 4 is a developing device employing a contact development method and a reversal development method, using toner 90, a non-magnetic single-component developer, as the developer. In this embodiment, the normal charge polarity of the toner 90, which is the primary charge polarity of the toner 90 when developing an electrostatic image, is negative. The developing device 4 includes a developer container 41, a developing roller 42 as a developing member (developer carrier), a supply roller 43 as a supply member, and a regulating blade 44 as a regulating member. The developer container 41 contains the toner 90. The developing roller 42 carries the toner 90 and transports it to the photosensitive drum 1. The supply roller 43 supplies the toner 90 to the developing roller 42 and scrapes the toner 90 from the developing roller 42. The regulating blade 44 regulates the amount of toner 90 on the developing roller 42 (the thickness of the toner layer). The developing roller 42 and the supply roller 43 are each rotated by a driving force transmitted from a driving device 55 serving as a driving unit provided in the apparatus main body 110 of the image forming apparatus 100. The toner 90 is charged to the normal negative polarity due to friction with the developing roller 42, the supply roller 43, and the regulating blade 44.

[0026] The developing roller 42 is an elastic roller having a core and an elastic layer formed on the outer periphery of the core using a conductive elastic rubber or the like as an elastic member. In this embodiment, the developing roller 42 has a cylindrical outer periphery with an outer diameter of 16 mm, and both ends are supported by the developer container 41 so as to be rotatable. During image formation, the developing roller 42 contacts the photosensitive drum 1 and is driven to rotate in the direction of arrow R2 (clockwise) in the figure. That is, the developing roller 42 rotates so that the surface (outer surface) of the photosensitive drum 1 and the surface (outer surface) of the developing roller 42 move forward at a development position (development section) A where the photosensitive drum 1 and the developing roller 42 face each other (contact each other). In this embodiment, the developing device 4 is configured so that the peripheral speed of the developing roller 42 is 140% of the peripheral speed of the photosensitive drum 1 in order to obtain an appropriate image density. Therefore, in this embodiment, the developing roller 42 is rotated at a peripheral speed of 252 mm / sec in the high-speed mode and at a peripheral speed of 84 mm / sec in the low-speed mode. The rotational axis of the developing roller 42 is approximately parallel to the rotational axis of the photosensitive drum 1.

[0027] The supply roller 43 is an elastic sponge roller with an outer diameter of 18 mm, including a core 43a and a foamed elastic layer (foamed elastic layer) 43b formed around the outer periphery of the core 43a using a foamed elastic material such as urethane sponge. The foamed cells on the surface of the foamed layer 43b are open, making it easier to retain and transport the toner 90. The supply roller 43 is positioned to contact the developing roller 42 with a predetermined penetration depth, forming a predetermined nip N, which is the contact area between the developing roller 42 and the supply roller 43. The hardness of the foamed layer 43b of the supply roller 43 is lower than that of the developing roller 42. Therefore, in the nip N, the foamed layer 43b of the supply roller 43 is deformed into a concave shape by the developing roller 42. The supply roller 43 is driven to rotate in the direction of arrow R3 (clockwise) in the figure. In other words, the supply roller 43 rotates such that the surface (outer surface) of the developing roller 42 and the surface (outer surface) of the supply roller 43 move in opposite directions in the nip N. As the supply roller 43 rotates, it supplies toner 90 onto the developing roller 42. As the supply roller 43 rotates, it also scrapes (scrapes) off from the developing roller 42 any toner 90 that remains on the developing roller 42 without being supplied to the electrostatic image at the development position A. The supply roller 43 scrapes off the toner 90 from the developing roller 42 through openings in the foam layer 43b on its surface and returns it to the inside of the developer container 41. The foam layer 43b of the supply roller 43 deforms just before the nip portion N in the rotation direction of the supply roller 43. This deformation causes the toner 90 that has remained on the surface and inside of the foam layer 43b to be discharged in the direction of arrow T1 in the figure into a region X (herein also referred to as the "discharge region") between the developing roller 42 and the supply roller 43 just before the nip portion N. The surface of the foam layer 43b of the supply roller 43 passes through the nip portion N, and the deformation of the foam layer 43b recovers immediately after the nip portion N in the rotation direction of the supply roller 43. At this time, toner 90 in the region Y between the development roller 42 and the supply roller 43 immediately after the nip portion N is sucked into the foam layer 43b in the direction of arrow T2 in the figure. In this embodiment, the supply roller 43 is rotationally driven at a driving speed v of 240 rpm in the high-speed mode, and at a driving speed v of 80 rpm in the low-speed mode.The rotational axis of the supply roller 43 is substantially parallel to the rotational axis of the developing roller .

[0028] The regulating blade 44 is formed of a flexible metal plate. The regulating blade 44 is disposed so that its longitudinal direction is substantially parallel to the rotational axis direction of the developing roller 42, one end of the lateral direction is fixed to the developing container 41, and the other end (free end) of the lateral direction is in contact with the developing roller 42. The toner 90 inside the developing container 41 is supplied onto the developing roller 42 by the supply roller 43, and as the developing roller 42 rotates, the toner 90 is thinned to a predetermined layer thickness by the regulating blade 44, held on the developing roller 42, and used for development.

[0029] A predetermined developing bias (developing voltage) is applied to the developing roller 42 by a developing power supply E2 (FIG. 3) serving as a developing bias application unit. In this embodiment, a DC voltage of -350 V, for example, is applied to the developing roller 42 as the developing bias. At the developing position A, an electrostatic force acts on the toner 90 due to the potential difference between the developing bias (potential of the developing roller 42) and the potential of the exposed portion of the surface of the photosensitive drum 1 (light portion potential Vl), and the toner 90 adheres to the exposed portion on the photosensitive drum 1, thereby developing an electrostatic image on the photosensitive drum 1.

[0030] A predetermined supply bias (supply voltage) is applied to the supply roller 43 by a supply power source E3 (FIG. 3) serving as a supply bias application unit. In this embodiment, a DC voltage having the same polarity as the developing bias and an absolute value greater than that of the developing bias is applied to the supply roller 43 as the supply bias. An electrostatic force acts on the toner 90 due to the potential difference between the supply bias (potential of the supply roller 43) and the developing bias (potential of the developing roller 42), urging the toner 90 from the supply roller 43 toward the developing roller 42. Furthermore, a predetermined regulating bias (regulating voltage) is applied to the regulating blade 44 by a regulating power source E4 (FIG. 3) serving as a regulating bias application unit. In this embodiment, a DC voltage having the same polarity as the developing bias and an absolute value greater than that of the developing bias is applied to the regulating blade 44 as the regulating bias. The potential difference between the regulating bias (potential of the regulating blade 44) and the developing bias (potential of the developing roller 42) causes an electrostatic force to act on the toner 90, urging the toner 90 from the regulating blade 44 toward the developing roller 42 and promoting the imparting of charge to the toner 90.

[0031] The arrangement of the components of the developing device 4 is not limited to that in this embodiment. For example, the developing device 4 may be configured such that the regulating blade 44 is located below the developing roller 42, the developing roller 42 rotates in the direction opposite to that indicated by the arrow R2 in this embodiment, and the supply roller 43 rotates in the direction opposite to that indicated by the arrow R3 in this embodiment.

[0032] In addition, in this embodiment, the image forming apparatus 100 has a contact / separation mechanism 80 for controlling (switching) the contact / non-contact state (contact / separation state) between the photosensitive drum 1 and the developing roller 42. This prevents unnecessary contact between the photosensitive drum 1 and the developing roller 42 during non-image formation. During image formation, the developing roller 42 rotates while in contact with the surface of the photosensitive drum 1 at a development position A (hereinafter also referred to as "development contact"). During non-image formation, the developing roller 42 is spaced apart (hereinafter also referred to as "development separation") from the surface of the photosensitive drum 1 at a predetermined distance G (FIG. 2(b)). However, in this embodiment, even during non-image formation, the developing roller 42 rotates while in contact with the surface of the photosensitive drum 1 at the development position A during the inter-sheet step during continuous image formation and the preparatory operation described below. Furthermore, in this embodiment, when the developing roller 42 is spaced apart from the photosensitive drum 1 during non-image formation, the rotation of the developing roller 42 is stopped. The contact / separation mechanism 80 will be described in detail later.

[0033] 3. Control Configuration 3 is a block diagram showing an outline of the control configuration of image forming apparatus 100 of this embodiment. Image forming apparatus 100 has a control unit 60 that controls the operation of image forming apparatus 100. Control unit 60 has a CPU 61 as an arithmetic processing unit (arithmetic processing section), a memory 62 as a storage unit (storage section) composed of ROM, RAM, nonvolatile memory, etc., and an input / output unit (not shown) that sends and receives signals between control unit 60 and devices outside control unit 60. Control programs, control data, etc. are stored in ROM. Calculation results by CPU 61 and detection results by various sensors are saved in RAM. Various setting information, usage history information, etc. are saved in nonvolatile memory.

[0034] The control unit 60 is connected to each component of the image forming apparatus 100. For example, the control unit 60 is connected to a charging power supply E1, a developing power supply E2, a supply power supply E3, a regulating power supply E4, a transfer power supply E5, a drive unit 55, an exposure unit 3, a contact / separation mechanism 80, and the like. The control unit 60 receives a print signal (image information and a print instruction) transmitted from an external device such as a host computer, and controls each component of the image forming apparatus 100 to perform an image forming operation. The control unit 60 also controls each component of the image forming apparatus 100 to perform a preparatory operation involving the rotation of the developing roller 42 and the supply roller 43 during non-image formation, which will be described later. The "preparatory operation" is an operation for suppressing density unevenness caused by insufficient scraping of toner from the developing roller 42 by the supply roller 43.

[0035] The image forming apparatus 100 executes a job (printing operation), which is a series of operations that starts with a single start command and forms and outputs an image on one or more recording materials S. A job generally includes an image forming process, a pre-rotation process, a sheet-to-sheet process when forming images on multiple recording materials S, and a post-rotation process. The image forming process is a period during which an electrostatic image of the image to be actually formed on the recording material S, a toner image, and the toner image are formed and transferred, and this period is referred to as the image formation time. More specifically, the timing of the image formation process differs depending on the position where each of the electrostatic image formation, toner image formation, and toner image transfer processes is performed. The pre-rotation process is a period during which preparatory operations are performed before the image forming process, from when a start command is input until the actual start of image formation. The sheet-to-sheet process is a period corresponding to the interval between recording materials S when image formation is performed continuously on multiple recording materials S (continuous image formation). The post-rotation process is a period during which a tidying up operation (preparatory operation) is performed after the image forming process. Non-image formation refers to periods other than image formation, and includes the above-mentioned pre-rotation process, paper interval process, post-rotation process, and also the pre-multi-rotation process, which is a preparatory operation when the image forming device 100 is turned on or when it returns from a sleep state.

[0036] 4. Configuration and operation of the contact / separation mechanism Next, we will explain the configuration and operation of the contact / separation mechanism 80, which serves as a position movement mechanism that changes the position of the developing roller 42 in this embodiment. Figures 2(a) and (b) are schematic cross-sectional views of the developing device 4 in this embodiment, with Figure 2(a) showing the developing device 4 in a state where it is disposed at the contact position and Figure 2(b) showing the developing device 4 in a state where it is disposed at the separation position.

[0037] The contact / separation mechanism 80 has a lever 81 as an action receiving portion provided in the developing device 4, a moving portion 82 as an action portion provided in the apparatus main body 110 of the image forming apparatus 100, and a contact / separation driving portion 83 provided in the apparatus main body 110 for driving the moving portion 82. The developing device 4 is coupled to a frame (not shown) that fixes the position of the photosensitive drum 1 so as to be swingable about a rotation axis that is approximately parallel to the rotation axis direction of the photosensitive drum 1. The developing device 4 is biased in a direction that rotates the developing roller 42 so as to approach the photosensitive drum 1. In this embodiment, a tension spring (not shown) is provided as a biasing member serving as a biasing means, with both ends attached to the frame that fixes the position of the photosensitive drum 1 and the developing device 4, respectively. The contact / separation mechanism 80 can swing the developing device 4 by operating the moving unit 82 with the contact / separation drive unit 83 to move the lever 81, thereby moving the developing device 4 between the contact position (FIG. 2(a)) and the separation position (FIG. 2(b)). As shown in FIG. 2(a), when the developing device 4 is located at the contact position (first position), the developing roller 42 comes into contact with the photosensitive drum 1. Also, as shown in FIG. 2(b), when the developing device 4 is located at the separation position (second position), the developing roller 42 is separated from the photosensitive drum 1.

[0038] The movement of the developing device 4 from the separated position to the contact position is caused by the spring force of the tension spring and a rotational moment centered on the drive input to the developing device 4 when the developing device 4 is driven. When the contact / separation drive unit 83 moves the moving unit 82 in the direction of arrow P1 in the figure (in the direction toward the photosensitive drum 1), the lever 81 is held by the moving unit 82, and the state in which the developing device 4 is held at the separated position is released. Then, the spring force and rotational moment cause the developing device 4 to swing, and the developing roller 42 moves toward the photosensitive drum 1. This moves the developing device 4 to the contact position, and the developing roller 42 can be brought into contact with the photosensitive drum 1. Conversely, to move the developing device 4 from the contact position to the separated position, the contact / separation drive unit 83 moves the moving unit 82 in the direction of arrow P2 in the figure (in the direction away from the photosensitive drum 1), thereby moving the lever 81 in the same direction. As a result, the developing device 4 is moved to the separated position, and the developing device 4 is held at the separated position, so that the developing roller 42 can be separated from the photosensitive drum 1.

[0039] The movement of the moving unit 82 is achieved by the moving unit 82 receiving a driving force from a motor or a solenoid, which serves as a driving source provided in the contact / separation drive unit 83, via a drive transmission member provided in the contact / separation drive unit 83. In this embodiment, when the developing device 4 moves from the separated position to the contact position (contact operation), the holding force of the lever 81 by the moving unit 82 is released, and the developing roller 42 contacts the photosensitive drum 1 due to the spring biasing force. Therefore, in this embodiment, the developing roller 42 moves from the separated position to the contact position at a speed faster than the moving speed of the moving unit 82. On the other hand, when the developing device 4 moves from the contact position to the separated position (separation operation), the developing device 4 is moved by the movement of the moving unit 82. Therefore, in this embodiment, the developing roller 42 moves from the contact position to the separated position at a speed approximately equal to the moving speed of the moving unit 82.

[0040] In this embodiment, during image formation, the developing device 4 is disposed at the contact position, and the developing roller 42 is in contact with the photosensitive drum 1. Furthermore, in this embodiment, during non-image formation (for example, standby state, sleep state, power-off state) excluding the inter-sheet process during continuous image formation and the preparatory operation described below, the developing device 4 is disposed at the separated position, and the developing roller 42 is in a separated state from the photosensitive drum 1. In this way, by contacting the developing roller 42 with the photosensitive drum 1 only when necessary, wear on the photosensitive drum 1 can be suppressed and the performance of the photosensitive drum 1 can be maintained for a longer period of time.

[0041] Furthermore, in this embodiment, when the developing device 4 is placed in the separated position, a clutch (not shown) serving as a rotational drive transmission disconnecting means provided in the driving device 55 cuts off the transmission of the rotational drive force from the driving device 55 to the developing device 4. This stops the rotation of the developing roller 42 and the supply roller 43. In this way, by stopping the rotation of the developing roller 42 and the supply roller 43 when the developing device 4 is in the separated position, deterioration of the components of the developing device 4 and the toner 90 can be suppressed.

[0042] 5. Uneven density Next, density unevenness caused by improper scraping of developer from the developing member by the supplying member will be described.

[0043] As described above, for example, when the amount of toner 90 in the developer container 41 is large (the developer filling amount is large) and the drive speed v of the supply roller 43 during image formation is slow, the supply roller 43 may not properly scrape the toner 90 from the development roller 42 (herein simply referred to as "scraping failure"). This makes it difficult to form a uniform toner layer on the development roller 42, and density unevenness, an image defect in which the density of the developed image is not uniform, may occur. This image defect is also called a hazy image. Furthermore, as described above, in the conventional configuration, in order to suppress the occurrence of density unevenness, it may be necessary to rotate the development roller 42 and the supply roller 43 for a long time during non-image formation.

[0044] In other words, when the drive speed v of the supply roller 43 is slow, the amount of toner 90 discharged from the supply roller 43 per unit time decreases, making the toner 90 around the supply roller 43 less mobile and compacted. This prevents the toner 90 from being discharged from the supply roller 43 in the direction of arrow T1 toward the discharge area X shown in FIG. 2(a), resulting in an excessive amount of toner 90 contained within the supply roller 43. In this state, it becomes difficult for the supply roller 43 to scrape off the toner 90, resulting in a poor scraping condition, and the amount of toner 90 on the developing roller 42 before being regulated by the regulating blade 44 increases. As a result, it becomes difficult for the regulating blade 44 to form a uniform toner layer on the developing roller 42, which can result in uneven density, an image defect in which the density of the developed image is non-uniform.

[0045] In particular, if a long time elapses between image formation in low-speed mode and the next image formation, density unevenness is likely to occur. This is because, over time, the toner 90 solidifies due to its own weight, making it even more difficult to move. This further hinders the toner 90 from being discharged from the supply roller 43 to the discharge area X in the direction indicated by arrow T1 during the next image formation, accelerating the poor scraping condition. Furthermore, when image formation is performed continuously in low-speed mode, the toner 90 in the discharge area X becomes compacted, increasing the amount of toner 90 contained within the supply roller 43. Therefore, in this case as well, poor scraping is likely to occur, making density unevenness more likely to occur.

[0046] In order to prevent uneven density, it is effective to rotate the supply roller 43 at a drive speed v higher than that in the low-speed mode before forming an image. This is because it is possible to eliminate the compaction of the toner 90 that is promoted in the low-speed mode and to eliminate poor scraping. However, this requires that the rotation time of the development roller 42 and the supply roller 43 be extended before forming an image.

[0047] That is, when the supply roller 43 is rotated at a slow drive speed (first drive speed) v1 during image formation and then rotated at a fast drive speed (second drive speed faster than the first drive speed) v2 during non-image formation, a large amount of toner 90 is expelled from the supply roller 43. At this time, the pressure of the toner 90 momentarily increases in the expulsion area X, causing the expelled toner 90 to enter the nip N between the development roller 42 and the supply roller 43 from the expulsion area X. In this state, it is difficult for the cells on the surface of the supply roller 43 to open and scrape off the toner 90, which can result in uneven density. To resolve this poor scraping and prevent uneven density, the development roller 42 must be rotated to scrape out the toner 90 that has entered the nip N. However, if the toner 90 remains compacted after being scraped off, the toner 90 will continue to enter the nip N. Therefore, it is necessary to extend the rotation time of the developing roller 42 and the supply roller 43 not only to eliminate the compacted state of the toner 90 by rotating the supply roller 43 at a high driving speed v2, but also to scrape out the toner 90 that has entered the nip portion N.

[0048] Here, if the rotation time of the developing roller 42 and the supply roller 43 during non-image formation is increased in order to suppress the occurrence of density unevenness, downtime during which image output is not possible (user waiting time) will increase. Furthermore, there is a possibility that the toner 90 will deteriorate. On the other hand, simply shortening the rotation time of the developing roller 42 and the supply roller 43 during non-image formation will make it difficult to sufficiently suppress the occurrence of density unevenness. Thus, with the conventional configuration, it has been difficult to simultaneously shorten the rotation time of the developing roller 42 and the supply roller 43 during non-image formation and suppress the occurrence of density unevenness.

[0049] Therefore, it is necessary to reduce the rotation time of the developing roller 42 and the supply roller 43 when no image is being formed, while suppressing the occurrence of uneven density caused by the supply roller 43 not being able to scrape the toner 90 from the developing roller 42 properly.

[0050] 6. Preparatory Movement Next, the control of the "preparatory operation" as an operation mode during non-image formation for suppressing density unevenness in this embodiment will be described. Fig. 4 is a flowchart showing an outline of the control of the preparatory operation in this embodiment. The control unit 60 determines whether to perform the preparatory operation and performs the preparatory operation according to the procedure shown in Fig. 4.

[0051] First, when a print signal (job start instruction) is input to the image forming apparatus 100 (S101), the control unit 60 determines whether the image formation mode of the previous image formation was the low-speed mode (first mode) or the high-speed mode (second mode) (S102). If the control unit 60 determines that the image formation mode was the high-speed mode (No in S102), it executes image formation without performing a preparatory operation (S105). This is because a preparatory operation to suppress the occurrence of density unevenness is not necessary in this case. On the other hand, if the control unit 60 determines that the image formation mode was the low-speed mode (Yes in S102), it proceeds to determining the conditions for the next preparatory operation. This is because a state in which density unevenness occurs may occur due to the slow drive speed v of the supply roller 43.

[0052] The control unit 60 determines whether the elapsed time tp since the end of the previous image formation exceeds a predetermined threshold (elapsed time threshold) tth (S103). For example, the control unit 60 stores the timing (typically, the year, month, day, and time) of the end of the last image formation of the job in the memory 62 (RAM or nonvolatile memory) each time a job is executed. The control unit 60 then compares the timing (typically, the year, month, day, and time) of the input of the print signal with the timing of the end of the previous image formation to determine the elapsed time tp since the end of the previous image formation. Here, the threshold tth is preset and stored in the memory 62 (ROM or nonvolatile memory). The timing of the end of image formation is typically represented by the timing when the rotation of the photosensitive drum 1 stops upon the end of the job. However, this is not limited to this. For example, any timing that allows estimation of the unused time of the developing device 4 can be used, such as the timing when the rotation of the developing roller 42 and the supply roller 43 stops, or the timing when the recording material S on which the last image of the job is formed is discharged from the image forming apparatus 100. The method for estimating the idle time of the developing device 4 is not limited to the above method. For example, the control unit 60 may count the elapsed time from the time when image formation is completed. When the control unit 60 determines that the elapsed time tp is equal to or less than the threshold value tth (No in S103), the control unit 60 executes image formation without executing a preparatory operation (S105). This is because a preparatory operation to suppress density unevenness is unnecessary in this case. On the other hand, when the control unit 60 determines that the elapsed time tp exceeds the threshold value tth (Yes in S103), the control unit 60 executes a preparatory operation before image formation (S104). This is because density unevenness is likely to occur in this case. Then, the control unit 60 executes image formation after executing the preparatory operation (S105). In this embodiment, the threshold value tth is set to 6 hours. However, the present invention is not limited to this, and the threshold value tth can be appropriately set depending on, for example, the configuration of the image forming apparatus 100 (developing device 4) and the characteristics of the toner 90.

[0053] As described above, in this embodiment, the preparatory operation is performed before the next image formation only after image formation in low-speed mode, in which density unevenness is likely to occur. Also, in this embodiment, the preparatory operation is performed before the next image formation only when the elapsed time tp since the end of the previous image formation exceeds a predetermined threshold tth, in which case density unevenness is likely to occur. This makes it possible to suppress the occurrence of density unevenness while shortening the rotation time of the developing roller 42 and the supply roller 43, which is added for the preparatory operation, i.e., downtime (user waiting time).

[0054] However, the effects of the present invention are not limited to this control flow. For example, a preparatory operation may be performed when image formation in low-speed mode is performed continuously on more than a predetermined number of sheets. In this case, for example, a preparatory operation can be performed before image formation in a job following a job in which image formation in low-speed mode is performed continuously on more than a predetermined number of sheets. Also, for example, in a job in which image formation in low-speed mode is performed continuously on more than a predetermined number of sheets, a preparatory operation can be performed in the inter-sheet step after the predetermined number of images have been formed, and then image formation can be resumed. In this case, the inter-sheet step can be extended to perform the preparatory operation.

[0055] In this embodiment, the developing roller 42 and the supply roller 43 are driven to rotate by the same drive source of the drive device 55. In this embodiment, the developing roller 42 and the supply roller 43 start rotating at approximately the same time and stop rotating at approximately the same time. In this embodiment, the drive speed of the developing roller 42 during the preliminary operation is set in accordance with the drive speed v of the supply roller 43 so that the ratio of the circumferential speed of the supply roller 43 to the circumferential speed of the developing roller 42 is approximately constant. However, the effects of the present invention are not limited to such a configuration or setting. For example, the drive speed of the developing roller 42 may be constant regardless of the drive speed v of the supply roller 43.

[0056] The following describes the control of the preparatory operation in an example where the previous image formation was performed in low-speed mode and a preparatory operation is performed before the current image formation. It is also assumed that the photosensitive drum 1 is continuously driven to rotate from the start of a job operation until the end of the job operation. Therefore, when the developing roller 42 and the supply roller 43 are driven to rotate, the photosensitive drum 1 is also driven to rotate. It is also assumed that when the developing roller 42 contacts the photosensitive drum 1, the surface potential of the photosensitive drum 1 at the development position A is charged to the dark potential Vd. It is also assumed that when the developing roller 42 and the supply roller 43 rotate, a developing bias, a supply bias, and a regulating bias are applied to the developing roller 42, the supply roller 43, and the regulating blade 44, respectively.

[0057] 7. Control of the Example An example of control of the preparatory operation that can be performed by the image forming apparatus 100 of this embodiment will be described.

[0058] Example 1 5 is a timing chart of the preparatory operation in Example 1. In the timing chart, the elapsed time tp while the image forming apparatus 100 is stopped is shown shortened (the same applies to the other examples and comparative examples).

[0059] During the previous image formation, the supply roller 43 was driven to rotate at a first drive speed (80 rpm) corresponding to the low-speed mode, and the developing roller 42 was driven to rotate at a speed corresponding to the low-speed mode. The rotational drive of the supply roller 43 and the developing roller 42 was stopped at the end of the previous image formation. Furthermore, at the end of the previous image formation, the developer separation operation was started while the developing roller 42 and the supply roller 43 were rotating. The developer separation operation ended and the developing device 4 was positioned in the separation position, and the rotation of the developing roller 42 and the supply roller 43 stopped approximately simultaneously. Furthermore, after the elapsed time tp, when a print signal is input to the image forming apparatus 100, a determination is made as to whether or not to perform a preparatory operation, and the preparatory operation is performed. The time from the start timing tS to the end timing tE of the preparatory operation is downtime (user waiting time). In this embodiment, this time is approximately the same as the rotation time of the developing roller 42 and the supply roller 43 during the preparatory operation. These are also assumed to be the same in the other examples and comparative examples described below.

[0060] In Example 1, the developing roller 42 is rotationally driven during the preparatory operation. Also, in Example 1, the supply roller 43 is rotationally driven by increasing the drive speed v in multiple stages during the preparatory operation. First, the supply roller 43 is rotationally driven for 5 seconds at a second drive speed (160 rpm) higher than the first drive speed (80 rpm). The first drive speed (80 rpm) is the drive speed v corresponding to the low-speed mode and is the drive speed v used in the previous image formation. Thereafter, the supply roller 43 is rotationally driven for 15 seconds at a third drive speed (240 rpm) higher than the second drive speed (160 rpm). Therefore, in Example 1, the total rotation time of the developing roller 42 and the supply roller 43 during the preparatory operation is 20 seconds. Also, during the preparatory operation, the contact / separation mechanism 80 releases the development device 4 from the separated position slightly after the development roller 42 and the supply roller 43 start to rotate, and the development contact operation is completed at timing t1. At this time, the toner 90 in the developing device 41 is vibrated once.

[0061] The drive speed v of the supply roller 43 during the preliminary operation may be higher than the drive speed v of the supply roller 43 during the previous image formation, and may be higher than the drive speed v of the supply roller 43 during the previous image formation. Typically, at least one drive speed v (such as the highest drive speed v) is the drive speed v during image formation in high-speed mode (the highest drive speed v during image formation). However, the highest drive speed v among the multiple drive speeds v of the supply roller 43 during the preliminary operation may be higher or lower than the drive speed v during image formation in high-speed mode. Also, typically, the period during which the supply roller 43 is driven at the highest drive speed v among the multiple drive speeds v of the supply roller 43 during the preliminary operation is set longer than the periods during which the supply roller 43 is driven at the other drive speeds v. This reduces the speed difference before and after changing the drive speed v, as described below, and effectively eliminates the consolidation of the toner 90 in the discharge area X. Also, when the drive speed v of the supply roller 43 is increased in multiple stages during the preliminary operation, the number of stages is not limited to two and may be three or more (typically five or fewer).

[0062] The first effect of the preliminary operation of the first embodiment will be described. By increasing the drive speed v of the supply roller 43 to a value higher than the first drive speed (80 rpm) used during image formation in the previous low-speed mode, the consolidation of the toner 90 in the discharge area X can be eliminated, thereby eliminating the poor scraping condition. In other words, by increasing the drive speed v of the supply roller 43 in this manner, the amount of toner 90 discharged from the supply roller 43 per unit time increases compared to when image formation was performed in the low-speed mode, making it easier for the toner 90 in the developer container 41 to move. As a result, the consolidation of the toner 90 in the discharge area X is eliminated, the amount of toner 90 contained in the foam layer 43b of the supply roller 43 decreases, and the toner 90 on the developer roller 42 can be sufficiently scraped off.

[0063] Next, a second effect of the preparatory operation of the first embodiment will be described. In the first embodiment, by increasing the drive speed v of the supply roller 43 in multiple stages, the speed difference before and after the change in drive speed v can be dispersed and reduced. As a result, the amount of toner 90 instantaneously discharged from the supply roller 43 to the discharge area X is reduced, and the instantaneous pressure change in the discharge area X can be reduced. This makes it possible to suppress the intrusion of toner 90 from the discharge area X into the nip N. This eliminates the need to extend the rotation time of the development roller 42 in order to scrape out the toner 90 that has entered the nip N and eliminate the scraping failure state. Therefore, the rotation times of the development roller 42 and the supply roller 43 can be shortened.

[0064] In this way, in the first embodiment, by increasing the drive speed v of the supply roller 43 in multiple stages in the preliminary operation, it is possible to suppress the occurrence of density unevenness even when the rotation time of the development roller 42 and the supply roller 43 is short.

[0065] <Example 2> FIG. 6 is a timing chart of the preparatory operation in the second embodiment.

[0066] In the second embodiment, the driving speed v of the supply roller 43 is not changed in multiple steps in the preliminary operation, which was performed in the first embodiment to suppress the occurrence of density unevenness in a short time. In the second embodiment, the supply roller 43 is rotated and driven at a driving speed v of 240 rpm corresponding to the high-speed mode from the beginning in the preliminary operation. This driving speed (240 rpm) is a higher driving speed v than the driving speed (80 rpm) corresponding to the low-speed mode in the previous image formation. On the other hand, in the second embodiment, in order to suppress the occurrence of density unevenness in a short time, a developer contact operation is performed multiple times as a position movement operation during the preliminary operation. When this developer contact operation is performed, the developing roller 42 is rotated and driven. In the second embodiment, the developer contact operation is performed a total of four times at timings t1 to t4. Before the developer contact operation at timings t2 to t4, a developer separation operation is performed so that the developer contact operation can be performed. In Example 2, the rotation time of the developing roller 42 and the supply roller 43 from the completion of one development contact operation to the completion of the next development contact operation (until the rotation of the developing roller 42 and the supply roller 43 stops in the case of the last development contact operation) is 5 seconds. Therefore, in Example 2, the rotation time of the developing roller 42 and the supply roller 43 in the preliminary operation is 20 seconds in total.

[0067] The number of times of development contact operations in the preliminary operation is not limited to four, and can be set appropriately in consideration of the effect of suppressing density unevenness due to the preliminary operation and the effect of reducing the rotation time of the development roller 42 and the supply roller 43 in the preliminary operation. The number of times is preferably two or more and about ten or less, and is typically two or more and five or less.

[0068] The first effect of the preliminary operation of the second embodiment will be described. By increasing the drive speed v of the supply roller 43 from 80 rpm during image formation in the previous low-speed mode to 240 rpm, the consolidation of the toner 90 in the discharge area X can be eliminated, thereby eliminating the poor scraping condition. In other words, by increasing the drive speed v of the supply roller 43 in this manner, the amount of toner 90 discharged from the supply roller 43 per unit time increases compared to when image formation was performed in the low-speed mode, making it easier for the toner 90 in the developer container 41 to move. As a result, the consolidation of the toner 90 in the discharge area X is eliminated, the amount of toner 90 contained in the foam layer 43b of the supply roller 43 decreases, and the toner 90 on the developer roller 42 can be sufficiently scraped off.

[0069] Next, a second effect of the preliminary operation of the second embodiment will be described. In the second embodiment, multiple developer contact operations are performed to vibrate the toner 90 in the developer container 41, thereby loosening the toner 90 and further reducing the compaction of the toner 90 in the discharge area X. If the speed difference before and after the change in the drive speed v of the supply roller 43 is large, a momentary pressure change in the discharge area X causes toner 90 to enter the nip N from the discharge area X, resulting in poor scraping. However, if the development roller 42 and the supply roller 43 are rotated alone, a long rotation time for the development roller 42 and the supply roller 43 is required to resolve this. By contrast, multiple developer contact operations further reduce the compaction of the toner 90, thereby reducing the amount of toner 90 that subsequently enters the nip N. Therefore, the toner 90 that has entered the nip N from the discharge area X is scraped back into the discharge area X, quickly eliminating the poor scraping.

[0070] In this way, in Example 2, by increasing the driving speed v of the supply roller 43 in the preliminary operation and performing the development contact operation multiple times, it is possible to suppress the occurrence of density unevenness even with a short rotation time of the development roller 42 and the supply roller 43.

[0071] Example 3 FIG. 7 is a timing chart of the preparatory operation in the third embodiment.

[0072] In the third embodiment, similarly to the first embodiment, the supply roller 43 is rotated at a driving speed v that is increased in multiple stages during the preliminary operation. First, the supply roller 43 is rotated for 5 seconds at a second driving speed (160 rpm) that is higher than the first driving speed (80 rpm). The first driving speed (80 rpm) is the driving speed v that corresponds to the low-speed mode and is the driving speed v used in the previous image formation. Thereafter, the supply roller 43 is rotated for 5 seconds at a third driving speed (240 rpm) that is higher than the second driving speed (160 rpm).

[0073] Furthermore, in Example 3, similar to Example 2, in order to suppress the occurrence of density unevenness in a short time, the developer contact operation is performed multiple times during the preparatory operation. When this developer contact operation is performed, the developing roller 42 is rotationally driven. In Example 3, the developer contact operation is performed a total of two times at timings t1 and t2. Before the developer contact operation at timing t2, a developer separation operation is performed so that the developer contact operation can be performed. In Example 3, the rotation time of the developing roller 42 and the supply roller 43 from the completion of one developer contact operation to the completion of the next developer contact operation (in the case of the last developer contact operation, until the rotation of the developing roller 42 and the supply roller 43 stops) is 5 seconds each. Therefore, in Example 3, the rotation time of the developing roller 42 and the supply roller 43 during the preparatory operation is 10 seconds in total.

[0074] In this way, in the third embodiment, when the development contact operation is performed multiple times during the preliminary operation, the driving speed v of the supply roller 43 is increased as the number of development contact operations increases. In the third embodiment, the total rotation time of the development roller 42 and the supply roller 43 during the preliminary operation is set shorter than in the first and second embodiments.

[0075] The first effect of the preliminary operation of the third embodiment will be described. By increasing the drive speed v of the supply roller 43 to a value higher than the first drive speed (80 rpm) used during image formation in the previous low-speed mode, the consolidation of the toner 90 in the discharge area X can be eliminated, thereby eliminating the poor scraping condition. In other words, by increasing the drive speed v of the supply roller 43 in this manner, the amount of toner 90 discharged from the supply roller 43 per unit time increases compared to when image formation was performed in the low-speed mode, making it easier for the toner 90 in the developer container 41 to move. As a result, the consolidation of the toner 90 in the discharge area X is eliminated, the amount of toner 90 contained in the foam layer 43b of the supply roller 43 decreases, and the toner 90 on the developer roller 42 can be sufficiently scraped off.

[0076] Next, a second effect of the preparatory operation of the third embodiment will be described. In the third embodiment, by increasing the drive speed v of the supply roller 43 in multiple stages, the speed difference before and after the change in drive speed v can be dispersed and reduced. This reduces the amount of toner 90 instantaneously discharged from the supply roller 43 to the discharge area X, thereby reducing the instantaneous pressure change in the discharge area X. This in turn reduces the intrusion of toner 90 from the discharge area X into the nip N. This eliminates the need to extend the rotation time of the development roller 42 in order to scrape out the toner 90 that has entered the nip N and eliminate the scraping failure. This makes it possible to shorten the rotation time of the development roller 42 and the supply roller 43.

[0077] Next, a third effect of the preliminary operation of the third embodiment will be described. In the third embodiment, multiple developer contact operations are performed to vibrate the toner 90 in the developer container 41, thereby loosening the toner 90 and further reducing the compaction of the toner 90 in the discharge area X. If the speed difference before and after the change in the drive speed v of the supply roller 43 is large, a momentary pressure change in the discharge area X causes toner 90 to enter the nip N from the discharge area X, resulting in poor scraping. However, if the development roller 42 and the supply roller 43 are rotated alone, a long rotation time for the development roller 42 and the supply roller 43 is required to resolve this. By contrast, multiple developer contact operations further reduce the compaction of the toner 90, thereby reducing the amount of toner 90 that subsequently enters the nip N. Therefore, the toner 90 that has entered the nip N from the discharge area X is scraped back into the discharge area X, quickly eliminating the poor scraping.

[0078] In the preliminary operation, the time from when the drive speed v of the supply roller 43 is changed to the second drive speed (160 rpm) to when the immediately subsequent development contact operation is completed is defined as td1. Furthermore, the time from when the drive speed v of the supply roller 43 is changed to the third drive speed (240 rpm) to when the immediately subsequent development contact operation is completed is defined as td2. Here, the timing at which the drive speed v of the supply roller 43 is changed to a predetermined drive speed v is represented by the timing at which the change in the drive speed of the supply roller 43 is started (or the timing at which driving is started) with the predetermined drive speed v as the target. However, in this embodiment, the time from when the change in the drive speed v of the supply roller 43 is started to when the change is completed is sufficiently short compared, for example, with the time from when the development contact operation is started to when the development contact operation is completed. In this case, it is desirable that the times td1 and td2 each be within two revolutions of the supply roller 43. This is because the instantaneous pressure increase in the discharge area X when the drive speed v of the supply roller 43 is increased is high during the period corresponding to two rotations of the supply roller 43 after the drive speed v of the supply roller 43 is increased. During the period corresponding to one rotation of the supply roller 43 after the drive speed v of the supply roller 43 is increased, a large amount of toner 90 is discharged, causing the pressure in the discharge area X to increase. It then takes time for the pressure to decrease, at least for one rotation of the supply roller 43. Therefore, by applying vibration to the toner 90 due to the developer contact operation during this period, the pressure in the discharge area X can be more effectively reduced. In Example 3, the times td1 and td2 were each set to 0.3 seconds. In Example 3, even at the third drive speed (240 rpm), which is the fastest drive speed v of the supply roller 43 during the preliminary operation, the time corresponding to two rotations of the supply roller 43 is 0.5 seconds. Therefore, in Example 3, the times td1 and td2 satisfy the above relationship.

[0079] Furthermore, during the preparatory operation, the time for which the supply roller 43 is rotationally driven at the second driving speed (160 rpm) and the time for which the supply roller 43 is rotationally driven at the third driving speed (240 rpm) are each preferably equal to or greater than two revolutions of the supply roller 43. This is because the occurrence of density unevenness can be more effectively suppressed by waiting for the momentary increase in pressure in the discharge area X, which occurs when the driving speed v of the supply roller 43 is increased, to settle down before the next speed change or image formation is performed. In the third embodiment, the time for which the supply roller 43 is rotationally driven at the second driving speed (160 rpm) and the time for which the supply roller 43 is rotationally driven at the third driving speed (240 rpm) are each 5 seconds. In the third embodiment, even at the second driving speed (160 rpm), which is the slowest driving speed v of the supply roller 43 during the preparatory operation, two revolutions of the supply roller 43 are 0.75 seconds. Therefore, in the third embodiment, the time for which the supply roller 43 is rotationally driven at each driving speed satisfies the above relationship. Although not limited to this, in Example 3, the time for rotating the supply roller 43 at each drive speed is often sufficient if it is 50 revolutions or less of the supply roller 43, and from the viewpoint of reducing downtime, 30 revolutions or less is preferable, and typically 20 revolutions or less.

[0080] In the second embodiment, the time from when the drive speed v of the supply roller 43 is changed in the preliminary operation until the development contact operation immediately thereafter is completed can be set in the same manner as td1 and td2 in the third embodiment. In the second embodiment, the rotation time of the supply roller 43 from when one development contact operation is completed until the next development contact operation is completed can be set in the same manner as the rotation time at each drive speed v in the third embodiment.

[0081] In this way, in Example 3, when the development contact operation is performed multiple times during the preliminary operation, the driving speed v of the supply roller 43 is increased as the number of development contact operations increases, thereby making it possible to suppress the occurrence of density unevenness even with a short rotation time of the development roller 42 and the supply roller 43.

[0082] 8. Evaluation Results The control results were evaluated for Examples 1 to 3 using the following method. For comparison with Examples 1 to 3, the following Comparative Examples 1 to 5 were also evaluated using the same method. Note that in the image forming apparatuses of the Comparative Examples, elements having the same or corresponding functions or configurations as elements of the image forming apparatuses of the Examples will be described using the same reference numerals.

[0083] In an environment with a temperature of 15°C and a relative humidity of 10%, 100 consecutive images were formed in high-speed mode with a print rate of 1%. Then, 10 consecutive solid white images were formed in low-speed mode, and 12 hours later, a halftone image with a density of 25% was formed in low-speed mode as an evaluation image for density unevenness. Density unevenness was judged visually based on the following criteria. ○: No uneven density ×: Density unevenness occurs

[0084] <Comparative Example 1> Comparative Example 1 is an example in which the preparatory operation was not performed in the image forming apparatus 100 having substantially the same configuration as the present embodiment.

[0085] <Comparative Example 2> Comparative Example 2 is an example in which the following preparatory operation is performed in an image forming apparatus 100 having substantially the same configuration as the present embodiment. FIG. 8 is a timing chart of the preparatory operation in Comparative Example 2. Unlike Example 1, in Comparative Example 2, in the preparatory operation, the supply roller 43 is rotated and driven from the beginning at a drive speed v of 240 rpm, which corresponds to the high-speed mode, compared to 80 rpm during the previous image formation in the low-speed mode. The drive speed v of the supply roller 43 is constant during the preparatory operation. The rotation time of the developing roller 42 and the supply roller 43 during the preparatory operation is 20 seconds, the same as in Examples 1 and 2. Before the image forming apparatus 100 receives a print signal and starts the preparatory operation, the developing device 4 is in the separated position. In Comparative Example 2, unlike Example 2, the development contact operation is performed only once, immediately after the development roller 42 and the supply roller 43 start to rotate, which is necessary for the rotation of the development roller 42 and the supply roller 43.

[0086] <Comparative Example 3> Comparative Example 3 is an example in which the following preparatory operation is performed in an image forming apparatus 100 having substantially the same configuration as the present embodiment. FIG. 9 is a timing chart of the preparatory operation in Comparative Example 3. In Comparative Example 3, as in Comparative Example 2, in the preparatory operation, the supply roller 43 is rotated and driven from the beginning at a drive speed v of 240 rpm, which corresponds to the high-speed mode, compared to 80 rpm during the previous image formation in the low-speed mode. The drive speed v of the supply roller 43 is constant during the preparatory operation. The rotation time of the developing roller 42 and the supply roller 43 during the preparatory operation is 10 seconds, the same as in Example 3. Before the image forming apparatus 100 receives a print signal and starts the preparatory operation, the developing device 4 is in the separated position. In Comparative Example 3, unlike Example 3, the development contact operation is performed only once, immediately after the development roller 42 and the supply roller 43 start to rotate, which is necessary for the rotation of the development roller 42 and the supply roller 43.

[0087] <Comparative Example 4> Comparative Example 4 is an example in which the following preparatory operation is performed in an image forming apparatus 100 having substantially the same configuration as this embodiment. FIG. 10 is a timing chart of the preparatory operation in Comparative Example 4. In Comparative Example 4, as in Comparative Examples 2 and 3, in the preparatory operation, supply roller 43 is driven to rotate at a drive speed v of 240 rpm corresponding to the high-speed mode from the beginning, as opposed to 80 rpm during the previous image formation in the low-speed mode. The drive speed v of supply roller 43 is constant during the preparatory operation. The rotation time of developing roller 42 and supply roller 43 in the preparatory operation is 30 seconds, which is longer than in Examples 1 to 3, Comparative Example 2, and Comparative Example 3.

[0088] <Comparative Example 5> Comparative Example 5 is an example in which the following preparatory operation is performed in an image forming apparatus 100 having substantially the same configuration as this embodiment. FIG. 11 is a timing chart of the preparatory operation in Comparative Example 5. In Comparative Example 5, the drive speed v of supply roller 43 in the preparatory operation is different from that in Comparative Example 4. In Comparative Example 4, in the preparatory operation, supply roller 43 is driven to rotate at a drive speed v of 160 rpm, compared to 80 rpm during the previous image formation in low-speed mode. The drive speed v of supply roller 43 during the preparatory operation is constant. The rotation time of developing roller 42 and supply roller 43 in the preparatory operation is 30 seconds, the same as in Comparative Example 4.

[0089] <Comparison between Examples 1 to 3 and Comparative Examples 1 to 5> Table 1 shows the evaluation results of density unevenness for Examples 1 to 3 and Comparative Examples 1 to 5, and the value of the time (downtime) that the start of image formation was delayed due to the preparatory operation. Note that this downtime value is the same as the rotation time of the developing roller 42 and the supply roller 43 that was added by performing the preparatory operation.

[0090] [Table 1]

[0091] As shown in Table 1, in Comparative Example 1, no downtime occurred because no preparatory operation was performed, but density unevenness occurred. In Comparative Example 1, the drive speed v of supply roller 43 during the previous image formation was slow, so toner 90 in discharge area X became compacted, increasing the amount of toner contained in supply roller 43, resulting in poor scraping and causing density unevenness.

[0092] Furthermore, in Comparative Example 4, a preliminary operation was performed, so no density unevenness occurred, but in Comparative Examples 2 and 3, density unevenness occurred despite the preliminary operation. The duration of the preliminary operation in Comparative Examples 2 and 3 was shorter than that in Comparative Example 4. Therefore, it was not possible to prevent toner 90 from entering the nip N from the discharge area X due to a momentary pressure change in the discharge area X when the drive speed v of the supply roller 43 was increased. This resulted in poor scraping, which is thought to be the cause of density unevenness.

[0093] Furthermore, in Comparative Example 5, a preliminary operation was performed, but density unevenness occurred. In Comparative Example 5, the drive speed v of supply roller 43 during the preliminary operation was slower than in Comparative Example 4. Therefore, the speed difference between the drive speed v of supply roller 43 during the previous image formation and the drive speed v of supply roller 43 during the preliminary operation was small, making it difficult for toner 90 to enter nip N from discharge area X. However, because the drive speed v of supply roller 43 was slow, the compacted state of toner 90 in discharge area X could not be resolved, and the amount of toner contained in supply roller 43 remained large, resulting in poor scraping and density unevenness.

[0094] In Comparative Example 4, the drive speed v of supply roller 43 during the preparatory operation is high, and the rotation time of development roller 42 and supply roller 43 is sufficiently long. This prevents the compaction of toner 90 in discharge area X and the intrusion of toner 90 from discharge area X into nip N, preventing density unevenness. However, to prevent density unevenness caused by toner 90 intrusion from discharge area X into nip N, it is necessary to lengthen the rotation time of development roller 42 and supply roller 43 during the preparatory operation.

[0095] On the other hand, in Example 1, even with the same downtime as Comparative Example 2, which was shorter than Comparative Examples 4 and 5, no density unevenness occurred. In Example 1, the driving speed v of the supply roller 43 is increased during the preliminary operation, thereby eliminating the compacted state of the toner 90 in the discharge area X. Furthermore, in Example 1, the driving speed v of the supply roller 43 is increased in multiple stages, thereby dispersing and reducing the speed difference. This reduces the instantaneous pressure change in the discharge area X and prevents the toner 90 from entering the nip N from the discharge area X. As a result, Example 1 can reduce downtime more than Comparative Example 4. In this way, Example 1 can reduce the rotation time of the developing roller 42 and supply roller 43, which are added during non-image formation, more than Comparative Example 4, while suppressing the occurrence of density unevenness.

[0096] Furthermore, in Example 2, even with the same downtime as Comparative Example 2, which was shorter than Comparative Example 4, no density unevenness occurred. In Example 2, the driving speed v of the supply roller 43 is increased in the preparatory operation, thereby eliminating the compacted state of the toner 90 in the discharge area X. In Example 2, the compacted state of the toner 90 in the discharge area X can be further reduced by vibrating the toner 90 in the developer container 41 through multiple developer contact operations in the preparatory operation. This reduces the time required for the toner 90 that has entered the nip portion N from the discharge area X to be scraped back into the discharge area X by the rotation of the developing roller 42, thereby reducing downtime compared to Comparative Example 4. In this way, in Example 2, the occurrence of density unevenness can be suppressed while shortening the rotation time of the developing roller 42 and the supply roller 43, which are added during non-image formation, compared to Comparative Example 4.

[0097] Furthermore, in Example 3, even with the same downtime as Comparative Example 3, which was shorter than Comparative Examples 2, 4, and 5, no density unevenness occurred. Furthermore, Example 3 also achieved a shorter downtime than Examples 1 and 2. In Example 3, the driving speed v of the supply roller 43 is increased during the preliminary operation, thereby eliminating the consolidation of the toner 90 in the discharge area X. Furthermore, in Example 3, the driving speed v of the supply roller 43 is increased in multiple stages, thereby dispersing and reducing the speed difference. This reduces the instantaneous pressure change in the discharge area X and prevents the toner 90 from entering the nip N from the discharge area X. As a result, Example 3 achieves a shorter downtime than Comparative Example 4, Example 1, and Example 2. Furthermore, in Example 3, the consolidation of the toner 90 in the discharge area X is further reduced by vibrating the toner 90 in the developer container 41 through multiple developer contact operations during the preliminary operation. This makes it possible to reduce the time required for the toner 90 that has entered the nip portion N from the discharge area X to be scraped back into the discharge area X by the rotation of the developing roller 42, thereby reducing downtime more than in Comparative Example 4, Example 1, and Example 2. In this way, in Example 3, it is possible to reduce the rotation time of the developing roller 42 and the supply roller 43 that are added during non-image formation more than in Comparative Example 4, Example 1, and Example 2, while suppressing the occurrence of density unevenness.

[0098] 9. Action and Effects As described above, in this embodiment, image forming apparatus 100 includes image carrier (photosensitive drum) 1 on whose surface an electrostatic image is formed, developing member (developing roller) 42 that rotates to supply toner to the electrostatic image, developer container 41 that stores toner, supply member (supply roller) 43 that has a foam layer 43b that contacts developing member 42 to form nip portion N and rotates to supply toner stored in developer container 41 to developing member 42, regulating member (regulating blade) 44 that regulates the amount of toner supplied to developing member 42 by supply member 43, drive unit (drive device) 55 that rotationally drives developing member 42 and supply member 43, and control unit 60 that can control drive unit 55. In this embodiment, control unit 60 can control drive unit 55 to perform image formation, in which developing member 42 supplies toner to the electrostatic image to develop the electrostatic image and form a toner image on the surface of image carrier 1, in a first mode in which the electrostatic image is developed while supply member 43 is rotationally driven at a first drive speed. Furthermore, in this embodiment, when a first image formation is performed in the above-mentioned first mode and a second image formation is performed following the first image formation, the control unit 60 can control the drive unit 55 to perform a preliminary operation after the first image formation and before the second image formation, in which the developing member 42 is rotationally driven and the supply member 43 is rotationally driven at a second drive speed (e.g., 160 rpm) higher than the first drive speed (e.g., 80 rpm) during a first period (e.g., a period when the drive speed v in Figure 5 is 160 rpm), and the developing member 42 is rotationally driven and the supply member 43 is rotationally driven at a third drive speed (e.g., 240 rpm) higher than the second drive speed during a second period after the first period (e.g., a period when the drive speed v in Figure 5 is 240 rpm) (Examples 1 and 3). In this embodiment, the control unit 60 can control the drive unit 55 to rotate the supply member 43 at the same drive speed as the highest drive speed during image formation (for example, 240 rpm corresponding to high-speed mode) during the first period or the second period of the preparatory operation. Furthermore, the control unit 60 can control the drive unit 55 so that the second period during the preparatory operation is longer than the first period.

[0099] In addition, in this embodiment, the image forming apparatus 100 has a position movement mechanism (contact / separation mechanism) 80 that can move the position of the developing member 42 relative to the image carrier 1 between a first position (contact position) and a second position (separated position) where the distance between the developing member 42 and the image carrier 1 is different from the first position. In this embodiment, when a first image formation is performed in the first mode and a second image formation is performed following the first image formation, the control unit 60 can control the drive unit 55 and the position movement mechanism 80 to perform a preliminary operation of performing a first position movement operation (e.g., a development contact operation at timing t1 in Figure 6) to move the position of the developing member 42 from the second position to the first position, and a second position movement operation (e.g., a development contact operation at timing t2 in Figure 6) to move the position of the developing member 42 from the second position to the first position after the first position movement operation, while rotating the developing member 42 and rotating the supply member 43 at a driving speed (e.g., 240 rpm) higher than the first driving speed (e.g., 80 rpm) after the first image formation and before the second image formation (Examples 2 and 3). Furthermore, the control unit 60 can control the drive unit 55 to rotate the supply member 43 at a second drive speed (e.g., 160 rpm) higher than the first drive speed (e.g., 80 rpm) when performing the first position movement operation during the preparatory operation, and to rotate the supply member 43 at a third drive speed (e.g., 240 rpm) higher than the second drive speed when performing the second position movement operation (Example 3). In this case, the control unit 60 can control the drive unit 55 and the position movement mechanism 80 so that, during the preparatory operation, the first position movement operation is completed at a timing (e.g., timing t1 in FIG. 7) after the drive speed of the supply member 43 is changed to the second drive speed, and the second position movement operation is completed at a timing (e.g., timing t2 in FIG. 7) after the drive speed of the supply member 43 is changed to the third drive speed.In this case, the control unit 60 can control the drive unit 55 and the position movement mechanism 80 so that, during the preparatory operation, the time from when the drive speed of the supply member 43 is changed to the second drive speed to when the first position movement operation is completed and the time from when the drive speed of the supply member 43 is changed to the third drive speed to when the second position movement operation is completed are each within the time required for the supply member 43 to make two revolutions. The control unit 60 can also control the drive unit 55 so that, during the preparatory operation, the time for which the supply member 43 is rotationally driven at the second drive speed and the time for which the supply member 43 is rotationally driven at the third drive speed are each the time required for the supply member 43 to make at least two revolutions. In this embodiment, the control unit 60 can control the drive unit 55 to rotate the supply member 43 at the same drive speed as the highest drive speed during image formation (for example, 240 rpm corresponding to high-speed mode) during at least one period of the preparatory operation. In this embodiment, the first position is a position where the developing member 42 and the image carrier 1 are in contact with each other, and the second position is a position where the developing member 42 and the image carrier 1 are separated from each other.

[0100] Furthermore, in this embodiment, the control unit 60 can control the preparatory operation not to be performed before the second image formation if the elapsed time since the first image formation is performed is a first time, and to be performed before the second image formation if the elapsed time since the first image formation is performed is a second time that is longer than the first time.

[0101] In addition, in this embodiment, the control unit 60 can control the drive unit 55 to perform the image formation in the first mode (low speed mode) and a second mode (high speed mode) in which an electrostatic image is developed while the supply member 43 is rotationally driven at a drive speed (e.g., 240 rpm) higher than the first drive speed (e.g., 80 rpm), and can control the drive unit 55 so that when the first image formation is in the second mode, a preliminary operation is not performed before the second image formation.

[0102] As described above, according to this embodiment, it is possible to reduce the rotation time of the developing roller 42 and the supply roller 43 when no image is being formed, while suppressing the occurrence of density unevenness caused by the supply roller 43 not properly scraping the toner 90 from the developing roller 42. This reduces downtime (the waiting time of the user) and extends the life of the developing device 4 by suppressing deterioration of the components and toner of the developing device 4.

[0103] In this embodiment, the execution conditions are set so that the preparatory operation is performed only before the next image formation in the low-speed mode, which is a mode in which density unevenness is likely to occur. This makes it possible to reduce the rotation time of the developing roller 42 and the supply roller 43, which are added during non-image formation, while suppressing the occurrence of density unevenness.

[0104] In this embodiment, the execution conditions are set so that the preparatory operation is performed only before image formation after the elapsed time since the previous image formation exceeds a predetermined threshold value tth, which is the time when density unevenness is likely to occur. This makes it possible to reduce the rotation time of the developing roller 42 and the supply roller 43, which are added when no image is being formed, while suppressing the occurrence of density unevenness.

[0105] [Other embodiments] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0106] In the above-described embodiment, the developer contact operation is used as the position movement operation. This allows the toner 90 in the developer container 41 to be efficiently vibrated over substantially the entire area in the direction of the rotation axis of the developer roller 42 via the developer roller 42. However, the effects of the present invention are not limited to this. For example, the developer separation operation may be used as the position movement operation to vibrate the toner 90 in the developer container 41. In this case, for example, the image forming apparatus 100 may be provided with an abutting member that the developing device 4 abuts against when the developing roller 42 moves in a direction away from the photosensitive drum 1. This allows the toner 90 in the developer container 41 to be vibrated via a member (such as the developer container 41) other than the developing roller 42 that abuts against the abutting member. Even if an abutting member is not provided, the toner 90 in the developer container 41 can be sufficiently vibrated, for example, when the moving speed of the developing device 4 due to the moving part 82 of the abutment / separation mechanism 80 is increased or decreased (stopped) sufficiently rapidly during the developer separation operation.

[0107] In addition, the execution conditions for the preliminary operation, the driving speed v of the supply roller 43, the rotation time of the development roller 42 and the supply roller 43, and the number of position movement operations can be set appropriately depending on the configuration of the development device 4, the fluidity of the toner 90, the remaining amount of toner 90 in the development container 41, etc.

[0108] In the above embodiment, the image forming apparatus is a monochrome image forming apparatus, but the present invention can also be applied to a color image forming apparatus. For example, in a configuration having a plurality of image forming units, each having an image carrier and a developing device, the present invention can be applied to the developing device of each image forming unit.

[0109] In addition, in the above-described embodiment, the developing roller and the supply roller are described as being rotated by a driving force transmitted from a common driving source, but it is also possible to provide independent driving units that drive the developing roller and the supply roller, respectively. [Explanation of symbols]

[0110] 1 Photosensitive drum 4. Developing device 41 Developer container 42 Developing roller 43 Supply roller 44 Regulatory Blade 60 Control Unit 80 Contact / separation mechanism 90 toner

Claims

1. an image carrier on whose surface an electrostatic image is formed; a developing member that rotates to supply toner to the electrostatic image; a supply member that has a foam layer that contacts the developing member to form a nip portion, and that rotates to supply toner to the developing member; a drive unit that rotates the developing member and the supply member; a control unit capable of controlling the drive unit; and The control unit the drive unit is capable of controlling image formation in a first mode in which the developing member supplies toner to the electrostatic image to develop the electrostatic image and form a toner image on the surface of the image carrier, the image formation being performed in a first mode in which the electrostatic image is developed while the supply member is rotationally driven at a first drive speed; an image forming apparatus capable of controlling the drive unit to perform a preparatory operation in which, when a first image formation is performed in the first mode and a second image formation is performed following the first image formation, the developing member is rotationally driven and the supply member is rotationally driven at a second drive speed higher than the first drive speed during a first period after the first image formation and before the second image formation is performed, and the developing member is rotationally driven and the supply member is rotationally driven at a third drive speed higher than the second drive speed during a second period after the first period and after the first image formation and before the second image formation is performed.

2. 2. The image forming apparatus according to claim 1, wherein the control unit controls the drive unit so that the supply member is rotated at a drive speed equal to the highest drive speed during image formation during the first period or the second period of the preparatory operation.

3. 2. The image forming apparatus according to claim 1, wherein the control unit controls the drive unit so that the second period is longer than the first period in the preparatory operation.

4. an image carrier on whose surface an electrostatic image is formed; a developing member that rotates to supply toner to the electrostatic image; a supply member that has a foam layer that contacts the developing member to form a nip portion, and that rotates to supply toner to the developing member; a drive unit that rotates the developing member and the supply member; a position movement mechanism that can move the position of the developing member relative to the image carrier between a first position and a second position where the distance between the developing member and the image carrier is different from that of the first position; a control unit capable of controlling the drive unit and the position movement mechanism; and The control unit the drive unit is capable of controlling image formation in a first mode in which the developing member supplies toner to the electrostatic image to develop the electrostatic image and form a toner image on the surface of the image carrier, the image formation being performed in a first mode in which the electrostatic image is developed while the supply member is rotationally driven at a first drive speed; an image forming apparatus capable of controlling the drive unit and the position movement mechanism to perform a preliminary operation of moving the position of the developing member from the second position to the first position after the first image formation and the second image formation, while rotating the developing member and the supply member at a driving speed higher than the first driving speed, after the first image formation and before the second image formation, and a preliminary operation of moving the position of the developing member from the second position to the first position after the first position movement operation, while rotating the developing member and the supply member at a driving speed higher than the first driving speed;

5. The image forming apparatus according to claim 4, characterized in that the control unit controls the drive unit so that, during the preliminary operation, the supply member is rotationally driven at a second drive speed higher than the first drive speed when performing the first position movement operation, and the supply member is rotationally driven at a third drive speed higher than the second drive speed when performing the second position movement operation.

6. The image forming apparatus according to claim 5, characterized in that the control unit controls the drive unit and the position movement mechanism so that, during the preliminary operation, the first position movement operation is completed at a timing after the drive speed of the supply member is changed to the second drive speed, and the second position movement operation is completed at a timing after the drive speed of the supply member is changed to the third drive speed.

7. The image forming apparatus according to claim 6, characterized in that the control unit controls the drive unit and the position movement mechanism so that, during the preliminary operation, the time from when the drive speed of the supply member is changed to the second drive speed to when the first position movement operation is completed, and the time from when the drive speed of the supply member is changed to the third drive speed to when the second position movement operation is completed, are each within the time it takes for the supply member to rotate two revolutions.

8. The image forming apparatus according to claim 5, wherein the control unit controls the drive unit so that, during the preliminary operation, the time for which the supply member is rotationally driven at the second drive speed and the time for which the supply member is rotationally driven at the third drive speed are each a time required for the supply member to rotate at least two times.

9. 5. The image forming apparatus according to claim 4, wherein the control unit controls the drive unit so that the supply member is rotated at a drive speed equal to the highest drive speed during image formation during at least one period of the preparatory operation.

10. 5. The image forming apparatus according to claim 4, wherein the first position is a position where the developing member and the image carrier are in contact with each other, and the second position is a position where the developing member and the image carrier are separated from each other.

11. The image forming apparatus according to any one of claims 1 to 10, characterized in that the control unit controls the preparatory operation not to be performed before the second image formation when the elapsed time since the first image formation is performed is a first time, and controls the preparatory operation to be performed before the second image formation when the elapsed time since the first image formation is performed is a second time that is longer than the first time.

12. The image forming apparatus of any one of claims 1 to 10, characterized in that the control unit is capable of controlling the drive unit to perform the image formation in the first mode and a second mode in which the electrostatic image is developed while the supply member is rotationally driven at a drive speed higher than the first drive speed, and when the first image formation is in the second mode, the preparatory operation is not performed before the second image formation.

Images