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 copier, and a facsimile apparatus using an electrophotographic method.

Background Art

[0002] In an electrophotographic image forming apparatus, an electrostatic image formed on an image carrier is developed by a developing device. As the developing device, there is one having a rotatable developing member that carries a developer and conveys it to the image carrier, a supply member that supplies the developer to the developing member and scrapes the developer from the developing member, and a regulating member that regulates the amount of the developer on the developing member. As the supply member, a rotatable supply roller capable of containing a developer in a foamed layer is used.

[0003] In order to perform stable image output using such a developing device, it is required to make the layer thickness of the developer layer on the developing member uniform. However, for example, when the filling amount of the developer in the developing device is large and the driving speed of the supply member at the time of image formation is slow, it may 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 the developer discharged from the supply member per unit time decreases, so the developer around the supply member becomes difficult to move and is in a consolidated state. Then, the discharge of the developer from the supply member is hindered, and the amount of the developer contained in the supply member becomes excessive. In such a state, it becomes difficult to scrape the developer by the supply member, and the amount of the 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, and an image defect may occur in which the density of the developed image becomes non-uniform. Here, the image defect caused by the poor scraping of the developer from the developing member by such a supply member is simply referred to as "density unevenness".

[0004] In response to such a problem, Patent Document 1 proposes increasing the driving speed of the supply member at the time of non-image formation compared to the driving speed at the time of image formation.

Prior Art Document

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-178113 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, in conventional configurations, a long rotation time for the developing and supplying members may be required during non-image formation to suppress the occurrence of density unevenness. This is because, in conventional configurations, the developer discharged from the supplying member during non-image formation enters the space between the developing and supplying members, and it takes time to scrape it out.

[0007] Therefore, the objective of the present invention is to shorten the rotation time of the developing member and the supply member during non-image formation, while suppressing the occurrence of density unevenness caused by poor scraping of the developer from the developing member by the supply member. [Means for solving the problem]

[0008] The above objective is achieved by the image forming apparatus according to the present invention. In summary, the present invention comprises an image carrier on which an electrostatic image is formed on its surface; a developing member that supplies toner to the electrostatic image by rotating; a supply member having a foamed layer that contacts the developing member and forms a nip portion, and supplies toner to the developing member by rotating; 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 forming, in which the developing member supplies toner to the electrostatic image to form a toner image on the surface of the image carrier, in a first mode in which the supply member is rotated at a first driving speed to develop the electrostatic image. The image forming apparatus is capable of performing a first image formation in the first mode, and then performing a second image formation after the first image formation, characterized in that the drive unit can be controlled to perform a preliminary operation in which, during a first period after the first image formation and before the second image formation, the developing member is rotated and the supply member is rotated at a second drive speed higher than the first drive speed, and during a second period after the first period and after the first image formation and before the second image formation, the developing member is rotated and the supply member is rotated at a third drive speed higher than the second drive speed.

[0009] According to another aspect of the present invention, the present invention comprises an image carrier on which an electrostatic image is formed on its surface; a developing member that supplies toner to the electrostatic image by rotating; a supply member having a foamed layer that contacts the developing member and forms a nip portion, and that supplies toner to the developing member by rotating; a drive unit that rotates the developing member and the supply member; a position movement mechanism capable of moving the position of the developing member relative to the image carrier to a first position and a second position where the distance between the developing member and the image carrier is different from the first position; and a control unit capable of controlling the drive unit and the position movement mechanism, wherein the control unit develops the electrostatic image by supplying toner to the electrostatic image with the developing member and forms a toner image on the surface of the image carrier, and the supply member rotates at a first driving speed. The provided image forming apparatus is characterized in that the drive unit can be controlled to perform a first mode in which the electrostatic image is developed while the drive unit is rotated, and when a first image is formed in the first mode and a second image is formed after the first image is formed, the drive unit and the position movement mechanism can be controlled to perform a preliminary operation in which, after the first image is formed and before the second image is formed, the developing member is rotated and the supply member is rotated at a drive speed higher than the first drive speed, the developing member is rotated, and the position movement mechanism is controlled to perform a first position movement operation in which the position of the developing member is moved from a second position to a first position, and a second position movement operation in which the position of the developing member is moved from a second position to a first position after the first position movement operation. [Effects of the Invention]

[0010] According to the present invention, it is possible to shorten the rotation time of the developing member and the supply member when not forming an image, while suppressing the occurrence of density unevenness caused by poor scraping of the developer from the developing member by the supply member. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic cross-sectional view showing the general configuration of an image forming apparatus. [Figure 2] This is a schematic cross-sectional view showing the general configuration of a developing apparatus. [Figure 3] This is a block diagram illustrating the control configuration of an image forming apparatus. [Figure 4] This is a flowchart diagram of the control system used to determine whether to perform a preliminary action. [Figure 5] This is a timing chart diagram of the preliminary operation in Example 1. [Figure 6] This is a timing chart of the preliminary operation in Example 2. [Figure 7] This is a timing chart of the preliminary operation in Example 3. [Figure 8] This is a timing chart of the preliminary operation in Comparative Example 2. [Figure 9] This is a timing chart of the preliminary operation in Comparative Example 3. [Figure 10] This is a timing chart of the preliminary operation in Comparative Example 4. [Figure 11] This is a timing chart of the preliminary operation in Comparative Example 5. [Modes for carrying out the invention]

[0012] Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment can be appropriately changed 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 the image forming apparatus Referring to Figure 1, the overall configuration and image forming operation of the image forming apparatus 100 of this embodiment will be described. Figure 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 the present embodiment is a laser beam printer capable of forming a monochromatic black image on a sheet-like recording material S using an electrophotographic method. The image forming apparatus 100 performs image formation based on image information (image signal) input from an external device such as a host computer. The main components of the image forming apparatus 100 include 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, and the like.

[0015] The photosensitive drum 1, which is a cylindrical (drum type) photosensitive member (electrophotographic photosensitive member) as an image carrier, is rotationally driven by a driving device 55 about its axis in the direction of arrow R1 (counterclockwise direction) in the figure. In the present embodiment, in order to ensure fixing properties according to the paper type and the like, the image forming operation is executed by selecting either a high-speed mode, which is an image forming mode with a relatively high image forming speed, or a low-speed mode, which is an image forming mode with a relatively low image forming speed. In the present embodiment, the high-speed mode is the normal operation of the image forming apparatus 100. However, the low-speed mode may be the normal operation of the image forming apparatus 100. In the present embodiment, in the high-speed mode, the photosensitive drum 1 is rotationally driven at a peripheral speed (moving speed of the outer peripheral surface) of 180 mm / sec. Further, in the present embodiment, in the low-speed mode, the photosensitive drum 1 is rotationally driven at a peripheral speed of 60 mm / sec.

[0016] The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative polarity in this embodiment) by a charging roller 2 which is a roller-type charging member as a charging means. In this embodiment, the charging roller 2 is a conductive roller provided with a conductive elastic layer on a core metal. The charging roller 2 is arranged to contact the photosensitive drum 1 with a predetermined pressure and rotates passively as the photosensitive drum 1 rotates. During the charging process, a predetermined charging bias (charging voltage) is applied to the charging roller 2 by a charging power source E1 (FIG. 3) as a charging bias applying section. Thereby, discharge occurs between the charging roller 2 and the photosensitive drum 1, and the surface of the photosensitive drum 1 is charged to a predetermined charging potential (dark portion potential) Vd. In this embodiment, a DC voltage of, for example, -1100V is applied to the charging roller 2 as a charging bias, and the charging potential Vd of the photosensitive drum 1 after the charging process becomes -600V.

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

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

[0019] Opposite the photosensitive drum 1 is a transfer roller 51, which is a roller-type transfer member serving as a transfer means. The transfer roller 51 is pressed against the photosensitive drum 1 with a predetermined pressure, forming a transfer portion (transfer nip portion) TN, which is the contact area between the photosensitive drum 1 and the transfer roller 51. In this embodiment, the transfer roller 51 is rotationally driven by a drive device 55. The toner image formed on the photosensitive drum 1 is electrostatically transferred at the transfer portion TN onto the recording material S, which is being transported while being held between the photosensitive drum 1 and the transfer roller 51, by the action of the transfer roller 51. During transfer, a predetermined transfer bias (transfer voltage), which is a DC voltage with 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 supply E5 (Figure 3), which serves as a transfer bias application unit. The recording material (recording medium, transfer material, sheet) S, such as paper or a plastic sheet, is fed out 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 fed out by the transport roller 50 at a predetermined control timing synchronized with the position of the toner image on the photosensitive drum 1, and transported to the transfer section TN.

[0020] The recording material S onto which the toner image has been transferred is transported to a fixing device 52, which serves as a fixing means. The fixing device 52 includes a fixing roller 52a equipped with a heat source and a pressure roller 52b that presses against the fixing roller 52a. The fixing device 52 uses the fixing roller 52a and the pressure roller 52b to pressurize and heat the recording material S carrying the unfixed toner image, thereby fixing (melting and solidifying) the toner image onto the recording material S. The recording material S with the fixed toner image is discharged (output) as an image formed product to the outside of the main body 110 of the image forming apparatus 100.

[0021] Furthermore, any toner remaining on the photosensitive drum 1 after transfer (transfer residue toner) is removed and recovered from the photosensitive drum 1 by a cleaning device 7, which serves as a cleaning means.

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

[0023] Furthermore, in this embodiment, the photosensitive drum 1, the charging roller 2 acting thereon as process means, the developing device 4, and the cleaning device 7 together constitute a process cartridge 120 that can be attached to and detached 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 part of the image forming apparatus 100 excluding the process cartridge 120.

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

[0025] The developing device 4 supplies toner 90 as a developer to the electrostatic image on the photosensitive drum 1, making it visible as a toner image. In this embodiment, the developing device 4 is a developing device that employs a contact developing method and an inversion developing method, using toner 90, which is a non-magnetic one-component developer, as the developer. In this embodiment, the normal charge polarity of toner 90, which is the main charge polarity of toner 90 when developing the electrostatic image, is negative polarity. The developing device 4 includes a developing 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 developing 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 toner 90 to the developing roller 42 and scrapes off the toner 90 from the developing roller 42. The regulating blade 44 regulates the amount of toner 90 on the developing roller 42 (thickness of the toner layer). The developing roller 42 and the supply roller 43 rotate by a driving force transmitted from a drive unit 55, which is a drive unit provided in the main body 110 of the image forming apparatus 100. The toner 90 becomes charged to a negative polarity, which is its normal charging polarity, due to friction with the developing roller 42, the supply roller 43, and the regulating blade 44.

[0026] The developing roller 42 is composed of an elastic roller having a core metal and an elastic layer formed on the outer circumference of the core metal as an elastic member, such as conductive elastic rubber. In this embodiment, the developing roller 42 has a cylindrical outer surface with an outer diameter of 16 mm and is supported at both ends by the developing container 41 so that it can rotate. During image formation, the developing roller 42 comes into contact with the photosensitive drum 1 and is driven to rotate in the direction of arrow R2 in the figure (clockwise direction). In other words, 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 in the forward direction at the developing position (developing section) A where the photosensitive drum 1 and the developing roller 42 face each other (come into contact). In this embodiment, the developing apparatus 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 driven to rotate at a peripheral speed of 252 mm / sec in high-speed mode and at a peripheral speed of 84 mm / sec in low-speed mode. The rotation axis direction of the developing roller 42 is approximately parallel to the rotation axis direction of the photosensitive drum 1.

[0027] The supply roller 43 is an elastic sponge roller with an outer diameter of 18 mm, having a core metal 43a and a foamed layer (foamed elastic layer) 43b formed of urethane sponge or the like as a foamed elastic member on the outer circumference of the core metal 43a. On the surface of this foamed layer 43b, foam cells are open, making it easy to hold and transport the toner 90. The supply roller 43 is positioned to contact the developing roller 42 with a predetermined amount of penetration, forming a predetermined nip portion 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 the hardness of the developing roller 42. Therefore, at the nip portion 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 direction) in the figure. In other words, the supply roller 43 rotates so that at the nip portion N, the surface (outer circumference) of the developing roller 42 and the surface (outer circumference) of the supply roller 43 move in opposite directions. The supply roller 43, by rotating, supplies toner 90 onto the developing roller 42. The supply roller 43 also, by rotating, scrapes off (removes) any toner 90 that remained on the developing roller 42 at the developing position A without being supplied to the electrostatic image. The supply roller 43 scrapes off the toner 90 from the developing roller 42 through the openings in the foam layer 43b on its surface and returns it to the inside of the developing container 41. The foam layer 43b deforms just before the nip portion N in the direction of the supply roller 43's rotation. This deformation causes the toner 90 remaining on the surface and inside the foam layer 43b to be discharged in the direction of arrow T1 in the diagram into the region between the developing roller 42 and the supply roller 43 just before the nip portion N (also referred to here as the "discharge region") X. 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 is restored immediately after the nip portion N in the rotational direction of the supply roller 43. At that time, the toner 90 in the region Y between the developing roller 42 and the supply roller 43 immediately after the nip portion N is drawn into the foam layer 43b in the direction of arrow T2 in the figure. In this embodiment, the supply roller 43 is driven to rotate at a drive speed v of 240 rpm in high-speed mode and at a drive speed v of 80 rpm in low-speed mode.The rotation axis direction of the supply roller 43 is approximately parallel to the rotation axis direction of the developing roller 42.

[0028] The regulating blade 44 is made of a flexible metal sheet. The regulating blade 44 is positioned so that its longitudinal direction is substantially parallel to the rotation axis direction of the developing roller 42, with one end in the short direction fixed to the developing container 41 and the other end (free end) in the short direction 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 is thinned to a predetermined layer thickness by the regulating blade 44, held on the developing roller 42, and used for developing.

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

[0030] Furthermore, a predetermined supply bias (supply voltage) is applied to the supply roller 43 by the supply power supply E3 (Figure 3), which acts as a supply bias application unit. In this embodiment, a DC voltage with the same polarity as the development bias and a larger absolute value than the development bias is applied to the supply roller 43 as the supply bias. Due to the potential difference between the supply bias (potential of the supply roller 43) and the development bias (potential of the development roller 42), an electrostatic force acts on the toner 90, biasing the toner 90 from the supply roller 43 toward the development roller 42. In addition, a predetermined regulating bias (regulating voltage) is applied to the regulating blade 44 by the regulating power supply E4 (Figure 3), which acts as a regulating bias application unit. In this embodiment, a DC voltage with the same polarity as the development bias and a larger absolute value than the development 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, biasing the toner 90 from the regulating blade 44 towards the developing roller 42, and promoting the transfer of charge to the toner 90.

[0031] Note that the arrangement of each component of the developing apparatus 4 is not limited to the arrangement in this embodiment. For example, the developing apparatus 4 may be configured such that a regulating blade 44 is located below the developing roller 42, the developing roller 42 rotates in the opposite direction to arrow R2 in the figure in this embodiment, and the supply roller 43 rotates in the opposite direction to arrow R3 in the figure in this embodiment.

[0032] Furthermore, in this embodiment, the image forming apparatus 100 has a contact / separation mechanism 80 for controlling (switching) the contact / non-contact state (contact / separated state) between the photosensitive drum 1 and the developing roller 42. This suppresses unnecessary contact between the photosensitive drum 1 and the developing roller 42 when not forming an image. When forming an image, the developing roller 42 rotates in contact with the surface of the photosensitive drum 1 at the developing position A (hereinafter also referred to as "developing contact"). When not forming an image, the developing roller 42 is separated from the surface of the photosensitive drum 1 by a predetermined distance G (Figure 2(b)) (hereinafter also referred to as "developing separation"). However, in this embodiment, even when not forming an image, during the inter-paper process in continuous image formation and in the preliminary operation described later, the developing roller 42 rotates in contact with the surface of the photosensitive drum 1 at the developing position A. Furthermore, in this embodiment, when the developing roller 42 is separated from the photosensitive drum 1 when not forming an image, the rotation of the developing roller 42 is stopped. Details of the contact-separation mechanism 80 will be described later.

[0033] 3. Control Configuration Figure 3 is a block diagram illustrating the schematic control configuration of the image forming apparatus 100 in this embodiment. The image forming apparatus 100 has a control unit 60 that controls the operation of the image forming apparatus 100. The control unit 60 includes a CPU 61 as an arithmetic processing unit, a memory 62 composed of ROM, RAM, non-volatile memory, etc., as a storage unit, and an input / output unit (not shown) that exchanges signals between the control unit 60 and devices outside the control unit 60. The ROM stores control programs and control data. The RAM stores calculation results from the CPU 61 and detection results from various sensors. The non-volatile memory stores various setting information and usage history information.

[0034] The control unit 60 is connected to various parts 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-to-separation mechanism 80, and the like. The control unit 60 receives print signals (image information and print instructions) transmitted from an external device such as a host computer and controls various parts of the image forming apparatus 100 to execute image forming operations. The control unit 60 also controls various parts of the image forming apparatus 100 to perform preliminary operations involving the rotation of the developing roller 42 and the supply roller 43 during non-image forming times, which will be described later. The "preliminary operations" are operations to suppress the occurrence of density unevenness caused by poor scraping of toner from the developing roller 42 by the supply roller 43.

[0035] The image forming apparatus 100 executes a job (print operation), which is a series of operations that form and output an image on one or more recording materials S, initiated by a single start instruction. A job generally includes an image forming process, a pre-rotation process, a paper-to-paper process when forming an image on multiple recording materials S, and a post-rotation process. The image forming process is the period during which the electrostatic image, toner image, and toner image transfer of the image to be actually formed and output on the recording materials S are performed, and this period is referred to as the image forming time. More specifically, the timing of the image forming time differs depending on the position where each of these processes—electrostatic image formation, toner image formation, and toner image transfer—is performed. The pre-rotation process is the period during which preparatory operations are performed before the image forming process, from when the start instruction is input until the image is actually formed. The paper-to-paper process is the period corresponding to the space between recording materials S when image forming is performed continuously on multiple recording materials S (continuous image forming). The post-rotation process is the period during which tidying operations (preparatory operations) are performed after the image forming process. Non-image forming periods refer to periods other than image forming periods, and include the pre-rotation process, inter-paper process, post-rotation process, and pre-multi-rotation processes which are preparatory operations when the image forming apparatus 100 is powered on or when it returns from sleep mode.

[0036] 4. Configuration and operation of the contact-separation mechanism Next, the configuration and operation of the contact-separation mechanism 80, which serves as a position-moving mechanism for changing the position of the developing roller 42 in this embodiment, will be described. Figures 2(a) and 2(b) are schematic cross-sectional views of the developing apparatus 4 in this embodiment, where Figure 2(a) shows the developing apparatus 4 in the contact position and Figure 2(b) shows the developing apparatus 4 in the separated position.

[0037] The contact-separation mechanism 80 includes a lever 81 as an action receiving part provided on the developing device 4, a movable part 82 as an action part provided on the main body 110 of the image forming apparatus 100, and a contact-separation drive part 83 provided on the main body 110 that drives the movable part 82. The developing device 4 is coupled to a frame (not shown) that fixes the position of the photosensitive drum 1 so that it can swing about a rotation axis substantially parallel to the rotation axis direction of the photosensitive drum 1. The developing device 4 is also biased in a direction that causes the developing roller 42 to rotate towards the photosensitive drum 1. In this embodiment, a tension spring (not shown), which is a biasing member as a biasing means, is provided, with both ends attached to the frame that fixes the position of the photosensitive drum 1 and the developing device 4, respectively. The contact-to-separation mechanism 80 moves the developing device 4 between a contact position (Figure 2(a)) and a separated position (Figure 2(b)) by oscillating the developing device 4 through the operation of the moving part 82 by the contact-to-separation drive unit 83, which moves the lever 81. As shown in Figure 2(a), when the developing device 4 is positioned in the contact position (first position), the developing roller 42 comes into contact with the photosensitive drum 1. Also, as shown in Figure 2(b), when the developing device 4 is positioned in the separated position (second position), the developing roller 42 separates from the photosensitive drum 1.

[0038] The development device 4 moves from the separated position to the contact position by the spring biasing force of the tension spring and the rotational moment centered on the drive input to the development device 4 when it is driven. When the contact-separation drive unit 83 moves the moving part 82 in the direction of arrow P1 in the figure (towards the photosensitive drum 1), the lever 81 is held by the moving part 82 and the state in which the development device 4 is held in the separated position is released. Then, due to the spring biasing force and the rotational moment, the development device 4 swings and the development roller 42 moves toward the photosensitive drum 1. This moves the development device 4 to the contact position and puts the development roller 42 in contact with the photosensitive drum 1. Conversely, in order to move the development device 4 from the contact position to the separated position, the contact-separation drive unit 83 moves the moving part 82 in the direction of arrow P2 in the figure (away from the photosensitive drum 1), thereby moving the lever 81 in the same direction. This allows the developing device 4 to be moved to a separated position and held in that position, thereby separating the developing roller 42 from the photosensitive drum 1.

[0039] The movement of the movable part 82 is performed by the movable part 82 receiving driving force from a motor or solenoid, which serves as a drive source, provided by the contact-separation drive unit 83, via a drive transmission member provided by 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 movable part 82 is released, and the developing roller 42 comes into contact with the photosensitive drum 1 due to the spring biasing force. Therefore, in this embodiment, at this time, the developing roller 42 moves from the separated position to the contact position at a speed faster than the moving speed of the movable part 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 movable part 82. Therefore, in this embodiment, at this time, the developing roller 42 moves from the contact position to the separated position at a speed approximately the same as the moving speed of the movable part 82.

[0040] In this embodiment, during image formation, the developing device 4 is positioned in contact with the photosensitive drum 1, and the developing roller 42 is in contact with the photosensitive drum 1. In addition, in this embodiment, during non-image formation periods (e.g., standby state, sleep state, power-off state), excluding the inter-paper process during continuous image formation and the preliminary operations described later, the developing device 4 is positioned in a distanced position, and the developing roller 42 is in a distanced state from the photosensitive drum 1. By bringing the developing roller 42 into contact with the photosensitive drum 1 only when necessary, wear on the photosensitive drum 1 is 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 positioned at a distanced location, the transmission of rotational driving force from the drive unit 55 to the developing device 4 is interrupted by a clutch (not shown) which serves as a rotational drive transmission release means provided in the drive unit 55. As a result, the rotation of the developing roller 42 and the supply roller 43 stops. In this way, by stopping the rotation of the developing roller 42 and the supply roller 43 when the developing device 4 is at a distanced location, deterioration of the components of the developing device 4 and the toner 90 can be suppressed.

[0042] 5. Uneven concentration Next, we will explain the unevenness in concentration caused by insufficient scraping of the developer from the developing material by the supply material.

[0043] As mentioned above, for example, when the amount of toner 90 in the developing container 41 is large (a large amount of developer is filled), and the drive speed v of the supply roller 43 during image formation is slow, poor scraping of the toner 90 from the developing roller 42 by the supply roller 43 (here, also simply referred to as "poor scraping") may occur. This makes it difficult to form a uniform toner layer on the developing roller 42, and density unevenness, an image defect in which the density of the developed image is uneven, may occur. This image defect is also called a hazy image. Furthermore, as mentioned above, in conventional configurations, a long rotation time for the developing roller 42 and the supply roller 43 during non-image formation may be required to suppress the occurrence of density unevenness.

[0044] In other words, if 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 it difficult for the toner 90 around the supply roller 43 to move and causing it to become compacted. As a result, the discharge of toner 90 from the supply roller 43 in the direction of arrow T1 to the discharge area X shown in Figure 2(a) is hindered, resulting in an excess of toner 90 contained within the supply roller 43. In this state, the supply roller 43 has difficulty scraping off the toner 90, resulting in a scraping failure, and the amount of toner 90 on the developing roller 42 before being restricted by the regulating blade 44 increases. Consequently, it becomes difficult for the regulating blade 44 to form a uniform toner layer on the developing roller 42, which can cause uneven density, an image defect where the density of the developed image is uneven.

[0045] In particular, if there is a long elapsed time between image formation in low-speed mode and the next image formation, density unevenness is likely to occur. This is because, as time passes, the toner 90 solidifies due to its own weight, making it even more difficult to move. As a result, during the next image formation, the ejection of toner 90 from the supply roller 43 in the direction of arrow T1 to the ejection area X is further hindered, promoting a state of inadequate scraping. Also, if image formation is performed continuously in low-speed mode, the toner 90 in the ejection area X becomes compacted, and the amount of toner 90 contained in the supply roller 43 increases. Therefore, in this case as well, a state of inadequate scraping is likely to occur, and density unevenness is likely to occur.

[0046] To suppress the occurrence of density unevenness, it is effective to rotate the supply roller 43 at a higher drive speed v than in low-speed mode before image formation. This is because it eliminates the compaction state of the toner 90 that was promoted in low-speed mode, thereby eliminating the scraping failure state. However, this requires increasing the rotation time of the developing roller 42 and the supply roller 43 before image formation.

[0047] In other words, 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 discharged from the supply roller 43. At this time, the pressure due to the toner 90 increases instantaneously in the discharge region X, causing the discharged toner 90 to enter the nip section N between the developer roller 42 and the supply roller 43 from the discharge region X. In this state, it is difficult to scrape off the toner 90 with the openings of the cells on the surface of the supply roller 43, which can result in uneven density. To resolve this poor scraping condition and suppress the occurrence of uneven density, it is necessary to rotate the developer roller 42 to scrape off the toner 90 that has entered the nip section N. However, if the toner 90 being scraped off remains in a compacted state, more toner 90 will enter the nip section N. Therefore, it is necessary to increase the rotation time of the developing roller 42 and the supply roller 43 not only to relieve the compaction of the toner 90 by rotating the supply roller 43 at a high drive speed v2, but also to scrape out the toner 90 that has entered the nip section N.

[0048] In this case, if the rotation time of the developing roller 42 and supply roller 43 during non-image formation is increased to suppress the occurrence of density unevenness, the downtime (user waiting time) during which image output cannot be performed will increase. In addition, the deterioration of the toner 90 may progress. On the other hand, if the rotation time of the developing roller 42 and supply roller 43 during non-image formation is simply shortened, it becomes difficult to sufficiently suppress the occurrence of density unevenness. Thus, in the conventional configuration, it was difficult to achieve both shortening the rotation time of the developing roller 42 and supply roller 43 during non-image formation and suppressing the occurrence of density unevenness.

[0049] Therefore, it is necessary to shorten the rotation time of the developing roller 42 and the supply roller 43 when not forming an image, while suppressing the occurrence of density unevenness caused by poor scraping of toner 90 from the developing roller 42 by the supply roller 43.

[0050] 6. Preparatory actions Next, the control of the "preliminary operation" as an operating mode during non-image formation to suppress the occurrence of density unevenness in this embodiment will be described. Figure 4 is a flowchart illustrating the schematic of the control of the preliminary operation in this embodiment. The control unit 60 performs the decision to execute the preliminary operation and the execution of the preliminary operation according to the procedure shown in Figure 4.

[0051] First, when the print signal (job start instruction) is input to the image forming apparatus 100 (S101), the control unit 60 determines whether the image forming mode of the previous image forming was the low-speed mode (first mode) or the high-speed mode (second mode) (S102). If the control unit 60 determines that it was the high-speed mode (No in S102), it performs image forming without performing any preparatory operations (S105). In this case, preparatory operations to suppress the occurrence of density unevenness are unnecessary. On the other hand, if the control unit 60 determines that it was the low-speed mode (Yes in S102), it proceeds to determine the conditions for the next preparatory operation. In this case, it is possible that density unevenness is occurring because the drive speed v of the supply roller 43 is slow.

[0052] The control unit 60 determines whether the elapsed time tp since the completion of the previous image formation exceeds a predetermined threshold (elapsed time threshold) tth (S103). For example, each time a job is executed, the control unit 60 stores the timing (typically year, month, day, and time) when the last image formation of the job was completed in memory 62 (RAM or non-volatile memory). The control unit 60 can then determine the elapsed time tp since the completion of the previous image formation by comparing the timing (typically year, month, day, and time) when the print signal was input with the timing when the previous image formation was completed. Here, the threshold tth is pre-set and stored in memory 62 (ROM or non-volatile memory). The timing when image formation is completed is typically represented by the timing when the rotation of the photosensitive drum 1 stops upon completion of the job. However, it is not limited to this. For example, any timing that allows estimation of the waiting 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 has been formed is ejected from the image forming device 100. Furthermore, the method for estimating the waiting time of the developing device 4 is not limited to the method described above; for example, the elapsed time from the time when image formation is completed may be counted. If the control unit 60 determines that the elapsed time tp is less than or equal to the threshold tth (No in S103), it performs image formation without performing any preliminary operations (S105). In this case, preliminary operations to suppress the occurrence of density unevenness are unnecessary. On the other hand, if the control unit 60 determines that the elapsed time tp exceeds the threshold tth (Yes in S103), it performs preliminary operations before image formation (S104). In this case, density unevenness is likely to occur. Then, after performing the preliminary operations, the control unit 60 performs image formation (S105). In this embodiment, the threshold tth was set to 6 hours. However, it is not limited to this, and the threshold tth can be appropriately set according to, for example, the configuration of the image forming apparatus 100 (developing device 4) and the characteristics of the toner 90.

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

[0054] It should be noted that the control flow is not limited to the above in order to obtain the effects of the present invention. For example, a preliminary operation may be performed when image formation in low-speed mode is performed continuously for more than a predetermined number of frames. In this case, for example, the preliminary operation can be performed before image formation of the next job after a job in which image formation in low-speed mode has been performed continuously for more than a predetermined number of frames. Also, for example, in a job in which image formation in low-speed mode is performed continuously for more than a predetermined number of frames, a preliminary operation can be performed in the inter-paper process after the predetermined number of images have been formed, and then image formation can be resumed. In this case, the inter-paper process can be extended to perform the preliminary operation.

[0055] In this embodiment, the developing roller 42 and the supply roller 43 are rotationally driven by the same drive source of the drive unit 55. In this embodiment, the developing roller 42 and the supply roller 43 start rotating and stop rotating almost simultaneously. In this embodiment, the driving speed of the developing roller 42 during the preparatory operation is set such that the ratio of the peripheral speed of the supply roller 43 to the peripheral speed of the developing roller 42 is approximately constant, depending on the driving speed v of the supply roller 43. However, the present invention is not limited to such configurations and settings in order to obtain its effects. For example, the driving speed of the developing roller 42 may be kept constant regardless of the driving speed v of the supply roller 43.

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

[0057] 7. Control of the Examples An example of controlling preliminary operations that can be performed in the image forming apparatus 100 of this embodiment will be described.

[0058] <Example 1> Figure 5 is a timing chart of the preliminary operation in Example 1. Note that in the timing chart, the elapsed time tp when the image forming apparatus 100 is stopped is shown in a shortened form (the same applies to other examples and comparative examples).

[0059] In the previous image formation cycle, the supply roller 43 is driven to rotate at a first drive speed (80 rpm) corresponding to the low-speed mode, and the developing roller 42 is driven to rotate at a speed corresponding to the low-speed mode. At the end of the previous image formation cycle, the rotation of the supply roller 43 and the developing roller 42 is stopped. At the end of the previous image formation cycle, the developing separation operation is started while the developing roller 42 and the supply roller 43 are still rotating. Approximately simultaneously with the completion of the developing separation operation and the positioning of the developing device 4 in the separation position, the rotation of the developing roller 42 and the supply roller 43 stops. After an elapsed time tp, when a print signal is input to the image forming device 100, a decision is made to execute a preliminary operation, and the preliminary operation is executed. The time from the start timing tS to the end timing tE of the preliminary operation is the 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 preliminary operation. These conditions are the same in the other embodiments and comparative examples described below.

[0060] In Example 1, the developing roller 42 is driven to rotate during the preparatory operation. Also in Example 1, the supply roller 43 is driven to rotate during the preparatory operation with its drive speed v increased in multiple stages. First, the supply roller 43 is driven to rotate for 5 seconds at a second drive speed (160 rpm), which is 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. Subsequently, the supply roller 43 is driven to rotate for 15 seconds at a third drive speed (240 rpm), which is 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, a little after the start of rotation of the developing roller 42 and the supply roller 43, the contact-separation mechanism 80 releases the separation position of the developing device 4, and the developing contact operation is completed at timing t1. At this time, the toner 90 in the developing device 41 is subjected to a single vibration.

[0061] The drive speed v of the supply roller 43 during the preparatory operation only needs to be higher than the drive speed v of the supply roller 43 during the previous image formation, and there should be at least one timing when the drive speed v is 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 set to 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 preparatory 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 preparatory operation is made longer than the period during which it is driven at the other drive speeds v. This reduces the speed difference before and after the change in drive speed v, as will be described later, and effectively eliminates the compaction of the toner 90 in the discharge area X. Furthermore, the number of stages in which the drive speed v of the supply roller 43 is increased in multiple stages during the preparatory operation is not limited to two stages, but may be three or more stages (typically five stages or less).

[0062] The first effect of the preliminary operation in Example 1 will now be explained. By increasing the drive speed v of the supply roller 43 compared to the first drive speed (80 rpm) used during image formation in the previous image formation mode, which was the low-speed mode, the compaction of the toner 90 in the discharge area X can be eliminated, thereby eliminating the scraping failure. In other words, by increasing the drive speed v of the supply roller 43 in this way, the amount of toner 90 discharged from the supply roller 43 per unit time increases compared to image formation in the low-speed mode, making it easier for the toner 90 in the developing container 41 to move. As a result, the compaction 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 developing roller 42 can be sufficiently scraped off.

[0063] Next, we will explain the second effect of the preliminary operation in Example 1. In Example 1, 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 instantaneous amount of toner 90 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 ingress of toner 90 from the discharge area X into the nip area N. This eliminates the need to increase the rotation time of the developing roller 42 in order to scrape out the toner 90 that has entered the nip area N and resolve the scraping failure. As a result, the rotation time of the developing roller 42 and the supply roller 43 can be shortened.

[0064] Thus, in Example 1, by increasing the drive speed v of the supply roller 43 in multiple stages during the preliminary operation, it is possible to suppress the occurrence of density unevenness even with a short rotation time of the developing roller 42 and the supply roller 43.

[0065] <Example 2> Figure 6 is a timing chart of the preliminary operations in Example 2.

[0066] In Example 2, the practice of changing the drive speed v of the supply roller 43 in multiple stages during the preliminary operation, which was performed in Example 1 to suppress the occurrence of density unevenness in a short time, is not performed. In Example 2, during the preliminary operation, the supply roller 43 is rotated at a drive speed v of 240 rpm, which corresponds to the high-speed mode, from the beginning. This drive speed (240 rpm) is higher than the drive speed (80 rpm) which corresponds to the low-speed mode in the previous image formation. On the other hand, in Example 2, in order to suppress the occurrence of density unevenness in a short time, the development contact operation as a positional movement operation is performed multiple times during the preliminary operation. The development roller 42 is rotated when this development contact operation is performed. In Example 2, the development contact operation is performed a total of four times at timings t1 to t4. Before the development contact operation at timings t2 to t4, a development separation operation is performed to enable the development contact operation. In Example 2, the rotation time of the developing roller 42 and the supply roller 43 from the completion of one developing contact operation until the completion of the next developing contact operation (or until the rotation of the developing roller 42 and the supply roller 43 stops in the case of the last developing contact operation) is 5 seconds each. Therefore, in Example 2, the total rotation time of the developing roller 42 and the supply roller 43 during the preliminary operation is 20 seconds.

[0067] The number of times the developing roller 42 and supply roller 43 are brought into contact with the developing roller during the preliminary operation is not limited to four times, but can be set appropriately in consideration of the effect of suppressing density unevenness by the preliminary operation and the effect of reducing the rotation time of the developing roller 42 and supply roller 43 during the preliminary operation. Two to ten times is preferable, and typically it is two to five times.

[0068] The first effect of the preliminary operation in Example 2 will now be explained. By increasing the drive speed v of the supply roller 43 from 80 rpm during image formation in the previous image formation mode (low-speed mode) to 240 rpm, the compaction of the toner 90 in the discharge area X can be eliminated, thereby eliminating the scraping failure. In other words, by increasing the drive speed v of the supply roller 43 in this way, the amount of toner 90 discharged from the supply roller 43 per unit time increases compared to image formation in low-speed mode, making it easier for the toner 90 in the developing container 41 to move. As a result, the compaction 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 developing roller 42 can be sufficiently scraped off.

[0069] Next, we will explain the second effect of the preliminary operation in Example 2. In Example 2, by vibrating the toner 90 in the developing container 41 through multiple developing contact operations, the toner 90 is loosened, and the compaction of the toner 90 in the discharge area X can be further reduced. Due to the large speed difference before and after the change in the driving speed v of the supply roller 43, if some toner 90 remains to enter the nip area N from the discharge area X due to the instantaneous pressure change in the discharge area X, a scraping failure will occur. However, with only the rotational operation of the developing roller 42 and the supply roller 43, a long rotation time of the developing roller 42 and the supply roller 43 is required to resolve this. In contrast, because the compaction of the toner 90 is further reduced by multiple developing contact operations, less toner 90 enters the nip area N afterward. Therefore, the toner 90 that has entered the nip area N from the discharge area X can be scraped back into the discharge area X, and the scraping failure can be resolved more quickly.

[0070] Thus, in Example 2, by increasing the drive speed v of the supply roller 43 in the preliminary operation and performing the developing contact operation multiple times, the occurrence of density unevenness can be suppressed even with a short rotation time of the developing roller 42 and the supply roller 43.

[0071] <Example 3> Figure 7 is a timing chart of the preliminary operations in Example 3.

[0072] In Example 3, similar to Example 1, the supply roller 43 is rotated by increasing the drive speed v in multiple stages during the preparatory operation. First, the supply roller 43 is rotated at a second drive speed (160 rpm), which is higher than the first drive speed (80 rpm), for 5 seconds. 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. Subsequently, the supply roller 43 is rotated at a third drive speed (240 rpm), which is higher than the second drive speed (160 rpm), for 5 seconds.

[0073] Furthermore, in Example 3, similar to Example 2, multiple development contact operations are performed during the preliminary operation to suppress the occurrence of density unevenness in a short time. The development roller 42 is rotated when these development contact operations are performed. In Example 3, the development contact operations are performed a total of two times at timings t1 and t2. Before the development contact operation at timing t2, a development separation operation is performed to enable the development contact operation. In Example 3, the rotation time of the development roller 42 and the supply roller 43 from the completion of one development contact operation until the completion of the next development contact operation (or until the rotation of the development roller 42 and the supply roller 43 stops in the case of the last development contact operation) is 5 seconds each. Therefore, in Example 3, the total rotation time of the development roller 42 and the supply roller 43 during the preliminary operation is 10 seconds.

[0074] Thus, in Example 3, when the developing contact operation is performed multiple times during the preliminary operation, the drive speed v of the supply roller 43 is increased as the number of developing contact operations progresses. Furthermore, in Example 3, the total rotation time of the developing roller 42 and the supply roller 43 during the preliminary operation is shorter than in Examples 1 and 2.

[0075] The first effect of the preliminary operation in Example 3 will now be explained. By increasing the drive speed v of the supply roller 43 compared to the first drive speed (80 rpm) used during image formation in the previous image formation mode, which was the low-speed mode, the compaction of the toner 90 in the discharge area X can be eliminated, thereby eliminating the scraping failure. In other words, by increasing the drive speed v of the supply roller 43 in this way, the amount of toner 90 discharged from the supply roller 43 per unit time increases compared to image formation in the low-speed mode, making it easier for the toner 90 in the developing container 41 to move. As a result, the compaction 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 developing roller 42 can be sufficiently scraped off.

[0076] Next, we will explain the second effect of the preliminary operation in Example 3. In Example 3, 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 instantaneous amount of toner 90 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 ingress of toner 90 from the discharge area X into the nip area N. This eliminates the need to increase the rotation time of the developing roller 42 in order to scrape out the toner 90 that has entered the nip area N and resolve the scraping failure. As a result, the rotation time of the developing roller 42 and the supply roller 43 can be shortened.

[0077] Furthermore, a third effect of the preliminary operation in Example 3 will be explained. In Example 3, by vibrating the toner 90 in the developing container 41 through multiple developing contact operations, the toner 90 is loosened, and the compaction of the toner 90 in the discharge area X can be further reduced. Due to the large speed difference before and after the change in the driving speed v of the supply roller 43, if some toner 90 remains to enter the nip area N from the discharge area X due to the instantaneous pressure change in the discharge area X, a scraping failure will occur. However, with only the rotational operation of the developing roller 42 and the supply roller 43, a long rotation time of the developing roller 42 and the supply roller 43 is required to resolve this. In contrast, because the compaction of the toner 90 is further reduced by multiple developing contact operations, less toner 90 enters the nip area N afterward. Therefore, the toner 90 that has entered the nip area N from the discharge area X can be scraped back into the discharge area X, and the scraping failure can be resolved more quickly.

[0078] In the preliminary operation, td1 is defined as the time from when the drive speed v of the supply roller 43 is changed to the second drive speed (160 rpm) until the development contact operation immediately following is completed. Also, in the preliminary operation, td2 is defined as the time from when the drive speed v of the supply roller 43 is changed to the third drive speed (240 rpm) until the development contact operation immediately following is completed. 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 to that predetermined drive speed v is initiated (or the timing at which the drive is initiated). However, in this embodiment, the time from when the change in the drive speed v of the supply roller 43 is initiated until the change is completed is sufficiently short compared to, for example, the time from when the development contact operation is started until the development contact operation is completed. At this time, it is desirable that time td1 and time td2 each be within two rotations of the supply roller 43. This is because the instantaneous increase in pressure in the discharge region X when the drive speed v of the supply roller 43 is increased is due to the longer period between increasing the drive speed v of the supply roller 43 and the two rotations of the supply roller 43. During the period between increasing the drive speed v of the supply roller 43 and the one rotation of the supply roller 43, a large amount of toner 90 is discharged, causing the pressure in the discharge region X to increase, and then it takes at least one rotation of the supply roller 43 for that pressure to decrease. Therefore, by applying vibration to the toner 90 through the developing contact operation during this time, the pressure in the discharge region X can be reduced more effectively. In Example 3, time td1 and time td2 were set to 0.3 sec each. 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 preparatory operation, two rotations of the supply roller 43 take 0.5 sec. Therefore, in Example 3, time td1 and time td2 satisfy the above relationship.

[0079] Furthermore, in the preliminary operation, it is desirable that the time spent rotating the supply roller 43 at the second drive speed (160 rpm) and the time spent rotating it at the third drive speed (240 rpm) be at least the equivalent of two rotations of the supply roller 43. This is because, by allowing the instantaneous increase in pressure in the discharge region X when the drive speed v of the supply roller 43 is increased to settle down before performing the next speed change or image formation, the occurrence of density unevenness can be suppressed more effectively. In Example 3, the time spent rotating the supply roller 43 at the second drive speed (160 rpm) and the time spent rotating it at the third drive speed (240 rpm) are both 5 seconds. In Example 3, even at the second drive speed (160 rpm), which is the slowest drive speed v of the supply roller 43 during the preliminary operation, two rotations of the supply roller 43 are 0.75 seconds. Therefore, in Example 3, the time spent rotating the supply roller 43 at each drive speed satisfies the above relationship. Although not limited thereto, in Example 3, the time for rotating the supply roller 43 at each driving speed is often sufficient to be 50 rotations or less of the supply roller 43, and from the viewpoint of suppressing downtime, 30 rotations or less is preferable, and typically 20 rotations or less.

[0080] In Example 2, 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 following is completed can be set in the same way as td1 and td2 in Example 3. Also, the rotation time of the supply roller 43 from the completion of one development contact operation to the completion of the next development contact operation in Example 2 can be set in the same way as the rotation time at each drive speed v in Example 3.

[0081] Thus, in Example 3, when the developing 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 developing contact operations progresses, thereby suppressing the occurrence of density unevenness even with the short rotation time of the developing roller 42 and the supply roller 43.

[0082] 8. Evaluation Results The control results for Examples 1 to 3 were evaluated using the following method. Furthermore, for comparison with Examples 1 to 3, Comparative Examples 1 to 5 below were also evaluated using the same method. In addition, for the image forming apparatus of the comparative examples, elements having the same or corresponding functions or configurations as those in the image forming apparatus of the examples are denoted by the same reference numerals.

[0083] Under conditions of 15°C and 10% relative humidity, 100 consecutive images with a print density of 1% were formed in high-speed mode. Subsequently, 10 consecutive images of solid white were formed in low-speed mode, and after 12 hours, 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 unevenness in concentration occurred. ×: Uneven density present.

[0084] <Comparative Example 1> Comparative Example 1 is an example in which the image forming apparatus 100 has substantially the same configuration as that of this embodiment, but no preliminary operations are performed.

[0085] <Comparative Example 2> Comparative Example 2 is an example of an image forming apparatus 100 with substantially the same configuration as in this embodiment, but with the following preparatory operations. Figure 8 is a timing chart of the preparatory operations in Comparative Example 2. In Comparative Example 2, unlike Example 1, in the preparatory operations, the supply roller 43 is driven at a drive speed v of 240 rpm from the start, corresponding to the high-speed mode, compared to 80 rpm during the previous image forming in low-speed mode. The drive speed v of the supply roller 43 is constant during the preparatory operations. The rotation time of the developing roller 42 and the supply roller 43 during the preparatory operations is 20 sec, the same as in Example 1 and Example 2. Before the image forming apparatus 100 receives the print signal and the preparatory operations begin, the developing device 4 is in a separated position. In Comparative Example 2, unlike Example 2, the developing contact operation is performed only once, immediately after the rotation of the developing roller 42 and the supply roller 43 begins, which is necessary for the rotation of the developing roller 42 and the supply roller 43.

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

[0087] <Comparative Example 4> Comparative Example 4 is an example of an image forming apparatus 100 with substantially the same configuration as in this embodiment, but with the following preparatory operation. Figure 10 is a timing chart of the preparatory operation in Comparative Example 4. In Comparative Example 4, as with Comparative Examples 2 and 3, during the preparatory operation, the supply roller 43 is driven at a drive speed v of 240 rpm from the beginning, corresponding to the high-speed mode, compared to 80 rpm during the previous image forming 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 30 sec, which is longer than in Examples 1-3, Comparative Examples 2 and 3.

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

[0089] <Comparison of Examples 1-3 and Comparative Examples 1-5> Table 1 shows the evaluation results of density unevenness for Examples 1-3 and Comparative Examples 1-5, as well as the value of the time (downtime) during which 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 supply roller 43 added by 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, it is thought that the driving speed v of the supply roller 43 during the previous image formation was slow, causing the toner 90 in the ejection area X to become compacted, increasing the amount of toner contained in the supply roller 43, resulting in a poor scraping condition and density unevenness.

[0092] Furthermore, in Comparative Example 4, no density unevenness occurred because a preparatory operation was performed, but in Comparative Examples 2 and 3, density unevenness occurred despite the preparatory operation being performed. Comparative Examples 2 and 3 had shorter preparatory operation times compared to Comparative Example 4. Therefore, when the drive speed v of the supply roller 43 was increased, the entry of toner 90 from the discharge area X into the nip area N, which occurred due to the instantaneous pressure change in the discharge area X, could not be eliminated. As a result, it is thought that a state of inadequate scraping occurred, resulting in density unevenness.

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

[0094] In Comparative Example 4, the drive speed v of the supply roller 43 during the preparatory operation is fast, and the rotation time of the developing roller 42 and the supply roller 43 is sufficiently long. As a result, both the compaction of the toner 90 in the discharge area X and the intrusion of toner 90 from the discharge area X into the nip area N are eliminated, and density unevenness does not occur. However, in order to suppress the occurrence of density unevenness due to the intrusion of toner 90 from the discharge area X into the nip area N, it is necessary to increase the rotation time of the developing roller 42 and the 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 drive speed v of the supply roller 43 is increased during the preparatory operation to eliminate the compacted state of the toner 90 in the discharge area X. Furthermore, in Example 1, by increasing the drive speed v of the supply roller 43 in multiple stages, the speed difference is dispersed and reduced, resulting in smaller instantaneous pressure changes in the discharge area X and suppressing the intrusion of toner 90 from the discharge area X into the nip area N. As a result, in Example 1, the downtime can be shortened compared to Comparative Example 4. Thus, in Example 1, the rotation time of the developing roller 42 and supply roller 43 added during non-image formation is shortened compared to 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 that of Comparative Example 4, no density unevenness occurred. In Example 2, the drive speed v of the supply roller 43 can be increased during the preparatory operation to eliminate the compacted state of the toner 90 in the discharge area X. In addition, 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 developing container 41 through multiple developing contact operations during the preparatory operation. As a result, the time required for the rotation of the developing roller 42 to scrape the toner 90 that has entered the nip section N from the discharge area X back into the discharge area X can be shortened, and the downtime can be shortened compared to Comparative Example 4. Thus, in Example 2, the rotation time of the developing roller 42 and supply roller 43 added during non-image formation is shortened compared to Comparative Example 4, while suppressing the occurrence of density unevenness.

[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. Also, in Example 3, the downtime was shorter compared to Examples 1 and 2. In Example 3, the drive speed v of the supply roller 43 is increased in the preliminary operation to eliminate the compacted state of the toner 90 in the discharge area X. In addition, in Example 3, by increasing the drive speed v of the supply roller 43 in multiple stages, the speed difference is dispersed and reduced, so the instantaneous pressure change in the discharge area X is small, and the intrusion of toner 90 from the discharge area X into the nip section N can be suppressed. As a result, in Example 3, the downtime can be shortened compared to Comparative Examples 4, 1 and 2. Furthermore, in Example 3, by vibrating the toner 90 in the developing container 41 through multiple developing contact operations in the preliminary operation, the compacted state of the toner 90 in the discharge area X can be further reduced. As a result, the time required for the developing roller 42 to rotate and scrape the toner 90 that has entered the nip area N from the ejection area X back into the ejection area X is reduced, thus shortening the downtime compared to Comparative Example 4, Example 1, and Example 2. In this way, Example 3 can reduce the rotation time of the developing roller 42 and supply roller 43 added during non-image formation compared to Comparative Example 4, Example 1, and Example 2, while suppressing the occurrence of density unevenness.

[0098] 9. Effects Thus, in this embodiment, the image forming apparatus 100 includes an image carrier (photosensitive drum) 1 on which an electrostatic image is formed on its surface, a developing member (developing roller) 42 that supplies toner to the electrostatic image by rotating, a developing container 41 that contains toner, a supply member (supply roller) 43 that has a foamed layer 43b that contacts the developing member 42 to form a nip portion N and supplies toner contained in the developing container 41 to the developing member 42 by rotating, a regulating member (regulating blade) 44 that regulates the amount of toner supplied to the developing member 42 by the supply member 43, a drive unit (drive device) 55 that rotates the developing member 42 and the supply member 43, and a control unit 60 that can control the drive unit 55. In this embodiment, the control unit 60 can control the drive unit 55 to perform image forming, in which the electrostatic image is developed and a toner image is formed on the surface of the image carrier 1 by supplying toner to the electrostatic image with the developing member 42, in a first mode in which the electrostatic image is developed while the supply member 43 is rotated at a first driving speed. Furthermore, in this embodiment, when the control unit 60 performs first image formation in the first mode and then performs second image formation after the first image formation, it is possible to control the drive unit 55 to perform a preliminary operation in which, after the first image formation and before the second image formation, the developing member 42 is rotated during a first period (for example, the period when the drive speed v in Figure 5 is 160 rpm) and the supply member 43 is rotated at a second drive speed (for example, 160 rpm) that is higher than the first drive speed (for example, 80 rpm), and during a second period after the first period (for example, the period when the drive speed v in Figure 5 is 240 rpm), the developing member 42 is rotated during a second period (for example, the period when the drive speed v in Figure 5 is 240 rpm) and the supply member 43 is rotated at a third drive speed (for example, 240 rpm) that is higher than the second drive speed (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 the high-speed mode) during the first or second period of the preliminary operation. The control unit 60 can also control the drive unit 55 during the preliminary operation so that the second period is longer than the first period.

[0099] Furthermore, 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) which is at a different distance from the image carrier 1 than the first position. In this embodiment, when the control unit 60 performs first image formation in the first mode and then performs second image formation after the first image formation, it is possible to control the drive unit 55 and the position movement mechanism 80 to perform a preliminary operation in which, after the first image formation and before the second image formation, the developing member 42 is rotated and the supply member 43 is rotated at a higher drive speed (e.g., 240 rpm) than the first drive speed (e.g., 80 rpm), a first position movement operation (e.g., development contact operation at timing t1 in Figure 6) moves the position of the developing member 42 from the second position to the first position, and a second position movement operation (e.g., development contact operation at timing t2 in Figure 6) moves the position of the developing member 42 from the second position to the first position after the first position movement operation (Examples 2 and 3). Furthermore, the control unit 60 can control the drive unit 55 in the preliminary operation so that when performing the first position movement operation, it rotates the supply member 43 at a second drive speed (e.g., 160 rpm) that is higher than the first drive speed (e.g., 80 rpm), and when performing the second position movement operation, it rotates the supply member 43 at a third drive speed (e.g., 240 rpm) that is higher than the second drive speed (Example 3). In this case, the control unit 60 can control the drive unit 55 and the position movement mechanism 80 in the preliminary operation so that the first position movement operation is completed at the timing after the drive speed of the supply member 43 has been changed to the second drive speed (e.g., timing t1 in Figure 7), and the second position movement operation is completed at the timing after the drive speed of the supply member 43 has been changed to the third drive speed (e.g., timing t2 in Figure 7).In this case, the control unit 60 can control the drive unit 55 and the position movement mechanism 80 so that, in the preliminary operation, the time from when the drive speed of the supply member 43 is changed to the second drive speed until 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 until the second position movement operation is completed, are each within the time it takes for the supply member 43 to rotate twice. The control unit 60 can also control the drive unit 55 so that, in the preliminary operation, the time it takes for the supply member 43 to rotate at the second drive speed and the time it takes for the supply member 43 to rotate at the third drive speed are each within the time it takes for the supply member 43 to rotate at least twice. In this embodiment, the control unit 60 can also control the drive unit 55 so that, during at least one period of the preliminary operation, the supply member 43 rotates at the same drive speed as the highest drive speed during image formation (for example, 240 rpm corresponding to the high-speed mode). Furthermore, in this embodiment, the first position is the position where the developing member 42 and the image carrier 1 are in contact, and the second position is the position where the developing member 42 and the image carrier 1 are separated.

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

[0101] Furthermore, in this embodiment, the control unit 60 can control the drive unit 55 to perform the image formation in a first mode (low-speed mode) and a second mode (high-speed mode) in which the electrostatic image is developed while the supply member 43 is rotated at a higher drive speed (e.g., 240 rpm) than the first drive speed (e.g., 80 rpm). If the first image formation is performed in the second mode, the control unit 60 can control the drive unit 55 so as not to perform any preliminary operations before the second image formation.

[0102] As described above, this embodiment makes it possible to shorten the rotation time of the developing roller 42 and the supply roller 43 when not forming an image, while suppressing the occurrence of density unevenness caused by poor scraping of toner 90 from the developing roller 42 by the supply roller 43. Therefore, it is possible to shorten downtime (user waiting time) and extend the lifespan of the developing device 4 by suppressing deterioration of the components and toner of the developing device 4.

[0103] Furthermore, in this embodiment, the execution conditions were set so that the preliminary operation is performed only before image formation after image formation in low-speed mode, where density unevenness is likely to occur. This makes it possible to further shorten the rotation time of the developing roller 42 and supply roller 43, which are added during non-image formation times, while suppressing the occurrence of density unevenness.

[0104] Furthermore, in this embodiment, the execution conditions were set so that the preliminary operation is performed only before image formation after the elapsed time since the previous image formation exceeds a predetermined threshold tth, which is prone to density unevenness. This makes it possible to further shorten the rotation time of the developing roller 42 and supply roller 43 that are added during non-image formation periods while suppressing the occurrence of density unevenness.

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

[0106] In the above-described embodiment, a developing contact operation was used as the position movement operation. This allows vibration to be efficiently applied to the toner 90 in the developing container 41 via the developing roller 42 over substantially the entire range in the direction of the rotation axis of the developing roller 42. However, the present invention is not limited to this. For example, a developing separation operation may be used as the position movement operation to apply vibration to the toner 90 in the developing container 41. In that case, for example, the image forming apparatus 100 can be provided with a stopper member that the developing apparatus 4 abuts against when the developing roller 42 moves away from the photosensitive drum 1. This allows vibration to be applied to the toner 90 in the developing container 41 via a component other than the developing roller 42 that abuts against the stopper member (such as the developing container 41). Furthermore, even without providing a stopper member, for example, if the increase or decrease (stopping) of the moving speed of the developing apparatus 4 by the moving part 82 of the contact separation mechanism 80 is sufficiently rapid during the developing separation operation, the toner 90 in the developing container 41 can be sufficiently vibrated.

[0107] Furthermore, depending on the configuration of the developing device 4, the fluidity of the toner 90, the remaining amount of toner 90 in the developing container 41, the execution conditions in the preliminary operation, the drive speed v of the supply roller 43, and the rotation time and number of positional movement operations of the developing roller 42 and the supply roller 43 can be set as appropriate.

[0108] Furthermore, although the image forming apparatus in the above-described embodiment was a monochrome image forming apparatus, the present invention can also be applied to a color image forming apparatus. For example, in a configuration comprising multiple 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] Furthermore, although the above-described embodiment was explained as being rotationally driven by a driving force transmitted from a common drive source, the developing roller and the supply roller may each be driven by independently provided drive units. [Explanation of Symbols]

[0110] 1 Photosensitive drum 4. Developing device 41 Developing 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 which an electrostatic image is formed on its surface, A developing member that supplies toner to the electrostatic image by rotating, A supply member having a foamed layer that contacts the developing member to form a nip portion, and which supplies toner to the developing member by rotating, A drive unit that rotates the developing member and the supply member, A control unit capable of controlling the aforementioned drive unit, It has, The control unit, The drive unit can be controlled to perform image formation, which involves developing the electrostatic image by supplying toner to the electrostatic image using the developing member to form a toner image on the surface of the image carrier, in a first mode in which the electrostatic image is developed while the supply member is rotated at a first driving speed. An image forming apparatus characterized in that, when a first image is formed in the first mode and a second image is formed after the first image is formed, the drive unit can be controlled to perform a preliminary operation in which, during a first period after the first image is formed and before the second image is formed, the developing member is rotated and the supply member is rotated at a second drive speed higher than the first drive speed, and during a second period after the first period and after the first image is formed and before the second image is formed, the developing member is rotated and the supply member is rotated at a third drive speed higher than the second drive speed.

2. The image forming apparatus according to claim 1, characterized in that the control unit controls the drive unit to rotate the supply member at the same drive speed as the highest drive speed during image forming, during the first or second period of the preliminary operation.

3. The image forming apparatus according to claim 1, characterized in that the control unit controls the drive unit in the preliminary operation such that the second period is longer than the first period.

4. An image carrier on which an electrostatic image is formed on its surface, A developing member that supplies toner to the electrostatic image by rotating, A supply member having a foamed layer that contacts the developing member to form a nip portion, and which supplies toner to the developing member by rotating, A drive unit that rotates the developing member and the supply member, A position-shifting mechanism capable of moving the position of the developing member relative to the image carrier to a first position and a second position where the distance between the developing member and the image carrier is different from the first position, A control unit capable of controlling the drive unit and the position movement mechanism, It has, The control unit, The drive unit can be controlled to perform image formation, which involves developing the electrostatic image by supplying toner to the electrostatic image using the developing member to form a toner image on the surface of the image carrier, in a first mode in which the electrostatic image is developed while the supply member is rotated at a first driving speed. An image forming apparatus characterized in that, when a first image is formed in the first mode and a second image is formed after the first image is formed, the drive unit and the position movement mechanism can be controlled to perform a preliminary operation in which, after the first image is formed and before the second image is formed, the developing member is rotationally driven and the supply member is rotationally driven at a drive speed higher than the first drive speed, the developing member is moved from a second position to a first position, and the position movement mechanism is controlled to perform a preliminary operation in which, after the first position movement operation, the developing member is moved from a second position to a first position.

5. The image forming apparatus according to claim 4, characterized in that the control unit controls the drive unit so that, in the preliminary operation, it rotates the supply member at a second drive speed higher than the first drive speed when performing the first position movement operation, and rotates the supply member 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 such that, in the preliminary operation, the first position movement operation is completed at the 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 the 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, in the preliminary operation, the time from when the drive speed of the supply member is changed to the second drive speed until 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 until the second position movement operation is completed, are each within the time it takes for the supply member to rotate twice.

8. The image forming apparatus according to claim 5, characterized in that the control unit controls the drive unit in the preliminary operation such that the time for rotating the supply member at the second drive speed and the time for rotating the supply member at the third drive speed are each equal to the time for the supply member to complete at least two rotations.

9. The image forming apparatus according to claim 4, characterized in that the control unit controls the drive unit to rotate the supply member at the same drive speed as the highest drive speed during image forming during at least one period of the preliminary operation.

10. The image forming apparatus according to claim 4, characterized in that the first position is a position in which the developing member and the image carrier are in contact, and the second position is a position in which the developing member and the image carrier are separated.

11. The image forming apparatus according to any one of claims 1 to 10, characterized in that the control unit does not perform the preliminary operation before the second image forming if the elapsed time since the first image forming is a first time, and performs the preliminary operation before the second image forming if the elapsed time since the first image forming is a second time that is longer than the first time.

12. The control unit can control the drive unit to perform the image formation in a first mode and a second mode in which the electrostatic image is developed while the supply member is rotated at a drive speed higher than the first drive speed, and the image forming apparatus according to any one of claims 1 to 10, wherein if the first image formation is in the second mode, the preliminary operation is not performed before the second image formation.

Images