Image forming apparatus

Abstract

To provide an image forming apparatus that has a plurality of modes different in the rotational peripheral speed ratio between a photoconductor drum and a developing roller, and can form a high-quality image while preventing the occurrence of banding in all the modes.SOLUTION: An image forming apparatus is used which comprises: a rotatable image carrier; a rotatable developer carrier that develops an electrostatic latent image with developer at a nip part formed between the image carrier and the developer carrier; a driving unit that drives to rotate the image carrier and the developer carrier while allowing individual variation in the peripheral speed of each carrier; and a control unit that controls the driving unit. The control unit controls, in an executable manner, a first image formation mode in which a rotational peripheral speed ratio being the ratio between the surface movement speed of the developer carrier and the surface movement speed of the image carrier is 1 or less, and a second image formation mode in which the rotational peripheral speed ratio is larger than 1.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an image forming apparatus.

Background Art

[0002] In image forming apparatuses such as copiers, printers, and facsimiles using an electrophotographic image forming method (electrophotographic process), an electrophotographic photosensitive member (hereinafter referred to as "photosensitive member") as an image carrier is uniformly charged by a charging means, and the charged photosensitive member is exposed to form an electrostatic image on the photosensitive member. The electrostatic image formed on the photosensitive member is visualized as a toner image with toner of a developer by a developing means. Then, the toner image formed on the photosensitive member is transferred to a recording material such as a recording paper or a plastic sheet, and the toner image transferred onto the recording material is fixed to the recording material by applying heat or pressure to the toner image to perform image recording.

[0003] Such an image forming apparatus generally requires replenishment of a developer and maintenance of various process means. In order to facilitate the replenishment work of the developer and the maintenance of various process means, a process cartridge is made by integrating a photosensitive member, a charging means, a developing means, a cleaning means for cleaning the photosensitive member, etc. into a frame and making it detachable from the image forming apparatus main body. According to the process cartridge system, an image forming apparatus with excellent usability can be provided.

[0004] In recent years, one of the diverse market demands is to increase the image density and expand the color tone in order to obtain richer images. To achieve this purpose, in addition to a mode for obtaining a general image density, there is a mode in which the peripheral speed ratio of the photosensitive drum and the developing roller is changed, the supply amount of the developer to the photosensitive drum is increased, and the amount of the developer on the recording material is increased to achieve high density and color tone expansion (Patent Document 1).

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-181964 [Summary of the Invention] [Problems to be Solved by the Invention]

[0006] However, with the recent acceleration of processes, it has become necessary to increase the rotational speeds of the photosensitive drum and the developing roller. When the rotational speeds of the photosensitive drum and the developing roller increase, torque fluctuations and vibrations of the developing container are likely to occur, and the rotation of the developing roller becomes unstable. When the rotation of the developing roller becomes unstable, the rotational peripheral speed ratio with the photosensitive drum fluctuates, and density unevenness called banding may occur in the image.

[0007] The present invention has been made in view of the above problems, and an object thereof is to provide an image forming apparatus having a plurality of modes with different rotational peripheral speed ratios between a photosensitive drum and a developing roller, which can form a high-quality image while suppressing the occurrence of banding in any mode. [Means for Solving the Problems]

[0008] The present invention employs the following configuration. That is, a rotatable image carrier, a rotatable developer carrier that supplies a developer to the surface of the image carrier in a nip portion formed between the image carrier and the developer carrier, A supply member that supplies the developer to the developer carrier; Developing voltage application means for applying a developing voltage to the developer carrier; Supply voltage application means for applying a supply voltage to the supply member; a drive unit that rotationally drives the image carrier and the developer carrier with their respective peripheral speeds variably, The developing voltage application means and the supply voltage application means; the drive unit And; a control unit that controls the drive unit, and is provided with the control unit i) the Image bear surface movement speed of the carrier ForThe above-mentioned Dev image Agent The surface movement speed of the carrier Of is controllable to execute a first image forming mode in which the rotational peripheral speed ratio, which is a ratio, is 1 or less, and a second image forming mode in which the rotational peripheral speed ratio is greater than 1. Do, ii) Control so that the surface movement speed of the developer carrier in the second image forming mode is smaller than the surface movement speed of the developer carrier in the first image forming mode; iii) Control so that the difference between the developing voltage and the supply voltage is larger when the second image forming mode is executed than when the first image forming mode is executed;

Figure 1

Effect of the Invention

[0009] According to the present invention, in an image forming apparatus having a plurality of modes with different rotational peripheral speed ratios between a photosensitive drum and a developing roller, it is possible to provide an image formation capable of forming a high-quality image while suppressing the occurrence of banding in any mode.

Brief Description of the Drawings

[0010]

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0011] Hereinafter, with reference to the drawings, modes for carrying out the present invention will be exemplarily and specifically described based on examples. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment are to be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions, unless otherwise specifically described. That is, the scope of the present invention is not intended to be limited to the following embodiments.

[0012] [Example] (Image forming apparatus) The overall configuration of an embodiment of an electrophotographic image forming apparatus (hereinafter referred to as an image forming apparatus) will be described. FIG. 1 is a cross-sectional view of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 is a full-color laser beam printer adopting an in-line system and an intermediate transfer system. The image forming apparatus 100 can form a full-color image on a recording material (for example, recording paper, plastic sheet, cloth, etc.) according to image information. The image information is input into the image forming apparatus main body from an image reading device connected to the image forming apparatus main body or a host device such as a personal computer communicably connected to the image forming apparatus main body.

[0013] The image forming apparatus 100 has image forming units SY, SM, SC, and SK for forming images of respective colors of yellow (Y), magenta (M), cyan (C), and black (K), with a plurality of process cartridges 7. In this embodiment, the image forming units SY, SM, SC, and SK are arranged in a row in a direction intersecting the vertical direction.

[0014] The process cartridge 7 is detachably attached to the image forming apparatus 100 via attachment means such as an attachment guide and a positioning member provided on the image forming apparatus main body. In this embodiment, all the process cartridges for respective colors have the same shape, and toners of respective colors of yellow (Y), magenta (M), cyan (C), and black (K) are accommodated in the process cartridges 7 for respective colors. Note that, in this embodiment, the process cartridge will be described, but the developing device 3 may be configured to be detachably attached to the image forming apparatus main body alone.

[0015] The photosensitive drum 1 is rotationally driven by a driving means (driving source) (not shown). The photosensitive drum 1 A scanner unit 30 (exposure means) is arranged around it. The scanner unit 30 is an exposure means for irradiating a laser based on image information to form an electrostatic image (electrostatic latent image) on the photosensitive drum 1. For the write-out of laser exposure, in the main scanning direction (the direction orthogonal to the conveyance direction of the recording material 12), it is performed based on the position signal in a polygon scanner called BD (Beam Detect) for each scanning line. On the other hand, in the sub-scanning direction (the conveyance direction of the recording material 12), it is performed with a delay for a predetermined time from a TOP signal starting from a switch (not shown) in the conveyance path of the recording material 12. Thereby, in the four process stations Y, M, C, and K, laser exposure can always be performed on the same position on the photosensitive drum 1.

[0016] Facing the four photosensitive drums 1, an intermediate transfer belt 31 as an intermediate transfer body for transferring the toner image on the photosensitive drum 1 to the recording material 12 is arranged. The intermediate transfer belt 31 formed of an endless belt as the intermediate transfer body abuts on all the photosensitive drums 1 and circulates (rotates) in the direction of the arrow B in the figure (counterclockwise). On the inner peripheral surface side of the intermediate transfer belt 31, four primary transfer rollers 32 as primary transfer means are arranged in parallel so as to face each photosensitive drum 1. And a bias of a polarity opposite to the normal charging polarity of the toner is applied to the primary transfer roller 32 from a primary transfer bias power supply (high-voltage power supply) as primary transfer bias applying means (not shown). Thereby, the toner image on the photosensitive drum 1 is transferred (primary transfer) onto the intermediate transfer belt 31.

[0017] Also, a secondary transfer roller 33 as secondary transfer means is arranged on the outer peripheral surface side of the intermediate transfer belt 31. And a bias of a polarity opposite to the normal charging polarity of the toner is applied to the secondary transfer roller 33 from a secondary transfer bias power supply (high-voltage power supply) as secondary transfer bias applying means (not shown). Thereby, the toner image on the intermediate transfer belt 31 is transferred (secondary transfer) to the recording material 12. For example, when forming a full-color image, the above-described process is sequentially performed in the image forming units SY, SM, SC, and SK, and the toner images of each color are sequentially superimposed and primary transferred onto the intermediate transfer belt 31.

[0018] Subsequently, in synchronization with the movement of the intermediate transfer belt 31, the recording material 12 is conveyed to the secondary transfer unit. Then, by the action of the secondary transfer roller 33 that is in contact with the intermediate transfer belt 31 via the recording material 12, the four-color toner image on the intermediate transfer belt 31 is collectively secondarily transferred onto the recording material 12.

[0019] The recording material 12 onto which the toner image has been transferred is conveyed to a fixing device 34 as a fixing means. By applying heat and pressure to the recording material 12 in the fixing device 34, the toner image is fixed to the recording material 12.

[0020] (Process cartridge) The overall configuration of the process cartridge 7 mounted on the image forming apparatus 100 of this embodiment will be described. FIG. 2 is a cross-sectional (main cross-sectional) view of the process cartridge 7 of this embodiment as viewed along the longitudinal direction (rotation axis direction) of the photosensitive drum 1. In this embodiment, except for the type (color) of the developer accommodated, the configurations and operations of the process cartridges 7 for each color are substantially the same.

[0021] The process cartridge 7 includes a photosensitive unit 13 including a photosensitive drum 1 as an image carrier and a developing unit 3 including a developing roller 4 as a developer carrier.

[0022] The photosensitive drum 1 is rotatably attached to the photosensitive unit 13 via a bearing (not shown). The photosensitive drum 1 is rotationally driven in the direction of arrow A (clockwise) in accordance with the image forming operation by receiving the driving force of a driving motor as drum driving means 401 (driving source a). In this embodiment, as the photosensitive drum 1 that is the center of the image forming process, an organic photosensitive drum in which a subbing layer, a carrier generation layer, and a carrier transfer layer, which are functional films, are sequentially coated on the outer peripheral surface of an aluminum cylinder is used. An organic photosensitive drum in which a subbing layer, a carrier generation layer, and a carrier transfer layer, which are functional films, are sequentially coated on the outer peripheral surface of an aluminum cylinder is used.

[0023] In addition, a charging roller 2 and a cleaning blade 6 as a cleaning member are disposed on the photoreceptor unit 13 so as to contact the circumferential surface of the photosensitive drum 1. The cleaning blade 6 contacts the photosensitive drum 1 counter-abuttingly, and the transfer residual toner removed from the surface of the photosensitive drum 1 by the cleaning blade 6 falls and is accommodated in the cleaning frame 14.

[0024] The charging roller 2, which is a charging member, is driven to rotate passively by pressing and contacting the roller portion of the conductive rubber against the photosensitive drum 1. Here, a predetermined DC voltage is applied from a charging bias application unit 303 (charging voltage application means), which is a high-voltage power source, to the core metal of the charging roller 2. As a result, a uniform dark portion potential (Vd) is formed on the surface of the photosensitive drum 1.

[0025] When the spot pattern of the laser beam emitted corresponding to the image data by the laser beam from the scanner unit 30 described above exposes the charged photosensitive drum 1, at the exposed portion, the surface charges disappear due to the carriers from the carrier generation layer, and the potential decreases. As a result, an electrostatic latent image is formed on the photosensitive drum 1, where the exposed portion has a predetermined bright portion potential (Vl) and the unexposed portion has a predetermined dark portion potential (Vd) as the surface potential.

[0026] On the other hand, the developing unit 3 is composed of a toner storage container, which is a developer container for storing a non-magnetic one-component developer (toner 9) as a developer. Inside the toner storage container, there are a developing roller 4 as a developer carrier for carrying the toner 9 and a supply roller 5 as a supply member for supplying the toner 9 to the developing roller 4. Further, a developing blade 8 for regulating the coat amount and charging the toner on the developing roller 4 supplied by the supply roller 5 is disposed.

[0027] The developing blade 8 is made of a thin plate-like member, forms a contact pressure by utilizing the spring elasticity of the thin plate, and its surface is brought into contact with the toner 9 and the developing roller 4. The toner 9 is triboelectrically charged by the rubbing between the developing blade 8 and the developing roller 4, is given an electric charge, and at the same time, the layer thickness is regulated. Also, in this embodiment, a predetermined voltage is applied to the developing blade 8 from a blade bias power source (not shown) to stabilize the toner coating.

[0028] Further, there is a toner storage chamber 21 in the direction in which the toner scraped off from the developing roller 4 by the developing blade 8 falls, and the toner is stored therein. Also, inside the toner storage chamber 21, a toner conveying member 22 for stirring and conveying the toner 9 is provided. That is, the toner conveying member 22 stirs the toner 9 stored in the toner storage chamber 21 and conveys the toner 9 in the direction of arrow G in the figure toward the upper part of the toner supply roller 5.

[0029] The toner conveying member 22 is composed of a rotating shaft 22a and a conveying sheet 22b. The conveying sheet 22b is attached over substantially the entire axial direction (longitudinal direction) of the rotating shaft 22a. The conveying sheet 22b is a rectangular sheet member, and can be made of, for example, a flexible resin sheet such as a polyester film or a polycarbonate film with a thickness of 50 μm to 250 μm. Also, the length of the conveying sheet 22b in the radial direction of the rotation radius of the rotating shaft 22a in the natural state is set to be longer than the distance from the rotation center of the rotating shaft 22a in the same direction to the wall surface W1 of the toner storage portion 21.

[0030] The developing roller 4 has a configuration in which an elastic layer made of a conductive elastic body is provided on the outer peripheral surface of a SUS (stainless steel) core metal so as to have a diameter of 12 mm. In this embodiment, polyurethane rubber is adopted as the elastic body. The developing roller 4 contacts and abuts against the photosensitive drum 1, and the surfaces of the two oppose each other and move in the same direction (in this embodiment, the direction from bottom to top) at the opposing portion (contact portion). They rotate respectively as described above. In this embodiment, the hardness of the developing roller 4 is made lower than that of the photosensitive drum 1, and it is arranged so that the penetration amount becomes 50 μm.

[0031] In order to keep the contact nip with the photosensitive drum 1 uniform, for the elastic rubber layer of the developing roller 4, it is preferable to use one with an MD-1 hardness measured by an MD-1 hardness meter of Kobunshi Keisoku Co., Ltd. of 50° or less. If the MD-1 hardness is too hard, it becomes difficult to sufficiently secure the contact nip with the photosensitive drum 1, and it becomes difficult to obtain the effects of the present invention. In this embodiment, one with a rubber hardness of 38° was used.

[0032] Also, the surface roughness of the developing roller 4 is set to 1.4 μm or more in terms of the center line average roughness Ra, so that the necessary amount of toner 9 is retained on the surface of the developing roller 4. If Ra becomes smaller than 1.4 μm, it becomes impossible to develop sufficient toner 9 onto the photosensitive drum 1, and it becomes difficult to obtain sufficient image density. In this embodiment, one with Ra = 2.0 was used. The center line average roughness Ra was measured by "Surfcorder SE350" manufactured by Kosaka Laboratory Ltd.

[0033] Also, in this embodiment, for the predetermined DC bias (Vdc) applied from the developing bias application unit 301 (developing voltage application means), which is a high-voltage power supply, to the developing roller 4, the toner 9 negatively charged by triboelectrification transfers only to the bright part potential part from the potential difference in the developing part where it contacts the photosensitive drum 1, and the electrostatic latent image is visualized. The difference between the DC bias Vdc and the bright part potential Vl is called the developing contrast, and by changing the magnitude of this, the amount of toner developed from the developing roller to the photosensitive drum 1 is controlled, and the image density, line width, etc. are controlled.

[0034] The supply roller 5 is disposed to form a predetermined contact portion (nip portion) N on the circumferential surface of the developing roller 4. Further, the supply roller 5 is an elastic sponge roller having a foam layer formed on the outer periphery of a conductive mandrel, and the supply roller 5 and the developing roller 4 are in contact with each other with a predetermined intrusion amount. The intrusion amount here refers to the amount of depression △E when the toner supply roller 5 is deformed into a concave shape by the developing roller 4. In this embodiment, the intrusion amount of the supply roller 5 into the developing roller 4, that is, the amount of depression △E in which the supply roller 5 is made concave by the developing roller 5, is set to 1 mm.

[0035] Further, the supply roller 5 rotates in the direction of the illustrated arrow E so that the surfaces thereof move in opposite directions at the contact portion N facing the developing roller 4. Thereby, while recovering the residual toner on the developing roller 4, the toner 9 is supplied to the developing roller 4.

[0036] Further, a predetermined DC bias (supply voltage) is applied to the supply roller 5 from a supply roller bias application unit 302 (supply member voltage application means) which is a high-voltage power source, making it easier for the negatively charged toner 9 to be supplied from the supply roller 5 to the developing roller 4.

[0037] (Regarding banding occurrence) Here, the occurrence of banding in the image is examined. The banding image is generated by a variation in the rotational peripheral speed ratio between the photosensitive drum 1 and the developing roller 4. The rotational peripheral speeds of the photosensitive drum 1 and the developing roller 4 can be represented by the surface movement speeds of these members respectively. The rotational peripheral speed ratio between the developing roller 4 and the photosensitive drum 1 can be represented as the ratio of the surface movement speed of the developing roller 4 to the surface movement speed of the photosensitive drum 1. The variation in the rotational peripheral speed ratio between the photosensitive drum 1 and the developing roller 4 occurs, for example, in the following cases.

[0038] In this embodiment, the toner conveyance member 22 and the developing roller 4 are connected by a gear (not shown), and by receiving the driving force of a driving motor as developing driving means 402 (driving source b), the toner conveyance member 22 and the developing roller 4 are configured to be driven. Therefore, in this embodiment, the developing roller 4 and the toner conveyance member 22 rotate together. When the peripheral speed of the developing roller 4 increases, the peripheral speed of the toner conveyance member 22 also increases. The developing driving means 402 may be regarded as a driving unit that operates according to the control of a control unit 201, which will be described later, in combination with the drum driving means 401 described above. When the peripheral speed of the developing roller 4 increases, the peripheral speed of the toner conveyance member 22 also increases. The developing driving means 402 may be regarded as a driving unit that operates according to the control of a control unit 201, which will be described later, in combination with the drum driving means 401 described above.

[0039] Using FIG. 4, the case where a change in the load on the conveyance sheet 22b of the toner conveyance member 22 occurs will be described. As shown in FIG. 4(a), when the conveyance sheet 22b contacts the wall surface W1 of the toner storage chamber 21 during rotation and deforms, and toner 9 is placed on the upper part of the conveyance sheet 22b, the load is the largest. On the other hand, as shown in FIG. 4(b), when the conveyance sheet 22b is released from the deformed state and restored to the natural state (original shape) by the elastic restoring force of the conveyance sheet 22b itself, the load is the lightest.

[0040] When the load on the conveyance sheet 22b changes in this way, the load applied to the developing roller 4 changes through the connecting gear, and the rotational peripheral speed of the developing roller 4 may temporarily fluctuate. Then, the rotational peripheral speed ratio between the photosensitive drum 1 and the developing roller 4 changes. As a result, the amount of toner developed from the developing roller 4 to the photosensitive drum 1 changes, and a banding image occurs.

[0041] Here, when the rotational peripheral speed ratio between the developing roller 4 and the photosensitive drum 1 is 1 or less, that is, when the photosensitive drum 1 rotates faster than the developing roller 4, a force acts in a direction to assist the rotation of the developing roller 4 by the photosensitive drum 1. That is, when the rotation of the developing roller 4 is likely to generate vibration, a force to assist the rotation acts on the developing roller 4 from the photosensitive drum 1. As a result, the rotation vibration of the developing roller 4 is suppressed, and the occurrence of a banding image can be prevented.

[0042] On the other hand, when the rotational peripheral speed ratio between the developing roller 4 and the photosensitive drum 1 is greater than 1, that is, when the developing roller 4 rotates faster than the photosensitive drum 1, when the rotational vibration of the developing roller 4 is likely to occur, since no force to assist the rotation acts on the developing roller 4 from the photosensitive drum 1, a situation where the rotational vibration of the developing roller 4 is likely to occur arises. In this case, while maintaining the rotational peripheral speed ratio between the developing roller 4 and the photosensitive drum 1, by reducing the process speed, the torque fluctuation can be moderated, the rotational vibration of the developing roller 4 can be suppressed, and the occurrence of banding can be prevented.

[0043] In addition, in the present embodiment, the toner conveyance member 22 and the developing roller 4 are configured to be driven by receiving the driving force of a drive motor as the developing drive means 402 (drive source b), but they may be independently driven.

[0044] Also, in the present embodiment, regarding the mode in which the rotational peripheral speed ratio between the developing roller 4 and the photosensitive drum 1 is greater than 1 (the high-concentration mode described later), the peripheral speed of the developing roller 4 is made smaller than that in the mode in which the rotational peripheral speed ratio between the developing roller 4 and the photosensitive drum 1 is 1 or less (the normal mode described later). As a result, as described above, the rotational peripheral speed of the toner conveyance member 22 also becomes smaller. Thereby, the torque fluctuation can be moderated, and furthermore, the occurrence of banding can be prevented.

[0045] Also, in this embodiment, regarding the high-concentration mode where the rotational peripheral speed ratio of the developing roller 4 to the photosensitive drum 1 is greater than 1, the peripheral speed of the developing roller 4 is made smaller compared to the normal mode where the rotational peripheral speed ratio of the developing roller 4 to the photosensitive drum 1 is 1 or less. As a result, as described above, a configuration is adopted in which the rotational peripheral speed of the toner conveying member 22 is also reduced, but it is not limited to this. For example, in an image forming mode for increasing the high concentration and the selection range of color tones by increasing the rotational peripheral speed ratio of the photosensitive drum 1 to the developing roller 4, which will be described later, the rotational speed of the toner conveying member 22 may be controlled to be increased. After suppressing the occurrence of banding by reducing the peripheral speed of the developing roller 4 to a certain extent, the conveying ability of the toner is increased by increasing the rotational speed of the toner conveying member 22 compared to the normal mode where the rotational peripheral speed ratio of the developing roller 4 to the photosensitive drum 1 is 1 or less. That is, the toner conveying member 22 may be provided with a toner conveying ability sufficient to handle high-concentration images, which will be described later, by increasing the rotational speed of the toner conveying member 22.

[0046] (Image forming mode) The image forming apparatus of this embodiment can operate in two image forming modes. The first mode (the first image forming mode) is an image forming mode for obtaining a normal image density (hereinafter referred to as the "normal mode"). The second mode (the second image forming mode) is an image forming mode for increasing the high concentration and the selection range of color tones by increasing the rotational peripheral speed ratio of the photosensitive drum 1 to the developing roller 4 while reducing the dark portion potential on the photosensitive drum (hereinafter referred to as the "high-concentration mode").

[0047] The specific control differences between the normal mode and the high-concentration mode in this embodiment are shown in Table 1 below. Here, the biases applied by the developing bias application unit 301, the supply roller bias application unit 302, and the charging bias application unit 303, and the light amount of the laser light from the scanner unit 30 are controlled by the control unit 201 based on the information obtained by the image forming mode information acquisition unit 200. The image forming mode information acquisition unit 200 acquires information input from an operation panel (not shown), a printer driver, or a host PC to the image forming apparatus 100.

[0048] As the control unit 201, for example, a control circuit or a computer having arithmetic resources such as a processor and a memory can be used. The control unit 201 performs various controls related to image formation, including exposure control, voltage control, drive control, and the like. The control unit 201 can also perform variable control of the rotation speed by the above-described toner conveyance member 22.

[0049] For example, the control unit 201 can variably control the peripheral speed when the developing roller 4 is rotationally driven by controlling the driving force of the driving motor as the developing driving means 402. The control unit 201 can further variably control the peripheral speed when the photosensitive drum 1 is rotationally driven by controlling the driving force of the driving motor as the drum driving means 401, or can variably control the peripheral speeds of the developing roller 4 and the photosensitive drum 1 individually. With such a configuration, the control unit 201 can change the rotational peripheral speed ratio between the developing roller 4 and the photosensitive drum 1, and can execute a first mode in which the rotational peripheral speed ratio is 1 or less and a second mode in which the ratio is greater than 1. Further, when executing the second mode, the control unit 201 may perform control such that the surface movement speed of the photosensitive drum 1 is smaller than when executing the first mode. [Table 1]

[0050] The rotational peripheral speed ratio in this embodiment represents the rotational peripheral speed of the developing roller 4 when the rotational peripheral speed of the photosensitive drum 1 is set to 1. Specifically, in the normal mode, the rotational peripheral speed of the photosensitive drum 1 is set to 300 mm / sec and the rotational peripheral speed of the developing roller 4 is set to 270 mm / sec. That is, in the normal mode, the rotational peripheral speed of the developing roller 4 is smaller than that of the photosensitive drum 1. By establishing this relationship, the photosensitive drum 1 assists the rotation of the developing roller 4, and the occurrence of banding can be suppressed. In order to fully exhibit this effect and effectively reduce the occurrence of banding caused by fluctuations in the rotational peripheral speed of the developing roller 4, it is desirable to set the rotational peripheral speed ratio to 0.95 or less.

[0051] Also, in order to obtain a sufficient density even in the normal mode, it is desirable to set the rotational speed ratio to 0.7 or more.

[0052] On the other hand, in the high-density mode, the rotational speed of the photosensitive drum 1 is set to 100 mm / sec, and the rotational speed of the developing roller is set to 120 mm / sec. As shown in FIG. 3, this is because when the rotational speed of the developing roller 4 is higher than that of the photosensitive drum 1, the amount of toner developed from the developing roller 4 onto the photosensitive drum increases for the same development contrast, and the image density becomes higher. Thus, the density required for the high-density mode can be obtained without degrading the character quality. For the high-density mode, it is desirable that the rotational speed ratio is 1.5 or less. This is because an increase in the rotational speed ratio means an increase in the rotational speed of the developing roller, and banding due to fluctuations in the rotational speed of the developing roller 4 is likely to occur.

[0053] Also, in this embodiment, as shown in Table 1, the value of the development contrast in the high-density mode is set larger than the value of the development contrast in the normal mode. Thereby, in the high-density mode, the selection range of higher density and color tone can be increased.

[0054] Also, in both the normal mode and the high-density mode, the supply roller bias is applied so that the difference between the development potential and the supply roller bias becomes a negative electric field in the toner supply direction. And the difference between the development potential and the supply roller bias is made larger in the high-density mode than in the normal mode. Thereby, the amount of toner supplied to the developing roller in the high-density mode increases, a further high density can be obtained, and a shortage of the toner supply amount due to continuous printing of high-density images can be prevented.

[0055] In the configuration and control described above, in the high-density mode, while maintaining the state where the rotation of the developing roller 4 is faster, the process speed is decelerated by reducing the rotational peripheral speed of each of the developing roller 4 and the photosensitive drum 1. As a result, the torque fluctuation becomes gentle, and the rotational vibration of the developing roller 4 can be suppressed, so that the occurrence of banding can be prevented. Therefore, the normal mode and the high-density mode in this embodiment are set to the control shown in Table 1, and while preventing banding, a high-quality image required by the user can be provided.

[0056] (Experiment) In order to verify the above effects, the following verification experiments were conducted. A two-sheet intermittent printing durability test was performed in an environment of a temperature of 23°C and a humidity of 50%. In this printing durability test, the E character with an image ratio of 1% was printed. 30,000 sheets were printed in this mode, and a halftone image was printed regularly to check whether horizontal streaks caused by banding occurred. It was marked as "×" if it occurred, and "〇" if it did not occur.

[0057] Also, in the high-density mode, solid black was printed, and the density of solid black was measured using a SPECTORDEN SITOMETER 500 manufactured by X-Rite.

[0058] Also, in order to compare with the effects of this configuration, experiments of Comparative Examples 1 to 3 in which the control values were changed as shown in Table 2 were conducted from the examples.

Table 2

[0059] In Comparative Example 1, the rotational peripheral speed ratio in the normal mode is 1.2. In Comparative Example 2, the rotational peripheral speed ratio in the high-density mode is 0.9. In Comparative Example 3, the rotational speed of the photosensitive drum in the high-density mode is the same as 300 mm / sec in the normal mode.

[0060] The results are shown in Table 3. In the banding evaluation, "×" was used when horizontal streaks caused by banding occurred, and "〇" was used when they did not occur. In the density evaluation by measuring solid black in the high-concentration mode, "〇" was used if the density was 1.5 or more, and "×" was used if the density was less than 1.5.

Table 3

[0061] In the configuration of this embodiment, the occurrence of banding could be suppressed in both the normal mode and the high-concentration mode, and the density required in the high-concentration mode could be obtained.

[0062] On the other hand, in the configuration of Comparative Example 1, since the rotation speed of the developing roller 4 was higher than that of the photosensitive drum 1 in the normal mode, when uneven rotation of the developing roller 4 occurred, the rotational assist by the photosensitive drum 1 did not work, resulting in the occurrence of a banding image.

[0063] Also, in the configuration of Comparative Example 2, the rotation speed of the photosensitive drum 1 remained high in the high-concentration mode as it was in the normal mode. Therefore, the torque fluctuation could not be alleviated, and the rotation blur of the developing roller 4 could not be suppressed, resulting in the occurrence of a banding image.

[0064] Also, in the configuration of Comparative Example 3, the occurrence of a banding image could be suppressed, but toner 9 was not sufficiently supplied in the high-concentration mode, and the required density could not be obtained.

[0065] As described above, in the normal mode, by setting the rotational peripheral speed ratio so that the rotational speed of the developing roller 4 is lower than that of the photosensitive drum 1, even when uneven rotational peripheral speed of the developing roller 4 is likely to occur, the occurrence of a banding image can be suppressed. Also, in the high-concentration mode, even when the rotational speed of the developing roller 4 is increased relative to the rotational speed of the photosensitive drum 1, by slowing down the process speed, the occurrence of a banding image can be suppressed. It can cut. By having these two modes, it becomes possible to answer the needs of users who seek high-density images while suppressing the generation of banding images.

[0066] Therefore, according to the image forming apparatus of the present invention, in an image forming apparatus having a plurality of modes with different rotational peripheral speed ratios between the photosensitive drum 1 and the developing roller 4, it is possible to form a high-quality image while suppressing the occurrence of banding in any mode. Thus, in both the normal mode for obtaining a general image density and the high-density mode for realizing an increase in high density and color tone, it is possible to achieve the density required by the user while suppressing image banding.

Explanation of Reference Numerals

[0067] 1: Photosensitive drum, 2: Charging roller, 4: Developing roller, 21: Toner storage chamber, 30: Scanner unit, 100: Image forming apparatus, 301: Developing bias application unit, 303: Charging bias application unit

Claims

1. A rotatable image carrier, A rotatable developer carrier that supplies a developer to the surface of the image carrier in a nip portion formed between the image carrier and the developer carrier, A supply member that supplies the developer to the developer carrier, Developing voltage application means for applying a developing voltage to the developer carrier, Supply voltage application means for applying a supply voltage to the supply member, A drive unit that rotationally drives the image carrier and the developer carrier so that their circumferential speeds can be variably adjusted individually, A control unit that controls the developing voltage application means, the supply voltage application means, and the drive unit, comprising, The control unit, i) Controls to be capable of executing a first image forming mode in which a rotational circumferential speed ratio, which is the ratio of the surface movement speed of the developer carrier to the surface movement speed of the image carrier, is 1 or less, and a second image forming mode in which the rotational circumferential speed ratio is greater than 1, ii) Controls so that the surface movement speed of the developer carrier in the second image forming mode is smaller than the surface movement speed of the developer carrier in the first image forming mode, iii) Controls so that the difference between the developing voltage and the supply voltage is greater when executing the second image forming mode than when executing the first image forming mode, An image forming apparatus characterized by the above.

2. When executing the second image forming mode, the control unit controls so that the surface movement speed of the image carrier is smaller than that in the first image forming mode The image forming apparatus according to claim 1, characterized by the above.

3. Exposure means for exposing the surface of the image carrier based on image information to form an electrostatic latent image further comprising, When the difference between the developing voltage and the surface potential formed on the surface of the image carrier by being exposed by the exposure means is greater when the second image forming mode is executed than when the first image forming mode is executed, the control unit controls so that the image forming apparatus according to claim 1 or 2, characterized in that

4. In the first image forming mode, the control unit controls so that the rotational peripheral speed ratio of the developer carrier to the image carrier is 0.95 or less. The image forming apparatus according to any one of claims 1 to 3, characterized in that

5. In the first image forming mode, the control unit controls so that the rotational peripheral speed ratio of the developer carrier to the image carrier is 0.7 or more. The image forming apparatus according to claim 4, characterized in that

6. In the second image forming mode, the control unit controls so that the rotational peripheral speed ratio of the developer carrier to the image carrier is 1.5 or less. The image forming apparatus according to any one of claims 1 to 5, characterized in that

7. having a developer container that stores the developer supplied to the developer carrier, further comprising a conveying member that is disposed inside the developer container and stirs and conveys the developer by rotating about a rotation axis, The image forming apparatus according to any one of claims 1 to 6, characterized in that the control unit controls so that the rotation speed of the conveying member changes between when the first image forming mode is executed and when the second image forming mode is executed.

8. The image forming apparatus according to claim 7, characterized in that the control unit controls so that the rotation speed of the conveying member is higher when the second image forming mode is executed than when the first image forming mode is executed.

Images