Image forming apparatus

Abstract

To provide a configuration that can improve the efficiency in removing toner on a recovery roller while reducing downtime.SOLUTION: A carrier recovery device 5Y recovers a carrier attached to a photoconductor drum 28Y. A bias including AC voltage is applied to a recovery roller 52 of the carrier recovery device 5Y. The recovery roller 52 is arranged downstream of a developing position where an electrostatic latent image formed on the photoconductor drum 28Y is developed and is arranged upstream of a transfer position where a toner image carried by the photoconductor drum 28Y is transferred to an intermediate transfer belt 24 in terms of the direction of rotation of the photoconductor drum 28Y. During an image forming operation, a first bias is applied to the recovery roller 52, and in a predetermined mode (toner removal mode) during non-image formation, a second bias is applied to the recovery roller 52. The duty ratio of AC voltage of the second bias is lower than the duty ratio of AC voltage of the first bias.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile, or a multifunction machine having a plurality of these functions. [Background technology]

[0002] As an image forming apparatus, a configuration in which a toner image is formed using a two-component developer containing a non-magnetic toner and a magnetic carrier has been conventionally known. In this configuration, normally, in the developing process, an electrostatic latent image on a photosensitive drum is developed into a toner image by toner, but a certain percentage of the carrier may also adhere to the photosensitive drum (carrier adhesion). If carrier adhesion occurs, it will affect the output image, so for example, Patent Document 1 discloses a configuration equipped with a carrier recovery device that recovers the carrier that has adhered to the photosensitive drum.

[0003] The carrier recovery device described in Patent Document 1 includes a recovery roller and a magnet roller provided inside the recovery roller, and is further configured to apply a voltage in which a DC voltage and an AC voltage are superimposed to the recovery roller. As a result, the carrier on the photosensitive drum is recovered to the recovery roller by the magnetic force of the magnet roller and the electrostatic force due to the applied voltage. When the carrier is recovered by the carrier recovery device in this way, some toner may also be recovered by the recovery roller. For this reason, in the case of the configuration described in Patent Document 1, a toner removal mode is executed when no image is formed, and in the toner removal mode, the rotation speed of the photosensitive drum is reduced compared to when an image is formed, and an AC voltage is applied to the recovery roller to return the toner attached to the recovery roller to the photosensitive drum. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2020-148920 A Summary of the Invention [Problem to be solved by the invention]

[0005] When the rotation speed of the photosensitive drum is reduced in the toner removal mode as in the configuration described in Patent Document 1, it is necessary to change the rotation speed of the photosensitive drum between image formation and toner removal mode. This lengthens the execution time of the toner removal mode, and the downtime during which image formation cannot be performed increases accordingly. On the other hand, when the toner removal mode is performed without reducing the rotation speed of the photosensitive drum, it is necessary to extend the execution time of the toner removal mode in order to sufficiently remove the toner adhering to the collection roller, and as a result, the downtime increases.

[0006] An object of the present invention is to provide a configuration capable of improving the efficiency of toner removal by a collection roller while suppressing downtime. [Means for solving the problem]

[0007] One aspect of the present invention is an image forming apparatus capable of performing an image forming operation, the image forming apparatus comprising: a developing device having a rotatable image carrier on which an electrostatic latent image is formed, a developing container containing a developer including a toner and a carrier, and a developer carrier that carries the developer to develop the electrostatic latent image formed on the image carrier into a toner image; a transfer member to which the toner image carried on the image carrier is transferred, a rotatable sleeve disposed opposite the image carrier, and a magnet non-rotatably disposed inside the sleeve, the carrier recovery device recovering the carrier on the image carrier; a bias application unit that applies a bias including an AC voltage to the sleeve; and a control unit that controls the bias application unit, the sleeve being located downstream of a development position where the electrostatic latent image formed on the image carrier is developed in terms of a rotation direction of the image carrier, and the toner image carried on the image carrier being transferred to the transfer unit. the control unit controls the bias application unit to apply a first bias to the sleeve during an image formation operation, and controls the bias application unit to apply a second bias to the sleeve in a predetermined mode during non-image formation; when the duration of a voltage of the opposite polarity to the toner charge polarity with respect to the DC potential of the sleeve is t1, and the duration of a voltage of the same polarity as the toner charge polarity with respect to the DC potential of the sleeve is t2, during one period of the bias AC voltage applied to the sleeve by the bias application unit, and when the ratio of t1 to one period (t1+t2) of the bias AC voltage applied to the sleeve by the bias application unit is taken as a duty ratio, the duty ratio of the AC voltage of the second bias is lower than the duty ratio of the AC voltage of the first bias.

[0008] One aspect of the present invention is an image forming apparatus capable of performing an image forming operation, the image forming apparatus comprising: a developing device having a rotatable image carrier on which an electrostatic latent image is formed; a developing container containing a developer including toner and a carrier; and a developer carrier carrying the developer to develop the electrostatic latent image formed on the image carrier into a toner image; a transfer member to which the toner image carried on the image carrier is transferred; a rotatable sleeve disposed opposite the image carrier; and a magnet non-rotatably disposed inside the sleeve, the carrier recovery device recovering the carrier on the image carrier; a bias application unit applying a bias including an AC voltage to the sleeve; and a control unit controlling the bias application unit, the sleeve being located downstream of a development position where the electrostatic latent image formed on the image carrier is developed in terms of a rotation direction of the image carrier, and the toner image carried on the image carrier being transferred to the developing container; the control unit controls the bias application unit to apply a first bias to the sleeve during an image formation operation, and controls the bias application unit to apply a second bias to the sleeve in a predetermined mode during non-image formation; when the duration of one period of the bias AC voltage applied by the bias application unit to the sleeve is t1, and the duration of the voltage of the opposite polarity to the toner charge polarity with respect to the DC potential of the sleeve is t2, and when the ratio of t1 to one period (t1+t2) of the bias AC voltage applied to the sleeve by the bias application unit is taken as a duty ratio, the duty ratio of the AC voltage of the second bias is lower than 50%.

[0009] According to one aspect of the present invention, there is provided an image forming apparatus capable of performing an image forming operation, the image forming apparatus including: a developing device having a rotatable image carrier on which an electrostatic latent image is formed; a developing container containing a developer including a toner and a carrier; and a developer carrier carrying the developer to develop the electrostatic latent image formed on the image carrier into a toner image; a transfer member to which the toner image carried on the image carrier is transferred; a rotatable sleeve disposed opposite the image carrier; and a magnet non-rotatably disposed inside the sleeve, the carrier recovery device recovering the carrier on the image carrier; a bias application unit applying a bias in which a DC voltage and an AC voltage are superimposed to the sleeve; and a control unit controlling the bias application unit, the sleeve being configured to apply a bias to the electrostatic latent image formed on the image carrier in a direction in which the electrostatic latent image is formed, the electrostatic latent image being formed on the image carrier in a direction in which the electrostatic latent image is formed, the electrostatic latent image being formed on the image carrier in a direction in which the electrostatic latent image is formed on the image carrier. The image forming apparatus is characterized in that the control unit controls the bias application unit to apply a first bias to the sleeve during image formation operation, and controls the bias application unit to alternately apply a second bias and a third bias to the sleeve in a predetermined mode when image formation is not being performed, the first bias being a DC voltage whose polarity is the same as the charging polarity of the toner, the second bias being a DC voltage superimposed on an AC voltage such that the polarity of the potential of the sleeve relative to the image carrier is negative, and the third bias being a DC voltage superimposed on an AC voltage such that the polarity of the potential of the sleeve relative to the image carrier is positive. Effect of the Invention

[0010] According to the present invention, it is possible to improve the efficiency of toner removal by the collection roller while suppressing downtime. [Brief description of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to a first embodiment. [Diagram 2] 1 is a schematic cross-sectional view of a photosensitive drum according to a first embodiment. [Diagram 3] FIG. 2 is a cross-sectional view showing a schematic configuration of an image forming unit according to the first embodiment. [Figure 4] FIG. 2 is a schematic cross-sectional view of the developing device according to the first embodiment. [Diagram 5] FIG. 2 is a control block diagram of the image forming apparatus according to the first embodiment. [Figure 6] FIG. 2 is a cross-sectional view showing a schematic configuration of the carrier recovery device according to the first embodiment. [Figure 7] 6 is a graph showing the charge distribution of the toner adhering to the collection roller. [Figure 8] FIG. 11 is a diagram showing a voltage pattern in a toner removal mode in a comparative example. [Figure 9] FIG. 4 is a schematic diagram showing a first voltage pattern and a second voltage pattern in a toner removal mode according to the first embodiment. [Figure 10] FIG. 2A is a diagram for explaining toner that moves when a first voltage pattern is applied, and FIG. 2B is a diagram for explaining toner that moves when a second voltage pattern is applied in the first embodiment. [Figure 11] 5 is a flowchart of an image forming mode and a toner removal mode according to the first embodiment. [Figure 12] 5 is a flowchart of a toner removal mode according to the first embodiment. [Figure 13] 11 is a flowchart of a toner removal mode according to a comparative example. [Figure 14] FIG. 11 is a schematic diagram showing a first voltage pattern and a second voltage pattern in a toner removal mode according to a second embodiment. [Figure 15] 10 is a flowchart of an image forming mode and a toner removal mode according to a second embodiment. [Figure 16] 13A is a schematic diagram showing a first voltage pattern and a second voltage pattern in a toner removal mode in a high humidity environment, a normal humidity environment, and a low humidity environment according to a third embodiment; FIG. [Figure 17] 13 is a graph showing the relationship between the first and second voltage patterns and the environmental humidity in the toner removal mode according to the third embodiment. [Figure 18]13 is a flowchart of an image forming mode and a toner removal mode according to a third embodiment. [Figure 19] 13 is a graph showing the relationship between the first and second voltage patterns and the environmental humidity in the toner removal mode according to the fourth embodiment. [Figure 20] 13 is a flowchart of an image forming mode and a toner removal mode according to a fourth embodiment. [Figure 21] 13A is a schematic diagram showing a first voltage pattern and a second voltage pattern in a toner removal mode in a high humidity environment, a normal humidity environment, and a low humidity environment according to a fifth embodiment. FIG. [Figure 22] 13 is a graph showing the relationship between the DC bias difference and environmental humidity in a toner removal mode according to the fifth embodiment. [Figure 23] 13 is a flowchart of an image forming mode and a toner removal mode according to a fifth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] <First embodiment> The first embodiment will be described with reference to Figures 1 to 13. First, the schematic configuration of an image forming apparatus 100 of this embodiment will be described with reference to Figures 1 to 3. Note that Figure 1 is a schematic cross-sectional view of the entire image forming apparatus 100, Figure 2 is a schematic cross-sectional view of the periphery of a photosensitive drum 28Y in an image forming unit PY, and Figure 3 is a schematic cross-sectional view of the image forming unit PY.

[0013] [Image forming device] The image forming apparatus 100 is a full-color image forming apparatus, and in this embodiment, for example, is an MFP (Multi-Function Peripheral) having a copy function, a printer function, and a scan function. The image forming apparatus 100 forms a toner image on a recording material in response to image information from an external device, such as a document reading device connected to the image forming apparatus main body or a personal computer (PC) connected to the image forming apparatus main body so as to be able to communicate with the image forming apparatus main body. As shown in FIG. 1, the image forming apparatus 100 has image forming units PY, PM, PC, and PK arranged in parallel, which respectively perform image forming processes for four colors of toner images, namely yellow, magenta, cyan, and black.

[0014] The image forming units PY, PM, PC, and PK of the respective colors include primary chargers 21Y, 21M, 21C, and 21K, developing devices 1Y, 1M, 1C, and 1K, optical writing units (exposure devices) 22Y, 22M, 22C, and 22K, photosensitive drums 28Y, 28M, 28C, and 28K, cleaning devices 26Y, 26M, 26C, and 26K, and carrier recovery devices 5Y, 5M, 5C, and 5K. The image forming apparatus 100 also includes a transfer device 2 and a fixing device 3. Since the image forming units PY, PM, PC, and PK of the respective colors have the same configuration, the following description will be given using the image forming unit PY as a representative.

[0015] The photosensitive drum 28Y, which serves as an image carrier that carries an electrostatic latent image on its surface, is a photosensitive member having a photosensitive layer made of a resin such as polycarbonate containing an organic photoconductor (OPC), and is configured to rotate at a predetermined speed. The primary charger 21Y is made of a corona discharge electrode disposed around the photosensitive drum 28Y, and charges the surface of the photosensitive drum 28Y with ions generated.

[0016] The optical writing unit 22Y incorporates a scanning optical device and exposes the charged photosensitive drum 28Y based on image data, thereby lowering the potential of the exposed portion and forming a charge pattern (electrostatic latent image) corresponding to the image data. The developing device 1Y transfers the contained developer to the photosensitive drum 28Y in a developing section D (see FIG. 3) facing the photosensitive drum 28Y, developing the electrostatic latent image formed on the photosensitive drum 28Y. The developer is a mixture of carrier and toner corresponding to each color, and the electrostatic latent image is visualized by the toner.

[0017] The transfer device 2 has primary transfer rollers 23Y, 23M, 23C, and 23K, an intermediate transfer belt 24 as an intermediate transfer body (transfer member), and a secondary transfer roller 25. The intermediate transfer belt 24 is wound around the primary transfer rollers 23Y, 23M, 23C, and 23K and a plurality of rollers, and is supported so as to be capable of running. The primary transfer rollers 23Y, 23M, 23C, and 23K correspond to the colors Y (yellow), M (magenta), C (cyan), and K (black) in order from the top of FIG. 1. The secondary transfer roller 25 is disposed outside the intermediate transfer belt 24, and is configured so that a recording material can pass between the secondary transfer roller 25 and the intermediate transfer belt 24. The recording material is, for example, a sheet such as paper or a plastic sheet.

[0018] The toner images of each color formed on the photosensitive drums 28Y, 28M, 28C, and 28K are sequentially primarily transferred onto the intermediate transfer belt 24 at primary transfer sections T1Y, T1M, T1C, and T1K as transfer sections by primary transfer rollers 23Y, 23M, 23C, and 23K, forming color toner images in which yellow, magenta, cyan, and black layers are superimposed. The formed toner images are transferred by secondary transfer roller 25 to a recording material conveyed from a cassette or the like containing the recording material at secondary transfer section T2. ​​Pressure and heat are applied to the recording material to which the toner images have been transferred in fixing device 3. This melts the toner on the recording material, and the color image is fixed to the recording material.

[0019] After the toner images are primarily transferred to the intermediate transfer belt 24, the surfaces of the photosensitive drums 28Y, 28M, 28C, and 28K are cleaned by the cleaning devices 26Y, 26M, 26C, and 26K, respectively. The cleaning devices 26Y, 26M, 26C, and 26K clean the surfaces of the photosensitive drums 28Y, 28M, 28C, and 28K, for example, by bringing cleaning blades into contact with the surfaces of the photosensitive drums 28Y, 28M, 28C, and 28K, respectively. The photosensitive drums 28Y, 28M, 28C, and 28K whose surfaces have been cleaned are prepared for the next image forming process.

[0020] 2 and 3, the carrier recovery devices 5Y, 5M, 5C, and 5K recover carriers attached to the photosensitive drums 28Y, 28M, 28C, and 28K downstream of the developing unit D and upstream of the primary transfer units T1Y, T1M, T1C, and T1K in the rotational direction of the photosensitive drums 28Y, 28M, 28C, and 28K. As shown in FIG. 3, each of the carrier recovery devices 5Y, 5M, 5C, and 5K includes a recovery roller 52 that is disposed facing the photosensitive drums 28Y, 28M, 28C, and 28K and rotates, and a magnet roller 51 that is disposed inside the recovery roller 52 in a non-rotating manner and serves as a magnetic field generating unit that attracts the carriers to the surface of the recovery roller 52 by magnetic force. A detailed description of the carrier recovery devices 5Y, 5M, 5C, and 5K will be given later.

[0021] The developer storage units 27Y, 27M, 27C, and 27K are provided corresponding to the developing devices 1Y, 1M, 1C, and 1K, respectively, and are loaded with replaceable bottles containing developers corresponding to the respective colors of yellow, magenta, cyan, and black, in order from the top. The developer storage units 27Y, 27M, 27C, and 27K are configured to be able to transport (supply) developers to the developing devices 1Y, 1M, 1C, and 1K corresponding to the colors of the developers stored therein.

[0022] For example, the toner weight ratio of the developer stored in the bottle is 80 to 95%, and the toner weight ratio of the developer in the developing devices 1Y, 1M, 1C, and 1K is 5 to 10%. Therefore, when toner is consumed by development in the developing devices 1Y, 1M, 1C, and 1K, developer containing toner corresponding to the consumed amount is replenished, and the toner weight ratio of the developer in the developing devices 1Y, 1M, 1C, and 1K is maintained constant.

[0023] [Developing device] Next, the developing devices 1Y, 1M, 1C, and 1K will be described with reference to Fig. 4. Since the developing devices 1Y, 1M, 1C, and 1K have the same configuration, the developing device 1Y will be described below as a representative. The developing device 1Y has a first developing roller 30, a second developing roller 31, a peeling roller 32, a developer supply screw 42, a developer stirring screw 43, and a developer recovery screw 44, and these members are contained in a developing container 60.

[0024] The first developing roller 30 is a developer carrier that is driven to rotate, and is disposed adjacent to the photosensitive drum 28Y so that its rotation axis is substantially parallel to the rotation axis of the photosensitive drum 28Y. The first developing roller 30 has a rotating first sleeve 33 and a first magnet (fixed magnet) 36 that is disposed inside the first sleeve 33 in a non-rotating manner and attracts the developer to the surface of the first sleeve 33 by magnetic force. The first developing roller 30 attracts (carries) the developer pumped up from the developer supply screw 42 by magnetic force, and develops the electrostatic latent image formed on the rotating photosensitive drum 28Y (image carrier) with the developer.

[0025] The first sleeve 33 is a non-magnetic cylindrical member, and is driven to rotate around a rotation shaft 39. The rotation direction of the first sleeve 33 is clockwise as indicated by the arrow in Fig. 4, which is the opposite direction to the rotation direction of the photosensitive drum 28Y in this embodiment. Therefore, the first sleeve 33 and the photosensitive drum 28Y rotate in the same direction at positions facing each other.

[0026] The first magnet 36 is disposed inside the first sleeve 33, and has a plurality of sector-shaped magnetic poles and sector-shaped non-magnetic pole portions. A space that allows the first sleeve 33 to rotate is disposed between the inner periphery of the first sleeve 33 and the outer periphery of the first magnet 36.

[0027] The developer attracted onto the first sleeve 33 is transported toward the photosensitive drum 28Y by the rotational movement of the first sleeve 33, and develops the latent image formed on the photosensitive drum 28Y. After developing the latent image formed on the photosensitive drum 28Y, the developer on the first sleeve 33 is transported to the vicinity of the second developing roller 31 by the rotational movement of the first sleeve 33. Then, in the vicinity of the closest position between the first developing roller 30 and the second developing roller 31, the developer is peeled off from the first sleeve 33 and transferred onto the second sleeve 34 by the magnetic field generated by the first magnet 36 contained in the first developing roller 30 and the second magnet 37 contained in the second developing roller 31.

[0028] The second developing roller 31 is a developer carrier that is driven to rotate, and is disposed downstream of the first developing roller 30 in the rotation direction of the photosensitive drum 28Y and above the center of rotation of the first developing roller 30 in the vertical direction, and receives the developer from the first developing roller 30 by magnetic force. Similar to the first developing roller 30, the second developing roller 31 is disposed adjacent to the photosensitive drum 28Y such that its rotation axis is substantially parallel to the rotation axis of the photosensitive drum 28Y. Therefore, the rotation axes of the second developing roller 31 and the first developing roller 30 are substantially parallel to each other.

[0029] The second developing roller 31 has a rotating second sleeve 34 and a second magnet (fixed magnet) 37 that is non-rotatingly disposed inside the second sleeve 34 and attracts the developer to the surface of the second sleeve 34 by magnetic force. The second developing roller 31 receives the developer from the first developing roller 30 (first sleeve 33) based on the magnetic force, attracts (carries) the developer, and develops the electrostatic latent image formed on the rotating photosensitive drum 28Y with the developer. A peeling roller 32, which will be described later, is located on the side of the second developing roller 31.

[0030] The second sleeve 34 is a non-magnetic cylindrical member, and is driven to rotate around the rotation shaft 40. The rotation direction of the second sleeve 34 is clockwise as shown by the arrow in FIG. 4, which is opposite to the rotation direction of the photosensitive drum 28Y in this embodiment. Therefore, the second sleeve 34 and the photosensitive drum 28Y rotate in the same direction when they face each other. Moreover, the second sleeve 34 and the first sleeve 33 rotate in opposite directions when they face each other.

[0031] The second magnet 37 is disposed inside the second sleeve 34, and has a plurality of sector-shaped magnetic poles and sector-shaped non-magnetic pole portions. A space is disposed between the inner circumference of the second sleeve 34 and the outer circumference of the second magnet 37, allowing the second sleeve 34 to rotate.

[0032] The developer attracted onto the second sleeve 34 is transported toward the photosensitive drum 28Y by the rotational operation of the second sleeve 34, and develops the latent image formed on the photosensitive drum 28Y. After the latent image formed on the photosensitive drum 28Y is developed, the developer remaining on the second sleeve 34 is transported to the vicinity of the peeling roller 32 by the rotational operation of the second sleeve 34. Then, in the vicinity of the closest position between the second developing roller 31 and the peeling roller 32, the developer is transferred from the second sleeve 34 to the third sleeve 35 of the peeling roller 32 by the magnetic field generated by the second magnet 37 contained in the second developing roller 31 and the third magnet 38 contained in the peeling roller 32.

[0033] The peeling roller 32 as a peeling section is disposed on the opposite side to the photosensitive drum 28Y with respect to the rotation center of the second sleeve 34, and peels off the developer from the second developing roller 31 after the electrostatic latent image on the photosensitive drum 28Y is developed by the second developing roller 31. Specifically, the peeling roller 32 is a developer carrier that is driven to rotate, and is disposed between the second developing roller 31 and the developer recovery screw 44 such that its rotation center is above the rotation center of the second developing roller 31.

[0034] The peeling roller 32 is disposed so that its rotation axis is substantially parallel to the rotation axis of the second developing roller 31. The peeling roller 32 has a rotating third sleeve 35 and a third magnet (fixed magnet) 38 that is non-rotatingly disposed inside the third sleeve 35 and that attracts the developer to the surface of the third sleeve 35 by magnetic force, and is configured to receive the developer from the second developing roller 31 based on the magnetic force.

[0035] The third sleeve 35 is a non-magnetic cylindrical member, and is driven to rotate around the rotation shaft 41. The rotation direction of the third sleeve 35 is counterclockwise as indicated by the arrow in Fig. 4, which is the opposite direction to the rotation direction of the second sleeve 34 in this embodiment. Therefore, the third sleeve 35 and the second sleeve 34 rotate in the same direction at positions facing each other.

[0036] The third magnet 38 is disposed inside the third sleeve 35, and has a plurality of sector-shaped magnetic poles and sector-shaped non-magnetic pole portions. A space that allows the third sleeve 35 to rotate is disposed between the inner periphery of the third sleeve 35 and the outer periphery of the third magnet 38.

[0037] The developer attracted onto the third sleeve 35 is transported downstream in the rotation direction by the rotation of the third sleeve 35, and is peeled off from the third sleeve 35 by the third magnet 38 contained in the peeling roller 32 at a position close to the developer recovery screw 44, and falls by its own weight toward the guide member 45 located vertically below. The developer that has fallen onto the guide member 45 is then guided by its own weight toward the developer recovery screw 44.

[0038] The guide member 45 and the developer recovery screw 44 configure a developer recovery section 47 as a recovery section that recovers the developer peeled off from the third sleeve 35 on the peeling roller 32. In the developer recovery section 47, the developer recovery screw 44 is located below the rotation center of the peeling roller 32 in the vertical direction, and conveys the developer delivered (recovered) from the peeling roller 32 while stirring it. The guide member 45 as a guide section is located below the rotation center of the peeling roller 32 in the vertical direction, and guides the developer peeled off by the peeling roller 32 toward the developer recovery screw 44.

[0039] The developer recovery screw 44, which serves as a recovery member and a transport section, transports the recovered developer to a developer circulating section 46, which will be described next. That is, the developer recovery screw 44 is a screw transport member used to transport the recovered developer in one direction while stirring it as it slides down the inclined surface of the guide member 45.

[0040] The developer circulating section 46 is a supply section for supplying the developer to the first developing roller 30, and includes a regulating member 64, a developer supply screw 42, and a developer stirring screw 43. In the developer circulating section 46, the developer is stirred in the developer supply screw 42 and the developer stirring screw 43 and transported in a substantially horizontal direction, and is supplied to the first developing roller 30. As described above, the developer collected by the developer collecting section 47 falls by its own weight and is introduced into the developer circulating section 46. That is, the developer circulating section 46 is located lower than the developer collecting section 47 in the vertical direction.

[0041] The developer supply screw 42, developer stirring screw 43, and developer recovery screw 44 are screw transport members that transport the developer in one direction while stirring it, and the developer supply screw 42 and developer stirring screw 43 are located below the rotation center of the developer recovery screw 44 in the vertical direction. The developer supply screw 42, developer stirring screw 43, and developer recovery screw 44 are also arranged so that their rotation axes are approximately parallel to each other. The rotation axes of these screws are also approximately parallel to the rotation axis of the first developing roller 30.

[0042] The developer supply screw 42 is located between the first developing roller 30 and the developer stirring screw 43, and a partition wall 48 of the developing container 60 is disposed between the developer supply screw 42 and the developer stirring screw 43. The partition wall 48 of the developing container 60 extends along the rotation axis direction of the developer supply screw 42 and the developer stirring screw 43. The partition wall 48 is provided with a communication port (not shown) that communicates between a first conveying path 61, through which the developer is conveyed by the developer supply screw 42, and a second conveying path 62, through which the developer is conveyed by the developer stirring screw 43.

[0043] The developer stirred by the developer recovery screw 44 passes through a communication port (not shown) formed in a partition wall 63 of the developing container 60 between the developer recovery screw 44 and the developer supply screw 42, and falls by its own weight toward the developer supply screw 42. The above-mentioned guide member 45 is formed integrally with the partition wall 63, and the developer recovery screw 44 is disposed above the partition wall 63.

[0044] The developer transport directions of the developer supply screw 42 and the developer stirring screw 43 are opposite to each other. The start side (upstream end side in the developer transport direction) and the end side (downstream end side in the developer transport direction) of the first transport path 61 in which the developer supply screw 42 is arranged communicate with the end side and the start side of the second transport path 62 in which the developer stirring screw 43 is arranged via a communication port provided in the partition wall 48. Therefore, the developer circulates in the rotation direction of the developer supply screw 42 and the developer stirring screw 43 shown by the arrows in FIG. 4 and in the approximately horizontal direction within the developing container 60, and a part of the developer is supplied toward the first developing roller 30.

[0045] The developer supply port 65 is disposed above the developer stirring screw 43 in the developing container 60, and is connected to the developer storage unit 27Y (see FIG. 1). The developer supply port 65 is configured to be able to supply the developer stored in a bottle loaded in the developer storage unit 27Y to the second conveying path 62 in which the developer stirring screw 43 is disposed.

[0046] As described above, the toner weight ratio of the developer stored in the bottle of developer storage section 27Y is greater than the toner weight ratio of the developer in developing device 1Y, so by adjusting the developer supplied to developer stirring screw 43, it is possible to maintain the toner weight ratio of the developer in developing device 1 constant.

[0047] The toner concentration detection sensor 49 is disposed to detect the toner concentration in the developer contained in the developer circulating section 46. The toner concentration detection sensor 49 is a sensor that detects the magnetic permeability of the developer. The toner concentration corresponds to the amount of toner consumed in the developing device 1Y, and is therefore used to control the supply of developer from the developer storage section 27Y. For example, when it is detected that the toner concentration has fallen below a predetermined value, developer is supplied from the developer storage section 27Y. Note that since the magnetic permeability of the developer changes depending on the toner concentration, it is possible to detect the toner concentration using the magnetic permeability.

[0048] The regulating member 64 is disposed adjacent to the first developing roller 30, and is used to regulate the amount of developer supplied from the developer circulating unit 46 to the first developing roller 30. The regulating member 64 can be configured to regulate the amount of developer attracted to the first developing roller 30 based on, for example, the gap between the surface of the first sleeve 33 of the first developing roller 30 and the end of the regulating member 64.

[0049] The circulation path of the developer in the developing container 60 is such that the developer is transported in a substantially horizontal direction while being stirred in the developer circulating section 46, and then is supplied to the first developing roller 30, and is transferred from the first developing roller 30 to the second developing roller 31 above by magnetic force. Next, the developer is transferred from the second developing roller 31 to the peeling roller 32 on the side of the second developing roller 31 again by magnetic force, and then is peeled off from the peeling roller 32 by a third magnet 38 contained in the peeling roller 32, and is then collected in the developer collecting section 47 and introduced again into the developer circulating section 46.

[0050] As described above, in this embodiment, a two-component development method is used as the development method, and the developer is a mixture of non-magnetic toner with a normal negative charging polarity and a magnetic carrier. The non-magnetic toner is a resin such as polyester or styrene acrylic that contains colorants, wax components, etc., and is pulverized or polymerized to form a powder, to which fine powders such as titanium oxide and silica are added on the surface. The magnetic carrier is a core made of resin particles kneaded with ferrite particles or magnetic powder, and a resin coating is applied to the surface layer. In this embodiment, the toner concentration in the developer in the initial state (weight ratio of the toner contained in the developer) is 8%.

[0051] The developing process of the developing device 1Y will be further described. As described above, the developing device 1Y transports the developer to the development area by the first sleeve 33 and the second sleeve 34. The photosensitive drum 28Y is uniformly charged to a charging potential by a DC voltage applied to the primary charger 21Y or a voltage obtained by superimposing an AC voltage on a DC voltage, and then the image portion is exposed to a laser from the optical writing unit 22Y, and the potential at the exposed position becomes the exposure potential. The negative polarity toner present in the developer is transported to the electrostatic latent image on the photosensitive drum 28Y, which is at the exposure potential, by an electric field created by the charging potential and exposure potential of the first sleeve 33, the second sleeve 34, and the photosensitive drum 28Y, and is developed.

[0052] [Control configuration] 5 is a block diagram showing a part of the control configuration of the image forming apparatus 100 in this embodiment. In this embodiment, the voltage applied to the primary chargers 21Y to 21K is generated by the charging high voltage board 110 through the control unit 101, and is a voltage obtained by superimposing a DC voltage of -500 [V] and an AC voltage that is a sine wave with a peak-to-peak voltage of 1200 [V] and a frequency of 2 [kHz]. As a result, the charging potential of the surfaces of the photosensitive drums 28Y, 28M, 28C, and 28K is -500 [V]. In addition, by being exposed by the optical writing units 22Y, 22M, 22C, and 22K, the exposure potential of the surfaces of the photosensitive drums 28Y, 28M, 28C, and 28K is -150 [V].

[0053] Further, a DC voltage or a voltage obtained by superimposing an AC voltage on a DC voltage, which is generated by a development high voltage board 120 through the control unit 101, is applied to the first sleeve 33 and the second sleeve 34 of the developing devices 1Y, 1M, 1C, and 1K. In this embodiment, a voltage obtained by superimposing an AC voltage, which is a square wave having a duty ratio of 60%, a peak-to-peak voltage of 1400 [V], and a frequency of 12 [kHz] on a DC voltage of -350 [V], is applied to the first sleeve 33 and the second sleeve 34. The voltage applied to the collection roller 52 of the carrier collection devices 5Y, 5M, 5C, and 5K will be described later.

[0054] Moreover, the image forming apparatus 100 of this embodiment has a relative humidity detection unit 140 as a humidity detection unit capable of detecting the relative humidity around the image forming apparatus 100. The relative humidity detection unit 140 is configured with a humidity sensor, detects the relative humidity of the atmosphere around the image forming apparatus 100, and transmits the detected humidity to the control unit 101.

[0055] The image processing unit 150 processes image information from an external device 170 such as a PC or a mobile terminal, and converts it into image formation information. The image forming units PY, PM, PC, and PK of each color form toner images based on this image formation information. At this time, the image processing unit 150 calculates how much toner of each color is printed, and calculates the printing rate (image ratio) for each image of each color. The dirt amount calculation unit 160 integrates the image ratio of the image formed, and predicts the dirt amount of the collection roller 52. This point will be described in detail later. The dirt amount of the collection roller 52 may be calculated by detecting the current between the development roller and the photosensitive drum of the development device. That is, the charge amount of the toner may be calculated from this current, and the dirt amount of the collection roller may be calculated based on the charge amount.

[0056] [Carrier recovery device] Next, the carrier recovery devices 5Y, 5M, 5C, and 5K will be described with reference to FIG. 3 and FIG. 6. Since the carrier recovery devices 5Y, 5M, 5C, and 5K have the same configuration, the carrier recovery device 5Y will be described below as a representative. As described above, the carrier recovery device 5Y is disposed at a position facing the photosensitive drum 28Y downstream of the area (developing section D) where the photosensitive drum 28Y is adjacent to the developing device 1Y and upstream of the area (primary transfer section T1Y) where the photosensitive drum 28Y is adjacent to the primary transfer roller 23Y with respect to the rotation direction of the photosensitive drum 28Y. As shown in FIG. 6, the carrier recovery device 5Y includes a recovery roller 52 as a rotatable sleeve disposed facing the photosensitive drum 28Y, a magnet roller 51 as a magnetic field generating section (magnet) fixed inside the recovery roller 52, a recovery chamber 53, and a conveying screw 54 that conveys the carriers collected in the recovery chamber 53. These are disposed in a recovery container 55.

[0057] The magnet roller 51 has a plurality of magnetic poles (magnet pieces) 51a, 51b, and 51c (three in this embodiment). The magnetic pole 51a is disposed at a position facing the photosensitive drum 28Y via the collection roller 52. The magnetic pole 51a is a magnetic pole for attracting carriers attached to the outer peripheral surface of the photosensitive drum 28Y, and for this reason, the magnetic pole 51a is disposed near the closest position P1 between the photosensitive drum 28Y and the collection roller 52. The magnetic pole 51b is disposed downstream of the magnetic pole 51a in the rotation direction of the collection roller 52, and has a different polarity from the magnetic pole 51a. The magnetic pole 51c is disposed adjacent to the magnetic pole 51b downstream of the magnetic pole 51b in the rotation direction of the collection roller 52, and has the same polarity as the magnetic pole 51b.

[0058] The recovery roller 52 is disposed downstream of a development position where an electrostatic latent image formed on the photosensitive drum 28Y is developed, and upstream of a transfer position where a toner image carried on the photosensitive drum 28Y is transferred to the intermediate transfer belt 24, with respect to the rotation direction of the photosensitive drum 28Y. The recovery roller 52 rotates in the direction of the arrow in FIG. 6, and causes carrier adhering to the photosensitive drum 28Y to be attracted to the surface of the recovery roller 52 by a magnetic field created by a magnetic pole 51a disposed around a position close to the photosensitive drum 28Y. The attracted carrier is transported with the rotation of the recovery roller 52, and is peeled off into a recovery chamber 53 by a repulsive magnetic field created by magnetic poles 51b and 51c.

[0059] In this embodiment, a carrier recovery blade 56 as a blade is disposed so as to face the recovery roller 52 with a gap therebetween downstream of the position where the recovery roller 52 faces the photosensitive drum 28Y in the rotation direction of the recovery roller 52. The carrier recovery blade 56 removes the carrier adhering to the recovery roller 52. In this embodiment, the carrier recovery blade 56 is disposed at a position facing the magnetic pole 51c across the recovery roller 52. Therefore, the carrier that stands up due to the magnetic field of the magnetic pole 51c can be scraped off by the carrier recovery blade 56. As a result, the carrier that does not fall from the recovery roller 52 due to the repulsive magnetic field of the magnetic poles 51b and 51c and remains on the recovery roller 52 can be scraped off by the carrier recovery blade 56 into the recovery chamber 53 of the recovery container 55.

[0060] The transport screw 54, which serves as a carrier transport member, has a rotating shaft made of a non-magnetic metal and resin blades formed in a spiral shape around the rotating shaft. The transport screw 54 rotates to transport the carrier that has fallen from the collection roller 52 in the direction of its rotation axis. In this embodiment, the rotation axis of the transport screw 54 and the rotation axis of the collection roller 52 are substantially parallel to each other.

[0061] A bias including an AC voltage, in this embodiment, a voltage obtained by superimposing an AC voltage on a DC voltage, is applied to the recovery roller 52 from a recovery high voltage substrate 130 serving as a voltage application section (bias application section). As shown in Fig. 5, this recovery high voltage substrate is controlled by a control section 101, and can apply an AC voltage superimposed on a DC voltage in accordance with the operation of the carrier recovery device 5Y.

[0062] [Normal image formation] The voltage (first bias) applied from the recovery high voltage board 130 to the recovery roller 52 during normal image formation (first mode, during image formation operation) will be described. The DC component of the voltage applied to the recovery roller 52 during normal image formation is a voltage having a difference of 100 [V] with respect to the charging potential of the photosensitive drum 28Y. This is to suppress the movement of the negative toner developing the electrostatic latent image on the photosensitive drum 28Y to the recovery roller 52. That is, in this embodiment, the DC voltage applied to the recovery roller 52 in the first mode is a voltage whose polarity is the same as the normal charging polarity of the toner (negative polarity in this embodiment). In other words, during normal image formation, the polarity of the DC potential of the photosensitive drum 28Y and the polarity of the DC potential of the recovery roller 52 are the same as the charging polarity of the toner, and the absolute value of the DC potential of the recovery roller 52 is greater than the absolute value of the DC potential of the photosensitive drum 28Y. As described above, since the charging potential is −500 [V], the DC voltage applied to the collection roller 52 in the first mode is −600 [V].

[0063] In the first mode, an AC voltage is applied to the recovery roller 52 in addition to the DC voltage in order to improve the recovery of the carrier adhering to the photosensitive drum 28Y. In this embodiment, the AC component of the voltage applied to the recovery roller 52 during normal image formation is a square wave with a duty ratio of 50%, a peak-to-peak voltage of 1500 [V], and a frequency of 5 [kHz]. The carrier recovery device 5Y recovers the carrier adhering to the photosensitive drum 28Y by the force of the magnetic field formed by the magnet roller 51 and the force of the electric field formed between the recovery roller 52 and the photosensitive drum 28Y by the voltage applied to the recovery roller 52.

[0064] [Toner adhesion on the collection roller] Next, the adhesion of toner to the collection roller 52 will be described. As described above, a voltage is applied to the collection roller 52 to facilitate the collection of carrier on the surface of the photosensitive drum 28Y at the closest position P1 between the photosensitive drum 28Y and the collection roller 52. At this time, the collection roller 52 also collects so-called fog toner that has adhered to the surface of the photosensitive drum 28Y. That is, toner adheres to the collection roller 52.

[0065] FIG. 7 is a diagram showing the distribution of the charge amount of the toner adhering to the collection roller 52. The toner inside the developing device 1Y is negatively charged, but the toner adhering to the collection roller 52 is mostly weakly charged toner with no charge, centered on 0 charge (i.e., toner with a charge amount of 0 or close to 0), or toner with an opposite charge (i.e., toner with a positive charge), as shown in FIG. 7. Such toner adheres to the background of the image and becomes fog toner. The toner in a normal charged state adheres to the photosensitive drum 28Y by electrostatic force, but the weakly charged toner or the oppositely charged toner has a weak adhesion force to the photosensitive drum 28Y by electrostatic force. For this reason, it is easy to transfer to the collection roller 52 at the closest position P1 between the collection roller 52 and the photosensitive drum 28Y.

[0066] Since the toner is a non-magnetic material, the toner transferred to the recovery roller 52 is not affected by the magnetic force generated by the magnetic poles of the magnet roller 51, and moves while remaining fixed to the surface of the recovery roller 52. However, when the toner reaches the peeling pole portions, namely magnetic poles 51b and 51c, it is rubbed by the carrier remaining in the peeling pole portions, accumulates together with the carrier, and is finally recovered in the recovery chamber 53.

[0067] However, if the amount of carrier on the recovery roller 52 is insufficient, the carrier remaining at the peeling pole portion will be insufficient, and the toner cannot be sufficiently recovered at the peeling pole portion. In this case, the toner will accumulate on the recovery roller 52. Then, when the gap between the photosensitive drum 28Y and the recovery roller 52 is filled with the toner on the recovery roller 52, the toner on the recovery roller 52 will adhere to the photosensitive drum 28Y again (so-called dripping occurs), and as a result, the toner will be transferred to the output product and image noise will occur. Therefore, in this embodiment, the toner adhering to the recovery roller 52 is electrically returned to the surface of the photosensitive drum 28Y, and the toner on the recovery roller 52 is removed.

[0068] [Removing toner from the collection roller] In this embodiment, a toner removal mode can be executed as a second mode (predetermined mode) in addition to the normal image formation (first mode) in order to improve the toner removal efficiency of the collection roller 52. In the toner removal mode, the toner on the collection roller 52 is removed by moving the toner from the collection roller 52 to the photosensitive drum 28Y by an electric field.

[0069] At this time, the charge distribution of the toner on the collection roller 52 is different from that of the toner in the developing device 1Y during normal image creation, and has a high ratio of not only normally charged toner but also reversely charged toner, as shown in Fig. 7. For this reason, when -600 [V] is applied to the collection roller 52 for the photosensitive drum 28Y charged to -500 [V] as in the developing device 1Y, only normally charged toner is removed, and reversely charged toner remains on the collection roller 52, and therefore removal is not sufficient.

[0070] Therefore, in the conventional technology shown in Patent Document 1, as shown in FIG. 8 as a comparative example, the potential difference (DC bias difference) between the photosensitive drum and the recovery roller is set to 0, and only an AC voltage (AC bias) is applied to vibrate the toner and move it onto the photosensitive drum, where it is removed. The comparative example in FIG. 8 shows an AC voltage with a duty ratio of 50% and a peak-to-peak voltage of 2400 [V]. However, this alone is insufficient for the efficiency of toner removal, so the peripheral speed of the photosensitive drum is lowered to compensate for the insufficient removal efficiency. For this reason, in the comparative example, a lot of downtime was required, such as switching the speed of the photosensitive drum, which was a factor in reducing productivity.

[0071] [Toner Removal Mode of the Present Embodiment] For this reason, in the toner removal mode (second mode) of the present embodiment, the efficiency of toner transfer from the recovery roller 52 to the photosensitive drum 28Y is increased compared to the comparative example, and the toner is transferred onto the photosensitive drum 28Y only by bias (second bias and third bias in the present embodiment) without reducing the circumferential speed of the photosensitive drum 28Y. That is, in the toner removal mode (second mode) of the present embodiment, while rotating the photosensitive drum 28Y at the same speed as the first mode, in an AC voltage in which a voltage of a polarity opposite to the normal charging polarity of the toner and a voltage of the same polarity as the normal charging polarity of the toner are one cycle, a first voltage pattern (third bias) in which the duty ratio, which is the ratio of the duration of the voltage of the normal charging polarity of the toner to the voltage of the opposite polarity, is greater than 50% (i.e., higher than the duty ratio during normal image formation (first mode)) and a second voltage pattern (second bias) in which the duty ratio is smaller than 50% (i.e., lower than the duty ratio during normal image formation (first mode)) are switched at a constant period to apply to the recovery roller 52. In the AC voltage having one cycle (one period) of a voltage of the normal charging polarity of the toner, a voltage of the opposite polarity, and a voltage of the same polarity as the normal charging polarity of the toner, the duration of the voltage of the opposite polarity to the normal charging polarity of the toner is t1, and the duration of the voltage of the same polarity as the normal charging polarity of the toner is t2 (see FIG. 9). In this case, in the AC voltage having one cycle of a voltage of the normal charging polarity of the toner, a voltage of the opposite polarity to the normal charging polarity of the toner, and a voltage of the same polarity as the normal charging polarity of the toner, the duty ratio, which is the ratio of the duration of the voltage of the normal charging polarity of the toner to the voltage of the opposite polarity, is expressed as "t1 / (t1+t2)".

[0072] The bias patterns (first and second voltage patterns) of the toner removal mode in this embodiment are shown in Fig. 9. The bias pattern is configured with an alternating current voltage (AC bias) and is made up of two biases: removal pattern 1 (first voltage pattern) with a duty ratio of 65% and removal pattern 2 (second voltage pattern) with a duty ratio of 35% or less. Fig. 9 shows a schematic diagram of how these two patterns are switched at regular intervals.

[0073] The duty ratio of removal pattern 1 (third bias) may be a pattern greater than 50% (i.e., a pattern higher than the duty ratio during normal image formation (first mode)), and the duty ratio of removal pattern 2 (second bias) may be a pattern less than 50% (i.e., a pattern lower than the duty ratio during normal image formation (first mode)). In the toner removal mode, a combination of removal pattern 1 and removal pattern 2 may be used. In this embodiment, the duty ratio of removal pattern 1 is 65%, and the duty ratio of removal pattern 2 is 35%. The AC voltage in removal pattern 1 and removal pattern 2 is a square wave with a peak-to-peak voltage of 2400 [V] and a frequency of 5 [kHz].

[0074] In this embodiment, similarly to Comparative Example 1, the difference in surface potential between the photosensitive drum 28Y and the collection roller 52 is set to 0V. That is, in the toner removal mode, the control unit 101 controls the collection high voltage board 130 so that the difference in surface potential between the photosensitive drum 28Y and the collection roller 52 is 0V, and then switches between removal pattern 1 and removal pattern 2 at a constant period and applies them to the collection roller 52. That is, in the toner removal mode, the absolute value of the DC potential of the collection roller 52 is set to be the same as the absolute value of the DC potential of the photosensitive drum 28Y. As a method for controlling the surface potential of the photosensitive drum 28Y to 0V, for example, the charging operation of the primary charger 21Y that operates during normal image formation is stopped, and the surface of the photosensitive drum 28Y is exposed to the laser of the optical writing unit 22Y to neutralize the surface of the photosensitive drum 28Y to set the surface potential to 0V. In addition, the DC bias component of the collection roller 52 is set to 0V (earth). In this embodiment, the potential difference between the photosensitive drum 28Y and the collection roller 52 is 0 V. Furthermore, if the potential difference between the photosensitive drum 28Y and the collection roller 52 is 0 V, the photosensitive drum 28Y can be set to −500 V and the collection roller 52 can be set to −500 V, and the same effect can be obtained even if the potential value is not 0 V.

[0075] In FIG. 9, in the removal pattern 1 with a duty ratio of 65%, the duration of the voltage of the polarity opposite to the normal charging polarity of the toner (positive voltage in this embodiment, 900V in FIG. 9) is long, and therefore the peak value of the voltage of the same polarity as the normal charging polarity of the toner (negative voltage in this embodiment, −1700V in FIG. 9) is high. The graph in FIG. 9 shows the time change of the AC voltage, but even if the duty ratio is changed, the area showing the positive voltage and the area showing the negative voltage are the same. Therefore, if the duration of the positive voltage is extended, the peak value of the negative voltage becomes high. Therefore, in the removal pattern 1, an AC voltage with a high peak value of the negative voltage is applied between the collection roller 52 and the photosensitive drum 28Y. Therefore, as shown in FIG. 10(a), in the removal pattern 1 with a duty ratio of 65%, the force of moving the toner of the same normal charging polarity (negative polarity in this embodiment) as the developing device 1Y to the photosensitive drum 28Y is strong, and the removal efficiency of the normal charging toner is very high. On the other hand, if only the removal pattern 1 is used, the efficiency of removing the oppositely charged toner decreases, and the removal efficiency decreases.

[0076] In FIG. 9, in the removal pattern 2 with a duty ratio of 35%, the duration of the voltage of the polarity opposite to the normal charging polarity of the toner (positive voltage in this embodiment, 1560V in FIG. 9) is short, and therefore the peak value of the voltage of the same polarity as the normal charging polarity of the toner (negative voltage in this embodiment, −840V in FIG. 9) is low. In other words, the peak value of the voltage of the polarity opposite to the normal charging polarity of the toner (positive voltage in this embodiment) is high. Therefore, in the removal pattern 2, an AC voltage with a high peak value of the positive voltage is applied between the collection roller 52 and the photosensitive drum 28Y. Therefore, as shown in FIG. 10(b), in the removal pattern 2 with a duty ratio of 35%, the force that moves the toner of the opposite charging polarity to the normal charging toner (reverse charging toner, positive toner) to the photosensitive drum 28Y is strong, and the removal efficiency of the reverse charging toner is high. On the other hand, the removal efficiency of the normal charging toner is reduced by the removal pattern 2 alone.

[0077] For this reason, in this embodiment, the bias patterns of the removal pattern 1 and the removal pattern 2 are applied alternately to the collection roller 52. This makes it possible to move not only the normally charged toner but also the reversely charged toner to the photosensitive drum 28Y. In this way, since both the normally charged toner and the reversely charged toner on the collection roller 52 can be moved to the photosensitive drum 28Y, the toner on the collection roller 52 can be efficiently removed. As a result, unlike the comparative example, the toner on the collection roller 52 can be efficiently removed without reducing the rotation speed of the photosensitive drum 28Y, and downtime is not increased. It was confirmed that the same effect was obtained as the toner removal efficiency of the collection roller 52 even in this embodiment in which the rotation speed of the photosensitive drum 28Y is not reduced when the bias patterns of the removal pattern 1 and the removal pattern 2 are applied alternately to the collection roller 52. On the other hand, even if the rotation speed of the photosensitive drum 28Y is reduced when the bias patterns of the removal pattern 1 and the removal pattern 2 are applied alternately to the collection roller 52, the toner removal efficiency of the collection roller 52 is increased. When the bias patterns of removal pattern 1 and removal pattern 2 are alternately applied to the recovery roller 52, if the rotation speed of the photosensitive drum 28Y is not reduced, the time required for control can be shortened (e.g., from 60 seconds to 30 seconds) compared to when the rotation speed of the photosensitive drum 28Y is reduced.

[0078] From the viewpoint of increasing the efficiency of toner removal by the collection roller 52, the application time of removal pattern 1 and removal pattern 2 is preferably equal to or longer than the time required for the collection roller 52 to rotate once. The duty ratio of removal pattern 1 is preferably equal to or greater than 60%. The duty ratio of removal pattern 2 is preferably equal to or less than 40%. In this embodiment, in order to more efficiently remove toner by the collection roller 52, the change timing for switching between removal pattern 1 and removal pattern 2 is set to twice the rotation period of the collection roller 52. In this embodiment, the application times of removal pattern 1 with a duty ratio of 65% and removal pattern 2 with a duty ratio of 35% are set to be equal.

[0079] As described above, in the present embodiment, in the toner removal mode, the bias patterns of removal pattern 1 and removal pattern 2 are alternately applied to the collection roller 52. That is, in the present embodiment, in order to move not only the oppositely charged toner but also the normally charged toner to the photosensitive drum 28Y in the toner removal mode, the bias patterns of removal pattern 1 as well as removal pattern 2 are applied to the collection roller 52.

[0080] 7, most of the toner adhering to the collection roller 52 is weakly charged toner or oppositely charged toner. Therefore, if the oppositely charged toner can be moved to the photosensitive drum 28Y in the toner removal mode, the toner removal efficiency of the collection roller 52 can be sufficiently improved. Therefore, in the toner removal mode, instead of applying the bias patterns of removal pattern 1 and removal pattern 2 alternately to the collection roller 52, a modified example may be used in which only the bias pattern of removal pattern 2 is applied to the collection roller 52.

[0081] [Sequence before entering toner removal mode] Next, the operation up to entering the toner removal mode of this embodiment will be described with reference to Figures 5 and 11. First, when image processing unit 150 receives image information from external device 170 (S100), image processing unit 150 sends the image information to dirt amount calculation unit 160 (S101). Control unit 101 causes dirt amount calculation unit 160 to accumulate the image ratio (print rate) at which an image is formed from the image information, and calculates the current dirt amount (total dirt amount) on collection roller 52 from the accumulated value (S102).

[0082] The amount of dirt is assumed to be proportional to the image ratio and is calculated as dirt amount (N) = image ratio × α. Note that α is a constant, and is set to a fixed value of 1 in this embodiment, but may be a variable, such as being made to vary depending on humidity. Therefore, the total amount of dirt is an integrated value of the image ratio (%). The total amount of dirt (N) calculated by the dirt amount calculation unit 160 is sent to the control unit 101, which determines whether the amount of dirt is higher than a predetermined value (S103). In this embodiment, the predetermined value is set to 10,000.

[0083] If the total amount of dirt (N) is equal to or less than a predetermined value (NO in S103), the toner removal mode is not performed and the image forming operation continues, and the process proceeds to S108, which will be described later. On the other hand, if the total amount of dirt (N) exceeds a predetermined value (YES in S103), the image forming operation is temporarily interrupted (S104) and the toner removal mode is performed (S105). The contents of the toner removal mode will be described later. After the toner removal mode ends, the total amount of dirt (N) is reset (S106), and the image forming operation is resumed (S107). The above operations are continued until the image forming job ends (S108).

[0084] An image forming job is a period from the start of an image forming operation based on an image forming signal for forming an image on a recording material to the completion of the image forming operation. Specifically, it refers to the period from pre-rotation (preparatory operation before image formation) after receiving an image forming signal (input of an image forming job) to post-rotation (operation after image formation), and includes the period of the image forming operation and periods of non-image formation such as between sheets.

[0085] [Toner Removal Mode Sequence of the Present Embodiment] Next, the sequence of the toner removal mode in this embodiment will be described with reference to FIG. 12. When the toner removal mode is entered in S105 of FIG. 11, first, the surface potential of the photosensitive drum 28Y is controlled to 0V (S1000). As a method of controlling the surface potential of the photosensitive drum 28Y to 0V, for example, the charging operation of the primary charger 21Y that operates during normal image formation is stopped, and the surface of the photosensitive drum 28Y is exposed to the laser of the optical writing unit 22Y to discharge the surface of the photosensitive drum 28Y to set the surface potential to 0V. Next, the control unit 101 changes the voltage applied to the collection roller 52 to a 65% duty AC bias (removal pattern 1) using the collection high voltage board 130 (S1001). This removes the normally charged toner attached to the collection roller 52.

[0086] After applying the bias of removal pattern 1 for the time required for two revolutions of the collection roller 52 (YES in S1002), the control unit 101 changes the voltage applied to the collection roller 52 to the AC bias of 35% duty (removal pattern 2) using the collection high voltage board 130 (S1003). This removes the reversely charged toner adhering to the collection roller 52.

[0087] After applying the bias of removal pattern 2 for the time required for two revolutions of the collection roller 52 (YES in S1004), the sequence from S1001 to S1004 is continued until a predetermined time has elapsed (S1005). The predetermined time here is a time that is preset as the execution time of the toner removal mode. The predetermined time is, for example, 60 seconds.

[0088] [Toner Removal Mode Sequence in Comparative Example] Next, the sequence of the toner removal mode in the comparative example will be described with reference to FIG. 13. When the toner removal mode is entered in S105 of FIG. 11, first, the control unit 101 changes the applied voltage of the recovery roller 52 to an AC bias (50% duty) using the recovery high voltage board 130 (S9000). Then, the surface potential of the photosensitive drum 28Y is controlled to 0V (S9001). At this time, the photosensitive drum 28Y is stopped. Next, the photosensitive drum 28Y and the recovery roller 52 start to rotate in a state in which a peripheral speed ratio different from that during normal image formation is applied (S9002). Specifically, the peripheral speed of the photosensitive drum 28Y is made slower than that during image formation to provide a peripheral speed ratio between the photosensitive drum 28Y and the recovery roller 52. The sequence continues until a predetermined time has elapsed (S9003).

[0089] In this way, in the comparative example, the photosensitive drum 28Y and the collection roller 52 are stopped once to change the speed in the toner removal mode sequence, and then the driving is restarted, which causes a lot of downtime. On the other hand, in the toner removal mode sequence in this embodiment, the photosensitive drum 28Y and the collection roller 52 are not stopped, and only a bias is applied, so downtime is short and toner can be removed efficiently.

[0090] [Experiment 1] In order to confirm the effect of this embodiment, the following experiment was conducted. In the experiment, using the image forming apparatus of this embodiment described above, actual image formation was performed in Comparative Example 1 in which the toner removal mode was not executed, Comparative Example 2 in which the sequence of the toner removal mode described above with reference to Figs. 8 and 13 was executed, and Example 1 in which the sequence of the toner removal mode of this embodiment was executed, and the stains on the collection roller 52 were checked. Downtimes were also compared at the same time. The results are shown in Table 1. Note that "drip" in Table 1 refers to toner on the collection roller 52 adhering again to the photosensitive drum 28Y, and the stains on the collection roller 52 were judged based on whether drips occurred or not. [Table 1]

[0091] As is clear from Table 1, in Comparative Example 1, there was no downtime, but dripping occurred. On the other hand, in Comparative Example 2, there was no dripping, but the downtime increased significantly. In contrast, in Example 1, it was possible to prevent dripping, as in Comparative Example 2, while suppressing downtime. That is, in Example 1, it was possible to greatly improve the toner removal efficiency compared to Comparative Example 1, and it was possible to achieve the same toner removal efficiency while reducing downtime compared to Comparative Example 2.

[0092] <Second embodiment> The second embodiment will be described with reference to Figures 14 and 15. In this embodiment, unlike the first embodiment, the voltage pattern applied to the collection roller 52 in the toner removal mode is offset by offsetting the direct current voltage (DC component) instead of changing the duty ratio. The other configurations and functions are the same as those of the first embodiment.

[0093] In the toner removal mode (second mode) of this embodiment, while rotating the photosensitive drum 28Y at the same speed as in the first mode, a first voltage pattern in which an AC voltage is superimposed on a DC voltage such that the polarity of the potential of the recovery roller 52 relative to the photosensitive drum 28Y is negative, and a second voltage pattern in which an AC voltage is superimposed on a DC voltage such that the polarity of the potential of the recovery roller 52 relative to the photosensitive drum 28Y is positive, are switched at a constant period and applied to the recovery roller 52.

[0094] The bias patterns (first and second voltage patterns) of the toner removal mode in this embodiment are shown in Fig. 14. The bias pattern is composed of an alternating current voltage (AC bias) and a direct current voltage (DC bias), and is composed of two biases: removal pattern 3 (first voltage pattern) with a DC component of -1000V, and removal pattern 4 (second voltage pattern) with a DC component of +1000V. Fig. 14 shows a schematic diagram of how these two patterns are switched at a constant cycle.

[0095] In the toner removal mode of this embodiment, the duty ratio of the AC voltage (AC component) in removal pattern 3 and removal pattern 4 is 50%. The absolute value of the DC voltage in removal pattern 3 is the same as that in removal pattern 4. That is, the absolute value of both is 1000V. However, the magnitude of the DC voltage may be different between removal pattern 3 and removal pattern 4. As in the first embodiment, the application time of removal pattern 3 and removal pattern 4 is preferably longer than the time required for the collection roller to rotate once, and the change timing for switching between removal pattern 1 and removal pattern 2 is set to twice the rotation period of the collection roller 52. Furthermore, the application time of removal pattern 3 and removal pattern 4 is set to be equal. The sequence until entering the toner removal mode of this embodiment is the same as that of the first embodiment described in FIG. 11.

[0096] [Toner Removal Mode Sequence of the Present Embodiment] Next, the sequence of the toner removal mode in this embodiment will be described with reference to Fig. 15. When the toner removal mode is entered in S105 of Fig. 11, first, the surface potential of the photosensitive drum 28Y is controlled to 0V (S2000). Next, the control unit 101 changes the voltage applied to the collection roller 52 to a 50% duty AC bias and a -1000V DC bias (removal pattern 3) using the collection high voltage board 130 (S2001). This removes the normally charged toner adhering to the collection roller 52.

[0097] After applying the bias of removal pattern 3 for the time required for two revolutions of the collection roller 52 (YES in S2002), the control unit 101 changes the voltage applied to the collection roller 52 to a 50% duty AC bias and a +1000V DC bias (removal pattern 4) using the collection high voltage board 130 (S2003). This removes the reversely charged toner adhering to the collection roller 52.

[0098] After applying the bias of the removal pattern 4 for a time equivalent to two revolutions of the collection roller 52 (YES in S2004), the sequence from S2001 to S2004 is continued until a predetermined time has elapsed (S2005). The predetermined time here is a time preset as an execution time of the toner removal mode. The predetermined time is, for example, 60 seconds. It has been confirmed that the same effect can be obtained in terms of the toner removal efficiency of the collection roller 52 even in this embodiment in which the rotation speed of the photosensitive drum 28Y is not reduced when the bias patterns of the removal pattern 3 and the removal pattern 4 are applied alternately to the collection roller 52. On the other hand, even if the rotation speed of the photosensitive drum 28Y is reduced when the bias patterns of the removal pattern 3 and the removal pattern 4 are applied alternately to the collection roller 52, the toner removal efficiency of the collection roller 52 is improved. When the bias patterns of removal pattern 3 and removal pattern 4 are alternately applied to the recovery roller 52, if the rotation speed of the photosensitive drum 28Y is not reduced, the time required for control can be shortened (e.g., from 60 seconds to 30 seconds) compared to when the rotation speed of the photosensitive drum 28Y is reduced.

[0099] [Experiment 2] In order to confirm the effect of this embodiment, an experiment similar to Experiment 1 described in the first embodiment was conducted. Comparative Examples 1 and 2 and Example 1 are the same as Experiment 1. In Experiment 2, Example 2 in which the toner removal mode sequence of this embodiment was executed was also investigated. The results are shown in Table 2. [Table 2]

[0100] As is clear from Table 2, in Example 2, like Example 1, downtime was suppressed, while, like Comparative Example 2, the occurrence of dripping was prevented.

[0101] <Third embodiment> The third embodiment will be described with reference to Fig. 16(a) to Fig. 18. In this embodiment, as in the first embodiment, the duty ratio is changed as the voltage pattern applied to the collection roller 52 in the toner removal mode, but the duty ratio is further changed according to the environment. The other configurations and functions are the same as those of the first embodiment.

[0102] Here, in a high humidity environment, the ratio of reverse charge toner contained in the toner on the photosensitive drum 28Y is high, and as a result, the ratio of reverse component toner adhering to the collection roller 52 is high. On the other hand, in a low humidity environment, the reverse component is small. For this reason, in this embodiment, the duty ratio of the bias applied to the collection roller 52 is changed depending on the humidity.

[0103] That is, in the toner removal mode (second mode) of this embodiment, the control unit 101 changes the bias pattern (first voltage pattern and second voltage pattern) of the toner removal mode based on the relative humidity detected by the relative humidity detection unit 140 as the humidity detection unit. Specifically, when the relative humidity detected by the relative humidity detection unit 140 is a first humidity, the control unit 101 applies a voltage in which the duty ratio of the first voltage pattern is a first value and the duty ratio of the second voltage pattern is a second value to the collection roller 52 in the second mode. On the other hand, when the relative humidity detected by the relative humidity detection unit 140 is a second humidity higher than the first humidity, the control unit 101 applies a voltage in which the duty ratio of the first voltage pattern is a third value smaller than the first value and the duty ratio of the second voltage pattern is a fourth value smaller than the second value to the collection roller 52 in the second mode.

[0104] The bias patterns (first voltage pattern and second voltage pattern) in the toner removal mode in this embodiment are shown in Figs. 16(a) to 16(c). In the first embodiment, biases with duty ratios of 65% and 35% were applied regardless of the environmental humidity. In contrast, in this embodiment, in a high humidity environment, for example, when the humidity is 95%, bias patterns with duty ratios of 60% (first voltage pattern) and 30% (second voltage pattern) are applied to the collection roller 52 by switching at regular intervals, as shown in Fig. 16(a). In a normal humidity environment, for example, when the humidity is 50%, bias patterns with duty ratios of 65% (first voltage pattern) and 35% (second voltage pattern) are applied to the collection roller 52 by switching at regular intervals, as shown in Fig. 16(b). Furthermore, in a low humidity environment, for example, when the humidity is 5%, bias patterns with duty ratios of 70% (first voltage pattern) and 40% (second voltage pattern) are applied to the collection roller 52 by switching at regular intervals.

[0105] For example, if a duty ratio of 65% in a normal humidity environment is a first value, and a duty ratio of 35% is a second value, then a duty ratio of 60% in a high humidity environment corresponds to a third value, and a duty ratio of 30% corresponds to a fourth value. The same applies to the duty ratio in the relationship between a low humidity environment and a normal humidity environment, and the duty ratio in the relationship between a low humidity environment and a high humidity environment.

[0106] In this embodiment, the duty ratio of each bias pattern according to humidity is changed continuously as shown in FIG. 17. That is, the control unit 101 determines the bias pattern in the toner removal mode by referring to the table of FIG. 17 from the relative humidity detected by the relative humidity detection unit 140. The bias pattern may be determined according to a plurality of relative humidity categories, not according to the table of FIG. 17. For example, when the relative humidity is 0% or more and less than 30%, a bias pattern with a duty ratio of 70% (first voltage pattern) and 40% (second voltage pattern) may be determined as a low humidity environment, when the relative humidity is 30% or more and less than 70%, a bias pattern with a duty ratio of 65% (first voltage pattern) and 35% (second voltage pattern) may be determined as a normal humidity environment, and when the relative humidity is 70% or more, a bias pattern with a duty ratio of 60% (first voltage pattern) and 30% (second voltage pattern) may be determined as a high humidity environment. That is, the bias pattern may be determined in stages.

[0107] In any case, in this embodiment, the duty ratio is changed according to the relative humidity, so that it is possible to more effectively remove toner from the collection roller 52. The sequence up to the start of the toner removal mode in this embodiment is the same as that in the first embodiment described with reference to FIG.

[0108] [Toner Removal Mode Sequence of the Present Embodiment] Next, the sequence of the toner removal mode in this embodiment will be described with reference to FIG. 18. In S105 of FIG. 11, when the toner removal mode is entered, first, the surface potential of the photosensitive drum 28Y is controlled to 0V (S3000). The method of controlling the surface potential of the photosensitive drum 28Y to 0V is the same as that of the first embodiment. Next, in this embodiment, the control unit 101 checks the humidity information of the relative humidity detection unit 140, and determines the duty ratio of the two types of AC bias to be applied from the table of FIG. 17 (S3001). Next, the control unit 101 changes the voltage applied to the collection roller 52 using the collection high voltage board 130 to the AC bias (first voltage pattern) of the normally charged toner removal duty determined in S3001 (S3002). Thereby, the normally charged toner attached to the collection roller 52 is removed.

[0109] After applying the bias of the first voltage pattern for a time period corresponding to two revolutions of the collection roller 52 (S3003), the control unit 101 changes the voltage applied to the collection roller 52 using the collection high voltage board 130 to an AC bias (second voltage pattern) having the reversely charged toner removal duty determined in S3001 (S3004). This removes the reversely charged toner adhering to the collection roller 52.

[0110] After the bias of the second voltage pattern is applied for a time period corresponding to two revolutions of the collection roller 52 (S3005), the sequence from S3002 to S3005 is continued until a predetermined time period has elapsed (S3006). The predetermined time period is a time period that is preset as the execution time of the toner removal mode. The predetermined time period is, for example, 30 seconds.

[0111] [Experiment 3] In order to confirm the effects of this embodiment, experiments similar to Experiments 1 and 2 described in the first and second embodiments were conducted. Comparative Examples 1 and 2 and Example 1 were the same as Experiment 1, and Example 2 was the same as Experiment 2. In Experiment 3, Example 3 in which the sequence of the toner removal mode of this embodiment was executed was also investigated. In Experiment 3, the stains on the collection roller 52 when the humidity was changed from normal humidity to high humidity were also investigated in Comparative Examples 1 and 2 and Examples 1, 2, and 3. The results are shown in Table 3. [Table 3]

[0112] As is clear from Table 3, in Example 3, like Example 1, downtime was suppressed, while dripping was prevented like Comparative Example 2. In Comparative Example 2, Examples 1 and 2, dripping occurred slightly in a high humidity environment, but not enough to affect the image. In contrast, dripping did not occur in Example 3, even in a high humidity environment. This shows that in this embodiment, the toner removal efficiency can be maintained even when the environmental humidity changes.

[0113] <Fourth embodiment> The fourth embodiment will be described with reference to Figures 19 and 20. In this embodiment, as in the first embodiment, the duty ratio is changed as the voltage pattern applied to the collection roller 52 in the toner removal mode, but further, the application time of the voltage pattern is changed according to the environment. The other configurations and functions are the same as those of the first embodiment.

[0114] In the above-mentioned third embodiment, the duty ratio of the bias applied to the collection roller 52 was changed according to the environmental humidity, but in this embodiment, the ratio of the time for which the bias is applied to the collection roller 52 is changed according to the environmental humidity. Furthermore, in the above-mentioned first embodiment, the time for which the first voltage pattern and the second voltage pattern are applied are the same regardless of the environmental humidity, whereas in this embodiment, since the ratio of inverted toner is high in a high humidity environment, the bias for removing inverted toner is applied for a longer period. That is, the application time of the second voltage pattern is made longer. On the other hand, the time for removing the bias for normally charged toner is shortened accordingly. That is, the application time of the first voltage pattern is made shorter.

[0115] That is, in the toner removal mode (second mode) of this embodiment, the control unit 101 changes the bias pattern (first voltage pattern and second voltage pattern) of the toner removal mode based on the relative humidity detected by the relative humidity detection unit 140 as a humidity detection unit. Specifically, when the relative humidity detected by the relative humidity detection unit 140 is a first humidity, the control unit 101 applies a voltage to the collection roller 52 in the second mode, in which the first voltage pattern is applied for a first time and the second voltage pattern is applied for a second time. On the other hand, when the relative humidity detected by the relative humidity detection unit 140 is a second humidity higher than the first humidity, the control unit 101 applies a voltage to the collection roller 52 in the second mode, in which the first voltage pattern is applied for a third time shorter than the first time and the second voltage pattern is applied for a fourth time longer than the second time.

[0116] More specifically, in the normal humidity environment, as in the first embodiment, the application time of the bias (first voltage pattern) with a duty ratio of 65% and the application time of the bias (second voltage pattern) with a duty ratio of 35% are the same, and each is twice the rotation period of the collection roller 52. On the other hand, in a high humidity environment, for example, when the humidity is 95%, the application time of the bias (first voltage pattern) with a duty ratio of 65% is changed to one time the rotation period of the collection roller 52, and the application time of the bias (second voltage pattern) with a duty ratio of 35% is changed to three times the rotation period of the collection roller 52. Also, in a low humidity environment, for example, when the humidity is 5%, the application time of the bias (first voltage pattern) with a duty ratio of 65% is changed to three times the rotation period of the collection roller 52, and the application time of the bias (second voltage pattern) with a duty ratio of 35% is changed to one time the rotation period of the collection roller 52.

[0117] For example, if the application times of the first and second voltage patterns in a normal humidity environment are the first and second times (twice the rotation period of the collection roller 52), respectively, the application time of the first voltage pattern in a high humidity environment (one time the rotation period of the collection roller 52) corresponds to a third time, and the application time of the second voltage pattern (three times the rotation period of the collection roller 52) corresponds to a fourth time. The same applies to the application times in the relationship between a low humidity environment and a normal humidity environment, and the application times in the relationship between a low humidity environment and a high humidity environment.

[0118] In this embodiment, the timing for switching between the bias patterns according to humidity is continuously changed as shown in Fig. 19. That is, the control unit 101 determines the application time of the bias pattern in the toner removal mode by referring to the table of Fig. 19 based on the relative humidity detected by the relative humidity detection unit 140. Note that the application time of the bias pattern may be determined according to a plurality of relative humidity categories, without using the table of Fig. 19. For example, a relative humidity of 0% or more and less than 30% may be determined as a low humidity environment, and the application time of the first voltage pattern may be determined as three times the rotation period of the collection roller 52, and the application time of the second voltage pattern may be determined as one time the rotation period of the collection roller 52; a relative humidity of 30% or more and less than 70% may be determined as a normal humidity environment, and the application time of the first voltage pattern and the application time of the second voltage pattern may be determined as twice the rotation period of the collection roller 52; and a relative humidity of 70% or more may be determined as a high humidity environment, and the application time of the first voltage pattern may be determined as one time the rotation period of the collection roller 52, and the application time of the second voltage pattern may be determined as three times the rotation period of the collection roller 52.

[0119] In any case, in this embodiment, the duty ratio is appropriately changed according to the relative humidity, so that it is possible to more effectively remove toner from the collection roller 52. The sequence up to the start of the toner removal mode in this embodiment is the same as that in the first embodiment described with reference to FIG.

[0120] [Toner Removal Mode Sequence of the Present Embodiment] Next, the sequence of the toner removal mode in this embodiment will be described with reference to FIG. 20. In S105 of FIG. 11, when the toner removal mode is entered, first, the surface potential of the photosensitive drum 28Y is controlled to 0V (S4000). The method of controlling the surface potential of the photosensitive drum 28Y to 0V is the same as that of the first embodiment. Next, in this embodiment, the control unit 101 checks the humidity information of the relative humidity detection unit 140, and determines the application time of the two types of AC bias to be applied from the table of FIG. 19 (S4001). Next, the control unit 101 changes the voltage applied to the collection roller 52 to an AC bias of 65% duty (first voltage pattern) using the collection high voltage board 130 (S4002). This removes the normally charged toner attached to the collection roller 52. At this time, the control unit 101 applies the first voltage pattern to the collection roller 52 for the time determined in S4001 (S4003).

[0121] Next, the control unit 101 changes the voltage applied to the collection roller 52 to a 35% duty AC bias (second voltage pattern) using the collection high voltage board 130 (S4004). This removes the reversely charged toner adhering to the collection roller 52. At this time, the control unit 101 applies the second voltage pattern to the collection roller 52 for the time determined in S4001 (S4005). The sequence from S4002 to S4005 is continued until a predetermined time has elapsed (S4006). The predetermined time here is a time that is preset as the execution time of the toner removal mode. The predetermined time is, for example, 60 seconds. It was confirmed that the same effect was obtained as the toner removal efficiency of the collection roller 52 even in this embodiment in which the rotation speed of the photosensitive drum 28Y is not reduced when the bias patterns corresponding to the humidity are alternately applied to the collection roller 52. On the other hand, even if the rotation speed of the photosensitive drum 28Y is reduced when the bias patterns corresponding to the humidity are alternately applied to the collection roller 52, the toner removal efficiency of the collection roller 52 is improved. If the rotation speed of the photosensitive drum 28Y is not reduced when the bias patterns corresponding to the humidity are alternately applied to the collection roller 52, the time required for control can be shortened (for example, from 60 seconds to 30 seconds) compared to the case where the rotation speed of the photosensitive drum 28Y is reduced.

[0122] [Experiment 4] In order to confirm the effect of this embodiment, experiments similar to Experiments 1, 2, and 3 described in the first, second, and third embodiments were conducted. Comparative Examples 1 and 2 and Example 1 were the same as Experiment 1, Example 2 was the same as Experiment 2, and Example 3 was the same as Experiment 3. In Experiment 4, Example 4 in which the sequence of the toner removal mode of this embodiment was executed was also investigated. In Experiment 4, as in Experiment 3, the stains on the collection roller 52 when the humidity was changed from normal humidity to high humidity were also investigated. The results are shown in Table 4. [Table 4]

[0123] As is clear from Table 4, in Example 4, like Example 1, downtime was suppressed, while dripping was prevented like Comparative Example 2. Moreover, in Example 4, dripping did not occur even in a high humidity environment like Example 3. This shows that the toner removal efficiency can be maintained even when the environmental humidity changes in this embodiment.

[0124] <Fifth embodiment> The fifth embodiment will be described with reference to Fig. 21(a) to Fig. 23. In this embodiment, as in the first embodiment, the duty ratio is changed as the voltage pattern applied to the collection roller 52 in the toner removal mode, but further, the DC component of the bias applied to the collection roller 52 is changed according to the environmental humidity. The other configurations and operations are the same as those of the first embodiment described above.

[0125] In the above third and fourth embodiments, the duty ratio and the application time ratio of the bias applied to the collection roller 52 are changed according to the environmental humidity, but in this embodiment, the DC component of the bias applied to the collection roller 52 is changed according to the environmental humidity. More specifically, in the above first embodiment, the DC components applied to the first and second voltage patterns are the same regardless of the environmental humidity, whereas in this embodiment, the ratio of inverted toner is high in a high humidity environment (e.g., humidity 95%), so the DC bias is applied in a direction to remove the inverted toner. That is, in the inverted toner removal bias, a negative DC bias (e.g., -500V) is applied to the collection roller 52, so the collection voltage of the inverted toner increases by the amount of the DC bias applied. On the other hand, in this embodiment, the ratio of inverted toner is low in a low humidity environment (e.g., humidity 5%), so the DC bias is applied in a direction to remove the normally charged toner. That is, in the removal bias for the normally charged toner, a positive DC bias (for example, +500 V) is applied to the collection roller 52, so that the collection voltage for the inverted toner is reduced by the amount of the applied DC bias.

[0126] That is, in the toner removal mode (second mode) of this embodiment, the control unit 101 changes the bias pattern (first voltage pattern and second voltage pattern) of the toner removal mode based on the relative humidity detected by the relative humidity detection unit 140 serving as a humidity detection unit. Specifically, when the relative humidity detected by the relative humidity detection unit 140 is high or low, the control unit 101 applies a DC bias to the collection roller 52 in accordance with the environmental humidity in each of the first voltage pattern and the second voltage pattern in the second mode.

[0127] More specifically, as shown in Fig. 21B, in a normal humidity environment (humidity 50%), the application time of the bias (first voltage pattern) with a duty ratio of 65% and the DC component of the bias (second voltage pattern) with a duty ratio of 35% are the same as in the first embodiment, and the potentials of the recovery roller 52 and the photosensitive drum 28 are the same. On the other hand, in a high humidity environment, for example, when the humidity is 95%, as shown in Fig. 21A, the difference in the DC bias of the recovery roller 52 with respect to the photosensitive drum 28 is changed to -500 V, which has a large effect of recovering reversely charged toner. Also, in a low humidity environment, for example, when the humidity is 5%, as shown in Fig. 21C, the difference in the DC bias of the recovery roller 52 with respect to the photosensitive drum 28 is changed to +500 V, which has a large effect of recovering normally charged toner.

[0128] In this embodiment, the difference in the DC bias of the collection roller 52 with respect to the photosensitive drum 28 according to the humidity is continuously changed as shown in FIG. 22. That is, the control unit 101 determines the difference in the DC bias in the toner removal mode by referring to the graph in FIG. 22 from the relative humidity detected by the relative humidity detection unit 140. The difference in the DC bias may be determined according to a plurality of relative humidity categories, not according to the graph in FIG. 22. For example, a relative humidity of 0% or more and less than 30% may be determined as a low humidity environment, with the difference in the DC bias being -500V, a relative humidity of 30% or more and less than 70% may be determined as a normal humidity environment, with the difference in the DC bias being 0V, and a relative humidity of 70% or more may be determined as a high humidity environment, with the difference in the DC bias being +500V.

[0129] In any case, in this embodiment, the difference in the DC bias is appropriately changed according to the relative humidity, so that it is possible to more effectively remove toner from the collection roller 52. The sequence up to the start of the toner removal mode in this embodiment is the same as that in the first embodiment described with reference to FIG.

[0130] [Toner Removal Mode Sequence of the Present Embodiment] Next, the sequence of the toner removal mode in this embodiment will be described with reference to FIG. 23. In FIG. 23, when the toner removal mode is entered, first, the surface potential of the photosensitive drum 28Y is controlled to 0V (S5000). The method of controlling the surface potential of the photosensitive drum 28Y to 0V is the same as that in the first embodiment. Next, in this embodiment, the control unit 101 checks the humidity information of the relative humidity detection unit 140, and determines the difference in the DC bias in the toner removal mode from the graph in FIG. 22 (S5001). Next, the control unit 101 changes the voltage applied to the collection roller 52 to an AC bias of 65% duty (first voltage pattern) using the collection high voltage board 130 (S5002). This removes the normally charged toner attached to the collection roller 52. At this time, the control unit 101 applies the first voltage pattern to the collection roller 52 until the time for two revolutions of the collection roller 52 has elapsed (S5003).

[0131] Next, the control unit 101 changes the voltage applied to the collection roller 52 to an AC bias (second voltage pattern) of 35% duty using the collection high voltage board 130 (S5004). This removes the reversely charged toner attached to the collection roller 52. At this time, the control unit 101 applies the second voltage pattern to the collection roller 52 with the difference of the DC bias determined in S5001 (S5005). At this time, the control unit 101 applies the second voltage pattern to the collection roller 52 until the time for two revolutions of the collection roller 52 has elapsed (S5005). Then, the control unit 101 continues the sequence from S5002 to S5005 until a predetermined time has elapsed since the start of the toner removal mode (S5006). The predetermined time here is a time previously set as the execution time of the toner removal mode, and is, for example, 60 seconds. It was confirmed that the same effect was obtained in terms of the toner removal efficiency of the collection roller 52 even in the present embodiment in which the rotation speed of the photosensitive drum 28Y is not reduced when the bias patterns corresponding to the humidity are alternately applied to the collection roller 52. On the other hand, even if the rotation speed of the photosensitive drum 28Y is reduced when the bias patterns corresponding to the humidity are alternately applied to the collection roller 52, the toner removal efficiency of the collection roller 52 is improved. When the rotation speed of the photosensitive drum 28Y is not reduced when the bias patterns corresponding to the humidity are alternately applied to the collection roller 52, the time required for control can be shortened (for example, from 60 seconds to 30 seconds) compared to the case in which the rotation speed of the photosensitive drum 28Y is reduced.

[0132] [Experiment 5] In order to confirm the effect of this embodiment, experiments similar to Experiments 1, 2, 3, and 4 described in the first, second, third, and fourth embodiments were conducted. Comparative Examples 1 and 2 and Example 1 were the same as Experiment 1, Example 2 was the same as Experiment 2, Example 3 was the same as Experiment 3, and Example 4 was the same as Experiment 4. In Experiment 5, Example 5 in which the sequence of the toner removal mode of this embodiment was executed was also investigated. In Experiment 5, as in Experiments 3 and 4, the stains on the collection roller 52 when the humidity was changed from normal humidity to high humidity were also investigated. The results are shown in Table 5. [Table 5]

[0133] As is clear from Table 5, in Example 5, downtime was suppressed as in Example 1, while dripping was prevented as in Comparative Example 2. Furthermore, in Example 5, dripping did not occur even in a high humidity environment as in Examples 3 and 4. This shows that the toner removal efficiency can be maintained in this embodiment even when the environmental humidity changes.

[0134] <Other embodiments> The present invention is not limited to the configuration of each of the above-mentioned embodiments. For example, the image forming apparatus 100 is not limited to an MFP, and may be a copier, a printer, or a facsimile machine. The configurations of the developer supply screw 42, the developer stirring screw 43, and the developer recovery screw 44 are not particularly limited as long as they can transport the developer, and for example, a spiral blade or a paddle-shaped blade can be applied. The developing device of each of the above-mentioned embodiments has been described as having two developing rollers, but may have one developing roller. [Explanation of symbols]

[0135] 1Y, 1M, 1C, 1K... Developing device 5Y, 5M, 5C, 5K... Carrier recovery device 24... Intermediate transfer belt (transfer member) 28Y, 28M, 28C, 28K...Photosensitive drum (image carrier) 51 Magnet roller (magnetic field generating part) 52 Collection roller 100 Image forming apparatus 101...Control section 130: Recovery high voltage board (voltage application section)

Claims

1. An image forming apparatus capable of performing an image forming operation, a rotatable image carrier on which an electrostatic latent image is formed; a developing device including a developer container that contains a developer containing a toner and a carrier, and a developer carrier that carries the developer in order to develop the electrostatic latent image formed on the image carrier into a toner image; a transfer member onto which the toner image carried on the image carrier is transferred; a carrier recovery device that has a rotatable sleeve disposed opposite to the image carrier and a magnet non-rotatably disposed inside the sleeve, and recovers the carrier on the image carrier; a bias application unit that applies a bias including an AC voltage to the sleeve; A control unit that controls the bias application unit; Equipped with the sleeve is disposed, with respect to a rotation direction of the image carrier, downstream of a development position where the electrostatic latent image formed on the image carrier is developed and upstream of a transfer position where the toner image carried on the image carrier is transferred to the transfer member; the control unit controls the bias application unit to apply a first bias to the sleeve during an image forming operation, and controls the bias application unit to apply a second bias to the sleeve in a predetermined mode during non-image formation; In one period of the bias AC voltage applied to the sleeve by the bias application unit, the duration of a voltage having a polarity opposite to the charge polarity of the toner with respect to the DC potential of the sleeve is denoted as t1, the duration of a voltage having a polarity the same as the charge polarity of the toner with respect to the DC potential of the sleeve is denoted as t2, and the ratio of t1 to one period (t1+t2) of the bias AC voltage applied to the sleeve by the bias application unit is denoted as a duty ratio, the duty ratio of the AC voltage of the second bias is lower than the duty ratio of the AC voltage of the first bias.

1. An image forming apparatus comprising:

2. The duty ratio of the AC voltage of the second bias is lower than 50%.

2. The image forming apparatus according to claim 1,

3. The duty ratio of the AC voltage of the second bias is 40% or less.

2. The image forming apparatus according to claim 1,

4. The duty ratio of the AC voltage of the first bias is 50%.

2. The image forming apparatus according to claim 1,

5. the control unit controls the bias application unit to alternately apply the second bias and the third bias to the sleeve in the predetermined mode; The duty ratio of the AC voltage of the third bias is higher than the duty ratio of the AC voltage of the first bias.

2. The image forming apparatus according to claim 1,

6. The duty ratio of the AC voltage of the second bias is lower than 50%; The duty ratio of the AC voltage of the third bias is higher than 50%.

6. The image forming apparatus according to claim 5,

7. The duty ratio of the AC voltage of the second bias is 40% or less, The duty ratio of the AC voltage of the third bias is 60% or more.

6. The image forming apparatus according to claim 5,

8. The duty ratio of the AC voltage of the first bias is 50%.

6. The image forming apparatus according to claim 5,

9. In the predetermined mode, the absolute value of the DC potential of the sleeve is the same as the absolute value of the DC potential of the image carrier.

2. The image forming apparatus according to claim 1,

10. During the image forming operation, the polarity of the DC potential of the image carrier and the polarity of the DC potential of the sleeve are the same as the charging polarity of the toner, and the absolute value of the DC potential of the sleeve is greater than the absolute value of the DC potential of the image carrier.

2. The image forming apparatus according to claim 1,

11. the first bias is a DC voltage superimposed on an AC voltage, The second bias is an AC voltage only.

2. The image forming apparatus according to claim 1,

12. An image forming apparatus capable of performing an image forming operation, a rotatable image carrier on which an electrostatic latent image is formed; a developing device including a developer container that contains a developer containing a toner and a carrier, and a developer carrier that carries the developer in order to develop the electrostatic latent image formed on the image carrier into a toner image; a transfer member onto which the toner image carried on the image carrier is transferred; a carrier recovery device that has a rotatable sleeve disposed opposite to the image carrier and a magnet non-rotatably disposed inside the sleeve, and recovers the carrier on the image carrier; a bias application unit that applies a bias including an AC voltage to the sleeve; A control unit that controls the bias application unit; Equipped with the sleeve is disposed, with respect to a rotation direction of the image carrier, downstream of a development position where the electrostatic latent image formed on the image carrier is developed and upstream of a transfer position where the toner image carried on the image carrier is transferred to the transfer member; the control unit controls the bias application unit to apply a first bias to the sleeve during an image forming operation, and controls the bias application unit to apply a second bias to the sleeve in a predetermined mode during non-image formation; In one period of the bias AC voltage applied to the sleeve by the bias application unit, the duration of a voltage having a polarity opposite to the charge polarity of the toner with respect to the DC potential of the sleeve is denoted as t1, the duration of a voltage having a polarity the same as the charge polarity of the toner with respect to the DC potential of the sleeve is denoted as t2, and the ratio of t1 to one period (t1+t2) of the bias AC voltage applied to the sleeve by the bias application unit is denoted as a duty ratio, the duty ratio of the AC voltage of the second bias is lower than 50%.

1. An image forming apparatus comprising:

13. The duty ratio of the AC voltage of the second bias is 40% or less.

13. The image forming apparatus according to claim 12.

14. The duty ratio of the AC voltage of the first bias is 50%.

13. The image forming apparatus according to claim 12.

15. the control unit controls the bias application unit to alternately apply the second bias and the third bias to the sleeve in the predetermined mode; The duty ratio of the AC voltage of the third bias is higher than 50%.

13. The image forming apparatus according to claim 12.

16. The duty ratio of the AC voltage of the second bias is 40% or less, The duty ratio of the AC voltage of the third bias is 60% or more.

16. The image forming apparatus according to claim 15.

17. In the predetermined mode, the absolute value of the DC potential of the sleeve is the same as the absolute value of the DC potential of the image carrier.

13. The image forming apparatus according to claim 12.

18. During the image forming operation, the polarity of the DC potential of the image carrier and the polarity of the DC potential of the sleeve are the same as the charging polarity of the toner, and the absolute value of the DC potential of the sleeve is greater than the absolute value of the DC potential of the image carrier.

13. The image forming apparatus according to claim 12.

19. the first bias is a DC voltage superimposed on an AC voltage, The second bias is an AC voltage only.

13. The image forming apparatus according to claim 12.

20. An image forming apparatus capable of performing an image forming operation, a rotatable image carrier on which an electrostatic latent image is formed; a developing device including a developer container that contains a developer containing a toner and a carrier, and a developer carrier that carries the developer in order to develop the electrostatic latent image formed on the image carrier into a toner image; a transfer member onto which the toner image carried on the image carrier is transferred; a carrier recovery device that has a rotatable sleeve disposed opposite to the image carrier and a magnet non-rotatably disposed inside the sleeve, and recovers the carrier on the image carrier; a bias applying unit that applies a bias in which a DC voltage and an AC voltage are superimposed on the sleeve; A control unit that controls the bias application unit; Equipped with the sleeve is disposed, with respect to a rotation direction of the image carrier, downstream of a development position where the electrostatic latent image formed on the image carrier is developed and upstream of a transfer position where the toner image carried on the image carrier is transferred to the transfer member; the control unit controls the bias application unit to apply a first bias to the sleeve during an image forming operation, and controls the bias application unit to alternately apply a second bias and a third bias to the sleeve in a predetermined mode during non-image formation; the first bias has a DC voltage having the same polarity as the charging polarity of the toner; the second bias is a DC voltage superimposed on an AC voltage such that the polarity of the potential of the sleeve with respect to the image carrier is negative; The third bias is a DC voltage having a positive polarity as a potential of the sleeve with respect to the image carrier, and an AC voltage is superimposed on the DC voltage.

1. An image forming apparatus comprising:

Images