Image forming apparatus

Abstract

To provide an image forming apparatus that can detect, at low cost, a change in the electrification performance of an electrifier.SOLUTION: According to an embodiment, an image forming apparatus has a photoreceptor, an electrifier, a developing unit, a toner sensor, and a controller. The electrifier electrifies the photoreceptor with electrification bias voltage. The developing unit forms a toner image on the photoreceptor with developing bias voltage when the developing bias voltage rises, the developing bias voltage being applied after a predetermined time from the application of the electrification bias voltage. The toner sensor detects the toner image developed by the developing unit. The controller executes predetermined operations according to a change in the density of the toner image detected by the toner sensor.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] An embodiment of the present invention relates to an image forming apparatus. [Background technology]

[0002] Conventionally, image forming apparatuses using electrophotography charge a photoconductor using a charger in an image forming process. When the image forming process is repeatedly performed, the charger of the image forming apparatus may change in charging characteristics due to deterioration or dirt. For example, the charging characteristics of a charging roller, which is an example of a charger, change depending on the electrical resistance of the rubber layer that occupies most of the roller. In general, the electrical resistance of the rubber layer of a charging roller tends to increase with continued use. If the electrical resistance of the rubber layer of the charging roller is too high, the charging ability of the charging roller decreases. In image forming apparatuses, the charging ability of the charger may deteriorate without the user noticing, causing a decrease in the charging potential, resulting in problems such as fogged images.

[0003] In order to prevent such problems, it is necessary to predict the change in the electrical resistance of the charger. In a conventional technique, a mechanism for detecting current is provided in a power supply unit that applies a charging bias voltage, making it possible to directly detect an increase in electrical resistance due to deterioration of the charging roller. However, in order to directly detect the electrical resistance of the charger, a special mechanism must be incorporated in the power supply unit, which is expensive as a power supply. For this reason, there is a demand for an image forming apparatus that can detect a change in the charging ability of the charger without incurring high costs. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2014-102333 A Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide an image forming apparatus capable of detecting a change in the charging ability of a charger without incurring any cost. [Means for solving the problem]

[0006] According to an embodiment, an image forming apparatus includes a photoconductor, a charger, a developer, a toner sensor, and a controller. The charger charges the photoconductor with a charging bias voltage. The developer supplies toner to the photoconductor that forms a toner image on the photoconductor when a developing bias voltage rises, by a developing bias voltage that is applied a predetermined time after the charging bias voltage is applied. The toner sensor detects the toner image developed by the developer. The controller executes a predetermined operation in response to a change in density of the toner image detected by the toner sensor. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram showing an example of the configuration of a digital multifunction peripheral as an image forming apparatus according to an embodiment. [Diagram 2] FIG. 2 is a diagram showing an example of the configuration of a main part of a printer in a digital multifunction peripheral as an image forming apparatus according to an embodiment. [Diagram 3] FIG. 3 is a diagram for explaining a charging bias and a developing bias of a printer in a digital multifunction peripheral as an image forming apparatus according to an embodiment. [Figure 4] FIG. 4 is a block diagram showing an example of the configuration of a control system in a digital multifunction peripheral as an image forming apparatus according to an embodiment. [Diagram 5] FIG. 5 is a diagram showing the relationship between the charging bias voltage, the surface potential of the photosensitive drum, and the developing bias voltage in the image forming apparatus according to the embodiment. [Figure 6] FIG. 6 is a diagram showing an example of a toner band formed immediately after the development bias voltage is turned on in the image forming apparatus according to the embodiment. [Figure 7]FIG. 7 is a diagram showing the relationship between the charging bias voltage, the surface potential of the photosensitive drum, and the developing bias voltage when the electrical resistance of the charger in the image forming apparatus according to the embodiment increases. [Figure 8] FIG. 8 is a diagram showing an example of a toner band formed immediately after turning on the development bias power supply in a case where the electrical resistance of the charger in the image forming apparatus according to the embodiment has increased. [Figure 9] FIG. 9 is a diagram for explaining an example of a process in which the digital multifunction peripheral 1 serving as the image forming apparatus according to the embodiment detects a toner band using a toner adhesion sensor. [Figure 10] FIG. 10 is a flowchart for explaining an example of operation including a process for determining deterioration of a charger by a digital multifunction peripheral as an image forming apparatus according to the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, the present embodiment will be described with reference to the drawings. First, a configuration of a digital multi-functional peripheral (MFP) 1 as an image forming apparatus according to the embodiment will be described. FIG. 1 is a diagram showing an example of the configuration of a digital multifunction peripheral 1 as an image forming apparatus according to an embodiment. As shown in FIG. 1, the digital multifunction peripheral 1 includes a printer 2, an operation panel 3, a scanner 4, and a system controller 5.

[0009] The printer 2 is an image forming device that forms an image on a recording medium. The printer 2 included in the digital multifunction device 1 is an image forming device that forms an image on a recording medium by electrophotography. The printer 2 forms an image (toner image) on a recording medium such as paper using toner. The recording medium on which the printer 2 forms an image may be any medium on which an image can be formed, and is not limited to paper, and may be cloth, a plastic film, or a sheet, etc.

[0010] The scanner 4 is installed on the upper part of the main body of the digital multifunction device 1. The scanner 4 is a device that optically reads an image of an original. For example, the scanner 4 reads an image of an original set on a platen glass. The scanner 4 may also be configured to include a scanner that reads an image of an original conveyed by an automatic document feeder (ADF).

[0011] The operation panel 3 is a user interface. The operation panel 3 has a display, a touch panel, operation buttons, etc. The operation panel 3 displays operation guides and various warnings (alerts) on the display. The operation panel 3 accepts operation instructions from the user via the touch panel and operation buttons, etc. For example, the operation panel 3 has a touch panel on the display screen of the display, and detects a portion of the display screen that the user touches.

[0012] The system controller 5 controls the entire digital multifunction peripheral 1. The system controller 5 receives operation instructions input to the operation panel 3 and controls the operation of each section. The system controller 5 also receives operation instructions from external devices connected via an interface and controls the operation of each section. For example, when an instruction is given to form an image on a recording medium, the system controller 5 controls the printer 2 to cause the printer 2 to form an image on the recording medium.

[0013] The configuration of the printer 2 will now be described. As shown in FIG. 1, the printer 2 has a medium supply mechanism 13, a conveying mechanism 15, multiple image forming stations SY, SM, SC, SK, an intermediate transfer belt 21, a secondary transfer roller 22, a support roller 23, a toner adhesion sensor 241, a temperature and humidity sensor 242, a transfer belt cleaner 25, and a fixing unit 26.

[0014] The medium supply mechanism 13 has a plurality of paper feed cassettes 321, 322, 323. Any number of paper feed cassettes may be used. Each of the paper feed cassettes 321, 322, 323 stores paper as the recording medium M. The paper as the recording medium M stored in each paper feed cassette may be of different sizes or types. Pickup rollers 341, 342, 343 are disposed in each of the paper feed cassettes 321, 322, 323. The pickup rollers 341, 342, 343 pick up sheets of paper as the recording medium one by one from the paper feed cassettes 321, 322, 323, respectively. The pickup rollers 341, 342, 343 supply the picked up recording medium M to the transport mechanism 15.

[0015] The conveying mechanism 15 conveys the recording medium M. The conveying mechanism 15 has first conveying rollers 521, 522, and 523, a second conveying roller 54, and a registration roller 56 in a conveying path before an image is formed on the recording medium M. The conveying mechanism 15 conveys the recording medium M supplied by the pickup rollers 341, 342, and 343 from the first conveying rollers 521, 522, and 523 to the second conveying roller 54. In the conveying mechanism 15, the second conveying roller 54 further conveys the recording medium M to the registration roller 56.

[0016] The registration rollers 56 of the transport mechanism 15 transport the recording medium M to a secondary transfer position, which will be described later, in accordance with the timing at which an image is transferred from the intermediate transfer belt 21 to the recording medium M at the secondary transfer position. The transport mechanism 15 configures a transport path so as to transport the recording medium M onto which the image has been transferred from the intermediate transfer belt 21 to the fixing device 26. The transport mechanism 15 further includes a third transport roller 58 for discharging the paper to a paper discharge section, and a transport mechanism for transporting the recording medium M to an inversion section that inverts the recording medium M.

[0017] Each of the image forming stations SY, SM, SC, and SK forms an image using toner. In this embodiment, the image forming station SY forms a yellow image using yellow toner. The image forming station SM forms a magenta image using magenta toner. The image forming station SC forms a cyan image using cyan toner. The image forming station SK forms a black image using black toner. Each of the image forming stations SY, SM, SC, and SK transfers the image formed with toner to the intermediate transfer belt 21.

[0018] The intermediate transfer belt 21 is a medium that holds the images transferred by each of the image forming stations SY, SM, SC, and SK. The intermediate transfer belt 21 is an endless belt as shown in Fig. 1. The intermediate transfer belt 21 moves in the direction indicated by the arrow a in Fig. 1. The intermediate transfer belt 21 moves the images transferred by each of the image forming stations SY, SM, SC, and SK to a position where the secondary transfer roller 22 and the support roller 23 face each other.

[0019] The secondary transfer roller 22 and the support roller 23 constitute a transfer section (secondary transfer section) that transfers an image from the intermediate transfer belt 21 to the recording medium. The position where the secondary transfer roller 22 and the support roller 23 face each other is the secondary transfer position where an image is transferred from the intermediate transfer belt 21 to the recording medium. The secondary transfer roller 22 and the support roller 23 sandwich the intermediate transfer belt 21 and the recording medium at the secondary transfer position.

[0020] The support roller 23 supports the intermediate transfer belt 21. The support roller 23 is a drive roller that drives the intermediate transfer belt 21. The secondary transfer roller 22 faces the support roller 23 across the intermediate transfer belt 21. The secondary transfer roller 22 transfers (secondary transfer) an image formed with toner on the transfer surface of the intermediate transfer belt 21 onto the surface of a recording medium.

[0021] The toner adhesion amount sensor 241 is a sensor that detects the amount (concentration) of toner. The toner adhesion amount sensor 241 detects the amount of toner adhesion on the intermediate transfer belt 21. The toner adhesion amount sensor 241 is disposed facing the transfer surface of the intermediate transfer belt 21. The toner adhesion amount sensor 241 is provided between the transfer position (primary transfer position) of an image by each image forming station and the secondary transfer position in the movement direction (conveyance direction) a of the intermediate transfer belt 21. The toner adhesion amount sensor 241 outputs the detected amount of toner adhesion to the system controller 5.

[0022] For example, the toner adhesion sensor 241 detects the image density of a test pattern formed on the intermediate transfer belt 21 in image quality maintenance control that adjusts the image density (toner concentration). The digital multifunction printer 1 adjusts the image density by adjusting various set values ​​using the detection result of the test pattern by the toner adhesion sensor 241. The toner adhesion sensor 241 is also used to detect a toner band formed when a developing bias voltage, which will be described later, is activated.

[0023] The temperature and humidity sensor 242 detects the temperature and humidity inside the printer 2. The temperature and humidity sensor 242 is disposed at a position where it can detect the temperature and humidity as information indicating the environment inside the printer 2.

[0024] 1, the transfer belt cleaner 25 is disposed between the secondary transfer position and the primary transfer position in the moving direction a of the intermediate transfer belt 21. The transfer belt cleaner 25 removes toner on the intermediate transfer belt 21. For example, the transfer belt cleaner 25 removes toner remaining on the transfer surface of the intermediate transfer belt 21 after an image is transferred from the intermediate transfer belt 21 to a recording medium.

[0025] The fixing device 26 fixes the image formed by the toner transferred to the recording medium to the recording medium. The fixing device 26 is disposed on the transport path of the recording medium after it has passed the secondary transfer position. The fixing device 26 has a pressure roller and a heating roller that face each other. The fixing device 26 applies heat and pressure to the recording medium by transporting the recording medium between the opposing heating roller and pressure roller. The fixing device 26 fixes the toner image transferred to the recording medium by heating it in a pressurized state.

[0026] Next, the configuration of each of the image forming stations SY, SM, SC, and SK in the printer 2 as the image forming apparatus according to the embodiment will be described in detail. Fig. 2 is a diagram showing an example of the configuration of each of the image forming stations SY, SM, SC, and SK in the printer 2. Fig. 3 is a diagram showing the relationship between the surface potential Va of the photoconductor drum 122 at the development position of the image forming station SK (SY, SM, SC) and the surface potential Vb of the development roller 114.

[0027] 2, each of the image forming stations SY, SM, SC, and SK includes an exposure unit 100, a developing unit 110, a photoconductor drum 122, a charger 126, a primary transfer roller 128, a photoconductor cleaner 130, and a static eliminator 132. In this embodiment, each of the image forming stations SY, SM, SC, and SK has the configuration shown in FIG.

[0028] The photoconductor drum 122 is an image carrier having a photoconductor layer 124 on its surface. The photoconductor drum 122 rotates in a direction (indicated by arrow b in FIG. 2) that matches the movement of the intermediate transfer belt 21 in the moving direction a. Around the photoconductor drum 122, a charger 126, an exposure unit 100, a developer 110, a primary transfer roller 128, the intermediate transfer belt 21, a photoconductor cleaner 130, and a static eliminator 132 are arranged.

[0029] The charger 126 uniformly charges the photoconductor layer 124 on the surface of the photoconductor drum 122. The charger 126 uniformly charges the photoconductor layer 124 on the surface of the photoconductor drum 122 to a negative polarity, for example. In the configuration example shown in Fig. 3, the charger 126 is a charging roller to which a charging bias voltage is applied. The charging roller 126 charges the surface of the photoconductor drum 122 by rotating in contact with the photoconductor drum 122 with the charging bias voltage applied to it.

[0030] The exposure unit 100 forms a static electricity pattern (electrostatic latent image) corresponding to an image on the surface of the photoconductor drum 122. The exposure unit 100 irradiates the surface of the photoconductor drum 122 with light L, the emission of which is controlled based on image data. For example, the exposure unit 100 irradiates the surface of the photoconductor drum 122 with light L, which is emitted based on image data, via an optical system such as a polygon mirror. The exposure unit 100 may be configured to include a device that emits a plurality of laser beams that are guided to the photoconductor drums 122 of a plurality of image forming stations. Furthermore, the exposure unit 100 may be a light emitting device provided for each of the plurality of image forming stations.

[0031] The developing unit 110 develops the electrostatic latent image formed on the surface of the photoconductor drum 122 with a developer. The developing unit 110 supplies the developer to the surface of the photoconductor drum 122 exposed by the exposure unit 100. The developing unit 110 of each image forming station develops an image in a corresponding color. For example, the developing unit 110 of the image forming station SY develops the electrostatic latent image on the photoconductor drum 122 with yellow toner. The developing unit 110 of the image forming station SM develops the electrostatic latent image on the photoconductor drum 122 with magenta toner. The developing unit 110 of the image forming station SC develops the electrostatic latent image on the photoconductor drum 122 with cyan toner. The developing unit 110 of the image forming station SK develops the electrostatic latent image on the photoconductor drum 122 with black toner.

[0032] In the exemplary configuration shown in FIG. 2, the developing unit 110 includes a developer container 112, a developing roller 114, a first mixer 116, a second mixer 118, and a toner concentration sensor 120. The developer container 112 is a container that contains the developer. The developer is a mixture of toner and a carrier made of magnetic fine particles. When the developer is stirred, the toner becomes triboelectrically charged. As a result, the toner adheres to the surface of the carrier by electrostatic force. A developing roller 114, a first mixer 116, and a second mixer 118 are disposed inside the developer container 112. A toner concentration sensor 120 is also disposed inside the developer container 112. The toner concentration sensor 120 detects the toner concentration in the developer contained in the developer container 112. The toner concentration is expressed, for example, by the ratio of toner to carrier in the developer in the developer container 112 (toner / carrier). The system controller 5 controls the toner concentration detected by the toner concentration sensor 120 to a predetermined value.

[0033] The developing roller 114 has a magnetic body (e.g., a magnet) in which positive and negative poles are alternately arranged along a circumference. In FIG. 2, the developing roller 114 rotates counterclockwise. The first mixer 116 and the second mixer 118 mix the developer in the developer container 112. The first mixer 116 and the second mixer 118 also transport the developer. The second mixer 118, which is disposed below the developing roller 114, supplies the developer to the surface of the developing roller 114.

[0034] The developer adheres to the surface of the developing roller 114 in the form of spikes in accordance with the magnetic field distribution generated by the magnetic body of the developing roller 114. The developing roller 114 rotates while carrying the developer. The layer of developer adhering to the developing roller 114 is limited to a predetermined thickness by a blade provided at a predetermined distance from the surface of the developing roller 114. The developer carried by the developing roller 114, limited to the predetermined thickness by the blade, moves to a position (developing position) facing the surface of the photosensitive drum 122.

[0035] A developing bias voltage is applied to the developing roller 114 that carries the developer. The surface potential Vb of the developing roller 114 shown in FIG. 3 is controlled by the developing bias voltage. Moreover, the surface potential Va of the photoconductor drum 122 shown in FIG. 3 is controlled by the charging bias voltage. The toner in the developer carried by the developing roller 114 adheres to the surface of the photoconductor drum 122 due to the potential difference between the potential Vb of the developing roller 114 and the surface potential Va of the photoconductor drum 122.

[0036] The developer carried by the developing roller 114 approaches the surface of the photoconductor drum 122 at a development position as a result of the developing roller 114 rotating in a predetermined direction. The toner contained in the developer carried by the developing roller 114 develops the electrostatic latent image on the photoconductor drum 122 when it approaches the surface of the photoconductor drum 122. As a result, a toner image obtained by developing the electrostatic latent image with toner is formed on the photoconductor drum 122.

[0037] The potential difference between the potential Vb of the developing roller 114 and the surface potential Va of the photoconductor drum 122 is related to the density (image density) of the toner moving from the developing roller 114 to the electrostatic latent image on the photoconductor drum 122. For example, the digital multifunction printer 1 performs image quality maintenance control that adjusts the image density using the result of detection by the toner adhesion amount sensor 241 of the image density (toner density) of a test pattern formed on the intermediate transfer belt 21. In the image quality maintenance control, the system controller 5 adjusts the developing bias voltage or the charging bias voltage.

[0038] Furthermore, the system controller 5 corrects the charging bias voltage when the charging characteristics of the charger (charging roller) 126 change due to an increase in electrical resistance as described below. Furthermore, the system controller 5 may adjust the developing bias voltage depending on the environment (temperature, humidity) inside the printer 2 detected by the temperature and humidity sensor 242. When adjusting the developing bias voltage depending on the environment inside the printer 2, the system controller 5 may correct the charging bias voltage according to the developing bias voltage.

[0039] An image (toner image) developed with toner on the surface of the photosensitive drum 122 moves to a position corresponding to the primary transfer roller 128 by the rotation of the photosensitive drum 122. The primary transfer roller 128 faces the photosensitive drum 122 with the intermediate transfer belt 21 sandwiched therebetween. The primary transfer roller 128 abuts against the surface of the photosensitive drum 122 with the intermediate transfer belt 21 sandwiched therebetween. The primary transfer roller 128 transfers the toner image on the surface of the photosensitive drum 122 to the intermediate transfer belt 21 (primary transfer).

[0040] The photoconductor cleaner 130 is disposed downstream, in the circumferential direction of the photoconductor drum 122, of the position where the toner image on the surface of the photoconductor drum 122 is transferred onto the intermediate transfer belt 21. The photoconductor cleaner 130 removes the toner on the surface of the photoconductor drum 122. That is, the photoconductor cleaner 130 removes the toner remaining on the surface of the photoconductor drum 122 after the primary transfer of the toner image from the photoconductor drum 122 to the intermediate transfer belt 21 is executed.

[0041] The static eliminator 132 is disposed downstream of the position of the photoconductor cleaner 130 in the circumferential direction of the photoconductor drum 122. The static eliminator 132 irradiates light onto the surface of the photoconductor drum 122. In this way, the static eliminator 132 removes the electric charge remaining on the photoconductor layer 124 on the surface of the photoconductor drum 122.

[0042] Next, the configuration of a control system in the digital multifunction peripheral 1 serving as the image forming apparatus according to the embodiment will be described. FIG. 4 is a block diagram showing an example of the configuration of a control system in a digital multifunction peripheral 1 serving as an image forming apparatus according to an embodiment. 4, the system controller 5 includes a processor 101, a ROM 102, a RAM 103, a storage device (memory) 104, and a communication interface (I / F) 105. The processor 101 of the system controller 5 is connected to each unit in the digital multifunction peripheral 1 via various interfaces.

[0043] The processor 101 executes a program to realize various processes. The processor 101 is, for example, a CPU. The processor 101 is connected to a ROM 102, a RAM 103, a storage device 104, a communication interface (I / F) 105, and the like. The processor 101 is also connected to each part in the printer 2, an operation panel 3, and a scanner 4 via the interfaces.

[0044] The ROM 102 is a non-rewritable non-volatile memory. The ROM 102 operates as a program memory for storing programs. The RAM 103 operates as a working memory or a buffer memory. The processor 101 executes various processes by using the RAM 103 to execute programs stored in the ROM 102 or the storage device 104.

[0045] The storage device 104 is a rewritable non-volatile memory. For example, the storage device 104 is configured with a storage device such as an HDD (hard disk drive) or an SSD (solid state drive). The storage device 104 stores data such as control data, control programs, and setting information. The storage device 104 also stores image data and the like.

[0046] The communication I / F 105 is an interface for performing data communication with an external device. For example, the communication I / F 105 communicates with a user terminal such as a PC or a mobile terminal via a network. The communication I / F 105 may input an image print request (print job) from a user terminal such as a PC.

[0047] The communication I / F 105 includes an interface for communicating with the server SV. The server SV acquires setting information and log information in the digital multifunction device 1. For example, the server SV acquires information detected by various sensors from the system controller 5. The server SV may also acquire information indicating the length of a toner band (described later) detected by the toner adhesion amount sensor 241. The server SV may also provide various operational instructions to the system controller 5. For example, the server SV acquires various information from the system controller 5 and supplies control instructions such as updating setting information according to the acquired information.

[0048] As shown in FIG. 4, the printer 2 includes a power supply 140 in addition to the components shown in FIGS. 3, the power supply 140 has a high voltage power supply 141, a developing bias power supply 142, a charging bias power supply 143, a primary transfer bias power supply 144, and a secondary transfer bias power supply 145. However, the developing bias power supply 142, the charging bias power supply 143, and the primary transfer bias power supply 144 are provided for each of the image forming stations SY, SM, SC, and SK.

[0049] A high-voltage power supply 141 supplies a high voltage to various bias power supplies 142, 143, 144, and 145. The high voltage is, for example, a voltage of several hundreds of volts to several kV. The high-voltage power supply 141 generates the high voltage from an input voltage of, for example, several tens of volts.

[0050] The developing bias power supply 142 supplies a developing bias voltage to the developing device 110. The developing bias power supply 142 converts the high voltage generated by the high voltage power supply 141 into a developing bias voltage having a voltage value set by the system controller 5. The developing bias power supply 142 supplies the developing bias voltage specified by the system controller 5 to the developing device 110.

[0051] The charging bias power supply 143 supplies a charging bias voltage to the charging roller 126 serving as a charger. The charging bias power supply 143 converts the high voltage generated by the high voltage power supply 141 into a charging bias voltage having a voltage value set by the system controller 5. The charging bias power supply 143 supplies the charging bias voltage designated by the system controller 5 to the charger 126.

[0052] The primary transfer bias power supply 144 supplies a primary transfer bias voltage to the primary transfer roller 128. The primary transfer bias power supply 144 converts the high voltage generated by the high voltage power supply 141 into a primary transfer bias voltage having a voltage value set by the system controller 5. The primary transfer bias power supply 144 supplies the primary transfer bias voltage specified by the system controller 5 to the primary transfer roller 128.

[0053] The secondary transfer bias power supply 145 supplies a secondary transfer bias voltage to the secondary transfer roller 22. The secondary transfer bias power supply 145 converts the high voltage generated by the high voltage power supply 141 into a secondary transfer bias voltage having a voltage value set by the system controller 5. The secondary transfer bias power supply 145 supplies the secondary transfer bias voltage of the value specified by the system controller 5 to the secondary transfer roller 22.

[0054] Next, the relationship between the charging bias voltage and the developing bias voltage at the start-up of the digital multifunction peripheral 1 as the image forming apparatus according to the embodiment will be described. Fig. 5 is a diagram showing the relationship between the charging bias voltage Vc, the surface potential (photoconductor surface potential) Va of the photoconductor drum 122, and the developing bias voltage Vb in the image forming apparatus according to the embodiment. Fig. 6 is a diagram showing an example of a band-shaped toner image (hereinafter referred to as a toner band) TBa formed by the surface potential Va of the photoconductor drum 122 at the developing position and the developing bias voltage Vb immediately after the developing bias voltage Vb is turned on.

[0055] 5, the charging bias voltage Vc rises (starts to rise) to a steady value after being turned on. When the charging bias voltage Vc reaches the discharge start voltage Vcs, the surface potential Va of the photoconductor drum 122 starts to rise (starts to rise). After rising, the surface potential Va of the photoconductor drum 122 rises to a steady value in response to the rise in the charging bias voltage Vc.

[0056] The developing bias voltage Vb is turned on when a predetermined time T has elapsed since the charging bias voltage Vc was turned on. Here, the predetermined time T is set to a time slightly shorter than the time it takes for the surface of the photoconductor drum 122 to move from the charging position to the developing position. The charging position is the position where the surface of the photoconductor drum 122 contacts the charging roller 126. The developing position is the position where the surface of the photoconductor drum 122 is closest to the developing roller 114. The photoconductor drum 122 rotates at a predetermined peripheral speed set by the controller 5. Therefore, the time it takes for the surface of the photoconductor drum 122, charged by the charging roller 126, to move from the charging position to the developing position can be determined from the speed at which the photoconductor drum 122 rotates.

[0057] At the development position, the surface potential Va of the photoconductor drum 122 charged by the charging roller 126 and the surface potential Vb of the development roller 114 vary as shown in Fig. 6. The surface potential Va of the photoconductor drum 122 at the development position shown in Fig. 6 is the surface potential of the photoconductor drum 122 shown in Fig. 5 delayed by the time from the charging position to the development position.

[0058] The predetermined time T, which is the timing for turning on the development bias voltage, is set to a time slightly shorter than the time it takes for the surface of the photoconductor drum 122 to move from the charging position to the development position. Therefore, at the development position, the surface potential Vb of the development roller 114 rises earlier than the surface potential Va of the photoconductor drum 122. If the surface potential of the photoconductor drum 122 rises before the surface potential of the development roller, a phenomenon may occur in which carriers in the developer adhere to the photoconductor drum 122. To prevent such a phenomenon from occurring, the potential Vb of the development roller 114 is controlled to rise slightly earlier than the surface potential Va of the photoconductor drum 122 at the development position.

[0059] 6, when the potential Vb of the developing roller 114 starts to rise, the surface potential Va of the photoconductor drum 122 at the development position has not yet risen. When the potential of the developing roller 114 starts to rise, no electric field is formed at the development position, so the toner does not move to the surface of the photoconductor drum 122.

[0060] After the potential Vb of the developing roller 114 starts to rise, the surface potential Va of the photoconductor drum 122 at the developing position rises later than the potential Vb of the developing roller 114. For this reason, the potential Vb of the developing roller 114 rises earlier than the surface potential Va of the photoconductor drum 122, and the electric field in the direction in which the toner moves toward the photoconductor drum 122 gradually becomes stronger at the developing position. The stronger the electric field formed at the developing position, the more the amount of toner that moves from the developing roller 114 to the surface of the photoconductor drum 122 increases.

[0061] When the surface potential Va of the photoconductor drum 122 starts to rise, the potential difference with the potential Vb of the developing roller 114 gradually decreases. When the potential difference between the surface potential Va of the photoconductor drum 122 and the potential Vb of the developing roller 114 decreases, the electric field in the direction in which the toner moves to the photoconductor drum 122 gradually weakens. The weaker the electric field formed at the development position, the less toner moves from the developing roller 114 to the surface of the photoconductor drum 122. When the surface potential Va of the photoconductor drum 122 at the development position reaches a steady value, the surface potential Va of the photoconductor drum 122 becomes higher than the potential Vb of the developing roller 114, and the toner no longer develops.

[0062] 6, a toner band TBa is formed on the surface of the photoconductor drum 122 due to the relationship between the surface potential Va of the photoconductor drum 122 and the potential Vb of the developing roller 114. The toner band TBa formed on the surface of the photoconductor drum 122 is transferred to the intermediate transfer belt 21 and detected by the toner adhesion amount sensor 241. In a normal (normal, initial) state, the relationship between the surface potential of the photoconductor drum 122 and the potential of the developing roller 114 is a predetermined relationship, so that the toner band has a predetermined length in the transport direction.

[0063] Next, the relationship between the charging bias voltage, the surface potential of the photosensitive drum 122, and the developing bias voltage when the electrical resistance of the charging roller 126 in the digital multifunction peripheral 1 according to the embodiment increases will be described. Fig. 7 is a diagram showing the relationship between the charging bias voltage Vc', the surface potential Va' of the photoconductor drum 122, and the developing bias voltage Vb when the electrical resistance of the charging roller 126 increases. Fig. 8 is a diagram showing an example (second example) of a toner band TBb formed by the surface potential Va' of the photoconductor drum 122 charged by the charging bias voltage as shown in Fig. 7 and the developing bias voltage Vb. Here, Figures 5 and 6 show the charging bias voltage Vc, the surface potential Va of the photoconductor drum 122, the developing bias voltage Vb, and the toner band TBa in a normal (normal, initial) state. Also, Figures 7 and 8 show a state in which the electrical resistance of the charger 126 is increased compared to the normal state shown in Figures 5 and 6.

[0064] The electric resistance of the charger 126 may increase with use. For example, the electric resistance of the rubber layer that forms the charging roller 126 increases with use. When the electric resistance of the charging roller 126 increases, the rise of the charging bias voltage becomes slower. Also, the charging roller 126 has the property that the discharge start voltage increases as the electric resistance increases. For this reason, when the electric resistance of the charging roller 126 increases, the rise of the surface potential of the photoconductor drum 122 is delayed compared to the normal state.

[0065] Furthermore, a general property of high-voltage power supplies is that as the load resistance increases, the voltage rise time of the high-voltage power supply becomes longer. When the rise time of the voltage of the high-voltage power supply becomes longer, the rise of the charging bias voltage becomes slower. When the rise of the charging bias voltage becomes slower, the rise of the surface potential of the photoconductor drum 122 charged by the charging roller also becomes slower. Therefore, when the electrical resistance of the charging roller 126 increases due to deterioration or the like, the rise timing and waveform of the surface potential of the photoconductor drum 122 change.

[0066] According to the example shown in Fig. 7, the charging bias voltage Vc' rises to a steady value slower than the charging bias voltage Vc shown in Fig. 5. As a result, the surface potential Va' of the photoconductor drum 122 shown in Fig. 7 takes longer to reach a steady value (rises slower) than the surface potential Va of the photoconductor drum 122 shown in Fig. 6.

[0067] Furthermore, when the charging roller 126 deteriorates (electrical resistance increases), the discharge start voltage increases, and the amount of discharge charge that moves to the surface of the photoconductor drum 122 decreases. In the example shown in Fig. 7, the discharge start voltage Vcs' at the charging bias voltage Vc' is higher than the discharge start voltage Vcs in the normal state shown in Fig. 5. As a result, the surface potential Va' of the photoconductor drum 122 shown in Fig. 7 starts to rise later than the surface potential Va of the photoconductor drum 122 shown in Fig. 6 by the delay time d.

[0068] The surface potential Va' of the photoconductor drum 122 shown in Fig. 7 starts to rise later and takes longer to reach a steady value than the surface potential Va of the photoconductor drum 122 shown in Fig. 6. As a result, immediately after the development bias voltage Vb is turned on, the surface potential Va' of the photoconductor drum 122 at the development position and the potential Vb of the development roller 114 have the relationship shown in Fig. 8. During the period when the potential Vb of the development roller 114 at the development position is higher than the surface potential Va' of the photoconductor drum 122 (the period when the potential difference is greater than or equal to the potential difference that moves the toner), a toner band TBb is formed on the surface of the photoconductor drum 122.

[0069] 8 and 6, the period during which the potential Vb is higher than the potential Va' is longer than the period during which the potential Vb is higher than the potential Va. As a result, the length in the transport direction of the toner band TBb where the toner concentration is equal to or higher than a predetermined concentration (hereinafter referred to as the toner band length) is longer than that of the toner band TBa. That is, when the electrical resistance of the charger (charging roller) 126 increases, the rise of the surface potential of the photoconductor drum 122 becomes slower, and the length of the toner band becomes longer.

[0070] Next, a process of detecting a toner band in order to determine deterioration of the charger 126 in the digital multifunction peripheral 1 according to the embodiment will be described. As described above, if the electrical resistance of the charger (charging roller) 126 increases, the rise in the surface potential of the photoconductor drum 122 becomes slower, and the length of the toner band becomes longer. The digital multifunction printer 1 detects the toner band TB formed immediately after the development bias voltage is turned on (when the development bias is started) using the toner adhesion sensor 241. The system controller (hereinafter, controller) 5 obtains information indicating the length of the toner band formed when the development bias is started based on the detection value of the toner adhesion sensor 241.

[0071] The controller 5 determines (predicts) the degree of deterioration (increase in electrical resistance) of the charging roller 126 by comparing the information indicating the length of the toner band with a predetermined reference value. The controller 5 executes a predetermined operation according to the degree of deterioration of the charging roller 126 determined from the information indicating the length of the toner band. For example, the controller 5 issues an alert as a predetermined operation when it is determined that the charging roller 126 has deteriorated.

[0072] FIG. 9 is a diagram for explaining an example of a detection process for detecting a toner band using the toner adhesion amount sensor 241 in the digital multifunction peripheral 1. In FIG. 9 illustrates timings ta, tb, ..., ti at which the toner adhesion sensor 241 detects the toner forming the toner band TB. The timings ta, tb, ..., ti are set based on the timing at which the developing bias voltage is turned on. The timing ta is based on the timing at which the developing bias voltage is turned on and is set according to the time it takes for the toner to move from the development position to the detection position of the toner adhesion sensor 241. The timings tb, ..., ti are set to be a predetermined time interval from the timing ta.

[0073] In the example shown in Fig. 9, the toner band TBa is formed in a normal state (a state in which the charging roller is not deteriorated), and the toner band TBa is formed in a state in which the charging roller is deteriorated. The controller 5 judges the toner band TBa to be in a normal (normal) state, and judges the toner band TBb to be in a state in which deterioration has occurred and a predetermined operation (alert or correction) is required. The controller 5 judges whether or not the state is normal based on information indicating the length of the toner band TB in the transport direction. The controller 5 can obtain information indicating the length of the toner band for judging whether or not the state is normal by various methods.

[0074] For example, the controller 5 acquires the sum of the detection values ​​by the toner adhesion sensor 241 at each timing as information indicating the length of the toner band. The controller 5 determines the degree of deterioration of the charging roller 126 based on the sum of the detection values ​​by the toner adhesion sensor 241 at each timing. Specifically, when the sum of the detection values ​​by the toner adhesion sensor 241 (the length of the toner band) exceeds a predetermined threshold, the controller 5 determines that the electrical resistance of the charger 126 has increased and executes a predetermined operation.

[0075] The controller 5 may also determine whether the length of the toner band is equal to or greater than a predetermined threshold value depending on whether the toner band is detected at a predetermined determination timing. For example, as shown in Fig. 9, the determination timing Tt is set based on the timing at which the developing bias voltage is turned on. The determination timing Tt is set to a timing at which a toner band with a length equal to or greater than a predetermined threshold value can be detected without detecting a toner band with a length less than the predetermined threshold value. If the controller 5 detects toner with a predetermined density or greater at the determination timing Tt (if the detection value of the toner adhesion amount sensor 241 is equal to or greater than a predetermined value), the controller 5 determines that the electrical resistance of the charger 126 has increased and executes a predetermined operation.

[0076] Furthermore, the controller 5 may determine the degree of deterioration of the charger 126 by using a detection value of the toner band TBa in the normal state detected by the toner adhesion amount sensor 241 as a reference value. For example, the controller 5 stores a detection value of the toner band in the normal state detected by the toner adhesion amount sensor 241 as a reference value in the storage device 104. The controller 5 may execute a predetermined operation when the difference between the detection value detected by the toner adhesion amount sensor 241 and the reference value stored in the storage device 104 exceeds a predetermined threshold value.

[0077] Next, a predetermined operation that the digital multifunction peripheral 1 serving as the image forming apparatus according to the embodiment executes depending on the degree of deterioration (increase in electrical resistance) of the charging roller 126 will be described. As described above, the controller 5 determines the deterioration degree of the charging roller 126 based on information indicating the length of the toner band TB detected by the toner adhesion amount sensor 241. The controller 5 executes a predetermined operation according to the deterioration degree of the charger 126. The controller 5 may execute a process other than issuing an alert as the predetermined operation according to the deterioration degree of the charger 126. For example, the controller 5 may execute a process of correcting a setting value such as a charging bias voltage according to the deterioration degree of the charger 126.

[0078] When the electrical resistance of the charging roller 126 increases, the charging performance generally deteriorates, which may also cause a decrease in the surface potential of the photoconductor drum 122. According to the example shown in Fig. 7, the steady value of the surface potential Va' of the photoconductor drum 122 is lower than the steady value of the surface potential Va of the photoconductor drum 122. In other words, Fig. 7 illustrates that when the electrical resistance of the charging roller 126 increases, the surface potential of the photoconductor drum 122 decreases compared to the normal state.

[0079] In order to correct the decrease in the surface potential of the photoconductor drum 122, when deterioration (increase in electrical resistance) of the charging roller 126 is detected, correction control such as increasing the charging bias voltage may be performed. In other words, the controller 5 can prevent the decrease in the surface potential of the photoconductor drum 122 by correcting the charging bias voltage in accordance with the deterioration of the charging roller 126. As a result, the digital multifunction printer 1 can extend the actual usage period of the charging roller and the like.

[0080] Furthermore, the controller 5 may set a plurality of thresholds (or determination timings) for the information indicating the length of the toner band corresponding to a plurality of operations. The controller 5 may set a first threshold (warning threshold) for issuing an alert and a second threshold (correction threshold) for correcting the charging bias voltage. This allows the controller 5 to issue an alert when the length of the toner band exceeds the first threshold, and to correct the charging bias voltage when the length of the toner band exceeds the second reference value.

[0081] Furthermore, the first threshold for the information indicating the length of the toner band may be set to a value greater than the second threshold. In this case, the controller 5 corrects the charging bias voltage when the length of the toner band reaches the second threshold, and issues an alert when the length reaches the first threshold that is equal to or greater than the second threshold. This allows the digital multifunction peripheral to extend its life by correcting the charging bias voltage, while issuing an alert when deterioration that cannot be suppressed by correcting the charging bias voltage occurs.

[0082] Next, an operation example including a deterioration determination process for determining deterioration of the charger 126 in the digital multifunction peripheral 1 as the image forming apparatus according to the embodiment will be described. FIG. 10 is a flowchart for explaining an example of operation including a deterioration determination process for determining deterioration of the charger 126 by the digital multifunction peripheral 1 as the image forming apparatus according to the embodiment.

[0083] When the controller 5 starts the printer 2, it turns on the charging bias voltage applied from the charging bias power supply 143 to the charging roller 126 (ACT11). The controller 5 measures the elapsed time since the charging bias voltage was turned on, and monitors whether a predetermined time T has elapsed since the charging bias voltage was turned on (ACT12).

[0084] When a predetermined time T has elapsed (ACT12, YES), the controller 5 turns on the developing bias voltage applied from the developing bias power supply 142 to the developing roller 114 (ACT13). After turning on the developing bias voltage, the controller 5 transfers the toner image (toner band) formed on the surface of the photoconductor drum 122 onto the intermediate transfer belt 21 (ACT14).

[0085] The controller 5 detects the toner (toner band) on the intermediate transfer belt 21 by the toner adhesion amount sensor 241 at a predetermined timing based on the time when the developing bias voltage is turned on (ACT15). The controller 5 obtains information indicating the length of the toner band transferred to the intermediate transfer belt 21 from the detection value detected by the toner adhesion amount sensor 241. The controller 5 determines whether or not a predetermined operation (alert, correction of charging bias voltage) is required depending on the degree of deterioration of the charging roller 126 based on the information indicating the length of the toner band (ACT16, 18).

[0086] For example, the controller 5 obtains the sum of the detection values ​​detected by the toner adhesion amount sensor 241 at a plurality of timings at a predetermined interval as information indicating the length of the toner band. In this case, the controller 5 determines whether or not to execute a predetermined operation depending on whether or not the information indicating the length of the toner band (the length of the toner band) is equal to or greater than a predetermined threshold. As a specific example, the storage device 104 stores a first threshold (warning threshold) and a second threshold (correction threshold) shorter than the first threshold as thresholds for the information indicating the length of the toner band.

[0087] In this case, the controller 5 determines whether an alert is required based on whether the information indicating the length of the toner band (the length of the toner band) is equal to or greater than a first threshold (a warning threshold) (ACT 16).The controller 5 also determines whether correction of the charging bias voltage is required based on whether the information indicating the length of the toner band (the length of the toner band) is equal to or greater than a second threshold (a correction threshold) (ACT 18).

[0088] Furthermore, the controller 5 may perform a predetermined operation depending on whether or not the toner band is detected at a predetermined determination timing. As a specific example, the storage device 104 stores a first determination timing and a second determination timing. The first determination timing is a timing corresponding to a threshold value of the toner band length at which an alert is issued (determination timing for warning). The second determination timing is a timing corresponding to a threshold value of the toner band length at which the charging bias voltage is corrected (determination timing for correction). The second determination timing is a timing earlier than the first determination timing.

[0089] In this case, the controller 5 determines whether an alert is required based on whether the toner adhesion sensor 241 detects a toner band (toner having a predetermined density or more) at the first determination timing (ACT 16). Also, the controller 5 determines whether the charging bias voltage needs to be corrected based on whether the toner adhesion sensor 241 detects a toner band (toner having a predetermined density or more) at the second determination timing (ACT 18).

[0090] When the controller 5 determines that an alert is not necessary by any of the above-mentioned processes (ACT16, NO), the controller 5 proceeds to ACT18. Furthermore, when the controller 5 determines that an alert is necessary (ACT16, YES), it executes a process of issuing an alert (ACT17). For example, as the process of issuing an alert, the controller 5 transmits a warning to an external device (such as a terminal device of an administrator) via the communication interface 105, indicating that the charging roller 126 has deteriorated. As the process of issuing an alert, the controller 5 may also display a warning to the effect that the charging roller 126 has deteriorated on a display of the operation panel 3.

[0091] When the controller 5 determines that the charging bias voltage does not need to be corrected (ACT18, NO), the controller 5 proceeds to ACT20. Furthermore, when the controller 5 determines that the charging bias voltage needs to be corrected (ACT18, YES), it executes the correction of the charging bias voltage (ACT19). For example, the controller 5 corrects the set value of the charging bias voltage so as to increase it by a predetermined adjustment value. Furthermore, the controller 5 may correct the set value of the charging bias voltage by executing image quality maintenance control.

[0092] When the processing of ACT11-19 is completed, the controller 5 notifies the server SV of information indicating the detection result of the toner band via the communication interface 105 (ACT20). The server SV saves the information acquired from the digital multifunction device 1 in a storage device as log data. Here, the controller 5 may notify the server SV of the detection result of the toner band, which is the value detected by the toner adhesion amount sensor 241. The controller 5 may also notify the server SV of the detection result of the toner band together with information such as the temperature and humidity detected by the temperature and humidity sensor 242 as environmental data within the device. This allows the server SV to accumulate information indicating the detection result of the toner band together with the operating environment of the printer 2. When the process of ACT11-20 ends, the controller 5 makes the printer 2 ready to perform normal print control.

[0093] The above-mentioned deterioration determination process for the charging roller (ACT13-20) may be performed in each of the multiple image forming stations, or may be performed in one specific image forming station. When deterioration of the charging roller is determined in a specific image forming station, the image forming apparatus may perform deterioration determination process for the charging roller in other image forming stations. For example, the deterioration determination process for the charging roller may be performed in the image forming station SK used for black monochrome printing. In this case, when deterioration of the charging roller in the image forming station SK is determined, deterioration determination process for the charging roller is performed in the other image forming stations SY, SM, and SC. This can reduce the time required for deterioration determination process for the charging roller 126.

[0094] The above-mentioned charging roller deterioration determination process (ACT13-20) may be executed at a specific timing. For example, the image forming apparatus may execute the above-mentioned charging roller deterioration determination process every time a predetermined period of time elapses. The image forming apparatus may execute the charging roller deterioration determination process when the number of images printed (the number of printed sheets of paper) exceeds a predetermined number. The image forming apparatus may execute the charging roller deterioration determination process when an execution instruction is received from an operator.

[0095] As described above, the image forming apparatus according to the embodiment can execute a predetermined operation such as issuing an alert or performing correction control in accordance with the length of the toner band formed when the developing bias is started. As a result, according to the embodiment, it is possible to provide an image forming apparatus that can detect a change in the charging ability of the charger at low cost without incorporating a device that detects the electrical resistance of the charger.

[0096] Image forming apparatuses according to the embodiments will be listed below. [1] A photoconductor; a charger for charging the photoconductor with a charging bias voltage; a developing device that forms a toner image on the photoconductor when a developing bias voltage rises by a developing bias voltage that is applied a predetermined time after the charging bias voltage is applied; a toner sensor that detects the toner image developed by the developing unit; a controller that executes a predetermined operation in response to a change in density of the toner image detected by the toner sensor; An image forming apparatus comprising: [2] the toner sensor detects the toner image at a predetermined position as the toner image is conveyed in a predetermined conveying direction; the controller outputs an alert when the length of the toner image in the transport direction detected by the toner sensor is equal to or greater than a warning threshold value. [1] The image forming apparatus according to the present invention. [3] An interface for communicating with an external device; the controller notifies an external device of a warning when a length of the toner image in a transport direction detected by the toner sensor is equal to or greater than a warning threshold value; [2] The image forming apparatus according to the present invention. [4] the controller further corrects a charging bias voltage to be output to the charger when a length of the toner image in the transport direction detected by the toner sensor is equal to or greater than a correction threshold that is shorter than the warning threshold. [2] The image forming apparatus according to the present invention. [5] the toner sensor detects the toner image at a predetermined position as the toner image is conveyed in a predetermined conveying direction; the controller further corrects a charging bias voltage to be output to the charger when a length of the toner image in a transport direction detected by the toner sensor is equal to or greater than a correction threshold value. [1] The image forming apparatus according to the present invention. [6] the toner sensor detects the toner image at a predetermined position as the toner image is conveyed in a predetermined conveying direction; the controller outputs an alert when the toner sensor detects a toner having a predetermined density or more at a timing for determining a warning. [1] The image forming apparatus according to the present invention. [7] An interface for communicating with an external device; the controller notifies an external device of a warning when the toner sensor detects a toner having a predetermined density or more at a warning determination timing. [6] The image forming apparatus according to claim 1. [8] the controller further corrects a charging bias voltage to be output to the charger when the toner sensor detects toner having a predetermined density or more at a correction determination timing earlier than the warning determination timing. [6] The image forming apparatus according to claim 1. [9] the toner sensor detects the toner image at a predetermined position as the toner image is conveyed in a predetermined conveying direction; the controller corrects a charging bias voltage to be output to the charger when the toner sensor detects a toner having a predetermined density or more at a determination timing for correction; [1] The image forming apparatus according to the present invention.

[10] The charger is a charging roller that rotates in contact with the photoconductor. [1] The image forming apparatus according to the present invention.

[0097] Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, and are included in the scope of the invention and its equivalents described in the claims. [Explanation of symbols]

[0098] 1...digital multifunction peripheral (image forming apparatus), 2...printer, 3...operation panel, 4...scanner, 5...system controller, 13...medium supply mechanism, 15...conveying mechanism, 21...intermediate transfer belt (medium), 22...secondary transfer roller, 100...exposure device, 101...processor, 104...storage device, 105...communication interface, 110...developer, 112...developer container, 114...developing roller, 120...toner concentration sensor, 122...photosensitive drum, 124...photosensitive layer, 126...charger, 128...primary transfer roller, 142...developing bias power supply, 143...charging bias power supply, 241...toner adhesion amount sensor.

Claims

1. A photoconductor; a charger for charging the photoconductor with a charging bias voltage; a developing device that forms a toner image on the photoconductor when a developing bias voltage rises by a developing bias voltage that is applied a predetermined time after the charging bias voltage is applied; a toner sensor that detects the toner image developed by the developing unit; a controller that executes a predetermined operation in response to a change in density of the toner image detected by the toner sensor; An image forming apparatus comprising:

2. the toner sensor detects the toner image at a predetermined position as the toner image is conveyed in a predetermined conveying direction; the controller outputs an alert when the length of the toner image in the transport direction detected by the toner sensor is equal to or greater than a warning threshold value. The image forming apparatus according to claim 1 .

3. the controller further corrects a charging bias voltage to be output to the charger when a length of the toner image in the transport direction detected by the toner sensor is equal to or greater than a correction threshold that is shorter than the warning threshold. The image forming apparatus according to claim 2 .

4. the toner sensor detects the toner image at a predetermined position as the toner image is conveyed in a predetermined conveying direction; the controller outputs an alert when the toner sensor detects a toner having a predetermined density or more at a timing for determining a warning. The image forming apparatus according to claim 1 .

5. the controller further corrects a charging bias voltage to be output to the charger when the toner sensor detects toner having a predetermined density or more at a correction determination timing earlier than the warning determination timing. The image forming apparatus according to claim 4 .

Images