JP2015069193A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a failure caused by reduction in electric field in a nip part, such as reduction in sheet separation performance from an image carrier or reduction in toner transferability from the image carrier to a sheet, even when an image forming apparatus is left in a high-humidity environment for a long time and a resistance value of a transfer belt decreases.SOLUTION: A voltage application unit is controlled so that a third current opposite a second current flows into a nip part. A potential detection unit is controlled so as to detect a potential on an image carrier charged by the third current. According to a detection result of the potential detection unit, a current value of a first current or the second current is determined.

Description

  The present invention relates to an image forming apparatus.

  In recent years, belt transfer type image forming apparatuses are known. In the belt transfer system, the transfer belt is run so as to be in contact with the photosensitive drum, and the paper is conveyed in synchronization with the toner image formed on the photosensitive drum. A transfer voltage having a polarity opposite to the toner charging polarity (transfer polarity) is applied to the transfer belt, and the toner image on the photosensitive drum is transferred to the paper side by electrostatic attraction.

  Patent Document 1 describes an image forming apparatus of a direct transfer type. In the image forming apparatus described in Patent Document 1, the direct transfer unit includes a direct transfer belt, a K photoconductor, a transfer roller, a transfer belt roller, and the like for performing monochrome printing. When performing monochrome printing, a black toner image is formed by a K photoconductor, and is directly transferred onto a sheet on a transfer belt at a transfer roller point by a direct transfer method (paragraphs 0014, 0015, etc.).

  However, the direct transfer method has the following problems. That is, when the transfer belt is left in a high humidity environment for a long time, the transfer belt absorbs moisture in the air and its electric resistance is lowered. FIG. 9 is a graph showing changes in the surface resistance of the transfer belt depending on the standing time in a high humidity environment (30 [° C.], 80 [%]). A curve L0 indicates a change in surface resistance of a new transfer belt, and a curve L1 indicates a change in surface resistance of the transfer belt after printing 2 million sheets. In any of the transfer belts, the resistance value decreases as the standing time in a high humidity environment becomes longer.

  When the surface resistance of the transfer belt decreases, when a voltage is applied to the transfer roller and a current flows through the nip portion, not only in the direction from the transfer roller to the K photoconductor but also directly along the transfer belt, In some cases, current also flows upstream and downstream in the moving direction. That is, current leaks in the direction along the moving direction of the transfer belt directly, and the ratio of current from the transfer roller to the K photoconductor decreases. For this reason, when the leading edge of the paper (dielectric) is separated from the photosensitive drum, the electric field (separation electric field) generated between the transfer belt and the leading edge of the paper is reduced, and the recording paper is separated from the photosensitive drum. Performance will be degraded. In addition, since the electric field (transfer electric field) generated when the toner image formed on the photosensitive drum is transferred to a sheet is reduced, the toner transfer performance is also deteriorated.

  When the transfer belt usage history (for example, the total number of prints in the image forming apparatus) increases, discharge products and the like are formed on the inner peripheral surface of the transfer belt and the transfer belt easily absorbs moisture. The phenomenon of performance degradation tends to get worse.

  Furthermore, it affects the resistance value of the transfer belt and the photosensitive drum and the leading edge of the paper depending on many parameters such as humidity environment, transfer belt leaving time, transfer belt usage history, and the number of outputs from the start of printing. The transfer electric field varies in various ways. Therefore, it is difficult to control the separation performance of the paper from the photosensitive drum in a high humidity environment.

  FIG. 10 is a graph showing a change in separation failure occurrence rate (also referred to as “separation jam occurrence rate”) due to a standing time in a high humidity environment (30 [° C.], 80 [%]). A curve L2 shows a change in separation failure occurrence rate when a new transfer belt is used, and a curve L3 shows a change in separation failure occurrence rate when a transfer belt after printing 2 million sheets is used. Regardless of which transfer belt is used, the separation failure occurrence rate increases as the standing time in a high humidity environment becomes longer. Although not shown, the toner transfer performance also decreases as the standing time in a high-humidity environment increases, as with the paper separation performance.

JP 2012-168437 A

  The present invention has been made in view of such problems. Even when the image forming apparatus is left in a high-humidity environment for a long time and the resistance value of the transfer belt is lowered, the image forming apparatus is An object of the present invention is to provide an image forming apparatus capable of suppressing failures due to a decrease in electric field in the nip, such as a decrease in paper separation performance and a transfer performance of toner from an image carrier to a paper.

The present invention provides the following means. That is,
An image carrier for carrying an image to be transferred to paper;
A transfer member that forms a nip portion facing the image carrier;
At least the second current among the first current for separating the paper from the image carrier and the second current for transferring the image carried on the image carrier to the paper by applying a voltage to the transfer member. A voltage application section for flowing a current of
A potential detector for detecting a potential on the image carrier;
A control unit for controlling the operation of the image forming apparatus,
The controller is
Controlling the voltage application unit to flow a third current in a direction opposite to the second current to the nip portion;
Controlling the potential detector to detect a potential on the image carrier charged by the third current;
An image forming apparatus, wherein a current value of the first current or the second current is determined according to a detection result by the potential detection unit.

  According to the present invention, even when the image forming apparatus is left in a high-humidity environment for a long time and the resistance value of the transfer belt is lowered, the separation performance of the paper from the image carrier is reduced or the image carrier is separated from the image carrier. It is possible to provide an image forming apparatus capable of suppressing a failure due to a decrease in an electric field in a nip portion, such as a decrease in toner transfer performance to a sheet.

1 is a control block diagram of an image forming apparatus showing an embodiment according to the present invention. It is a figure which shows the specific structure of the image formation part vicinity which shows embodiment which concerns on this invention. 5 is a flowchart illustrating a control method in the image forming apparatus according to the embodiment of the present invention. 10 is a flowchart showing a modification of the control method in the image forming apparatus according to the embodiment of the present invention. 6 is a flowchart illustrating a control method in another example 1 of the image forming apparatus according to the embodiment of the present invention. It is a flowchart which shows the control method in the other Example 2 of the image forming apparatus which concerns on embodiment of this invention. It is a figure which shows the result of the experiment (Experiment 1) which confirms the effectiveness of this invention. It is a figure which shows the result of the experiment (Experiment 2) which confirms the effectiveness of this invention. It is a figure which shows the change of the surface resistance of the transfer belt with respect to leaving time. It is a figure which shows the change of the separation defect occurrence rate with respect to leaving time.

  Embodiments of the present invention will be described below with reference to the drawings.

[Configuration of image forming apparatus]
An image forming apparatus 100 shown in FIG. 1 forms an image on a sheet by an electrophotographic process. As illustrated in FIG. 1, the image forming apparatus 100 includes a document reading unit 110, an operation display unit 120, an image processing unit 130, an image writing unit 135, an image forming unit 140, a conveyance unit 150, a fixing unit 160, and a communication unit 171. A storage unit 172, a voltage application unit 180, a potential detection unit 190, and a control unit 200. The backup roller 63 and the voltage application unit 180 will be described later.

  The control unit 200 includes a CPU (Central Processing Unit) 201, a ROM (Read Only Memory) 202, a RAM (Random Access Memory) 203, and the like. The CPU 201 reads a program corresponding to the processing content from the ROM 202 and develops it in the RAM 203, and controls the operation of each block of the image forming apparatus 100 in cooperation with the developed program. At this time, various data stored in the storage unit 172 are referred to. The storage unit 172 includes, for example, a nonvolatile semiconductor memory (so-called flash memory) or a hard disk drive.

  The control unit 200 transmits and receives various data to and from an external device (for example, a personal computer) connected to a communication network such as a LAN (Local Area Network) or a WAN (Wide Area Network) via the communication unit 171. Do. For example, the control unit 200 receives image data transmitted from an external device, and forms an image on a sheet based on the received image data. The communication unit 171 includes a communication control card such as a LAN card, for example.

  The document reading unit 110 optically scans the document conveyed on the contact glass, forms an image of reflected light from the document on a light receiving surface of a CCD (Charge Coupled Device) sensor, and reads the document. The document is conveyed onto the contact glass by an automatic document feeder (ADF). However, the document may be manually placed on the contact glass.

  The operation display unit 120 has a touch panel screen. Input operations for various instructions and settings performed by the user can be performed via a touch panel screen. These instructions and setting information are handled by the control unit 200 as job information. The job information includes, for example, a paper size and the number of prints.

  The image processing unit 130 includes a circuit that performs analog-digital (A / D) conversion processing and a circuit that performs digital image processing. The image processing unit 130 generates digital image data from the analog image signal acquired by the CCD sensor of the document reading unit 110 by A / D conversion processing and outputs the digital image data to the image writing unit 135.

  The image writing unit 135 emits laser light based on the digital image data generated by the image processing unit 130, and irradiates the photoconductive drum of the image forming unit 140 with the emitted laser light. An electrostatic latent image is formed on the drum (exposure process).

  The image forming unit 140 is configured to execute a charging process performed before the exposure process, a development process performed after the exposure process, a transfer process after the development process, and a cleaning process after the transfer process in addition to the exposure process described above. It has. In the charging step, the image forming unit 140 uniformly charges the surface of the photosensitive drum by corona discharge from the charging device. In the developing step, the image forming unit 140 forms a toner image on the photosensitive drum by attaching toner contained in the developer in the developing device to the electrostatic latent image on the photosensitive drum.

  In the transfer process, the image forming unit 140 applies the transfer voltage from the voltage applying unit 180 to transfer the toner image on the photosensitive drum onto the sheet conveyed by the conveying unit 150. In the cleaning process, the image forming unit 140 removes toner remaining on the photosensitive drum after the transfer process by bringing a cleaning device such as a brush into contact with the photosensitive drum.

  The fixing unit 160 includes a fixing roller and a pressure roller. The pressure roller is disposed in pressure contact with the fixing roller. A fixing nip portion is formed at a pressure contact portion between the fixing roller and the pressure roller. The fixing unit 160 fixes the toner image on the paper by applying heat and pressure to the toner image on the paper introduced into the fixing nip (heat fixing) (fixing step). As a result, a fixed toner image is formed on the paper. The sheet heated and fixed by the fixing unit 160 is discharged to the outside of the image forming apparatus 100.

  The potential detection unit 190 is a surface potential sensor, for example, and is disposed in the vicinity of the photosensitive drum 1. The potential detection unit 190 detects the potential value (surface potential value) on the surface of the photosensitive drum 1 charged by corona discharge from the charging device in a non-contact manner, and outputs the detection signal to the control unit 200.

  Next, a specific configuration near the image forming unit 140 will be described with reference to FIG. In FIG. 2, reference numeral 1 denotes a photosensitive drum that functions as an image carrier. A charging device 2 that functions as a charging unit, an image writing unit 135, and development along the rotation direction (arrow direction) of the photosensitive drum 1. The apparatus 4, the potential detection unit 190, the transfer conveyance path 5 that guides the paper P to the transfer region, the transfer belt 6 (transfer member) that transfers the toner image formed on the photosensitive drum 1 to the paper P, and the photosensitive drum 1. A cleaning device 7 is provided to remove residual toner. Further, a fixing unit 160 is provided downstream of the transfer belt 6 in the sheet conveyance direction, and fixes the toner image on the sheet P.

The transfer belt 6 is obtained by applying PTFE (polytetrafluoroethylene) having a thickness of 3 [μm] as a coating layer to the surface of a substrate composed of 0.5 [mm] chloroplane rubber or the like. Is used. The transfer belt 6 has a volume resistivity of 9.5 [log (= 10 ^9.5 ) under a predetermined environment (temperature: 20 [° C.], relative humidity: 50 [%], voltage application: 500 [V]). ) Ω · cm], and the surface resistivity is 10.5 [log (= 10 ^10.5 ) Ω / □].

  The transfer belt 6 is stretched between the driven roller 61, the driving roller 62, and other rollers, and the surface of the transfer belt 6 contacts a part of the outer peripheral surface of the photosensitive drum 1 below the photosensitive drum 1. Are arranged as follows. That is, a nip portion NP as a transfer region is formed between the transfer belt 6 and the photosensitive drum 1. The paper P is conveyed while being pressed against the photosensitive drum 1 by the transfer belt 6 at the nip portion NP.

  A backup roller 63 that can apply a transfer voltage to the transfer belt 6 is disposed inside the transfer belt 6 that contacts a part of the outer peripheral surface of the photosensitive drum 1. The backup roller 63 is connected to a voltage application unit 180 as a power source for applying a transfer voltage to the transfer belt 6. The control unit 200 controls the voltage application unit 180 so that a constant current flows from the voltage application unit 180 to the backup roller 63. By applying a positive transfer voltage to the transfer belt 6, the negative toner image on the photosensitive drum 1 is transferred to the paper P in contact with the photosensitive drum 1.

  The paper P is stored in the paper feed cassette 9 and supplied to the transfer transport path 5 through the paper feed transport path 90. A gate 91 is provided downstream of the fixing unit 160 to switch between when the paper P is discharged to the outside and when the paper P is fed to the duplex conveyance path 92 for duplex printing. The paper P that has entered the double-sided conveyance path 92 once proceeds to the reverse conveyance path 93, where it is reversed and merges from the refeed conveyance path 94 to the transfer conveyance path 5.

  The configuration of the image forming apparatus 100 and the configuration around the image forming unit 140 have been described above. Hereinafter, a control method of the image forming apparatus 100 will be described.

[Control Method of Image Forming Apparatus 100]
The image forming apparatus 100 according to the present invention is characteristic in that the control shown in the flowchart of FIG. 3 is executed. The control in FIG. 3 is control for determining the current values of the separation current and the transfer current.

  The separation current (first current) is an operation for forming an image on a sheet (image forming operation), and separates the leading end portion of the sheet from the photosensitive drum 1 when the sheet passes through the nip portion NP. Refers to current. Since the electric field (separation electric field) is generated at the leading end of the sheet and separated from the photosensitive drum 1 by the electrostatic adsorption force between the transfer belt 6 and the leading end of the sheet (dielectric), the direction of the separation current is the backup. The direction from the roller 63 toward the photosensitive drum 1 may be used, or the opposite direction may be used. The transfer current (second current) refers to a current for transferring the toner image carried on the photosensitive drum 1 to a sheet. The transfer current is a current that flows when a negative toner image on the photosensitive drum 1 is transferred to a sheet in contact with the photosensitive drum 1 by applying a positive transfer voltage to the transfer belt 6. Therefore, the direction of the transfer current is the direction from the backup roller 63 toward the photosensitive drum 1. Further, the current values of the separation current and the transfer current determined in FIG. 3 are values used as control target values when the control unit 200 controls the voltage application unit 180 to flow a current through the nip portion NP. This current value includes not only the current value of the current flowing in the direction from the backup roller 63 toward the photosensitive drum 1 but also the current value of the current leaked in the direction along the moving direction of the transfer belt 6.

Various timings can be considered as the timing for executing the control of FIG. For example, it may be executed when the image forming apparatus 100 is turned on after the power is turned on, or may be executed every time an image is formed on a predetermined number of sheets (for example, 10 pages). Further, when the absolute humidity around the transfer belt 6 is not so high (for example, the absolute humidity is 1500 [× 10 ⁻² g / m ³ ] or less), or when the transfer belt 6 is left in a high humidity environment for a short time. In the case where the leaving time is 5 hours or less, for example, the control operation shown in the flowchart of FIG. 3 may not be executed. Here, as an example, a case will be described in which the control of FIG. 3 is executed after the image forming apparatus 100 is turned on and before the image forming operation is started.

  A detailed control method will be described below with reference to the flowchart of FIG.

  When the control is started, the control unit 200 controls the voltage application unit 180 so that a current (charging current, third current) in a direction opposite to the transfer current flows through the nip NP (S301). In the image forming apparatus 100 according to the present embodiment, a transfer current for transferring a toner image onto a sheet is transferred from the transfer belt 6 to which a positive transfer voltage is applied to the negatively charged photosensitive drum 1. It flows in the direction of heading. Therefore, the control unit 200 controls the voltage application unit 180 so that a charging current flows in a direction from the uncharged photosensitive drum 1 to the transfer belt 6 by applying a negative voltage to the transfer belt 6. . At this time, the photosensitive drum 1 is negatively charged by the charging current.

Here, if the resistance value of the transfer belt 6 is sufficiently large, almost all of the charging current flows in the direction from the photosensitive drum 1 toward the transfer belt 6. However, when the image forming apparatus 100 is left in a high-humidity environment for a long time and the resistance value of the transfer belt 6 decreases, a part of the charging current flows upstream or downstream in the moving direction of the transfer belt 6. Flow (current leakage). For this reason, when the image forming apparatus 100 is left in a high humidity environment for a long time, the charge amount of the photosensitive drum 1 is normal (when the image forming apparatus 100 is not left in a high humidity environment for a long time). Smaller than That is, the potential V ₁ of the on the photosensitive drum 1, as compared to the potential V ₀ on the photosensitive drum 1 in passing the same charging current in the image forming apparatus 100 in the normal, the absolute value becomes smaller . That is, the degree of decrease in the resistance value of the transfer belt 6 is the degree of decrease in the absolute value of the potential V ₁ of the photosensitive drum 1 when compared with the absolute value of the potential V ₀ on the photosensitive drum 1 at the normal time. Reflected. Therefore, by detecting the electric potential V ₁ of the on the photosensitive drum 1, it is possible to estimate the resistance value of the transfer belt 6.

Next, the control unit 200 controls the photosensitive drum 1 to rotate the portion charged by the charging current on the photosensitive drum 1 until it is located at a position that can be detected by the potential detection unit 190. Then, the potential detector 190 is controlled to detect the potential on the photosensitive drum 1 charged by the charging current (S302). That is, the control unit 200 controls the potential detection unit 190 so as to detect the potential V ₁ on the photosensitive drum 1.

  Thereafter, the control unit 200 determines the current values of the separation current and the transfer current according to the detection result by the potential detection unit 190 (S303). The significance of this step is as follows. When the image forming apparatus 100 is left in a high-humidity environment for a long time, if the separation current or transfer current having the same current value as normal is supplied, the leading edge of the sheet is separated from the photosensitive drum 1 at the nip portion NP. The absolute value of the separation electric field and the transfer electric field for transferring the toner image carried on the photosensitive drum 1 to the paper becomes small, and the paper separation performance and the toner transfer performance deteriorate. Therefore, the current value of the separation current and the transfer current that does not cause such a failure is calculated.

More specifically, in consideration of a portion of the separation current or transfer current that leaks along the moving direction of the transfer belt 6, a value larger than the current value of the separation current and transfer current in the normal state is set in a high humidity environment. It is determined as the current value of the separation current and the transfer current after standing under. At this time, with reference to the potential V ₁ on the photosensitive drum 1 reflecting the decrease in the resistance value of the transfer belt 6, the separation current and transfer large enough to compensate for leakage of the separation current and transfer current. Calculate the current value of the current. Further, as the absolute value of the potential V ₁ of the on the photosensitive drum 1 is small, that the leakage current when a current of the charging current is large in S301, i.e., indicates that the resistance value of the transfer belt 6 is lowered more ing. Accordingly, as the absolute value of the potential V ₁ of the on the photosensitive drum 1 is small, the absolute value of the separation current and transfer current to compensate for leakage of the separation current and the transfer current is determined to a large value.

Various methods can be considered as methods for calculating the current values of the separation current and the transfer current. Here, as an example, the current values of the separation current and the transfer current are determined to be values that are inversely proportional to the potential on the photosensitive drum 1. To do. That usually separated current and the current value of the transfer current, respectively I _a0 and I _b0 during _separation current and the current value of the transfer current, respectively I _a and I _b are determined by the control of FIG. _3, the normal (transfer belt 6), the potential on the photosensitive drum 1 after flowing the same charging current as in S301 is V ₀ , and the potential on the photosensitive drum 1 after flowing the charging current in S301. Is V ₁ , the current values I _a and I _b of the separation current and the transfer current determined by the control of FIG. 3 are calculated by the following formulas 1 and 2.
Formula 1: I _a = I _a0 × (V ₀ / V ₁ )
Formula 2: I _b = I _b0 × (V ₀ / V ₁ )
In addition, the derivation method of Formula 1 and Formula 2 is mentioned later.

When the current values of the separation current and the transfer current are calculated in S303, the control in FIG. 3 ends. Thereafter, when the image forming operation is started, the control unit 200 determines the voltage so that the separation current when separating the leading edge of the sheet from the photosensitive drum 1 becomes the separation current value I _a determined in the control of FIG. The application unit 180 is controlled. The control unit 200, as the transfer current at the time of transferring the toner image on the photosensitive drum 1 to the paper, the transfer current value I _b determined in the control of FIG. 3, controls the voltage application unit 180 To do.

  Note that the current values of the separation current and the transfer current calculated in S303 are not limited to values that are strictly inversely proportional to the potential on the photosensitive drum 1, but are values that are generally inversely proportional to the potential on the photosensitive drum 1. Also good.

[Effect of this embodiment]
As described above, in the embodiment of the present invention, the control unit 200 executes the control shown in the flowchart of FIG. That is, the control unit 200 controls the voltage application unit 180 so that a charging current (third current) opposite to the transfer current (second current) flows through the nip NP (S301). The potential detector 190 is controlled so as to detect the potential on the charged photosensitive drum 1 (image carrier) (S302), and according to the detection result by the potential detector 190, the separation current (first current) and The value of the transfer current (second current) is determined (S303).

  By controlling in this way, it is possible to determine the separation current and the current value of the transfer current in consideration of the change in the resistance value of the transfer belt 6. For this reason, even when the image forming apparatus 100 is left in a high humidity environment for a long time and the resistance value of the transfer belt 6 is reduced, the separation performance of the paper from the image carrier and the paper from the image carrier are reduced. It is possible to suppress a failure due to a decrease in the electric field at the nip NP, such as a decrease in toner transfer performance to the toner.

  In the control of FIG. 3, the control unit 200 increases the absolute values of the separation current and the transfer current as the absolute value of the potential on the photosensitive drum 1 detected by the potential detection unit 190 is smaller. To decide. When the resistance value of the transfer belt 6 is greatly reduced by controlling in this way, the absolute value of the transfer current is determined to be larger, so that a failure due to a decrease in the electric field in the nip portion NP can be more reliably performed. It becomes possible to suppress.

  More specifically, the control unit 200 determines the current value of the transfer current as a value that is inversely proportional to the potential on the photosensitive drum 1 detected by the potential detection unit 190. By controlling in this way, it is possible to more reliably suppress a failure due to a decrease in the electric field in the nip portion NP (see the experimental result of Experiment 1 described later (FIG. 7)).

[Method for Deriving Equation 1 and Equation 2]
Equations 1 and 2 were derived based on the following considerations.

Previously, a method of deriving the equation 1 for calculating the current value I _a of the transfer current after leaving the image forming apparatus 100 in a high-humidity environment.

First, in the transfer process of the image forming operation, in order to form a transfer electric field having a sufficient strength for transferring the toner on the photosensitive drum 1 to a sheet, a current I _a0 is transferred from the transfer belt 6 to the photosensitive drum 1. Suppose that a transfer current of a value needs to flow.

However, when the image forming apparatus 100 is left in a high humidity environment for a long time, when the resistance value of the transfer belt 6 decreases and a transfer current flows through the nip portion NP, A part leaks along the moving direction of the transfer belt 6. For this reason, even when a current having a current value I _a0 is passed through the nip NP as a transfer current, the current value of the current that actually flows from the transfer belt 6 to the photosensitive drum 1 and contributes to the formation of the transfer electric field is _{Suppose that} only a certain percentage of p × I _a0 (0 <p <1). In this case, since the current value of the current flowing from the transfer belt 6 to the photosensitive drum 1 is smaller than I _a0 , a sufficiently strong transfer electric field cannot be formed.

Therefore, the I _a by increasing the current value of the transfer current to be input by the ratio p minute leaks out of the current value, if it is possible to obtain the I _a such that p × I _{a =} I _a0, transfer The current value that flows from the belt 6 to the photosensitive drum 1 and contributes to the formation of the transfer electric field becomes I _a0 , and a transfer electric field having a sufficient strength can be formed. Therefore, the above-described ratio p is calculated by the method described below.

First, it is assumed that the electric potential on the photosensitive drum 1 detected by the electric potential detector 190 is V ₀ after a charging current having a current value I ₀ is passed through the nip portion NP during normal operation. On the other hand, when the image forming apparatus 100 is left in a high humidity environment for a long time and then a charging current having a current value I ₀ is passed through the nip portion NP, the photosensitive drum 1 detected by the potential detection unit 190 is detected. Is assumed to be V ₁ . At this time, a part of the current value of the charging current leaks along the moving direction of the transfer belt 6, so that the current value of the current flowing from the photosensitive drum 1 to the transfer belt 6 is p × I ₀ .

Here, since the potential on the photosensitive drum 1 after flowing the charging current is proportional to the current value of the current flowing from the photosensitive drum 1 to the transfer belt 6, V ₀ : V ₁ = I ₀ : (p XI ₀ ) and can be calculated as p = V ₁ / V ₀ .

Therefore, _{I a} such, such that p × _I a _{= I a0} as described _above, can be obtained as _{I a = I a0 / p =} I a0 × (V 0 / V 1), Equation 1 is derived It was done.

  Although the transfer current has been described above, the same consideration can be made for the separation current, and Equation 2 is derived.

[Modification]
In the control of FIG. 3, the current values of the separation current and the transfer current are determined, but only one of the current values may be determined.

  In the above embodiment, the example in which the photosensitive drum 1 functions as the image carrier of the present invention has been described, but the present invention is not limited to this. For example, in the case of the intermediate belt transfer type (indirect transfer type) image forming apparatus 100, the intermediate transfer belt 6 may function as an image carrier. The present invention can be applied to both the monochrome image forming apparatus 100 that forms a monochromatic image and the color image forming apparatus 100 that forms a color image.

  In the above description, in step S301, the voltage application unit 180 is controlled so that a current in the direction opposite to the transfer current flows as the charging current, and the photosensitive drum 1 is charged to a negative polarity. On the contrary, the control unit 200 may control the voltage application unit 180 so as to flow a current in the same direction as the transfer current, and charge the photosensitive drum 1 to the positive polarity. Even in such a case, the potential on the photosensitive drum 1 is a value reflecting a change in the resistance value of the transfer belt 6 as in the case where the photosensitive drum 1 is negatively charged. It is. Therefore, by detecting the potential on the photosensitive drum 1 by the potential detection unit 190 in S302, an appropriate transfer current value can be calculated.

  However, in the present embodiment, it is more preferable to charge the photosensitive drum 1 to a negative polarity by controlling the voltage application unit 180 so that a current in the direction opposite to the transfer current flows as the charging current. The reason will be described below.

  The photosensitive drum 1 used in the present embodiment is negatively charged by the image forming unit 140, and a part of the negative charge is removed by the image writing unit 135 to form an electrostatic latent image. That is, the photosensitive drum 1 used in the present embodiment has a characteristic that negative charges are easily removed when exposed. In general, a photoconductor having such characteristics has a characteristic that, when charged to a positive polarity, it is difficult to remove charges even when exposed.

  Therefore, when the control unit 200 controls the charging current to flow in the same direction as the transfer current and charges the photosensitive drum 1 to the positive polarity, after the potential is detected by the potential detection unit 190, some method is used. Therefore, it is necessary to remove the positive charge on the photosensitive drum 1. If the charges cannot be sufficiently removed, there is a possibility that the image forming operation may be adversely affected.

  On the other hand, as described with reference to FIG. 3, when the control unit 200 controls the charging current to flow in the direction opposite to the transfer current and charges the photosensitive drum 1 in the negative polarity, the potential detection unit After the electric potential is detected by 190, the image writing unit 135 can expose and easily remove the electric charge. Alternatively, the entire photosensitive drum 1 may be controlled to be uniformly charged during the image forming operation without removing the charge charged by the charging current.

  Further, in the present embodiment, the photosensitive drum 1 that carries negatively charged toner and carries toner during the image forming operation is described. However, the photosensitive drum 1 that carries positively charged toner and carries toner during the image forming operation is described. You may use. In this case as well, the relationship that it is more preferable to control the voltage application unit 180 so that a current in the direction opposite to the transfer current flows as the charging current is the same.

  In addition, in the embodiment described above, the current values of the separation current and the transfer current determined by the control in FIG. 3 are the same even when the paper type and basis weight of the paper passing through the nip portion NP are different. Although constant, the current values of the separation current and the transfer current may be determined according to the paper type (for example, plain paper, high-quality paper, rough paper, etc.) and the basis weight. For example, the current values calculated by Equation 1 and Equation 2 may be multiplied by a predetermined coefficient corresponding to the paper type and basis weight of the paper to determine the current values of the separation current and the transfer current. By controlling in this way, it is possible to determine the current values of the separation current and the transfer current according to the characteristics of the paper, and it is possible to more reliably suppress a failure due to a decrease in the electric field in the nip portion NP.

  Furthermore, the current values of the separation current and the transfer current may be determined according to the use history of the transfer belt 6 (for example, the total number of prints in the image forming apparatus). For example, the current values calculated by Equation 1 and Equation 2 may be multiplied by a predetermined coefficient corresponding to the usage history of the transfer belt 6 to determine the current values of the separation current and the transfer current. By controlling in this way, even if the resistance value varies depending on the usage history of the transfer belt 6, the current values of the separation current and the transfer current can be determined appropriately, and at the nip portion NP. It becomes possible to more reliably suppress a failure due to a decrease in the electric field.

  FIG. 4 is a flowchart for explaining a control method for determining the current value of the separation current and the transfer current according to the paper type and basis weight of the paper as described above and the use history of the transfer belt 6. Since S401 and S402 are the same control as S301 and S302 of FIG. 3, description thereof is omitted. In step S <b> 403, the control unit 200 obtains a coefficient stored in the storage unit 172 according to the paper type and basis weight of the paper. Similarly, in S <b> 404, the control unit 200 acquires a coefficient corresponding to the use history of the transfer belt 6 stored in the storage unit 172. In S <b> 405, the control unit 200 determines a value obtained by multiplying the current value calculated using Expression 1 and Expression 2 by the coefficient acquired in S <b> 403 and S <b> 404 as the current value of the separation current and the transfer current.

[Another Embodiment 1 of Image Forming Apparatus 100]
In the control method described with reference to FIG. 3, the control unit 200 passes the charging current to the nip portion NP only once, detects the potential on the photosensitive drum 1, and uses Equation 1 to Control is performed so as to determine the current value of the transfer current.

In the present embodiment, the electric current value I ₀ of the charging current is changed according to the detection result of the electric potential on the photosensitive drum 1 by the electric potential detector 190 until the electric potential on the photosensitive drum 1 coincides with the target electric potential. Then, the control of passing the charging current and detecting the potential on the photosensitive drum 1 is repeated. Hereinafter, a detailed control method in the present embodiment will be described with reference to FIG.

  FIG. 5 is a flowchart for explaining a control method for determining the current values of the separation current and the transfer current in this embodiment.

  Since S501 and S502 are the same control as S301 and S302 of FIG. 3, description thereof is omitted.

In S503, the control unit 200, the potential V ₁ of the on the photosensitive drum 1 detected by the potential detection unit 190 determines whether or not equal to the target potential V _G. Here, the target potential V _G is a target value of the potential V ₁ on the photosensitive drum 1 after flowing the charging current to the nip portion NP. That is, the potential on the photosensitive drum 1 after passing a charging current to the nip portion NP calculates the current value of the separation current and transfer current based on the current value I _G of charging current such that V _G the potential at which the it is possible to form a field of appropriate strength to the nip portion NP in the image forming operation, the target potential V _G. Here, the potential V ₀ on the photosensitive drum 1 after a charging current similar to S301 is passed through the nip portion NP in the normal state is set as a target potential.

Then, in this embodiment, until the potential V ₁ of the on the photosensitive drum 1 becomes the target potential V _G, while changing the current value I ₀ of the charging current, flowing charging current (S501), the photosensitive drum 1 by repeating the control that detects the potential of the upper (S502), it calculates the current value I _G of a suitable charging current.

Further, _{V 1} is consistent with _{V G;} A (S503 Yes), it is not always necessary to exactly _{V 1} is limited to the case that is the same value as the _{V G.} For example, if a certain allowable range is provided for V _G and V ₁ is within the allowable range, it may be determined that V ₁ matches V _G.

Or; (No S503), the control unit 200, the absolute value of _{V 1} is the absolute value greater than the _{V G} in S503, the control unit 200, if _{the V 1} is determined not identical to the _{V G} It is determined whether or not (S504).

Control unit 200, if the absolute value of _{V 1} is judged to be smaller than the absolute value of _{V G} (S504; Yes), the control unit 200, the absolute value I of the charging current supplied to the nip portion NP ₀ is changed to a larger value (for example, 5 μA is increased) (S505), and a charging current is again passed through the nip portion NP (S501).

The significance of S505 is as follows. The absolute value of V ₁ is the absolute value is smaller than the V _G; case of determining the current value of (S504 Yes), on the basis of V ₁ separate current and transfer current strength appropriate to the nip portion NP in the image forming operation An electric field weaker than that is generated. For this reason, problems such as a decrease in the separation performance of the sheet from the photosensitive drum 1 and a decrease in the transfer performance of the toner from the photosensitive drum 1 to the sheet occur. Therefore, when the flow charge current again nip NP, so that the potential V ₁ of the on the photosensitive drum 1 approaches the target potential V _G, the absolute value I ₀ of the charging current supplied to the nip portion NP to larger values Change (S505). By controlling in this way, it is expected that the absolute value of the potential V ₁ on the photosensitive member detected again by the potential detection unit 190 will be a larger value.

On the other hand, in S504, if the control unit 200, the absolute value of _{V 1} is judged to be larger than the absolute value of _{V G} (S504; No), the control unit 200, the absolute value I of the charging current ₀ is set to a smaller value (for example, 5 μA is reduced) (S506), and a charging current is again passed through the nip NP (S501).

The significance of S506 is as follows. The absolute value of V ₁ is the absolute value greater than the V _G; case of determining the current value of (S504 No), on the basis of V ₁ separate current and transfer current strength appropriate to the nip portion NP in the image forming operation An electric field stronger than that is generated. When an excessive electric field is generated in the nip portion NP, there arises a problem that the quality of the image formed on the paper is deteriorated due to the reason that the toner is charged with a polarity opposite to that in the normal state. Therefore, when the flow charge current again nip NP, so that the potential V ₁ of the on the photosensitive drum 1 approaches the target potential V _G, the absolute value I ₀ of the charging current supplied to the nip portion NP to a smaller value Change (S506). By this control, the absolute value of the potential V ₁ of the on the photosensitive member to be detected again by the potential detection unit 190 is expected to be a smaller value.

As described above, _{V 1} until it is determined to be the same as the _{V G} in S503, the control unit 200 repeatedly executes the control of S501～S506.

Then, in S503, the control unit 200, if _{the V 1} is determined to be identical to _{V G} (S503; Yes), on the basis of the current value _{I 0} of the charging current supplied to the nip in S501, the separation current and Determine the current value of the transfer current. At this time, various methods for calculating the current values of the separation current and the transfer current based on the current value of the charging current are conceivable. Here, as an example, the potential V ₁ on the photosensitive drum ₁ is set to the target potential. the current value I _G of the charging current when a V _G, the current value of a predetermined coefficient k _a and the value obtained by multiplying the k _a separate current I _a and the transfer current I _b. That is, the current values of the separation current and the transfer current are calculated by the following formulas 3 and 4.
Formula 3: I _a = k _a × I _G
Formula 4: I _b = k _a × I _G

When the current values of the separation current and the transfer current are calculated in S507, the control in FIG. 5 ends. Thereafter, the image forming operation is started, the control unit 200, as separate currents in separating the leading edge of the sheet from the photosensitive drum 1 becomes the current value I _a separate current determined in the control of FIG. 5 The voltage application unit 180 is controlled. The control unit 200, as the transfer current at the time of transferring the toner image on the photosensitive drum 1 to the paper, the current value I _b of the transfer current determined in the control of FIG. 5, the voltage application unit 180 To control.

  As described above, in this embodiment, the charging current is supplied to the photosensitive drum 1 until the electric potential on the photosensitive drum 1 after supplying the charging current to the nip portion NP reaches a predetermined target potential. The control for detecting the potential is repeatedly executed. By controlling in this way, even when the image forming apparatus 100 is left in a high humidity environment for a long time and the resistance value of the transfer belt 6 decreases, A separation current and a transfer current for forming an electric field having an appropriate strength can be calculated more reliably. In other words, the quality of the image formed on the paper is reduced while more reliably suppressing problems such as a reduction in the separation performance of the paper from the image carrier and a reduction in the transfer performance of the toner from the image carrier to the paper. The occurrence of problems can be suppressed.

[Another Embodiment 2 of Image Forming Apparatus 100]
In the embodiment described above, the timing for executing the control for determining the current values of the separation current and the transfer current is before the image forming operation is started after the image forming apparatus 100 is turned on. In this embodiment, in addition to starting the image forming operation, control for determining the current values of the separation current and the transfer current is executed every time an image is formed on a predetermined number of sheets.

FIG. 6 is a flowchart for explaining a control method for determining the current values of the separation current and the transfer current in this embodiment. Although not shown in FIG. 6, before the image forming operation is started, the control unit 200 executes the control shown in FIG. 3 and an appropriate separation current corresponding to the decrease in the resistance value of the transfer belt 6. and it calculates the current value I _a and I _b of the transfer current.

When a job for forming an image on a sheet is input, the control unit 200 starts an image forming operation. In the image forming operation, the control unit 200 controls each unit such as the image writing unit 135, the image forming unit 140, and the fixing unit 160 to charge, expose, develop, transfer, clean, and fix processes. , Etc. are executed to form an image on the sheet (S601). In the transfer process, by using the current value I _a and I _b of suitable separation current and the transfer current, which are calculated by the control unit 200, separates the leading end of the sheet from the photosensitive drum 1, the toner on the photosensitive drum 1 Control to transfer the image onto the paper.

  When the image forming operation is performed on the sheet, the control unit 200 determines whether or not the job is completed (S602). If the control unit 200 determines that the job has been completed (S602; Yes), the image forming operation ends.

  On the other hand, when the control unit 200 determines that the job is not completed (S602; No), the control unit 200 further determines whether an image has been formed on a predetermined number of sheets (see FIG. S603). Specifically, a counter (not shown) counts the number of sheets on which the image forming operation has been performed, and the control unit 200 refers to the count stored in the counter to determine a predetermined number of sheets (for example, 10). It is determined whether an image is formed on the sheet.

  When the control unit 200 determines that an image is not formed on a predetermined number of sheets (S603; No), the process returns to S601 to perform control to form an image on the next sheet again.

  On the other hand, if it is determined that an image has been formed on a predetermined number of sheets (S603; Yes), the control unit 200 executes the control of S604 to S606 to determine the current values of the separation current and the transfer current. Execute control to The control method in S604 to S606 is the same control method as S301 to S303 in FIG.

  When the current values of the separation current and the transfer current are calculated in S606, control is performed to form an image on the paper again using the current values (S601).

  As described above, in this embodiment, after the image carried on the image carrier is transferred to a predetermined number of sheets by the voltage application unit 180, the current values of the separation current and the transfer current are determined. Execute control. That is, the control unit 200 controls the voltage application unit 180 so that a charging current (third current) flows through the nip NP, and detects a potential on the image carrier charged by the charging current. 190 is controlled, and the current values of the charging current (first current) and the transfer current (second current) for the subsequent sheets are determined according to the detection result by the potential detection unit 190.

  In addition, the absolute value of the potential on the image carrier detected by the potential detection unit 190 after the image carried on the image carrier is transferred to a predetermined number of sheets is determined for the predetermined number of sheets. If the absolute value of the potential on the image carrier detected by the potential detector 190 before transferring the image carried on the body is larger than the charging current (first current) for the subsequent paper or The absolute value of the transfer current (second current) is determined to a smaller value.

  By controlling in this way, even when the resistance value of the transfer belt 6 fluctuates during execution of the image forming operation, the photosensitive drum 1 on the photosensitive drum 1 after the charging current is passed through the nip portion NP. From the potential change, it is possible to estimate the change in the resistance value of the transfer belt 6 and calculate the current value of the separation current and the transfer current for forming an electric field having an appropriate strength in the nip portion NP ( Refer to the experimental results of Experimental Examples 1 to 4 in Experiment 2 described later (FIG. 8)).

  In particular, the current value of the separation current and the transfer current is appropriately calculated even when the moisture adsorbed on the transfer belt 6 evaporates and the resistance value of the transfer belt 6 increases during the image forming operation. Therefore, problems such as deterioration in the quality of the image formed on the sheet due to the generation of an excessive electric field at the nip portion NP can be suppressed (experimental results of Experimental Example 4 and Comparative Example in Experiment 2). See (Figure 8)).

  In the above description, the control for determining the current values of the separation current and the transfer current is executed every time an image is formed on a fixed number of sheets (10 sheets) as the predetermined number of sheets. The predetermined number need not be a fixed number, but may be a number that changes.

  For example, the amount of moisture adsorbed on the transfer belt 6 is relatively large at a time shortly after the start of the image forming operation. Therefore, when the moisture is evaporated by forming an image on the paper and the resistance value of the transfer belt 6 increases, the resistance value for each paper sheet increases greatly. For this reason, it is desirable to execute the control for determining the current values of the separation current and the transfer current relatively frequently (for example, every five sheets) at a time shortly after the start of the image forming operation. On the other hand, at a time when a long time has elapsed since the start of the image forming operation, the moisture adsorbed on the transfer belt 6 has already evaporated to some extent, so that the transfer belt 6 every time an image is formed on one sheet of paper. The increase in resistance value is small. For this reason, at the time when a long period has elapsed since the start of the image forming operation, there is no problem even if the frequency of executing the control for determining the current values of the separation current and the transfer current is relatively small (for example, 20 sheets). every). Further, in such a case, by reducing the frequency, it is possible to omit the time for executing the control for determining the current values of the separation current and the transfer current, thereby improving the productivity of the image forming operation. Can do.

  As described above, the predetermined number of sheets is set to 5 at the time immediately after the start of the image forming operation, and the predetermined number of sheets is set to 20 at the time when a long time has elapsed since the start of the image forming operation. As described above, the predetermined number of sheets may be decreased as time elapses after the image forming operation is started.

〔Experimental result〕
Finally, the results of Experiment 1 and Experiment 2 performed by the present inventors for confirming the effectiveness of the present invention will be described. In both experiments, an image forming operation is performed on 20 sheets using the current value of the separation current and the transfer current calculated based on the potential detected by the potential detection unit 190, and the separability at that time. And transferability was evaluated. The releasability was evaluated by the presence or absence of occurrence of a photoreceptor separation jam. The transferability was evaluated by evaluating the density of a black solid image.

As a high humidity environment, the temperature was 30 ° C. and the relative humidity was 80% (the absolute humidity was about 2400 [× 10 ⁻² g / m ³ ]). As the use history of the transfer belt 6, a history of performing image forming operation on 2 million sheets was used. Further, as the paper, a paper (thin paper) having a basis weight of 40 g / m ² and poor separation was used.

(Experiment 1)
In Experiment 1, before the first printing (image forming operation) after turning on the power of the apparatus, the control of FIG. 3 is executed, and the current values of the separation current and the transfer current calculated from the relationship of Formula 1 are obtained. Using this, an image forming operation was performed, and separation property and transfer property were evaluated.

  FIG. 7 is a table showing the conditions and results of Experiment 1. Experimental conditions will be described. The experimental result described as “normal time” is an experimental result when the current values of the separation current and the transfer current at normal time are used without being left in a high humidity environment. In Experimental Example 1 and Experimental Example 2, in the image forming apparatus 100 left in a high-humidity environment, the control in FIG. 3 is executed before the first printing (image forming operation) is performed after the apparatus is turned on. These are experimental results using the current values of the separation current and the transfer current calculated from the relationship of Equation 1. In Experimental Example 1 and Experimental Example 2, the time of standing in a high humidity environment is different, with Experimental Example 1 being 7 hours and Experimental Example 2 being 20 hours, respectively. On the other hand, in Comparative Example 1 and Comparative Example 2, in the image forming apparatus 100 left in a high humidity environment, the experimental results using the current values of the separation current and the transfer current in the normal state without executing the control of FIG. It is.

The experimental results will be described. Under normal experimental conditions, both separation and transfer were good. This is because the resistance value of the transfer belt 6 does not decrease because the image forming apparatus 100 is not left in a high humidity environment. The normal detection potential was -120V. The predetermined potential is a potential used as a target potential V _G in the control of FIG.

  On the other hand, in both Comparative Example 1 and Comparative Example 2, both the separability and transferability deteriorated. Since the resistance value of the transfer belt 6 is lowered because the image forming apparatus 100 is left in a high humidity environment, when the current value of the separation current and the transfer current at the normal time is used, the nip portion NP is sufficiently strong. This is probably because the electric field was not formed.

  In Experimental Example 1 and Experimental Example 2, both separation and transferability were good. By using the current values of the separation current and the transfer current calculated from the relationship of Formula 1, even when the resistance value of the transfer belt 6 decreases and the current leaks, the nip portion NP has a sufficient strength. This is probably because an electric field could be formed.

A method for calculating the current values of the separation current and the transfer current from the relationship of Formula 1 by executing the control of FIG. 3 will be specifically described below. In Experimental Example 1, the potential V ₁ on the photosensitive drum 1 detected by the potential detection unit 190 after flowing the charging current was −105V. Since the potential V ₀ on the photosensitive drum 1 after flowing the charging current in the normal time is −120 V, the current values I _a0 and I _b0 of the separation current and the transfer current in the normal time are +60 μA and +100 μA, respectively. From Formula 1 and Formula 2, the current values I _a and I _b of the separation current and the transfer current to be calculated are
I _a = + 60 × (−120) / (− 105) = + 69 μA
I _b = + 100 × (−120) / (− 105) = + 114 μA
It becomes. Similarly, in Experimental Example 2,
I _a = + 60 × (−120) / (− 80) = + 90 μA
I _b = + 100 × (−120) / (− 80) = + 150 μA
Is calculated.

(Experiment 2)
In Experiment 2, first, the control of FIG. 3 is executed before the image forming operation is started to calculate appropriate current values of the separation current and the transfer current, and then 10 sheets after the image forming operation is started. Each time printing is performed, the control of FIG. 6 is executed between the sheets to calculate the current values of the separation current and the transfer current. An image forming operation was performed using the calculated current values of the separation current and the transfer current, and the separation property and the transfer property were evaluated. In any experiment, after the image forming apparatus 100 was left in a high humidity environment for 20 hours, the control of FIG. 3 was performed to start the image forming operation.

  FIG. 8 is a table showing the conditions and results of Experiment 2. Experimental conditions will be described. The experimental result described as “at the start of printing” is the experimental result immediately after the start of the image forming operation.

  Experimental examples 1 to 4 are experimental results using current values of separation current and transfer current calculated by executing the control of FIG. 5 between paper sheets. In Experimental Examples 1 to 4, the number of prints is different, 20 sheets, 50 sheets, 100 sheets, and 300 sheets, respectively. On the other hand, the comparative example is an experimental result in which the current values of the separation current and the transfer current calculated by executing the control of FIG. 3 are continuously used without executing the control of FIG.

  The experimental results will be described. Under the experimental conditions at the start of printing, both separation and transfer were good. It is considered that an electric field having a sufficient strength could be formed at the nip portion NP by using the current values of the separation current and the transfer current calculated from the relationship of Formula 1.

  In Experimental Examples 1 to 4, both the separation property and the transfer property were good. This is considered for the reason explained below. In Experimental Examples 1 to 4, the absolute value of the detected potential increases as the number of printed sheets increases. From this, it is estimated that the moisture adsorbed on the transfer belt 6 evaporates and the resistance value of the transfer belt 6 increases during the image forming operation. In order to cope with the increase in the resistance value of the transfer belt 6, the image forming operation is performed using the separation current and the current value of the transfer current calculated by executing the control of FIG. Thereby, it is considered that an electric field having an appropriate strength can be formed at the nip portion NP, and both the separation property and the transfer property can be kept good.

  On the other hand, in the comparative example, the separability deteriorated and the transferability also deteriorated (transfer repelling occurred). Although the resistance value of the transfer belt 6 has increased during the image forming operation, the current values of the separation current and the transfer current calculated by the control of FIG. This is probably because an excessive electric field was generated in the nip.

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 6 Transfer belt 63 Backup roller 100 Image forming apparatus 140 Image forming part 180 Voltage application part 190 Potential detection part 200 Control part NP Nip part

Claims (8)

An image carrier for carrying an image to be transferred to paper;
A transfer member that forms a nip portion facing the image carrier;
At least the second current among the first current for separating the paper from the image carrier and the second current for transferring the image carried on the image carrier to the paper by applying a voltage to the transfer member. A voltage application section for flowing a current of
A potential detector for detecting a potential on the image carrier;
A control unit for controlling the operation of the image forming apparatus,
The controller is
Controlling the voltage application unit to flow a third current in a direction opposite to the second current to the nip portion;
Controlling the potential detector to detect a potential on the image carrier charged by the third current;
An image forming apparatus, wherein a current value of the first current or the second current is determined according to a detection result by the potential detection unit. The controller is
The absolute value of the first current or the second current is determined to be larger as the absolute value of the potential on the image carrier detected by the potential detector is smaller. The image forming apparatus according to 1. The controller is
3. The current value of the first current or the second current is determined to be a value inversely proportional to the potential on the image carrier detected by the potential detection unit. Image forming apparatus. The controller is
When the potential on the image carrier detected by the potential detector is not a predetermined potential,
In response to the potential on the image carrier detected by the potential detection unit, the voltage application unit is controlled so that the third current flows again to the nip portion,
Controlling the potential detector to detect a potential on the image carrier charged by the third current;
The image forming apparatus according to claim 1, wherein a current value of the first current or the second current is determined according to a detection result by the potential detection unit. The controller is
After transferring the image carried on the image carrier to a predetermined number of sheets by the voltage application unit,
Controlling the voltage application unit to flow the third current through the nip,
Controlling the potential detector to detect a potential on the image carrier charged by the third current;
5. The current value of the first current or the second current for subsequent sheets is determined according to a detection result by the potential detection unit. 6. Image forming apparatus. The controller is
The absolute value of the potential on the image carrier detected by the potential detector after transferring the image carried on the image carrier to a predetermined number of sheets is the image carrier on the predetermined number of sheets. If the absolute value of the potential on the image carrier detected by the potential detector before transferring the image carried thereon is larger than the first current or the second current for subsequent sheets The image forming apparatus according to claim 5, wherein the absolute value of the current is determined to be a smaller value. The controller is
The image forming apparatus according to claim 1, wherein the current value of the first current or the second current is determined according to a paper type or basis weight of the paper. The controller is
The image forming apparatus according to claim 1, wherein the current value of the first current or the second current is determined according to a use history of the transfer member.

Images