JP7250469B2 - image forming device - Google Patents

Description

The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine using an electrophotographic method or an electrostatic recording method.

2. Description of the Related Art Conventionally, in an image forming apparatus using an electrophotographic method or the like, a toner image is electrostatically transferred from an image bearing member such as a photosensitive member or an intermediate transfer member to a recording material such as paper. This transfer is often performed by applying a transfer voltage to a transfer member such as a transfer roller that forms a transfer portion in contact with the image bearing member. If the transfer voltage is too low, insufficient transfer may occur, resulting in "low image density" in which a desired image density cannot be obtained. Also, if the transfer voltage is too high, discharge occurs at the transfer section, and the polarity of the toner charge in the toner image is reversed due to the effects of the discharge, resulting in "blank spots" in which the toner image is partially not transferred. may occur. Therefore, in order to form a high-quality image, it is required to apply an appropriate transfer voltage to the transfer member.

Japanese Patent Application Laid-Open No. 2002-200000 discloses the following transfer voltage control in a configuration in which transfer is performed by applying a transfer voltage to a transfer member under constant voltage control. Immediately before the start of continuous image formation, a predetermined voltage is applied to the transfer portion where there is no recording material, the current value is detected, and the voltage value at which a predetermined target current is obtained is obtained. Then, a recording material allotted voltage corresponding to the type of recording material is added to this voltage value to set a transfer voltage value to be applied under constant voltage control during transfer. With such control, a transfer voltage corresponding to a desired target current can be applied under constant voltage control regardless of fluctuations in the electric resistance value of the transfer portion such as the transfer member and fluctuations in the electric resistance value of the recording material. .

Here, the types of recording materials include, for example, types based on the difference in surface smoothness of the recording materials, such as high-quality paper and coated paper, and types based on differences in the thickness of the recording materials, such as thin paper and thick paper. . The recording material allotted voltage can be obtained in advance according to the type of recording material, for example. However, because there are so many types of recording materials on the market, and even if the environment (temperature and humidity) is the same, the electrical resistance of recording materials fluctuates depending on the time they are left in the environment. It is often difficult to determine in advance with high accuracy. If the transfer voltage is not an appropriate value including the variation in the electric resistance of the recording material, image defects such as low image density and white spots may occur as described above.

In order to solve such problems, Japanese Patent Application Laid-Open Nos. 2003-300001 and 2004-200020 disclose the upper limit value of the current supplied to the transfer portion in a configuration in which the transfer voltage is applied under constant voltage control while the recording material is passing through the transfer portion. and a lower limit have been proposed. With such control, the current supplied to the transfer section while the recording material is passing through the transfer section can be set to a value within a predetermined range. can be suppressed. In Patent Literature 2, the upper limit is obtained based on environmental information. In Japanese Patent Application Laid-Open No. 2002-200011, the upper limit and lower limit are determined according to the front and back sides of the recording material, the type of the recording material, and the size of the recording material, in addition to the environment.

Japanese Patent Application Laid-Open No. 2004-117920 Japanese Patent No. 4161005 JP 2008-275946 A

However, the current that flows in the transfer portion when the recording material is passing through the transfer portion includes a "paper passing portion current (passing portion current)" and a "non-sheet passing portion current (non-passing portion current)". There is The paper-passing portion current is a current that flows in a region (“paper-passing portion (passing region)”) of the transfer portion through which the recording material passes in a direction substantially perpendicular to the conveying direction of the recording material. The non-paper-passing portion current is a current that flows in a region (“non-paper-passing portion (non-passing region)”) of the transfer portion in a direction substantially orthogonal to the conveying direction of the recording material. The non-paper-passing portion occurs because the transfer member, such as the transfer roller, stably conveys and transfers the toner image to recording media of various sizes. This is because it is made larger than the maximum width of the recording material guaranteed by .

The current that can be detected when the recording material is passing through the transfer portion is the sum of the paper-passing portion current and the non-sheet-passing portion current. In order to suppress the image defects as described above, it is important that the paper-passing portion current has a value within an appropriate range, but it is not possible to detect only the paper-passing portion current. Moreover, the electrical resistance of the transfer member forming the non-sheet-passing portion fluctuates under various conditions. These various conditions include variations in products, environment (temperature/humidity), temperature/humidity absorption of members, cumulative usage time (operation status and repeated usage status of the image forming apparatus), and the like. Therefore, even if the upper limit and lower limit of the transfer current (“transfer current range”) are determined in advance for each size of the recording material, the appropriate transfer current range changes due to fluctuations in the electrical resistance of the transfer member. The methods described in Patent Documents 2 and 3 do not deal with fluctuations in the electrical resistance of the transfer member that forms the non-sheet passing portion.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an image forming apparatus capable of setting a permissible range of current flowing through a transfer member in accordance with variations in electrical resistance of the transfer member.

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention comprises an image carrier carrying a toner image, an intermediate transfer belt onto which the toner image is transferred from the image carrier, and a voltage applied from the intermediate transfer belt to a recording material at a transfer portion. a transfer member for transferring a toner image, a power source for applying a voltage to the transfer member, a current detection section for detecting current flowing through the transfer member , When the detection result detected by the current detection unit is within a predetermined range determined based on the type of recording material, constant voltage control is performed so that the voltage applied to the transfer member becomes the target voltage. and a control unit, wherein , when the detection result is out of the predetermined range while the recording material is passing through the transfer unit, the control unit determines that the detection result is within the predetermined range. and the constant voltage control is executed with the adjusted target voltage. or the voltage applied to the transfer member when the current is supplied to the transfer member with no recording material in the transfer portion . This image forming apparatus is characterized by setting a value .

According to another aspect of the present invention, an image carrier carrying a toner image; an intermediate transfer belt onto which the toner image is transferred from the image carrier; a transfer member for transferring a toner image onto the transfer member; a power supply for applying a voltage to the transfer member; a current detection section for detecting current flowing through the transfer member; When the detection result detected by the current detection unit is within a predetermined range determined based on the type of recording material, constant voltage control is performed so that the voltage applied to the transfer member becomes the target voltage. and a control unit, wherein, when the detection result is out of the predetermined range while the recording material is passing through the transfer unit, the control unit determines that the detection result is within the predetermined range. and the constant voltage control is executed with the adjusted target voltage. the detection result of the current detection unit is detected based on the current flowing through the transfer member at the beginning or the voltage applied to the transfer member when the current is supplied to the transfer member when there is no recording material in the transfer unit. An image forming apparatus characterized by correcting is provided.

According to another aspect of the present invention, an image carrier carrying a toner image; an intermediate transfer belt onto which the toner image is transferred from the image carrier; a transfer member for transferring a toner image onto the transfer member; a power supply for applying a voltage to the transfer member; a current detection section for detecting current flowing through the transfer member; When the detection result detected by the current detection unit is within a predetermined range determined based on the type of recording material, constant voltage control is performed so that the voltage applied to the transfer member becomes the target voltage. and a control unit, wherein, when the detection result is out of the predetermined range while the recording material is passing through the transfer unit, the control unit determines that the detection result is within the predetermined range. and the constant voltage control is executed with the adjusted target voltage. or the voltage applied to the transfer member when the current is supplied to the transfer member in a state where there is no recording material in the transfer portion. There is provided an image forming apparatus characterized by:
According to another aspect of the present invention, an image carrier carrying a toner image; an intermediate transfer belt onto which the toner image is transferred from the image carrier; a transfer member for transferring a toner image onto the transfer member; a power supply for applying a voltage to the transfer member; a current detection section for detecting current flowing through the transfer member; When the detection result detected by the current detection unit is within a predetermined range determined based on the type of recording material, constant voltage control is performed so that the voltage applied to the transfer member becomes the target voltage. and a control unit, wherein, when the detection result is out of the predetermined range while the recording material is passing through the transfer unit, the control unit determines that the detection result is within the predetermined range. and the constant voltage control is executed with the adjusted target voltage. or the voltage applied to the transfer member when the current is supplied to the transfer member with no recording material in the transfer portion. There is provided an image forming apparatus characterized by:

According to the present invention, it is possible to set the allowable range of the current flowing through the transfer member according to the variation of the electrical resistance of the transfer member.

1 is a schematic configuration diagram of an image forming apparatus; FIG. FIG. 3 is a schematic diagram of a configuration relating to secondary transfer; FIG. 2 is a schematic block diagram showing a control mode of main parts of the image forming apparatus; FIG. 4 is a flow chart of control in Example 1; 5 is a graph showing an example of the relationship between the voltage and current of the secondary transfer portion; FIG. FIG. 3 is a schematic diagram showing an example of table data of recording material allotted voltages; FIG. 10 is a schematic diagram showing an example of table data of sheet passing portion current ranges; FIG. 10 is a flow chart of control in Example 2; 4 is a schematic diagram showing an example of table data of secondary transfer current target values; FIG. FIG. 3 is a schematic diagram for explaining a paper-passing portion current and a non-sheet-passing portion current; It is a table for explaining a subject. 10 is a table for explaining problems in Example 3. FIG. FIG. 5 is a diagram for explaining the relationship between recording material shared voltage and penetration. FIG. 10 is a flowchart of control in Example 3; FIG. 10 is a schematic diagram for explaining a method of deriving a recording material allotted voltage; FIG. 10 is a schematic diagram showing an example of upper limit table data of recording material allotted voltages; FIG. 11 is a flowchart of control in Example 5; FIG. 10 is a schematic diagram showing an example of table data of correction coefficients of non-sheet-passing portion current; 5 is a graph for explaining changes in the secondary transfer current range depending on the thickness of the recording material; FIG. FIG. 10 is a schematic diagram showing another example of table data of correction coefficients of non-sheet-passing portion current; FIG. 11 is a flow chart of control in Embodiment 7; FIG. 11 is a flow chart of control in Example 8; It is a schematic diagram for explaining a problem.

Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

[Example 1]
1. Overall Configuration and Operation of Image Forming Apparatus FIG. 1 is a schematic configuration diagram of an image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment has the functions of a tandem-type multifunction machine (copier, printer, facsimile machine) that employs an intermediate transfer system and is capable of forming a full-color image using an electrophotographic system. ).

Image forming apparatus 100 includes, as a plurality of image forming units (stations), first, second, third, and fourth image forming units SY, SM, which respectively form yellow, magenta, cyan, and black images. It has SC and SK. For elements having the same or corresponding functions or configurations in the image forming units SY, SM, SC, and SK, the suffixes Y, M, C, and K are omitted from the symbols indicating that they are elements for one of the colors. may be described in a comprehensive manner. In this embodiment, the image forming section S includes a photosensitive drum 1, a charging roller 2, an exposure device 3, a developing device 4, a primary transfer roller 5, and a drum cleaning device 6, which will be described later.

The image forming section S has a photosensitive drum 1 that is a rotatable drum-shaped (cylindrical) photosensitive member (electrophotographic photosensitive member) as a first image carrier that carries a toner image. The photosensitive drum 1 is rotationally driven in the direction of an arrow R1 (counterclockwise) in the figure. The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential with a predetermined polarity (negative polarity in this embodiment) by a charging roller 2 which is a roller-type charging member as charging means. The charged surface of the photosensitive drum 1 is scanned and exposed by an exposure device (laser scanner device) 3 as exposure means based on image information, and an electrostatic image (electrostatic latent image) is formed on the photosensitive drum 1. be.

The electrostatic image formed on the photosensitive drum 1 is developed (visualized) by supplying toner as a developer by a developing device 4 as developing means, and a toner image is formed on the photosensitive drum 1 . In this embodiment, the exposure portion (image portion) on the photosensitive drum 1, which has been uniformly charged and then exposed to light, the absolute value of the potential of which has decreased is charged to the same polarity as the charging polarity of the photosensitive drum 1. Toner adheres (reversal development method). In this embodiment, the normal charge polarity of the toner, which is the charge polarity of the toner during development, is negative. The electrostatic image formed by the exposure device 3 is an aggregate of small dot images, and the density of the toner image formed on the photosensitive drum 1 can be changed by changing the density of the dot image. In this embodiment, the toner image of each color has a maximum density of about 1.5 to 1.7, and the amount of toner applied at the maximum density is about 0.4 to 0.6 mg/cm ² . It's becoming

An intermediate transfer belt 7, which is an intermediate transfer member composed of an endless belt, is arranged as a second image bearing member for carrying a toner image so as to be able to contact the surfaces of the four photosensitive drums 1. ing. The intermediate transfer belt 7 is stretched around a driving roller 71 , a tension roller 72 , and a secondary transfer counter roller 73 as a plurality of stretching rollers. The driving roller 71 transmits driving force to the intermediate transfer belt 7 . A tension roller 72 controls the tension of the intermediate transfer belt 7 to be constant. The secondary transfer counter roller 73 functions as a member (counter electrode) facing the secondary transfer roller 8, which will be described later. The driving roller 71 is driven to rotate, so that the intermediate transfer belt 7 rotates (circulates) in the direction indicated by an arrow R2 (clockwise) at a conveying speed (peripheral speed) of about 300 to 500 mm/sec. The tension roller 72 is applied with a force of a spring as a biasing means to push the intermediate transfer belt 7 from the inner peripheral surface side to the outer peripheral surface side. A tension of about 2 to 5 kg is applied. On the inner circumferential surface side of the intermediate transfer belt 7, primary transfer rollers 5, which are roller-type primary transfer members as primary transfer means, are arranged corresponding to the respective photosensitive drums 1. As shown in FIG. The primary transfer roller 5 is pressed toward the photosensitive drum 1 via the intermediate transfer belt 7 to form a primary transfer portion (primary transfer nip) N1 where the photosensitive drum 1 and the intermediate transfer belt 7 are in contact with each other. . The toner image formed on the photosensitive drum 1 is electrostatically transferred (primary transfer) onto the rotating intermediate transfer belt 7 by the action of the primary transfer roller 5 at the primary transfer portion N1. During the primary transfer process, the primary transfer roller 5 is supplied with a primary transfer voltage (primary transfer bias), which is a DC voltage having a polarity opposite to the normal charge polarity of the toner, from a primary transfer power source (not shown). is applied. For example, when forming a full-color image, yellow, magenta, cyan, and black toner images formed on the photosensitive drums 1 are sequentially transferred onto the intermediate transfer belt 7 so as to be superimposed.

A secondary transfer roller 8 , which is a roller-type secondary transfer member as a secondary transfer means, is arranged at a position facing the secondary transfer facing roller 73 on the outer peripheral surface side of the intermediate transfer belt 7 . The secondary transfer roller 8 is pressed toward the secondary transfer opposing roller 73 via the intermediate transfer belt 7 to form a secondary transfer portion (secondary transfer nip) where the intermediate transfer belt 7 and the secondary transfer roller 8 are in contact. ) to form N2. The toner image formed on the intermediate transfer belt 7 is transferred to the paper ( It is electrostatically transferred (secondary transfer) onto a recording material (sheet, transfer material) P such as paper. During the secondary transfer process, the secondary transfer roller 8 is supplied with a secondary transfer voltage (secondary transfer bias), which is a DC voltage having a polarity opposite to the normal charge polarity of the toner, from a secondary transfer power supply (high voltage power supply circuit) 20 . ) is applied. The recording material P is accommodated in a recording material cassette (not shown) or the like, and is fed one by one from the recording material cassette by a feeding roller (not shown) or the like and sent to the registration rollers 9 . After being temporarily stopped by the registration roller 9, the recording material P is supplied to the secondary transfer portion N2 in synchronization with the toner image on the intermediate transfer belt 7. As shown in FIG.

The recording material P onto which the toner image has been transferred is conveyed to a fixing device 10 as fixing means by a conveying member or the like. The fixing device 10 heats and presses the recording material P bearing the unfixed toner image, thereby fixing (melting or fixing) the toner image on the recording material P. FIG. After that, the recording material P is discharged (output) to the outside of the apparatus main body of the image forming apparatus 100 .

Toner remaining on the surface of the photosensitive drum 1 after the primary transfer process (primary transfer residual toner) is removed from the surface of the photosensitive drum 1 and collected by a drum cleaning device 6 as a photosensitive member cleaning means. Toner remaining on the surface of the intermediate transfer belt 7 after the secondary transfer process (secondary transfer residual toner) and adherents such as paper dust are removed from the intermediate transfer belt 7 by a belt cleaning device 74 as intermediate transfer body cleaning means. Removed from the surface and collected.

Here, in this embodiment, the intermediate transfer belt 7 is an endless belt having a three-layer structure of a resin layer, an elastic layer, and a surface layer from the inner peripheral side to the outer peripheral side. Polyimide, polycarbonate, or the like can be used as a resin material forming the resin layer. The thickness of the resin layer is preferably 70-100 μm. Urethane rubber, chloroprene rubber, or the like can be used as the elastic material forming the elastic layer. The thickness of the elastic layer is preferably 200-250 μm. Further, as the material of the surface layer, it is desirable to use a material that reduces the adhesive force of the toner to the surface of the intermediate transfer belt 7 and facilitates transfer of the toner onto the recording material P at the secondary transfer portion N2. For example, one or more resin materials such as polyurethane, polyester, epoxy resin, etc. can be used. Alternatively, one or more of elastic materials such as elastic materials (elastic rubber, elastomer), butyl rubber and the like can be used. In addition to these materials, one or two or more types of powders or particles such as fluororesin, or one or two or more types of these powders or particles, may be used to reduce surface energy and improve lubricity. having different particle sizes can be dispersed and used. The thickness of the surface layer is preferably 5-10 μm. The intermediate transfer belt 7 is added with a conductive agent for adjusting electrical resistance such as carbon black to adjust electrical resistance, and preferably has a volume resistivity of 1×10 ⁹ to 1×10 ¹⁴ Ω·cm.

Further, in this embodiment, the secondary transfer roller 8 includes a metal core (base material) and an elastic layer formed around the metal core with ion-conducting foamed rubber (NBR rubber). be. In this embodiment, the secondary transfer roller 8 has an outer diameter of 24 mm and a surface roughness Rz of 6.0 to 12.0 (μm). In this embodiment, the electrical resistance of the secondary transfer roller 8 is 1×10 ⁵ to 1×10 ⁷ Ω when measured by applying 2 kV at N/N (23° C., 50% RH). has an Asker-C hardness of about 30 to 40°. In this embodiment, the width of the secondary transfer roller 8 in the longitudinal direction (rotational axis direction) (the length in the direction substantially perpendicular to the conveying direction of the recording material P) is about 310 to 340 mm. In this embodiment, the width in the longitudinal direction of the secondary transfer roller 8 is the maximum width ( maximum width). In this embodiment, since the recording material P is conveyed with reference to the longitudinal center of the secondary transfer roller 8, all of the recording materials P guaranteed to be conveyed by the image forming apparatus 100 are aligned in the longitudinal direction of the secondary transfer roller 8. Pass within the length range. As a result, recording materials P of various sizes can be stably conveyed, and toner images can be stably transferred to recording materials P of various sizes.

FIG. 2 is a schematic diagram of a configuration relating to secondary transfer. The secondary transfer roller 8 forms a secondary transfer portion N2 by coming into contact with the secondary transfer facing roller 73 via the intermediate transfer belt 7 . A secondary transfer power supply 20 having a variable output voltage value is connected to the secondary transfer roller 8 . The secondary transfer counter roller 73 is electrically grounded (connected to the ground). While the recording material P is passing through the secondary transfer portion N2, a secondary transfer voltage, which is a DC voltage having a polarity opposite to the normal charging polarity of the toner, is applied to the secondary transfer roller 8, and the secondary transfer portion The toner image on the intermediate transfer belt 7 is transferred onto the recording material P by supplying the secondary transfer current to N2. In this embodiment, a secondary transfer current of +20 to +80 μA, for example, is supplied to the secondary transfer portion N2 during the secondary transfer.

In this embodiment, the upper limit value and lower limit value (“secondary transfer current range”) of the secondary transfer current when the recording material P is passing through the secondary transfer portion N2 are determined based on various kinds of information. . As will be described later in detail, these various types of information include the following information. First, information about conditions specified by an external device 200 (FIG. 3) such as an operation unit 31 (FIG. 3) provided in the device main body of the image forming device 100 or a personal computer communicably connected to the image forming device 100 is. Also, it is information about the detection result of the environment sensor 32 (FIG. 3). Also, it is information about the electrical resistance of the secondary transfer portion N2 detected before the recording material P reaches the secondary transfer portion N2. While the recording material P is passing through the secondary transfer portion N2, the secondary transfer current flowing through the secondary transfer portion N2 is detected, and the secondary transfer current is determined to be within the range of the secondary transfer current. The secondary transfer voltage output from the secondary transfer power supply 20 under constant voltage control is controlled so that In this embodiment, in order to perform such control, the secondary transfer power supply 20 is provided with current detection means (secondary transfer current) for detecting the current (secondary transfer current) flowing through the secondary transfer portion N2 (secondary transfer power supply 20). A current detection circuit 21 is connected as a detection unit. Further, the secondary transfer power source 20 is connected with a voltage detection circuit 22 as voltage detection means (detection section) for detecting the voltage (transfer voltage) output from the secondary transfer power source 20 . In this embodiment, the secondary transfer power source 20, the current detection circuit 21, and the voltage detection circuit 22 are provided in the same high-voltage board.

2. Control Mode FIG. 3 is a schematic block diagram showing a control mode of the main part of the image forming apparatus 100 of this embodiment. A control unit (control circuit) 50 includes a CPU 51 as control means, which is a central element for arithmetic processing, and a memory (storage medium) such as a RAM 52 and a ROM 53 as storage means. A RAM 52, which is a rewritable memory, stores information input to the control unit 50, detected information, calculation results, etc., and a ROM 53 stores a control program, a data table obtained in advance, and the like. Data can be transferred and read between the CPU 51 and memories such as the RAM 52 and the ROM 53 .

An external device 200 such as an image reading device (not shown) provided in the image forming apparatus 100 or a personal computer is connected to the control unit 50 . An operation unit (operation panel) 31 provided in the image forming apparatus 100 is connected to the control unit 50 . The operation unit 31 includes a display unit that displays various information to an operator such as a user or a service staff under the control of the control unit 50, and an input unit that allows the operator to input various settings related to image formation to the control unit 50. and A secondary transfer power source 20 , a current detection circuit 21 , and a voltage detection circuit 22 are connected to the control section 50 . In this embodiment, the secondary transfer power source 20 applies a secondary transfer voltage, which is a constant-voltage controlled DC voltage, to the secondary transfer roller 8 . An environment sensor 32 is also connected to the control unit 50 . In this embodiment, the environment sensor 32 detects the temperature and humidity inside the housing of the image forming apparatus 100 . Information on temperature and humidity detected by the environment sensor 32 is input to the control unit 50 . The environment sensor 32 is an example of an environment detection unit that detects at least one of temperature and humidity inside or outside the image forming apparatus 100 . Based on image information from the image reading device and the external device 200 and control commands from the operation unit 31 and the external device 200, the control unit 50 comprehensively controls each unit of the image forming device 100 to execute an image forming operation. Let

Here, the image forming apparatus 100 executes a job (printing operation), which is a series of operations for forming and outputting an image on a single or a plurality of recording materials P, which is started by one start instruction (printing instruction). do. A job generally includes an image forming process, a pre-rotation process, an inter-paper process when forming images on a plurality of recording materials P, and a post-rotation process. The image forming process is a period in which electrostatic image formation of an image to be actually formed and output on the recording material P, formation of a toner image, primary transfer, and secondary transfer of the toner image are performed. formation period) refers to this period. More specifically, the timing of image formation differs depending on the position where each step of electrostatic image formation, toner image formation, primary transfer, and secondary transfer of the toner image is performed. The pre-rotation process is a period from when the start instruction is input to when the image formation is actually started, during which preparatory operations are performed before the image forming process. The paper interval process is a period corresponding to the interval between recording materials P when image formation is continuously performed on a plurality of recording materials P (continuous image formation). The post-rotation process is a period during which an arrangement operation (preparation operation) is performed after the image forming process. The non-image forming period (non-image forming period) is a period other than the image forming period, and includes the pre-rotation process, the inter-paper process, the post-rotation process, and further, when the power of the image forming apparatus 100 is turned on or from the sleep state. It includes a pre-multi-rotation step, etc., which is a preparatory operation at the time of return. In this embodiment, control for determining the upper limit value and the lower limit value (“secondary transfer current range”) of the secondary transfer current is executed during non-image formation.

3. Change in Appropriate Secondary Transfer Current Range Due to Fluctuations in Non-Paper-Passing Portion Current Here, the aforementioned problems will be described in more detail. As shown in FIG. 10, the current flowing through the secondary transfer portion N2 when the recording material P is passing through the secondary transfer portion N2 is the current at the paper passing portion (I_paper passing portion) and the current at the non-paper passing portion N2. There is a current (I_non-sheet passing portion). The current that can be detected while the recording material P is passing through the secondary transfer portion N2 is the sum of the paper-passing portion current and the non-sheet-passing portion current. In order to suppress image defects such as low image density and white spots mentioned above, it is important that the current in the paper-passage area is within an appropriate range, but it is not possible to detect the current in the paper-passage area alone. . Therefore, the appropriate upper and lower limits of the secondary transfer current (“secondary transfer current range”) are obtained in advance for each size of the recording material P, and the secondary transfer portion N2 is adjusted according to the size of the recording material P. It is conceivable to control the secondary transfer current during passage of the recording material P to a value within the secondary transfer current range. However, even if an appropriate secondary transfer current range is determined in advance, the electrical resistance of the secondary transfer roller 8 forming the non-sheet-passing portion varies under various conditions. These various conditions include variations in products, environment (temperature/humidity), temperature/humidity absorption of members, cumulative usage time (operation status and repeated usage status of the image forming apparatus), and the like. Therefore, the appropriate secondary transfer current range changes due to fluctuations in the electrical resistance of the secondary transfer roller 8 .

Further description will be made with reference to FIG. FIG. 11A shows the secondary transfer current range for each size of the recording material P determined in advance by experiment or the like. In order to sufficiently suppress image defects, the range of the current that can be applied to the sheet passing portion when the recording material P is passing through the secondary transfer portion N2 is the recording material P having a width (297 mm) corresponding to A4 size. (Paper) was 15 to 20 μA. Also, if the recording material P (paper) has a width (148.5 mm) corresponding to the A5R size, the width is shorter than that of the A4 size, which is 7.5 to 10 μA. The width in the longitudinal direction of the secondary transfer roller 8 of the device in which the range of the paper-passing portion current was determined was 338 mm. When the recording material P passes through the secondary transfer portion N2, the range of the current flowing through the non-sheet passing portion is 3.6 to 4.4 μA for A4 size, and 16 μA for A5R size. It was 6 to 20.3 μA. Therefore, the range of current that can be passed through the secondary transfer portion N2 when the recording material P is passing through the secondary transfer portion N2 (“secondary transfer current range”) is 18.6 to 18.6 for A4 size. 24.4 μA, and 24.1 to 30.3 μA for A5R size.

However, for example, when the electrical resistance of the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment) is low, the current flowing through the non-sheet passing portion increases. FIG. 11B is an example of an appropriate secondary transfer current range when the electrical resistance of the secondary transfer portion N2 is lower than the state when the secondary transfer current range shown in FIG. 11A is determined. indicates Even if the electrical resistance of the secondary transfer portion N2 is lowered, the range of current that can be passed through the paper-passing portion does not change. However, when the electrical resistance of the secondary transfer portion N2 decreases, the secondary transfer current, which is the sum of the current in the paper-passing portion and the current in the non-paper-passing portion, increases its upper limit and Both lower bounds shift higher. For example, consider a case where the secondary transfer current is 24.5 μA when the recording material P of A5R size is passing through the secondary transfer portion N2. In this case, if the electrical resistance of the secondary transfer roller 8 is the same as the state when the secondary transfer current range shown in FIG. Therefore, an appropriate current flows through the paper passing portion. However, if the electrical resistance of the secondary transfer roller 8 is as low as the state in which the secondary transfer current range shown in FIG. , the secondary transfer current is smaller than the lower limit (26.9 μA) of the appropriate secondary transfer current range. As a result, the current flowing through the paper-passing portion may be insufficient, resulting in an image defect.

In other words, if the secondary transfer current value is near the lower limit of the electrical resistance of the non-paper-passing portion, even if there is no problem as long as the electrical resistance of the non-paper-passing portion is When the electrical resistance is low, the current in the paper-passing portion deviates from the lower limit at which image defects can be suppressed. Conversely, when the electrical resistance of the secondary transfer portion N2 increases, the current flowing through the non-sheet passing portion decreases. In this case, both the upper limit value and the lower limit value of the secondary transfer current are shifted lower. Therefore, in the case of a secondary transfer current value near the upper limit when the electrical resistance of the non-paper-passing portion is a certain value, even if there is no problem as long as the electrical resistance of the non-paper-passing portion is When the electrical resistance is high, the current in the paper-passing portion deviates from the upper limit for suppressing image defects.

4. Secondary Transfer Voltage Control Next, control of the secondary transfer voltage in this embodiment will be described. FIG. 4 is a flow chart showing an outline of the procedure for controlling the secondary transfer voltage in this embodiment. FIG. 4 shows a simplified procedure for controlling the secondary transfer voltage among the controls executed by the control unit 50 when executing a job. omitted.

Referring to FIG. 4A, first, when the control unit 50 acquires job information from the operation unit 31 or the external device 200, it starts the operation of the job (S101). In this embodiment, the information of this job includes image information specified by the operator, size (width, length) of the recording material P on which the image is formed, and information related to the thickness of the recording material P (thickness or basis weight), and information related to the surface properties of the recording material P, such as whether the recording material P is coated paper or not. In other words, information on paper size (width, length) and paper type category (plain paper, cardboard, etc. (including information related to thickness)) is included. The control unit 50 writes the information of this job to the RAM 52 (S102).

Next, the control unit 50 acquires environment information detected by the environment sensor 32 (S103). The ROM 53 also stores information indicating the correlation between the environment information and the target current Itarget for transferring the toner image on the intermediate transfer belt 7 onto the recording material P. FIG. Based on the environment information read in S103, the control unit 50 obtains the target current Itarget corresponding to the environment from the information indicating the relationship between the environment information and the target current Itarget, and writes it to the RAM 52 (S104).

The reason why the target current Itarget is changed according to the environmental information is that the charge amount of the toner changes depending on the environment. The information indicating the relationship between the environmental information and the target current Itarget is obtained in advance through experiments or the like. In addition to the environment, the toner charge amount may be affected by usage history such as the timing of supplying toner to the developing device 4 and the amount of toner discharged from the developing device 4 . In order to suppress these effects, the image forming apparatus 100 is configured so that the charge amount of the toner in the developing device 4 is within a certain range. However, if a factor affecting the charge amount of the toner on the intermediate transfer belt 7 is known other than the environmental information, the target current Itarget may also be changed according to that information. Alternatively, the image forming apparatus 100 may be provided with a measuring means for measuring the charge amount of the toner, and the target current Itarget may be changed based on the information on the charge amount of the toner obtained by this measuring means.

Next, before the toner image on the intermediate transfer belt 7 and the recording material P onto which the toner image is transferred reach the secondary transfer portion N2, the control portion 50 receives information about the electrical resistance of the secondary transfer portion N2. Acquire (S105). In this embodiment, information about the electrical resistance of the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment) is obtained by ATVC control (Active Transfer Voltage Control). That is, a predetermined voltage or current is supplied from the secondary transfer power source 20 to the secondary transfer roller 8 while the secondary transfer roller 8 and the intermediate transfer belt 7 are in contact with each other. Then, a current value when a predetermined voltage is supplied or a voltage value when a predetermined current is supplied is detected to acquire the relationship between voltage and current (voltage/current characteristics). The relationship between this voltage and current changes according to the electrical resistance of the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment). In the configuration of this embodiment, the relationship between the voltage and the current is not such that the current changes linearly (proportionally) to the voltage, but the current is represented by a second or higher polynomial of the voltage as shown in FIG. It will change as it is done. Therefore, in this embodiment, the predetermined voltage or current supplied when acquiring the information on the electrical resistance of the secondary transfer portion N2 is set at three points so that the relationship between the voltage and the current can be represented by a polynomial. The above multi-steps were used.

Next, the controller 50 obtains a voltage value to be applied from the secondary transfer power supply 20 to the secondary transfer roller 8 (S106). In other words, the control unit 50 calculates the target current Itarget written in the RAM 52 in S104 and the relationship between the voltage and the current obtained in S105 when there is no recording material P at the secondary transfer portion N2. A voltage value Vb required to flow Itarget is obtained. This voltage value Vb corresponds to the secondary transfer partial voltage. The ROM 53 also stores information for obtaining the recording material apportionment voltage Vp as shown in FIG. In this embodiment, this information is set as table data indicating the relationship between the moisture content in the atmosphere and the recording material apportionment voltage Vp for each basis weight category of the recording material P. FIG. Note that the control unit 50 can obtain the amount of moisture in the atmosphere based on environmental information (temperature/humidity) detected by the environment sensor 32 . The control unit 50 obtains the recording material apportionment voltage Vp from the above table data based on information on the basis weight of the recording material P included in the job information acquired in S102 and the environment information acquired in S103. . Then, the control unit 50 sets the initial value of the secondary transfer voltage Vtr to be applied from the secondary transfer power supply 20 to the secondary transfer roller 8 while the recording material P is passing through the secondary transfer portion N2 to the above Vb and Vb+Vp is obtained by adding Vp and Vb+Vp, and this is written in the RAM 52 . In this embodiment, the initial value of the secondary transfer voltage Vtr is obtained before the recording material P reaches the secondary transfer portion N2, and preparations are made for the timing when the recording material P reaches the secondary transfer portion N2.

Note that the table data for obtaining the recording material apportionment voltage Vp as shown in FIG. 6 is obtained in advance by experiments or the like. Here, the recording material assigned voltage (the transfer voltage corresponding to the electric resistance of the recording material P) Vp changes depending on the surface properties of the recording material P in addition to information related to the thickness of the recording material P (basis weight). Sometimes. Therefore, the table data may be set so that the recording material apportionment voltage Vp is also changed by information related to the surface properties of the recording material P. FIG. In this embodiment, information related to the thickness of the recording material P (further information related to the surface properties of the recording material P) is included in the job information acquired in S101. However, the image forming apparatus 100 may be provided with measuring means for detecting the thickness of the recording material P and the surface properties of the recording material P, and the recording material apportionment voltage Vp may be obtained based on the information obtained by this measuring means. .

Next, the control unit 50 performs processing for determining the upper limit value and lower limit value (“secondary transfer current range”) of the secondary transfer current when the recording material P is passing through the secondary transfer portion N2 ( S107). FIG. 4B shows the procedure for determining the secondary transfer current range in S107 of FIG. 4A. The ROM 53 stores, as shown in FIG. 7, the range of current that can be passed through the sheet passing portion when the recording material P passes through the secondary transfer portion N2 from the viewpoint of suppressing image defects ("sheet passing portion current Information for obtaining the range (passing portion current range) is stored. In this embodiment, this information is set as table data showing the relationship between the amount of moisture in the atmosphere and the upper and lower limits of the current that can be passed through the paper passing portion. Note that this table data is obtained in advance by experiments or the like. Referring to FIG. 4(b), control unit 50 obtains the range of current that can be passed through the paper passing portion from the table data based on the environment information acquired in S103 (S201).

It should be noted that the range of the current that can be passed through the sheet-passing portion varies depending on the width of the recording material P. FIG. In this embodiment, the table data is set assuming a recording material P having a width (297 mm) corresponding to A4 size. Here, from the viewpoint of suppressing image defects, the range of current that can be passed through the paper-passing portion may change depending on the thickness and surface properties of the recording material P, in addition to the environmental information. Therefore, the table data may be set so that the range of the electric current changes depending on information related to the thickness of the recording material P (basis weight) and information related to the surface properties of the recording material P. The range of current that can be passed through the paper-passing portion may be set as a calculation formula. In addition, the range of electric current that can be applied to the paper-passing portion may be set as a plurality of table data or calculation formulas for each recording material P size.

Next, the control unit 50 corrects the range of current that may be applied to the sheet passing portion acquired in S201 based on the information about the width of the recording material P included in the job information acquired in S102 (S202 ). The current range obtained in S201 corresponds to the width (297 mm) corresponding to the A4 size. For example, when the width of the recording material P actually used for image formation is a width (148.5 mm) corresponding to A5 longitudinal feed, that is, half the width corresponding to A4 size, the upper limit value obtained in S201 and The current range is corrected in proportion to the width of the recording material P so that the lower limits are halved.

Next, the control unit 50 obtains the current flowing through the non-sheet passing portion based on the following information (S203). Information about the width of the recording material P included in the job information acquired in S102, and the relationship between the voltage and the current at the secondary transfer portion N2 when there is no recording material P in the secondary transfer portion N2 obtained in S105. and information on the secondary transfer voltage Vtr obtained in S106. For example, if the width of the secondary transfer roller 8 is 338 mm, and the width of the recording material P obtained in S102 is the width (148.5 mm) corresponding to A5 longitudinal feed, the width of the non-sheet passing portion is the width of the secondary transfer roller 8 minus the width of the recording material P is 189.5 mm. Assume that the secondary transfer voltage Vtr obtained in S106 is, for example, 1000 V, and the current corresponding to the secondary transfer voltage Vtr is 40 μA from the relationship between the voltage and the current obtained in S105. In this case, the current flowing through the non-sheet passing portion corresponding to the secondary transfer voltage Vtr is calculated by the following proportional calculation:
40 μA×189.5 mm/338 mm=22.4 μA
can be obtained from That is, the current 40 μA corresponding to the secondary transfer voltage Vtr is reduced by the ratio of the width of the non-paper passing portion of 189.5 mm to the width of the secondary transfer roller 8 of 338 mm. can be asked for.

Next, the control unit 50 adds the non-sheet passing portion current obtained in S203 to each of the upper limit value and the lower limit value of the sheet passing portion current obtained in S202, so that the recording material P passes through the secondary transfer portion N2. An upper limit value and a lower limit value (“secondary transfer current range”) of the secondary transfer current when the current is set are obtained (S204). For example, consider a case where the upper limit value of the range of current that may be passed through the sheet passing portion corresponding to the A4 size width acquired in S201 is 20 μA and the lower limit value is 15 μA. In this case, when the width of the recording material P actually used for image formation is equivalent to the width of A5 longitudinal feed, the upper limit of the range of current that can be passed through the paper passing portion is 10 μA, and the lower limit is 7.5 μA. Become. When the current flowing through the non-paper-passing portion obtained in S203 is 22.4 μA as in the above example, the upper limit of the secondary transfer current range is 32.4 μA and the lower limit is 29.9 μA.

Referring to FIG. 4A, controller 50 controls current detecting circuit 21 to continue until recording material P reaches secondary transfer portion N2 and remains at secondary transfer portion N2 after recording material P reaches secondary transfer portion N2. is compared with the secondary transfer current range obtained in S107 (S108, S109). Then, the control unit 50 corrects the secondary transfer voltage Vtr output by the secondary transfer power source 20 as necessary (S110, S111). That is, if the detected secondary transfer current value is within the secondary transfer current range obtained in S107 (above the lower limit value and below the upper limit value), the control unit 50 determines that the secondary transfer power supply 20 is The next transfer voltage Vtr is maintained unchanged (S110). On the other hand, if the detected secondary transfer current value is out of the secondary transfer current range obtained in S107 (below the lower limit value or exceeds the upper limit value), the control unit 50 The secondary transfer voltage Vtr output by the secondary transfer power source 20 is corrected so that the voltage Vtr is (S111). In the present embodiment, when the upper limit is exceeded, the secondary transfer voltage Vtr is lowered, and when the secondary transfer current falls below the upper limit, correction of the secondary transfer voltage Vtr is stopped, and 2 The next transfer voltage Vtr is maintained. Typically, the secondary transfer voltage Vtr is stepped down with a predetermined step width. Further, in this embodiment, when the secondary transfer current is below the lower limit, the secondary transfer voltage Vtr is increased, and when the secondary transfer current exceeds the lower limit, correction of the secondary transfer voltage Vtr is stopped. of secondary transfer voltage Vtr is maintained. Typically, the secondary transfer voltage Vtr is increased stepwise with a predetermined step width. More specifically, the control unit 50 repeats the processing of S108 to S111 while the recording material P is passing through the secondary transfer portion N2, and when the secondary transfer current reaches a value within the secondary transfer current range, the secondary The correction of the transfer voltage Vtr is stopped and the current secondary transfer voltage Vtr is maintained.

Further, the control unit 50 repeats the processes of S108 to S111 until all the images of the job are transferred to the recording material P and output (S112).

As described above, the image forming apparatus 100 of this embodiment includes the detection section 21 that detects the current flowing through the transfer member 8 . The image forming apparatus 100 also includes a control section 50 that performs constant voltage control so that the voltage applied to the transfer member 8 while the recording material P is passing through the transfer section N2 becomes a predetermined voltage. The control unit 50 can change the voltage applied to the transfer member 8 so that the detection result detected by the detection unit 21 during transfer is within a predetermined range. Then, the control section 50 changes the predetermined range based on the detection result detected by the detection section 21 when the voltage is applied to the transfer member 8 with no recording material P in the transfer section N2. In this embodiment, the control unit 50 determines the predetermined range based on the information about the current that flows through the transfer member 8 when the predetermined voltage is applied to the transfer member 8 with no recording material P in the transfer portion N2. change. In particular, in this embodiment, the control unit 50 determines the voltage-current characteristic, which is the relationship between the voltage and the current flowing through the transfer member 8 when a voltage is applied to the transfer member 8 with no recording material P in the transfer portion N2. get. Further, based on the acquired voltage-current characteristics, the control unit 50 acquires the current flowing through the transfer member 8 when the predetermined voltage is applied to the transfer member 8 with no recording material P at the transfer portion N2. . Then, the control unit 50 changes the predetermined range based on the acquired current. In this embodiment, the control unit 50 also provides information about the current that flows through the transfer member 8 when the predetermined voltage is applied to the transfer member 8 in a state where there is no recording material P in the transfer portion N2, and The predetermined range is changed based on the size information in the width direction substantially orthogonal to the conveying direction. Here, in this embodiment, when forming an image on a predetermined recording material P, the control unit 50 can set the predetermined range as follows. That is, when the current indicated by the information regarding the current flowing through the transfer member 8 when the above-described predetermined voltage is applied to the transfer member 8 in a state where the recording material P is not present in the transfer portion N2 is the first current, the above-described predetermined range is set. Set to the first predetermined range. Further, when the current indicated by the information regarding the current flowing through the transfer member 8 when the above-described predetermined voltage is applied to the transfer member 8 in a state where the recording material P is not present in the transfer portion N2 is a second current higher than the first current. Then, the predetermined range is set to the second predetermined range. At this time, the absolute value of the upper limit of the first predetermined range is smaller than the absolute value of the upper limit of the second predetermined range. For example, as shown in FIG. 11A, when an image is formed on a recording material P of A4 size, the electric resistance of the transfer member 8 is a certain value, and the current that flows when a predetermined voltage is applied is the first value. In the case of current, the first predetermined range of transfer current is 18.6-24.4 μA. On the other hand, for example, as shown in FIG. 11B, when an image is formed on a recording material P of A4 size, the electric resistance of the transfer member 8 is a value smaller than the above-mentioned certain value, and the above-mentioned predetermined voltage is applied. If the current that sometimes flows is a second current higher than the first, then the following is done. That is, in this case, the second predetermined range of the transfer current is 19.2-25 μA. Thus, the absolute value of the upper limit of the first predetermined range (24.4 μA) is smaller than the absolute value of the upper limit of the second predetermined range (25 μA). Also, the absolute value of the lower limit of the first predetermined range (18.6 μA) is smaller than the absolute value of the lower limit of the second predetermined range (19.2 μA).

Further, in this embodiment, the image forming apparatus 100 includes a storage unit 53 that stores information about the predetermined range corresponding to the recording material P. FIG. In this embodiment, the control unit 50 controls the information about the current flowing through the transfer member 8 when the voltage is applied to the transfer member 8 in a state where there is no recording material P in the transfer portion N2, and the information stored in the storage unit 53. and changing the predetermined range based on information relating to the predetermined range. For example, when an image is formed on an A4 size recording material P as the first recording material, the first predetermined range of the transfer current is 18.6 to 24.4 μA (see FIG. 11 ( a)), 19.2 to 25 μA (FIG. 11(b)). On the other hand, when an image is formed on a recording material P of A5R size (width smaller than A4) as the second recording material, the second predetermined range of the transfer current is 24.1 to 24.1 depending on the electrical resistance of the transfer member 8. 30.3 μA (FIG. 11(a)) and 26.9 to 33.1 μA (FIG. 11(b)). Thus, the absolute value of the upper limit of the first predetermined range (24.4 μA or 25 μA) is smaller than the absolute value of the upper limit of the second predetermined range (30.3 μA or 33.1 μA). Also, the absolute value of the lower limit of the first predetermined range (18.6 μA or 19.2 μA) is smaller than the absolute value of the lower limit of the second predetermined range (24.1 μA or 26.9 μA). Also, the first difference, which is the difference between the upper limit value and the lower limit value of the first predetermined range, is smaller than the second difference, which is the difference between the upper limit value and the lower limit value of the second predetermined range.

Further, in this embodiment, when the length in the width direction substantially orthogonal to the conveying direction of the recording material P is a predetermined length, the control unit 50 sets the predetermined range to one of the following: can be different. It is at least one of the temperature or humidity of at least one of the inside and outside of the image forming apparatus 100, the index value related to the thickness of the recording material P, and the index value related to the surface roughness of the recording material. In this embodiment, the control unit 50 controls the detection result of the detection unit 21 when three or more levels of different voltages or currents are supplied from the power supply 20 to the transfer unit N2 in a state where there is no recording material P in the transfer unit N2. Based on this, the voltage-current characteristics are obtained. Further, in this embodiment, the voltage-current characteristics are expressed by a polynomial of the second or higher order of the voltage.

As described above, in the present embodiment, when the recording material P is passing through the secondary transfer portion N2, the current flowing through the non-sheet-passing portion is changed before the recording material P reaches the secondary transfer portion N2. Prediction is made by obtaining information about the electrical resistance of the secondary transfer portion N2. Then, by adding the predicted current flowing through the non-sheet-passing portion and the range of current that can be passed through the sheet-passing portion from the viewpoint of suppressing image defects, the recording material P passes through the secondary transfer portion N2. Determines the secondary transfer current range when Further, the secondary transfer voltage when the recording material P is passing through the secondary transfer portion N2 is controlled so that the value falls within the secondary transfer current range. This makes it possible to output an appropriate image regardless of the electrical resistance of the recording material P and the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment) that fluctuates under various circumstances.

In this embodiment, in S107, the current flowing through the secondary transfer portion N2 when the voltage is applied to the secondary transfer portion N2 when the recording material is not passing through the secondary transfer portion N2 is calculated. , the range of allowable current flowing through the secondary transfer portion N2 during transfer (during paper feeding) is changed. However, the present invention is not limited to this. For example, the allowable range of current flowing through the secondary transfer portion N2 during transfer (when paper is passed) is set constant, and when a voltage is applied to the secondary transfer portion N2 when no paper is passed, the current flows through the secondary transfer portion N2. Based on the current, the current detection result during paper feeding may be corrected. In other words, the control unit 50 causes the detection unit 21 to perform detection at the time of transfer based on the detection result detected by the detection unit 21 when a voltage is applied to the transfer member 8 in a state where there is no recording material P in the transfer unit N2. It is possible to correct the detection result and change the voltage applied to the transfer member 8 so that the corrected value is within a predetermined range.

[Example 2]
Another embodiment of the present invention will now be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of the first embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted. do.

In the first embodiment, the range of the current that can be passed through the sheet-passing portion while the recording material P is passing through the secondary transfer portion N2 has a range from the upper limit value to the lower limit value. However, the range of current that can be passed through the paper-passing portion is relatively narrow, and the current is set substantially constant at the target current (that is, the upper limit and lower limit of the current range in Example 1 are substantially the same). ) may be desired. In this case, the secondary transfer voltage applied to the secondary transfer roller 8 while the recording material P is passing through the secondary transfer portion N2 keeps the current flowing through the secondary transfer roller 8 at a substantially constant value. So-called constant current control is performed. In this case as well, the current flowing through the non-paper-passing portion may fluctuate due to fluctuations in the electrical resistance of the non-paper-passing portion, with respect to the current in the paper-passing portion that is desired to be controlled to be constant. Therefore, the secondary transfer current value, which is the sum of the current flowing in the sheet-passing portion to be controlled and the current flowing in the non-sheet-passing portion, fluctuates. In other words, the phenomenon that the secondary transfer current value, which is the sum of the current in the paper-passing portion and the current in the non-paper-passing portion, changes due to fluctuations in the electrical resistance of the non-paper-passing portion is In addition, it is also a problem to be considered when controlling the secondary transfer current value to a substantially constant value.

Therefore, in the present embodiment, in the structure in which the current to be supplied to the sheet passing portion is controlled to be substantially a constant value at the target current, the recording material P reaches the secondary transfer portion N2 in the same manner as in the first embodiment. The electrical resistance of the next transfer portion N2 is detected. Then, based on the detection result, a target value of the secondary transfer current (“secondary transfer current target value”) when the recording material P is passing through the secondary transfer portion N2 is obtained.

FIG. 8 is a flow chart showing an outline of the procedure for controlling the secondary transfer voltage in this embodiment. The processing of S301 to S312 in FIG. 8A is the same as S101 to S112 in FIG. 4A in the first embodiment. However, in this embodiment, the processing of S307 in FIG. processing) is different from that of the first embodiment. Further, in the present embodiment, the process of S309 in FIG. 8A (comparison with secondary transfer current target value) corresponding to S109 in FIG. processing) is different from that of the first embodiment. FIG. 8(b) shows the procedure of processing for determining the secondary transfer current target value in S307 of FIG. 8(a). In the following, differences from the first embodiment will be described, and descriptions of the same processes as in the first embodiment will be omitted.

In the present embodiment, the ROM 53 stores a current ("energy current") that may be applied to the sheet passing portion when the recording material P is passing through the secondary transfer portion N2 from the viewpoint of suppressing image defects, as shown in FIG. Information for obtaining the value of paper portion current (passing portion current) is stored. In this embodiment, this information is set as table data showing the relationship between the amount of moisture in the atmosphere and the value of the electric current that can be passed through the paper passing portion. The relationship between the amount of water and the current value is determined in advance by experiments or the like. It should be noted that the current value that may be applied to the paper-passing portion varies depending on the width of the recording material P. FIG. In this embodiment, the table data is set assuming a recording material P having a width (297 mm) corresponding to A4 size. Also, in this embodiment, the width of the secondary transfer portion N2 is 338 mm, which corresponds to the width of the secondary transfer roller 8 . Therefore, the target current Itarget when there is no recording material P at the secondary transfer portion N2 is 338/297 times (≈1.14 times) the value of the current shown in the table data of FIG. Here, from the viewpoint of suppressing image defects, the current value that may be applied to the paper-passing portion may change depending on the thickness and surface properties of the recording material P in addition to the environmental information. Therefore, the table data may be set so that the current value changes depending on information related to the thickness of the recording material P (basis weight) and information related to the surface properties of the recording material P. A current value that may be applied to the paper-passing portion may be set as a calculation formula. Further, the current values that may be applied to the paper-passing portion may be set as a plurality of table data or calculation formulas for each recording material P size. Also, as described in the first embodiment, the reason why the target current Itarget is changed according to the environmental information is that the charge amount of the toner changes depending on the environment. Therefore, the target current Itarget may be changed in another manner similar to that described in the first embodiment. In this embodiment, the table data shown in FIG. 9 is referred to in S304 of FIG.

Referring to FIG. 8A, the control unit 50 determines the target value of the secondary transfer current ("secondary transfer current target value") when the recording material P is passing through the secondary transfer portion N2. (S307). Referring to FIG. 8B, control unit 50 determines the amount of current that may be supplied to the sheet passing portion acquired in S304 based on the information about the width of recording material P included in the job information acquired in S302. The value (the target current Itarget is obtained from this current value in S304) is corrected (S401). The current value acquired in S304 corresponds to a width (297 mm) corresponding to A4 size. For example, when the width of the recording material P actually used for image formation is a width (148.5 mm) equivalent to A5 longitudinal feed, that is, half the width equivalent to A4 size, the current value obtained in S304 is The current value is corrected to be proportional to the width of the recording material P so as to be halved.

Next, the control unit 50 obtains the current flowing through the non-sheet passing portion based on the following information (S402). Information about the width of the recording material P included in the job information acquired in S302, and the relationship between the voltage and the current at the secondary transfer portion N2 when there is no recording material P in the secondary transfer portion N2 obtained in S305. and information on the secondary transfer voltage Vtr (=Vb+Vp) obtained in S306. As in the first embodiment, the control unit 50 controls the recording material P at the secondary transfer portion N2 based on the target current Itarget written in the RAM 52 in S304 and the relationship between the voltage and the current obtained in S305. A voltage value Vb required to flow the target current Itarget in the absence state is obtained. Also, the control unit 50 acquires Vp as in the first embodiment. The process of S402 in FIG. 8B is the same as the process of S203 in FIG. 4B in the first embodiment.

Next, the control unit 50 adds the sheet-passing-portion current obtained in S401 to the sheet-non-passing-portion current obtained in S402 to obtain a secondary transfer current when the recording material P is passing through the secondary transfer portion N2. A current target value is obtained (S403). For example, consider a case where the current value that may be applied to the sheet passing portion corresponding to the width corresponding to A4 size obtained in S304 is 18 μA. In this case, when the width of the recording material P actually used for image formation is equivalent to that of A5 longitudinal feed, the current value that may be applied to the paper passing portion is 9 μA. Then, when the current flowing through the non-sheet passing portion obtained in S402 is 22.4 μA as in the example described in the first embodiment, the secondary transfer current target value is 31.4 μA.

Referring to FIG. 8A, next, the controller 50 controls the secondary transfer current value detected by the current detection circuit 21 while the recording material P is present in the secondary transfer portion N2 and the secondary transfer current value obtained in S403. A secondary transfer current target value is compared (S308, S309). Then, the control unit 50 corrects the secondary transfer voltage Vtr output by the secondary transfer power source 20 as necessary (S310, S311). Here, in this embodiment, the secondary transfer voltage Vtr determined in S306 is applied for a predetermined period (initial period) after the recording material P reaches the secondary transfer portion N2. This is because, in the case of a system in which the electrical resistance varies greatly depending on the presence or absence of the recording material P, when a voltage is applied by constant current control from the state in which the recording material P does not exist, the voltage value fluctuates greatly and the flowing current is rather unstable. This is because it may become Therefore, in this embodiment, a certain voltage is applied at the beginning of the period in which the recording material P passes through the secondary transfer portion N2. After a predetermined period (for example, a period until the marginal portion of the leading edge has passed) elapses after the leading edge of the recording material P in the conveying direction enters the secondary transfer portion N2, there is a secondary transfer current value. A voltage was applied so that a constant current value was obtained. If the detected secondary transfer current value is substantially the same as the secondary transfer current target value obtained in S403 (they may differ within an allowable error range for control), the control unit 50 performs the secondary transfer. The secondary transfer voltage Vtr output by the power supply 20 is maintained without being changed (S310). On the other hand, when the detected secondary transfer current value deviates from the secondary transfer current target value obtained in S403, the control unit 50 outputs the secondary transfer power supply 20 so as to achieve the secondary transfer current target value. The secondary transfer voltage Vtr is corrected (S311). In this embodiment, when the secondary transfer current value becomes substantially the same as the secondary transfer current target value, correction of the secondary transfer voltage Vtr is stopped and the secondary transfer voltage Vtr at that time is maintained.

Thus, in this embodiment, the controller 50 controls the first period during which the predetermined leading edge of the recording material P is passing through the transfer portion N2 among the periods during which the recording material P is passing through the transfer portion N2. , constant voltage control is performed so that a predetermined voltage is applied to the transfer member 8 . Further, in the second period following the first period, the controller 50 controls the current flowing through the transfer member 8 based on the detection result of the detection unit 21 so that the current flowing through the transfer member 8 becomes a predetermined current. . The control unit 50 changes the predetermined current based on the information about the current flowing through the transfer member 8 when the predetermined voltage is applied to the transfer member 8 with no recording material P in the transfer portion N2.

As described above, in this embodiment, when the recording material P is passing through the secondary transfer portion N2, the current flowing through the non-sheet-passing portion is changed to Prediction is made by obtaining information about the electrical resistance of the secondary transfer portion N2. Then, by adding the predicted current flowing through the non-paper-passing portion and the current value that can be passed through the paper-passing portion from the viewpoint of suppressing image defects, the recording material P passes through the secondary transfer portion N2. Determines the secondary transfer current target value when Further, the secondary transfer voltage when the recording material P is passing through the secondary transfer portion N2 is controlled so as to achieve the secondary transfer current target value. This makes it possible to output an appropriate image regardless of the electrical resistance of the recording material P and the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment) that fluctuates under various circumstances.

[Example 3]
Another embodiment of the present invention will now be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of the first embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted. do.

In Examples 1 and 2, the relationship between the voltage and the current as information about the electrical resistance of the secondary transfer portion N2 was acquired as the voltage or current for measurement at three or more stages. This is because the relationship between the voltage and the current is such that the current is represented by a second or higher polynomial of the voltage. However, when the number of data to be acquired increases, the time required for control until the recording material P reaches the secondary transfer portion N2 increases, which may affect the productivity of image output.

Therefore, in the present embodiment, the image forming apparatus 100 considers the operation of acquiring information about the electrical resistance of the secondary transfer portion N2 until the recording material P reaches the secondary transfer portion N2 as the following first mode. , the second mode, and . The first mode is a mode in which the control time is relatively long, which is performed in the pre-multi-rotation process such as when the power of the image forming apparatus 100 is turned on or after returning from jam processing. The second mode is a mode in which the control time is shorter than in the first mode, which is performed at a timing other than the above, typically in the pre-rotation process of each job. That is, in the pre-rotation process of each job, when the relationship between the voltage and the current of the secondary transfer portion N2 is obtained by the processing of S105 of FIG. 4 in the first embodiment and S305 of FIG. mode can be run.

In the first mode, data is obtained by setting the voltage or current for measurement in multiple stages of three or more points. The method for obtaining the relationship between voltage and current in the first mode is the same as that described in the first embodiment.

On the other hand, in the second mode, one or two points of voltage or current are measured. Then, referring to the result of the first mode performed before the second mode (typically, the first mode performed last) and the result of the current second mode, the voltage and current Seek relationships.

For example, it is assumed that, as a result of the last performed first mode, the relationship between the voltage V and the current I at the secondary transfer portion N2 is a quadratic function as shown in Equation 1 below. Here, a, b, and c in Equation 1 below are coefficients obtained from the result of the first mode.
I=aV ² +bV+c (Formula 1)

In addition, it is assumed that the current flowing through the secondary transfer portion N2 is I2 as a result of the second mode in which the voltage or current for measurement is set to one voltage V0 after the first mode.

Further, by applying the voltage V0 to the above equation 1, the current I1 is calculated by the following equation 2.
I1=aV1 ² +bV1+c (Formula 2)

In this case, the relationship between the voltage V and the current I at the secondary transfer portion N2 as a result of the second mode is obtained by the following formula 3 by proportional calculation of I1 and I2.
I=I2/I1*( ^aV2 +bV+c) (Formula 3)

Thus, in this embodiment, the control unit 50 can selectively execute the following first mode and second mode. In the first mode, the transfer member 8 is detected based on the detection result of the detection unit 21 when three or more levels of different voltages or currents are supplied from the power source 20 to the transfer unit N2 in a state where there is no recording material P in the transfer unit N2. In this mode, the voltage-current characteristic, which is the relationship between the voltage when the voltage is applied and the current flowing through the transfer member 8, is obtained. The second mode is performed prior to the detection result of the detection unit 21 when the voltage or current of a level lower than that in the first mode is supplied from the power supply to the transfer unit in the state where there is no recording material P in the transfer unit N2. This is a mode for obtaining the voltage-current characteristics based on the result of the first mode obtained by the above.

As described above, in this embodiment, the same effect as in Embodiments 1 and 2 can be obtained, and the time required for the control performed before the recording material P reaches the secondary transfer portion N2 can be shortened. A decrease in output productivity can be suppressed.

[Example 4]
Another embodiment of the present invention will now be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of the first embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted. do.

As described in Examples 1 to 3, image defects such as low image density and white spots can be suppressed by providing a current range for the sheet passing portion. However, there is an image defect called "penetration" that is difficult to predict whether or not it will occur only by providing a current range for the sheet passing portion. The punch-through is an image defect in which when the recording material P passing through the secondary transfer portion N2 is discharged, the toner in the corresponding portion is not transferred to the recording material P, resulting in dot-like white spots. FIG. 12 is a table showing an example of the relationship between the sheet passing portion current and the presence or absence of punch-through examined as follows. "X" indicates that punch-through occurred, and "○" indicates that it did not occur. The experimental environment was NL (temperature 23° C., humidity 5%). As the recording material P, commercially available A4 size paper was used. Then, using paper immediately after being taken out of a commercially available individual package (opening) and after leaving in the NL environment for 24 hours or more (after leaving), the current of the paper passing part was shaken to prevent penetration. An experiment was conducted to check the presence or absence of From the results of FIG. 12, it can be seen that punch-through occurs at a lower paper-passing current in the case of using the paper that has been left standing than in the case of using the paper that has just been taken out of the packaging. As described above, even if the type of the recording material P is the same, for example, the sheet passing portion current that causes the punch-through differs depending on the standing state. Therefore, it is difficult to suppress punch-through, which is a different problem from low image density and white spots, simply by providing a paper-passing portion current range.

Here, with regard to penetration, it has been found by experiments that the thicker the recording material P, the larger the value of the recording material shared voltage when penetration occurs. FIG. 13 is a graph showing an overview of the relationship between the thickness of the recording material P and the voltage (absolute value) assigned to the recording material during secondary transfer. In this embodiment, the upper limit value (threshold value) of the recording material allotted voltage is set for each paper type (thickness) by using the above relationship. This makes it possible to control the secondary transfer current in the same manner as in the first to third embodiments while suppressing the occurrence of penetration.

FIG. 14 is a flow chart showing an overview of the procedure for controlling the secondary transfer voltage in this embodiment. The processes of S501 to S508 in FIG. 14 are the same as S101 to S108 in FIG. 4A in the first embodiment, respectively. Also, in this embodiment, the procedure of the processing for determining the secondary transfer current range in S507 is the same as the processing of S201 to S204 shown in FIG. 4B in the first embodiment.

The control unit 50 determines whether the secondary transfer current value detected by the current detection circuit 21 while the recording material P is passing through the secondary transfer portion N2 is less than the lower limit value of the secondary transfer current range obtained in S507. It is determined whether or not (S509). If the controller 50 determines that it is less than the lower limit value (“Yes”) in S509, it obtains the actual recording material apportionment voltage Vpth (S510). Here, the actual recording material apportionment voltage Vpth is different from the recording material apportionment voltage Vp determined in advance and stored in the ROM 53 as shown in FIG. 6, and is an actual calculated value during the secondary transfer. A method of calculating the actual recording material apportionment voltage Vpth will be described with reference to FIG. As shown in FIG. 15(a), during the secondary transfer, the secondary transfer voltage Vtr is applied to the secondary transfer roller 8, the secondary transfer opposing roller 73, and the recording material P, and the paper passing portion current flows. there is In FIG. 15A, Vtr is the secondary transfer voltage, Vpth is the actual voltage allotted to the recording material, and Vbth is the actual voltage allotted to the secondary transfer portion (mainly the secondary transfer roller 8 and the secondary transfer counter roller 73 are allotted). voltage). As shown in FIG. 15A, the actual recording material shared voltage Vpth can be derived by subtracting the actual secondary transfer partial charged voltage Vbth from the secondary transfer voltage Vtr. Further description will be made with reference to FIG. The control unit 50 can obtain the actual recording material apportionment voltage Vpth based on the following information. Information about the width of the recording material P included in the job information acquired in S502, and the relationship between the voltage and the current at the secondary transfer portion N2 when there is no recording material P in the secondary transfer portion N2 obtained in S505. and information on the secondary transfer voltage Vtr obtained in S506. That is, as shown in the left diagram of FIG. 15B, the paper-passing portion current Ip when the secondary transfer voltage Vtr is applied varies from the detected secondary transfer current Itr to the non-paper-passing portion current ( obtained by the same processing as S203 in FIG. 4B). Further, as shown in the center diagram of FIG. 15B, the actual secondary transfer partial voltage Vbth when the sheet passing portion current Ip flows is can ask. Then, as shown in the right diagram of FIG. 15B, by calculating the difference between the secondary transfer voltage Vtr and the actual secondary transfer partial voltage Vbth, the actual recording material shared voltage Vpth is obtained. can be done.

Next, the control unit 50 determines whether or not the actual recording material apportionment voltage Vpth is equal to or lower than the upper limit value (threshold value) (S511). In this embodiment, the upper limit of the actual recording material apportionment voltage Vpth is set for each piece of information related to the thickness of the recording material P (thickness or basis weight). Specifically, for each paper type category (basis weight) such as “thin paper, plain paper, thick paper 1, thick paper 2 (thick paper thicker than thick paper 1), etc.,” the upper limit of the actual recording material shared voltage Vpth Values are set in advance and stored in the ROM 53 as table data as shown in FIG. Based on the information of the paper type category (grammage) included in the job information acquired in S502, the control unit 50 stores the upper limit value of the actual recording material apportionment voltage Vpth corresponding to the paper type category in the above table. Select from the data and use. Incidentally, the method of setting the upper limit value of the actual recording material apportionment voltage Vpth is not limited to the method of this embodiment. For example, the relational expression between the thickness of the recording material P and the actual recording material assigned voltage Vpth (upper limit value, threshold value) at which penetration occurs is stored in the ROM 53, and the thickness information of the recording material P is stored for each job. The upper limit value of the actual recording material apportionment voltage Vpth may be set by directly acquiring it. As a method of acquiring the thickness information of the recording material P, there is a method in which the operator directly inputs the thickness of the recording material P in S501, or a method in which a thickness sensor using ultrasonic waves or the like is installed in the registration roller 9 in the conveying direction of the recording material P. For example, there is a method in which the sensor is provided upstream and measured for each job. If the controller 50 determines in S511 that the actual recording material apportionment voltage Vpth is equal to or lower than the upper limit (“Yes”), it increases the secondary transfer voltage Vtr (S512). At this time, the secondary transfer voltage Vtr is typically increased by a predetermined step width. On the other hand, if the control unit 50 determines in S511 that the actual recording material apportionment voltage Vpth exceeds the upper limit value (“No”), the control unit 50 does not change the secondary transfer voltage Vtr and maintains it (S513). .

If the control unit 50 determines in S509 that the value is equal to or greater than the lower limit value (“No”), the control unit 50 determines that the current detected by the current detection circuit 21, the current of the recording material P passing through the secondary transfer unit N2, is detected by the secondary transfer unit N2. It is determined whether or not the transfer current value exceeds the upper limit of the secondary transfer current range obtained in S507 (S514). If the controller 50 determines in S514 that the upper limit is exceeded (“Yes”), it lowers the secondary transfer voltage Vtr (S515). At this time, typically, the secondary transfer voltage Vtr is lowered by a predetermined step width. On the other hand, if the control unit 50 determines in S514 that the upper limit value is not exceeded (“No”), the control unit 50 does not change the secondary transfer voltage Vtr and maintains it (S516). After that, the control unit 50 repeats the processes of S508 to S516 until all the images of the job are transferred to the recording material P and output (S517).

In this embodiment, the control described above makes it possible to control the secondary transfer current in the same manner as in the first to third embodiments while suppressing the occurrence of penetration. Here, in this embodiment, even if the secondary transfer current is less than the lower limit value of the secondary transfer current range, the secondary transfer voltage Vtr may not be increased. has priority over suppression of This is based on consideration of the mechanism of secondary transfer current shortage and penetration. In other words, in this embodiment, the lower limit of the secondary transfer current range is set on the assumption that the duty (high image ratio) is higher than that of the average user, and a large amount of secondary transfer current is required. there is Therefore, even if the secondary transfer current falls below the lower limit of the secondary transfer current range, the transfer failure may not become obvious in the output image. However, the punch-through occurs depending on the recording material assigned voltage Vp, and becomes apparent regardless of whether the output image is a solid image or a halftone image. For this reason, in this embodiment, suppression of punch-through is prioritized over suppression of low image density and white spots.

As described above, in this embodiment, the control unit 50 controls the current flowing through the transfer member 8 when a voltage is applied to the transfer member 8 with no recording material P in the transfer portion N2, and the conveying direction of the recording material P. When the absolute value of the value obtained based on the information about the width in the substantially orthogonal direction and the current flowing through the transfer member 8 detected by the detection unit 21 during transfer exceeds a predetermined threshold value, the transfer Even if the absolute value of the current flowing through the transfer member 8 is less than the lower limit value of the predetermined range, the absolute value of the voltage applied to the transfer member 8 is adjusted so that the current flowing through the transfer member 8 during transfer is within the predetermined range. Do not make it bigger. Here, when the recording material P is passing through the transfer portion N2, the current flowing in the non-passing area of the transfer portion N2 in the width direction substantially orthogonal to the conveying direction of the recording material P, through which the recording material P does not pass, is referred to as non-passing current. current. At this time, in the present embodiment, the control unit 50 controls the non-passing portion current obtained based on the current flowing through the transfer member 8 when the voltage is applied to the transfer member 8 with no recording material P in the transfer portion N2. , and the current flowing through the transfer member 8 during transfer, the voltage assigned to the recording material P during transfer is obtained as the above value. Further, the threshold value is set according to an index value (thickness, basis weight, etc.) relating to the thickness of the recording material P. As shown in FIG. Typically, the thickness indicated by the index value is thicker than the first thickness than the threshold for the recording material P whose thickness is the first thickness indicated by the index value. The above threshold for the material P is larger.

In the present embodiment, the control of limiting the increase in the secondary transfer voltage Vtr in accordance with the actual recording material apportionment voltage Vpth is combined with the control of the first embodiment. may be combined. In that case, even if the secondary transfer current is less than the secondary transfer current target value, the secondary transfer voltage Vtr cannot be increased if the actual recording material shared voltage Vpth exceeds the upper limit value. should be avoided.

[Example 5]
Another embodiment of the present invention will now be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of the first embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted. do.

1. Effect of Thickness of Recording Material As described above, in order to solve the problem that the appropriate transfer current range changes due to fluctuations in the electrical resistance of the transfer member, the secondary transfer current is set before the recording material P reaches the secondary transfer portion N2. This can be dealt with by detecting the electrical resistance of the portion N2. However, when the recording material P used for image formation is a relatively thick recording material P such as thick paper, the thickness of the recording material P lowers the pressure at the non-paper passing portion. Therefore, the actual non-sheet passing portion current may deviate from the value predicted before the recording material P reaches the secondary transfer portion N2.

FIG. 23 is a graph showing changes in the pressure distribution of the secondary transfer portion N2 in the direction substantially orthogonal to the conveying direction of the recording material P caused by the passage of the recording material P. FIG. In the example shown in FIG. 23, the width of the recording material P is 300 mm. A plot indicated by a dashed line in FIG. 23 is the result of measuring the pressure distribution at the secondary transfer portion N2 when the recording material P is not present at the secondary transfer portion N2. On the other hand, the plot indicated by the solid line in FIG. 23 indicates that the recording material P having a basis weight of 300 g/m ² and a width of 105 mm passes through the vicinity of the center of the secondary transfer portion N2 in the direction substantially perpendicular to the conveying direction of the recording material P. This is the result of measuring the pressure distribution of the secondary transfer portion N2 when The pressure distribution (broken line in FIG. 23) at the secondary transfer portion N2 when the recording material P does not exist at the secondary transfer portion N2 is substantially uniform in the direction substantially orthogonal to the recording material P conveying direction. However, when the recording material P exists in the secondary transfer portion N2, the pressure of the paper passing portion (near the center of the solid line in FIG. 23) is higher than when the recording material P does not exist. On the other hand, the pressure in the non-sheet passing portion (region other than the center of the solid line in FIG. 23) is lower than when the recording material P is not present. The lower the pressure of the secondary transfer portion N2, the smaller the contact area between the intermediate transfer belt 7 and the secondary transfer roller 8 in the conveying direction of the recording material P. Therefore, even if the same secondary transfer voltage is applied, the current flowing will decrease. It gets smaller. If the transfer current range is determined based on the non-sheet passing portion current predicted from the electrical resistance of the secondary transfer portion N2 detected before the recording material P reaches the secondary transfer portion N2 without considering this phenomenon, The transfer current range may be higher than necessary. As a result, when the transfer current becomes too large, image defects due to the discharge phenomenon are likely to occur.

As described above, even when the recording material P having a relatively large thickness such as thick paper is used, the secondary transfer current when the recording material P is passing through the secondary transfer portion N2 is out of the appropriate range. Therefore, it is required to suppress the occurrence of image defects due to the occurrence of such defects.

2. Secondary Transfer Voltage Control Next, control of the secondary transfer voltage in this embodiment will be described. FIG. 17 is a flow chart showing an outline of the procedure for controlling the secondary transfer voltage in this embodiment. FIG. 17 shows a simplified procedure for controlling the secondary transfer voltage among the controls executed by the control unit 50 when executing a job. omitted.

In this embodiment, information about the thickness of the recording material P and the width of the recording material P is acquired based on information input from the operation unit 31 or the external device 200 . However, it is also possible to provide detection means for detecting the thickness and width of the recording material P in the image forming apparatus 100 and perform control based on information acquired by this detection means.

Referring to FIG. 17A, first, when control unit 50 acquires job information from operation unit 31 or external device 200, it starts the operation of the job (S601). In this embodiment, the information of this job includes image information specified by the operator, size (width, length) of the recording material P on which the image is formed, and information related to the thickness of the recording material P (thickness or basis weight), and information related to the surface properties of the recording material P, such as whether the recording material P is coated paper or not. In other words, information on paper size (width, length) and paper type category (plain paper, cardboard, etc. (including information related to thickness)) is included. The control unit 50 writes the information of this job to the RAM 52 (S602).

Next, the control unit 50 acquires environment information detected by the environment sensor 32 (S603). The ROM 53 also stores information indicating the correlation between the environment information and the target current Itarget for transferring the toner image on the intermediate transfer belt 7 onto the recording material P. FIG. Based on the environment information read in S603, the control unit 50 obtains the target current Itarget corresponding to the environment from the information indicating the relationship between the environment information and the target current Itarget, and writes it to the RAM 52 (S604).

The reason why the target current Itarget is changed according to the environmental information is that the charge amount of the toner changes depending on the environment. The information indicating the relationship between the environmental information and the target current Itarget is obtained in advance through experiments or the like. In addition to the environment, the toner charge amount may be affected by usage history such as the timing of supplying toner to the developing device 4 and the amount of toner discharged from the developing device 4 . In order to suppress these effects, the image forming apparatus 100 is configured so that the charge amount of the toner in the developing device 4 is within a certain range. However, if a factor affecting the charge amount of the toner on the intermediate transfer belt 7 is known other than the environmental information, the target current Itarget may also be changed according to that information. Alternatively, the image forming apparatus 100 may be provided with a measuring means for measuring the charge amount of the toner, and the target current Itarget may be changed based on the information on the charge amount of the toner obtained by this measuring means.

Next, before the toner image on the intermediate transfer belt 7 and the recording material P onto which the toner image is transferred reach the secondary transfer portion N2, the control portion 50 receives information about the electrical resistance of the secondary transfer portion N2. Acquire (S605). In this embodiment, information about the electrical resistance of the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment) is obtained by ATVC control (Active Transfer Voltage Control). That is, a predetermined voltage or current is supplied from the secondary transfer power source 20 to the secondary transfer roller 8 while the secondary transfer roller 8 and the intermediate transfer belt 7 are in contact with each other. Then, a current value when a predetermined voltage is supplied or a voltage value when a predetermined current is supplied is detected to acquire the relationship between voltage and current (voltage/current characteristics). The relationship between this voltage and current changes according to the electrical resistance of the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment). In the configuration of this embodiment, the relationship between the voltage and the current is not such that the current changes linearly (proportionally) to the voltage, but the current is represented by a second or higher polynomial of the voltage as shown in FIG. It is something that changes to be done. Therefore, in this embodiment, the predetermined voltage or current supplied when acquiring the information on the electrical resistance of the secondary transfer portion N2 is set at three points so that the relationship between the voltage and the current can be represented by a polynomial. The above multi-steps were used.

Next, the controller 50 obtains the voltage value to be applied from the secondary transfer power supply 20 to the secondary transfer roller 8 (S606). In other words, the control unit 50 calculates the target current Itarget written in the RAM 52 in S604 and the relationship between the voltage and the current obtained in S605 when there is no recording material P at the secondary transfer portion N2. A voltage value Vb required to flow Itarget is obtained. This voltage value Vb corresponds to the secondary transfer partial charge voltage. The ROM 53 also stores information for obtaining the recording material apportionment voltage Vp as shown in FIG. In this embodiment, this information is set as table data indicating the relationship between the moisture content in the atmosphere and the recording material apportionment voltage Vp for each basis weight category of the recording material P. FIG. Note that the control unit 50 can obtain the amount of moisture in the atmosphere based on environmental information (temperature/humidity) detected by the environment sensor 32 . The control unit 50 obtains the recording material apportionment voltage Vp from the table data based on the information on the basis weight of the recording material P included in the job information acquired in S602 and the environment information acquired in S603. . Then, the control unit 50 sets the initial value of the secondary transfer voltage Vtr to be applied from the secondary transfer power supply 20 to the secondary transfer roller 8 while the recording material P is passing through the secondary transfer portion N2 to the above Vb and Vb+Vp is obtained by adding Vp and Vb+Vp, and this is written in the RAM 52 . In this embodiment, the initial value of the secondary transfer voltage Vtr is obtained before the recording material P reaches the secondary transfer portion N2, and preparations are made for the timing when the recording material P reaches the secondary transfer portion N2.

Note that the table data for obtaining the recording material apportionment voltage Vp as shown in FIG. 6 is obtained in advance by experiments or the like. Here, the recording material assigned voltage (the transfer voltage corresponding to the electric resistance of the recording material P) Vp changes depending on the surface properties of the recording material P in addition to information related to the thickness of the recording material P (basis weight). Sometimes. Therefore, the table data may be set so that the recording material apportionment voltage Vp is also changed by information related to the surface property of the recording material P. FIG. In this embodiment, information related to the thickness of the recording material P (further information related to the surface properties of the recording material P) is included in the job information acquired in S601. However, the image forming apparatus 100 may be provided with measuring means for detecting the thickness of the recording material P and the surface properties of the recording material P, and the recording material apportionment voltage Vp may be obtained based on the information obtained by this measuring means. .

Next, the control unit 50 performs processing for determining the upper limit value and lower limit value (“secondary transfer current range”) of the secondary transfer current when the recording material P is passing through the secondary transfer portion N2 ( S607). FIG. 17(b) shows the procedure of processing for determining the secondary transfer current range in S607 of FIG. 17(a). The ROM 53 stores, as shown in FIG. 7, the range of current that can be passed through the sheet passing portion when the recording material P passes through the secondary transfer portion N2 from the viewpoint of suppressing image defects ("sheet passing portion current Information for obtaining the range (passing portion current range) is stored. In this embodiment, this information is set as table data showing the relationship between the amount of moisture in the atmosphere and the upper and lower limits of the current that can be passed through the paper passing portion. Note that this table data is obtained in advance by experiments or the like. Referring to FIG. 17(b), control unit 50 obtains the range of current that can be passed through the sheet passing portion from the table data based on the environmental information acquired in S603 (S701).

It should be noted that the range of the current that can be passed through the sheet-passing portion varies depending on the width of the recording material P. As shown in FIG. In this embodiment, the table data is set on the assumption that the recording material P has a width (297 mm) corresponding to A4 size. Here, from the viewpoint of suppressing image defects, the range of current that can be passed through the paper-passing portion may change depending on the thickness and surface properties of the recording material P, in addition to the environmental information. Therefore, the table data may be set so that the range of electric current changes depending on information (basis weight) related to the thickness of the recording material P and information related to the surface properties of the recording material P. The range of current that can be passed through the paper-passing portion may be set as a calculation formula. Also, the range of the current that can be passed through the sheet-passing portion may be set for each size of the recording material P as a plurality of table data or calculation formulas.

Next, the control unit 50 corrects the range of the current that may be supplied to the sheet passing portion acquired in S701 based on the information about the width of the recording material P included in the job information acquired in S602 (S702 ). The current range obtained in S701 corresponds to the width (297 mm) corresponding to the A4 size. For example, if the width of the recording material P actually used for image formation is a width (148.5 mm) equivalent to A5 longitudinal feed, that is, half the width equivalent to A4 size, then the upper limit value and the lower limit value acquired in S701 are are halved, the range of the current is corrected in proportion to the width of the recording material P. 7. That is, let Ip_max be the upper limit of the paper passing portion current before correction, Ip_min be the lower limit, and Lp_bas be the width of the recording material P when the table data of FIG. 7 is determined. Also, let Lp be the width of the recording material P that is actually conveyed, Ip_max_aft be the upper limit of the corrected sheet passing portion current, and Ip_min_aft be the lower limit. At this time, the upper limit value and the lower limit value of the sheet passing portion current after correction can be obtained by the following equations 4 and 5, respectively.
Ip_max_aft=Lp/Lp_bas*Ip_max (Formula 4)
Ip_min_aft=Lp/Lp_bas*Ip_min (Formula 5)

Next, the control unit 50 obtains the current flowing through the non-sheet passing portion based on the following information (S703). Information about the width of the recording material P included in the job information acquired in S602, and the relationship between the voltage and the current at the secondary transfer portion N2 when there is no recording material P in the secondary transfer portion N2 obtained in S605. and information on the secondary transfer voltage Vtr obtained in S606. For example, if the width of the secondary transfer roller 8 is 338 mm, and the width of the recording material P obtained in S602 is the width (148.5 mm) corresponding to A5 longitudinal feed, the width of the non-sheet passing portion is the width of the secondary transfer roller 8 minus the width of the recording material P is 189.5 mm. Assume that the secondary transfer voltage Vtr obtained in S606 is, for example, 1000 V, and the current corresponding to the secondary transfer voltage Vtr is 40 μA from the relationship between the voltage and the current obtained in S605. In this case, the current flowing through the non-sheet passing portion corresponding to the secondary transfer voltage Vtr is calculated by the following proportional calculation:
40 μA×189.5 mm/338 mm=22.4 μA
can be obtained from That is, the current 40 μA corresponding to the secondary transfer voltage Vtr is reduced by the ratio of the width of the non-paper passing portion of 189.5 mm to the width of the secondary transfer roller 8 of 338 mm. can be asked for.

If the thickness of the recording material P is relatively small, the value obtained in S703 can be used as the non-sheet passing portion current. However, as the thickness of the recording material P increases, the pressure on the non-paper passing portion when the recording material P is present in the secondary transfer portion N2 decreases, thereby reducing the non-paper passing portion current. Therefore, in this embodiment, the control unit 50 performs control for correcting the non-sheet passing portion current according to the thickness of the recording material P (S704). Let Inp_bef be the non-passing portion current before correction, Inp_aft be the non-sheet passing portion current after correction, and e (%) be the correction coefficient obtained in S703. At this time, the corrected non-sheet-passing-portion current can be obtained by the following equation (6).
Inp_aft=e*Inp_bef (Formula 6)

Here, in the present embodiment, the correction coefficient e in the above equation 6 is obtained in advance by experiment or the like and stored in the ROM 53, as shown in FIG. It is determined based on table data showing the relationship between the width of the material P and the correction coefficient e. The control unit 50 determines the correction coefficient e by referring to table data shown in FIG. do. The greater the thickness of the recording material P, the lower the pressure in the non-sheet passing portion. Taking this into account, the correction coefficient e is set such that the thicker the recording material P, the smaller the non-sheet passing portion current after correction. Further, the larger the width of the recording material P, the less likely the contact between the intermediate transfer belt 7 and the secondary transfer roller 8 in the non-passage portion, and the lower the pressure in the non-passage portion. Taking this into consideration, the correction coefficient e is set such that the larger the width of the recording material P, the smaller the non-sheet passing portion current after correction. For example, when the width of the recording material P is equivalent to A5 longitudinal feed (148.5 mm) and the basis weight of the recording material P is 350 g/m ² , the non-sheet passing portion current Inp_bef before correction is set to 85%. is the corrected non-sheet passing portion current Inp_aft. On the other hand, for example, when the width of the recording material P is equivalent to A5 longitudinal feed (148.5 mm) similar to the above, and the basis weight of the recording material P is 52 g/m ² , the non-passage before correction The non-sheet-passing-portion current Inp_aft after correction is obtained by maintaining the portion current Inp_bef at 100%.

Next, the control unit 50 sets the upper limit value and lower limit value (“secondary transfer current range”) of the secondary transfer current when the recording material P is passing through the secondary transfer portion N2 as follows. The determined secondary transfer current range is stored in the RAM 52 (S705). That is, the control unit 50 adds the corrected non-sheet-passing-portion current obtained in S704 to each of the upper limit value and the lower limit value of the sheet-passing-portion current obtained in S702. An upper limit value and a lower limit value (“secondary transfer current range”) of the secondary transfer current during passage are obtained. That is, let I_max be the upper limit of the secondary transfer current while the recording material P is passing through the secondary transfer portion N2, and I_min be the lower limit. At this time, the upper limit value and lower limit value of the secondary transfer current can be obtained by the following equations 7 and 8, respectively.
I_max=Ip_max_aft+Inp_aft (Formula 7)
I_min=Ip_min_aft+Inp_aft (Formula 8)

For example, consider a case where the upper limit of the range of current that may be passed through the sheet passing portion corresponding to the width of A4 size acquired in S701 is 20 μA and the lower limit is 15 μA. In this case, when the width of the recording material P actually used for image formation is equivalent to the width of A5 longitudinal feed, the upper limit of the range of current that can be passed through the paper passing portion is 10 μA, and the lower limit is 7.5 μA. Become. Then, when the current flowing through the non-sheet passing portion obtained in S703 is 22.4 μA as in the above example, if the recording material P is thick paper with a basis weight of 350 g/m ² , the above 22.4 μA is corrected to 85%, and 19 μA is the corrected non-sheet passing portion current. In this case, the secondary transfer current range has an upper limit of 29 μA and a lower limit of 26.5 μA. On the other hand, when the current flowing through the non-paper-passing portion obtained in S703 is 22.4 μA as described above, and the recording material P is paper with a basis weight of 52 g/m ² , the corrected non-paper-passing portion current is It is maintained at 22.4 μA, which is the non-sheet passing portion current before correction. Therefore, in this case, the upper limit value of the secondary transfer current range is 32.4 μA and the lower limit value is 29.9 μA.

Referring to FIG. 17A, next, control unit 50 controls current detection circuit 21 to continue until recording material P reaches secondary transfer portion N2 while recording material P is present at secondary transfer portion N2. is compared with the secondary transfer current range obtained in S607 (S608, S609). Then, the control unit 50 corrects the secondary transfer voltage Vtr output by the secondary transfer power source 20 as necessary (S610, S611). That is, if the detected secondary transfer current value is within the secondary transfer current range obtained in S607 (above the lower limit value and below the upper limit value), the control unit 50 determines that the secondary transfer power supply 20 is The next transfer voltage Vtr is maintained unchanged (S610). On the other hand, if the detected secondary transfer current value is out of the secondary transfer current range determined in S607 (below the lower limit value or exceeds the upper limit value), the control unit 50 The secondary transfer voltage Vtr output by the secondary transfer power source 20 is corrected so that the voltage Vtr is (S611). In the present embodiment, when the upper limit is exceeded, the secondary transfer voltage Vtr is lowered, and when the secondary transfer current falls below the upper limit, correction of the secondary transfer voltage Vtr is stopped, and 2 The next transfer voltage Vtr is maintained. Typically, the secondary transfer voltage Vtr is stepped down with a predetermined step width. Further, in this embodiment, when the secondary transfer current is below the lower limit, the secondary transfer voltage Vtr is increased, and when the secondary transfer current exceeds the lower limit, correction of the secondary transfer voltage Vtr is stopped. of secondary transfer voltage Vtr is maintained. Typically, the secondary transfer voltage Vtr is increased stepwise with a predetermined step width. More specifically, the control unit 50 repeats the processing of S608 to S611 while the recording material P is passing through the secondary transfer portion N2, and when the secondary transfer current reaches a value within the secondary transfer current range, the secondary The correction of the transfer voltage Vtr is stopped and the current secondary transfer voltage Vtr is maintained.

Further, the control unit 50 repeats the processes of S608 to S611 until all the images of the job are transferred to the recording material P and output (S612).

A change in the secondary transfer current range due to the control of this embodiment will be further described. Consider a case where the results of detecting the electrical resistance of the secondary transfer portion N2 before the recording material P reaches the secondary transfer portion N2 are approximately the same, and the secondary transfer voltage required for the secondary transfer is approximately the same. At this time, the secondary transfer current range in the case of using a recording material P with a width smaller than the maximum width is higher than the secondary transfer current range in the case of using the recording material P with the maximum width (absolute current (to increase the value). However, this shift amount decreases as the thickness of the recording material P increases.

For example, consider a case where a paper (thin paper) with a basis weight of 52 g/m ² and a paper (thick paper) with a basis weight of 350 g/m ² are used as the recording material P, respectively. Also, the results of detecting the electric resistance of the secondary transfer portion N2 before the recording material P reaches the secondary transfer portion N2 are almost the same in both cases, and it is assumed that a current of 30 μA flows when 1000 V is applied. . At this time, with paper having a basis weight of 52 g/m ² , the secondary transfer current range for A4 size (width 297 mm) is 24.9 to 19.9 μA, but for A5 longitudinal feed size (width 148.5 mm) In this case, the secondary transfer current range is 32.3 to 29.8 μA. That is, with paper having a basis weight of 52 g/m ² , when the width of the recording material P is reduced, the secondary transfer current range shifts to a higher level as a whole, and the lower limit is increased by about 10 μA. On the other hand, for paper with a basis weight of 350 g/m ² , the secondary transfer current range for A4 size (width 297 mm) is 24.1 to 19.1 μA, but for A5 longitudinal feed size (width 148.5 mm) is 29 to 26.5 μA. In other words, with paper having a basis weight of 350 g/m ² , when the width of the recording material P becomes smaller, the secondary transfer current range shifts to a higher level as a whole. The amount of shift is smaller than in the case of 52 g/m ² paper.

Actually, as shown in FIG. 6, the thicker the recording material P, the higher the electric resistance, and the higher the secondary transfer voltage Vtr required for secondary transfer. Therefore, when using thick paper and when using thin paper, the secondary transfer voltage Vtr required for the secondary transfer is higher when using thick paper. When the secondary transfer voltage Vtr is large, the secondary transfer current is also large when there is no recording material P at the secondary transfer portion N2, and the amount of change in the secondary transfer current range when the size of the recording material P changes is also large. . FIG. 19 shows the lower limit value of the secondary transfer current range for A5 longitudinal feed size when the initial secondary transfer voltage Vtr determined in S606 of FIG. 17A changes in the configuration of this embodiment. and the lower limit value of the secondary transfer current range for A4 size. The dashed line in FIG. 19 is the plot for paper with a basis weight of 52 g/m ² , and the solid line is the plot for paper with a basis weight of 350 g/m ² . If the thickness of the recording material P is different, the initial secondary transfer voltage Vtr changes. However, when the secondary transfer voltage Vtr is changed by several levels and the difference in the lower limit value of the secondary transfer current range due to the difference in the width of the recording material P is plotted, the following results are obtained. That is, the difference in the lower limit value of the secondary transfer current range due to the difference in the width of the recording material P for a certain secondary transfer voltage Vtr is smaller for the thicker recording material P as shown in FIG. there is

In the present embodiment, the current that flows when the voltage is actually applied to the secondary transfer portion N2 is detected as the information about the electrical resistance of the secondary transfer portion N2 when there is no recording material P in the secondary transfer portion N2. obtained by doing However, the present invention is not limited to this. For example, the electrical resistance of the secondary transfer portion N2 is obtained from environmental information such as the relationship between the output value of the environment sensor 32 and the electrical resistance of the secondary transfer portion N2. Information for this can be created as table data or the like. Based on the output value of the environment sensor 32, the electrical resistance of the secondary transfer portion N2 can be obtained by referring to the table data.

As described above, in this embodiment, the control unit 50 controls the detection result detected by the detection unit 21 when the voltage is applied to the transfer member 8 in a state where there is no recording material P in the transfer unit N2, and the transfer unit N2. The predetermined range is changed based on the information about the thickness of the recording material P passing through. Here, the width of the recording material P having the maximum width in the direction substantially perpendicular to the conveying direction of the recording material P among the recording materials P onto which the toner image can be transferred at the transfer portion N2 is defined as the maximum width. At this time, in the present embodiment, the control unit 50 controls the electric resistance indicated by the detection result detected by the detection unit 21 when the voltage is applied to the transfer member 8 in a state where there is no recording material P in the transfer unit N2. In the case of electrical resistance, the absolute value of the upper limit value of the predetermined range can be changed as follows based on the width of the recording material P passing through the transfer portion N2. That is, when the thickness of the recording material P passing through the transfer portion N2 is the first thickness, the upper limit value of the predetermined range with respect to the change from the maximum width of the width of the recording material P passing through the transfer portion N2 is When the amount of change is the first amount and the thickness of the recording material P passing through the transfer portion N2 is the second thickness larger than the first thickness, the amount of change in the upper limit value of the predetermined range is The upper limit of the predetermined range is changed so that the second amount is smaller than the first amount.

In other words, in this embodiment, the controller 50 changes the predetermined range as follows. That is, the electrical resistance indicated by the detection result detected by the detection unit 21 when a voltage is applied to the transfer member 8 in a state where the recording material P is not present in the transfer portion N2 is the predetermined electrical resistance, and the transfer portion N2 is passed through. When the thickness of the recording material P is the first thickness (for example, thin paper with a basis weight of 52 g/m ² in the above example), the width of the recording material P in the direction substantially perpendicular to the conveying direction of the recording material P is the first thickness. 1 (for example, the width corresponding to A4 size in the above example), the predetermined range is set to the first predetermined range (for example, 24.9 to 19.9 μA in the above example), and the width of the recording material P is If the second width is smaller than the first width (for example, the width corresponding to A5 vertical feed size in the above example), the above predetermined range is changed to the second predetermined range (for example, 32.3 to 29.8 μA in the above example). set to At this time, in this embodiment, the absolute value of the upper limit value of the second predetermined range is greater than the absolute value of the upper limit value of the first predetermined range. Also, in this embodiment, at this time, the absolute value of the lower limit of the second predetermined range is greater than the absolute value of the lower limit of the first predetermined range. Further, the control unit 50 determines that the electric resistance indicated by the detection result detected by the detection unit 21 when a voltage is applied to the transfer member 8 in a state where the recording material P is not present in the transfer portion N2 is the predetermined electric resistance, and When the thickness of the recording material P passing through the transfer portion N2 is a second thickness that is larger than the first thickness (for example, thick paper with a basis weight of 350 g/m ² in the above example), the width of the recording material P is When the width is the first width, the predetermined range is set to the third predetermined range (for example, 24.1 to 19.1 μA in the above example), and when the width of the recording material P is the second width. The predetermined range is set to a fourth predetermined range (for example, 29 to 26.5 μA in the above example). At this time, in this embodiment, the absolute value of the upper limit value of the fourth predetermined range is greater than the absolute value of the upper limit value of the third predetermined range. Further, in this embodiment, at this time, the absolute value of the lower limit value of the fourth predetermined range is greater than the absolute value of the lower limit value of the third predetermined range. Then, in this embodiment, the difference in the absolute value of the upper limit between the first predetermined range and the second predetermined range (for example, 7.4 μA (=32.3-24.9) in the above example) Also, the difference in the absolute values of the upper limits between the third predetermined range and the fourth predetermined range (for example, 4.9 μA (=29−24.1) in the above example) is smaller. Further, in this embodiment, the difference in the absolute value of the lower limit between the first predetermined range and the second predetermined range (for example, 9.9 μA (=29.8-19.9) in the above example) Also, the difference in the absolute values of the lower limits between the third predetermined range and the fourth predetermined range (for example, 7.4 μA (=26.5−19.1) in the above example) is smaller.

Further, in this embodiment, a storage unit 53 for storing information about the predetermined range corresponding to the recording material P is provided. Then, the control unit 50 controls the detection result detected by the detection unit 21 when a voltage is applied to the transfer member 8 with no recording material P in the transfer unit N2 and the thickness of the recording material P passing through the transfer unit N2. The predetermined range is changed based on the information regarding the height and the information regarding the predetermined range stored in the storage unit 53 . Further, in this embodiment, the control unit 50 controls the detection result of the detection unit 21 when three or more levels of different voltages or currents are supplied from the power source to the transfer unit N2 in a state where there is no recording material P in the transfer unit N2. Based on this, the voltage-current characteristic, which is the relationship between the voltage applied to the transfer member 8 and the current flowing through the transfer member 8, is obtained, and based on this voltage-current characteristic, it is determined that there is no recording material P at the transfer portion N2. A current that flows through the transfer member 8 when a predetermined voltage is applied to the transfer member 8 in this state is obtained, and the predetermined range is changed based on the obtained current. Further, in this embodiment, the voltage-current characteristic is represented by a second-order or higher polynomial.

As described above, in the present embodiment, when the recording material P is passing through the secondary transfer portion N2, the current flowing through the non-sheet-passing portion is changed before the recording material P reaches the secondary transfer portion N2. Prediction is made by obtaining information about the electrical resistance of the secondary transfer portion N2. At this time, the predicted value of the current flowing through the non-sheet passing portion is changed based on the information about the width of the recording material P, and the predicted value is corrected based on the information about the thickness of the recording material P. More specifically, the correction is performed so that the current flowing through the non-sheet passing portion decreases as the thickness of the recording material P increases. This makes it possible to more accurately predict the current flowing through the non-sheet passing portion. Then, by adding the predicted current flowing through the non-sheet-passing portion and the range of current that can be passed through the sheet-passing portion from the viewpoint of suppressing image defects, the recording material P passes through the secondary transfer portion N2. Determines the secondary transfer current range when Further, the secondary transfer voltage when the recording material P is passing through the secondary transfer portion N2 is controlled so that the value falls within the secondary transfer current range. As a result, even when a relatively thick recording material P such as thick paper is used, the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment) and the recording material, which fluctuate under various circumstances. An appropriate image can be output regardless of the electrical resistance of P.

[Example 6]
Another embodiment of the present invention will now be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of the first embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted. do.

In Example 5, the table data shown in FIG. Here, the reason why the change in the non-sheet passing portion current due to the difference in the thickness of the recording material P can be remarkably confirmed is that the index value related to the thickness of the recording material P is equal to or greater than a predetermined threshold value (for example, if the basis weight is a predetermined basis weight or more). Therefore, for example, only when the basis weight of the recording material P is equal to or greater than a predetermined basis weight, it is possible to correct the non-sheet passing portion current in the process of S704 in FIG. 17B. In this embodiment, only when the basis weight of the recording material P is equal to or greater than a predetermined basis weight, which is larger than that in the fifth embodiment, the non-sheet passing portion current is corrected in the process of S704 in FIG. 17B. do.

That is, in this embodiment, the table data used in the process of S704 in FIG. 17B is changed from the table data in FIG. 18 in the fifth embodiment to the table data in FIG. In the table data of FIG. 20, the correction coefficient e is 100% when the basis weight of the recording material P is less than 200 g/m ² . Therefore, in this embodiment, the correction of ^the non-sheet-passing portion current in the process of S704 in FIG. Only if ² or more.

In this manner, when the thickness of the recording material P passing through the transfer portion N2 is equal to or greater than a predetermined thickness, the control unit 50 sets the secondary transfer current range based on the thickness of the recording material P passing through the transfer portion N2. (predetermined range) can be changed.

As described above, in this embodiment, only in the case of using the recording material P having a thickness at which the change in the current in the non-sheet-passing portion is particularly remarkable, the detection result of the electric resistance of the secondary transfer portion and the recording material P The predicted value of the non-sheet-passing area current is corrected based on the width of the . As a result, the same effects as those of the fifth embodiment can be obtained, and control can be simplified.

[Example 7]
Another embodiment of the present invention will now be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of the first embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted. do.

In the present embodiment, in a structure in which the current to be supplied to the sheet passing portion is controlled to be substantially a constant value at the target current, the secondary transfer is performed before the recording material P reaches the secondary transfer portion N2 as in the fifth embodiment. The electric resistance of the portion N2 is detected. Then, based on the detection result and information about the width of the recording material P, a predicted value of the non-sheet passing portion current when the recording material P is passing through the secondary transfer portion N2 is obtained, and the predicted value is used as the recording material. Correction is made based on information about the thickness of P. As a result, the target value of the secondary transfer current (“secondary transfer current target value”) when the recording material P is passing through the secondary transfer portion N2 is obtained.

FIG. 21 is a flow chart showing an outline of the procedure for controlling the secondary transfer voltage in this embodiment. The processing of S801 to S812 of FIG. 21(a) is the same as S601 to S612 of FIG. 17(a) in the fifth embodiment. However, in this embodiment, the process of S807 in FIG. processing) is different from that of the fifth embodiment. Further, in this embodiment, the process of S809 in FIG. 21A (comparison with secondary transfer current target value) corresponding to S609 in FIG. processing) is different from that of the fifth embodiment. FIG. 21(b) shows the procedure of processing for determining the secondary transfer current target value in S807 of FIG. 21(a). Hereinafter, points different from the fifth embodiment will be described in particular, and descriptions of processes similar to the fifth embodiment will be omitted.

In the present embodiment, the ROM 53 stores a current ("energization current") that may be applied to the sheet passing portion when the recording material P is passing through the secondary transfer portion N2 from the viewpoint of suppressing image defects, as shown in FIG. Information for obtaining the value of paper portion current (passing portion current) is stored. In this embodiment, this information is set as table data showing the relationship between the amount of moisture in the atmosphere and the value of the electric current that can be passed through the paper passing portion. The relationship between the amount of water and the current value is determined in advance by experiments or the like. It should be noted that the current value that may be applied to the paper-passing portion varies depending on the width of the recording material P. FIG. In this embodiment, the table data is set on the assumption that the recording material P has a width (297 mm) corresponding to A4 size. Also, in this embodiment, the width of the secondary transfer portion N2 is 338 mm, which corresponds to the width of the secondary transfer roller 8 . Therefore, the target current Itarget when there is no recording material P at the secondary transfer portion N2 is the value of the current shown in the table data of FIG. 9 multiplied by 338/297 (≈1.14 times). In this embodiment, in S804 of FIG. 21(a), the table data shown in FIG.

Here, from the viewpoint of suppressing image defects, the current value that may be applied to the paper-passing portion may change depending on the thickness and surface properties of the recording material P in addition to the environmental information. Therefore, the table data may be set so that the current value changes depending on information related to the thickness of the recording material P (basis weight) and information related to the surface properties of the recording material P. A current value that may be applied to the paper-passing portion may be set as a calculation formula. Further, the current values that may be applied to the paper-passing portion may be set as a plurality of table data or calculation formulas for each recording material P size. Also, as described in the fifth embodiment, the reason why the target current Itarget is changed according to the environmental information is that the charge amount of the toner changes depending on the environment. Therefore, the target current Itarget may be changed in another modification mode similar to that described in the fifth embodiment.

Referring to FIG. 21A, control unit 50 determines the target value of the secondary transfer current ("secondary transfer current target value") when recording material P is passing through secondary transfer portion N2. (S807). Referring to FIG. 21B, based on the information about the width of the recording material P included in the job information acquired in S802, the control unit 50 determines the amount of current that may be supplied to the sheet passing portion acquired in S804. The value (the target current Itarget is obtained from this current value in S804) is corrected (S901). The current value acquired in S804 corresponds to the width (297 mm) corresponding to A4 size. For example, when the width of the recording material P actually used for image formation is a width (148.5 mm) equivalent to A5 longitudinal feed, that is, half the width equivalent to A4 size, the current value acquired in S804 is The current value is corrected to be proportional to the width of the recording material P so as to be halved. That is, Ip_tag is the paper passing portion current before correction obtained from the table data of FIG. 9, Lp_bas is the width of the recording material P when the table of FIG. The subsequent sheet passing portion current is set to Ip_tag_aft. At this time, the sheet passing portion current after correction can be obtained by the following equation 9.
Ip_tag_aft=Lp/Lp_bas*Ip_tag (Formula 9)

Next, the control unit 50 obtains the current flowing through the non-sheet passing portion based on the following information (S902). Information about the width of the recording material P included in the job information acquired in S802, and the relationship between the voltage and the current at the secondary transfer portion N2 when there is no recording material P in the secondary transfer portion N2 obtained in S805. and information on the secondary transfer voltage Vtr (=Vb+Vp) obtained in S806. That is, as in the fifth embodiment, the control unit 50 controls the target current Itarget written in the RAM 52 in S804 and the relationship between the voltage and the current obtained in S805 so that the recording material P is not present in the secondary transfer portion N2. A voltage value Vb required to flow the target current Itarget is obtained. Also, the control unit 50 acquires Vp as in the fifth embodiment. The processing of S902 of FIG. 21B is the same as the processing of S703 of FIG. 17B in the fifth embodiment.

Next, as in the fifth embodiment, the control unit 50 performs control to correct the non-sheet passing portion current according to the thickness of the recording material P (S903). Let Inp_bef be the non-passing portion current before correction, Inp_aft be the non-sheet passing portion current after correction, and e (%) be the correction coefficient obtained in S902. At this time, the corrected non-sheet-passing-portion current can be obtained by the following equation 6, which is the same as in the fifth embodiment.
Inp_aft=e*Inp_bef (Formula 6)

Here, in this embodiment, the correction coefficient e in the above equation 6 is determined based on the table data as shown in FIG. 18 similar to that of the fifth embodiment.

Next, the control unit 50 obtains the secondary transfer current target value while the recording material P is passing through the secondary transfer portion N2 as follows, and stores the obtained secondary transfer current target value in the RAM 52. Store (S904). In other words, the control unit 50 adds the paper-passing-portion current obtained in S901 to the paper-non-passing-portion current obtained in S902 to obtain the secondary transfer current when the recording material P is passing through the secondary transfer portion N2. Find the target value. That is, the secondary transfer current target value Itarget_aft can be obtained by the following equation (10).
Itarget_aft=Ip_tag_aft+Inp_aft (Formula 10)

For example, consider a case where the current value that may be applied to the sheet passing portion corresponding to the width corresponding to A4 size obtained in S804 is 18 μA. In this case, when the width of the recording material P actually used for image formation is equivalent to that of A5 longitudinal feed, the current value that may be applied to the paper passing portion is 9 μA. Then, when the current flowing through the non-sheet-passing portion obtained in S902 is 22.4 μA as in the example explained in the fifth embodiment, when the recording material P is thick paper with a basis weight of 350 g/m ² , , 19 μA obtained by correcting the above 22.4 μA to 85% is the non-sheet passing portion current after correction. In this case, the secondary transfer current target value is 28 (=9+19) μA. On the other hand, when the current flowing through the non-paper-passing portion obtained in S902 is 22.4 μA as described above, and the recording material P is paper with a basis weight of 52 g/m ² , the non-paper-passing current after correction is corrected. It is maintained at 22.4 μA, which is the previous non-sheet current. Therefore, in this case, the secondary transfer current target value is 31.4 (=9+22.4) μA.

Referring to FIG. 21A, next, while the recording material P is present in the secondary transfer portion N2, the control unit 50 controls the secondary transfer current value detected by the current detection circuit 21 and the secondary transfer current value obtained in S904. A secondary transfer current target value is compared (S808, S809). Then, the control unit 50 corrects the secondary transfer voltage Vtr output by the secondary transfer power supply 20 as necessary (S810, S811). Here, in this embodiment, the secondary transfer voltage Vtr determined in S806 is applied for a predetermined period (initial period) after the recording material P reaches the secondary transfer portion N2. This is because, in the case of a system in which the electrical resistance varies greatly depending on the presence or absence of the recording material P, when a voltage is applied by constant current control from the state in which the recording material P does not exist, the voltage value fluctuates greatly and the flowing current is rather unstable. This is because it may become Therefore, in this embodiment, a certain voltage is applied at the beginning of the period in which the recording material P passes through the secondary transfer portion N2. After a predetermined period (for example, a period until the marginal portion of the leading edge has passed) elapses after the leading edge of the recording material P in the conveying direction enters the secondary transfer portion N2, there is a secondary transfer current value. A voltage was applied so that a constant current value was obtained. If the detected secondary transfer current value is substantially the same as the secondary transfer current target value obtained in S904 (they may be different within an allowable error range for control), the control unit 50 performs the secondary transfer. The secondary transfer voltage Vtr output by the power supply 20 is maintained without being changed (S810). On the other hand, if the detected secondary transfer current value deviates from the secondary transfer current target value obtained in S904, the control unit 50 outputs the secondary transfer power source 20 so as to achieve the secondary transfer current target value. The secondary transfer voltage Vtr is corrected (S811). In this embodiment, when the secondary transfer current value becomes substantially the same as the secondary transfer current target value, correction of the secondary transfer voltage Vtr is stopped and the secondary transfer voltage Vtr at that time is maintained.

As described above, in this embodiment, the controller 50 keeps the voltage applied to the transfer member 8 constant so that the current flowing through the transfer member 8 when the recording material P is passing through the transfer portion N2 becomes a predetermined current. Control current. In this embodiment, the control unit 50 controls the detection result detected by the detection unit 21 when a voltage is applied to the transfer member 8 with no recording material P in the transfer unit N2, and the Based on the information about the thickness of the recording material P, the predetermined current is changed. At this time, the controller 50 controls the transfer member 8 to move a predetermined amount during a first period during which a predetermined leading edge of the recording material P passes through the transfer portion N2 among the periods during which the recording material P passes through the transfer portion N2. Constant voltage control of the voltage applied to the transfer member 8 is performed so that the voltage is applied. Further, the control unit 50 performs the constant current control during the second period following the first period.

As described above, in the present embodiment, as in the fifth embodiment, it is possible to more accurately predict the current flowing through the non-sheet-passing portion. In this embodiment, by adding the predicted current flowing through the paper non-passing portion and the current value that may be passed through the paper passing portion from the viewpoint of suppressing image defects, the secondary transfer portion N2 is transferred to the recording material. Determine the secondary transfer current target value when P is passing. Further, the secondary transfer voltage when the recording material P is passing through the secondary transfer portion N2 is controlled so as to achieve the secondary transfer current target value. As a result, even when a relatively thick recording material such as thick paper is used, the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment) and the recording material P, which fluctuate under various circumstances, It is possible to output an appropriate image regardless of the electrical resistance of

[Example 8]
Another embodiment of the present invention will now be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of the first embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted. do.

In Examples 5 to 7, the range of current that can be applied to the paper passing portion when the recording material P is passing through the secondary transfer portion N2 (“paper passing portion current range”) and the current of the non-paper passing portion A secondary transfer current range (or secondary transfer current target value) was obtained by adding the predicted value (after correction based on the thickness of the recording material P). Then, the secondary transfer voltage was controlled so that the secondary transfer current measured during the secondary transfer was within the range of the secondary transfer current (or the secondary transfer current target value). On the other hand, by subtracting the predicted value of the non-paper passing portion current (after correction based on the thickness of the recording material P) from the secondary transfer current measured during the secondary transfer, the paper passing portion current is obtained. The secondary transfer voltage may be controlled so that the paper section current falls within a predetermined range of the paper section current.

FIG. 22 is a flow chart showing an outline of the procedure for controlling the secondary transfer voltage in this embodiment. The processing of S1 to S6 in FIG. 22 is the same as the processing of S601 to S606 in FIG. 17A in the fifth embodiment. Also, the processing of S7 in FIG. 22 is the same as the processing of S701 in FIG. 17B in the fifth embodiment. Hereinafter, points different from the fifth embodiment will be described in particular, and descriptions of processes similar to the fifth embodiment will be omitted.

In S7, the control unit 50 obtains a paper-passing-portion current range corresponding to A4 size in the same manner as in the processing of S701 in FIG. 17B in the fifth embodiment. After that, the control unit 50 controls the secondary transfer current when the secondary transfer voltage Vtr is applied while the recording material P is present at the secondary transfer portion N2 after the recording material P reaches the secondary transfer portion N2. It is detected by the current detection circuit 21 (S8).

Then, the control unit 50 obtains the current flowing through the non-sheet passing portion based on the following information (S9). Information about the width of the recording material P included in the job information acquired in S2, and the relationship between the voltage and the current at the secondary transfer portion N2 when there is no recording material P in the secondary transfer portion N2 obtained in S5. and information on the currently applied secondary transfer voltage Vtr. The process of obtaining the non-sheet passing portion current in S9 is the same as the process of S703 in FIG. 17B in the fifth embodiment. However, in S9, the currently applied secondary transfer voltage (the initial value obtained in S6) is used as the secondary transfer voltage Vtr. That is, the secondary transfer voltage Vtr used to obtain the current flowing through the non-sheet passing portion in S9 is the initial value obtained in S6 at the timing when the first recording material P of the job enters the secondary transfer portion N2. . After that, when the secondary transfer voltage Vtr is changed according to the following flow, the changed secondary transfer voltage Vtr is used to obtain the current flowing through the non-sheet passing portion.

Next, the control unit 50 performs control for correcting the non-sheet passing portion current according to the thickness of the recording material P (S10) in the same manner as the processing of S704 in FIG. 17B in the fifth embodiment. Let Inp_bef be the non-sheet-passing portion current obtained in S9, Inp_aft be the non-sheet-passing portion current after correction, and e (%) be the correction coefficient. At this time, the corrected non-sheet-passing-portion current can be obtained by the following equation 6, which is the same as in the fifth embodiment.
Inp_aft=e*Inp_bef (Formula 6)

Here, in this embodiment, the correction coefficient e in the above equation 6 is determined based on the table data as shown in FIG. 18 similar to that of the fifth embodiment.

Next, the control unit 50 subtracts the corrected non-paper-passing-portion current obtained in S10 from the secondary transfer current detected in S8, and calculates the current as the paper-passing-portion current (S11). That is, if the secondary transfer current is Itr and the paper-passing portion current is Ip, the paper-passing portion current can be obtained by the following equation (11).
Ip=Itr-Inp_aft (Formula 11)

The sheet passing portion current Ip obtained by the above equation 11 is a current value corresponding to the width of the recording material P that is actually conveyed, whereas the sheet passing portion current range obtained in S7 is a reference recording It corresponds to the width corresponding to the size of the material P (A4 size in this embodiment). Therefore, in this embodiment, the controller 50 converts the paper passing portion current Ip obtained by the above equation 11 into a current value corresponding to the width corresponding to the size of the recording material P serving as a reference (S12). Let Lp_bas be the width of the recording material P when the table data in FIG. 7 is determined, Lp be the width of the recording material P that is actually conveyed, and Ip_aft be the sheet passing portion current after conversion. At this time, the paper passing portion current after conversion can be obtained by the following equation 12.
Ip_aft=Lp_bas/Lp*Ip (Formula 12)

Next, the control unit 50 compares the converted sheet passing portion current Ip_aft obtained in S12 with the sheet passing portion current range obtained in S7 (S13). Then, the control unit 50 corrects the secondary transfer voltage Vtr output by the secondary transfer power supply 20 as necessary (S14, S15). In other words, when the paper-passing-portion current Ip_aft after conversion is within the paper-passing-portion current range obtained in S7 (above the lower limit and below the upper limit), the control unit 50 determines that the secondary transfer power supply 20 is outputting The secondary transfer voltage Vtr is maintained unchanged (S14). On the other hand, if the converted sheet-passing-portion current Ip_aft is out of the sheet-passing-portion current range (below the lower limit value or exceeds the upper limit value) obtained in S7, the control unit 50 The secondary transfer voltage Vtr output by the secondary transfer power supply 20 is corrected so that (S15). In other words, if the converted sheet passing portion current Ip_aft exceeds the upper limit of the sheet passing portion current range, the secondary transfer voltage Vtr is lowered. When the secondary transfer voltage Vtr falls below the upper limit value, correction of the secondary transfer voltage Vtr is stopped, and Vtr at that time is maintained. Typically, the secondary transfer voltage Vtr is stepped down with a predetermined step width. Further, when the converted sheet passing portion current Ip_aft is below the lower limit value of the sheet passing portion current range, the secondary transfer voltage Vtr is increased. Then, when the lower limit is exceeded, correction of the secondary transfer voltage Vtr is stopped, and Vtr at that time is maintained. More specifically, in this embodiment, if the control unit 50 changes the secondary transfer voltage Vtr in S15 while the recording material P is passing through the secondary transfer portion N2, the process returns to S8. Then, a flow (S8 to S12) is performed to obtain the converted paper passing portion current Ip_aft for the changed secondary transfer voltage Vtr. Then, this flow is repeated until the converted sheet passing portion current Ip_aft reaches the value within the sheet passing portion current range obtained in S7. Then, when the current falls within the current range of the sheet passing portion, correction of the secondary transfer voltage Vtr is stopped, and Vtr at that time is maintained.

Further, the control unit 50 repeats the processes of S8 to S15 until all the images of the job are transferred to the recording material P and output (S16).

Note that when constant current control of the secondary transfer voltage is performed as in the seventh embodiment, the paper feeding voltage obtained by subtracting the predicted value of the non-paper passing portion current from the measured value of the secondary transfer current as in the present embodiment. Part current based control can also be applied. In this case, the target current value of the paper passing portion is determined in the process corresponding to S7 in this embodiment, and it is determined whether or not the current in the paper passing part matches the above target value in the process corresponding to S13 in this embodiment. You should do it.

As described above, in the present embodiment, as in the fifth embodiment, it is possible to more accurately predict the current flowing through the non-sheet-passing portion. In this embodiment, by subtracting the predicted current flowing through the non-sheet-passing portion from the measured secondary transfer current, the sheet-passing portion current to be controlled can be obtained accurately. Further, the secondary transfer voltage when the recording material P is passing through the secondary transfer portion N2 is controlled so that the value of the sheet passing portion current falls within a predetermined range of the sheet passing portion current. As a result, even when a relatively thick recording material P such as thick paper is used, the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment) and the recording material, which fluctuate under various circumstances. An appropriate image can be output regardless of the electrical resistance of P.

[others]
Although the present invention has been described with reference to specific examples, the present invention is not limited to the above-described examples.

In the above-described embodiments, the recording material is conveyed with reference to the center of the transfer member in the direction substantially perpendicular to the conveying direction. and the present invention can be equally applied.

Also, the present invention is equally applicable to a monochrome image forming apparatus having only one image forming station. In this case, the present invention is applied to a transfer portion where a toner image is transferred from an image bearing member such as a photosensitive drum onto a recording material.

7 intermediate transfer belt 8 secondary transfer roller 20 secondary transfer power source 21 current detection circuit 22 voltage detection circuit 50 control section

Claims (25)

an image carrier that carries a toner image;
an intermediate transfer belt onto which the toner image is transferred from the image carrier;
a transfer member to which a voltage is applied to transfer the toner image from the intermediate transfer belt to a recording material at a transfer portion;
a power supply that applies a voltage to the transfer member;
a current detection unit that detects a current flowing through the transfer member;
When the detection result detected by the current detection unit while the recording material is passing through the transfer unit is within a predetermined range determined based on the type of the recording material, the current is applied to the transfer member. a control unit that performs constant voltage control so that the voltage applied is the target voltage,
When the detection result is out of the predetermined range while the recording material is passing through the transfer unit, the control unit adjusts the target voltage so that the detection result is within the predetermined range. and performing the constant voltage control at the adjusted target voltage,
The control section controls current flowing to the transfer member when a voltage is applied to the transfer member in a state where there is no recording material in the transfer section, or current flowing in the transfer member when there is no recording material in the transfer section. An image forming apparatus, wherein an upper limit value and a lower limit value of the predetermined range are set based on the voltage applied to the transfer member when supplied. The control unit acquires current information about a current that flows through the transfer member when the target voltage is applied to the transfer member in a state in which there is no recording material in the transfer unit, and based on the acquired current information. 2. The image forming apparatus according to claim 1, wherein said upper limit value and said lower limit value are set. The control unit acquires a voltage-current characteristic that is a relationship between a current flowing through the transfer member and the voltage when a voltage is applied to the transfer member in a state where no recording material is present in the transfer unit. 2. The image forming apparatus according to claim 1, wherein the upper limit value and the lower limit value are set based on the voltage-current characteristics. The control unit acquires first current information Inp relating to a current flowing through the transfer member when the target voltage is applied to the transfer member in a state where there is no recording material in the transfer unit, and acquires the acquired first current information Inp. The second current information Ip is obtained based on the current information Inp, the size information in the width direction substantially perpendicular to the conveying direction of the recording material, and the detection result detected by the current detection unit when the toner image is transferred to the recording material. 2. The image forming apparatus according to claim 1, wherein the upper limit value and the lower limit value are set based on the acquired second current information Ip. When forming an image on a predetermined recording material, the control unit indicates current information about the current flowing through the transfer member when the target voltage is applied to the transfer member in a state where there is no recording material in the transfer unit. When the current is the first current, the upper limit value is set to the first upper limit value, and the current flows to the transfer member when the target voltage is applied to the transfer member in a state where there is no recording material in the transfer portion. When the current indicated by the current information about the current is a second current higher than the first current, the upper limit is set to a second upper limit, and the first upper limit is smaller than the second upper limit. 2. The image forming apparatus according to claim 1, wherein: further comprising a storage unit that stores first range information about the predetermined range corresponding to a recording material of a predetermined size;
Based on the first range information stored in the storage unit and the size information in the width direction substantially orthogonal to the conveying direction of the recording material passing through the transfer unit during the transfer, the control unit performs acquiring second range information about the predetermined range corresponding to the size of the recording material passing through the transfer portion during the transfer, and based on the acquired second range information and the acquired second current information; 5. The image forming apparatus according to claim 4, wherein the upper limit value and the lower limit value are set. The control unit changes the upper limit value and the lower limit value according to at least one of an index value related to the thickness of the recording material and an index value related to the surface roughness of the recording material. 2. The image forming apparatus according to claim 1. The control unit controls current flowing through the transfer member or applied to the transfer member when three or more levels of different voltages or currents are supplied from the power source to the transfer unit in a state where there is no recording material in the transfer unit. 4. The image forming apparatus according to claim 3, wherein the voltage-current characteristics are obtained based on voltage. The control unit
The controller controls the current flowing through the transfer member or applied to the transfer member when three or more levels of different voltages or currents are supplied from the power supply to the transfer unit in a state where there is no recording material in the transfer unit. a first mode for obtaining the voltage-current characteristic based on the voltage;
current flowing through the transfer member when the control unit supplies a voltage or current at a level lower than that in the first mode from the power source to the transfer unit in a state where there is no recording material in the transfer unit, or the transfer member a second mode of obtaining the voltage-current characteristic based on the voltage applied to and the result of the preceding first mode;
4. The image forming apparatus according to claim 3, wherein the image forming apparatus is capable of selectively executing 4. The image forming apparatus according to claim 3, wherein the current in the voltage-current characteristics is represented by a polynomial of second or higher order of voltage. The control unit acquires a value related to the voltage assigned to the recording material based on the target voltage and voltage information Vbth acquired based on the voltage-current characteristics, and the absolute value of the acquired value is a predetermined value. If the threshold value is exceeded, control is performed so that the absolute value of the voltage applied to the transfer member is not increased even if the value of the current flowing through the transfer member during the transfer is less than the lower limit value. 4. The image forming apparatus according to claim 3, characterized by: 12. The image forming apparatus according to claim 11, wherein the threshold is set according to an index value relating to the thickness of the recording material. The threshold value for the recording material indicated by the index value is greater than the threshold for the recording material having the first thickness, and the threshold value for the recording material indicated by the index value is the second thickness greater than the first thickness. 13. The image forming apparatus according to claim 12, wherein the larger one is larger than the other. The control section can change the upper limit based on the width of the recording material in a direction substantially perpendicular to the conveying direction of the recording material. In one case, the amount of change in the upper limit with respect to the change in the width of the recording material passing through the transfer portion from the width of the recording material having the maximum width is a first amount, and the recording material passing through the transfer portion is the first amount. is a second thickness larger than the first thickness, the upper limit value for the change in the width of the recording material passing through the transfer portion from the width of the recording material having the maximum width 8. The image forming apparatus according to claim 7, wherein the upper limit value is changed so that the amount of change is a second amount smaller than the first amount. When the thickness of the recording material passing through the transfer unit is the first thickness, the control unit controls the width of the recording material in the direction substantially perpendicular to the conveying direction of the recording material to be the first width. the upper limit is set to a first upper limit, and if the width of the recording material is a second width smaller than the first width, the upper limit is set to a second upper limit; The upper limit is greater than the first upper limit,
When the thickness of the recording material passing through the transfer unit is a second thickness larger than the first thickness, the control unit controls the width of the recording material to be the first width. The upper limit is set to the third upper limit, the upper limit is set to the fourth upper limit when the width of the recording material is the second width, and the fourth upper limit is the third upper limit. and the difference between the third upper limit and the fourth upper limit is smaller than the difference between the first upper limit and the second upper limit. Item 8. The image forming apparatus according to item 7. The control unit sets the upper limit value and the lower limit value based on the thickness of the recording material passing through the transfer unit when the thickness of the recording material passing through the transfer unit is equal to or greater than a predetermined thickness. 8. The image forming apparatus according to claim 7, wherein: an image carrier that carries a toner image;
an intermediate transfer belt onto which the toner image is transferred from the image carrier;
a transfer member to which a voltage is applied to transfer the toner image from the intermediate transfer belt to a recording material at a transfer portion;
a power supply that applies a voltage to the transfer member;
a current detection unit that detects a current flowing through the transfer member;
When the detection result detected by the current detection unit while the recording material is passing through the transfer unit is within a predetermined range determined based on the type of the recording material, the current is applied to the transfer member. a control unit that performs constant voltage control so that the voltage applied is the target voltage,
When the detection result is out of the predetermined range while the recording material is passing through the transfer unit, the control unit adjusts the target voltage so that the detection result is within the predetermined range. and performing the constant voltage control at the adjusted target voltage,
The control section controls current flowing to the transfer member when a voltage is applied to the transfer member in a state where there is no recording material in the transfer section, or current flowing in the transfer member when there is no recording material in the transfer section. An image forming apparatus, wherein the detection result of the current detection unit is corrected based on the voltage applied to the transfer member when supplied. an image carrier that carries a toner image;
an intermediate transfer belt onto which the toner image is transferred from the image carrier;
a transfer member to which a voltage is applied to transfer the toner image from the intermediate transfer belt to a recording material at a transfer portion;
a power supply that applies a voltage to the transfer member;
a current detection unit that detects a current flowing through the transfer member;
When the detection result detected by the current detection unit while the recording material is passing through the transfer unit is within a predetermined range determined based on the type of the recording material, the current is applied to the transfer member. a control unit that performs constant voltage control so that the voltage applied is the target voltage,
When the detection result is out of the predetermined range while the recording material is passing through the transfer unit, the control unit adjusts the target voltage so that the detection result is within the predetermined range. and performing the constant voltage control at the adjusted target voltage,
The control section controls current flowing to the transfer member when a voltage is applied to the transfer member in a state where there is no recording material in the transfer section, or current flowing in the transfer member when there is no recording material in the transfer section. An image forming apparatus, wherein the upper limit value of the predetermined range is set based on the voltage applied to the transfer member when supplied. The control unit acquires current information about a current that flows through the transfer member when the target voltage is applied to the transfer member in a state in which there is no recording material in the transfer unit, and based on the acquired current information. 19. The image forming apparatus according to claim 18, wherein the upper limit value is set. The control unit acquires a voltage-current characteristic that is a relationship between a current flowing through the transfer member and the voltage when a voltage is applied to the transfer member in a state where no recording material is present in the transfer unit. 19. The image forming apparatus according to claim 18, wherein the upper limit value is set based on the voltage-current characteristics. The control unit acquires first current information Inp relating to a current flowing through the transfer member when the target voltage is applied to the transfer member in a state where there is no recording material in the transfer unit, and acquires the acquired first current information Inp. The second current information Ip is obtained based on the current information Inp, the size information in the width direction substantially perpendicular to the conveying direction of the recording material, and the detection result detected by the current detection unit when the toner image is transferred to the recording material. 19. The image forming apparatus according to claim 18, wherein the upper limit value is set based on the acquired second current information Ip. an image carrier that carries a toner image;
an intermediate transfer belt onto which the toner image is transferred from the image carrier;
a transfer member to which a voltage is applied to transfer the toner image from the intermediate transfer belt to a recording material at a transfer portion;
a power supply that applies a voltage to the transfer member;
a current detection unit that detects a current flowing through the transfer member;
When the detection result detected by the current detection unit while the recording material is passing through the transfer unit is within a predetermined range determined based on the type of the recording material, the current is applied to the transfer member. a control unit that performs constant voltage control so that the voltage applied is the target voltage,
When the detection result is out of the predetermined range while the recording material is passing through the transfer unit, the control unit adjusts the target voltage so that the detection result is within the predetermined range. and performing the constant voltage control at the adjusted target voltage,
The control section controls current flowing to the transfer member when a voltage is applied to the transfer member in a state where there is no recording material in the transfer section, or current flowing in the transfer member when there is no recording material in the transfer section. An image forming apparatus, wherein the lower limit value of the predetermined range is set based on the voltage applied to the transfer member when supplied. The control unit acquires current information about a current that flows through the transfer member when the target voltage is applied to the transfer member in a state in which there is no recording material in the transfer unit, and based on the acquired current information. 23. The image forming apparatus according to claim 22, wherein said lower limit value is set. The control unit acquires a voltage-current characteristic that is a relationship between a current flowing through the transfer member and the voltage when a voltage is applied to the transfer member in a state where no recording material is present in the transfer unit. 23. The image forming apparatus according to claim 22, wherein the lower limit value is set based on the voltage-current characteristics. The control unit acquires first current information Inp relating to a current flowing through the transfer member when the target voltage is applied to the transfer member in a state where there is no recording material in the transfer unit, and acquires the acquired first current information Inp. The second current information Ip is obtained based on the current information Inp, the size information in the width direction substantially perpendicular to the conveying direction of the recording material, and the detection result detected by the current detection unit when the toner image is transferred to the recording material. 23. The image forming apparatus according to claim 22, wherein the lower limit value is set based on the acquired second current information Ip.

Images