JP2020064125A - Image forming apparatus - Google Patents

Abstract

To provide an image forming apparatus that controls a transfer voltage based on a result of detection of a current when a recording material passes through a transfer unit, and can change a change width in changing the transfer voltage according to a variation in electric resistance of a transfer member.SOLUTION: An image forming apparatus 100 comprises: an image carrier 7; a transfer member 8; a power supply 20 that applies a voltage to the transfer member 8; a detection unit 21 that detects a current flowing in the transfer member 8; and a control unit 50 that performs constant voltage control of the voltage applied to the transfer member 8, wherein the control unit 50 can change a transfer voltage applied to the transfer member 8 such that the current flowing in the transfer member 8 during transfer falls within a predetermined range. When the control unit 50 changes the transfer voltage applied to the transfer member 8 such that the current flowing in the transfer member 8 during transfer falls within the predetermined range, the control unit changes the transfer voltage for every predetermined change width of the transfer voltage, and changes the change width based on a result of detection of the current detected by the detection unit 21 when the voltage is applied to the transfer member 8 when a recording material P is not in a transfer unit N2.SELECTED DRAWING: Figure 4

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer or a fax machine which uses an electrophotographic system or an electrostatic recording system.

  Conventionally, in an image forming apparatus using an electrophotographic method or the like, a toner image is electrostatically transferred from an image carrier such as a photoconductor 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 contacts the image carrier to form a transfer portion. If the transfer voltage is too low, the transfer may not be performed sufficiently and a desired image density may not be obtained, which may result in “low image density”. Also, if the transfer voltage is too high, discharge occurs at the transfer section, and the polarity of the charge of the toner in the toner image is reversed due to the effect of the discharge, resulting in “white spots” in which the toner image is not partially 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.

  The amount of charge required for transfer varies depending on the size of the recording material and the area ratio of the toner image. Therefore, the transfer voltage is often applied by constant voltage control in which a constant voltage corresponding to a predetermined current density is applied. When the transfer voltage is applied by constant voltage control, the predetermined voltage is applied to the portion to which the target toner image is transferred, regardless of the current flowing outside the recording material or on the portion without the toner image on the recording material. This is because it is easy to secure a high transfer current. However, the electric resistance of the transfer member that constitutes the transfer portion changes according to product variations, member temperature, cumulative use time, etc., and the electric resistance of the recording material passing through the transfer portion also depends on the type of recording material, the surrounding environment. It changes according to (temperature and humidity). Therefore, when the transfer voltage is controlled to a constant voltage, it is necessary to adjust the transfer voltage according to the fluctuation of the electric resistance of the transfer member or the recording material.

  Patent Document 1 discloses the following control of the transfer voltage in the configuration in which the transfer voltage is applied to the transfer member by the constant voltage control to perform the transfer. Immediately before the start of continuous image formation, a predetermined voltage is applied to the transfer portion in the state where there is no recording material, the current value is detected, and the voltage value with which the predetermined target current is obtained is obtained. Then, a recording material sharing voltage according to the type of recording material is added to this voltage value to set a transfer voltage value to be applied by constant voltage control during transfer.

  Here, the types of recording materials include, for example, types due to differences in surface smoothness of recording materials such as high-quality paper and coated paper, and types due to differences in thickness of recording materials such as thin paper and thick paper. . The recording material sharing voltage can be obtained in advance, for example, according to the type of recording material. However, since there are many types of recording materials in circulation, or the electrical resistance of recording materials fluctuates depending on the time of being placed in the environment even if the environment (temperature and humidity) is the same, the recording material sharing voltage It is often difficult to accurately determine in advance. If the transfer voltage including the fluctuation of the electric resistance of the recording material is not an appropriate value, image defects such as low image density and blank areas may occur as described above.

  In order to solve such a problem, in Patent Documents 2 and 3, in a configuration in which a transfer voltage is applied by constant voltage control while a recording material is passing through the transfer unit, an upper limit value of a current supplied to the transfer unit. And it is proposed to set a lower limit. By such control, the current supplied to the transfer portion when the recording material is passing through the transfer portion can be set to a value within a predetermined range, so that the occurrence of image defects due to insufficient or excessive transfer current is prevented. Can be suppressed. In Patent Document 2, the upper limit value is obtained based on environmental information. In Patent Document 3, the upper limit value and the lower limit value are obtained depending on the front and back of the recording material, the type of the recording material, and the size of the recording material in addition to the environment.

JP, 2004-117920, A Japanese Patent No. 4161005 JP, 2008-275946, A

  As described above, when the recording material passes through the transfer portion, the current flowing through the transfer portion is detected, and the transfer voltage is set so that the current is within a predetermined range (upper limit value or lower limit value or higher). There is a way to control. When the transfer voltage is controlled by this method, there is a time lag from when it is detected that the current is out of the predetermined range until the transfer voltage is changed so that the current is within the predetermined range. Therefore, in the area of the recording material that passes through the transfer portion until the transfer voltage change is completed, the transfer current is out of the appropriate range, so that the density due to the excess or deficiency of the transfer current as shown in FIG. Image defects such as deterioration may occur.

  In order to suppress this phenomenon, it is possible to increase the fluctuation range of the transfer voltage per unit time. However, if the fluctuation range of the transfer voltage per unit time is increased, the amount of change in the current per unit time may become too large when the electric resistance of the transfer member is small. If the amount of change in the current per unit time becomes too large, the current may exceed the appropriate range and the insufficient state and the excessive state may be repeated. Therefore, a local image defect such as a density difference such as low density or a white spot due to a discharge phenomenon may occur.

  Therefore, an object of the present invention is to change the transfer voltage according to the fluctuation of the electric resistance of the transfer member in the configuration that controls the transfer voltage based on the detection result of the current when the recording material is passing through the transfer portion. It is an object of the present invention to provide an image forming apparatus capable of changing the change width when performing.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention relates to an image carrier that carries a toner image, a transfer member that contacts the image carrier and forms a transfer unit that transfers the toner image from the image carrier to a recording material, and the transfer member. A power source for applying a voltage to the transfer member, a detection unit for detecting a current flowing through the transfer member, and a transfer member for applying a predetermined voltage to the transfer member while a recording material passes through the transfer unit. A control unit that performs constant voltage control of the applied voltage, and the control unit can change the transfer voltage applied to the transfer member so that the current flowing through the transfer member falls within the predetermined range during transfer. In the image forming apparatus, the controller changes the transfer voltage when changing the transfer voltage applied to the transfer member so that the current flowing through the transfer member during transfer is within the predetermined range. Place of transfer voltage And changing the variation range based on a detection result detected by the detection unit when a voltage is applied to the transfer member in a state where there is no recording material in the transfer unit. Image forming apparatus.

  According to the present invention, when the transfer voltage is changed based on the change in the electric resistance of the transfer member in the configuration that controls the transfer voltage based on the detection result of the current when the recording material is passing through the transfer portion, The change width of can be changed.

1 is a schematic configuration diagram of an image forming apparatus. It is a schematic diagram of a configuration related to secondary transfer. FIG. 3 is a schematic block diagram showing a control mode of main parts of the image forming apparatus. FIG. 3 is a flowchart of the control of the first embodiment. It is a graph figure which shows an example of the relationship of the voltage and current of a secondary transfer part. It is a schematic diagram which shows an example of the table data of recording material sharing voltage. It is a schematic diagram which shows an example of the table data of a paper passing part electric current range. It is a schematic diagram which shows an example of the table data of the correction coefficient of a non-sheet passing part current. FIG. 6 is a graph for explaining a change in the secondary transfer current range depending on the thickness of the recording material. FIG. 7 is a flowchart of the control of the second embodiment. FIG. 8 is a flow chart diagram of control in Example 3; It is a schematic diagram which shows an example of table data of a secondary transfer current range. It is a schematic diagram for explaining the paper passing portion current and the non-paper passing portion current. 9 is a table for explaining changes in an appropriate current range due to changes in the electrical resistance of the transfer member. FIG. 6 is a schematic diagram for explaining a change in an appropriate current range depending on the thickness of a recording material. It is a schematic diagram for explaining an image defect due to an overshoot phenomenon. It is a graph figure for demonstrating an overshoot phenomenon.

  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 the present embodiment has a function of a tandem type multi-functional machine (copier, printer, facsimile machine) that can form a full-color image by using an electrophotographic method and adopts an intermediate transfer method. ).

  The image forming apparatus 100 includes, as a plurality of image forming units (stations), first, second, third, and fourth image forming units SY, SM, which form images of respective colors of yellow, magenta, cyan, and black. It has SC and SK. Regarding the elements having the same or corresponding functions or configurations in each image forming unit SY, SM, SC, SK, Y, M, C, K at the end of the reference numerals indicating the elements for any color are omitted. There is a general explanation. In this embodiment, the image forming section S is configured to include 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 unit S includes a photosensitive drum 1 that is a rotatable drum-type (cylindrical) photoconductor (electrophotographic photoconductor) as a first image carrier that carries a toner image. The photosensitive drum 1 is rotationally driven in the direction of arrow R1 (counterclockwise) in the figure. The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative polarity in this embodiment) by a charging roller 2 which is a roller type charging member as a charging means. The surface of the charged photosensitive drum 1 is scanned and exposed by an exposure device (laser scanner device) 3 as an exposure unit based on image information, and an electrostatic image (electrostatic latent image) is formed on the photosensitive drum 1. It

The electrostatic image formed on the photosensitive drum 1 is developed (visualized) by supplying toner as a developer by a developing device 4 as a developing unit, and a toner image is formed on the photosensitive drum 1. In this embodiment, the exposed portion (image portion) on the photosensitive drum 1 where the absolute value of the potential has been lowered by being uniformly charged and then exposed is charged to the same polarity as the photosensitive drum 1. Toner adheres (reverse development method). In this embodiment, the regular charging polarity of the toner, which is the charging 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 maximum density of the toner image of each color is about 1.5 to 1.7, and the applied amount of toner at the maximum density is about 0.4 to 0.6 mg / cm ^2. Has become.

  An intermediate transfer belt 7, which is an intermediate transfer member composed of an endless belt, is arranged as a second image carrier 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, which are a plurality of stretching rollers. The driving roller 71 transmits a driving force to the intermediate transfer belt 7. The tension roller 72 controls the tension of the intermediate transfer belt 7 to be constant. The secondary transfer counter roller 73 functions as a counter member (counter electrode) of the secondary transfer roller 8 described later. The driving roller 71 is driven to rotate, so that the intermediate transfer belt 7 rotates (rotates) in the direction of arrow R2 (clockwise) at a conveyance speed (peripheral speed) of about 300 to 500 mm / sec. The tension roller 72 is applied with a force that pushes the intermediate transfer belt 7 from the inner peripheral surface side to the outer peripheral surface side by the force of a spring as an urging unit, and this force causes the intermediate transfer belt 7 to move in the conveying direction. Has a tension of about 2 to 5 kg. On the inner peripheral surface side of the intermediate transfer belt 7, a primary transfer roller 5 which is a roller-type primary transfer member as a primary transfer means is arranged corresponding to each photosensitive drum 1. 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 come into 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) from a primary transfer power source (not shown), which is a DC voltage having a polarity opposite to the regular charging polarity of the toner. Is applied. For example, when forming a full-color image, the toner images of yellow, magenta, cyan, and black formed on the photosensitive drums 1 are sequentially transferred so as to be superimposed on the intermediate transfer belt 7.

  On the outer peripheral surface side of the intermediate transfer belt 7, a secondary transfer roller 8 which is a roller type secondary transfer member as a secondary transfer unit is arranged at a position facing the secondary transfer counter roller 73. The secondary transfer roller 8 is pressed against the secondary transfer counter roller 73 via the intermediate transfer belt 7, and the intermediate transfer belt 7 and the secondary transfer roller 8 come into contact with each other. ) Form N2. The toner image formed on the intermediate transfer belt 7 is conveyed by being sandwiched between the intermediate transfer belt 7 and the secondary transfer roller 8 by the action of the secondary transfer roller 8 at the secondary transfer portion N2 ( It is electrostatically transferred (secondary transfer) to a recording material (sheet, transfer material) P such as paper. During the secondary transfer process, the secondary transfer roller 8 receives a secondary transfer voltage (secondary transfer bias) which is a DC voltage having a polarity opposite to the normal charging polarity of the toner from the secondary transfer power supply (high voltage power supply circuit) 20. ) Is applied. The recording material P is contained 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 roller 9. The recording material P is temporarily stopped by the registration roller 9, and then is supplied to the secondary transfer portion N2 in time with the toner image on the intermediate transfer belt 7.

  The recording material P on which the toner image is transferred is conveyed to a fixing device 10 as a fixing unit by a conveying member or the like. The fixing device 10 fixes (melts and fixes) the toner image on the recording material P by heating and pressing the recording material P carrying the unfixed toner image. After that, the recording material P is discharged (output) to the outside of the main body of the image forming apparatus 100.

  Further, the toner (primary transfer residual toner) remaining on the surface of the photosensitive drum 1 after the primary transfer step is removed from the surface of the photosensitive drum 1 by a drum cleaning device 6 as a photosensitive member cleaning unit and collected. Further, the toner (secondary transfer residual toner) remaining on the surface of the intermediate transfer belt 7 after the secondary transfer step, and the adhered substances such as paper powder are removed from the intermediate transfer belt 7 by a belt cleaning device 74 as an intermediate transfer member cleaning unit. It is 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 surface side to the outer peripheral surface side. As the resin material forming the resin layer, polyimide, polycarbonate, or the like can be used. The thickness of the resin layer is preferably 70 to 100 μm. Further, as the elastic material forming the elastic layer, urethane rubber, chloroprene rubber or the like can be used. The thickness of the elastic layer is preferably 200 to 250 μm. Further, as the material of the surface layer, a material that reduces the adhesive force of the toner to the surface of the intermediate transfer belt 7 and facilitates the transfer of the toner to the recording material P at the secondary transfer portion N2 is desirable. For example, one kind or two or more kinds of resin materials such as polyurethane, polyester, and epoxy resin can be used. Alternatively, one kind or two or more kinds of elastic materials (elastic material rubber, elastomer), butyl rubber and the like can be used. Further, in addition to these materials, one or more kinds of materials such as powders and particles of fluororesin, etc., which reduce surface energy and improve lubricity, or one or more kinds of these powders and particles are used. The particles having different particle sizes can be dispersed and used. The thickness of the surface layer is preferably 5 to 10 μm. The electrical resistance of the intermediate transfer belt 7 is adjusted by adding a conductive agent for electrical resistance adjustment such as carbon black, and the volume resistivity is preferably 1 × 10 ⁹ to 1 × 10 ¹⁴ Ω · cm.

In addition, in this embodiment, the secondary transfer roller 8 is configured to include a core metal (base material) and an elastic layer formed of ion conductive foam rubber (NBR rubber) around the core metal. It In this embodiment, the outer diameter of the secondary transfer roller 8 is 24 mm, and the surface roughness Rz of the secondary transfer roller 8 is 6.0 to 12.0 (μm). Further, in this embodiment, the electric resistance value 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), the elastic layer. The hardness is about 30 to 40 ° in Asker-C hardness. Further, in this embodiment, the width of the secondary transfer roller 8 in the longitudinal direction (rotational axis direction) (the length in the direction substantially orthogonal to the conveying direction of the recording material P) is about 310 to 340 mm. In this embodiment, the width of the secondary transfer roller 8 in the longitudinal direction is the maximum width (the length in the direction substantially orthogonal to the carrying direction) of the recording material P that the image forming apparatus 100 guarantees the carrying ( Maximum width) longer. In this embodiment, since the recording material P is conveyed with the center of the secondary transfer roller 8 in the longitudinal direction as a reference, all the recording materials P guaranteed by the image forming apparatus 100 to be conveyed in the longitudinal direction of the secondary transfer roller 8. Pass within the length range. Thereby, it is possible to stably convey the recording materials P of various sizes and to stably transfer the toner image to the recording materials P of various sizes.

  FIG. 2 is a schematic diagram of a configuration related to secondary transfer. The secondary transfer roller 8 forms a secondary transfer portion N2 by contacting the secondary transfer counter roller 73 via the intermediate transfer belt 7. A secondary transfer power source 20 whose output voltage value is variable is connected to the secondary transfer roller 8. The secondary transfer counter roller 73 is electrically grounded (connected to the ground). When 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 regular charging polarity of the toner, is applied to the secondary transfer roller 8, and the secondary transfer portion is applied. By supplying the secondary transfer current to N2, the toner image on the intermediate transfer belt 7 is transferred onto the recording material P. In this embodiment, a secondary transfer current of, for example, +20 to +80 μA is applied to the secondary transfer portion N2 during secondary transfer.

  In the present embodiment, the upper limit value and the 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 information. . As will be described later in detail, the various types of information include the following types of information. First, information regarding conditions specified by the operation unit 31 (FIG. 3) provided in the apparatus body of the image forming apparatus 100 and the external device 200 (FIG. 3) such as a personal computer communicably connected to the image forming apparatus 100. Is. In addition, it is information regarding the detection result of the environment sensor 32 (FIG. 3). In addition, it is information regarding the electric resistance of the secondary transfer portion N2 that is detected before the recording material P reaches the secondary transfer portion N2. Then, while the recording material P is passing through the secondary transfer portion N2, while the secondary transfer current flowing through the secondary transfer portion N2 is being detected, the secondary transfer current becomes a value within the above-mentioned secondary transfer current range. As a result, the secondary transfer voltage output from the secondary transfer power supply 20 by the constant voltage control is controlled. Here, particularly in the present embodiment, the secondary transfer current range is changed based on the information regarding the thickness of the recording material P passing through the secondary transfer portion N2 and the width of the recording material P. In this embodiment, the information about the thickness of the recording material P and the width of the recording material P is acquired based on the 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 the information acquired by this detection means.

  In the present embodiment, in order to perform such control, the secondary transfer power source 20 has a current detection unit (secondary transfer current) that detects a current (secondary transfer current) flowing through the secondary transfer portion N2 (secondary transfer power source 20). A current detection circuit 21 as a detection unit) is connected. Further, the secondary transfer power supply 20 is connected to a voltage detection circuit 22 as a voltage detection unit (detection unit) that detects a voltage (transfer voltage) output from the secondary transfer power supply 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 substrate.

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

  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 person 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 are configured. In addition, the control unit 50 is connected to the secondary transfer power supply 20, a current detection circuit 21, and a voltage detection circuit 22. In the present embodiment, the secondary transfer power supply 20 applies a secondary transfer voltage, which is a constant voltage-controlled DC voltage, to the secondary transfer roller 8. Further, the environment sensor 32 is 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 the 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 detecting unit that detects at least one of temperature and humidity inside or outside the image forming apparatus 100. The control unit 50 comprehensively controls each unit of the image forming apparatus 100 based on the image information from the image reading device or the external device 200 and the control command from the operation unit 31 or the external device 200 to execute the image forming operation. Let

  Here, the image forming apparatus 100 executes a job (print 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 (print instruction). To do. The job generally includes an image forming step, a pre-rotating step, a sheet interval step when forming images on a plurality of recording materials P, and a post-rotating step. The image forming step is a period in which an electrostatic image of an image actually formed on the recording material P and output is formed, a toner image is formed, a primary transfer of the toner image is performed, and a secondary transfer of the toner image is performed. The formation period) means this period. More specifically, the timing of image formation is different at the positions where the steps of forming the electrostatic image, forming the toner image, primary transfer of the toner image, and secondary transfer are performed. The pre-rotation step is a period during which a preparatory operation before the image forming step is performed from the input of the start instruction to the actual start of image formation. The sheet interval process is a period corresponding to the recording material P between the recording materials P when images are continuously formed on a plurality of recording materials P (continuous image formation). The post-rotation step is a period during which the rearrangement operation (preparation operation) is performed after the image forming step. The non-image forming period (non-image forming period) is a period other than the image forming period, and includes the above-described pre-rotation step, sheet-to-sheet step, post-rotation step, and even when the image forming apparatus 100 is powered on or in a sleep state. The pre-multi-rotation step, which is a preparatory operation at the time of restoration, is included. In this embodiment, control is performed to determine the upper limit value and the lower limit value (“secondary transfer current range”) of the secondary transfer current during non-image formation.

3. Effect of overshoot A factor that changes the electrical resistance of recording materials such as paper is the change in the amount of water contained in the recording material. The greater the difference between the environment when the recording material is packed in the manufacturing process and the environment in which the image forming apparatus is placed when the user uses the image forming apparatus, the more the amount of water in the recording material changes. growing. For example, when the recording material has a water content of 5.0 to 6.0% while the recording material is used in a low humidity environment (eg, temperature 23 ° C., 5% RH). The water content of the recording material may be 3.0% or less. Further, when the change in the water content of the recording material is the same, the change in the electric resistance of the recording material is larger as the thickness of the recording material is larger, and the change amount of the transfer voltage for adjusting the transfer current to an appropriate range is smaller. growing. For example, when the weight of the thick paper having a basis weight of 300 g / m ² or more changes from the state of the water content of 5.0 to 6.0% to the state of 3.0% or less as described above, the transfer current is set to an appropriate range. The amount of change in the transfer voltage required for adjustment may be 1000 V or more. At this time, if it is attempted to change the transfer voltage all at once during transfer, an overshoot phenomenon in which the transfer voltage once greatly increases to a value that exceeds the target and then drops to the target value appears (see FIG. 17). . When this overshoot phenomenon of the transfer voltage occurs, the overshoot phenomenon also occurs in the current flowing by applying the transfer voltage. Here, the magnitude of the overshoot current indicating the degree of the overshoot phenomenon occurring in the current is defined as follows. That is, as shown in FIG. 17, when the transfer voltage is changed (switched) for a predetermined time, the current value (absolute value) when the current converges to a steady state, and the transfer voltage change It is defined as the difference between the peak value (maximum absolute value) of the current that temporarily increases during operation.

  For example, in the configuration of the example that was tested, the overshoot current when the transfer voltage fluctuates by 1000 V may be several tens of μA. On the other hand, in the same configuration, the range of the appropriate transfer current is 15 to 20 μA when the width of the recording material corresponds to A4 size. That is, in the case of this configuration, when the recording material placed in a low humidity environment as described above is used, the transfer voltage may be as low as 1000 V in order to keep the transfer current within an appropriate range. Then, when the operation for raising the transfer voltage by the shortage was performed all at once, an overshoot current of several 10 μA was generated as described above.

  When the transfer voltage is controlled by detecting the current when the recording material is passing through the transfer portion, typically, the detection of the current and the change of the transfer voltage are performed as shown in FIG. . That is, the detection time (first period) for detecting the current flowing in the transfer portion and the response time (second time) for waiting for the response after the signal for changing the transfer voltage based on the detection result of the current in the detection time is output. Period) and are repeated. When an overshoot current of several tens of μA is generated as described above, the overshoot current does not converge to a steady state within the response time, and it may be determined that a current higher than the upper limit value is flowing in the next detection time. is there. In this case, the operation of lowering the transfer voltage is performed in the next response time. Therefore, in the steady state, the current should have been an appropriate value, but the operation to lower the transfer voltage was performed.As a result, the current was insufficient and image defects such as low density (density step) occurred. I will end up. Further, the transfer voltage overshoot phenomenon occurs not only when the transfer voltage is raised but also when the transfer voltage is lowered. Therefore, in the next detection time, it may be determined that a current lower than the lower limit value is flowing contrary to the above. In this case, the operation of raising the transfer voltage at the next response time is performed. Therefore, as a result of the operation to increase the transfer voltage, which should have been an appropriate current in the steady state, the current becomes rather excessive and local image defects such as white spots due to the discharge phenomenon occur. Will occur. Then, at the next detection time, it is again determined that a current higher than the upper limit value is flowing, and thereafter, the same current shortage state and excess state are repeated, and the transfer voltage control operation converges. Things that don't happen can happen. When the transfer current is excessively and deficiently repeated in this manner, a density difference such as a low density and a local image defect due to a discharge phenomenon are repeatedly generated, which leads to deterioration in image quality.

4. Appropriate change in secondary transfer current range due to fluctuations in non-sheet passing current By the way, when the recording material is passing through the transfer portion, the current flowing through the transfer portion is "paper passing current (passing portion current)". And "non-sheet passing portion current (non-passing portion current)". The sheet passing portion current is a current flowing in a region (“sheet passing portion (passing region)”) of the transfer portion where the recording material passes in a direction substantially orthogonal to the recording material transport direction. Further, the non-sheet passing portion current is a current flowing in an area ("non-sheet passing portion (non-passing area)") of the transfer portion where the recording material does not pass in a direction substantially orthogonal to the conveying direction of the recording material. The non-sheet passing portion occurs because a transfer member such as a transfer roller has a length in the longitudinal direction in order to stably convey and transfer a toner image to recording materials of various sizes. This is because the width is made larger than the maximum width of the recording material guaranteed by.

  Further description will be given with reference to FIG. The current flowing through the secondary transfer portion N2 when the recording material P is passing through the secondary transfer portion N2 includes a paper passing portion current (I_paper passing portion) and a non-paper passing portion current (I_non-paper passing portion). ), And there is. The current that can be detected when the recording material P is passing through the secondary transfer portion N2 is the sum of the paper passing portion current and the non-paper passing portion current. In order to suppress the above-described image defects such as low image density and white spots, it is important that the paper passing portion current is in an appropriate range, but it is not possible to detect only the paper passing portion current. . Therefore, appropriate upper and lower limit values (“secondary transfer current range”) of the secondary transfer current are determined in advance for each size of the recording material P, and the secondary transfer portion N2 is set in accordance with the size of the recording material P. It is conceivable to control the secondary transfer current while the recording material P is passing to a value within the range of the secondary transfer current. However, even if an appropriate secondary transfer current range is determined in advance, the electric resistance of the secondary transfer roller 8 forming the non-sheet passing portion varies under various conditions. These various conditions include product variations, environment (temperature / humidity), member temperature / humidity, cumulative use time (operation status of the image forming apparatus and repeated usage). Therefore, the appropriate secondary transfer current range changes due to the change in the electrical resistance of the secondary transfer roller 8.

  FIG. 14A shows a secondary transfer current range for each size of the recording material P, which is determined in advance by an experiment or the like. In order to sufficiently suppress image defects, the range of the current that may be applied to the paper 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. In the case of (paper), it was 15 to 20 μA. In the case of the recording material P (paper) having a width corresponding to the A5R size (148.5 mm), the width was smaller than that of the A4 size by the amount of 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 is passing through the secondary transfer portion N2, the range of the current flowing to the non-sheet passing portion is 3.6 to 4.4 μA for the A4 size, and 16 for the A5R size. It was 6 to 20.3 μA. Therefore, when the recording material P is passing through the secondary transfer portion N2, the range of the current that can be passed through the secondary transfer portion N2 (“secondary transfer current range”) is 18.6 to A4 size. 24.4 μA and A5R size were set to 24.1 to 30.3 μA.

  However, for example, when the electrical resistance of the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment) becomes low, the current flowing in the non-sheet passing portion increases. FIG. 14B is an example of an appropriate secondary transfer current range when the electric resistance of the secondary transfer portion N2 is lower than that in the state when the secondary transfer current range shown in FIG. 14A is determined. Indicates. Even if the electric resistance of the secondary transfer portion N2 becomes low, the range of the current that can be passed through the paper passing portion does not change. However, when the electric resistance of the secondary transfer portion N2 becomes low, the secondary transfer current, which is the sum of the paper passing portion current and the non-paper passing portion current, has an upper limit value and Any of the lower limits will shift higher. For example, consider a case where the secondary transfer current when the recording material P of A5R size passes through the secondary transfer portion N2 is 24.5 μA. In this case, if the electric resistance of the secondary transfer roller 8 is the same as the state when the secondary transfer current range shown in FIG. 14A is determined, the secondary transfer current is a value in an appropriate secondary transfer current range. Therefore, an appropriate current flows through the paper passing portion. However, when the electric resistance of the secondary transfer roller 8 is as low as the state in which the secondary transfer current range shown in FIG. 14B is appropriate, the secondary transfer current remains 24.5 μA. Then, the secondary transfer current is smaller than the lower limit value (26.9 μA) of the appropriate secondary transfer current range. Therefore, the current flowing through the sheet passing portion may be insufficient, and an image defect may occur.

  That is, in the case of the secondary transfer current value near the lower limit value when the electric resistance of the non-paper passing portion is a certain value, even if there is no problem as long as the electric resistance state of the non-paper passing portion is satisfied, When the electric resistance is low, the current in the sheet passing portion is out of the lower limit value that can suppress image defects. On the contrary, when the electric resistance of the secondary transfer portion N2 becomes high, 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 shift to a lower level. Therefore, in the case of a secondary transfer current value near the upper limit value when the electric resistance of the non-paper passing portion is a certain value, even if there is no problem as long as the electric resistance of the non-paper passing portion is sufficient, In the state where the electric resistance is high, the current of the sheet passing portion deviates from the upper limit value capable of suppressing image defects.

  In addition, when the recording material used for image formation is a relatively thick recording material such as thick paper, the pressure of the non-sheet passing portion decreases due to the thickness of the recording material. However, the recording material may deviate from the value predicted before reaching the transfer portion.

FIG. 15 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, which occurs when the recording material P passes. In the example shown in FIG. 15, the width of the recording material P is 300 mm. A plot shown by a broken line in FIG. 15 is a result of measuring the pressure distribution of the secondary transfer portion N2 when the recording material P does not exist in the secondary transfer portion N2. On the other hand, the plot shown by the solid line in FIG. 15 shows that the recording material P having a basis weight of 300 g / m ² and a width of 105 mm passes near the center of the secondary transfer portion N2 in the direction substantially orthogonal to the conveying direction of the recording material P. It is the result of measuring the pressure distribution of the secondary transfer portion N2 during the operation. The pressure distribution (broken line in FIG. 15) of the secondary transfer portion N2 when the recording material P does not exist in the secondary transfer portion N2 is substantially uniform in the direction substantially orthogonal to the conveying direction of the recording material P. However, when the recording material P is present in the secondary transfer portion N2, the pressure in the sheet passing portion (around the center of the solid line in FIG. 15) is higher than when the recording material P is not present. On the other hand, the pressure of the non-sheet passing portion (the area other than the center of the solid line in FIG. 15) is lower than that 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 conveyance direction of the recording material P, so that the current that flows even if the same secondary transfer voltage is applied. It gets smaller. Without considering this phenomenon, if the transfer current range is determined based on the non-sheet passing portion current predicted from the electric resistance of the secondary transfer portion N2 detected before the recording material P reaches the secondary transfer portion N2, The transfer current range may be unnecessarily high. As a result, when the transfer current becomes too large, an image defect due to a discharge phenomenon is likely to occur.

5. Secondary Transfer Voltage Control Next, the control of the secondary transfer voltage in this embodiment will be described. FIG. 4 is a flow chart showing the outline of the procedure for controlling the secondary transfer voltage in this embodiment. FIG. 4 shows a simplified procedure relating to the control of the secondary transfer voltage among the controls executed by the control unit 50 when executing a job, and many other controls when executing a job are not shown. 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 job operation (S101). In this embodiment, the job information includes image information designated by the operator, size (width, length) of the recording material P on which an image is formed, and information (thickness or thickness) related to the thickness of the recording material P. It includes information related to the surface property of the recording material P, such as basis weight) and whether or not the recording material P is coated paper. That is, the information includes the paper size (width, length) and the paper type category (plain paper, cardboard, etc. (including information related to the thickness)). The control unit 50 writes the information of this job in the RAM 52 (S102).

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

  The target current Ittarget is changed according to the environmental information because the charge amount of the toner changes depending on the environment. The information indicating the relationship between the environmental information and the target current Ittarget is obtained in advance by experiments or the like. Here, the charge amount of the toner may be affected not only by the environment but also by the history of use such as the timing of replenishing the toner to the developing device 4 and the amount of toner coming out of the developing device 4. In order to suppress these influences, the image forming apparatus 100 is configured so that the charge amount of the toner in the developing device 4 becomes a value within a certain range. However, if the factors that influence the charge amount of the toner on the intermediate transfer belt 7 are known in addition to the environmental information, the target current Ittarget may be changed depending on the information. Further, the image forming apparatus 100 may be provided with a measuring unit for measuring the toner charge amount, and the target current Ittarget may be changed based on the information on the toner charge amount obtained by the measuring unit.

  Next, the control unit 50 provides information about the electric resistance of the secondary transfer portion N2 before the toner image on the intermediate transfer belt 7 and the recording material P to which the toner image is transferred reach the secondary transfer portion N2. It is acquired (S105). In this embodiment, the information about the electric resistance of the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment) is acquired by ATVC control (Active Transfer Voltage Control). That is, in a state where the secondary transfer roller 8 and the intermediate transfer belt 7 are in contact with each other, the secondary transfer power source 20 supplies a predetermined voltage or current to the secondary transfer roller 8. Then, the current value when the predetermined voltage is being supplied or the voltage value when the predetermined current is being supplied is detected, and the relationship between the voltage and the current (voltage-current characteristic) is acquired. The relationship between the voltage and the current changes according to the electric resistance of the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment). In the configuration of the present embodiment, the relationship between the voltage and the current is not such that the current changes linearly (proportional) with respect to the voltage, but the current is expressed by a polynomial of the second or higher order as shown in FIG. It changes as it is done. Therefore, in this embodiment, the predetermined voltage or current supplied when acquiring the information about the electrical resistance of the secondary transfer portion N2 is three points so that the relationship between the voltage and the current can be expressed by a polynomial. The above is the multi-step.

  Next, the control unit 50 obtains the voltage value to be applied to the secondary transfer roller 8 from the secondary transfer power source 20 (S106). That is, the control unit 50, based on the target current Itarget written in the RAM 52 in S104 and the relationship between the voltage and the current obtained in S105, detects the target current in the state where the recording material P is not present in the secondary transfer unit N2. A voltage value Vb required to flow Ittarget is obtained. This voltage value Vb corresponds to the secondary transfer portion voltage bearing. Further, the ROM 53 stores information for obtaining the recording material sharing voltage Vp as shown in FIG. In the present embodiment, this information is set as table data showing the relationship between the moisture content of the atmosphere and the recording material sharing voltage Vp for each classification of the basis weight of the recording material P. The control unit 50 can obtain the amount of water in the atmosphere based on the environmental information (temperature / humidity) detected by the environmental sensor 32. The control unit 50 obtains the recording material sharing voltage Vp from the table data based on the basis weight information of the recording material P included in the job information acquired in S102 and the environmental information acquired in S103. . Then, the control unit 50 sets the above-mentioned Vb as the initial value of the secondary transfer voltage Vtr applied to the secondary transfer roller 8 from the secondary transfer power source 20 when the recording material P passes through the secondary transfer unit N2. The sum of Vp and Vb + Vp is obtained, 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 the recording material P is ready for the timing of reaching the secondary transfer portion N2.

  The table data for obtaining the recording material sharing voltage Vp as shown in FIG. 6 is obtained in advance by experiments or the like. Here, the recording material sharing voltage (transfer voltage of the electric resistance of the recording material P) Vp changes not only with the information (grammage) related to the thickness of the recording material P but also with the surface property of the recording material P. Sometimes. Therefore, the table data may be set such that the recording material sharing voltage Vp also changes depending on the information related to the surface property of the recording material P. Further, in this embodiment, the information related to the thickness of the recording material P (further, the information related to the surface property of the recording material P) is included in the job information acquired in S101. However, the image forming apparatus 100 may be provided with a measuring means for detecting the thickness of the recording material P and the surface property of the recording material P, and the recording material sharing voltage Vp may be obtained based on the information obtained by this measuring means. .

  Next, the control unit 50 performs a process of determining the upper limit value and the lower limit value (“secondary transfer current range”) of the secondary transfer current when the recording material P is passing through the secondary transfer unit N2 ( S107). FIG. 4B shows a procedure of processing for determining the secondary transfer current range in S107 of FIG. 4A. As shown in FIG. 7, the ROM 53 has a range of current that can be passed through the paper passing portion while the recording material P is passing through the secondary transfer portion N2 from the viewpoint of suppressing image defects (“paper passing portion current”). Information for determining the range (passage current range) ”is stored. In the present embodiment, this information is set as table data indicating the relationship between the moisture content of the atmosphere and the upper limit value and the lower limit value of the current that may be passed through the paper passing portion. It should be noted that this table data is obtained in advance by experiments or the like. Referring to FIG. 4B, the control unit 50 obtains the range of the current that may be passed through the sheet 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 paper passing portion changes depending on the width of the recording material P. In the present 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 the current that may be passed through the paper passing portion may change depending on the thickness and surface property of the recording material P as well as the environmental information. Therefore, the table data may be set such that the range of the current changes depending on the information (grammage) related to the thickness of the recording material P and the information related to the surface property of the recording material P. The range of the current that may be passed through the paper passing portion may be set as a calculation formula. Further, the range of the current that may be passed through the paper passing portion may be set as a plurality of table data or calculation formulas for each size of the recording material P.

Next, the control unit 50 corrects the range of the current that may be passed through the sheet passing portion acquired in S201 based on the width information 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 corresponding to A5 vertical feeding (148.5 mm), that is, a half width of a width corresponding to A4 size, the upper limit value and the lower limit value acquired in S201. Are halved, and the current is proportional to the width of the recording material P. That is, the upper limit value of the sheet passing portion current before correction obtained from the table data of FIG. 7 is Ip_max, the lower limit value is Ip_min, and the width of the recording material P when the table data of FIG. 7 is determined is Lp_bas. Further, it is assumed that the width of the recording material P actually conveyed is Lp, the upper limit value of the corrected sheet passing portion current is Ip_max_aft, and the lower limit value is Ip_min_aft. At this time, the upper limit value and the lower limit value of the corrected sheet passing portion current can be calculated by the following equations 1 and 2, respectively.
Ip_max_aft = Lp / Lp_bas * Ip_max (Equation 1)
Ip_min_aft = Lp / Lp_bas * Ip_min (Equation 2)

Next, the control unit 50 obtains the current flowing through the non-sheet passing portion based on the following information (S203). Information on the width of the recording material P included in the job information acquired in S102, the relationship between the voltage and current of the secondary transfer portion N2 obtained in S105 when the recording material P is not present in the secondary transfer portion N2. Of the secondary transfer voltage Vtr obtained in S106. For example, when the width of the secondary transfer roller 8 is 338 mm and the width of the recording material P acquired in S102 is the width corresponding to A5 vertical feed (148.5 mm), the width of the non-sheet passing portion is the secondary transfer roller. It is 189.5 mm obtained by subtracting the width of the recording material P from the width of 8. Then, it is assumed 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 obtained in S105 and the current. In this case, the current flowing in 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 flowing through the non-sheet passing portion is calculated by proportionally reducing the current 40 μA corresponding to the secondary transfer voltage Vtr by the ratio of the width 189.5 mm of the non-sheet passing portion to the width 338 mm of the secondary transfer roller 8. Can be asked.

When the thickness of the recording material P is relatively small, the value obtained in S203 can be used as the non-sheet passing portion current. However, as the thickness of the recording material P increases, the pressure in the non-sheet passing portion when the recording material P exists in the secondary transfer portion N2 decreases, and the non-sheet passing current decreases accordingly. Therefore, in the present embodiment, the control unit 50 performs control for correcting the non-sheet passing portion current according to the thickness of the recording material P (S204). The non-passing portion current before correction obtained in S203 is Inp_bef, the non-passing portion current after correction is Inp_aft, and the correction coefficient is e (%). At this time, the corrected non-sheet passing portion current can be obtained by the following Expression 3.
Inp_aft = e * Inp_bef (Equation 3)

Here, in the present embodiment, the correction coefficient e in the above expression 3 is recorded for each classification of the basis weight of the recording material P 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 the table data shown in FIG. 8 based on the information of the width of the recording material P and the basis weight of the recording material P included in the job information acquired in S102. To do. The thicker the recording material P, the lower the pressure in the non-sheet passing portion. In consideration of this, the correction coefficient e is set such that the larger the thickness of the recording material P, the smaller the corrected non-sheet passing portion current. Further, as the width of the recording material P is larger, the intermediate transfer belt 7 and the secondary transfer roller 8 in the non-sheet passing portion are less likely to come into contact with each other, and the pressure in the non-sheet passing portion becomes lower. In consideration of this, the correction coefficient e is set so that the larger the width of the recording material P, the smaller the corrected non-sheet passing portion current. For example, when the width of the recording material P is equivalent to A5 vertical 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%. Becomes the corrected non-sheet passing portion current Inp_aft. On the other hand, for example, in the case where the width of the recording material P is equivalent to the A5 vertical feed (148.5 mm) similar to the above, and the basis weight of the recording material P is 52 g / m ² , non-sheet passing before correction is performed. The sheet current Inp_bef maintained at 100% is the corrected non-sheet passing portion current Inp_aft.

Next, the control unit 50 determines the upper limit value and the lower limit value (“secondary transfer current range”) of the secondary transfer current when the recording material P is passing through the secondary transfer unit N2 as follows. The obtained secondary transfer current range is stored in the RAM 52 (S205). That is, the control unit 50 adds the corrected non-sheet passing portion current obtained in S204 to each of the upper limit value and the lower limit value of the sheet passing portion current obtained in S202, and the recording material P is set to the secondary transfer portion N2. An upper limit value and a lower limit value (“secondary transfer current range”) of the secondary transfer current when passing is determined. That is, the upper limit value of the secondary transfer current when the recording material P is passing through the secondary transfer portion N2 is I_max, and the lower limit value is I_min. At this time, the upper limit value and the lower limit value of the secondary transfer current can be calculated by the following equations 4 and 5, respectively.
I_max = Ip_max_aft + Inp_aft (Equation 4)
I_min = Ip_min_aft + Inp_aft (Equation 5)

For example, consider a case where the upper limit of the range of the current that may be passed through the sheet passing portion corresponding to the width corresponding to the A4 size acquired in S201 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 a width corresponding to the A5 vertical feed, the upper limit value of the range of the current that may be passed through the paper passing portion is 10 μA and the lower limit value is 7.5 μA. Become. Then, when the current flowing in the non-sheet passing portion obtained in S203 is 22.4 μA as in the above example, when the recording material P is a thick paper having a basis weight of 350 g / m ² , the above-mentioned 22.4 μA Is corrected to 85%, and the corrected non-sheet passing portion current becomes 19 μA. In this case, the upper limit value of the secondary transfer current range is 29 μA and the lower limit value is 26.5 μA. On the other hand, when the current flowing in the non-sheet passing portion obtained in S203 is 22.4 μA as described above and the recording material P is a paper having a basis weight of 52 g / m ² , the non-sheet passing portion current after correction is The current 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. 4A, next, the control unit 50 performs a process of determining the fluctuation width ΔV of the secondary transfer voltage during the secondary transfer (S108). In this embodiment, when the recording material P is passing through the secondary transfer portion N2 and the current detected by the current detection circuit 21 is out of the secondary transfer current range, the detected current is The secondary transfer voltage is changed so that the value falls within the range of the secondary transfer current. In this operation, as shown in FIG. 17, the current is detected during a predetermined detection time (first period), and the secondary transfer is performed based on the detection result during a predetermined response time (second period) following the detection period. This is done by repeating the operation of changing the voltage. In this operation, the control unit 50 outputs a signal for changing the voltage output to the secondary transfer power supply 20 based on the signal indicating the current detection result input from the current detection circuit 21 during the detection period. Done. It is preferable that the detection time and the response time are as short as possible because it is possible to reduce the time (area) in which the secondary transfer current may deviate from the secondary transfer current range and an image defect may occur. The detection time and the response time are preferably about 10 msec or less, depending on the performance of the high-voltage substrate. In this embodiment, the detection time and the response time are each set to 8 msec. However, as described above, if the overshoot current when the secondary transfer voltage is changed during the response time is large, a current deviating from the steady state is detected in the detection time subsequent to the response time, and the secondary current is detected. The transfer voltage control may not converge normally.

Therefore, in this embodiment, the secondary transfer voltage at the response time during the secondary transfer is based on the information about the electric resistance of the secondary transfer portion N2 detected before the recording material P reaches the secondary transfer portion N2. The variation width ΔV of is changed. In general, when the electric resistance of the secondary transfer portion N2 is relatively large, the fluctuation width ΔV of the secondary transfer voltage is set to be larger than when it is relatively small. As a result, the overshoot current can be suppressed within an appropriate range according to the electric resistance of the secondary transfer portion N2. Further, in the present embodiment, based on the range (difference between the upper limit value and the lower limit value) of the current supplied to the secondary transfer portion N2 during the secondary transfer, the secondary transfer voltage of the response time during the secondary transfer is determined. Change the fluctuation width ΔV. In general, when the current range is relatively large, the fluctuation width ΔV of the secondary transfer voltage is set to be larger than that when the current range is relatively small. As a result, the fluctuation width ΔV of the secondary transfer voltage is sufficiently increased while suppressing the overshoot current from becoming excessively large, and the secondary transfer current is set to a value within an appropriate range by changing the secondary transfer voltage. It is possible to reduce the time required for Specifically, in this embodiment, the control unit 50 obtains information on the relationship between the voltage and current of the secondary transfer portion N2 in the state where the recording material P is not present in the secondary transfer portion N2, which is obtained in S105, and S201. The fluctuation range ΔV of the secondary transfer voltage during the response time is determined based on the range of the current obtained in (S108). That is, in this embodiment, the fluctuation width ΔV of the secondary transfer voltage is calculated by the following equation 6.
ΔV = A * (Ip_max-Ip_min) * Vb / Itarget ... (Equation 6)

  In the above equation 6, Vb is the secondary transfer portion voltage bearing obtained in S105, Ittarget is the target current obtained in S104, and Vb / Itarget is 2 when the recording material P is not present in the secondary transfer portion N2. It corresponds to the electric resistance of the next transfer portion N2. Further, in the above expression 6, Ip_max and Ip_min are the upper limit value and the lower limit value of the sheet passing portion current range in the width (297 mm) corresponding to the A4 size obtained in S201, respectively. Further, in the above equation 6, A is the upper limit value and the lower limit value of the current that supplies the overshoot current generated when the secondary transfer voltage is changed as shown in FIG. 17 to the secondary transfer portion N2 during the secondary transfer. This is a correction coefficient for suppressing the difference to or below. The correction coefficient A has been previously obtained by experiments or the like. In this embodiment, the response time is 8 msec, but the correction coefficient A is set to about 1 to 2. As a result, in the present embodiment, when the secondary transfer voltage is changed, the state in which the current is below the lower limit value exceeds the upper limit value, and conversely, the state is above the upper limit value and below the lower limit value. It can be suppressed. In the present embodiment, the target current Ittarget is about 15 to 20 μA, while the fluctuation width ΔV of the secondary transfer voltage varies depending on the electric resistance of the secondary transfer portion N2 according to the above equation 6, but is about 20 to 100 V. It becomes a value of the degree. Further, the range of the current supplied to the secondary transfer portion N2 during the secondary transfer (difference between the upper limit value and the lower limit value) is the narrower the width of the recording material P when the type of the recording material P is the same. large. Therefore, the fluctuation width ΔV of the secondary transfer voltage may be changed depending on the width of the recording material.

  Next, the controller 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. Is detected by the current detection circuit 21 (S109). Further, the control unit 50 compares the detected secondary transfer current value with the secondary transfer current range obtained in S107, and corrects the secondary transfer voltage Vtr output from the secondary transfer power supply 20 as necessary. (S110, S111, S112). As described above, in the present embodiment, the operation of changing the secondary transfer voltage is performed by alternately setting the detection time and the response time during the secondary transfer with the detection time of 8 msec and the response time of 8 msec. . That is, the control unit 50 outputs the secondary transfer power source 20 when the secondary transfer current value detected during the detection time is the value of the secondary transfer current range obtained in S107 (lower limit value or more and upper limit value or less). The secondary transfer voltage Vtr that has been changed is maintained as it is (S111). On the other hand, when the secondary transfer current value detected during the detection time is out of the secondary transfer current range obtained in S107 (below the lower limit value or above the upper limit value), the control unit 50 performs the secondary transfer during the response time. The voltage Vtr is changed (S112). In this embodiment, when the detected secondary transfer current value is lower than the lower limit value, the secondary transfer voltage Vtr is increased by the fluctuation width ΔV, and conversely, when it is higher than the upper limit value, the secondary transfer voltage is increased. Vtr is reduced by the fluctuation width ΔV. At this time, the fluctuation width ΔV obtained in S108 is used as the fluctuation width ΔV of the secondary transfer voltage. Then, when the response time has elapsed and the recording material P is passing through the secondary transfer portion N2 (S113), the control unit 50 returns the process to S109 and detects the secondary transfer current (detection time). Then, the secondary transfer voltage is changed (response time) as necessary (S109 to S112). The detection time and the response time are repeated while the recording material P is present in the secondary transfer portion N2 (more specifically, while the image forming area of the recording material P passes through the secondary transfer portion N2). As a result, the secondary transfer voltage Vtr is corrected so that the secondary transfer current detected when the recording material P passes through the secondary transfer portion N2 falls within the secondary transfer current range obtained in S107. Go.

  Further, the control unit 50 repeats the processing of S109 to S113 until all the images of the job are transferred to the recording material P and output is completed (S114).

  The change in the secondary transfer current range due to the control of this embodiment will be further described. Let us consider a case where 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 about the same, and the secondary transfer voltages required for the secondary transfer are about the same. At this time, the secondary transfer current range when the recording material P having a width smaller than the maximum width is used is higher than the secondary transfer current range when the recording material P having the maximum width is used (absolute current Shift (to increase the value). However, this shift amount decreases as the thickness of the recording material P increases.

For example, consider a case where paper having a basis weight of 52 g / m ² (thin paper) and paper having a basis weight of 350 g / m ² (thick paper) are used as the recording material P, respectively. Further, the result of detecting the electric resistance of the secondary transfer portion N2 before the recording material P reaches the secondary transfer portion N2 is almost the same in any case, and it is assumed that a current of 30 μA flows when 1000 V is applied. . At this time, in the case of the paper having the basis weight of 52 g / m ² , the secondary transfer current range in the case of the A4 size (width 297 mm) is 24.9 to 19.9 μA, but the secondary transfer current range of the A5 vertical feed size (width 148.5 mm) In this case, the secondary transfer current range is 32.3 to 29.8 μA. That is, in the case of a paper having a basis weight of 52 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, and the lower limit value becomes about 10 μA higher. On the other hand, for a paper having 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 vertical feed size (width 148.5 mm) Is 29 to 26.5 μA. That is, in the case of a 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, but only the lower limit value is increased to about 6.5 μA. The shift amount is smaller than that in the case of 52 g / m ² paper.

In practice, as shown in FIG. 6, the recording material P having a larger thickness tends to have a higher electric resistance, and the secondary transfer voltage Vtr required at the time of the secondary transfer tends to become higher. Therefore, when thick paper is used and when thin paper is used, the secondary transfer voltage Vtr required at the time of secondary transfer is larger when thick paper is used. When the secondary transfer voltage Vtr is large, the secondary transfer current when the recording material P is not present in the secondary transfer portion N2 is also large, and the amount of change in the secondary transfer current range when the size of the recording material P is changed is also large. . FIG. 9 is a lower limit value of the secondary transfer current range in the case of the A5 vertical feed size when the initial secondary transfer voltage Vtr determined in S106 of FIG. 4A is changed in the configuration of this embodiment. FIG. 5 is a graph diagram in which the difference between the lower limit of the secondary transfer current range and the lower limit of the A4 size is plotted. The broken line in FIG. 9 is a plot for paper having a basis weight of 52 g / m ² , and the solid line is a plot for paper having 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 result is as follows. 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 at a certain secondary transfer voltage Vtr is smaller in the recording material P having a larger thickness as shown in FIG. There is.

  In this embodiment, information about the electric resistance of the secondary transfer portion N2 in the state where the recording material P is not present in the secondary transfer portion N2 is detected, and the current flowing when the voltage is actually applied to the secondary transfer portion is detected. 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 the environmental information in advance, such as the relationship between the output value of the environment sensor 32 and the electrical resistance of the secondary transfer portion N2. The information for this can be created as table data. Then, based on the output value of the environment sensor 32, the electric resistance of the secondary transfer portion N2 can be obtained by referring to the table data and the like.

  As described above, the image forming apparatus 100 according to the present exemplary embodiment applies the predetermined voltage to the transfer member 8 when the recording material P passes through the detection unit 21 that detects the current flowing in the transfer member 8 and the transfer unit N2. As described above, the control unit 50 performs constant voltage control of the voltage applied to the transfer member 8. The control unit 50 can change the transfer voltage applied to the transfer member 8 so that the current flowing through the transfer member 8 during transfer is within the predetermined range. Then, when changing the transfer voltage applied to the transfer member 8 so that the current flowing through the transfer member 8 falls within the predetermined range during transfer, the control unit 50 changes the transfer voltage by a predetermined change in the transfer voltage. It is designed to be performed for each width. Further, the control unit 50 changes the fluctuation range based on the detection result detected by the detection unit 21 when a voltage is applied to the transfer member 8 in the state where the recording material P is not present in the transfer unit N2. In the present embodiment, the control unit 50, when the current value flowing through the transfer member 8 when the predetermined voltage is applied to the transfer member 8 in the state where the recording material P is not present in the transfer unit N2 is the first current value, When the fluctuation range is set to the first fluctuation range and the current value flowing through the transfer member 8 is the second current value smaller than the first current value, the fluctuation range is larger than the first fluctuation range. Change to. The predetermined value of the amount of change in current may be a preset constant value. In this embodiment, the control unit 50 controls the maximum absolute value of the current flowing through the transfer member 8 after the change of one transfer voltage and before the change of the next transfer voltage, and the change of the one transfer voltage. The difference between the absolute value of the current flowing through the transfer member 8 which is in a steady state after the transfer voltage and before the next change of the transfer voltage is smaller than the difference between the upper limit value and the lower limit value of the predetermined range. Change the fluctuation range. Further, the control unit 50 can change the fluctuation range based on the difference between the upper limit value and the lower limit value of the predetermined range. Further, the control unit 50 can change the fluctuation range based on the width of the recording material P.

  As described above, in the present embodiment, the fluctuation width ΔV when the secondary transfer voltage is changed during the secondary transfer is detected in the state where the recording material P is not present in the secondary transfer portion N2. The variation width ΔV is adjusted to an appropriate value by changing the variation width ΔV on the basis of the information about the electric resistance. In some cases, it may be desirable to change the target current Ittarget and the appropriate secondary transfer current range corresponding to it according to the type of the recording material P and the image forming conditions (conveyance speed, image density). In that case, it is desirable to change the fluctuation width ΔV of the secondary transfer voltage in accordance with the target current Ittarget and an appropriate secondary transfer current range. By changing the variation width ΔV of the secondary transfer voltage in this way, the amount of change in the current flowing through the secondary transfer portion N2 due to the change of the secondary transfer voltage can be suppressed to a predetermined value or less. As a result, it is possible to prevent the current flowing through the secondary transfer portion N2 from changing excessively beyond the appropriate range during the secondary transfer, and to locally change the density difference such as low density or the white spot due to the discharge phenomenon. It is possible to suppress the occurrence of image defects.

  Further, in the present embodiment, when the recording material P passes through the secondary transfer portion N2, the current flowing through the non-sheet passing portion is changed to the secondary transfer portion before the recording material P reaches the secondary transfer portion N2. Predict by obtaining information about the electrical resistance of N2. At this time, the predicted value of the current flowing through the non-sheet passing portion is changed based on the information regarding the width of the recording material P, and the predicted value is corrected based on the information regarding the thickness of the recording material P. More specifically, the correction is performed such that the larger the thickness of the recording material P, the smaller the current flowing through the non-sheet passing portion. This makes it possible to more accurately predict the current flowing through the non-sheet passing portion. Then, the recording material P passes through the secondary transfer portion N2 by adding the predicted current flowing through the non-paper passing portion and the range of current that may be passed through the paper passing portion from the viewpoint of suppressing image defects. The range of the secondary transfer current is determined. In addition, the secondary transfer voltage when the recording material P passes through the secondary transfer portion N2 is controlled so that the value is within the range of the secondary transfer current. As a result, even when the recording material P having a relatively large thickness such as thick paper is used, the secondary transfer portion N2 (mainly the secondary transfer roller 8 in the present embodiment) and the recording material that fluctuate in various situations are used. An appropriate image can be output regardless of the electric resistance of P.

[Example 2]
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus according to the present exemplary embodiment are the same as those of the image forming apparatus according to the first exemplary embodiment. Therefore, in the image forming apparatus according to the present exemplary embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus according to the first exemplary embodiment will be denoted by the same reference numerals as those in the first exemplary embodiment, and detailed description thereof will be omitted. To do.

  In the first embodiment, the range of the current that can be passed through the paper passing portion when the recording material P is passing through the secondary transfer portion N2 (“paper passing portion current range”) and the predicted value of the non-paper passing portion current. The value of (after correction by the thickness of the recording material P) was added to obtain the secondary transfer current range. 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 range. On the other hand, the paper passing portion current is obtained by subtracting the predicted value of the non-paper passing portion current (after correction by the thickness of the recording material P) from the secondary transfer current measured at the time of the secondary transfer. The secondary transfer voltage may be controlled so that the paper portion current becomes a value within a predetermined paper passage portion current range.

  FIG. 10 is a flow chart showing the outline of the procedure for controlling the secondary transfer voltage in this embodiment. The processes of S301 to S306 of FIG. 10 are the same as the processes of S101 to S106 of FIG. The process of S307 of FIG. 10 is the same as the process of S201 of FIG. 4B in the first embodiment. Furthermore, the process of S308 of FIG. 10 is the same as the process of S108 of FIG. Hereinafter, differences from the first embodiment will be described in particular, and description of the same processing as that of the first embodiment will be omitted.

  The controller 50 obtains the fluctuation width ΔV of the secondary transfer voltage in S308, and then, while the recording material P is present in the secondary transfer portion N2 after the recording material P reaches the secondary transfer portion N2, the secondary transfer The secondary transfer current when the transfer voltage Vtr is applied is detected by the current detection circuit 21 (S309).

  Then, the control unit 50 determines the current flowing through the non-sheet passing portion based on the following information (S310). Information about the width of the recording material P included in the job information acquired in S302, and the relationship between the voltage and current of the secondary transfer portion N2 obtained in S305 when the recording material P is not present in the secondary transfer portion N2. Of the secondary transfer voltage Vtr currently applied. The process of obtaining the non-sheet passing portion current in S310 is the same as the process of S203 of FIG. 4B in the first embodiment. However, in S310, the secondary transfer voltage currently applied (the initial value is obtained in S306) 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 S310 is the initial value obtained in S306 at the timing when the first recording material P of the job rushes into the secondary transfer portion N2. . After that, when the secondary transfer voltage Vtr is changed in the following flow, the changed secondary transfer voltage Vtr is used to determine the current flowing through the non-sheet passing portion.

Next, the control unit 50 performs the control for correcting the non-sheet passing portion current according to the thickness of the recording material P, similarly to the processing of S204 of FIG. 4B in the first embodiment (S311). It is assumed that the non-sheet passing current before correction obtained in S310 is Inp_bef, the non-sheet passing current after correction is Inp_aft, and the correction coefficient is e (%). At this time, the corrected non-sheet passing portion current can be obtained by the following Equation 3 similar to the first embodiment.
Inp_aft = e * Inp_bef (Equation 3)

  Here, in the present embodiment, the correction coefficient e in the above equation 3 is determined based on the table data as shown in FIG. 8 similar to the first embodiment.

Next, the control unit 50 subtracts the corrected non-sheet passing portion current obtained in S311 from the secondary transfer current detected in S309 to calculate a current as a sheet passing portion current (S312). That is, assuming that the secondary transfer current is Itr and the sheet passing portion current is Ip, the sheet passing portion current can be obtained by the following equation 8.
Ip = Itr-Inp_aft (Equation 7)

The paper passing portion current Ip obtained by the above equation 7 is a current value corresponding to the width of the recording material P that is actually conveyed, whereas the paper passing portion current range obtained at S307 is the reference recording. The width corresponds to the size of the material P (A4 size in this embodiment). Therefore, in the present embodiment, the control unit 50 performs a process of converting the sheet passing portion current Ip obtained by the above Expression 7 into a current value corresponding to a width corresponding to the size of the recording material P serving as a reference (S313). The width of the recording material P when the table data of FIG. 7 is determined is Lp_bas, the width of the recording material P actually conveyed is Lp, and the converted sheet passing portion current is Ip_aft. At this time, the converted sheet passing portion current can be obtained by the following equation 8.
Ip_aft = Lp_bas / Lp * Ip (Equation 8)

  Next, the control unit 50 compares the converted sheet passing portion current Ip_aft obtained in S512 with the sheet passing portion current range obtained in S307 (S314). Then, the controller 50 corrects the secondary transfer voltage Vtr output from the secondary transfer power supply 20 as necessary (S315, S316). As described above, in the present embodiment, the operation of changing the secondary transfer voltage is performed by alternately setting the detection time and the response time during the secondary transfer with the detection time of 8 msec and the response time of 8 msec. . That is, the control unit 50 outputs the secondary transfer power source 20 when the converted sheet passing portion current Ip_aft is the value of the sheet passing portion current range obtained in S307 (lower limit value or more and upper limit value or less). The secondary transfer voltage Vtr is maintained as it is (S315). On the other hand, when the converted sheet passing portion current Ip_aft is out of the sheet passing portion current range obtained in S307 (below the lower limit value or above the upper limit value), the control unit 50 determines the value of the sheet passing portion current range. The secondary transfer voltage Vtr output from the secondary transfer power supply 20 is corrected so as to be (S316). In the present embodiment, when the converted sheet passing portion current Ip_aft is lower than the lower limit value of the sheet passing portion current range, the secondary transfer voltage Vtr is increased by the fluctuation width ΔV, and conversely exceeds the upper limit value. In this case, the secondary transfer voltage Vtr is reduced by the fluctuation width ΔV. At this time, the fluctuation width ΔV obtained in S308 is used as the fluctuation width ΔV of the secondary transfer voltage. Then, when the response time has elapsed and the recording material P is passing through the secondary transfer portion N2 (S317), the control unit 50 returns the process to S309 and detects the secondary transfer current (detection time). Then, the secondary transfer voltage is changed (response time) as necessary (S309 to S316). When the secondary transfer voltage Vtr is changed, the flow for obtaining the converted sheet passing portion current Ip_aft for the changed secondary transfer voltage Vtr is performed (S309 to S513). The detection time and the response time are repeated while the recording material P is present in the secondary transfer portion N2 (more specifically, while the image forming area of the recording material P passes through the secondary transfer portion N2). As a result, the secondary transfer is performed so that the paper passing portion current based on the secondary transfer current detected when the recording material P is passing through the secondary transfer portion N2 falls within the paper passing portion current range obtained in S307. The voltage Vtr is corrected.

  Further, the control unit 50 repeats the processing of S309 to S317 until all the images of the job are transferred onto the recording material P and output is completed (S318).

  As described above, in the present embodiment, by subtracting the predicted current flowing in the non-sheet passing portion from the measured secondary transfer current, the sheet passing portion current to be controlled can be accurately obtained. 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 paper passing portion current becomes a value within a predetermined paper passing portion current range. With such a method, the same effect as that of the first embodiment can be obtained.

[Example 3]
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus according to the present exemplary embodiment are the same as those of the image forming apparatus according to the first exemplary embodiment. Therefore, in the image forming apparatus according to the present exemplary embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus according to the first exemplary embodiment will be denoted by the same reference numerals as those in the first exemplary embodiment, and detailed description thereof will be omitted. To do.

  In Example 1 and Example 2, the secondary transfer voltage was controlled in consideration of the magnitude of the sheet passing portion current in the secondary transfer current. On the other hand, in this embodiment, the current flowing in the non-sheet passing portion is treated as a sufficiently small proportion of the secondary transfer current, and the secondary transfer current is controlled to fall within a predetermined range.

  FIG. 11 is a flow chart showing the outline of the procedure for controlling the secondary transfer voltage in this embodiment. The processing of S401 to S406 of FIG. 11 is the same as the processing of S101 to S106 of FIG. Further, the processing of S409 to S414 of FIG. 11 is the same as the processing of S109 to S114 of FIG. Hereinafter, differences from the first embodiment will be described in particular, and description of the same processing as that of the first embodiment will be omitted.

  After obtaining the secondary transfer voltage Vtr in S406, the control unit 50 determines the upper limit value and the lower limit value (“secondary transfer current range”) of the secondary transfer current when the recording material P passes through the secondary transfer portion N2. ]) Is determined (S407). In this embodiment, in the ROM 53, from the viewpoint of suppressing the image defect as shown in FIG. 12, the current that may be supplied to the secondary transfer portion N2 when the recording material P passes through the secondary transfer portion N2. Information for determining the range (“secondary transfer portion current range”) is stored. In the present embodiment, this information is set as table data indicating the relationship between the moisture content of the atmosphere and the upper limit value and the lower limit value of the current that may flow in the secondary transfer portion N2. It should be noted that this table data is obtained in advance by experiments or the like. The control unit 50 obtains the range of the current that may flow to the secondary transfer unit N2 from the table data based on the environment information acquired in S403.

  Of the secondary transfer current, it is the paper passing portion current that is desired to be controlled. In this embodiment, the current flowing in the non-sheet passing portion is treated as a sufficiently small proportion of the secondary transfer current, and if the secondary transfer current is controlled within a predetermined range, the sheet passing portion current is controlled within an appropriate range. It is supposed to be possible. Here, the range of the current that may be applied to the secondary transfer portion N2 from the viewpoint of suppressing image defects may change depending on the thickness and surface property of the recording material P as well as the environmental information. Therefore, the table data may be set such that the range of the current changes depending on the information (grammage) related to the thickness of the recording material P and the information related to the surface property of the recording material P. The range of the current that may flow in the secondary transfer portion N2 may be set as a calculation formula. Further, the range of the current that may be passed through the secondary transfer portion N2 may be set as a plurality of table data or calculation formulas for each size of the recording material P.

Next, the control unit 50 performs a process of determining the fluctuation width ΔV of the secondary transfer voltage during the secondary transfer (S408). The process of S408 is the same as S108 of FIG. 4A in the first embodiment. However, in this embodiment, the following expression 9 is used instead of the expression 6 used in the first embodiment.
ΔV = A * (Ip_max'-Ip_min ') * Vb / Itarget ... (Equation 9)

  In the above formula 9, Vb is the secondary transfer portion charge voltage obtained in S405, Itarget is the target current obtained in S404, and Vb / Itarget is 2 when the recording material P is not present in the secondary transfer portion N2. It corresponds to the electric resistance of the next transfer portion N2. Further, in the above Expression 9, Ip_max 'and Ip_min' are the upper limit value and the lower limit value of the secondary transfer current range obtained in S407, respectively. Further, in the above Expression 9, as in the case of Expression 6 in the first embodiment, A represents the overshoot current generated when the secondary transfer voltage is changed as shown in FIG. It is a correction coefficient for suppressing the difference between the upper limit value and the lower limit value of the current supplied to N2 to be equal to or less than the difference. The correction coefficient A is previously obtained through experiments and the like, and in the present embodiment, the response time is 8 msec as in the first embodiment, while the correction coefficient A is set to about 1 to 2. There is. As a result, in the present embodiment, when the secondary transfer voltage is changed, the state in which the current is below the lower limit value exceeds the upper limit value, and conversely, the state is above the upper limit value and below the lower limit value. It can be suppressed. In the present embodiment, the target current Ittarget is about 15 to 20 μA, while the fluctuation width ΔV of the secondary transfer voltage varies depending on the electric resistance of the secondary transfer portion N2 according to the above equation 6, but is about 20 to 100 V. It becomes a value of the degree.

  After that, the control unit 50 uses the fluctuation width ΔV of the secondary transfer voltage obtained in S408 to perform the same processing as S109 to S114 of FIG. 4A in the first embodiment (S409 to S414). ).

  As described above, according to the present embodiment, the same effect as that of the first embodiment can be obtained, and the control can be simplified.

[Other]
Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above embodiments.

  In the above-described embodiment, the recording material is conveyed with reference to the center of the transfer member in the direction substantially orthogonal to the conveying direction, but the present invention is not limited to this, and the recording material is conveyed with reference to one end side, for example. The present invention may be equally applied to the present invention.

  Further, the present invention is equally applicable to a monochrome image forming apparatus having only one image forming section. In this case, the present invention is applied to the transfer section where the toner image is transferred from the image carrier such as the photosensitive drum to the recording material.

7 Intermediate Transfer Belt 8 Secondary Transfer Roller 20 Secondary Transfer Power Supply 21 Current Detection Circuit 22 Voltage Detection Circuit 50 Control Section

Claims (5)

An image carrier that carries a toner image,
A transfer member that is in contact with the image carrier and forms a transfer unit that transfers a toner image from the image carrier to a recording material;
A power supply for applying a voltage to the transfer member,
A detection unit for detecting a current flowing through the transfer member,
A control unit for performing constant voltage control of a voltage applied to the transfer member so that a predetermined voltage is applied to the transfer member when a recording material passes through the transfer unit;
Have
In the image forming apparatus capable of changing the transfer voltage applied to the transfer member such that the current flowing through the transfer member during transfer is within the predetermined range,
When changing the transfer voltage applied to the transfer member so that the current flowing through the transfer member falls within the predetermined range during transfer, the controller changes the transfer voltage by a predetermined change in the transfer voltage. An image characterized by being performed for each width and changing the variation width based on a detection result detected by the detection unit when a voltage is applied to the transfer member in a state where there is no recording material in the transfer unit. Forming equipment.   If the current value flowing through the transfer member is the first current value when a predetermined voltage is applied to the transfer member in the state where there is no recording material in the transfer unit, the controller changes the fluctuation range by the first fluctuation. When the current value flowing in the transfer member is a second current value smaller than the first current value, the fluctuation width is changed to a second fluctuation width larger than the first fluctuation width. The image forming apparatus according to claim 1, wherein:   The control unit, after one of the change of the transfer voltage and the next maximum of the absolute value of the current flowing in the transfer member before the change of the transfer voltage, and after the change of the one of the transfer voltage and The variation so that the difference between the absolute value of the current flowing through the transfer member that is in a steady state before the next change of the transfer voltage and the difference between the upper limit value and the lower limit value of the predetermined range is smaller than the difference. The image forming apparatus according to claim 1, wherein the width is changed.   The image forming apparatus according to claim 1, wherein the control unit changes the fluctuation range based on a difference between an upper limit value and a lower limit value of the predetermined range.   The image forming apparatus according to claim 1, wherein the control unit changes the variation width based on a width of a recording material.