JP2020085946A - Image forming apparatus - Google Patents

Abstract

To provide an image forming apparatus capable of suppressing repeated occurrences of a similar image defect in a plurality of recording materials due to excessive or insufficient transfer current during continuous image formation for continuously forming images in the plurality of recording materials.SOLUTION: An image forming apparatus 100 includes: an image bearing member 7; a transfer member 8; application means 20 for applying transfer voltage for transferring to a transfer part N2; current detecting means 21 for detecting current flowing through the transfer part N2; control means 50 capable of performing constant voltage control of the transfer voltage at a predetermined voltage value and changing the transfer voltage so that the current detected by the detecting means 21 falls within a predetermined current range. When the transfer voltage is changed in continuous image formation during first recording material P is passing through the transfer part N2, the control means 50 is configured to determine an initial vale of transfer voltage during second recording material P is passing through the transfer part N2, the second recording material being passing through the transfer part N2 after the first recording material P, on the basis of transfer voltage after the change during the first recording material P is passing through the transfer part N2.SELECTED DRAWING: Figure 4

Description

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.

2. Description of the Related Art Conventionally, in an image forming apparatus using an electrophotographic method or the like, a toner image is electrostatically transferred from an image bearing member such as a 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 part, 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. It is because it is easy to secure the transfer current. However, the electrical resistance of the transfer member that constitutes the transfer unit changes according to product variations, member temperature, cumulative usage time, etc., and the electrical resistance of the recording material that passes through the transfer unit also depends on the type of recording material and the ambient 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 transfer voltage control in a configuration in which transfer is performed by applying a transfer voltage to the transfer member under constant voltage control. Immediately before the start of continuous image formation, a predetermined voltage is applied to the transfer section in the state where there is no recording material, the current value is detected, and the voltage value at which the predetermined target current is obtained is obtained. Then, a recording material sharing voltage corresponding to the type of recording material is added to this voltage value to set a transfer voltage value 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, there are many types of recording materials in circulation. Further, the electric resistance of the recording material varies depending on the wet state of the recording material (water content of the recording material), but the water content of the recording material varies depending on the time of being in the environment even if the environment (temperature/humidity) is the same. .. For these reasons, it is often difficult to accurately obtain the recording material sharing voltage 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, a current supplied to the transfer unit (transfer current It is proposed that the upper limit value and the lower limit value of () are set. The passing of the recording material through the transfer portion is also referred to as “paper passing”. By such control, the transfer current supplied to the transfer portion during the sheet passing can be set to a value within a predetermined range, so that it is possible to suppress the occurrence of an image defect due to insufficient or excessive transfer current. 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, there is a method of detecting the transfer current during sheet passing and controlling the transfer voltage so that the transfer current falls within a predetermined range (upper limit value or less and lower limit value or more). When the transfer voltage is controlled by this method, there is a time lag from when it is detected that the transfer current is out of the predetermined range until the transfer voltage is changed so that the transfer 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, and therefore the density decrease due to the excess or deficiency of the transfer current as shown in FIG. Image defects such as the above may occur. Then, as shown in FIG. 14, for example, when the above-described image defect occurs in the first recording material during continuous image formation, the similar image defect may occur in the subsequent recording material. is there. This is because the plurality of recording materials used during continuous image formation are often of the same type, and are often left in the same state.

Therefore, it is an object of the present invention to prevent similar image defects from repeatedly occurring in a plurality of recording materials due to excess or deficiency of transfer current during continuous image formation in which images are continuously formed on a plurality of recording materials. An image forming apparatus is provided.

The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention is directed to an image carrier that carries a toner image, a transfer member that forms a transfer unit that transfers the toner image from the image carrier to a recording material, and a transfer member that transfers the toner image to the transfer unit. An applying unit that applies a voltage, a detecting unit that detects a current flowing in the transfer unit, and a constant voltage control of the transfer voltage at a predetermined voltage value while the recording material is passing through the transfer unit, In an image forming apparatus having a control unit that can change the transfer voltage so that the current detected by the detection unit falls within a predetermined current range, the control unit can continuously form images on a plurality of recording materials. When the transfer voltage is changed while the first recording material is passing through the transfer portion during continuous image formation to be formed, the second recording that passes through the transfer portion after the first recording material is performed. Determining an initial value of the transfer voltage when the material passes through the transfer portion, based on the transfer voltage after the change when the first recording material passes through the transfer portion. A characteristic image forming apparatus.

According to the present invention, it is possible to prevent a similar image defect due to excess or deficiency of transfer current from repeatedly occurring in a plurality of recording materials during continuous image formation in which images are continuously formed on a plurality of recording materials.

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 illustrating a control mode of a main part of the image forming apparatus. FIG. 3 is a flowchart of control of the first embodiment. 6 is a table showing an example of table data of target currents. 6 is a table showing an example of table data of recording material sharing voltage. 6 is a table showing an example of table data in a predetermined current range of a secondary transfer current. FIG. 6 is a schematic diagram showing changes in transfer voltage and transfer current and a defective image in a comparative example. 5A and 5B are schematic diagrams showing changes in transfer voltage and transfer current and image defects in Example 1. FIG. FIG. 6 is a graph showing an example of the water content of the recording material in the recording material cassette. 6 is a schematic diagram showing changes in transfer voltage and transfer current in Example 2. FIG. FIG. 7 is a flowchart of the control of the second embodiment. It is a graph figure for demonstrating the change method of transfer voltage. FIG. 4 is a schematic diagram showing transitions of transfer voltage and transfer current and image defects for explaining the problem.

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

[Example 1]
1. Overall Configuration and Operation of Image Forming Apparatus FIG. 1 is a schematic configuration diagram of an image forming apparatus 100 of this embodiment. The image forming apparatus 100 according to the present embodiment has a function of a tandem type multi-function device (copier, printer, facsimile device) that can form a full-color image using an electrophotographic method and that employs an intermediate transfer method. ).

The image forming apparatus 100 has, 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 of the image forming sections 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 the developing device 4 as a developing unit, and a toner image is formed on the photosensitive drum 1. In the present embodiment, the exposed portion (image portion) on the photosensitive drum 1 where the absolute value of the potential has been lowered by being exposed after being uniformly charged is charged to the same polarity as the charging polarity of 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. Is becoming

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 capable of contacting the surfaces of the four photosensitive drums 1. ing. The intermediate transfer belt 7 is stretched around a driving roller 71 as a plurality of stretching rollers, a tension roller 72, and a secondary transfer counter roller 73. 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. When the driving roller 71 is driven to rotate, the intermediate transfer belt 7 rotates (rotates) in the arrow R2 direction (clockwise) at a conveyance speed (circumferential 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 means, 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 in the primary transfer portion N1. .. During the primary transfer process, a primary transfer voltage (primary transfer bias) is applied to the primary transfer roller 5 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 superposed 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 means is arranged at a position facing the secondary transfer counter roller 73. The secondary transfer roller 8 is pressed toward 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 in the secondary transfer portion N2. It is electrostatically transferred (secondary transfer) to (sheet, transfer material) P. The recording material P is typically paper (paper), but is not limited to this, and synthetic paper made of resin such as waterproof paper, a plastic sheet such as an OHP sheet, or cloth is used. Sometimes it is. During the secondary transfer process, the secondary transfer roller 8 receives a secondary transfer voltage (secondary transfer bias) from the secondary transfer power supply (high voltage power supply circuit) 20, which is a DC voltage having a polarity opposite to the regular charging polarity of the toner. ) Is applied. The recording material P is housed in the recording material cassette 11 or the like, and the feeding roller 12 is driven based on the feeding start signal to feed the recording material P one by one from the recording material cassette 11 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 the 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.

Toner remaining on the surface of the photosensitive drum 1 after the primary transfer process (primary transfer residual toner) is removed and collected from the surface of the photosensitive drum 1 by a drum cleaning device 6 as a photosensitive member cleaning unit. Further, adhered substances such as toner (secondary transfer residual toner) and paper dust remaining on the surface of the intermediate transfer belt 7 after the secondary transfer process 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 including a resin layer, an elastic layer, and a surface layer from the inner peripheral surface side to the outer peripheral surface side. Polyimide, polycarbonate or the like can be used as the resin material forming the resin layer. 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 type or two or more types 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 rubber, elastomer), butyl rubber and the like can be used. In addition to these materials, one or more kinds of powders and particles such as a material such as fluororesin having a small surface energy to improve lubricity, or one or more kinds of these powders and particles. Those 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 have 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). In addition, 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), and 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 to carry (the length in the direction substantially orthogonal to the carrying direction). 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 to be conveyed by the image forming apparatus 100 are 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 is in contact with the secondary transfer counter roller 73 via the intermediate transfer belt 7 to form a secondary transfer portion N2. The secondary transfer roller 8 is connected to a secondary transfer power source 20 as an applying unit, which has a variable output voltage value. 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. A roller corresponding to the secondary transfer facing roller 73 of the present embodiment is used as a transfer member, and a secondary transfer voltage having the same polarity as the normal charging polarity of the toner is applied to the roller, and the secondary transfer roller of the present embodiment is applied. The roller corresponding to No. 8 may be used as the counter electrode and electrically grounded.

In the present embodiment, based on the information regarding the electric resistance of the secondary transfer portion N2 (mainly the secondary transfer roller 8 in this embodiment) acquired without the toner image and the recording material P in the secondary transfer portion N2, The secondary transfer voltage applied to the secondary transfer roller 8 is set by constant voltage control during the secondary transfer. In addition, in the present embodiment, the secondary transfer current flowing through the secondary transfer portion N2 is detected during sheet passing. Then, the secondary transfer current is output from the secondary transfer power source 20 by constant voltage control so that the secondary transfer current has a value equal to or lower than a predetermined upper limit value and equal to or higher than a lower limit value (herein, also simply referred to as “predetermined current range”). The secondary transfer voltage is controlled. This predetermined current range can be set based on various information. The various types of information may include the following types of information, for example. 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. For example, the predetermined current range can be changed based on information about the thickness and width of the recording material P used for image formation. The information about the thickness of the recording material P and the width of the recording material P can be acquired based on the information input from the operation unit 31 or the external device 200. Alternatively, it is also possible to provide a detecting 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 detecting means.

In this embodiment, in order to perform such control, the secondary transfer power supply 20 has a current (secondary transfer power) flowing in the secondary transfer portion N2 (that is, the secondary transfer power supply 20 or the secondary transfer roller 8). A current detection circuit 21 as a current detection unit (detection unit) for detecting the current 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, etc., 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 transfer and read data with each other.

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 is 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 for 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 source 20 applies a secondary transfer voltage, which is a direct-current voltage controlled to a constant voltage, to the secondary transfer roller 8. Note that the constant voltage control is control that makes the value of the voltage applied to the transfer portion (that is, the transfer member) have a substantially constant voltage value. 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 detection 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-rotation step, a sheet interval step when images are formed on a plurality of recording materials P, and a post-rotation 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 differs 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 in 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 paper 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 interval step, post-rotation step, and further 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 the present embodiment, control for setting the initial value of the secondary transfer voltage during non-image formation, control for determining the upper limit value and the lower limit value (predetermined current range) of the secondary transfer current during paper passage, etc. are executed. It

3. Problem When controlling a transfer voltage by detecting a transfer current during sheet feeding, typically, the transfer current is detected and the transfer voltage is changed as follows. That is, the detection time (first period) for detecting the transfer current and the response time (second period) for waiting for the response after the signal for changing the transfer voltage is output based on the detection result of the transfer current in the detection time. And repeat.

Here, as described above, there is a time lag from the detection of the transfer current deviating from the predetermined range to the completion of the change of the transfer voltage. Therefore, an image defect may occur due to an excess or deficiency of the transfer current in a region where the transfer current is out of an appropriate range, which passes through the transfer unit until the change of the transfer voltage is completed.

FIG. 14 is a schematic diagram showing the transition of the transfer voltage and the transfer current and the state of occurrence of an image defect when the transfer voltage is changed when the transfer current detected during sheet feeding is below the lower limit value. It is shown in the figure. The "leading edge" and "rear edge" of the recording material refer to the leading edge and the trailing edge in the conveying direction of the recording material.

As shown in FIG. 14, at the transfer voltage V0 applied to the leading edge of the recording material, the transfer current during sheet passing becomes I0, which is below the lower limit value IL of the transfer current. Therefore, control is performed to gradually increase the transfer voltage from V0 so that the transfer current becomes the lower limit value IL. As a result, the thin image density (transfer missing) due to the small transfer current is eliminated, but the thin image density occurs in the section A in which the transfer current is below the lower limit value IL.

Then, as shown in FIG. 14, when the low image density occurs on the first recording material during continuous image formation, a similar low image density may occur on the subsequent recording materials. It is highly likely. This is because the plurality of recording materials used during continuous image formation are likely to be of the same type, and are also likely to be in the same state when left unattended. In FIG. 14, the image defect due to the insufficient transfer current has been described as an example, but the same problem may occur with respect to the image defect due to the excessive transfer current.

As described above, it is required to prevent the same image defect due to excess or deficiency of transfer current from being repeatedly generated in a plurality of recording materials during continuous image formation in which images are continuously formed on a plurality of recording materials.

4. Secondary Transfer Voltage Control Next, the secondary transfer voltage control in this embodiment will be described. FIG. 4 is a flowchart showing an outline of the procedure of the secondary transfer voltage control in this embodiment. FIG. 4 shows a simplified procedure relating to the secondary transfer voltage control of the control executed by the control unit 50 when executing a job, and the illustration of many other controls when executing a job is omitted. Has been done.

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 the present embodiment, the job information includes the following information. That is, the image information specified by the operator, the size (width, length) of the recording material P on which the image is formed, information related to the thickness of the recording material P (thickness or grammage), and the recording material P is coated paper. Information related to the surface property of the recording material P (paper type category). 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). Further, the ROM 53 stores information indicating a correlation between the environmental information and the target current Ittarget for transferring the toner image on the intermediate transfer belt 7 onto the recording material P as shown in FIG. There is. In the present embodiment, this information is set as table data indicating the target current Ittarget for each category of the moisture content of the atmosphere. This table data is obtained in advance by experiments or the like. The control unit 50 can determine the amount of water in the atmosphere based on the environmental information (temperature/humidity) detected by the environmental sensor 32. 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 Ittarget, and writes this 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 environment 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, in addition to the environmental information, if the factors that influence the charge amount of the toner on the intermediate transfer belt 7 are known, 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 of 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. The secondary transfer voltage is acquired and the secondary transfer voltage is set based on the result (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), and the secondary information is obtained based on the result. Set the transfer voltage. That is, in a state where the secondary transfer roller 8 and the intermediate transfer belt 7 are in contact with each other, a predetermined voltage or current is supplied from the secondary transfer power source 20 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 voltage-current characteristic, which is the relationship between the voltage and the current, 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). For example, when the relationship between the voltage and the current is such that the current does not change linearly (proportional) with respect to the voltage but the current changes as represented by a polynomial of second or higher order of voltage, The predetermined voltage or current is in multiple stages of three points or more. Next, the control unit 50 causes the target current Itarget to flow in the secondary transfer unit N2 without the recording material P based on the target current Itarget written in the RAM 52 in S104 and the acquired voltage-current characteristic. The voltage value Vb required for 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 table data for obtaining the recording material sharing voltage Vp is obtained in advance by experiments or the like. The control unit 50 can determine 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 initial value of the secondary transfer voltage Vn applied to the secondary transfer roller 8 from the secondary transfer power source 20 during sheet passing (n indicates the nth sheet in the job, and the initial value is 1). Then, Vb+Vp, which is the sum of Vb and Vp, is obtained, and this is written in the RAM 52. In this embodiment, the initial value of the secondary transfer voltage Vn 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 recording material sharing voltage (transfer voltage of the electric resistance of the recording material P) Vp may be changed by the surface property of the recording material P as well as the information (grammage) related to the thickness of the recording material P. There is. Therefore, the table data may be set such that the recording material sharing voltage Vp 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 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 (predetermined current range) of the secondary transfer current during sheet passing (S106). The ROM 53 stores information for obtaining the range of the current that may be supplied to the secondary transfer portion N2 during sheet passing from the viewpoint of suppressing image defects 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 upper limit value and the lower limit value of the current that may be passed through the secondary transfer portion N2 during sheet passing. This table data is obtained in advance by experiments or the like. The control unit 50 obtains a predetermined current range of the secondary transfer current during the sheet passing from the table data based on the environment information acquired in S103.

It should be noted that the range of the current that may be applied to the secondary transfer portion N2 during paper passing varies depending on the width of the recording material P. FIG. 7 shows, as an example, table data set assuming a sheet having a width (297 mm) corresponding to an A4 size and a basis weight of 90 g/m ² . Here, when the recording material P is passing through the secondary transfer portion N2, there are a paper passing portion current and a non-paper passing portion current as currents flowing through the transfer portion. The sheet passing portion current is a current flowing in a region (“sheet passing portion”) of the secondary transfer portion N2 where the recording material P passes in a direction substantially orthogonal to the conveying direction of the recording material P. Further, the non-sheet passing portion current is a current flowing in a region (“non-sheet passing portion”) of the secondary transfer portion N2 where the recording material P does not pass in a direction substantially orthogonal to the conveying direction of the recording material P. The current that can be detected during sheet passing is the sum of sheet passing portion current and non-sheet passing portion current. Therefore, an appropriate predetermined current range of the secondary transfer current during paper passing is obtained in advance for each size of the recording material P, and the secondary transfer current during paper passing is controlled to a value within the predetermined current range. By doing so, the current flowing through the paper passing portion can be controlled within an appropriate range.

In addition, from the viewpoint of suppressing image defects, the range of the current that may be applied to the secondary transfer portion N2 during paper passing may change depending on the thickness and surface property of the recording material P as well as the environmental information. Therefore, the range of the current that may be applied to the secondary transfer portion N2 during the sheet passing is selected according to the information (grammage) related to the thickness of the recording material P or the information related to the surface property of the recording material P. The table data may be set so that it can be done. In addition, the range of the current that may be applied to the secondary transfer portion N2 during the sheet passing may be set as a calculation formula. For example, the range of the current that may be applied to the secondary transfer portion N2 during paper passing is set in plural for each size of the recording material P, environmental information, information related to thickness (grammage), and recording material P It may be table data or a calculation formula that specifies the range of the current according to the information related to the surface property.

Next, when the n-th recording material P (initially n=1) of the recording material P reaches the secondary transfer portion N2 (S107), the control unit 50 applies the secondary transfer voltage Vn to the secondary transfer roller 8 during sheet passing. (Initially n=1) is applied (S108). Further, the control unit 50 acquires the detection result of the secondary transfer current In (initially n=1) by the current detection circuit 21 during sheet passing (S109). Then, the control unit 50 compares the secondary transfer current In with the predetermined current range determined in S106, and corrects the secondary transfer voltage output from the secondary transfer power source 20 as necessary (S110, S111). .. In the present embodiment, if the current detected by the current detection circuit 21 is out of the predetermined current range during sheet feeding, the secondary transfer voltage is adjusted so that the detected current falls within the predetermined current range. To change. In this operation, as shown in FIG. 13, 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 performed by repeating the operation of changing the voltage. Further, 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 detection result of the current input from the current detection circuit 21 during the detection period. It is done by that.

FIG. 13 schematically shows changes in the secondary transfer voltage and the secondary transfer current when the secondary transfer voltage is changed when the secondary transfer current detected during sheet feeding is below the lower limit value. Is shown in. As shown in FIG. 13, when the predetermined secondary transfer voltage is applied for 8 ms (response time + detection time) and the secondary transfer current is still below the lower limit value, the secondary transfer voltage is as follows. Is changed. That is, the secondary transfer voltage is changed to the secondary transfer voltage obtained by adding a predetermined voltage fluctuation width ΔV (100 V in this embodiment) to the predetermined secondary transfer voltage. Further, the change of the secondary transfer voltage is repeatedly executed until the secondary transfer current detected during sheet passing reaches the lower limit value. The same applies when the secondary transfer current detected during paper passing exceeds the upper limit value. For example, when a predetermined secondary transfer voltage is applied for 8 ms (response time + detection time), the secondary transfer current is applied. Is still above the upper limit, the secondary transfer voltage is changed as follows. That is, the secondary transfer voltage is changed to the secondary transfer voltage obtained by reducing the predetermined secondary transfer voltage by a predetermined voltage fluctuation width ΔV (100 V in this embodiment). Further, the change of the secondary transfer voltage is repeatedly executed until the secondary transfer current detected during the sheet passing reaches the upper limit value.

It is preferable that the detection time and the response time are as short as possible because it is possible to reduce a time (area) in which the secondary transfer current may deviate from a predetermined 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. As shown in FIG. 13, when the secondary transfer voltage is changed, if an overshoot occurs in which the secondary transfer voltage once rises to a value that exceeds the target value and then drops to the target value, Overshoot also occurs in the secondary transfer current. It is preferable to set the response time so that the secondary transfer current can be detected even after the overshoot occurs, after the convergence to the steady state.

In this way, when the secondary transfer current detected during the passage of the n-th sheet (initially n=1) of the recording material P is not within the predetermined current range (S110: NO), the predetermined transfer current is determined. The secondary transfer voltage Vn is corrected to Vn' so that it falls within the current range (S111). After that, when the image formation on the nth recording material P is completed (S112) and the image formation is performed on the n+1th recording material P (S113), the following processing is performed. That is, the control unit 50 sets the secondary transfer voltage Vn+1 applied to the leading end of the n+1th recording material P as the corrected secondary transfer voltage Vn′ during the passage of the nth recording material P ( S114). On the other hand, if the secondary transfer current detected during the passage of the n-th (initially n=1) recording material P is within the predetermined current range (S110: YES), the secondary transfer voltage Vn Is not corrected. After that, when the image formation on the nth recording material P is completed (S115) and the image formation is performed on the n+1th recording material P (S116), the following processing is performed. That is, the control unit 50 sets the secondary transfer voltage Vn+1 applied to the leading end of the n+1th recording material P to a voltage value substantially the same as the secondary transfer voltage Vn during the passage of the nth recording material P. Yes (S117). After that, when the image formation on all the recording materials P of the job is completed (S113, S116), the control unit 50 ends the operation of the job.

5. Effect FIG. 8 schematically shows the transition of the secondary transfer voltage and the secondary transfer current and the appearance of the image defect in the comparative example in which the secondary transfer voltage control of the present embodiment as described above is not performed. There is. Here, in an ambient environment (water content of 8.9 g/kg) at 23° C. and 50% RH, continuous image formation was performed by using A4 size paper having a basis weight of 90 g/m ² as the recording material P and one sheet. An example in which the secondary transfer current detected during the passage of the recording material P of the eye is below the lower limit is shown. In this case, the lower limit value of the predetermined current range is 30 μA and the upper limit value is 50 μA (FIG. 7). Further, in this case, the target current Ittarget is 40 μA (FIG. 5), and the secondary transfer partial voltage Vb obtained using this target current Ittarget is 1000V. In this case, the recording material sharing voltage Vp is 500V (FIG. 6), and the initial value of the secondary transfer voltage, which is the total value of this Vp and the above Vb, is 1500V. The secondary transfer current detected when the secondary transfer voltage is applied to the leading end of the first recording material P is 20 μA. This occurs when the passed recording material P has the same basis weight as the recording material P when the recording material sharing voltage Vp as shown in FIG. 6 is determined, but the electric resistance is extremely high. ..

In the example shown in FIG. 8, the secondary transfer current detected during the passage of the leading edge of the first recording material P is 20 μA, and the secondary transfer current is below the lower limit of 30 μA. Therefore, the secondary transfer voltage is changed to 1600V (1500V+ΔV (=100V)), and the secondary transfer current is detected again. After that, the secondary transfer voltage is changed so as to be increased every ΔV (=100V) until the secondary transfer current reaches the lower limit value. Here, it is assumed that the secondary transfer current reaches the lower limit value of 30 μA when the secondary transfer voltage reaches 2200V. That is, in this case, the change of the secondary transfer voltage is executed seven times. Then, after the secondary transfer current reaches the lower limit value, the change of the secondary transfer voltage is stopped, the secondary transfer voltage is maintained at 2200 V, and the toner is moved toward the rear end of the first recording material P. Secondary transfer of the image is performed.

As described above, in the example shown in FIG. 8, in the section A from the leading edge of the recording material P where the secondary transfer current is 20 μA to the position where the secondary transfer current reaches the lower limit value of 30 μA, the image due to insufficient transfer current is generated. A defect will occur.

Further, in this comparative example, as shown in FIG. 8, when the secondary transfer current detected during the passage of the first recording material P during continuous image formation is below the lower limit value, two sheets are printed. There is a high possibility that the secondary transfer current will fall below the lower limit value even during the passage of the recording material P after the eye. In the example shown in FIG. 8, the secondary transfer voltage of 1500 V is applied during the passage of the leading edge of the second recording material P, similarly to during the passage of the leading edge of the first recording material P. In this case, during the passage of the leading edge of the second recording material P, a secondary transfer current of 20 μA, which is almost the same as during the passage of the leading edge of the first recording material P, is detected. Therefore, in the second recording material P as well as in the first recording material P, the secondary transfer current reaches the lower limit value of 30 μA from the tip of the recording material P having the secondary transfer current of 20 μA. In the section B up to the position, an image defect occurs due to insufficient transfer current. The same image defect due to the shortage of the transfer current is also succeeded to the third and subsequent recording materials P (section C in FIG. 8 for the third recording material P).

As shown in FIG. 8, the reason why the similar shortage of transfer current occurs in the plurality of recording materials P during continuous image formation is considered as follows. That is, the plurality of recording materials P used during continuous image formation are highly likely to be of the same type. Further, the plurality of recording materials P have no significant difference in the time of being left after being taken out from the packed package, and it is highly possible that the recording materials P have substantially the same water content. That is, since the electric resistances of the recording materials P used during continuous image formation are likely to be substantially the same, it is highly likely that similar transfer current shortages occur when the same transfer voltage is applied.

Therefore, in this embodiment, when the secondary transfer current detected during the passage of the recording material P during continuous image formation is out of the predetermined current range and the secondary transfer voltage is corrected, The secondary transfer voltage applied to the leading end of the recording material P is determined based on the corrected secondary transfer voltage. In particular, in the present embodiment, the secondary transfer voltage applied to the leading edge of the next recording material P has a voltage value substantially the same as the corrected secondary transfer voltage. As a result, it is possible to prevent repetitive image defects due to insufficient transfer current on the plurality of recording materials P during continuous image formation.

FIG. 9 is a schematic view similar to FIG. 8 when continuous image formation is performed according to this embodiment. FIG. 9 shows an example in which continuous image formation is performed under the same conditions as the comparative example shown in FIG. That is, in an ambient environment (moisture content of 8.9 g/kg) at 23° C. and 50% RH, continuous image formation is performed using A4 size paper having a basis weight of 90 g/m ² as the recording material P. 2 shows an example in which the secondary transfer current detected during the passage of the recording material P is less than the lower limit value. In the example of FIG. 9, similarly to the example of FIG. 8, the lower limit value of the predetermined current range is 30 μA, the upper limit value is 50 μA, the target current Ittarget is 40 μA, the secondary transfer portion bearing voltage Vb is 1000 V, and the recording material sharing voltage Vp is The initial value (Vb+Vp) of the secondary transfer voltage of 500V is 1500V. Then, in the example shown in FIG. 9, it is assumed that the secondary transfer current detected during the passage of the leading edge of the first recording material P is 20 μA, as in the example shown in FIG.

In the example shown in FIG. 9, the first recording material P behaves similarly to the example shown in FIG. In other words, at the secondary transfer voltage of 1500 V applied to the leading edge of the first recording material P, the secondary transfer current detected during paper passage is 20 μA, which is below the lower limit of 30 μA. Therefore, the secondary transfer voltage is changed to be gradually increased, and when the corrected secondary transfer voltage finally reaches 2200 V, the detected secondary transfer current reaches the lower limit value of 30 μA. ..

Then, in the present embodiment, as shown in FIG. 9, the secondary transfer voltage applied to the leading end of the second recording material P is the passage of the first recording material P which is the preceding recording material P. It is determined based on the secondary transfer voltage after the correction. In particular, in this embodiment, the secondary transfer voltage applied to the leading edge of the second recording material P is the corrected secondary voltage during the passage of the first recording material P, which is the recording material P before that. The transfer voltage is 2200 V (that is, the secondary transfer voltage applied to the trailing edge of the first recording material P). As a result, the secondary transfer current detected during the passage of the second recording material P reaches the lower limit value of 30 μA from the leading edge of the second recording material P. Therefore, it is possible to suppress the occurrence of the image defect due to the shortage of the transfer current on the leading end side of the second recording material P, which has occurred in the example shown in FIG.

Similarly, as for the secondary transfer voltage applied to the leading edge of the third and subsequent recording materials P, the secondary transfer voltage applied during the passage of the preceding recording material P (that is, the immediately preceding recording The secondary transfer voltage applied to the rear end of the material P is taken over. This makes it possible to prevent the occurrence of image defects due to insufficient transfer current on the leading end side of each recording material P for the third and subsequent recording materials P.

As described above, in this embodiment, when the secondary transfer voltage is corrected so that the secondary transfer current detected during sheet feeding falls within a predetermined current range, the secondary transfer voltage is applied to the leading edge of the next recording material P. The secondary transfer voltage to be applied is determined based on the corrected secondary transfer voltage. Particularly, in this embodiment, the secondary transfer voltage applied to the leading edge of the next recording material P is set to a voltage value substantially the same as the corrected secondary transfer voltage. As a result, it is possible to suppress the occurrence of image defects due to excess or deficiency of the secondary transfer current in many recording materials P during continuous image formation.

In FIG. 9, the case where the secondary transfer current is below the lower limit value is taken as an example, but the same control can be performed when the secondary transfer current is above the upper limit value. For example, with the secondary transfer voltage applied to the leading edge of the first recording material P, the secondary transfer current detected during sheet passing may exceed the upper limit value. In this case, the secondary transfer voltage is changed to be gradually decreased so that the finally detected secondary transfer current reaches the upper limit value. Then, the secondary transfer voltage applied to the leading edge of the second recording material P is the corrected secondary transfer voltage during the passage of the first recording material P (that is, the secondary transfer voltage of the first recording material P). The secondary transfer voltage applied to the rear end).

Further, in the present embodiment, when the secondary transfer voltage is corrected during the passage of the recording material P at the time of continuous image formation, the secondary transfer voltage applied to the leading edge of the next recording material P is corrected as described above. However, the voltage value is not limited to this value. Any secondary transfer voltage that can suppress image defects based on the corrected secondary transfer voltage may be used. That is, if the secondary transfer current is corrected to fall below the lower limit value and increase the absolute value of the secondary transfer voltage while the recording material P is passing, the secondary transfer current does not exceed the upper limit value. It is sufficient that the absolute value of the secondary transfer voltage before correction is larger than the absolute value of the secondary transfer voltage. Further, when the secondary transfer current exceeds the lower limit value and is corrected so as to reduce the absolute value of the secondary transfer voltage during the passage of the recording material P, the secondary transfer current does not fall below the lower limit value. It is sufficient that the absolute value of the voltage value is smaller than the secondary transfer voltage before correction, which is set to.

Although the initial value of the secondary transfer voltage applied during the passage of the recording material P is described as the secondary transfer voltage applied to the leading end of the recording material P, the image forming area is described. The secondary transfer may be applied at the tip of (the area where the toner image can be transferred). Similarly, here, the secondary transfer voltage (including the corrected secondary transfer voltage) applied during the passage of the preceding recording material P is the secondary transfer voltage applied to the trailing end of each recording material P. Although the voltage is described as a voltage, any secondary transfer voltage applied to the rear end of the image forming area may be used.

Further, in this embodiment, when the secondary transfer voltage is corrected while the recording material P is being passed during the continuous image formation, the secondary transfer voltage applied to the leading end of the recording material P immediately after that is passed. Was determined based on the corrected secondary transfer voltage, but the present invention is not limited to this. For example, a recording material P that is passed after the recording material P that is passed immediately after (for example, a recording material P that is next to the recording material P that is passed immediately after) in relation to the transition of other controls. It may be determined based on the corrected secondary transfer voltage from the secondary transfer voltage applied to the tip of the. Further, the first recording material P whose secondary transfer voltage may be corrected during sheet passing during continuous image formation is not limited to the first recording material P during continuous image formation. When the secondary transfer voltage is corrected during the passage of an arbitrary first recording material P during continuous image formation, the second recording material P is passed to the front end of the second recording material P after the first recording material P. The secondary transfer voltage to be applied can be determined.

As described above, the image forming apparatus 100 according to the present exemplary embodiment includes the detection unit 21 that detects the current flowing in the transfer portion N2. Further, the image forming apparatus 100 performs constant voltage control of the transfer voltage at a predetermined voltage value while the recording material P is passing through the transfer portion N2, and the current detected by the detection means 21 is in a predetermined current range. The control means 50 is capable of changing the transfer voltage so that Then, the control unit 50 changes the transfer voltage when the first recording material P passes through the transfer portion N2 during continuous image formation in which images are continuously formed on a plurality of recording materials P, The initial value of the transfer voltage when the second recording material P passing through the transfer portion N2 after the first recording material P passes through the transfer portion N2 is the initial value of the transfer voltage when the first recording material P passes through the transfer portion N2. It is determined on the basis of the transfer voltage after the above-mentioned change in the case of In the present embodiment, when the control means 50 changes the transfer voltage so that the absolute value of the transfer voltage is increased while the first recording material P is passing through the transfer portion N2, The initial value of the transfer voltage when passing through the transfer portion N2 is set to a voltage value whose absolute value is larger than the initial value of the transfer voltage when the first recording material P passes through the transfer portion N2. Further, when the control unit 50 changes the transfer voltage so that the absolute value of the first recording material P is reduced while the first recording material P is passing through the transfer portion N2, the second recording material P is transferred to the transfer portion N2. The initial value of the transfer voltage when the first recording material P passes through the transfer portion N2 can be set to a voltage value whose absolute value is smaller than the initial value of the transfer voltage when the first recording material P passes through the transfer portion N2. In the present embodiment, the control unit 50 sets the initial value of the transfer voltage when the second recording material P passes the transfer portion N2 to the above-mentioned value when the first recording material P passes the transfer portion N2. The voltage value is almost the same as the transfer voltage after the change. Further, in the present embodiment, the control unit 50 transfers the next recording material P when the recording voltage is not changed while the one recording material P is passing the transfer portion N2 during continuous image formation. The initial value of the transfer voltage when passing through the portion N2 is set to be substantially the same as the transfer voltage when the one recording material P passes through the transfer portion N2.

As described above, according to the present exemplary embodiment, a plurality of similar image defects due to an excess or deficiency of the secondary transfer current that occurs until the secondary transfer current falls within a predetermined current range during continuous image formation may occur. It is possible to suppress the repetitive occurrence of the recording material P.

[Example 2]
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of the first embodiment. 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 are designated by the same reference numerals as those in the first exemplary embodiment, and detailed description thereof will be omitted. ..

In the first embodiment, when the secondary transfer voltage is corrected while the recording material P is being passed during continuous image formation, the secondary transfer voltage applied to the leading edge of the next recording material P is set to 2 after the correction. A voltage value approximately the same as the next transfer voltage was adopted. On the other hand, in this embodiment, as the secondary transfer voltage applied to the leading edge of the next recording material, the corrected secondary transfer voltage during the passage of the previous recording material P is multiplied by a predetermined coefficient. It adopts the voltage value.

In addition, in Example 1, continuous image formation was performed using an A4 size paper having a basis weight of 90 g/m ² as the recording material P in an ambient environment (water content of 8.9 g/kg) at 23° C. and 50% RH. The case has been described as an example. On the other hand, in the present embodiment, the environment around the image forming apparatus 100 is an extremely dry environment such as an environment at 23° C. and 5% RH (water content 0.88 g/kg). This will be explained as an example.

Under an extremely dry ambient environment such as the ambient environment of 23° C. and 5% RH, in the bundle of the recording materials P contained in the recording material cassette 11, the recording material P on the top and the bundle of the recording materials P are The water content of the recording material P at the center of the stacking direction may differ greatly. FIG. 10 shows the moisture content of paper in the bundle of paper stored in the recording material cassette 11 one by one from the top paper. Here, as an example, an example is shown in which the time after the paper is stored in the recording material cassette 11 is 2.5 hours. As shown in FIG. 10, the water content of the top paper is 4.0%, the water content of the fifth paper from the top is 5.5%, the 10th paper is 6.0%, and the 20th paper is 20%. The eyes are 6.2%, and the 100th sheet is 6.2%. In other words, the moisture content of the paper in the bundle of papers in the recording material cassette 11 in the extremely dry surrounding environment is greatly different between the first to fifth sheets from the top, and the tenth sheet to the following. Makes little difference. Immediately after taking out the above-mentioned bundle of papers from the pack, the moisture content of each paper is 6.2%, and the moisture content of the 20th and subsequent papers from the top of the bundle of papers in the recording material cassette 11 is high. Is the same as.

Therefore, in the present embodiment, the following secondary transfer voltage control is performed in an environment in which the amount of water contained in the recording material P in the bundle of recording materials P stored in the recording material cassette 11 is greatly biased. To do. That is, in the present embodiment, the secondary transfer voltage applied to the leading edge of the next recording material when the secondary transfer voltage is corrected while the recording material P is passing is the corrected secondary transfer voltage. Is multiplied by a predetermined coefficient to obtain a voltage value. Particularly, in the present embodiment, a coefficient that reduces the correction width from the secondary transfer voltage before correction is used.

FIG. 11 schematically shows changes in the secondary transfer voltage and the secondary transfer current value when continuous image formation is performed according to this embodiment. In FIG. 11, continuous image formation was performed using an A4 size paper having a basis weight of 90 g/m ² as the recording material P in an ambient environment (water content 0.88 g/kg) at 23° C. and 5% RH. An example in which the secondary transfer current detected during the passage of the first recording material P is below the lower limit is shown. In this case, the lower limit value of the predetermined current range is 50 μA and the upper limit value is 70 μA (FIG. 7). In addition, in this case, the target current Ittarget is 60 μA (FIG. 5), and the secondary transfer partial charge voltage Vb obtained using this target current Ittarget is 1500V. In this case, the recording material sharing voltage Vp is 1000V (FIG. 6), and the secondary transfer voltage, which is the sum of this Vp and the above Vb, is 2500V. The secondary transfer current detected when the secondary transfer voltage is applied to the leading edge of the first recording material P is 40 μA. The state of the water content of the recording material P stored in the recording material cassette 11 is the same as that described with reference to FIG.

In the example shown in FIG. 11, at the secondary transfer voltage of 2500 V applied to the leading edge of the first recording material P, the secondary transfer current detected during sheet feeding is 40 μA, which is below the lower limit value of 50 μA. There is. Therefore, the secondary transfer voltage is changed so as to be gradually increased in the same manner as in Example 1, and when the corrected secondary transfer voltage finally becomes 3200 V, the detected secondary transfer current becomes the lower limit. The value of 50 μA is reached.

Next, in this embodiment, the secondary transfer voltage applied to the leading edge of the second recording material P is 3130V obtained as follows. That is, in the present embodiment, the uncorrected secondary transfer voltage 2500 V applied to the leading end of the first recording material P and the corrected secondary transfer voltage during the passage of the first recording material P. The difference from 3200V is 700V. Then, in this embodiment, 630V obtained by multiplying the difference 700V by 9/10 is added to 2500V before the correction, and 3130V is set as the secondary transfer voltage applied to the leading end of the second recording material P. In this example, if the secondary transfer current detected during the passage of the leading edge of the first recording material P is below the lower limit value, as described above, This is because the electric resistance of the recording material P gradually decreases until the required secondary transfer voltage. Similarly, for the secondary transfer voltage applied to the leading end of the third recording material P, 3060V obtained by adding 560V obtained by multiplying the difference 700V by 8/10 to 2500V before correction is used as the second recording material. The secondary transfer voltage is applied to the tip of P. Similarly, the secondary transfer voltage applied to the leading ends of the fourth to tenth recording materials P is gradually reduced, and the secondary transfer voltage applied to the leading ends of the eleventh and subsequent recording materials P is 10. The voltage value is substantially the same as the secondary transfer voltage during the passage of the recording material P of the first sheet.

FIG. 12 is a flowchart showing the outline of an example of the procedure of the secondary transfer voltage control in this embodiment. Here, the procedure in the case of the example shown in FIG. 11 will be described. The processing of S201 to S210 of FIG. 11 is the same as the processing of S101 to S110 of FIG. 4, respectively. However, in FIG. 11, for convenience, the secondary transfer voltage applied to the leading edge of the first recording material P is V0, and the corrected secondary transfer voltage during the passage of the first recording material P is V1, 2 Secondary transfer voltages applied during the passage of the recording material P after the first sheet are V2, V3,...

When the secondary transfer current detected during the passage of the first recording material P is not within the predetermined current range (S210: NO), it is within the predetermined current range as in the first embodiment. Thus, the secondary transfer voltage V0 is corrected to V1 (S211). After that, when the image formation on the first recording material P is completed (S212) and the image formation on the second recording material P is performed (S213), the following processing is performed. That is, the control unit 50 sets the secondary transfer voltage applied to the leading end of the second recording material P to the secondary transfer voltage V0 before correction and the corrected secondary transfer voltage V0 during the passage of the first recording material P. The secondary transfer voltage V2 obtained from the following equation is set based on the next transfer voltage V1 (S214).
V2=V0+((V1-V0)×9/10)

After that, when the image formation on the second recording material P is completed (S215) and the image formation on the third recording material P is performed (S216), the following processing is performed. That is, the control unit 50 sets the secondary transfer voltage applied to the leading edge of the third recording material P to the secondary transfer voltage V0 before correction and the corrected secondary transfer voltage V0 during the passage of the first recording material P. Based on the next transfer voltage V1, the secondary transfer voltage V3 obtained from the following equation is set (S217).
V3=V0+((V1-V0)×8/10)

The same applies to the secondary transfer voltages applied to the leading ends of the recording materials P from the fourth sheet to the tenth sheet, and the secondary transfer voltages V4 to V10 obtained from the following equations, respectively. Then, the secondary transfer voltage applied to the leading edge of the 11th and subsequent recording materials P is set to a voltage value substantially the same as the secondary transfer voltage during the passage of the 10th recording material P (S218).
V4=V0+((V1-V0)×7/10)
V5=V0+((V1-V0)×6/10)
V6=V0+((V1-V0)×5/10)
V7=V0+((V1-V0)×4/10)
V8=V0+((V1-V0)×3/10)
V9=V0+((V1-V0)×2/10)
V10=V0+((V1-V0)×1/10)

On the other hand, when the secondary transfer current detected during the passage of the first recording material P is within the predetermined current range (S210: YES), the secondary transfer voltage is not corrected (S219). ~ S225).

Although details are omitted in FIG. 12, the control unit 50 ends the operation of the job when the image formation on all the recording materials P of the job is completed.

As described above, in this embodiment, the secondary transfer voltage applied to the leading edge of the second and subsequent recording materials P during continuous image formation is set to the secondary transfer voltage during the passage of the first recording material P. According to the correction amount, it is set to be smaller than the secondary transfer voltage during the passage of the preceding recording material P. Thereby, it becomes possible to consider the distribution of the water content of the recording material P in the bundle of the recording materials P in the recording material cassette 11. Particularly, in the present embodiment, the water content of the recording material P gradually increases from above the bundle of the recording materials P, and the water content of the recording material P when the bundle of the recording materials P is packed by the tenth sheet. It is almost the same as the quantity. In this embodiment, the secondary transfer voltage can be appropriately controlled with respect to the distribution of the water content of the recording material P in the bundle of the recording materials P in the recording material cassette 11 as described above. Therefore, in the first recording material P, an image defect due to insufficient transfer current can be suppressed, and in the second and subsequent recording materials P, an appropriate secondary transfer voltage can be applied to the change in the water content of the recording material P. Can be set.

In the present embodiment, the case where the secondary transfer current detected is below the lower limit value in an extremely dry ambient environment has been described as an example, but it is detected in an ambient environment with extremely high humidity, for example. The same control can be performed when the secondary transfer current exceeds the upper limit value. In this case, the recording material P in the recording material cassette 11 gradually decreases in water content from the top to the bottom, and the electric resistance gradually increases. In order to accommodate this, contrary to the present embodiment, the secondary transfer voltage applied to the leading edge of the second and subsequent recording materials P during continuous image formation is applied to the first recording material P during sheet feeding. The secondary transfer voltage may be set higher than the secondary transfer voltage during the passage of the preceding recording material P according to the correction amount of the secondary transfer voltage.

Further, the control of the secondary transfer voltage of the present embodiment can be executed when the surrounding environment satisfies a predetermined condition. For example, when the amount of water in the surrounding environment is smaller than a predetermined threshold value, it is possible to execute the control of gradually decreasing the secondary transfer voltage. Further, for example, when the amount of water in the surrounding environment is larger than another predetermined threshold value, the above-described control of gradually increasing the secondary transfer voltage can be executed. Then, when the surrounding environment does not satisfy the predetermined condition, the control described in the first embodiment can be executed.

As described above, in the present embodiment, when the control unit 50 changes the transfer voltage so that the absolute value of the transfer voltage increases while the first recording material P is passing through the transfer portion N2, The initial value of the transfer voltage when the recording material P passes the transfer portion N2 has a larger absolute value than the initial value of the transfer voltage when the first recording material P passes the transfer portion N2, and The recording material P has a voltage value smaller than the absolute value of the transfer voltage after the above-described change when the recording material P is passing through the transfer portion N2. In particular, in the present embodiment, when the control means 50 changes the transfer voltage so as to increase the absolute value of the transfer voltage when the first recording material P is passing through the transfer portion N2, the transfer portions N2 are sequentially transferred. The initial value of the transfer voltage when each of the plurality of second recording materials P passing therethrough passes through the transfer portion N2 is smaller than the initial value of the transfer voltage for the second recording material P that later passes through the transfer portion N2. The value. Further, when the control unit 50 changes the transfer voltage so that the absolute value of the first recording material P is reduced while the first recording material P is passing through the transfer portion N2, the second recording material P is transferred to the transfer portion N2. The initial value of the transfer voltage when passing the first recording material P is smaller than the initial value of the transfer voltage when the first recording material P passes the transfer portion N2, and the first recording material P is transferred to the transfer portion N2. It is possible to make the voltage value larger than the absolute value of the transfer voltage after the above change when passing through N2. At this time, when the first recording material P is changed so as to reduce the absolute value of the transfer voltage while the first recording material P is passing through the transfer portion N2, the control means 50 sequentially passes through the plurality of transfer portions N2. The initial value of the transfer voltage when each of the second recording materials P passes through the transfer portion N2 is set to be as large as the initial value of the transfer voltage for the second recording material P that subsequently passes through the transfer portion N2. You can Further, in the present embodiment, the control unit 50 sets the initial value of the transfer voltage when the second recording material P passes the transfer portion N2 to the initial value when the first recording material P passes the transfer portion N2. The voltage value obtained by multiplying the transfer voltage after the above change by a predetermined coefficient.

As described above, according to the present embodiment, the same effect as that of the first embodiment can be obtained, and it is appropriate according to the change in the water content of the recording material P when the surrounding environment is extremely dry. It is possible to set the secondary transfer voltage.

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

The present invention can be equally applied 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 (10)

An image carrier that carries a toner image,
A transfer member forming a transfer portion for transferring the toner image from the image carrier to a recording material,
An applying unit that applies a transfer voltage for the transfer to the transfer unit;
A detection means for detecting a current flowing through the transfer portion,
While the recording material is passing through the transfer section, the transfer voltage is controlled to a constant voltage value at a predetermined voltage value, and the transfer voltage is controlled so that the current detected by the detection unit falls within a predetermined current range. Changeable control means,
In the image forming apparatus having
When the first recording material is changing the transfer voltage while the first recording material is passing through the transfer portion during continuous image formation in which images are continuously formed on a plurality of recording materials, the control means sets the first The initial value of the transfer voltage when the second recording material passing through the transfer portion after the recording material of the first transfer material passes through the transfer portion, An image forming apparatus, wherein the image forming apparatus is determined based on the transfer voltage after the change. When the first recording material changes the transfer voltage so as to increase its absolute value while the first recording material is passing through the transfer portion, the second recording material causes the transfer portion to move. The initial value of the transfer voltage when passing the first recording material is set to a voltage value whose absolute value is larger than the initial value of the transfer voltage when the first recording material passes through the transfer portion. 1. The image forming apparatus according to 1. When the first recording material changes the transfer voltage so as to reduce the absolute value of the transfer voltage while the first recording material is passing through the transfer portion, the second recording material causes the transfer portion to move. The initial value of the transfer voltage when passing is set to a voltage value whose absolute value is smaller than the initial value of the transfer voltage when the first recording material passes through the transfer section. The image forming apparatus according to 1 or 2. The control means changes the initial value of the transfer voltage when the second recording material passes through the transfer portion after the change when the first recording material passes through the transfer portion. The image forming apparatus according to claim 1, wherein a voltage value that is substantially the same as the transfer voltage is used. When the control unit does not change the transfer voltage when one recording material is passing through the transfer unit during the continuous image formation, the control unit controls when the next recording material passes through the transfer unit. 5. The initial value of the transfer voltage is set to a voltage value that is substantially the same as the transfer voltage when the one recording material is passing through the transfer section. The image forming apparatus according to item 1. When the first recording material changes the transfer voltage so as to increase its absolute value while the first recording material is passing through the transfer portion, the second recording material causes the transfer portion to move. The initial value of the transfer voltage when passing the first recording material is larger in absolute value than the initial value of the transfer voltage when the first recording material passes through the transfer portion, and the first recording material is The image forming apparatus according to claim 1, wherein the image forming apparatus has a voltage value that is smaller than an absolute value of the transfer voltage after the change while passing through the transfer unit. When the first recording material changes the transfer voltage so as to increase its absolute value while the first recording material is passing through the transfer unit, the control unit sequentially passes through the plurality of transfer units. The initial value of the transfer voltage when each of the two recording materials passes through the transfer portion is set to a voltage value smaller than the initial value of the transfer voltage for the second recording material that later passes through the transfer portion. The image forming apparatus according to claim 6, wherein. When the first recording material changes the transfer voltage so as to reduce the absolute value of the transfer voltage while the first recording material is passing through the transfer portion, the second recording material causes the transfer portion to move. The initial value of the transfer voltage when passing the first recording material is smaller in absolute value than the initial value of the transfer voltage when the first recording material passes through the transfer portion, and the first recording material is 8. The image forming apparatus according to claim 1, wherein the voltage value is set to be larger than the absolute value of the transfer voltage after the change when passing through the transfer portion. .. When the first recording material changes the transfer voltage so as to reduce its absolute value while the first recording material is passing through the transfer unit, the control unit sequentially transfers the plurality of first transfer members to the transfer unit. The initial value of the transfer voltage when each of the two recording materials passes through the transfer section is set to a voltage value that is larger than the initial value of the transfer voltage for the second recording material that passes through the transfer section later. The image forming apparatus according to claim 8. The control means changes the initial value of the transfer voltage when the second recording material passes through the transfer portion after the change when the first recording material passes through the transfer portion. The image forming apparatus according to claim 6, wherein the transfer voltage is a voltage value obtained by multiplying the transfer voltage by a predetermined coefficient.