Image forming device

Abstract

To prevent generation of "transfer memory" when images are continuously formed on recording materials with different paper widths, while preventing harmful effects such as a reduction in productivity and an increase in size and cost of an apparatus.SOLUTION: In continuously feeding a recording material P with a narrow paper width and a recording material P with a wide paper width in this order, a control unit 150 of an image forming apparatus 100 controls to increase back contrast in feeding the recording material P with a wide paper width compared with back contrast in feeding the recording material P with a narrow paper width.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus using an electrophotographic system, such as an electrophotographic copying machine or an electrophotographic printer (for example, an LED printer or a laser beam printer). [Background technology]

[0002] In an electrophotographic image forming apparatus, the surface of a photoconductor, such as a rotatable photoconductor drum, is uniformly charged to a predetermined potential, and the charged surface of the photoconductor is exposed to light according to image information, forming an electrostatic latent image on the photoconductor. The electrostatic latent image formed on the photoconductor is then developed with toner to form a toner image on the photoconductor, which is then transferred to a sheet-like recording material, such as paper. The electrostatic latent image on the photoconductor is developed, for example, by the action of a development contrast, which is the potential difference between the potential of the image area (exposed area) on the surface of the photoconductor and a development bias applied to a developing member, such as a developing roller, that contacts the photoconductor to form a development area (development nip). The toner image is transferred from the photoconductor to the recording material using a transfer member, such as a transfer roller that contacts the photoconductor to form a transfer area (transfer nip). During the transfer process, a transfer bias of a polarity opposite to the normal charge polarity of the toner constituting the toner image is applied to the transfer member. Although the recording material is sometimes referred to as "paper," the recording material is not limited to paper and may be a plastic sheet, cloth, or the like. The length of the recording material in a direction substantially perpendicular to the conveying direction is also referred to as the "paper width (or simply width)." Conveying the recording material and passing it through the transfer section is also referred to as "paper passing."

[0003] When repeated image formation is performed on a rotating photoconductor, the history of the previous image formation may affect the next image formation. For example, when a charge is applied from a transfer member to the photoconductor during the transfer process, the surface of the photoconductor may not be uniformly charged to the desired potential during the subsequent charging process, resulting in uneven charging of the photoconductor surface. In this case, toner may be supplied from the developing member to the photoconductor even in non-image areas (non-exposed areas), resulting in an image defect known as "transfer memory."

[0004] Furthermore, it is known that the charging unevenness on the surface of the photoconductor due to the transfer process described above is more likely to occur in areas corresponding to "non-paper-passing areas" in the transfer section than in areas corresponding to "paper-passing areas." Here, "paper-passing areas" refer to areas through which the recording material passes in a direction substantially perpendicular to the recording material's transport direction in the transfer section, and "non-paper-passing areas" refer to areas through which the recording material does not pass in a direction substantially perpendicular to the recording material's transport direction in the transfer section. This is because the amount of charge imparted to the photoconductor from the transfer member is greater in non-paper-passing areas where the transfer member and the photoconductor are in direct contact than in paper-passing areas. Therefore, for example, if narrow paper is passed through the transfer section and then standard-size paper, which is wider than the narrow paper, is passed through the transfer section, "transfer memory" is more likely to occur in the areas of the standard-size paper that correspond to the non-paper-passing areas of the narrow paper.

[0005] Conventionally, the following method has been known as a countermeasure against "transfer memory" when recording materials of different widths are passed continuously: By widening the interval between passing sheets ("sheet interval"), image formation on the next recording material begins after the charging unevenness caused by the passing of the previous recording material is alleviated.

[0006] Also, as described in Patent Document 1, a method is known in which a pre-charge exposure device having a light source such as an LED uniformly irradiates light onto a photosensitive member (full-surface exposure, pre-charge exposure) to reduce charging unevenness. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 5197264 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the method of increasing the interval until the next paper feed to alleviate the charging unevenness caused by the previous paper feed requires a longer interval between papers, which increases the time it takes to complete a print job and can reduce productivity.

[0009] Furthermore, pre-charging exposure requires a light source such as an LED, a light guide for uniformly irradiating the light emitted from the light source in a direction approximately perpendicular to the direction of movement of the surface of the photoconductor, etc. This may result in an increase in the size and cost of the device (process cartridge or main body of the image forming device).

[0010] Therefore, the object of the present invention is to suppress the occurrence of "transfer memory" when forming images continuously on recording materials of different paper widths, while suppressing adverse effects such as reduced productivity and increased size and cost of the device. [Means for solving the problem]

[0011] The above object is achieved by the image forming apparatus according to the present invention. According to one aspect ofan exposure unit that exposes the charged surface of the photoreceptor to light to form an electrostatic image on the photoreceptor; a developing member that supplies toner to the electrostatic image on the photoreceptor in a developing unit to form a toner image on the photoreceptor; a developing voltage application unit that applies a developing voltage to the developing member; a transfer member that contacts the photoreceptor to form a transfer unit and transfers the toner image from the photoreceptor to a recording material passing through the transfer unit; a transfer voltage application unit that applies a transfer voltage to the transfer member; and a control unit that controls at least one of the charging voltage application unit, the developing voltage application unit, and the exposure unit, When the absolute value of the difference between the potential of the non-image portion of the surface of the photoreceptor and the potential of the developing voltage is defined as the back contrast, when transferring to a first recording material having a first width in a direction substantially perpendicular to the conveying direction of the recording material and then transferring to a second recording material having a second width larger than the first width, the second back contrast, which is the back contrast formed when the area of ​​the surface of the photoreceptor forming the transfer portion reaches the developing portion when the second recording material passes through the transfer portion, can be controlled to be larger than the first back contrast, which is the back contrast formed when the area of ​​the surface of the photoreceptor forming the transfer portion reaches the developing portion when the first recording material passes through the transfer portion. and the control unit controls the charging voltage application unit to change the surface potential of the photosensitive member formed by the charging process performed by the charging unit so that the second back contrast is greater than the first back contrast. An image forming apparatus characterized by is provided . According to another aspect of the present invention, a photosensitive member includes a rotatable photosensitive member, a charging unit that charges the surface of the photosensitive member, a charging voltage applying unit that applies a charging voltage to the charging unit, an exposure unit that exposes the charged surface of the photosensitive member to light to form an electrostatic image on the photosensitive member, a developing member that supplies toner to the electrostatic image on the photosensitive member in a developing unit to form a toner image on the photosensitive member, a developing voltage applying unit that applies a developing voltage to the developing member, and a transfer unit that contacts the photosensitive member to form a transfer unit that transfers the toner image from the photosensitive member to a recording material that passes through the transfer unit. In an image forming apparatus having a transfer member, a transfer voltage application unit that applies a transfer voltage to the transfer member, and a control unit that controls at least one of the charging voltage application unit, the developing voltage application unit, and the exposure unit, the control unit is configured to perform transfer onto a first recording material having a first width in a direction substantially perpendicular to a conveying direction of the recording material, and then to perform transfer onto a second recording material having a second width that is larger than the first width, when the absolute value of the difference between the potential of a non-image portion of the surface of the photosensitive member in the developing unit and the potential of the developing voltage is defined as a back contrast. and a second back contrast, which is the back contrast formed when a surface area of ​​the photosensitive member forming the transfer section reaches the developing section when the second recording material passes through the transfer section, can be controlled to be larger than a first back contrast, which is the back contrast formed when a surface area of ​​the photosensitive member forming the transfer section reaches the developing section when the first recording material passes through the transfer section, when transferring the first recording material to the second recording material; the exposure section exposes the charged surface of the photosensitive member with a first output that forms a potential of a non-image section and a second output that forms a potential of an image section to form an electrostatic image on the photosensitive member; and the control section controls the exposure section to change the first output so that only the second back contrast in a surface area of ​​the photosensitive member that does not contact the first recording material at the transfer section and that contacts the second recording material at the transfer section is larger than the first back contrast. [Effects of the Invention]

[0012] According to the present invention, it is possible to suppress the occurrence of "transfer memory" when forming images continuously on recording materials of different paper widths, while suppressing adverse effects such as reduced productivity and increased size and cost of the device. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Figure 2] FIG. 2 is a schematic block diagram showing a control mode of a main part of the image forming apparatus. [Figure 3] FIG. 2 is a schematic diagram of a measuring device for measuring the electrical resistance of a transfer roller. [Figure 4] FIG. 2 is a schematic diagram showing the state of a transfer unit for explaining the first embodiment. [Figure 5] FIG. 4 is a graph showing an example of the transition of the surface potential of the photosensitive drum. [Figure 6] FIG. 10 is a flowchart illustrating an example of a procedure for a print job. [Figure 7] FIG. 10 is a schematic diagram showing the state of a transfer unit for explaining a fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0015] [Example 1] 1. Overall configuration and operation of the image forming apparatus 1 is a schematic cross-sectional view of an image forming apparatus 100 according to this embodiment. The image forming apparatus 100 according to this embodiment is a laser beam printer that uses an electrophotographic method.

[0016] The image forming apparatus 100 includes an image forming unit S. The image forming unit S includes a photosensitive drum 1, which is a rotatable drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as an image carrier. The photosensitive drum 1 is driven by a drive source (not shown) to rotate in the direction of arrow R1 (clockwise) in FIG. 1 at a peripheral speed (process speed) of 250 mm / sec. The image forming unit S also includes a charging roller 2, an exposure device 3, a developing device 5, and a cleaning device 4, which are arranged around the photosensitive drum 1 in this order along the direction of rotation of the photosensitive drum 1. In this embodiment, the photosensitive drum 1 and the charging roller 2, developing device 5, and cleaning device 4, which act as process means, integrally form a process cartridge 6 that is detachable from the main body M of the image forming apparatus 100. The image forming apparatus 100 also includes a paper feed cassette 7, located at the bottom of the main body M, that stores sheet-like recording material P, such as paper. The image forming apparatus 100 also includes, in order along the transport path of the recording material P from the paper feed cassette 7, a paper feed roller 8, a pair of transport rollers 9, a top sensor 10, a pre-transfer guide 11, a transfer roller 12, a transport guide 13, a fixing device 14, a paper discharge roller 15, and a paper discharge tray 16.

[0017] During image formation, the surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by the charging roller 2, a roller-shaped charging member serving as a charging means (charging unit). During the charging process, a predetermined charging bias (charging voltage) is applied to the charging roller 2 by a charging power supply (high-voltage power supply) E1 ( FIG. 2 ) serving as a charging voltage application unit. Here, the position on the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 where charging is performed by the charging roller 2 is the charging position. In this embodiment, the charging roller 2 charges the surface of the photosensitive drum 1 by utilizing discharge generated in at least one of minute gaps formed upstream and downstream of the contact point between the charging roller 2 and the photosensitive drum 1 in the rotation direction of the photosensitive drum 1. However, for simplicity, the position on the photosensitive drum 1 that contacts the charging roller 2 may be considered to be the charging position.

[0018] The charged surface of the photosensitive drum 1 is irradiated with light L based on image information by an exposure device 3 serving as an exposure means (exposure unit). This removes the charge from the exposed portion of the photosensitive drum 1, and an electrostatic latent image (electrostatic image) corresponding to the image information is formed on the photosensitive drum 1. In this embodiment, the exposure device 3 is a laser scanner. This laser scanner has a spot diameter of approximately 60 μm on the photosensitive drum 1 and is configured to be able to form an image at a resolution of 600 dpi even with deviations in the main scanning direction and sub-scanning direction. The main scanning direction is a direction approximately perpendicular to the direction of movement of the surface of the photosensitive drum 1. The sub-scanning direction is a direction approximately parallel to the direction of movement of the surface of the photosensitive drum 1 and approximately perpendicular to the main scanning direction. Here, the position on the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 where light is irradiated by the exposure device 3 is the exposure position.

[0019] The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by the developing device 5 as a developing means, which supplies toner as a developer, and forms a toner image (toner image, developer image) on the photosensitive drum 1. The developing device 5 has a developer container 5a that contains toner, a developing roller 5b as a developer carrier (developing member), and a developing blade 5c as a developer regulating member. In this embodiment, during the development process, the developing roller 5b is brought into contact with the surface of the photosensitive drum 1 to form a development zone (development nip) D, which is the contact area between the photosensitive drum 1 and the developing roller 5b. During the development process, the developing roller 5b is rotated in the direction of arrow R2 in FIG. 1 (counterclockwise). The toner in the developer container 5a is supplied to the surface of the rotating developing roller 5b, and is triboelectrically charged by the developing blade 5c, forming a toner layer on the surface of the developing roller 5b. During the development process, a predetermined development bias (development voltage) is applied to the development roller 5b by a development power supply (high-voltage power supply) E2 (FIG. 2) serving as a development voltage application unit. This causes toner to adhere to the electrostatic latent image on the photosensitive drum 1 in the development unit D, forming a toner image. In this embodiment, toner charged with the same polarity as the charge polarity of the photosensitive drum 1 (negative in this embodiment) adheres to the exposed area (image area) of the photosensitive drum 1, where the absolute value of the potential has been reduced by exposure after uniform charging (reverse development method). In this embodiment, the normal charge polarity of the toner, which is the primary charge polarity of the toner during the development process, is negative. In this embodiment, a non-magnetic one-component developer is used as the developer. The position on the photosensitive drum 1 where toner is supplied from the development roller 5b in the rotational direction of the photosensitive drum 1 (the position where the development roller 5b abuts) is the development position, which corresponds to the position on the photosensitive drum 1 where the development unit D is formed.

[0020] A transfer roller 12, a roller-shaped transfer member serving as a transfer means, is disposed facing the photosensitive drum 1. The transfer roller 12 is pressed against the photosensitive drum 1, forming a transfer nip N, which is a contact portion between the photosensitive drum 1 and the transfer roller 12. The toner image formed on the photosensitive drum 1 is transferred by the action of the transfer roller 12 to a recording material P, which is conveyed while being sandwiched between the photosensitive drum 1 and the transfer roller 12, at the transfer nip N. The recording material P is stored in a paper feed cassette 7, which serves as a recording material storage unit. The recording material P is fed one sheet at a time by a paper feed roller 8, conveyed by a conveyance roller pair (a registration roller pair) 9, and conveyed to the transfer unit N while being guided by a pre-transfer guide 11. In this embodiment, the image forming apparatus 100 is provided with a plurality of paper feed cassettes 7, each capable of storing a recording material P of a different size. The image forming apparatus 100 can feed the recording material P from a paper feed cassette 7 designated by information on a print job (described later). In this embodiment, a paper width sensor 17 is disposed on the conveyance path of the recording material P from the paper feed cassette 7 to the transfer unit N as a paper width detection unit for detecting the paper width of the recording material P (the length in a direction substantially perpendicular to the conveyance direction of the recording material P). The paper width sensor 17 is disposed, for example, on the conveyance path from the paper feed roller 8 to the conveyance roller pair 9. Alternatively, a paper width sensor 17 may be disposed in each paper feed cassette 7. During the transfer process, a transfer bias (transfer voltage), which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the core metal 12a of the transfer roller 12 by a transfer power supply (high-voltage power supply) E3 (FIG. 2) serving as a transfer voltage application unit. As a result, the toner image on the photosensitive drum 1 is transferred to a predetermined position on the recording material P passing through the transfer unit N. Here, the position where toner is transferred to the recording material P on the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 (the position where the transfer roller 12 abuts) is the transfer position, which corresponds to the position on the photosensitive drum 1 that forms the above-mentioned transfer section N.

[0021] The recording material P bearing an unfixed toner image on its surface is conveyed along a conveying guide 13 to a fixing device 14 serving as a fixing means. The fixing device 14 has a drive roller 14a and a fixing roller 14c incorporating a heater 14b. The drive roller 14a is pressed against the fixing roller 14c, forming a fixing section (fixing nip) where the drive roller 14a and the fixing roller 14c contact each other. The fixing device 14 applies heat and pressure to the recording material P passing through the fixing section, thereby fixing (melting and adhering) the toner image onto the recording material P. After the toner image has been fixed, the recording material P is discharged (output) by a paper discharge roller 15 onto a paper discharge tray 16 provided on the top surface of the apparatus main body M.

[0022] On the other hand, toner (transfer residual toner) remaining on the surface of the photosensitive drum 1 without being transferred to the recording material P after the transfer step is removed from the surface of the photosensitive drum 1 and collected by the cleaning device 4. The cleaning device 4 has a cleaning blade 4a as a cleaning member and a waste toner container 4b. The cleaning device 4 scrapes the transfer residual toner from the surface of the rotating photosensitive drum 1 with the cleaning blade 4a that contacts the surface of the photosensitive drum 1 and collects it in the waste toner container 4b. Note that the image forming apparatus 100 of this embodiment is not provided with a pre-charge exposure device or the like as a discharging means for exposing the surface of the photosensitive drum 1 to electricity to discharge the surface of the photosensitive drum 1 after the transfer step and before the charging step.

[0023] The photosensitive drum 1 generally has a configuration in which a photosensitive material such as OPC (organic photoconductor), amorphous selenium, or amorphous silicon is provided on a drum-shaped (cylindrical) substrate (conductive substrate) made of aluminum, nickel, or the like. The photosensitive drum 1 used in this embodiment is a negatively charged OPC photosensitive drum with an outer diameter of 24 mm. This photosensitive drum 1 has a photosensitive layer formed on the surface of a conductive substrate made of an aluminum cylinder, in which a charge generation layer and a charge transport layer are stacked in this order from the conductive substrate side.

[0024] The transfer roller 12 is composed of a conductive shaft (core) 12a, which also serves as a power supply electrode, and an elastic layer 12b that cylindrically surrounds the outer periphery of the core. The elastic layer 12b is typically made of semiconductive rubber, such as EPDM, NBR, urethane rubber, epichlorohydrin, or silicone rubber. The transfer roller 12 used in this embodiment has an outer diameter of 14 mm, a core diameter of 5 mm, an elastic layer thickness of 4.5 mm, and a hardness of 30° (Asker C hardness). The core 12a of this transfer roller 12 is made of stainless steel (SUS), and the elastic layer 12b is made of a rubber mixture of NBR and epichlorohydrin. In this embodiment, the contact pressure (contact pressure) of the transfer roller 12 against the photosensitive drum 1 is 9.8 N (1 kgf).

[0025] In this embodiment, during image formation, the surface of the photosensitive drum 1 is uniformly charged by the charging roller 2 to a surface potential (non-image area potential, non-exposed area potential, dark area potential, charging potential) of −500 V. After being uniformly charged and exposed to a laser by the exposure device 3, the surface potential (image area potential, exposed area potential, light area potential) of the photosensitive drum 1 becomes approximately −125 V. Due to the contrast between the non-image area potential and the image area potential, an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 1, with the non-image area potential representing the non-image area and the image area potential representing the image area. In this embodiment, the toner is negatively charged in the developing device 5. During image formation, a DC voltage of −375 V is applied to the developing roller 5 b as a development bias. During image formation, a DC voltage of, for example, +2500 V is applied to the transfer roller 12 as a transfer bias.

[0026] Although this embodiment uses negatively charged toner, the present invention is not limited to this embodiment. The present invention can also be applied when positively charged toner is used. When toner whose normal charging polarity is positive is used, for example, the surface of the photosensitive drum 1 is uniformly charged to a positive polarity (reverse development method), and a negative transfer bias is applied to the transfer roller 12 during the transfer process. When toner whose normal charging polarity is positive is used, the magnitude (high / low) relationships of voltages and potentials related to the charging bias, development bias, transfer bias, etc. are generally opposite to those described in this embodiment. Those skilled in the art will be able to appropriately set these voltages and potentials when using positively charged toner based on the disclosures herein.

[0027] 2 is a schematic block diagram showing the control mode of the main parts of image forming apparatus 100 of this embodiment. Image forming apparatus 100 is provided with a control unit 150. Control unit 150 has a CPU 151 as an arithmetic control means which is a central element for performing arithmetic processing, a memory (storage element) 152 such as a ROM or RAM as a storage means, an input / output unit (not shown) that controls the exchange of signals between various elements connected to control unit 150, and the like. The RAM stores the detection results of sensors, arithmetic results, etc., and the ROM stores a control program, a pre-determined data table, etc.

[0028] The control unit 150 is a control unit capable of comprehensively controlling the operation of the image forming apparatus 100. The control unit 150 executes a predetermined image formation sequence by controlling the transmission and reception of various electrical information signals and drive timing. The control unit 150 is connected to each unit of the image forming apparatus 100. For example, in this embodiment, the control unit 150 is connected to a charging power supply E1, a developing power supply E2, a transfer power supply E3, an exposure device 3, a paper width sensor 17, and the like. The control unit 150 controls the ON / OFF and output values ​​of the various power supplies E1, E2, and E3, the exposure amount of the exposure device 3, and the like, based on signals (detection signals) input from various sensors such as the paper width sensor 17, thereby controlling the image formation operation and the like.

[0029] The image forming apparatus 100 is capable of executing a print job (printing operation), which is a series of operations for forming an image on one or more recording materials P, initiated by a single start instruction. In this embodiment, the start instruction is input to the image forming apparatus 100 from an external device (not shown), such as a personal computer. A print job generally includes an image formation process (printing process), a pre-rotation process, a paper spacing process when forming images on multiple recording materials P, and a post-rotation process. The image formation process is a period during which an electrostatic latent image is actually formed on the photosensitive drum 1, the electrostatic latent image is developed (a toner image is formed), the toner image is transferred, and the toner image is fixed. This period is referred to as the image formation time. More specifically, the timing of the image formation process differs depending on the positions where the electrostatic latent image is formed, the toner image is formed, the toner image is transferred, and the toner image is fixed. The pre-rotation process is a period during which preparatory operations are performed prior to the image formation process. The inter-sheet process (inter-image process) is the period corresponding to the interval between recording materials P when the image forming process is performed continuously on a plurality of recording materials P (during continuous image formation). The post-rotation process is the period during which an organizing operation (preparatory operation) is performed after the image forming process. The non-image formation time is the period other than when image formation is being performed, and includes the pre-rotation process, inter-sheet process, post-rotation process, and the pre-multiple rotation process which is a preparatory operation when the image forming apparatus 100 is turned on or when it returns from a sleep state.

[0030] 2.Method for measuring the volume resistance of the transfer roller Next, we will explain the method for measuring the volume resistance of the transfer roller 12. Figure 3 is a schematic diagram of a system for measuring the volume resistance of the transfer roller 12. The measurement environment was set to a temperature of 23°C and a humidity of 50%.

[0031] A pressure of 4.9 N was applied to both ends of the core 12a of the transfer roller 12 in the direction of the rotation axis of the transfer roller 12, and the transfer roller 12 was pressed against the metal drum with a pressure of 9.8 N. The voltage Vref across the reference resistance Rref when voltage V1 was applied to the core 12a of the transfer roller 12 was measured using a digital multimeter (FLUK). The applied voltage V1 to the core 12a of the transfer roller 12 was set to 1000 V, and the reference resistance Rref was set to 1000 Ω. The voltage across the reference resistance Rref was measured 10 seconds after the voltage was applied to the core 12a of the transfer roller 12, and the average value over the 10-second measurement period was defined as Vref. Here, the current flowing through the reference resistance Ref was defined as Iref, the voltage applied to the transfer roller 12 as Vrol, and the current flowing through the transfer roller 12 as Irol. In this case, the volume resistance Rm of the transfer roller 12 was calculated using the following formula: Rm = Vrol / Irol (Equation 1) where Vrol and Irol are Vrol=V1-Vref (Equation 2) Irol=Vref / Rref (Equation 3) Substituting formula 2 and formula 3 into formula 1, we get Rm=(V1-Vref)×Rref / Vref (Formula 4) Therefore, based on Equation 4, the volume resistance Rm of the transfer roller 5 can be calculated from Vref measured by the above-mentioned measurement method.

[0032] In this embodiment, from the viewpoint of transferability, the volume resistance Rm of the transfer roller 12 is 1.0×10 6 ~5.0×10 9 In this example, the volume resistivity Rm is preferably in the range of 1.0×10 7 The transfer roller 12 used was Ω.

[0033] 3. Transfer memory Next, the mechanism by which "transfer memory" occurs will be described. First, the surface potential of the photosensitive drum 1 before and after passing through the transfer portion N will be described. In this embodiment, the surface of the photosensitive drum 1 is uniformly charged to -500 V by the charging roller 2. In this embodiment, the potential of the non-image portion of the surface of the photosensitive drum 1 is maintained at approximately -500 V even immediately before passing through the transfer portion N. In this embodiment, a positive voltage (transfer bias) is applied to the transfer roller 12 at the transfer portion N, and a positive charge is imparted from the transfer roller 12 to the photosensitive drum 1. Therefore, the surface potential (non-image portion potential) of the photosensitive drum 1 after passing through the transfer portion N becomes higher than -500 V (higher in polarity opposite to the normal charging polarity of the toner). In this case, a positive charge may be imparted from the transfer roller 12 to the photosensitive drum 1 to the extent that the surface potential (non-image portion potential) of the photosensitive drum 1 locally becomes positive. In such a case, the surface of the photosensitive drum 1 cannot be uniformly charged to −500 V in the next charging step, and charging unevenness may occur on the surface of the photosensitive drum 1.

[0034] In this embodiment, a developing bias of −375 V is applied to the developing roller 5 b. For example, if the surface potential of the unevenly charged portion of the surface of the photosensitive drum 1 is −375 V or higher, the surface potential will be higher than the potential of the developing bias. In this case, toner is supplied from the developing roller 5 b to the photosensitive drum 1 even though it is a non-image area (non-exposed area). The toner supplied to the photosensitive drum 1 is transferred onto the recording material P at the transfer portion N. In this way, the history of the potential on the photosensitive drum 1 generated during the previous transfer process affects the next image formation, and an image defect known as “transfer memory” may occur.

[0035] Even if the potential of the non-image portion is lower than the potential of the development bias, if the absolute value of the difference between the potential of the non-image portion and the potential of the development roller is small, toner may be supplied from the development roller 5b to the photosensitive drum 1. This phenomenon is likely to occur in an image forming apparatus 100 that employs a "contact development method" in which the development roller 5b contacts the photosensitive drum 1 to form the development portion D.

[0036] Furthermore, charging unevenness, which is a cause of "transfer memory," is more likely to occur in areas corresponding to "non-paper passing areas" in the transfer unit N than in areas corresponding to "paper passing areas" in the transfer unit N. This is because, in the transfer unit N, more current flows from the transfer roller 12 to the photosensitive drum 1 in the non-paper passing areas where the transfer roller 12 and the photosensitive drum 1 are in direct contact than in the paper passing areas. Note that when the electrical resistance of the recording material P is high, such as when the recording material P has a high basis weight, or when the volume resistance of the transfer roller 12 is low, current is more likely to flow in the non-paper passing areas.

[0037] 4. Transfer memory when recording materials of different widths are fed consecutively One printing condition (paper passing condition) that is likely to cause "transfer memory" is when recording materials P of different paper widths are passed continuously. The occurrence of "transfer memory" in this case will be described.

[0038] FIG. 4 is a schematic diagram showing the state of the transfer unit N when narrow recording material P and then wide recording material P are passed in succession. Region A in FIG. 4 is a region on the photosensitive drum 1 that corresponds to a non-paper passing portion at the transfer unit N when the narrow recording material P passes, in a direction substantially perpendicular to the conveyance direction of the recording material P at the transfer unit N. As described above, this region A is prone to charging unevenness. In this region A, due to charging unevenness that occurs when the narrow recording material P passes, toner may be supplied from the developing roller 5b to the photosensitive drum 1 in a non-image portion (non-exposed portion) during the development process for forming an image on the next wide recording material P. In this state, if a wide recording material P is passed after the narrow recording material P, the toner supplied to the photosensitive drum 1 will be transferred onto the wide recording material P, resulting in a "transfer memory."

[0039] A conventional method for addressing "transfer memory" when recording materials P of different widths are passed continuously involves repeatedly performing a charging process before passing a wider recording material P, thereby reducing charging unevenness. If the paper-passing interval ("paper gap") between the preceding and succeeding recording materials P is made longer than the circumferential length of the photosensitive drum 1, the charging process can be performed at least twice to compensate for the potential history on the photosensitive drum 1 generated during the transfer process. For example, if the paper gap is twice the circumferential length of the photosensitive drum 1, the charging process can be performed three times. By repeatedly performing the charging process in this manner, the potential history on the photosensitive drum 1 generated during the transfer process can be reduced, thereby reducing the aforementioned charging unevenness. However, this method requires a longer paper gap, which increases the time required for a single print job and may reduce productivity.

[0040] Another known method is to reduce charging unevenness by uniformly irradiating light onto the photosensitive drum 1 (full-surface exposure, pre-charge exposure) using a pre-charge exposure device having a light source such as an LED. However, this method requires a light source such as an LED and a light guide, which can increase the size and cost of the device.

[0041] Therefore, it is desirable to suppress the occurrence of "transfer memory" when forming images continuously on recording materials P of different paper widths, while suppressing the drawbacks of conventional methods such as reduced productivity and increased size and cost of the equipment.

[0042] Therefore, in this embodiment, when recording materials P of different paper widths are continuously fed, the occurrence of "transfer memory" is suppressed by changing the back contrast, which will be described later, and it is possible to form good images without image defects. This will be explained in more detail below.

[0043] 5.Back contrast to prevent transfer memory Next, a method for suppressing "transfer memory" in this embodiment when recording materials P with different paper widths are continuously passed will be described.

[0044] In this embodiment, when recording materials P of different paper widths are continuously fed, the control unit 150 controls to increase the "back contrast (Vb)," which is the absolute value of the difference between the potential of the non-image portion on the surface of the photosensitive drum 1 in the development unit D and the potential of the development bias. This makes it possible to suppress the occurrence of "transfer memory" when images are continuously formed on recording materials P of different paper widths, while suppressing adverse effects such as a decrease in productivity and an increase in the size and cost of the device.

[0045] FIG. 5 is a graph showing, by taking a certain portion of the surface of the photosensitive drum 1 as an example, the transition of the surface potential of that portion in a comparison between a case where the back contrast is small and a case where the back contrast is large.

[0046] First, at time point 1 in FIG. 5, the surface of the photosensitive drum 1 is charged by the charging roller 2. When the back contrast is small, the surface of the photosensitive drum 1 is charged to -500 V, and when the back contrast is large, the surface of the photosensitive drum 1 is charged to -600 V. The potential of the development bias is set to -375 V. Therefore, when the back contrast is small, the potential is 125 V, and when the back contrast is large, the potential is 225 V. In this embodiment, as an example, when the back contrast is small, a DC voltage of -1000 V is applied as the charging bias to the charging roller 2, and when the back contrast is large, a DC voltage of -1100 V is applied as the charging bias to the charging roller 2.

[0047] Next, at time point 2 in FIG. 5, the surface potential of the photosensitive drum 1 increases when passing through the transfer portion N.

[0048] Next, at time point 3 in Figure 5, the surface of the photosensitive drum 1 is charged again by the charging roller 2. At this time, as shown in Figure 5, the surface of the photosensitive drum 1 after charging may not drop completely to the desired non-image area potential. This part of the surface of the photosensitive drum 1 where the potential after charging is locally higher than the desired non-image area potential is the part of uneven charging that causes "transfer memory."

[0049] Next, time point 4 in Figure 5 is the timing when the portion with the uneven charging passes through the developing unit D. The surface potential of the photosensitive drum 1 at this time is compared between when the back contrast is small and when it is large. When the back contrast is small, the surface potential of the photosensitive drum 1 is -375 V or higher, which is larger than the potential of the developing bias. Therefore, toner is supplied from the developing roller 5b to the photosensitive drum 1. On the other hand, when the back contrast is large, the surface potential of the photosensitive drum 1 is lower than -475 V, which is lower than the potential of the developing bias and there is a sufficient potential difference that toner is not supplied from the developing roller 5b to the photosensitive drum 1. Therefore, toner is not supplied from the developing roller 5b to the photosensitive drum 1.

[0050] That is, when the back contrast is large, although charging unevenness remains, the occurrence of "transfer memory" can be suppressed.

[0051] However, when the back contrast is increased, it is necessary to consider the negative effects of "fog" and "deterioration in the reproducibility of one dot." This point will be explained next.

[0052] "Fogging" is a phenomenon in which the polarity of toner charged negatively on the developing roller 5b is reversed to positive polarity at the developing section D, causing the toner to be supplied to non-image areas on the photosensitive drum 1. This toner whose polarity has been reversed to positive polarity is not transferred onto the recording material P at the transfer section N, but is collected by the cleaning device 4. Therefore, although this "fogging" is unlikely to cause image defects, it increases the amount of toner consumed. The magnitude of the back contrast is related to the factor that causes the polarity of the toner on the developing roller 5b to be reversed, and it is known that the greater the back contrast, the more easily the toner polarity is reversed.

[0053] "Deterioration in reproducibility of one dot" refers to a phenomenon in which the reproducibility of a toner image deteriorates in the development process for an electrostatic latent image formed on the surface of the photosensitive drum 1. The reproducibility of a toner image is particularly prone to deterioration in halftones formed with one dot, and line images with a one-dot width. This is because an electrostatic latent image with a small image area, such as a one-dot image, is easily affected by the electric field of the surrounding non-image area. Furthermore, since the electric field in the non-image area becomes stronger the greater the back contrast, it is known that the greater the back contrast, the more likely "deterioration in reproducibility of one dot" occurs.

[0054] For these reasons, it is desirable to control the back contrast as follows: In other words, under printing conditions in which "transfer memory" is unlikely to occur, the back contrast is reduced to suppress "fog" and "deterioration in the reproducibility of one dot." And under printing conditions in which "transfer memory" is likely to occur, the back contrast is increased to suppress "transfer memory."

[0055] Therefore, in this embodiment, when narrow-width recording material P and wide-width recording material P are passed through consecutively in this order, the control unit 150 controls the back contrast to be increased when the wide-width recording material P is passed through.

[0056] 6. Confirmation of effectiveness Next, an evaluation experiment for confirming the effects of this embodiment will be described.

[0057] The evaluation conditions are as follows: The recording material P (paper) was B5 size with a basis weight of 68 g / m 2 CS-068 (Canon), LTR size, basis weight 75 g / m 2Vitality (manufactured by Xerox) was used. The B5-size recording material P is also referred to as "B5-size paper," and the LTR-size recording material P is also referred to as "LTR-size paper." These recording materials P were left in an environment of 15°C temperature and 10% humidity for two days, and were used in a state where the moisture content was 3% as measured with a microwave moisture meter Moistrex MX8000 (manufactured by NDC Infrared Engineering Ltd.). The image forming apparatus 100 was installed in an environment of 15°C temperature and 10% humidity, and printing operations were performed under these conditions. The transfer bias applied to the transfer roller 12 when the recording material P was transported to the transfer section N was +2500V. The printing operations were performed with the recording material P transport speed at 250 mm / sec, the printing speed at 40 sheets / min, and the sheet interval at 50 mm.

[0058] One print job consisted of sequentially passing B5-size paper and LTR-size paper through the printer. An image was printed on the LTR-size paper, with 1-dot horizontal lines (lines extending in the main scanning direction) spaced 50 spaces apart. When printing on B5-size paper, the development bias potential was set to -375V, the non-image area potential after charging was set to -500V, and the back contrast was set to 125V. When printing on LTR-size paper, the development bias potential was set to -375V, and the back contrast conditions were varied by changing the non-image area potential after charging to confirm the effect. The back contrast was set to 125V, 150V, 175V, 200V, 225V, and 250V.

[0059] Then, on LTR size paper, we checked whether or not "transfer memory" occurred, and "1-dot reproducibility (whether or not the 1-dot horizontal line image was reproduced without any defects such as chipping)." Regarding "transfer memory," if an image defect associated with it occurred, it was evaluated as × (poor), and if it did not occur, it was evaluated as ○ (good). Regarding "1-dot reproducibility," if there was any defect such as chipping in the 1-dot horizontal line image, it was evaluated as × (poor), and if it was reproduced without any defects such as chipping, it was evaluated as ○ (good). The results of the evaluation experiment are shown in Table 1.

[0060] [Table 1]

[0061] As shown in Table 1, under printing conditions where "transfer memory" is likely to occur, the occurrence of "transfer memory" on wide recording material P was suppressed when the back contrast was 175V or higher. The printing conditions where "transfer memory" is likely to occur are when a narrow recording material P and a wide recording material P are passed consecutively in that order, with the wide recording material P being passed. Furthermore, when the back contrast was 250V or higher, the "reproducibility of one dot" deteriorated. In other words, in order to obtain good reproducibility of one dot while suppressing the occurrence of "transfer memory," it is preferable to set the back contrast to 175V or higher and 225V or lower.

[0062] Furthermore, as mentioned above, the back contrast is correlated with "fog," and the higher the back contrast, the worse the "fog" may become. Therefore, in order to suppress "fog" while suppressing the occurrence of "transfer memory," it is preferable to set the back contrast as small as possible within a range in which the occurrence of "transfer memory" can be suppressed.

[0063] Therefore, in this embodiment, when narrow recording material P and wide recording material P are passed consecutively in this order, the control unit 150 controls the back contrast when passing the wide recording material P to set it to 175V. In other words, in this embodiment, the control unit 150 controls the charging bias to set such a back contrast, and controls the non-image portion potential after charging. As a result, even under printing conditions that are prone to "transfer memory," it is possible to maintain "reproducibility of one dot," suppress "fog," and suppress the occurrence of "transfer memory."

[0064] Note that under printing conditions where "transfer memory" is unlikely to occur, it is preferable to set the back contrast as small as possible from the viewpoint of improving "reproducibility of one dot" and suppressing "fog." Therefore, in this embodiment, the control unit 150 controls the back contrast to be 125 V under printing conditions other than those where "transfer memory" is likely to occur. That is, in this embodiment, the control unit 150 controls the charging bias to set such a back contrast, thereby controlling the non-image portion potential after charging. As a result, under printing conditions where "transfer memory" is unlikely to occur, printing operations can be performed under conditions that are advantageous for improving "reproducibility of one dot" and suppressing "fog" as much as possible. Note that printing conditions where "transfer memory" is unlikely to occur include any printing conditions other than those where "transfer memory" is likely to occur, including when passing a narrow recording material P when passing a narrow recording material P and a wide recording material P in that order. In this embodiment, the printing conditions under which the above-mentioned "transfer memory" is likely to occur are when a recording material P with a narrow paper width and a recording material P with a wide paper width are passed in succession in this order, and the recording material P with a wide paper width is passed.

[0065] 7. Operation procedure 6 is a flowchart showing an outline of the procedure for a print job according to this embodiment. In this example, in one print job, when narrow recording material P and wide recording material P are passed consecutively in that order, control is performed to increase the back contrast when the wide recording material P is passed.

[0066] First, the control unit 150 acquires print job information from an external device (S101). Next, the control unit 150 determines whether or not control to increase the back contrast is necessary (S102). As described above, here, an example is taken of a case where control to increase the back contrast is performed when recording materials P of different paper widths are continuously fed in one print job. Therefore, the control unit 150 determines in S102 that control to increase the back contrast is not necessary for the image formed on the first recording material P of the print job ("No"). In this case, the control unit 150 sets process conditions that result in a normal back contrast (S103) and performs image formation (S105). Next, the control unit 150 determines whether or not formation of all images specified in the print job has been completed (S106). If the control unit 150 determines in S106 that formation of all images has been completed ("Yes"), the control unit 150 ends the print job. On the other hand, if the control unit 150 determines in S106 that the formation of all images has not been completed ("No"), the process returns to S102.

[0067] Then, in S102, the control unit 150 determines that control to increase the back contrast is necessary ("Yes") if the paper width of the recording material P on which the current image formation is performed is larger than the paper width of the recording material P on which the previous image formation was performed. Here, the control unit 150 can make this determination based on information about the size of each recording material P on which an image is to be formed in the job, which information is included in print job information input from an external device and stored in memory 152. The image forming apparatus 100 is also provided with a paper width sensor 17 so that it can detect the paper width of the recording material P that is actually passed through. The control unit 150 may make this determination based on the detection result of the paper width of the recording material P on which the previous image formation was performed and the detection result of the paper width of the recording material P on which the current image formation is performed, which are detected by the paper width sensor 17 and stored in memory 152. Alternatively, the control unit 150 may make this determination using both the print job information and the detection result of the paper width sensor 17. For example, if the width of the recording material P based on the print job information differs from the width of the recording material P detected by the paper width sensor 17, the above judgment can be made based on the width of the recording material P detected by the paper width sensor 17.

[0068] If the control unit 150 determines in S102 that control to increase the back contrast is necessary ("Yes"), it sets process conditions that result in a back contrast greater than the normal back contrast (S104). Here, the back contrast in the area on the surface of the photosensitive drum 1 relative to the rotation direction of the photosensitive drum 1 that passed through the transfer unit N when the recording material P on which the previous image was formed passed through the transfer unit N is defined as the first back contrast. Also, the back contrast in the area on the surface of the photosensitive drum 1 relative to the rotation direction of the photosensitive drum 1 that passes through the transfer unit N when the recording material P on which the current image is formed passes through the transfer unit N is defined as the second back contrast. In this embodiment, the control unit 150 changes the charging bias during the inter-paper period at the charging position to change the potential of the non-image portion on the surface of the photosensitive drum 1 after the charging process so that the second back contrast is greater than the first back contrast. Then, the control unit 150 performs image formation (S105).

[0069] Thereafter, the control unit 150, in the same manner as described above, ends the print job if all images specified in the print job have been formed; if not, the process returns to S102. In this embodiment, when narrow-width recording material P and wide-width recording material P are fed consecutively in this order, the control unit 150 controls the back contrast to increase by the amount of the recording material P after the back contrast change. Therefore, when consecutive images are formed on recording material P with a width equal to or smaller than the width of the recording material P when the back contrast change is performed, it is determined again in S102 that control to increase the back contrast is not necessary. Then, in S103, process conditions that result in the normal back contrast are set. However, the present invention is not limited to this configuration. Depending on the configuration of the image forming apparatus 100, charging one sheet of recording material P may not be enough to alleviate charging unevenness caused by image formation on the narrow-width recording material P. In this case, the back contrast may be controlled to be large over multiple sheets of recording material P after switching the back contrast, which is preset so as to alleviate the charging unevenness. These multiple sheets of recording material P have a paper width larger than the paper width of the recording material P before switching the back contrast, and a paper width equal to or smaller than the paper width of the recording material P when the back contrast is switched.

[0070] Here, the description is given assuming that control is performed to increase the back contrast when passing the wide recording material P when narrow and wide recording materials P are passed consecutively in this order in a single print job. However, the printing conditions under which "transfer memory" is likely to occur are not limited to when passing the wide recording material P when narrow and wide recording materials P are passed consecutively in this order in a single print job. For example, information on multiple print jobs input to the image forming apparatus 100 from one or more external devices connected to the image forming apparatus 100 may be stored in memory 152 or the like, and multiple print jobs may be performed consecutively. In this case, the paper gap between the last recording material P of a preceding print job and the first recording material P of a subsequent print job may be set to be the same as the paper gap within a single print job. In this case, even between print jobs, when narrow and wide recording materials P are fed consecutively in this order, the feeding of the wide recording material P creates printing conditions that are likely to cause "transfer memory." In this case, the narrow recording material P is the last recording material P of the preceding print job, and the wide recording material P is the first recording material P of the subsequent print job. Therefore, in this case, too, by controlling the back contrast to increase when feeding the wide recording material P of the subsequent print job, the same effect as in the example above can be achieved. Here, printing conditions that are likely to cause "transfer memory" between print jobs are, for example, when the paper spacing between print jobs is the same as the paper spacing within a single print job, as described above. In other words, this typically occurs when multiple print jobs are performed consecutively without stopping the rotation of the photosensitive drum 1. However, this is not limited to this, and the influence of uneven charging may remain strong depending on, for example, the width and number of narrow recording materials P passed in front of the wide recording materials P (see Examples 2 and 3).If the period corresponding to the paper gap between print jobs is relatively short, even if the photosensitive drum 1 temporarily stops between those print jobs, the effects of charging unevenness that occurred in the preceding print job may remain in the subsequent print job. Therefore, even if the photosensitive drum 1 temporarily stops between print jobs, it is possible to perform control to increase the back contrast when passing a wide recording material P in the subsequent print job, for example, according to the time corresponding to the paper gap between print jobs.

[0071] Furthermore, even when narrow and wide recording materials P are fed consecutively in this order in a single print job, control to increase the back contrast may not be performed when feeding the wide recording material P. For example, the time required for feeding the paper may be longer than usual, resulting in a longer gap between sheets. Furthermore, paper feeding may be intentionally delayed due to the time required for image processing. In these cases, even when narrow and wide recording materials P are fed consecutively in this order in a single print job, the effects of charging unevenness may be sufficiently mitigated if the period corresponding to the gap between these recording materials P is sufficiently long. Therefore, for example, control to increase the back contrast may not be performed depending on the time corresponding to the gap between the recording materials P.

[0072] As described above, in this embodiment, the image forming apparatus 100 includes a rotatable photosensitive member 1, a charging unit 2 that charges the surface of the photosensitive member 1, a charging voltage application unit E1 that applies a charging voltage for charging to the charging unit, an exposure unit 3 that exposes the charged surface of the photosensitive member 1 to light to form an electrostatic image on the photosensitive member, a developing member 5b that supplies toner to the electrostatic image on the photosensitive member at a development unit D to form a toner image on the photosensitive member, a developing voltage application unit E2 that applies a developing voltage for development to the developing member 5b, a transfer member 12 that contacts the photosensitive member 1 to form a transfer unit N and transfers the toner image from the photosensitive member 1 to a recording material P passing through the transfer unit N, a transfer voltage application unit E3 that applies a transfer voltage for transfer to the transfer member 12, and a control unit 150 that controls at least one of the charging voltage application unit E1, the developing voltage application unit E2, and the exposure unit 3. In this embodiment, when a transfer is performed on a first recording material P having a first width in a direction substantially perpendicular to the conveyance direction of the recording material P, followed by a transfer on a second recording material P having a second width larger than the first width, the control unit 150 can control the second back contrast, which is the back contrast formed when the surface area of ​​the photoreceptor 1 forming the transfer portion N reaches the developing unit D as the second recording material P passes through the transfer portion N, to be larger than the first back contrast, which is the back contrast formed when the surface area of ​​the photoreceptor 1 forming the transfer portion N reaches the developing unit D as the first recording material P passes through the transfer portion N. In this embodiment, the control unit 150 controls the charging voltage application unit E2 to change the surface potential of the photoreceptor 1 charged by the charging unit 2 so that the second back contrast is larger than the first back contrast. In this embodiment, the control unit 150 controls the second back contrast to be greater than the first back contrast when the transfer is performed successively on the first recording material P and the second recording material P in a single print job, which is a series of operations that perform transfer successively on multiple recording materials P starting with a single start instruction.However, the control unit 150 may perform control so that the second back contrast is larger than the first back contrast when successive transfers are performed to the first recording material P and the second recording material P without stopping the rotation of the photosensitive member 1. Furthermore, when successively performing transfer to a third recording material P having the second width after performing transfer to the second recording material P, the control unit 150 can control so that the third back contrast, which is the back contrast formed when the area of ​​the surface of the photosensitive member 1 forming the transfer portion N reaches the developing portion D as the third recording material P passes through the transfer portion N, is larger than the first back contrast.

[0073] As described above, in this embodiment, when narrow recording material P and wide recording material P are passed consecutively in this order, the back contrast is increased when the wide recording material P is passed. This makes it possible to suppress the occurrence of "transfer memory" in the wide recording material P. Therefore, according to this embodiment, it is possible to suppress the occurrence of "transfer memory" when forming images consecutively on recording materials P of different widths, while suppressing the adverse effects of conventional methods, such as reduced productivity and increased size and cost of the apparatus.

[0074] [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 embodiment 1. Therefore, elements in the image forming apparatus of this embodiment that have the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those in embodiment 1, and detailed descriptions thereof will be omitted.

[0075] 1. Overview of this Example In the first embodiment, when narrow recording material P and wide recording material P are passed consecutively in this order, the control unit 150 performs control to increase the back contrast when passing the wide recording material P. In the present embodiment, the control unit 150 further performs control to change the back contrast when passing the subsequently passed wide recording material P (hereinafter also referred to as the "subsequent paper"), depending on the paper width of the previously passed narrow recording material P (hereinafter also referred to as the "previous paper").

[0076] As described above, in the transfer section N, the current flowing from the transfer roller 12 to the photosensitive drum 1 is greater in the non-paper-passing section where the transfer roller 12 and the photosensitive drum 1 are in direct contact than in the paper-passing section. The current density of the current flowing in the non-paper-passing section varies depending on the size of the non-paper-passing section in the transfer section N. That is, the current density of the current flowing in the non-paper-passing section varies depending on the paper width; the wider the paper width, the higher the current density of the current flowing in the non-paper-passing section. Therefore, when a narrow preceding sheet and a wide succeeding sheet are passed consecutively, the narrower and wider the preceding sheet is, the more likely "transfer memory" will occur. Therefore, the optimal value of the back contrast required to suppress "transfer memory" in the succeeding sheet varies depending on the paper width of the preceding sheet. As described above, back contrast is correlated with "fog," and increasing the back contrast may worsen "fog." Therefore, in order to suppress "fog" while suppressing the occurrence of "transfer memory," it is preferable to set the optimal back contrast in accordance with the paper width of the preceding paper.

[0077] 2. Confirmation of effectiveness Next, an evaluation experiment for confirming the effects of this embodiment will be described.

[0078] The evaluation conditions are as follows: The recording material P (paper) was B5 size with a basis weight of 68 g / m 2 CS-068 (Canon), A4 size, 68g / m 2 CS-068 (Canon), LTR size, basis weight 75 g / m 2Vitality (manufactured by Xerox) was used. The B5-size recording material P is also referred to as "B5-size paper," the A4-size recording material P as "A4-size paper," and the LTR-size recording material P as "LTR-size paper." These recording materials P were left in an environment of 15°C temperature and 10% humidity for two days, and used in a state where the moisture content was 3% as measured with a microwave moisture meter Moistrex MX8000 (manufactured by NDC Infrared Engineering Ltd.). The image forming apparatus 100 was installed in an environment of 15°C temperature and 10% humidity, and printing operations were performed under these conditions. The transfer bias applied to the transfer roller 12 when the recording material P was transported to the transfer section N was +2500V. The printing operations were performed with a recording material P transport speed of 250 mm / sec, a printing speed of 40 sheets / min, and a paper gap of 50 mm.

[0079] One print job consisted of consecutively passing B5- or A4-size paper as the leading paper and LTR-size paper as the following paper. An image with 1-dot horizontal lines spaced 50 spaces apart was printed on the LTR-size paper. When printing on the leading paper, the development bias potential was -375V, the non-image area potential after charging was -500V, and the back contrast was 125V. When printing on the following paper, the development bias potential was -375V, and the back contrast conditions were changed by varying the non-image area potential after charging to confirm the effect. The back contrast was varied to 125V, 150V, 175V, 200V, 225V, and 250V.

[0080] Then, on LTR size paper, we checked whether or not "transfer memory" occurred, and "1-dot reproducibility (whether or not the 1-dot horizontal line image was reproduced without any defects such as chipping)." Regarding "transfer memory," if an image defect associated with it occurred, it was evaluated as × (poor), and if it did not occur, it was evaluated as ○ (good). Regarding "1-dot reproducibility," if there was any defect such as chipping in the 1-dot horizontal line image, it was evaluated as × (poor), and if it was reproduced without any defects such as chipping, it was evaluated as ○ (good). The results of the evaluation experiment are shown in Table 2.

[0081] [Table 2]

[0082] As shown in Table 2, when the preceding paper was B5 size, the occurrence of "transfer memory" on the following paper was suppressed when the back contrast was 175V or higher. On the other hand, when the preceding paper was A4 size, the occurrence of "transfer memory" on the following paper was suppressed when the back contrast was 200V or higher. Furthermore, whether the preceding paper was B5 size or A size, the "reproducibility of one dot" deteriorated when the back contrast was 250V or higher.

[0083] Furthermore, as mentioned above, the back contrast is correlated with "fog," and the higher the back contrast, the worse the "fog" may become. Therefore, in order to suppress "fog" while suppressing the occurrence of "transfer memory," it is preferable to set the back contrast as small as possible within a range in which the occurrence of "transfer memory" can be suppressed.

[0084] Therefore, in this embodiment, the control unit 150 controls the back contrast when passing the subsequent LTR size paper so that it is set to 175V if the preceding paper is B5 size, and to 200V if the preceding paper is A4 size. In other words, in this embodiment, the control unit 150 controls the charging bias to set such a back contrast, and controls the non-image portion potential after charging processing. This makes it possible to maintain "one dot reproducibility," suppress "fog," and suppress the occurrence of "transfer memory" in each case where the paper width of the preceding paper is different.

[0085] The amount of change in the back contrast according to the paper width of the preceding paper is an example, and the back contrast may be set according to recording materials P of other paper widths.

[0086] 3. Operation procedure The outline of the print job procedure according to this embodiment is similar to the outline of the print job procedure in the first embodiment described with reference to FIG. 6. However, in this embodiment, in S104, the amount of change in the back contrast is changed depending on the paper width of the preceding sheet of narrow recording material P. In this embodiment, the control unit 150 stores information about the paper width of the recording material P on which the image was formed in the memory 152. This paper width information can be stored, for example, by being sequentially overwritten. Also, the memory 152 stores information indicating the previously set relationship between the paper width of the preceding sheet and the back contrast as table data or the like. Therefore, in S104, the control unit 150 can set the back contrast based on the paper width information of the preceding sheet and the information indicating the relationship.

[0087] As described above, in this embodiment, the control unit 150 controls to change the value of the second back contrast that is larger than the first back contrast in accordance with the first width of the first recording material P. In this embodiment, the control unit 150 controls to change the value of the second back contrast that is larger than the first back contrast in a case where the first width is a second value that is larger than the first value, compared to a case where the first width is a first value.

[0088] As described above, in this embodiment, when a narrow preceding sheet and a wide succeeding sheet are fed consecutively, the back contrast is set to an optimum value according to the width of the preceding sheet, thereby suppressing the occurrence of "transfer memory" while suppressing "fog" according to the width of the preceding sheet.

[0089] [Example 3] 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 embodiment 1. Therefore, elements in the image forming apparatus of this embodiment that have the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those in embodiment 1, and detailed descriptions thereof will be omitted.

[0090] 1. Overview of this Example In the first embodiment, when narrow recording material P and wide recording material P are passed consecutively in this order, the control unit 150 performs control to increase the back contrast when passing the wide recording material P. In the present embodiment, the control unit 150 further performs control to change the back contrast when passing the subsequent wide recording material P, depending on the number of narrow recording material P that have passed.

[0091] As mentioned above, "transfer memory" occurs due to charging unevenness on the surface of the photosensitive drum 1 caused by the transfer process, and is more likely to occur when the paper interval is short when consecutive sheets are passed. This is because charging unevenness cannot be alleviated between sheets when the paper interval is short. If the next sheet is passed without alleviating the charging unevenness, the charging unevenness may become even greater. As charging unevenness becomes greater, the back contrast required to suppress the occurrence of "transfer memory" also increases. In other words, it is preferable to change the back contrast when passing the subsequent wide recording material P depending on the number of preceding narrow recording material P sheets that have been passed. This makes it possible to more effectively suppress the occurrence of "transfer memory."

[0092] 2. Confirmation of effectiveness Next, an evaluation experiment for confirming the effects of this embodiment will be described.

[0093] The evaluation conditions are as follows: The recording material P (paper) was B5 size with a basis weight of 68 g / m 2 CS-068 (Canon), LTR size, basis weight 75 g / m 2Vitality (manufactured by Xerox) was used. The B5-size recording material P is also referred to as "B5-size paper," and the LTR-size recording material P is also referred to as "LTR-size paper." These recording materials P were left in an environment of 15°C temperature and 10% humidity for two days, and were used in a state where the moisture content was 3% as measured with a microwave moisture meter Moistrex MX8000 (manufactured by NDC Infrared Engineering Ltd.). The image forming apparatus 100 was installed in an environment of 15°C temperature and 10% humidity, and printing operations were performed under these conditions. The transfer bias applied to the transfer roller 12 when the recording material P was transported to the transfer section N was +2500V. The printing operations were performed with the recording material P transport speed at 250 mm / sec, the printing speed at 40 sheets / min, and the sheet interval at 50 mm.

[0094] One print job consisted of a set of B5-size sheets being fed as the leading paper, followed by LTR-size sheets being fed consecutively. An image with 1-dot horizontal lines spaced 50 spaces apart was printed on the LTR-size paper. The number of B5-size sheets fed was varied between 1 and 9, 10 to 49, and 50 or more sheets to confirm the effect. When printing on the leading paper, the development bias potential was set to -375V, the non-image area potential after charging was set to -500V, and the back contrast was set to 125V. When printing on the following paper, the development bias potential was set to -375V, and the non-image area potential after charging was changed to change the back contrast conditions to confirm the effect. The back contrast was set to 125V, 150V, 175V, 200V, 225V, and 250V.

[0095] Then, on LTR size paper, we checked whether or not "transfer memory" occurred, and "1-dot reproducibility (whether or not the 1-dot horizontal line image was reproduced without any defects such as chipping)." Regarding "transfer memory," if an image defect associated with it occurred, it was evaluated as × (poor), and if it did not occur, it was evaluated as ○ (good). Regarding "1-dot reproducibility," if there was any defect such as chipping in the 1-dot horizontal line image, it was evaluated as × (poor), and if it was reproduced without any defects such as chipping, it was evaluated as ○ (good). The results of the evaluation experiment are shown in Table 3.

[0096] [Table 3]

[0097] As shown in Table 3, when the number of narrow preceding recording materials P passed was 1 to 9, the occurrence of "transfer memory" in the following wide recording material P was suppressed when the back contrast was 175V or higher. Furthermore, when the number of narrow preceding recording materials P passed was 10 to 49, the occurrence of "transfer memory" in the following wide recording material P was suppressed when the back contrast was 200V or higher. Furthermore, when the number of narrow preceding recording materials P passed was 50 or more (50 in this example), the occurrence of "transfer memory" in the following wide recording material P was suppressed when the back contrast was 225V or higher. Furthermore, regardless of the number of narrow preceding recording materials P passed, the "reproducibility of one dot" deteriorated when the back contrast was 250V or higher. These results indicate that the greater the number of preceding sheets passed, the greater the back contrast required to suppress the occurrence of "transfer memory" in the following sheet.

[0098] Furthermore, as mentioned above, the back contrast is correlated with "fog," and the higher the back contrast, the worse the "fog" may become. Therefore, in order to suppress "fog" while suppressing the occurrence of "transfer memory," it is preferable to set the back contrast as small as possible within a range in which the occurrence of "transfer memory" can be suppressed.

[0099] Therefore, in this embodiment, when the number of narrow-width recording materials P that have passed through as preceding sheets is 1 to 9, the control unit 150 controls the back contrast to be set to 175V when passing the wide-width recording materials P that are subsequent sheets. Furthermore, when the number of narrow-width recording materials P that have passed through as preceding sheets is 10 to 49, the control unit 150 controls the back contrast to be set to 200V when passing the wide-width recording materials P that are subsequent sheets. Furthermore, when the number of narrow-width recording materials P that have passed through as preceding sheets is 50 or more, the control unit 150 controls the back contrast to be set to 225V when passing the wide-width recording materials P that are subsequent sheets. In other words, in this embodiment, the control unit 150 controls the charging bias to set such a back contrast, and controls the non-image portion potential after charging. As a result, even when the number of narrow-width recording materials P that have passed through as preceding sheets increases, the "reproducibility of one dot" can be maintained, "fog" can be suppressed, and the occurrence of "transfer memory" can be suppressed.

[0100] 3. Operation procedure The outline of the print job procedure according to this embodiment is similar to the outline of the print job procedure in the first embodiment described with reference to FIG. 6. However, in this embodiment, in S104, the amount of change in the back contrast is changed depending on the number of narrow-width recording materials P that have passed through as preceding sheets. In this embodiment, the control unit 150 accumulates and stores the number of passed sheets for each width of the recording material P in the memory 152, which functions as a counter, each time an image is formed on the recording material P. The memory 152 also stores information indicating the relationship between the number of narrow-width recording materials P that have passed through as preceding sheets and the back contrast, as previously set, in advance, such as table data. Therefore, in S104, the control unit 150 can set the back contrast based on the information on the number of narrow-width recording materials P that have passed through as preceding sheets and the information indicating the relationship. Note that when control to increase the back contrast is performed, the information on the number of passed sheets for each width stored in the memory 152 as a counter is reset to an initial value (e.g., 0).

[0101] As described in the first embodiment, the control unit 150 typically controls the back contrast to be increased for only one sheet of recording material P after the back contrast is switched. However, the present invention is not limited to this embodiment, and the control unit 150 may control the back contrast to be increased for multiple sheets of recording material P after the back contrast is switched. For example, depending on the number of narrow-width preceding sheets of recording material P passed through, charging the single sheet of recording material P may not be enough to alleviate charging unevenness caused by image formation on the preceding sheets. Therefore, depending on the number of narrow-width preceding sheets of recording material P passed through, the control unit 150 may control the back contrast to be increased for multiple sheets of recording material P after the recording material P is switched, which is preset to alleviate charging unevenness. In this case, information indicating the relationship between the preset number of narrow-width preceding sheets of recording material P passed through and the number of wide-width succeeding sheets of recording material P passed through for which the back contrast is increased may be stored in advance in the memory 152 as table data or the like.

[0102] It is also possible to combine the control based on the paper width described in the second embodiment with the control based on the number of sheets passed described in this embodiment. For example, the background contrast can be increased when passing the succeeding wide recording material P if the number of narrow preceding recording materials P is the same, or the wider the paper width is if the number of preceding recording materials P is the same.

[0103] As described above, in this embodiment, the control unit 150 controls to change the value of the second back contrast, which is larger than the first back contrast, in accordance with the number of sheets of recording material P of the first width that have been continuously transferred before transferring to the second recording material P. In this embodiment, the control unit 150 controls to change the value of the second back contrast, which is larger than the first back contrast, in the case where the number of sheets is the second number that is larger than the first number, compared to the case where the number of sheets is the first number.

[0104] As described above, in this embodiment, when narrow preceding sheets and wide succeeding sheets are passed consecutively, the back contrast is set to an optimum value according to the number of narrow preceding sheets of recording material P that have passed. This makes it possible to suppress "fog" according to the number of narrow preceding sheets of recording material P that have passed, while also suppressing the occurrence of "transfer memory."

[0105] [Example 4] 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 embodiment 1. Therefore, elements in the image forming apparatus of this embodiment that have the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those in embodiment 1, and detailed descriptions thereof will be omitted.

[0106] In the image forming apparatus 100 of this embodiment, the exposure device (laser scanner) 3 emits a weak light in the non-image area (non-toner image forming area) on the surface of the photosensitive drum 1, to an extent that does not cause excess toner to adhere to the photosensitive drum 1, thereby optimizing the potential of the photosensitive drum 1. Such weak exposure of the non-image area (non-toner image forming area) is called "background exposure." Note that the background exposure itself can be appropriately performed using, for example, a known method or configuration that is available.

[0107] In this embodiment, the exposure device 3 is a laser scanner capable of simultaneously performing background exposure (weak exposure, first output, first laser power) and normal exposure for image formation (second output, second laser power). In this embodiment, during image formation, the surface of the photosensitive drum 1 is uniformly charged to −600 V by the charging roller 2. After uniform charging and normal laser exposure by the exposure device 3, the potential of the image area on the surface of the photosensitive drum 1 is approximately −125 V. After uniform charging and background laser exposure by the exposure device 3, the background exposure potential (non-image area potential) on the surface of the photosensitive drum 1 is approximately −500 V. The background exposure potential can be adjusted by changing the amount of background exposure. In this embodiment, the toner is negatively charged in the developing device 5. During image formation, a DC voltage of −375 V is applied to the developing roller 5b as a development bias.

[0108] In this embodiment, the absolute value of the difference between the background exposure potential and the potential of the developing bias is the back contrast. Therefore, if the background exposure amount is reduced, the background exposure potential is reduced and the back contrast is increased. Furthermore, for the same background exposure amount, if the non-image area potential after charging processing is reduced, the background exposure potential is reduced and the back contrast is increased.

[0109] As explained in the above embodiment, when narrow recording material P and wide recording material P are passed consecutively in this order, the occurrence of "transfer memory" can be suppressed by increasing the back contrast when passing the wide recording material P. Also, as explained in the above embodiment, in order to suppress "fog," it is preferable to set the back contrast as small as possible.

[0110] Therefore, in this embodiment, when passing a subsequent recording material P having a wide paper width, the control unit 150 increases the back contrast only in a portion that corresponds to a non-paper passing portion when a preceding recording material P having a narrow paper width is passed and also corresponds to a paper passing portion when a subsequent recording material P having a wide paper width is passed. This will be explained in more detail below.

[0111] FIG. 7 is a schematic diagram showing the state of the transfer unit N when narrow recording material P and then wide recording material P are sequentially passed through the transfer unit N. Region B in FIG. 7 corresponds to a non-paper passing portion of the transfer unit N when narrow recording material P is passed through, and a paper passing portion of the transfer unit N when wide recording material P is passed through, in a direction substantially perpendicular to the conveyance direction of the recording material P at the transfer unit N. In this embodiment, when a subsequent wide recording material P is passed through, the back contrast is increased only in region B. This minimizes "fog" and suppresses the occurrence of "transfer memory." Specifically, the non-image portion potential of the photosensitive drum 1 after charging is set to -600 V both when a preceding narrow recording material P is passed through and when a subsequent wide recording material P is passed through. Then, when a subsequent recording material P having a wider paper width is passed through, the background exposure amount is reduced (turned off in this embodiment) only in region B, the background exposure potential is set to -600 V, and the back contrast is set to 225 V. When a subsequent recording material P having a wider paper width is passed through, the background exposure amount is increased (turned on at a predetermined light amount) in regions other than region B, the background exposure potential is set to -500 V, and the back contrast is set to 125 V. In this embodiment, the background exposure is turned off when the background exposure amount is reduced, but the exposure amount may be reduced so as to form the desired potential.

[0112] In Example 1, the back contrast was increased in area A in Figure 4. On the other hand, in this example, the area where the back contrast is increased is limited to area B, which is narrower than area A in Example 1. This makes it possible to minimize "fog" while suppressing the occurrence of "transfer memory."

[0113] Thus, in this embodiment, the exposure unit 3 exposes the charged surface of the photosensitive member 1 with a first output that forms a potential in the non-image area and a second output that forms a potential in the image area to form an electrostatic image on the photosensitive member, and the control unit 150 controls the exposure unit 3 to change the first output so that only the second back contrast in the area of ​​the surface of the photosensitive member 1 that does not come into contact with the first recording material P at the transfer unit N and that comes into contact with the second recording material P at the transfer unit N is greater than the first back contrast.

[0114] As described above, in this embodiment, when a preceding sheet with a narrow paper width and a succeeding sheet with a wide paper width are passed consecutively, when the succeeding sheet with a wide paper width, the back contrast is increased only in the portion that corresponds to the non-paper passing portion when the preceding sheet with a narrow paper width, the recording material P, is passed, and also corresponds to the paper passing portion when the succeeding sheet with a wide paper width, the recording material P, is passed. This makes it possible to minimize the occurrence of "fog" and suppress the occurrence of "transfer memory."

[0115] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0116] For example, while the image forming apparatus in the above-described embodiment is a monochrome image forming apparatus with one image forming unit, the present invention can also be applied to, for example, a tandem-type image forming apparatus with multiple image forming units employing a direct transfer system. As is well known to those skilled in the art, this image forming apparatus forms different color toner images in multiple image forming units, each equipped with a photosensitive element, in the same manner as in the above-described embodiment. The toner images formed in the multiple image forming units are then transferred in a superimposed manner onto a recording material carried and transported on a recording material carrier, such as an endless belt. This transfer is often performed by applying a transfer voltage to a roller or other device that contacts the inner surface of the recording material carrier. In this case, the recording material carrier and the roller can be considered to constitute a transfer member. The recording material to which the toner image has been transferred is then fixed in the same manner as in the above-described embodiment and then discharged from the image forming apparatus. Even in such an image forming apparatus, "transfer memory" can occur in each image forming unit due to uneven charging of the photosensitive element of each image forming unit caused by the transfer process in the transfer unit of each image forming unit. Therefore, in such an image forming apparatus, it is possible to control the back contrast in each image forming unit in the same manner as in the above-described embodiment, and it is possible to obtain the effect of suppressing the occurrence of "transfer memory" in the same manner as in the above-described embodiment.

[0117] Furthermore, in the above-described embodiment, the image forming apparatus was not provided with a pre-charge exposure device. According to the present invention, it is possible to suppress the occurrence of "transfer memory" with a simple configuration without providing a pre-charge exposure device. However, the present invention can also be applied to cases where the image forming apparatus is provided with a pre-charge exposure device. In this case, by applying the present invention, it is possible to reduce the exposure amount by the pre-charge exposure device while suppressing the occurrence of "transfer memory," and to simplify the configuration of the pre-charge exposure device.

[0118] Furthermore, in a configuration where background exposure is performed, control may be performed to increase the back contrast of the area on the photosensitive member in the same manner as in the first embodiment.

[0119] Furthermore, in the above-described embodiment, the photosensitive member is a rotatable drum-shaped member, but it may also be an endless belt-shaped member supported by a plurality of support rollers.

[0120] In the above-described embodiment, the charging potential (charging bias) of the photosensitive member is changed to change the back contrast, but the developing bias may be changed, or both the charging potential (charging bias) of the photosensitive member and the developing bias may be changed. When the developing bias is changed, it is desirable to also change the exposure amount so as to maintain the developing contrast, which is the absolute value of the difference between the potential of the image portion of the photosensitive member and the developing bias (to reduce the change in the developing contrast due to the change in the developing bias). [Explanation of symbols]

[0121] 1 Photosensitive drum 2 Charging roller 3 Exposure equipment 4 Cleaning device 5. Developing device 12 Transfer roller 100 Image forming device 150 control section P recording material

Claims

1. a rotatable photoreceptor; a charging unit that charges the surface of the photoreceptor; a charging voltage application unit that applies a charging voltage to the charging unit; an exposure unit that exposes the charged surface of the photoreceptor to light to form an electrostatic image on the photoreceptor; a developing member that supplies toner to the electrostatic image on the photosensitive member in a developing section to form a toner image on the photosensitive member; a developing voltage applying section that applies a developing voltage to the developing member; a transfer member that contacts the photoreceptor to form a transfer portion and transfers a toner image from the photoreceptor to a recording material that passes through the transfer portion; a transfer voltage applying section that applies a transfer voltage to the transfer member; a control unit that controls at least one of the charging voltage application unit, the developing voltage application unit, and the exposure unit; In an image forming apparatus having When the absolute value of the difference between the potential of the non-image portion of the surface of the photosensitive member in the developing unit and the potential of the developing voltage is defined as a back contrast, when transferring to a first recording material having a first width in a direction substantially perpendicular to the recording material conveying direction and then transferring to a second recording material having a second width larger than the first width, the control unit is capable of controlling so that the second back contrast, which is the back contrast formed when the area of ​​the surface of the photosensitive member forming the transfer portion reaches the developing unit when the second recording material passes through the transfer unit, is larger than the first back contrast, which is the back contrast formed when the area of ​​the surface of the photosensitive member forming the transfer portion reaches the developing unit when the first recording material passes through the transfer unit, the control unit controls the charging voltage application unit to change the surface potential of the photosensitive member formed by the charging process performed by the charging unit so that the second back contrast is greater than the first back contrast. An image forming apparatus characterized by:

2. A rotatable photoreceptor; a charging unit that charges the surface of the photoreceptor; a charging voltage application unit that applies a charging voltage to the charging unit; an exposure unit that exposes the charged surface of the photoreceptor to light to form an electrostatic image on the photoreceptor; a developing member that supplies toner to the electrostatic image on the photosensitive member in a developing section to form a toner image on the photosensitive member; a developing voltage applying section that applies a developing voltage to the developing member; a transfer member that contacts the photoreceptor to form a transfer portion and transfers a toner image from the photoreceptor to a recording material that passes through the transfer portion; a transfer voltage applying section that applies a transfer voltage to the transfer member; a control unit that controls at least one of the charging voltage application unit, the developing voltage application unit, and the exposure unit; In an image forming apparatus having When the absolute value of the difference between the potential of the non-image portion of the surface of the photosensitive member in the developing unit and the potential of the developing voltage is defined as a back contrast, when transferring to a first recording material having a first width in a direction substantially perpendicular to the recording material conveying direction and then transferring to a second recording material having a second width larger than the first width, the control unit is capable of controlling so that the second back contrast, which is the back contrast formed when the area of ​​the surface of the photosensitive member forming the transfer portion reaches the developing unit when the second recording material passes through the transfer unit, is larger than the first back contrast, which is the back contrast formed when the area of ​​the surface of the photosensitive member forming the transfer portion reaches the developing unit when the first recording material passes through the transfer unit, the exposure unit exposes the charged surface of the photoconductor to a first output that forms a potential in a non-image area and a second output that forms a potential in an image area, thereby forming an electrostatic image on the photoconductor; the control unit controls the exposure unit to change the first output so that only the second back contrast in a region of the surface of the photosensitive member that does not come into contact with the first recording material at the transfer unit and that comes into contact with the second recording material at the transfer unit is larger than the first back contrast. An image forming apparatus characterized by:

3. 3. The image forming apparatus according to claim 1, wherein the control unit controls the second back contrast, which is greater than the first back contrast, to be changed in accordance with the first width.

4. The image forming apparatus according to claim 3, characterized in that the control unit controls the value of the second back contrast, which is greater than the first back contrast, to be larger when the first width is a second value greater than the first value than when the first width is a first value.

5. The image forming apparatus according to any one of claims 1 to 4, characterized in that the control unit controls to change the value of the second back contrast, which is larger than the first back contrast, depending on the number of sheets of recording material of the first width that have been continuously transferred before transferring to the second recording material.

6. The image forming apparatus according to claim 5, characterized in that the control unit controls the value of the second back contrast, which is greater than the first back contrast, to be larger when the number of sheets is a second number greater than the first number than when the number of sheets is a first number.

7. The image forming apparatus according to any one of claims 1 to 6, characterized in that the control unit performs control so that the second back contrast is greater than the first back contrast when performing continuous transfer onto the first recording material and the second recording material in a single print job, which is a series of operations that perform continuous transfer onto multiple recording materials starting with a single start instruction.

8. An image forming apparatus according to any one of claims 1 to 6, characterized in that the control unit performs control so that the second back contrast is greater than the first back contrast when continuous transfer is performed to the first recording material and the second recording material without stopping the rotation of the photosensitive member.

9. An image forming apparatus as described in any one of claims 1 to 8, characterized in that when transferring to a third recording material having the second width consecutively after transferring to the second recording material, the control unit is capable of controlling the third back contrast, which is the back contrast formed when the area of ​​the surface of the photosensitive body forming the transfer section reaches the development section when the third recording material passes through the transfer section, to be larger than the first back contrast.

Images